Chapter 13 EMI/Direction Finding

THE SCOPE OF THE PROBLEM Decibel (dB) — a logarithmic unit of tibility” and is the term typically used in As our lives become filled with tech- relative power measurement that ex- the commercial world. nology, the likelihood of electronic presses the ratio of two power levels. Induction — the transfer of electrical interference increases. Every lamp dim- Differential-mode signals — Signals signals via magnetic coupling. mer, garage-door opener or other new that arrive on two or more conductors Interference — the unwanted interac- technical “toy” contributes to the electri- such that there is a 180° phase difference tion between electronic systems. cal noise around us. Many of these de- between the signals on some of the con- Intermodulation — the undesired mix- vices also “listen” to that growing noise ductors. ing of two or more frequencies in a nonlin- and may react unpredictably to their elec- Electromagnetic compatibility (EMC) ear device, which produces additional tronic neighbors. — the ability of electronic equipment to frequencies. Sooner or later, nearly every Amateur be operated in its intended electromag- Low-pass filter — a filter designed to Radio operator will have a problem with netic environment without either causing pass all frequencies below a cutoff fre- interference. Most cases of interference interference to other equipment or sys- quency, while rejecting frequencies above can be cured! The proper use of “diplo- tems, or suffering interference from other the cutoff frequency. macy” skills and standard cures will usu- equipment or systems. Noise — any signal that interferes with ally solve the problem. Electromagnetic interference (EMI) the desired signal in electronic communi- This section of Chapter 13, by Ed Hare, — any electrical disturbance that inter- cations or systems. W1RFI, is only an overview. The ARRL feres with the normal operation of elec- Nonlinear — having an output that is RFI Book contains detailed information on tronic equipment. not in linear proportion to the input. the causes of and cures for nearly every Emission — electromagnetic energy Notch filter — a filter that rejects or type of interference problem.1 propagated from a source by radiation. suppresses a narrow band of frequencies Filter — a network of resistors, induc- within a wider band of frequencies. Pieces of the Problem tors and/or capacitors that offer little Passband — the band of frequencies Every interference problem has two resistance to certain frequencies while that a filter conducts with essentially no components — the equipment that is in- blocking or attenuating other frequen- attenuation. volved and the people who use it. A solu- cies. Radiated emission — radio-frequency tion requires that we deal with both the Fundamental overload — interference energy that is coupled between two sys- equipment and the people effectively. resulting from the fundamental signal of a tems by electromagnetic fields. First, define the term “interference” with- radio transmitter. Radio-frequency interference (RFI) out emotion. The ARRL recommends that Ground — a low-impedance electrical — interference caused by a source of the hams and their neighbors cooperate to connection to the earth. Also, a common radio-frequency signals. This is a subclass find solutions. The FCC shares this view. reference point in electronic circuits. of EMI. Harmonics — signals at exact integral Spurious emission — An emission, on Important Terms multiples of the operating (or fundamen- frequencies outside the necessary band- Bypass capacitor — a capacitor used tal) frequency. width of a transmission, the level of which to provide a low-impedance radio-fre- High-pass filter — a filter designed to may be reduced without affecting the in- quency path around a circuit element. pass all frequencies above a cutoff fre- formation being transmitted. Common-mode signals — signals that quency, while rejecting frequencies below Susceptibility — the characteristic of are in phase on both (or several) conduc- the cutoff frequency. electronic equipment that permits undes- tors in a system. Immunity — the ability of electronic ired responses when subjected to electro- Conducted signals — signals that equipment to reject interference from ex- magnetic energy. travel by electron flow in a wire or other ternal sources of electromagnetic energy. TVI — interference to television sys- conductor. This is the conjugate of the term “suscep- tems.

EMI/Direction Finding 13.1

Chapter 13.pmd 1 7/28/2006, 9:15 AM problem, or call channel 8 to see if the sta- you are not causing the interference! This Responsibility tion has a problem. If channel 8 was oper- is also a valuable troubleshooting tool: If When an interference problem occurs, ating properly, you would likely decide that you know your station is clean, you have we may ask “Who is to blame?” The ham your TV set is broken. Now, if you tune cut the size of the problem in half! If the and the neighbor often have different opin- your TV to channel 3, and see your local FCC ever gets involved, you can demon- ions. It is almost natural (but unproduc- shortwave radio station (quite possibly strate that you are not interfering with your tive) to fix blame instead of the problem. Amateur Radio), don’t blame the short- own equipment. No amount of wishful thinking (or de- wave station without some investigation. Apply EMI cures to your own consumer mands for the “other guy” to solve the In fact, many televisions respond to strong electronics equipment. When your neigh- problem) will result in a cure for interfer- signals outside the television bands. They bor sees your equipment working well, it ence. Each individual has a unique per- may be working as designed, but require demonstrates that filters work and cause spective on the situation, and a different added filters and/or shields to work prop- no harm. degree of understanding of the personal erly near a strong, local RF signal. To clean up your station, clean up the and technical issues involved. On the other Your neighbor will probably feel much mess! A rat’s nest of wires, unsoldered hand, each person has certain responsibili- better if you explain that you will help find connections and so on in your station can ties to the other and should be prepared to a solution, even if the interference is not contribute to EMI. To help build a better address those responsibilities fairly. your fault. This offer can change your relationship, you may want to show your image from neighborhood villain to hero, station to your neighbor. A clean station FCC Regulations especially if the interference is not caused looks professional; it inspires confidence A radio operator is responsible for the by your station. (This is often the case.) in your ability to solve the EMI problem. proper operation of the radio station. This Install a transmit filter (low-pass or responsibility is spelled out clearly in Part band-pass) and a reasonable station PREPARE YOURSELF 97 of the FCC regulations. If interference ground. (If the FCC becomes involved, is caused by a spurious emission from your Learn About EMI they will ask you about both items.) Show station, you must correct the problem In order to troubleshoot and cure EMI, your neighbor that you have installed the there. you need to learn more than just the ba- necessary filter on your transmitter and Fortunately, most cases of interference sics. This is especially important when explain that if there is still interference, it are not the fault of the transmitting sta- dealing with your neighbor. If you visit is necessary to try filters on the neighbor’s tion. Most interference problems involve your neighbor’s house and try a few dozen equipment, too. some kind of electrical noise or fundamen- things that don’t work (or make things Operating practices and station-design tal overload. worse), your neighbor may lose confi- considerations can affect EMI. Don’t dence in your ability to help cure the prob- overdrive a transmitter or amplifier; that Personal Diplomacy lem. If that happens, you may be asked to can increase its harmonic output. You can What happens when you first talk to leave. take steps to reduce the strength of your your neighbor sets the tone for all that signal at the victim equipment. This might follows. Any technical solutions cannot Local Help include reducing transmit power. Locate help if you are not allowed in your If you are not an expert (and even ex- the antenna as far as possible from suscep- neighbor’s house to explain them! If the perts can use moral support), you should tible equipment or its wiring (ac line, tele- interference is not caused by spurious find some local help. Fortunately, such phone, cable TV). Antenna orientation emissions from your station, however, you help is often available from your Section may be important. For example, if your should be a locator of solutions, not a pro- Technical Coordinator (TC). The TC HF dipole at 30 ft is coupling into the vider of solutions. knows of any local RFI committees and neighbor’s overhead cable-TV drop, that Your neighbor will probably not under- may have valuable contacts in the local coupling could be reduced 20 dB by stand all of the technical issues — at least utility companies. Even an expert can ben- changing to a vertical antenna — even not at first. Understand that, regardless of efit from a TC’s help. more by orienting the antenna so that the fault, an interference problem is annoying The easiest way to find your TC is drop is off its end. Try different modes; to your neighbor. Let your neighbor know through your ARRL Section Manager CW or FM usually do not generate nearly that you want to help find a solution and (SM). There is a list of SMs on page 16 of as much telephone interference as AM or that you want to begin by talking things any recent QST. He or she can quickly put SSB, for example. over. you in contact with the best source of local Talk about some of the more important help. Call Your Neighbor technical issues, in non-technical terms. Even if you can’t secure the help of a Now that you have learned more about Interference can be caused by unwanted local expert, a second ham can be a valu- EMI, located some local help (we’ll as- signals from your transmitter. Assure your able asset. Often a second party can help sume it’s the TC) and done all of your neighbor that you will check your station defuse any hostility. It is also helpful to homework, make contact with your thoroughly and correct any problems. You have someone to operate your station neighbor. First, arrange an appointment should also discuss the possible sus- while you and your neighbor run through convenient for you, the TC and your ceptibility of consumer equipment. You troubleshooting steps and try various neighbor. After you introduce the TC, may want to print a copy of the RFI cures. allow him or her to explain the issues to information found on ARRLWeb at: www. your neighbor. Your TC will be able to arrl.org/news/rfi/neighbors.html. Prepare Your Home answer most questions, but be prepared Here is a good analogy: If you tune your The first step toward curing an interfer- to assist with support and additional in- TV to channel 3, and see channel 8 instead, ence problem is to make sure your own formation as required. would you blame channel 8? No. You might signal is clean. You must eliminate all Invite the neighbor to visit your station. check another set to see if it has the same interference in your own house to be sure Show your neighbor some of the things

13.2 Chapter 13

Chapter 13.pmd 2 7/28/2006, 9:15 AM you do with your radio equipment. Point Table 13.1 out any test equipment you use to keep your station in good working order. Of EMI Survival Kit course, you want to show the filters you Filters: have installed on your transmitter. (2) 300-Ω high-pass filter (different brands recommended) Next, have the TC operate your station (2) 75-Ω high-pass filter (different brands recommended) on several different bands. Show your (2) Commercially available common-mode chokes (optional) neighbor that your home electronics (12) Assorted ferrite cores: 43, 63 and 75 material, FT-140 and FT-240 size (3) Telephone RFI filters (different brands recommended) equipment is working properly while your (2) Brute-force ac line filters station is in operation. Point out the filters (6) 0.01-μF ceramic capacitors you have installed to correct any suscepti- (6) 0.001-μF ceramic capacitors bility problems. (2) Speaker-lead filters (optional) At this point, tell your neighbor that the Miscellaneous: next step is to try some of these cures on • Hand tools, assorted screwdrivers, wire cutters, pliers his or her equipment. This is a good time • Hookup wire to emphasize that the problem is probably • Electrical tape not your fault, but that you and the TC will • Soldering iron and solder (use with caution!) try to help find a solution anyway. • Assorted lengths 75-Ω coaxial cable with connectors Table 13.1 is a list of the things needed • Spare F connectors, male to troubleshoot and solve most EMI prob- • F-connector female-female “barrel” • Alligator clips lems. Decide ahead of time which of these • Notebook and pencil items are needed and take them with you. • Portable multimeter At Your Neighbor’s Home You and the TC should now visit the neighbor’s home. Inspect the equipment installation and ask when the interference occurs, what equipment is involved and fects. Try all bands and modes that you recognize electrical noise or some kind of what frequencies or channels are affected. use. Ask the neighbor to demonstrate the equipment malfunction. If so, explain The answers are valuable clues. Next, problem. your findings to the neighbor and suggest either you or the TC should operate your The tests may show that your station that he or she contact appropriate service station while the other observes the ef- isn’t involved at all. You may immediately personnel.

EMC Fundamentals Knowledge is one of the most valuable radiated by the source and picked up by a filter that passes desired signals and tools for solving EMI problems. A suc- conductor attached to the victim (or di- shunts unwanted signals to the return line. cessful EMI cure usually requires famil- rectly by the victim’s circuitry) and is then Most desired signals, such as the TV sig- iarity with the relevant technology and conducted into the victim. EMI from in- nal inside a coaxial cable are differential- troubleshooting procedures. duction is rare. mode signals. In a common-mode circuit, many wires SOURCE-PATH-VICTIM DIFFERENTIAL VS of a multiwire system act as if they were a All cases of EMI involve a source of COMMON-MODE single wire. The result can be a good electromagnetic energy, a device that re- It is important to understand the differ- antenna, either as a radiator or as a recep- sponds to this electromagnetic energy ences between differential-mode and tor of unwanted energy. The return path is (victim) and a transmission path that common-mode conducted signals (see usually earth ground. Since the source and allows energy to flow from the source to Fig 13.1). Each of these conduction modes return conductors are usually well sepa- the victim. Sources include radio transmit- requires different EMI cures. Differential- rated, there is no reliable phase difference ters, receiver local oscillators, computing mode cures, (the typical high-pass filter, between the conductors and no convenient devices, electrical noise, lightning and for example) do not attenuate common- place to shunt unwanted signals. Toroid other natural sources. mode signals. On the other hand, a typical chokes are the answer to common-mode There are three ways that EMI can travel common-mode choke does not affect in- interference. (The following explanation from the source to the victim: radiation, terference resulting from a differential- applies to rod cores as well as toroids, but conduction and induction. Radiated EMI mode signal. since rod cores may couple into nearby propagates by electromagnetic radiation Differential-mode currents usually circuits, use them only as a last resort.) from the source, through space to the vic- have two easily identified conductors. In a Toroids work differently, but equally tim. A conducted signal travels over wires two-wire transmission line, for example, well, with coaxial cable and paired conduc- connected to the source and the victim. the signal leaves the generator on one line tors. A common-mode signal on a coaxial Induction occurs when two circuits are and returns on the other. When the two cable is usually a signal that is present on magnetically (and in some cases, electri- conductors are in close proximity, they the outside of the cable shield. When we cally) coupled. Most EMI occurs via con- form a transmission line and there is a 180° wrap the cable around a ferrite-toroid core, duction, or some combination of radiation phase difference between their respective the choke appears as a reactance in series and conduction. For example, a signal is signals. It’s relatively simple to build a with the outside of the shield, but it has no

EMI/Direction Finding 13.3

Chapter 13.pmd 3 7/28/2006, 9:15 AM source. When the distance from the source doubles, the strength of the electromag- netic field decreases to one-fourth of its strength at the original distance from the source. This characteristic can often be used to help solve EMI cases. You can often make a significant improvement by moving the victim equipment and the antenna farther away from each other. External Noise Most cases of interference reported to the FCC involve some sort of external noise source. The most common of these noise sources are electrical. External “noise” can also come from transmitters or from unlicensed RF sources such as computers, video games, electronic mice repellers and the like. Typically, such de- vices are legal under Part 15 of the FCC’s rules. Electrical noise is fairly easy to identify by looking at the picture of a susceptible Fig 13.1 — A shows a differential-mode, while B shows a common-mode signal. TV or listening on an HF receiver. A photo The two kinds of signals are described in the text. of electrical noise on a TV screen is shown in the TVI section of this chapter. On a effect on signals inside the cable because A strong fundamental signal can enter receiver, it usually sounds like a buzz, their field is (ideally) confined inside the equipment in several different ways. Most sometimes changing in intensity as the arc shield. With paired conductors such as commonly, it is conducted into the equip- or spark sputters a bit. If you determine zip-cord, signals with opposite phase set ment by wires connected to it. Possible the problem to be caused by external noise, up magnetic fluxes of opposite phase in the conductors include antennas and feed it must be cured at the source. Refer to the core. These “differential” fluxes cancel lines, interconnecting cables, power lines Electrical Noise section of this chapter and each other, and there is no net reactance for and ground wires. TV antennas and feed The ARRL RFI Book. the differential signal. To common-mode lines, telephone or speaker wiring and ac Spurious Emissions signals, however, the choke appears as a power leads are the most common points reactance in series with the line. of entry. All transmitters generate some (hope- Toroid chokes work less well with The effect of an interfering signal is fully few) RF signals that are outside their single-conductor leads. Because there is no directly related to its strength. The allocated frequency bands. These out-of- return current to set up a canceling flux, the strength of a radiated signal diminishes band signals are called spurious emis- choke appears as a reactance in series with with the square of the distance from the sions, or spurs. Spurious emissions can be both the desired and undesired signals. SOURCES OF EMI The basic causes of EMI can be grouped into several categories: • Fundamental overload effects • External noise • Spurious emissions from a transmitter • Intermodulation distortion or other external spurious signals As an EMI troubleshooter, you must determine which of these are involved in your interference problem. Once you do, it is easy to select the necessary cure. Fundamental Overload Most cases of interference are caused by fundamental overload. The world is filled with RF signals. Properly designed equipment should be able to select the desired signal, while rejecting all others. Unfortunately, because of design defi- Fig 13.2 — The spectral output of a typical amateur transmitter. The fundamental is at 7 MHz. There are visible harmonics at 14, 21 and 28 MHz. Unlabeled lines are ciencies such as inadequate shields or fil- non-harmonic spurious emissions. This transmitter complies with the stringent ters, some equipment is unable to reject FCC spectral-purity regulations regarding amateur transmitters with less than 5 W strong out-of-band signals. of RF output.

13.4 Chapter 13

Chapter 13.pmd 4 7/28/2006, 9:15 AM discrete signals or wideband noise. Har- millions. You probably will not see your weather? You should do the same when- monics, the most common spurious emis- exact problem and cure listed in this book or ever you operate. If you can readily repro- sions, are signals at exact multiples of the any other. You must diagnose the problem! duce the problem with your station, you operating (or fundamental) frequency. Troubleshooting an EMI problem is a can start to troubleshoot the problem. Other discrete spurious signals are usu- three-step process, and all three steps are ally caused by the superheterodyne mix- equally important: Diagnose the Problem ing process used in most modern • Identify the problem Look Around — Aside from the brain, transmitters. Fig 13.2 shows the spectral • Diagnose the problem eyes are a troubleshooter’s best tool. Look output of a transmitter, including harmon- • Cure the problem. around. Installation defects contribute to ics and mixing products. many EMI problems. Look for loose con- Transmitters may also produce broad- Identify the Problem nections, shield breaks in a cable-TV in- band noise and/or “parasitic” oscillations. Is It Really EMI? — Before trying to stallation or corroded contacts in a (Parasitic oscillations are discussed in the solve a suspected case of EMI, verify that telephone installation. Fix these first. RF Power Amplifiers chapter.) If these the symptoms actually result from exter- Problems that occur only on harmonics unwanted signals cause interference to nal causes. A variety of equipment mal- of the fundamental signal usually indicate another radio service, FCC regulations functions or external noise can look like the transmitter. Harmonics can also be require the owner to correct the problem. interference. “Your” EMI problem might generated in nearby semiconductors, such be caused by another ham or a radio trans- as an unpowered VHF receiver left con- TROUBLESHOOTING EMI mitter of another radio service, such as a nected to an antenna, or a corroded con- Most EMI cases are complex. They in- local CB or police transmitter. nection in a tower guy wire. Harmonics volve a source, a path and a victim. Each of Is It Your Station? — If it appears that can also be generated in the front-end these main components has a number of your station is involved, operate your sta- components of the TV or radio experienc- variables: Is the problem caused by harmon- tion on each band, mode and power level ing interference. ics, fundamental overload, conducted emis- that you use. Note all conditions that pro- Is the wiring connected to the victim sions, radiated emissions or a combination duce interference. If no transmissions pro- equipment resonant on one or more ama- of all of these factors? Should it be fixed duce the problem, your station may not be teur bands? If so, a common-mode choke with a low-pass filter, high-pass filter, com- the cause. (Although some contributing placed at the middle of the wiring may be mon-mode chokes or ac-line filter? How factor may have been missing in the test.) an easy cure. about shielding, isolation transformers, a Have your neighbor keep notes of when These are only a few of the questions different ground or antenna configuration? and how the interference appears: what you might need to ask. Any information By the time you finish with these ques- time of day, what station, what other ap- you gain about the systems involved will tions, the possibilities could number in the pliances were in use, what was the help find the EMI cause and cure.

Cures At Your Station — Make sure that your problem is solved, then start adding lines interference returns, you know that there own station and consumer equipment are and equipment back one at a time, fixing were multiple causes. clean. This cuts the size of the problem in the problems as you go along. If you are half! Once this is done, you won’t need to lucky, you will solve all of the problems in OVERVIEW OF TECHNIQUES diagnose or troubleshoot your station one pass. If not, at least you can point to later. Also, any cures successful at your one piece of equipment as the source of Shields house may work at your neighbor’s as the problem. Shields are used to set boundaries for well. If you do have problems in your own Multiple Causes — Many EMI prob- radiated energy. Thin conductive films, house, refer first to the Transmitter sec- lems have multiple causes. These are usu- copper braid and sheet metal are the most tion of this chapter, or continue through ally the ones that give new EMI common shield materials. Maximum the troubleshooting steps and specific troubleshooters the most trouble. If, for shield effectiveness usually requires solid cures and take care of your own problem example, a TVI problem is caused by har- sheet metal that completely encloses the first. monics from the transmitter, an arc in the source or susceptible circuitry or equip- Simplify the Problem — Don’t tackle a transmitting antenna, an overloaded TV ment. Small discontinuities, such as holes complex system — such as a telephone preamp, differential-mode fundamental or seams, decrease shield effectiveness. In system in which there are two lines run- overload generating harmonics in the TV addition, mating surfaces between differ- ning to 14 rooms — all at once. You could tuner, induced and conducted RF in the ent parts of a shield must be conductive. spend the rest of your life running in ac-power system and a common-mode sig- To ensure conductivity, file or sand off circles and never find the true cause of the nal picked up on the shield of the TV’s paint or other nonconductive coatings on problem. coaxial feed line, you would never find a mating surfaces. There’s a better way. In our hypotheti- cure by trying only one at a time! cal telephone system, first locate the tele- In this case, the solution requires that Filters phone jack closest to the telephone service you apply all of the cures at the same time. A major means of separating signals entrance. Disconnect the lines to more When troubleshooting, if you try a cure, relies on their frequency differences. Fil- remote jacks and connect one EMI-resis- leave it in place. When you finally try a ters offer little opposition to certain fre- tant telephone at the remaining jack. If the cure that really works, start removing the quencies while blocking others. Filters interference remains, try cures until the “temporary” attempts one at a time. If the vary in attenuation characteristics, fre-

EMI/Direction Finding 13.5

Chapter 13.pmd 5 7/28/2006, 9:15 AM quency characteristics and power-han- typical low-pass filter curve is shown in pedance path to ground for RF signals. dling capabilities. The names given to Fig 13.3. A schematic is shown in Fig 13.4. Bypass capacitors for HF signals are usu- various filters are based on their uses. These filters are difficult to construct prop- ally 0.01 μF, while VHF bypass capaci- Low-pass filters pass frequencies below erly so you should buy one. Many retail tors are usually 0.001 μF. some cutoff frequency, while attenuating Amateur Radio stores that advertise in QST AC-line filters, sometimes called frequencies above that cutoff frequency. A stock low-pass filters. “brute-force” filters, are used to filter RF High-pass filters pass frequencies energy from power lines. A schematic is above some cutoff frequency while attenu- shown in Fig 13.7. Only ac-rated capaci- ating frequencies below that cutoff fre- tors as specified in Fig 13.7 should be used quency. A typical high-pass filter curve is for the filter. These RFI capacitors must shown in Fig 13.5. Fig 13.6 shows a sche- be used in applications where a hazard matic of a typical high-pass filter. Again, could be present to a person who touches it is best to buy one of the commercially the associated equipment — if such a ca- available filters. pacitor were to fail. Y-Class capacitors are Bypass capacitors can be used to cure designed for connection between power EMI problems. A bypass capacitor is usu- lines and from line to ground. There are ally placed between a signal or power lead several sub-classes of Y-class capacitors, and circuit ground. It provides a low-im- Y2 being the most common. Y2 capaci-

Fig 13.3 — An example of a low-pass filter-response curve.

Fig 13.4 — A low-pass filter for amateur transmitting use. Complete construction information appears in the Transmitters chapter of The ARRL RFI Book.

Fig 13.5 — An example of a high-pass filter response curve.

Fig 13.6 — A differential-mode high-pass filter for 75-Ω coax. It rejects HF signals picked up by a TV antenna or that leak into a cable-TV system. It is ineffective against common-mode signals. All capacitors are high-stability, low-loss, NP0 ceramic discs. Values are in pF. The inductors are all #24 enameled wire on T-44-0 toroid cores. L4 and L6 are each 12 turns (0.157 μH). L5 is 11 turns (0.135μH).

Fig 13.7 — A “brute-force” ac-line filter.

Fig 13.8 — Several styles of common- mode chokes. Fig 13.9 — Impedance vs. frequency plots for “101” size ferrite beads.

13.6 Chapter 13

Chapter 13.pmd 6 7/28/2006, 9:15 AM tors are rated for nominal working volt- ages less than or equal to 250 Vac. Their peak impulse voltage rating is consider- ably higher however, up to 5 kV. Common-Mode Chokes Common-mode chokes may be the best- kept secret in Amateur Radio. The differ- ential-mode filters described earlier are not effective against common-mode signals. To eliminate common-mode signals prop- erly, you need common-mode chokes. They may help nearly any interference problem, from cable TV to telephones to audio inter- ference caused by RF picked up on speaker leads. Common-mode chokes usually have ferrite core materials. These materials are well suited to attenuate common-mode currents. Several kinds of common-mode chokes are shown in Fig 13.8. The optimum size and ferrite material are determined by the application and fre- quency. For example, an ac cord with a plug attached cannot be easily wrapped on a small ferrite core. The characteristics of ferrite materials vary with frequency, as shown by the graph in Fig 13.9. Fig 13.10 — A shows a stereo system grounded as an undesirable “ground loop.” Grounds B is the proper way to ground a multiple-component system. An electrical ground is not a huge sink that somehow swallows noise and un- wanted signals. Ground is a circuit con- When is Ground not a Ground? cept, whether the circuit is small, like a In many stations, it is impossible to get radio receiver, or large, like the propaga- a good RF connection to earth ground. tion path between a transmitter and cable- Most practical installations require sev- TV installation. Ground forms a universal eral feet of wire between the station reference point between circuits. ground connection and an outside ground This chapter deals with the EMC as- rod. Many troublesome harmonics are in pects of grounding. While grounding is not the VHF range. At VHF, a ground wire a cure-all for EMI problems, ground is an length can be several wavelengths long — important safety component of any elec- a very effective long-wire antenna! Any tronics installation. It is part of the light- VHF signals that are put on a long ground ning protection system in your station and wire will be radiated. This is usually not a critical safety component of your house the intended result of grounding. wiring. Any changes made to a grounding Take a look at the station shown in system must not compromise these impor- Fig 13.11. In this case, the ground wire tant safety considerations. Refer to the could very easily contribute to an interfer- Safety chapter for important information ence problem in the downstairs TV set. about grounding. While a station ground may cure some Many amateur stations have several transmitter EMI problems — either by grounds: a safety ground that is part of the putting the transmitter chassis at a low- ac-wiring system, another at the antenna impedance reference point or by rearrang- for lightning protection and perhaps an- ing the problem so the “hot spots” are other at the station for EMI control. These farther away from susceptible equipment grounds can interact with each other in — it is not the cure-all that some literature ways that are difficult to predict. has suggested. A ground is easy to install, and it may reduce stray fundamental or Ground Loops harmonic currents on your antenna lead; it All of these station grounds can form a is worth a try. large ground loop. This loop can act as a large loop antenna, with increased sus- SPECIFIC CURES Fig 13.11 — When a transmitter is located on an upper floor, the ground ceptibility to lightning or EMI problems. Now that you have learned some EMI lead may act as an antenna for VHF/ Fig 13.10 shows a ground loop and a fundamentals, you can work on technical UHF energy. It may be better to not use proper single-point ground system. solutions. A systematic approach will a normal ground.

EMI/Direction Finding 13.7

Chapter 13.pmd 7 7/28/2006, 9:15 AM identify the problem and suggest a cure. Armed with your EMI knowledge, a kit of filters and tools, your local TC and a de- termination to solve the problem, it is time to diagnose the problem. Most EMI problems can be solved by the application of standard cures. If you try these cures and they work, you may not need to troubleshoot the problem at all. Perhaps if you can install a low-pass filter on your transmitter or a common- mode choke on a TV, the problem will be solved. Here are some specific cures for differ- ent interference problems. You should also get a copy of The ARRL RFI Book. It’s comprehensive and picks up where this chapter leaves off. Here are several stan- dard cures. Transmitters We start with transmitters not because most interference comes from transmit- ters, but because your station transmitter is under your direct control. Many of the troubleshooting steps in other parts of this chapter assume that your transmitter is “clean” (free of unwanted RF output). Controlling Spurious Emissions — Start by looking for patterns in the interfer- ence. If the interference is only on frequen- cies that are multiples of your operating frequency, you clearly have interference from harmonics. (Although these harmon- ics may not come from your station!) If HF-transmitter spurs are interfering with a VHF service, a low-pass filter on the transmitter will usually cure the prob- lem. Install it after the amplifier (if used) and before the antenna tuner. (A second filter between the transmitter and ampli- fier may occasionally help as well.) Install a low-pass filter as your first step in any interference problem that involves an- other radio service. Interference from non-harmonic spuri- ous emissions is extremely rare in com- mercially built radios. Any such problem indicates a malfunction that should be re- paired. Television Interference (TVI) For a TV signal to look good, it must have about a 45 to 50 dB signal-to-noise Fig 13.12 — TVI Troubleshooting Flowchart ratio. This requires a good signal at the TV antenna-input connector. This brings up an important point: to have a good signal, you transmitter’s fundamental signal. All of these potential problems are made must be in a good signal area. The FCC • Signals within the TV channel from more severe because the TV set is hooked does not protect fringe-area reception. some source other than your station, up to two antenna systems: (1) the incom- TVI, or interference to any radio ser- such as electrical noise, an overloaded ing antenna and its feed line and (2) the vice, can be caused by one of several mast-mounted TV preamplifier or a ac power lines. These two “long-wire” things: transmitter in another service. antennas can couple a lot of fundamen- • Spurious signals within the TV channel • The TV set might be defective or tal or harmonic energy into the TV set! coming from your transmitter or station. misadjusted, making it look like there is The TVI Troubleshooting Flowchart in • The TV set may be overloaded by your an interference problem. Fig 13.12 is a good starting point.

13.8 Chapter 13

Chapter 13.pmd 8 7/28/2006, 9:15 AM Warning: Performing Repairs You are the best judge of a local situation, but the ARRL strongly recommends that you do not work on your neighbor’s equipment. The minute you take the back off a TV or open up a telephone, you may become liable for problems. Internal modifications to your neighbor’s equipment may cure the interference problem, but months later, when that 25-year-old clunker gives up the ghost, you may be held to blame. In some states, it is illegal for you to do any work on electronic equipment other than your own. — Ed Hare, W1RFI, ARRL Laboratory Supervisor

Fundamental Overload A television set can be overloaded by a strong, local RF signal. This happens be- cause the manufacturer did not install the necessary filters and shields to protect the TV set from other signals present on the air. These design deficiencies can some- times be corrected externally. Start by determining if the interference is affecting the video, the sound or both. If it is present only on the sound, it is prob- ably a case of audio rectification. (See the Stereos section of this chapter.) If it is present on the video, or both, it could be getting into the video circuitry or affect- Fig 13.13 — This sort of installation should cure any kind of conducted TVI. It will ing either the tuner or IF circuitry. not cure direct-pickup or spurious-emission problems. The first line of defense for an antenna- connected TV is a high-pass filter. Install a high-pass filter directly on the back of turers. Contact them directly for assistance 6-MHz wide. If the interference is only on the TV set. You may also have a problem in locating help at www.eia.org. channels that are multiples of your operat- with common-mode interference. The ing frequency, you clearly have interfer- second line of defense is a common-mode Spurious Emissions ence from harmonics. (It is not certain that choke on the antenna feed line — try this Start by analyzing which TV channels are these harmonics are coming from your first in a cable-television installation. affected. The TV Channel Chart, Fig 13.14, station, however.) These two filters can probably cure most shows the relationship of the ham alloca- You are responsible for spurious signals cases of TVI! tions and their harmonics to over-the-air produced by your station. If your station is Fig 13.13 shows a “bulletproof” installa- and cable channels. Each channel is generating any interfering spurious sig- tion. If this doesn’t cure the problem, the TV circuitry is picking up your signal directly. In that case, don’t try to fix it yourself — it is a problem for the TV manufacturer. Warning: Surplus Toroidal Cores VHF Transmitters — A VHF transmit- Don’t use an unknown core or an old TV yoke core to make a common-mode ter can interfere with over-the-air TV re- choke. Such cores may not be suitable for the frequency you want to remove. If ception. Most TV tuners are not very you try one of these “unknowns” and it doesn’t work, you may incorrectly selective and a strong VHF signal can conclude that a common-mode choke won’t help. Perhaps the correct material overload the tuner easily. In this case, a would have done the job. VHF notch or stop-band filter at the TV Ferrite beads are also used for EMI control, both as common-mode chokes and low-pass filters. It takes quite a few beads to be effective at the lower end can help by reducing the VHF fundamen- of the HF range, though. It is usually better to form a common-mode choke by tal signal that gets to the TV tuner. Star wrapping about 10 to 20 turns of wire or coaxial cable around an FT-140 Circuits is one company that sells tunable (1.4-inch OD) or FT-240 (2.4-inch OD) core of the correct material. Mix 43 is a 2 notch filters. good material for most of the HF and VHF ranges. — Ed Hare, W1RFI, ARRL The Electronic Industries Alliance (EIA) Laboratory Supervisor can help you contact equipment manufac-

EMI/Direction Finding 13.9

Chapter 13.pmd 9 7/28/2006, 9:15 AM Fig 13.14 — This chart shows CATV and broadcast channels used in the United States and their relationship to the harmonics of MF, HF, VHF and UHF amateur bands.

13.10 Chapter 13

Chapter 13.pmd 10 7/28/2006, 9:15 AM Fig 13.16 — Several turns of coax on a Fig 13.15 — Two examples of TVs experiencing electrical noise. ferrite core eliminate HF and VHF signals from the outside of a coaxial cable.

nals, the problem must be cured there. So, defense is a common-mode choke. Only if the problem occurs only when you trans- in rare cases is a high-pass filter neces- mit, go back and check your station. Refer sary. It is important to remember this, probably results from direct pickup inside to the section on Transmitters. You must because if your neighbor has several TVs the TV. In this case, contact the TV manu- first find out if the transmitter has any connected to cable and you suggest the facturer through the EIA. spurs. wrong filter (at $15 each), you may have a Interference to cable-TV installations If your transmitter and station check personal diplomacy problem of a whole from VHF transmitters is a special case. “clean,” then you must look elsewhere. new dimension. Fig 13.16 shows a com- Cable TV uses frequencies allocated to The most likely cause is TV susceptibility mon-mode choke. over-the-air services, such as Amateur — fundamental overload. This is usually Fig 13.13 shows a bulletproof installa- Radio. When the cable shielding is less indicated by interference to all channels, tion for cable TV. (The high-pass filter is than perfect, interference can result. or at least all VHF channels. If the prob- usually not needed.) If all of the cures The TV Channel Chart in Fig 13.14 lem is fundamental overload, see that sec- shown have been tried, the interference shows which cable channels coincide with tion earlier in this chapter. If not, read on. Electrical Noise Electrical noise is fairly easy to identify by looking at the picture or listening on an HF receiver. Electrical noise on a TV screen is shown in Fig 13.15. On a re- ceiver, it usually sounds like a buzz, some- times changing in intensity as the arc or spark sputters a bit. If you have a problem with electrical noise, go to the Electrical Noise section. Cable TV Cable TV has been a blessing and a curse for Amateur Radio TVI problems. On the plus side, the cable delivers a strong, consistent signal to the TV re- ceiver. It is also (in theory) a shielded system, so an external signal can’t get in and cause trouble. On the minus side, the cable forms a large, long-wire antenna that can pick up lots of external signals on its shield (in the common mode). Many TVs and VCRs and even some cable set-top converters are easily overloaded by such common-mode signals. Leakage into a cable-TV system is called ingress. Leakage out is called egress. If the cable isn’t leaking, there should be no external signals getting in- side the cable. So, an in-line filter such as Fig 13.17 — Part of page 18 from FCC Interference Handbook (1990 edition) a high-pass filter is not usually necessary. explains the facts and places responsibility for interference to non-radio For a cable-connected TV, the first line of equipment.

EMI/Direction Finding 13.11

Chapter 13.pmd 11 7/28/2006, 9:15 AM ham bands. If, for example, you have inter- Non-radio Devices affect a cure, you must locate the detec- ference to cable channel 18 from amateur Interference to non-radio devices is not tion point and correct the problem there. 2-m operation, suspect cable ingress. Con- the fault of the transmitter. (A portion of The telephone company lightning arres- tact the cable company; it may be their re- the FCC Interference Handbook, 1990 tor may be defective. Defective arrestors sponsibility to locate and correct the Edition, is shown in Fig 13.17.3) In es- can act like diodes, rectifying any nearby problem. The cable company is not respon- sence, the FCC views non-radio devices RF energy. Telephone-line amplifiers or sible, however, for leakage occurring in that pick up nearby radio signals as im- other electronic equipment may also be at customer-owned, cable-ready equipment properly functioning; contact the manu- fault. Leave the telephone company equip- that is tuned to the same frequency as the facturer and return the equipment. The ment to the experts, however. There are over-the-air signal. If there is interference FCC does not require that non-radio de- important safety issues that are the sole to a cable-TV installation, the cable com- vices include EMI protection and they responsibility of the telephone company. pany should be able to demonstrate inter- don’t offer legal protection to users of Inspect the installation. Years of expo- ference-free reception when using a these devices that are susceptible to inter- sure in damp basements, walls or crawl cable-company supplied set-top converter. ference. spaces may have caused deterioration. Be suspicious of anything that is corroded or TV Preamplifiers Telephones discolored. In many cases, homeowners Some television owners use a preampli- Telephones have probably become the have installed their own telephone wiring, fier — sometimes when it’s not needed. number one interference problem of Ama- often using substandard wiring. If you find Preamplifiers are only needed in weak- teur Radio. However, most cases of tele- sections of telephone wiring made from signal areas, and they often cause more phone interference can be cured by nonstandard cable, replace it with stan- trouble than they prevent. They are sub- correcting any installation defects and in- dard twisted-pair wire. Radio Shack, ject to the same overload problems as TVs, stalling telephone EMI filters where among others, sells several kinds of tele- and their location on the antenna mast usu- needed. phone wire. ally makes it difficult to install the appro- Telephones can improperly function as Next, evaluate each of the telephone priate cures. You may need to install a radio receivers. There are devices inside instruments. If you find a susceptible tele- high-pass or notch filter at the input of the many telephones that act like diodes. phone, install a telephone EMI filter on that preamplifier, as well as a common-mode When such a telephone is connected to the telephone. Several QST advertisers sell choke on the input, output and power-sup- telephone wiring (a large antenna), an AM small, attractive telephone EMI filters. ply wiring (if separate) to affect a com- radio receiver can be formed. When a If you determine that you have interfer- plete cure. nearby transmitter goes on the air, these ence only when you operate on one par- VCRs telephones can be affected. ticular ham band, the telephone wiring is Troubleshooting techniques were dis- probably resonant on that band. If pos- A VCR usually contains a television cussed earlier in the chapter. The sugges- sible, install a few strategically placed in- tuner, or has a TV channel output, so it is tion to simplify the problem applies line telephone EMI filters to break up the subject to all of the interference problems especially to telephone interference. Dis- resonance. of a TV receiver. It is also hooked up to an connect all telephones except one, right at Telephone Accessories — Answering antenna or cable system and the ac-line the service entrance if possible, and start machines, fax machines and some alarm wiring. The video baseband signal extends troubleshooting the problem there. systems are also prone to interference from 30 Hz to 3.5 MHz, with color in- If any one device, or bad connection in problems. All of the troubleshooting tech- formation centered around 3.5 MHz the phone system, detects RF and puts the niques and cures that apply to telephones and the FM sound subcarrier at 4.5 MHz. detected signal back onto the phone line also apply to these telephone devices. In The entire video baseband is frequency as audio, that audio cannot be removed addition, many of these devices connect to modulated onto the tape at frequencies up with filters. Once the RF has been detected the ac mains. Try a common-mode choke to 10 MHz. It is no wonder that some and turned into audio, it cannot be filtered and/or ac-line filter on the power cord VCRs are quite susceptible to EMI. out because the interference is at the same (which may be an ac cord set, a small trans- Many cases of VCR EMI can be cured. frequency as the desired audio signal. To former or power supply). Start by proving that the VCR is the sus- ceptible device. Temporarily disconnect the VCR from the television. If there is no interference to the TV, then the VCR is the most likely culprit. You need to find out how the interfering signal is getting into the VCR. Tempo- rarily disconnect the antenna or cable feed line from the VCR. If the interference goes away, then the antenna line is involved. In this case, you can probably fix the prob- lem with a common-mode choke or high- pass filter. Fig 13.13 shows a bulletproof VCR in- stallation. If you have tried all of the cures shown and still have a problem, the VCR is probably subject to direct pickup. In this case, contact the manufacturer through the EIA. Fig 13.18 — A typical modern stereo system.

13.12 Chapter 13

Chapter 13.pmd 12 7/28/2006, 9:15 AM Cordless Telephones — A cordless determine if the interfering signal is being is no interference with the headphones, telephone is an unlicensed radio device coupled into the amplifier by the speaker filtering the speaker leads will cure the that is manufactured and used under Part leads. Temporarily disconnect the speaker problem. 15 of the FCC regulations. The FCC does leads from the amplifier, and plug in a test The best way to eliminate RF signals not intend Part 15 devices to be protected set of headphones with short leads. If there from speaker leads is with common-mode from interference. These devices usually chokes. Fig 13.19 shows how to wrap have receivers with very wide front-end speaker wires around an FT-140-43 fer- filters, which make them very susceptible rite core to cure speaker-lead EMI. Use to interference. A label on the telephone the correct core material for the job. See or a paragraph in the owner’s manual the information about common-mode should explain that the telephone must not chokes earlier in this chapter. cause interference to other services and Another way to cure speaker-lead inter- must tolerate any interference caused to it. ference is with an LC filter as shown in It’s worthwhile to try a telephone filter Fig 13.20. on the base unit and properly filter its ac Interconnect cables can couple interfer- line cord. (You might get lucky!) The best ing signals into an amplifier or accesso- source of help is the manufacturer, but ries. The easiest cure here is also a they may point out that the Part 15 device common-mode choke. However, it may is not protected from interference. These also be necessary to add a differential- kinds of problems are difficult to fix after Fig 13.19 — This is how to make a mode filter to the input of the amplifier or the fact. The necessary engineering should speaker-lead common-mode choke. Be accessory. Fig 13.21 shows a home-brew be done when the device is designed. sure to use the correct ferrite material. version of such a filter. Intercoms and Public-Address Sys- Other Audio Devices tems — All of these problems also apply Other audio devices, such as stereos, to intercoms, public-address (PA) systems intercoms and public-address systems and similar devices. These systems usu- can also pick up and detect strong nearby ally have long speaker leads or intercon- transmitters. The FCC considers these nect cables that can pick up a lot of RF non-radio devices and does not protect energy from a nearby transmitter. The them from licensed radio transmitters cures discussed above do apply to these that may interfere with their operation. Fig 13.20 — An LC filter for speaker leads. systems, but you may also need to contact See Fig 13.17 for the FCC’s point of view. the manufacturer to see if they have any Use the standard troubleshooting tech- additional, specific information. niques discussed earlier in this chapter to isolate problems. In a multi-component Computers and Other Unlicensed RF stereo system (as in Fig 13.18), for ex- Sources ample, you must determine what combi- Computers and microprocessors can be nation of components is involved with the sources, or victims, of interference. These problem. First, disconnect all auxiliary devices contain oscillators that can, and do, components to determine if there is a prob- radiate RF energy. In addition, the internal lem with the main receiver/amplifier. functions of a computer generate different (Long speaker/interconnect cables are frequencies, based on the various data rates prime suspects.) Fig 13.21 — A filter for use at the input as software is executed. All of these signals Stereos — If the problem remains with of audio equipment. The components are digital — with fast rise and fall times should be installed inside of the the main amplifier isolated, determine if chassis at the connector by a qualified that are rich in harmonics. the interference level is affected by the technician. Don’t just think “computer” when volume control. If so, the interference is thinking of computer systems. Many getting into the circuit before the volume household appliances contain micropro- control, usually through accessory wiring. cessors: digital clocks, video games, cal- If the volume control has no effect on the Warning: Bypassing culators and more. level of the interfering sound, the interfer- Speaker Leads Computing devices are covered under ence is getting in after the control, usually Older amateur literature might tell Part 15 of the FCC regulations as uninten- through speaker wires. you to put a 0.01-μF capacitor tional emitters. The FCC has set up abso- Speaker wires are often resonant on the across the speaker terminals to cure lute radiation limits for these devices. FCC HF bands. In addition, they are often con- speaker-lead interference. Don’t do regulations state that the operator or owner nected directly to the output transistors, this! Some modern solid-state of Part 15 devices must take whatever where RF can be detected. Most amplifier amplifiers can break into a destruc- steps are necessary to reduce or eliminate designs use a negative feedback loop to tive, full-power, sometimes ultra- any interference they cause to a licensed improve fidelity. This loop can conduct sonic oscillation if they are con- radio service. This means that if your the detected RF signal back to the high- nected to a highly capacitive load. If neighbor’s video game interferes with gain stages of the amplifier. The combina- you do this to your neighbor’s your radio, the neighbor is responsible for tion of all of these factors makes the amplifier, you will have a whole new correcting the problem. (Of course, your kind of personal diplomacy problem! speaker leads the usual indirect cause of neighbor may appreciate your help in lo- — Ed Hare, W1RFI, ARRL Labora- interference to audio amplifiers. tory Supervisor cating a solution!) There is a simple test that will help The FCC has set up two levels of type

EMI/Direction Finding 13.13

Chapter 13.pmd 13 7/28/2006, 9:15 AM acceptance for computing devices. Class A is for computers used in a commercial environment. FCC Class B requirements are more stringent — for computers used in residential environments. If you buy a computer or peripheral, be sure that it is Class B certified or it will probably gener- ate interference to your amateur station or home-electronics equipment. If you find that your computer system is interfering with your radio (not uncom- mon in this digital-radio age), start by sim- plifying the problem. Temporarily switch off as many peripherals as possible and disconnect their cables from the back of the computer. If possible, use just the com- puter, keyboard and monitor. This test may indicate one or more peripherals as the source of the interference. When seeking cures, first ensure that all interconnection cables are shielded. Re- place any unshielded cables with well- shielded ones; this often significantly reduces RF noise from computer systems. The shield must also be terminated prop- erly at the connectors. Unfortunately, quite often the only way to find out is to take it apart. The second line of defense is the common-mode choke, made from a ferrite toroid. The toroids should be in- stalled as close to the computer and/or peripheral device as practical. Fig 13.22 shows the location of common-mode chokes in a complete computer system where both the computer and peripherals are noisy. In some cases, a switching power sup- ply may be a source of interference. A common-mode choke and/or ac-line filter may cure this problem. In extreme cases of computer interference you may need to improve the shielding of the computer. Refer to The ARRL RFI Book for more information about how to do this. Don’t forget that some peripherals (such as mo- dems) are connected to the phone line, so you may need to treat them like tele- phones. Automobiles As automobiles have become more tech- nologically sophisticated, questions about the compatibility of automobiles and ama- teur transmitters have increased in number and scope. The use of microprocessors in autos makes them computer systems on wheels, subject to all of the same problems as any other computer. Installation of ham equipment can cause problems, ranging from nuisances like a dome light coming on every time you transmit to serious ones such as damage to the vehicle electronic control module (ECM). Fig 13.22 — Where to locate ferrites in a computer system. At A, the computer is noisy, but the peripherals are quiet. At B, the computer is quiet, but external Only qualified service personnel should devices are noisy. At C, both the computer and externals are noisy. work on automotive EMC problems.

13.14 Chapter 13

Chapter 13.pmd 14 7/28/2006, 9:15 AM Many critical safety systems on modern shown in the TVI section of this chapter. equipment in an effort to locate the source cars should not be handled by amateurs. This kind of noise can come from power of the problem. Doing so is a potentially Even professionals can meet with mixed lines, electrical motors or switches, to FATAL action! Leave any investigation results. The ARRL (TIS) contacted each name just a few. Here is one quick diag- of these sources to the power company. If of the automobile manufacturers and nostic trick — if electrical noise seems to you have a problem and are not getting asked about their EMC policies, service come and go with the weather, the source help from the power company, contact us bulletins and best contacts to resolve EMI is probably outside, usually on the power at ARRL and we can help you work it out problems. About 20% of the companies lines. If electrical noise varies with the with them. Many power companies have never answered, and answers from the rest time of day, it is usually related to what qualified, knowledgeable people on staff ranged from good to poor. One company people are doing, so look to your own, or to correct any EMI problems that occur even said that the answers to those ques- your neighbors’, house and lifestyle. The with their equipment. However, if they tions were “proprietary.” ARRL RFI Book describes techniques for seem confused and unsure as how to pro- Some of the companies do have reason- locating RFI sources. ceed with your concern, ARRL can pro- able EMC policies, but these policies often Filters usually cure electrical noise. At vide them with information on noise- fall apart at the dealer level. The ARRL its source, the noise can usually be filtered locating techniques. All they need to do is has reports of problems with nearly every with a differential-mode filter. A differ- ask. You can be encouraged from the fact auto manufacturer. Check with your ential-mode filter can be as simple as a that such noise sources are usually simple dealer before you install a transceiver in a 0.01-μF ac-rated capacitor, such as to locate and correct — it will typically car. The dealer can direct you to any ser- Panasonic part ECQ-U2A103MN, or it cost the power company a lot less time and vice bulletins or information that is appli- can be a pi-section filter like that shown in money than they might expect. cable to your model. If you are not Fig 13.7. satisfied with the dealer’s response, con- For removing signals that arrive via In Conclusion tact the regional or factory customer ser- power lines, a common-mode choke is Remember that EMI problems can be vice representatives. usually the best defense. Wrap about 10 cured. With the proper technical knowl- For additional information about auto- turns of the ac-power cord around an edge and interpersonal skills, you can deal motive EMC, refer to the Automobiles FT-240-43 ferrite core; do this as close as effectively with the people and hardware chapter in The ARRL RFI Book, or see possible to the device you are trying to that make up any EMI problem. the automotive pages of the RFI section protect. on ARRLWeb: www.arrl.org/tis/info/ Electrical noise can also indicate a dan- Notes 1ARRL Order no. 6834 is available from ARRL rfigen.html. gerous electrical condition that needs to Publication Sales or your local Amateur be corrected. The ARRL has recorded sev- Radio equipment dealer. Electrical Noise eral cases where defective or arcing door- 2Star Circuits Model 23H tunes 6 m, Model Many electrical appliances and power bell transformers caused widespread 1822 tunes 2 m and Model 46FM tunes the FM broadcast band. Their address is Star lines can generate electrical noise. On a neighborhood electrical interference. This Circuits, PO Box 94917, Las Vegas, NV receiver, electrical noise usually sounds subject is well covered in the The ARRL 89193. like a rough buzz, heard across a wide fre- RFI Book.4 3The FCC Interference to Home Electronic quency range. The buzz will either have a Power Lines — Electrical noise fre- Entertainment Equipment Handbook is strong 60- or 120-Hz component, or its quently comes from lines and equipment available from the US Government Printing Office. See http://bookstore.gpo.gov. pitch will vary with the speed of a motor owned by the power company. DO NOT 4The ARRL RFI Book, ARRL Order no. 6834, that generates the noise. The appearance hammer on poles, shake guy wires, or is available from ARRL Publication Sales or of electrical noise on a television set is otherwise disturb any other utility-owned your local Amateur Radio equipment dealer.

FINDING NOISE SOURCES IN THE SHACK The radio amateur of yesteryear is to be envied to some degree because of the rela- Table 13.2 tively small amount of electrical noise that Components List caused problems. How different it is to- day, with every house full of electrical Resistors Value Capacitors Value (continued) equipment capable of emitting electro- R1 1 kΩ C9, C11 330 μF, 16 V electrolytic magnetic radiation to interfere with the R2, R6 100 Ω C10 0.1 μF poor radio amateur who is trying to listen R3, R4 47 kΩ Ω Semiconductors to signals on the bands. R5 100 k R7 10 Ω U1 LM741 This project detects the radiation that R8 10 kΩ, with switch U2 LM386 causes problems to the amateur, and the noise can be heard. When we say to other Capacitors Value Additional Items members of the household “Please don’t turn C1, C6 4.7 μF, 16 V electrolytic LS1 Small 8-Ω loudspeaker on that computer, vacuum cleaner or TV” C2, C5 0.01 μF Perforated board, 7 × 7 cm C3, C4 22 μF, 16 V electrolytic PP3 battery and clip they cannot understand why we are com- μ plaining, but this little device will allow you C7 0.047 F 3.5 mm mono-jack socket C8 10 μF, 16 V electrolytic Case to show them and let them hear the ‘noise’ Telephone pick-up coil with which we have to contend.

EMI/Direction Finding 13.15

Chapter 13.pmd 15 7/28/2006, 9:15 AM Fig 13.23 — The detector works by receiving stray radiation on a telephone pick-up coil and amplifying it to loudspeaker level.

CONSTRUCTION loud buzz from the speaker. Too much gain The circuit (Fig 13.23) uses a telephone may cause a feedback howl, in which case Table 13.3 pick-up coil as a detector, the output of you will need to adjust R8 to reduce the Readings (pick-up coil near which is fed into a LM741 IC preampli- gain. Complete the rest of the wiring and household items) fier, followed by a LM386 IC power am- test with a finger on the input, which should 29-MHz oscilloscope 0.56 V plifier. See Table 13.2 for the complete produce a click and a buzz. The pick-up Old computer monitor 0.86 V coil comes with a lead and 3.5 mm jack, so component list. Old computer with plastic case 1.53 V you will need a suitable socket. The project is built on a perforated New computer monitor 0.45 V board (Fig 13.24), with the component New tower PC with metal case 0.15 V leads pushed through the holes and joined RELATIVE NOISES with hook-up wire underneath. There is a Place a high-impedance meter set to a Old TV 1.2 V wire running around the perimeter of the low-ac-voltage range across the speaker New TV 0.4 V board to form an earth bus. leads to give a comparative readout be- Plastic-cased hairdryer 4.6 V Build from the loudspeaker backwards tween different items of equipment in the Vacuum cleaner 3.6 V to R8, apply power and touch the wiper of home. Sample readings are shown in Drill 4.9 V R8. If everything is OK you should hear a Table 13.3.

Fig 13.24 — The project is built on perforated board with point-to-point wiring underneath.

13.16 Chapter 13

Chapter 13.pmd 16 7/28/2006, 9:15 AM Radio Direction Finding Far more than simply finding the direc- mitting volunteers to provide evidence nal wavelength. In a single-turn loop, the tion of an incoming radio signal, radio leading to prosecution in serious cases of conductor should be less than 0.08-λ long. direction finding (RDF) encompasses a malicious amateur-to-amateur interfer- For example, a 28-MHz loop should be variety of techniques for determining the ence. RDF is an important part of the evi- less than 34 inches in circumference, giv- exact location of a signal source. The pro- dence-gathering process. ing a diameter of approximately 10 inches. cess involves both art and science. RDF The most basic RDF system consists of The loop may be smaller, but that will re- adds fun to ham radio, but has serious a directional antenna and a method of de- duce its voltage output. Maximum output purposes, too. tecting and measuring the level of the from a small loop antenna is in directions This section was written by Joe Moell, radio signal, such as a receiver with signal corresponding to the plane of the loop; KØOV. strength indicator. RDF antennas range these lobes are very broad. Sharp nulls, RDF is almost as old as radio communi- from a simple tuned loop of wire to an acre obtained at right angles to that plane, are cation. It gained prominence when the of antenna elements with an electronic more useful for RDF. British Navy used it to track the move- beam-forming network. Other sophisti- For a perfect bidirectional pattern, the ment of enemy ships in World War I. Since cated techniques for RDF use the Doppler loop must be balanced electrostatically then, governments and the military have effect, or measure the time of arrival dif- with respect to ground. Otherwise, it will developed sophisticated and complex ference of the signal at multiple antennas. exhibit two modes of operation, the mode RDF systems. Fortunately, simple equip- All of these methods have been used of a perfect loop and that of a non-direc- ment, purchased or built at home, is quite from 2 to 500 MHz and above. However, tional vertical antenna of small dimen- effective in Amateur Radio RDF. RDF practices vary greatly between the sions. This dual-mode condition results in In European and Asian countries, direc- HF and VHF/UHF portions of the spec- mild to severe inaccuracy, depending on tion-finding contests are foot races. The trum. For practical reasons, high gain the degree of imbalance, because the out- object is to be first to find four or five beams, Dopplers and switched dual anten- puts of the two modes are not in phase. transmitters in a large wooded park. nas find favor on VHF/UHF, while loops The theoretical true loop pattern is il- Young athletes have the best chance of and phased arrays are the most popular lustrated in Fig 13.25A. When properly capturing the prizes. This sport is known choices on 6 m and below. Signal propa- balanced, there are two nulls exactly 180° as foxhunting (after the British hill-and- gation differences between HF and VHF apart. When the unwanted antenna effect dale horseback events) or ARDF (Ama- also affect RDF practices. But many basic is appreciable and the loop is tuned to reso- teur Radio direction finding). transmitter-hunting techniques, discussed nance, the loop may exhibit little directiv- In North America and England, most later in this chapter, apply to all bands and ity, as shown in Fig 13.25B. By detuning RDF contests involve mobiles — cars, all types of portable RDF equipment. the loop to shift the phasing, you may trucks, and vans, even motorcycles. It may obtain a useful pattern similar to Fig be possible to drive all the way to the trans- RDF ANTENNAS FOR HF BANDS 13.25C. While not symmetrical, and not mitter, or there may be a short hike at the Below 50 MHz, gain antennas such as necessarily at right angles to the plane of end, called a sniff. These competitions are Yagis and quads are of limited value for the loop, this pattern does exhibit a pair of also called foxhunting by some, while oth- RDF. The typical installation of a tri- nulls. ers use bunny hunting, T-hunting or the bander on a 70-ft tower yields only a gen- By careful detuning and amplitude bal- classic term hidden transmitter hunting. eral direction of the incoming signal, due ancing, you can approach the unidirec- In the 1950s, 3.5 and 28 MHz were the to ground effects and the antenna’s broad tional pattern of Fig 13.25D. Even though most popular bands for hidden transmitter forward lobe. Long monoband beams at there may not be a complete null in the hunts. Today, most competitive hunts greater heights work better, but still can- pattern, it resolves the 180° ambiguity of worldwide are for 144-MHz FM signals, not achieve the bearing accuracy and re- Fig 13.25A. Korean War era military loop though other VHF bands are also used. Some peatability of simpler antennas designed antennas, sometimes available on today’s international foxhunts include 3.5-MHz specifically for RDF. events. Even without participating in RDF con- RDF Loops tests, you will find knowledge of the tech- An effective directional HF antenna niques useful. They simplify the search for can be as uncomplicated as a small loop a neighborhood source of power-line inter- of wire or tubing, tuned to resonance with ference or TV cable leakage. RDF must be a capacitor. When immersed in an elec- used to track down emergency radio bea- tromagnetic field, the loop acts much the cons, which signal the location of pilots and same as the secondary winding of a trans- boaters in distress. Amateur Radio enthusi- former. The voltage at the output is pro- asts skilled in transmitter hunting are in portional to the amount of flux passing demand by agencies such as the Civil Air through it and the number of turns. If the Patrol and the US Coast Guard Auxiliary loop is oriented such that the greatest for search and rescue support. amount of area is presented to the mag- The FCC’s Field Operations Bureau has netic field, the induced voltage will be created an Amateur Auxiliary, adminis- the highest. If it is rotated so that little or Fig 13.25 — Small loop field patterns tered by the ARRL Section Managers, to no area is cut by the field lines, the volt- with varying amounts of antenna effect deal with interference matters. In many age induced in the loop is zero and a null — the undesired response of a loop acting merely as a mass of metal areas of the country, there are standing occurs. connected to the receiver antenna agreements between Local Interference To achieve this transformer effect, the terminals. The horizontal lines show Committees and district FCC offices, per- loop must be small compared with the sig- the plane of the loop turns.

EMI/Direction Finding 13.17

Chapter 13.pmd 17 7/28/2006, 9:15 AM surplus market, use this controlled- usually unsatisfactory for RDF work. later in this chapter. However, it is almost antenna-effect principle. Multi-turn loops are generally used in- always desirable to be able to resolve the An easy way to achieve good electro- stead. They are easier to resonate with ambiguity immediately by having a unidi- static balance is to shield the loop, as practical capacitor values and give higher rectional antenna pattern available. shown in Fig 13.26. The shield, repre- output voltages. This type of loop may also You can modify a loop or loopstick sented by the dashed lines in the drawing, be shielded. If the total conductor length antenna pattern to have a single null by eliminates the antenna effect. The re- remains below 0.08 λ, the directional pat- adding a second antenna element. This sponse of a well-constructed shielded tern is that of Fig 13.25A. element is called a sense antenna, because loop is quite close to the ideal pattern of it senses the phase of the signal wavefront Fig 13.25A. Ferrite Rod Antennas for comparison with the phase of the loop For 160 through 30 m, single-turn loops Another way to get higher loop output output signal. The sense element must be that are small enough for portability are is to increase the permeability of the me- omnidirectional, such as a short vertical. dium in the vicinity of the loop. By wind- When signals from the loop and the sense ing a coil of wire around a form made of antenna are combined with 90° phase shift high-permeability material, such as ferrite between the two, a heart-shaped (cardioid) rod, much greater flux is obtained in the pattern results, as shown in Fig 13.28A. coil without increasing the cross-sectional Fig 13.28B shows a circuit for adding a area. sense antenna to a loop or loopstick. For Modern magnetic core materials make the best null in the composite pattern, sig- compact directional receiving antennas nals from the loop and sense antennas must practical. Most portable AM broadcast be of equal amplitude. R1 adjusts the level receivers use this type of antenna, com- of the signal from the sense antenna. monly called a loopstick. The loopstick is In a practical system, the cardioid pat- the most popular RDF antenna for por- tern null is not as sharp as the bidirectional table/mobile work on 160 and 80 m. null of the loop alone. The usual proce- As does the shielded loop discussed dure when transmitter hunting is to use the earlier, the loopstick responds to the mag- loop alone to obtain a precise line of bear- netic field of the incoming radio wave, and ing, then switch in the sense antenna and not to the electrical field. For a given size take another reading to resolve the ambi- of loop, the output voltage increases with guity. increasing flux density, which is obtained by choosing a ferrite core of high perme- Phased Arrays and Adcocks ability and low loss at the frequency of Two-element phased arrays are popular Fig 13.26 — Electrostatically-shielded interest. For increased output, the turns for amateur HF RDF base station installa- loop for RDF. To prevent shielding of may be wound over two rods taped to- tions. Many directional patterns are pos- the loop from magnetic fields, leave the gether. A practical loopstick antenna is sible, depending on the spacing and shield unconnected at one end. described later in this chapter. phasing of the elements. A useful example 1 1 A loop on a ferrite core has maximum is two /2-λ elements spaced /4-λ apart and signal response in the plane of the turns, fed 90° out of phase. The resultant pattern just as an air core loop. This means that is a cardioid, with a null off one end of the maximum response of a loopstick is axis of the two antennas and a broad peak broadside to the axis of the rod, as shown in the opposite direction. The directional in Fig 13.27. The loopstick may be frequency range of this antenna is limited shielded to eliminate the antenna effect; a to one band, because of the critical length U-shaped or C-shaped channel of alumi- of the phasing lines. num or other form of “trough” is best. The The best-known phased array for RDF shield must not be closed, and its length is the Adcock, named after the man who should equal or slightly exceed the length invented it in 1919. It consists of two ver- of the rod. tical elements fed 180° apart, mounted so the array may be rotated. Element spacing Sense Antennas is not critical, and may be in the range from Because there are two nulls 180° apart 0.1 to 0.75 λ. The two elements must be of in the directional pattern of a small loop or identical lengths, but need not be self- loopstick, there is ambiguity as to which resonant; shorter elements are commonly null indicates the true direction of the tar- used. Because neither the element spacing get station. For example, if the line of bear- nor length is critical in terms of wave- ing runs east and west from your position, lengths, an Adcock array may operate over you have no way of knowing from this more than one amateur band. single bearing whether the transmitter is Fig 13.29 is a schematic of a typical east of you or west of you. Adcock configuration, called the H- If bearings can be taken from two or Adcock because of its shape. Response to more positions at suitable direction and a vertically polarized wave is very similar Fig 13.27 — Field pattern for a ferrite- distance from the transmitter, the ambigu- to a conventional loop. The passing wave rod antenna. The dark bar represents the rod on which the loop turns are ity can be resolved and distance can be induces currents I1 and I2 into the vertical wound. estimated by triangulation, as discussed members. The output current in the trans-

13.18 Chapter 13

Chapter 13.pmd 18 7/28/2006, 9:15 AM mission line is equal to their difference. Consequently, the directional pattern has two broad peaks and two sharp nulls, like the loop. The magnitude of the difference current is proportional to the spacing (d) and length (l) of the elements. You will get somewhat higher gain with larger dimen- sions. The Adcock of Fig 13.30, designed for 40 m, has element lengths of 12 ft and spacing of 21 ft (approximately 0.15 λ). Fig 13.31 shows the radiation pattern of the Adcock. The nulls are broadside to the axis of the array, becoming sharper with increased element spacing. When element 3 spacing exceeds /4 λ, however, the antenna begins to take on additional unwanted nulls off the ends of the array axis. The Adcock is a vertically polarized antenna. The vertical elements do not re- spond to horizontally polarized waves, and the currents induced in the horizontal Fig 13.28 — At A, the members by a horizontally polarized wave directivity pattern of a loop (dotted arrows in Fig 13.29) tend to bal- antenna with sensing ance out regardless of the orientation of element. At B is a circuit the antenna. for combining the signals from the two elements. Since the Adcock uses a balanced feed Adjust C1 for resonance system, a coupler is required to match the with T1 at the operating unbalanced input of the receiver. T1 is an frequency. air-wound coil with a two-turn link wrapped around the middle. The combi- nation is resonated with C1 to the operat- ing frequency. C2 and C3 are null-clearing capacitors. Adjust them by placing a low- power signal source some distance from the antenna and exactly broadside to it. Adjust C2 and C3 until the deepest null is obtained. While you can use a metal support for Fig 13.29 — A simple Adcock antenna and its coupler. the mast and boom, wood is preferable because of its non-conducting properties. Similarly, a mast of thick-wall PVC pipe gives less distortion of the antenna pattern than a metallic mast. Place the coupler on

Fig 13.31 — The pattern of an Adcock array with element spacing of ½ Fig 13.30 — An experimental Adcock wavelength. The elements are aligned antenna on a wooden frame. with the vertical axis.

EMI/Direction Finding 13.19

Chapter 13.pmd 19 7/28/2006, 9:15 AM the ground below the wiring harness junc- known type is the Wullenweber antenna. It tion on the boom and connect it with a has a large number of elements arranged in short length of 300-Ω twin-lead-feed line. a circle, usually outside of a circular reflect- ing screen. Depending on the installation, Loops vs. Phased Arrays the circle may be anywhere from a few hun- Loops are much smaller than phased dred feet to more than a quarter of a mile in arrays for the same frequency, and are thus diameter. Although the Wullenweber is not the obvious choice for portable/mobile HF practical for most amateurs, some of the RDF. For base stations in a triangulation techniques it uses may be applied to amateur network, where the 180° ambiguity is not RDF. a problem, Adcocks are preferred. In gen- The device, which permits rotating the Fig 13.32 — One technique used in eral, they give sharper nulls than loops, antenna beam without moving the elements, electronic beam forming. By delaying but this is in part a function of the care has the classic name radio goniometer, or the signal from element A by an amount equal to the propagation delay, two used in constructing and feeding the indi- simply goniometer. Early goniometers were signals are summed precisely in phase, vidual antennas, as well as of the spacing RF transformers with fixed coils connected even though the signal is not in the of the elements. The primary construction to the array elements and a moving pickup broadside direction. considerations are the shielding and bal- coil connected to the receiver input. Both ancing of the feed line against unwanted amplitude and phase of the signal coupled signal pickup and the balancing of the into the pickup winding are altered with coil antenna for a symmetrical pattern. Users rotation in a way that corresponded to actu- report that Adcocks are somewhat less ally rotating the array itself. With sufficient which the wave is arriving. sensitive to proximity effects, probably elements and a goniometer, accurate RDF Altering the delay in small increments because their larger aperture offers some measurements can be taken in all compass steers the peak (or null) of the antenna. space diversity. directions. The system is not frequency sensitive, other than the frequency range limitations Skywave Considerations Beam Forming Networks of the array elements. Lumped-constant Until now we have considered the di- By properly sampling and combining networks are suitable for delay elements if rectional characteristics of the RDF loop signals from individual elements in a large the system is used only for receiving. only in the two-dimensional azimuthal array, an antenna beam is electronically Delay lines at installations used for trans- plane. In three-dimensional space, the re- rotated or steered. With an appropriate mitting and receiving employ rolls of co- sponse of a vertically oriented small loop number and arrangement of elements in axial cable of various lengths, chosen for is doughnut-shaped. The bidirectional null the system, it is possible to form almost the time delay they provide at all frequen- (analogous to a line through the doughnut any desired antenna pattern by summing cies, rather than as simple phasing lines hole) is in the line of bearing in the azi- the sampled signals in appropriate ampli- designed for a single frequency. muthal plane and toward the horizon in tude and phase relationships. Delay net- Combining signals from additional ele- the vertical plane. Therefore, maximum works and/or attenuation are added in line ments narrows the broad beamwidth of the null depth is achieved only on signals ar- with selected elements before summation pattern from the two elements and sup- riving at 0° elevation angle. to create these relationships. press unwanted sidelobes. Electronically Skywave signals usually arrive at non- To understand electronic beam forming, switching the delays and attenuations to zero wave angles. As the elevation angle first consider just two elements, shown as the various elements causes the formed increases, the null in a vertically oriented A and B in Fig 13.32. Also shown is the beam to rotate around the compass. The loop pattern becomes shallower. It is pos- wavefront of a radio signal arriving from a package of electronics that does this, in- sible to tilt the loop to seek the null in ele- distant transmitter. The wavefront strikes cluding delay lines and electronically vation as well as azimuth. Some amateur element A first, then travels somewhat far- switched attenuators, is the beam-forming RDF enthusiasts report success at estimat- ther before it strikes element B. Thus, there network. ing distance to the target by measurement is an interval between the times that the of the elevation angle with a tilted loop wavefront reaches elements A and B. METHODS FOR VHF/UHF RDF and computations based on estimated We can measure the differences in ar- Three distinct methods of mobile RDF height of the propagating ionospheric rival times by delaying the signal received are commonly in use by amateurs on layer. This method seldom provides high at element A before summing it with that VHF/UHF bands: directional antennas, accuracy with simple loops, however. from element B. If two signals are com- switched dual antennas and Dopplers. Most users prefer Adcocks to loops for bined directly, the amplitude of the sum Each has advantages over the others in skywave work, because the Adcock null is will be maximum when the delay for ele- certain situations. Many RDF enthusiasts present at all elevation angles. Note, how- ment A exactly equals the propagation employ more than one method when ever, that an Adcock has a null in all direc- delay, giving an in-phase condition at the transmitter hunting. tions from signals arriving from overhead. summation point. On the other hand, if one Thus for very high angles, such as under- of the signals is inverted and the two are Directional Antennas 250-mile skip on 80 and 40 m, neither added, the signals will combine in a 180° Ordinary mobile transceivers and hand- loops nor Adcocks will perform well. out-of-phase relationship when the ele- helds work well for foxhunting on the ment A delay equals the propagation popular VHF bands. If you have a light- Electronic Antenna Rotation delay, creating a null. Either way, once the weight beam and your receiver has an State-of-the-art fixed RDF stations for time delay is determined by the amount of easy-to-read S-meter, you are nearly ready government and military work use antenna delay required for a peak or null, we can to start. All you need is an RF attenuator arrays of stationary elements, rather than convert it to distance. Then trigonometry and some way to mount the setup in your mechanically rotatable arrays. The best- calculations provide the direction from vehicle.

13.20 Chapter 13

Chapter 13.pmd 20 7/28/2006, 9:15 AM Amateurs seldom use fractional wave- poor if a VHF RDF antenna is cross-polar- length loops for RDF above 60 MHz because ized to the transmitting antenna, because they have bidirectional characteristics and multipath and scattered signals (which low sensitivity, compared to other practical have indeterminate polarization) are VHF antennas. Sense circuits for loops are enhanced, relative to the cross-polarized difficult to implement at VHF, and signal direct signal. The installation of Fig 13.33 reflections tend to fill in the nulls. Typically features a slip joint at the boom-to-mast VHF loops are used only for close-in sniff- junction, with an actuating cord to rotate ing where their compactness and sharp nulls the boom, changing the polarization. are assets, and low gain is of no conse- Mechanical stops limit the boom rotation quence. to 90°. Phased Arrays Parasitic Array Performance for RDF The small size and simplicity of 2-ele- The directional gain of a mobile beam ment driven arrays make them a com- (typically 8 dB or more) makes it unex- mon choice of newcomers at VHF RDF. celled for both weak signal competitive Antennas such as phased ground planes hunts and for locating interference such as and ZL Specials have modest gain in one TV cable leakage. With an appropriate direction and a null in the opposite direc- Fig 13.33 — The mobile RDF installation receiver, you can get bearings on any sig- tion. The gain is helpful when the signal is of WB6ADC features a thin wire quad nal mode, including FM, SSB, CW, TV, for 144 MHz and a mechanical linkage weak, but the broad response peak makes that permits either the driver or front pulses and noise. Because only the re- it difficult to take a precise bearing. passenger to rotate the mast by hand. sponse peak is used, the null-fill problems As the signal gets stronger, it becomes and proximity effects of loops and phased possible to use the null for a sharper arrays do not exist. S-meter indication. However, combina- You can observe multiple directions of tions of direct and reflected signals (called arrival while rotating the antenna, allow- multipath) will distort the null or perhaps ing you to make educated guesses as to obscure it completely. For best results which signal peaks are direct and which with this type of antenna, always find clear are from non-direct paths or scattering. locations from which to take bearings. Skilled operators can estimate distance to the transmitter from the rate of signal Parasitic Arrays strength increase with distance traveled. Parasitic arrays are the most common The RDF beam is useful for transmitting, RDF antennas used by transmitter hunters if necessary, but use care not to damage an in high competition areas such as South- attenuator in the coax line by transmitting ern California. Antennas with significant through it. gain are a necessity due to the weak sig- The 3-dB beamwidth of typical mobile- nals often encountered on weekend-long mount VHF beams is on the order of 80°. T-hunts, where the transmitter may be This is a great improvement over 2-ele- over 200 miles distant. Typical 144-MHz ment driven arrays, but it is still not pos- installations feature Yagis or quads of sible to get pinpoint bearing accuracy. three to six elements, sometimes more. You can achieve errors of less than 10° by Quads are typically home-built, using data carefully reading the S-meter. In practice, from The ARRL Antenna Book and Trans- this is not a major hindrance to successful mitter Hunting (see Bibliography). mobile RDF. Mobile users are not as con- Two types of mechanical construction cerned with precise bearings as fixed sta- are popular for mobile VHF quads. The tion operators, because mobile readings model of Fig 13.33 uses thin gauge wire are used primarily to give the general di- (solid or stranded), suspended on wood rection of travel to “home in” on the sig- dowel or fiberglass rod spreaders. It is Fig 13.34 — KØOV uses this mobile nal. Mobile bearings are continuously lightweight and easy to turn rapidly by setup for RDF on several bands, with updated from new, closer locations. separate antennas for each band that hand while the vehicle moves. Many hunt- mate with a common lower mast Amplitude-based RDF may be very dif- ers prefer to use larger gauge solid wire section, pointer and 360° indicator. ficult when signal level varies rapidly. The (such as AWG 10) on a PVC plastic pipe Antenna shown is a heavy gauge wire transmitter hider may be changing power, frame (Fig 13.34). This quad is more rug- quad for 2 m. or the target antenna may be moving or ged and has somewhat wider frequency near a well-traveled road or airport. The range, at the expense of increased weight resultant rapid S-meter movement makes and wind resistance. It can get mashed desirable for ease of turning at high it hard to take accurate bearings with a going under a willow, but it is easily re- speeds. quad. The process is slow because the shaped and returned to service. A well-designed mobile Yagi or quad antenna must be carefully rotated by hand Yagis are a close second to quads in installation includes a method of selecting to “eyeball average” the meter readings. popularity. Commercial models work fine wave polarization. Although vertical po- for VHF RDF, provided that the mast is larization is the norm for VHF-FM com- Switched Antenna RDF Units attached at a good balance point. Light- munications, horizontal polarization is Three popular types of RDF systems are weight and small-diameter elements are allowed on many T-hunts. Results will be relatively insensitive to variations in sig-

EMI/Direction Finding 13.21

Chapter 13.pmd 21 7/28/2006, 9:15 AM nal level. Two of them use a pair of verti- finds its greatest use in the 120-MHz air- No RF attenuator is needed for close-in 1 cal dipole antennas, spaced /2 λ or less craft band, where AM is the standard mode. work in the TDOA case. apart, and alternately switched at a rapid Commercial manufacturers make portable Popular designs for practical do-it-your- rate to the input of the receiver. In use, the RDF sets with switched pattern antennas self TDOA RDF sets include the Simple indications of the two systems are similar, and built-in receivers for field portable use. Seeker (described elsewhere in this chap- but the principles are different. These sets can usually be adapted to the ter) and the W9DUU design (see article by amateur 144-MHz band. Other designs are Bohrer in the Bibliography). Articles with Switched Pattern Systems adaptable to any VHF receiver that covers plans for the Handy Tracker, a simple The switched pattern RDF set (Fig 13.35) the frequency of interest and has an AM TDOA set with a delay line to resolve the alternately creates two cardioid antenna detector built in or added. dual-null ambiguity instead of LEDs or a patterns with lobes to the left and the right. Switched pattern units work well for meter, are listed in the Bibliography. The patterns are generated in much the same RDF from small aircraft, for which the two way as in the phased arrays described above. vertical antennas are mounted in fixed Performance Comparison PIN RF diodes select the alternating pat- positions on the outside of the fuselage or Both types of dual antenna RDFs make terns. The combined antenna outputs go to a simply taped inside the windshield. The good on-foot “sniffing” devices and are receiver with AM detection. Processing left-right indication tells the pilot which excellent performers when there are rapid after the detector output determines the way to turn the aircraft to home in. Since amplitude variations in the incoming sig- phase or amplitude difference between the street vehicles generally travel only on nal. They are the units of choice for air- patterns’ responses to the signal. roads, fixed mounting of the antennas on borne work. Compared to Yagis and Switched pattern RDF sets typically them is undesirable. Mounting vehicular quads, they give good directional perfor- have a zero center meter as an indicator. switched-pattern arrays on a rotatable mance over a much wider frequency range. The meter swings negative when the sig- mast is best. Their indications are more precise than nal is coming from the user’s left, and those of beams with broad forward lobes. positive when the signal source is on the Time-of-Arrival Systems Dual-antenna RDF sets frequently give right. When the plane of the antenna is Another kind of switched antenna RDF inaccurate bearings in multipath situa- exactly perpendicular to the direction of set uses the difference in arrival times of tions, because they cannot resolve signals the signal source, the meter reads zero. the signal wavefront at the two antennas. of nearly equal levels from more than one The sharpness of the zero crossing indi- This narrow-aperture Time-Difference- direction. Because multipath signals are a cation makes possible more precise bear- of-Arrival (TDOA) technology is used for combined pattern of peaks and nulls, they ings than those obtainable with a quad or many sophisticated military RDF systems. appear to change in amplitude and bearing Yagi. Under ideal conditions with a well- The rudimentary TDOA implementation as you move the RDF antenna along the built unit, null direction accuracy is within of Fig 13.36 is quite effective for amateur bearing path or perpendicular to it, 1°. Meter deflection tells the user which use. The signal from transmitter 1 reaches whereas a non-multipath signal will have way to turn to zero the meter. For example, antenna A before antenna B. Conversely, constant strength and bearing. a negative (left) reading requires turning the signal from transmitter 3 reaches The best way to overcome this problem the antenna left. This solves the 180° am- antenna B before antenna A. When the is to take large numbers of bearings while biguity caused by the two zero crossings plane of the antenna is perpendicular to moving toward the transmitter. Taking in each complete rotation of the antenna the signal source (as transmitter 2 is in the bearings while in motion averages out the system. figure), the signal arrives at both antennas effects of multipath, making the direct sig- Because it requires AM detection of the simultaneously. nal more readily discernible. Some TDOA switched pattern signal, this RDF system If the outputs of the antennas are alter- RDF sets have a slow-response mode that nately switched at an audio rate to the re- ceiver input, the differences in the arrival times of a continuous signal produce phase changes that are detected by an FM dis- criminator. The resulting short pulses sound like a tone in the receiver output. The tone disappears when the antennas are equidistant from the signal source, giving an audible null. The polarity of the pulses at the dis- criminator output is a function of which antenna is closer to the source. Therefore, the pulses can be processed and used to drive a left-right zero-center meter in a manner similar to the switched pattern units described above. Left-right LED in- dicators may replace the meter for economy and visibility at night. RDF operations with a TDOA dual antenna RDF are done in the same manner as with a switched antenna RDF set. The Fig 13.36 — A dual-antenna TDOA RDF Fig 13.35 — In a switched pattern RDF system has a similar indicator to a set, the responses of two cardioid main difference is the requirement for an switched pattern unit, but it obtains antenna patterns are summed to drive FM receiver in the TDOA system and an bearings by determining which of its a zero center indicator. AM receiver in the switched pattern case. antennas is closer to the transmitter.

13.22 Chapter 13

Chapter 13.pmd 22 7/28/2006, 9:15 AM aids the averaging process. being moved on a circular track around Switched antenna systems generally do point P, with constant angular velocity. As not perform well when the incoming sig- the antenna approaches the transmitter on nal is horizontally polarized. In such cases, its track, the received frequency is shifted the bearings may be inaccurate or unread- higher. The highest instantaneous fre- able. TDOA units require a carrier type sig- quency occurs when the antenna is at point nal such as FM or CW; they usually cannot A, because tangential velocity toward the yield bearings on noise or pulse signals. transmitter is maximum at that point. Con- Unless an additional method is em- versely, the lowest frequency occurs when ployed to measure signal strength, it is the antenna reaches point C, where veloc- easy to “overshoot” the hidden transmit- ity is maximum away from the transmitter. ter location with a TDOA set. It is not Fig 13.38 shows a plot of the compo- uncommon to see a TDOA foxhunter walk nent of the tangential velocity that is in the Fig 13.38 — Frequency shift versus over the top of a concealed transmitter and direction of the transmitter as the antenna time produced by the rotating antenna walk away, following the opposite 180° moves around the circle. Comparing Figs movement toward and away from the null, because there is no display of signal 13.37 and 13.38, notice that at B in Fig signal source. amplitude. 13.38, the tangential velocity is crossing zero from the positive to the negative and Doppler RDF Sets the antenna is closest to the transmitter. RDF sets using the Doppler principle are The Doppler shift and resulting audio out- ence, because they are inconspicuous popular in many areas because of their ease put from the receiver discriminator follow (compared to beams) and effective at of use. They have an indicator that instan- the same plot, so that a negative-slope tracking the strong vertically polarized taneously displays direction of the signal zero-crossing detector, synchronized with signals that repeater jammers usually source relative to the vehicle heading, the antenna rotation, senses the incoming emit. either on a circular ring of LEDs or a digital direction of the signal. A Doppler does not provide superior readout in degrees. A ring of four, eight or The amount of frequency shift due to performance in all VHF RDF situations. If more antennas picks up the signal. Quarter- the Doppler effect is proportional to the the signal is too weak for detection by the wavelength monopoles on a ground plane RF frequency and the tangential antenna Doppler unit, the hunt advantage goes to are popular for vehicle use, but half-wave- velocity. The velocity is a function of the teams with beams. Doppler installations are length vertical dipoles, where practical, radius of rotation and the angular velocity not suitable for on-foot sniffing. The limi- perform better. (rotation rate). The radius of rotation must tations of other switched antenna RDFs Radio signals received on a rapidly be less than ¼ λ to avoid errors. To get a also apply: (1) poor results with horizon- moving antenna experience a frequency usable amount of FM deviation (compa- tally polarized signals, (2) no indication of shift due to the Doppler effect, a phenome- rable to typical voice modulation) with distance, (3) carrier type signals only and non well known to anyone who has ob- this radius, the antenna must rotate at ap- (4) inadvisability of transmitting through served a moving car with its horn proximately 30,000 RPM (500 Hz). This the antenna. sounding. The horn’s pitch appears higher puts the Doppler tone in the audio range Readout to the nearest degree is pro- than normal as the car approaches, and for easy processing. vided on some commercial Doppler units. lower as the car recedes. Similarly, the Mechanically rotating a whip antenna This does not guarantee that level of accu- received radio frequency increases as the at this rate is impractical, but a ring of racy, however. A well-designed four- antenna moves toward the transmitter and whips, switched to the receiver in succes- monopole set is typically capable of ±5° vice versa. An FM receiver will detect this sion with RF PIN diodes, can simulate a accuracy on 2 m, if the target signal is frequency change. rapidly rotating antenna. Doppler RDF vertically polarized and there are no 1 Fig 13.37 shows a /4-λ vertical antenna sets must be used with receivers having multipath effects. FM detectors. The DoppleScAnt and The rapid antenna switching can intro- Roanoke Doppler (see Bibliography) are duce cross modulation products when the mobile Doppler RDF sets designed for user is near strong off-channel RF sources. inexpensive home construction. This self-generated interference can tem- porarily render the system unusable. Doppler Advantages and Disadvantages While not a common problem with mobile Ring-antenna Doppler sets are the ulti- Dopplers, it makes the Doppler a poor mate in simplicity of operation for mobile choice for use in remote RDF installations RDF. There are no moving parts and no at fixed sites with high power VHF trans- manual antenna pointing. Rapid direction mitters nearby. indications are displayed on very short signal bursts. Mobile RDF System Installation Many units lock in the displayed direc- Of these mobile VHF RDF systems, the tion after the signal leaves the air. Power Doppler type is clearly the simplest from variations in the source signal cause no a mechanical installation standpoint. A difficulties, as long as the signal remains four-whip Doppler RDF array is easy to above the RDF detection threshold. A implement with magnetic mount antennas. Doppler antenna goes on top of any car Alternately, you can mount all the whips Fig 13.37 — A theoretical Doppler quickly, with no holes to drill. Many Local on a frame that attaches to the vehicle roof antenna circles around point P, continuously moving toward and away Interference Committee members choose with suction cups. In either case, setup is from the source at an audio rate. Dopplers for tracking malicious interfer- rapid and requires no holes in the vehicle.

EMI/Direction Finding 13.23

Chapter 13.pmd 23 7/28/2006, 9:15 AM on antennas that have more than one. If imbalance or off-axis response is found in the antennas, there are two op- tions available. One is to correct it, insofar as possible. A second option is to accept it and use some kind of indicator or correc- tion procedure to show the true directions of signals. Sometimes the end result of the calibration procedure is a compromise between these two options, as a perfect pattern may be difficult or impossible to attain. The same calibration suggestions apply Fig 13.40 — A window box allows the for fixed RDF installations, such as a base navigator to turn a mast mounted station HF Adcock or VHF beam. Of antenna with ease while remaining dry course it does no good to move it to an and warm. No holes in the vehicle are needed with a properly designed open field. Instead, calibrate the array in window box. its intended operating position, using a portable or mobile transmitter. Because of nearby obstructions or reflecting objects, your antenna may not indicate the precise front passenger and even a rear passenger direction of the transmitter. Check for can turn the mast when required. The in- imbalance and systemic error by taking stallation in Fig 13.34 uses a roof-hole readings with the test emitter at locations bushing made from mating threaded PVC in several different directions. pipe adapters and reducers. When it is not The test signal should be at a distance of in use for RDF, a PVC pipe cap provides 2 or 3 miles for these measurements, and a watertight cover. There is a pointer and should be in as clear an area as possible Fig 13.39 — A set of TDOA RDF 360° indicator at the bottom of the mast during transmissions. Avoid locations antennas is light weight and mounts for precise bearings. where power lines and other overhead readily through a sedan window wiring can conduct signal from the trans- without excessive overhang. DIRECTION-FINDING TECHNIQUES mitter to the RDF site. Once antenna ad- AND PROJECTS justments are optimized, make a table of The ability to locate a transmitter bearing errors noted in all compass direc- quickly with RDF techniques is a skill you tions. Apply these error values as correc- You can turn small VHF beams and dual- will acquire only with practice. It is very tions when actual measurements are made. antenna arrays readily by extending the important to become familiar with your mast through a window. Installation on equipment and its limitations. You must Preparing to Hunt each model vehicle is different, but usually also understand how radio signals behave Successfully tracking down a hidden the mast can be held in place with some sort in different types of terrain at the fre- transmitter involves detective work — of cup in the arm rest and a plastic tie at the quency of the hunt. Experience is the best examining all the clues, weighing the evi- top of the window, as in Fig 13.39. This teacher, but reading and hearing the sto- dence and using good judgment. Before technique works best on cars with frames ries of others who are active in RDF will setting out to locate the source of a signal, around the windows, which allow the door help you get started. note its general characteristics. Is the fre- to be opened with the antenna in place. Verify proper performance of your por- quency constant, or does it drift? Is the Check local vehicle codes, which limit how table RDF system before you attempt to signal continuous, and if not, how long are far your antenna may protrude beyond the track signals in unknown locations. Of transmissions? Do transmissions occur at line of the fenders. Larger antennas may primary concern is the accuracy and sym- regular intervals, or are they sporadic? have to be put on the passenger side of the metry of the antenna pattern. For instance, Irregular, intermittent signals are the most vehicle, where greater overhang is gener- a lopsided figure-8 pattern with a loop, difficult to locate, requiring patience and ally permissible. Adcock, or TDOA set leads to large bear- quick action to get bearings when the The window box (Fig 13.40) is an im- ing errors. Nulls should be exactly 180° transmitter comes on. provement over through-the-window apart and exactly at right angles to the loop mounts. It provides a solid, easy-turning plane or the array boom. Similarly, if feed- Refraction, Reflections and the Night mount for the mast. The plastic panel line pickup causes an off-axis main lobe Effect keeps out bad weather. You will need to in your VHF RDF beam, your route to the You will get best accuracy in tracking custom-design the box for your vehicle target will be a spiral instead of a straight ground wave signals when the propaga- model. Vehicle codes may limit the use of line. tion path is over homogeneous terrain. If a window box to the passenger side. Perform initial checkout with a low- there is a land/water boundary in the path, For the ultimate in convenience and powered test transmitter at a distance of a the different conductivities of the two versatility, cast your fears aside, drill a few hundred feet. Compare the RDF bear- media can cause bending (refraction) of hole through the center of the roof and ing indication with the visual path to the the wave front, as in Fig 13.41A. Even the install a waterproof bushing. A roof-hole transmitter. Try to “find” the transmitter most sophisticated RDF equipment will mount permits the use of large antennas with the RDF equipment as if its position not indicate the correct bearing in this situ- without overhang violations. The driver, were not known. Be sure to check all nulls ation, as the equipment can only show the

13.24 Chapter 13

Chapter 13.pmd 24 7/28/2006, 9:15 AM teaming up and assigning tasks. The driver concentrates on handling the vehicle, while the assistant (called the “navigator” by some teams) turns the beam, reads the meters and calls out bearings. The assis- tant is also responsible for maps and plot- ting, unless there is a third team member for that task. Maps and Bearing-Measurements Possessing accurate maps and knowing how to use them is very important for suc- cessful RDF. Even in difficult situations where precise bearings cannot be ob- tained, a town or city map will help in plot- ting points where signal levels are high and low. For example, power line noise tends to propagate along the power line and radiates as it does so. Instead of a single source, the noise appears to come from a multitude of sources. This renders many ordinary RDF techniques ineffec- tive. Mapping locations where signal am- plitudes are highest will help pinpoint the source. Several types of area-wide maps are Fig 13.41 — RDF errors caused by refraction (A) and reflection (B). The reading at suitable for navigation and triangulation. A is false because the signal actually arrives from a direction that is different from Street and highway maps work well for that to the source. At B, a direct signal from the source combines with a reflected mobile work. Large detailed maps are signal from the mountain ridge. The RDF set may average the signals as shown, or indicate two lines of bearing. preferable to thick map books. Contour maps are ideal for open country. Aeronau- tical charts are also suitable. Good sources of maps include auto clubs, stores cater- direction from which the signal is arriv- ger than the direct signal. In extreme cases, ing to camping/hunting enthusiasts and ing. RDFers have observed this phenom- triangulation from several locations will city/county engineering departments. enon on both HF and VHF bands. appear to “confirm” that the transmitter is A heading is a reading in degrees rela- Signal reflections also cause mislead- at the location of the reflecting object. The tive to some external reference, such as ing bearings. This effect becomes more direct signal may not be detectable until your house or vehicle; a bearing is the pronounced as frequency increases. you arrive at the reflecting point or another target signal’s direction relative to your T-hunt hiders regularly achieve strong sig- high location. position. Plotting a bearing on a hidden nal bounces from distant mountain ranges Objects near the observer such as con- transmitter from your vehicle requires that on the 144-MHz band. crete/steel buildings, power lines and you know the vehicle location, transmitter Tall buildings also reflect VHF/UHF chain-link fences will distort the incom- heading with respect to the vehicle and signals, making mid-city RDF difficult. ing wavefront and give bearing errors. vehicle heading with respect to true north. Hunting on the 440-MHz and higher ama- Even a dense grove of trees can sometimes First, determine your location, using teur bands is even more arduous because have an adverse effect. It is always best to landmarks or a navigation device such as of the plethora of reflecting objects. take readings in locations that are as open a GPS receiver. Next, using your RDF In areas of signal reflection and multi- and clear as possible, and to take bearings equipment, determine the bearing to the path, some RDF gear may indicate that the from numerous positions for confirma- hidden transmitter (0 to 359.9°) with re- signal is coming from an intermediate tion. Testing of RDF gear should also be spect to the vehicle. Zero degrees heading point, as in Fig 13.41B. High gain VHF/ done in clear locations. corresponds to signals coming from di- UHF RDF beams will show direct and re- Locating local signal sources on fre- rectly in front of the vehicle, signals from flected signals as separate S-meter peaks, quencies below 10 MHz is much easier the right indicate 90°, and so on. leaving it to the operator to determine during daylight hours, particularly with Finally, determine your vehicle’s true which is which. Null-based RDF anten- loop antennas. In the daytime, D-layer heading, that is, its heading relative to true nas, such as phased arrays and loops, have absorption minimizes skywave propaga- north. Compass needles point to magnetic the most difficulty with multi-path, be- tion on these frequencies. When the D north and yield magnetic headings. Trans- cause the multiple signals tend to make layer disappears after sundown, you may lating a magnetic heading into a true head- the nulls very shallow or fill them in en- hear the signal by a combination of ground ing requires adding a correction factor, tirely, resulting in no bearing indication at wave and high-angle skywave, making it called magnetic declination1, which is a all. difficult or impossible to obtain a bearing. positive or negative factor that depends on If the direct path to the transmitter is RDFers call this phenomenon the night your location. masked by intervening terrain, a signal re- effect. Declination for your area is given on flection from a higher mountain, building, While some mobile T-hunters prefer to US Geological Survey (USGS) maps, water tower, or the like may be much stron- go it alone, most have more success by though it undergoes long-term changes.

EMI/Direction Finding 13.25

Chapter 13.pmd 25 7/28/2006, 9:15 AM ing from Site 3 has narrowed down the probable area of the transmitter position. Computerized Transmitter Hunting A portable computer is an excellent tool for streamlining the RDF process. Some T-hunters use one to optimize VHF beam bearings, generating a two-dimensional plot of signal strength versus azimuth. Others have automated the bearing-tak- ing process by using a computer to cap- ture signal headings from a Doppler RDF set, vehicle heading from a flux-gate compass, and vehicle location from a GPS receiver (Fig 13.43). The computer program can compute averaged headings from a Doppler set to reduce multipath effects. Provided with perfect position and bear- ing information, computer triangulation could determine the transmitter location within the limits of its computational ac- curacy. Two bearings would exactly lo- cate a fox. Of course, there are always uncertainties and inaccuracies in bearing Fig 13.42 — Bearing sectors from three RDF positions drawn on a map for triangu- and position data. If these uncertainties lation. In this case, bearings are from loop antennas, which have 180° ambiguity. can be determined, the program can com- pute the uncertainty of the triangulated bearings. A “smart” computer program can evaluate bearings, triangulate the bearings of multiple hunters, discard those Add the declination to your magnetic the angle between bearings approaches 0° that appear erroneous, determine which heading to get a true heading. or 180°. locations have particularly great or small As an example, assume that the trans- There is always uncertainty in the fixes multipath problems and even “grade” the mitted signal arrives at 30° with respect to obtained by triangulation due to equipment performance of RDF stations. the vehicle heading, that the compass in- limitations, propagation effects and mea- By adding packet radio connections to a dicates that the vehicle’s heading is 15°, surement errors. Obtaining bearings from group of computerized base and mobile and the magnetic declination is +15°. Add three or more locations reduces the uncer- RDF stations, the processed bearing data these values to get a true transmitter bear- tainty. A good way to show the probable from each can be shared. Each station in ing (that is, a bearing with respect to true area of the transmitter on the triangulation the network can display the triangulated north) of 60°. map is to draw bearings as a narrow sector bearings of all. This requires a common Because of the large mass of surround- instead of as a single line. Sector width map coordinate set among all stations. The ing metal, it is very difficult to calibrate an represents the amount of bearing uncer- USGS Universal Transverse Mercator in-car compass for high accuracy at all tainty. Fig 13.42 shows a portion of a map (UTM) grid, consisting of 1×1-km grid vehicle headings. It is better to use a re- marked in this manner. Note how the bear- squares, is a good choice. motely mounted flux-gate compass sen- sor, properly corrected, to get vehicle headings, or to stop and use a hand com- pass to measure the vehicle heading from the outside. If you T-hunt with a mobile VHF beam or quad, you can use your manual compass to sight along the antenna boom for a magnetic bearing, then add the Fig 13.43 — Screen plot declination for true bearing to the fox. from a computerized RDF system showing Triangulation Techniques three T-hunt bearings (straight lines radiating If you can obtain accurate bearings from from small circles) and two locations separated by a suitable dis- the vehicle path tance, the technique of triangulation will (jagged trace). The grid give the expected location of the transmit- squares correspond to areas of standard ter. The intersection of the lines of bearing topographic maps. from each location provides a fix. Trian- gulation accuracy is greatest when stations are located such that their bearings inter- sect at right angles. Accuracy is poor when

13.26 Chapter 13

Chapter 13.pmd 26 7/28/2006, 9:15 AM The computer is an excellent RDF tool, monic charts are custom maps prepared reading changes only slightly or perhaps but it is no substitute for a skilled “navi- especially for government and military not at all as the RDF antenna rotates, no gator.” You will probably discover that agencies. matter how much attenuation you add. The using a computer on a high-speed T-hunt Skywave signals do not always follow cure is to shield the receiving equipment. requires a full-time operator in the ve- the great-circle path in traveling from a Something as simple as wrapping the re- hicle to make full use of its capabilities. transmitter to a receiver. For example, if ceiver in foil or placing it in a bread pan or the signal is refracted in a tilted layer of cake pan, covered with a piece of copper Skywave Bearings and the ionosphere, it could arrive from a di- or aluminum screening securely fastened Triangulation rection that is several degrees away from at several points, may reduce direct pickup Many factors make it difficult to obtain the true great-circle bearing. enough for you to get bearings. accuracy in skywave RDF work. Because Another cause of signals arriving off the Alternatively, you can replace the re- of Faraday rotation during propagation, great-circle path is termed sidescatter. It ceiver with a field-strength meter as you skywave signals are received with random is possible that, at a given time, the iono- close in, or use a heterodyne-type active polarization. Sometimes the vertical com- sphere does not support great-circle attenuator. Plans for these devices are at ponent is stronger, and at other times the propagation of the signal from the trans- the end of this chapter. horizontal. During periods when the verti- mitter to the receiver because the fre- cal component is weak, the signal may quency is above the MUF for that path. The Body Fade appear to fade on an Adcock RDF system. However, at the same time, propagation A crude way to find the direction of a At these times, determining an accurate may be supported from both ends of the VHF signal with just a hand-held trans- signal null direction becomes very diffi- path to some mutually accessible point off ceiver is the body fade technique, so cult. the great-circle path. The signal from the named because the blockage of your body For a variety of reasons, HF bearing source may propagate to that point on the causes the signal to fade. Hold your accuracy to within 1 or 2° is the exception Earth’s surface and hop in a sideways di- HT close to your chest and turn all the rather than the rule. Errors of 3 to 5° are rection to continue to the receiver. way around slowly. Your body is provid- common. An error of 3° at a thousand For example, signals from Central Eu- ing a shield that gives the hand-held a miles represents a distance of 52 miles. rope have propagated to New England by cardioid sensitivity pattern, with a sharp Even with every precaution taken in mea- hopping from an area in the Atlantic Ocean decrease in sensitivity to the rear. This surement, do not expect cross-country HF off the northwest coast of Africa, whereas null indicates that the source is behind triangulation to pinpoint a signal beyond a the great-circle path puts the reflection you (Fig 13.44). county, a corner of a state or a large met- point off the southern coast of Greenland. If the signal is so strong that you can’t ropolitan area. The best you can expect is Readings in error by as much as 50° or find the null, try tuning 5 or 10 kHz off to be able to determine where a mobile more may result from sidescatter. The ef- frequency to put the signal into the skirts RDF group should begin making a local fect of propagation disturbances may be of the IF passband. If your hand-held is search. that the bearing seems to wander some- dual-band (144/440 MHz) and you are Triangulation mapping with skywave what over a few minutes of time, or it may hunting on 144 MHz, try tuning to the signals is more complex than with ground be weak and fluttery. At other times, how- much weaker third harmonic of the signal or direct waves because the expected paths ever, there may be no telltale signs to in- in the 440-MHz band. are great-circle routes. Commonly avail- dicate that the readings are erroneous. The body fade null, which is rather shal- able world maps are not suitable, because low to begin with, can be obscured by re- the triangulation lines on them must be Closing In flections, multipath, nearby objects, etc. curved, rather than straight. In general, for On a mobile foxhunt, the objective is Step well away from your vehicle before flat maps, the larger the area encompassed, usually to proceed to the hidden T with trying to get a bearing. Avoid large build- and the greater the error that straight line minimum time and mileage. Therefore, do ings, chain-link fences, metal signs and triangulation procedures will give. not go far out of your way to get off-course the like. If you do not get a good null, move A highway map is suitable for regional bearings just to triangulate. It is usually to a clearer location and try again. triangulation work if it uses some form of better to take the shortest route along your conical projection, such as the Lambert initial line of bearing and “home in” on the conformal conic system. This maintains signal. With a little experience, you will the accuracy of angular representation, but be able to gauge your distance from the the distance scale is not constant over the fox by noting the amount of attenuation entire map. needed to keep the S-meter on scale. One alternative for worldwide areas As you approach the transmitter, the is the azimuthal-equidistant projection, signal will become very strong. To keep better known as a great-circle map. True the S-meter on scale, you will need to add bearings for great-circle paths are shown an RF attenuator in the transmission line as straight lines from the center to all from the antenna to the receiver. Simple points on the Earth. Maps centered on resistive attenuators are discussed in an- three or more different RDF sites may be other chapter. compared to gain an idea of the general In the final phases of the hunt, you will geographic area for an unknown source. probably have to leave your mobile and For worldwide triangulation, the best continue the hunt on foot. Even with an projection is the gnomonic, on which all attenuator in the line, in the presence of a great circle paths are represented by strong RF field, some energy will be Fig 13.44 — When performing the body straight lines and angular measurements coupled directly into the receiver cir- fade maneuver, a hand-held transceiver with respect to meridians are true. Gno- cuitry. When this happens, the S-meter exhibits this directional pattern.

EMI/Direction Finding 13.27

Chapter 13.pmd 27 7/28/2006, 9:15 AM Air Attenuators Fig 13.45 — The air attenuator for a VHF hand-held in use. Suspend the In microwave parlance, a signal that is radio by the wrist strap or a string too low in frequency to be propagated in a inside the tube. waveguide (that is, below the cutoff fre- quency) is attenuated at a predictable loga- rithmic rate. In other words, the farther To use this air attenuation scheme for inside the waveguide, the weaker the signal body fade bearings, hold the tube verti- gets. Devices that use this principle to re- cally against your chest and lower the duce signal strength are commonly known hand-held into it until the signal begins to as air attenuators. Plans for a practical weaken (Fig 13.45). Holding the receiver model for insertion in a coax line are in in place, turn around slowly and listen for Transmitter Hunting (see Bibliography). a sudden decrease in signal strength. If the With this principle, you can reduce the null is poor, vary the depth of the receiver level of strong signals into your hand-held in the tube and try again. You do not need transceiver, making it possible to use the to watch the S-meter, which will likely be body fade technique at very close range. out of sight in the tube. Instead, use noise Glen Rickerd, KC6TNF, documented this level to estimate signal strength. technique for QST. Start with a pasteboard For extremely strong signals, remove mailing tube that has sufficient inside di- the “rubber duck” antenna or extend the ameter to accommodate your hand-held. wrist strap with a shoelace to get greater Cover the outside of the tube completely depth of suspension in the tube. The depth with aluminum foil. You can seal the bot- that works for one person may not work tom end with foil, too, but it probably will for another. Experiment with known sig- not matter if the tube is long enough. For nals to determine what works best for you. durability and to prevent accidental shorts, wrap the foil in packing tape. You will also Note 1Declination is the term as denoted on land need a short, stout cord attached to the USGS topographic maps. Deviation and hand-held. The wrist strap may work for Variation are terms used on nautical and this, if long enough. aviation charts, respectively.

THE SIMPLE SEEKER

The Simple Seeker for 144 MHz is the (with respect to the switching waveform), tance RDF. They ensure maximum signal latest in a series of dual-antenna TDOA depending on which antenna is nearer the pickup and provide the best load for projects by Dave Geiser, W5IXM. Fig source. Thus, comparing the receiver out- transmitting. Fig 13.47 shows plans for a 13.36 and accompanying text shows its put phase to that of the switching wave- pair of dipoles mounted on an H frame of 1 principle of operation. It is simple to per- form determines which end of the null line /2-inch PVC tubing. Connect the 39-inch form rapid antenna switching with diodes, points toward the transmitter. The com- elements to the switcher with coaxial driven by a free-running multivibrator. mon name for a circuit to make this com- cables of exactly equal length. Spacing For best RDF performance, the switching parison is a phase detector, achieved in between dipoles is about 20 inches for pulses should be square waves, so anten- this unit with a simple bridge circuit. A 2 m, but is not critical. To prevent exter- nas are alternately connected for equal phase detector balance control is included, nal currents flowing on the coax shield times. The Simple Seeker uses a CMOS although it may not be needed. Serious from disrupting RDF operation, wrap version of the popular 555 timer, which imbalance indicates incorrect receiver three turns (about 2 inch diameter) of the demands very little supply current. A 9-V tuning, an off-frequency target signal, or incoming coax to form a choke balun. alkaline battery will give long life. See misalignment in the receiver IF stages. For receive-only work, dipoles are ef- Fig 13.46 for the schematic diagram. Almost any audio transformer with fective over much more than their useful PIN diodes are best for this application approximately 10:1 voltage step-up to a transmit bandwidth. A pair of appropri- because they have low capacitance and center-tapped secondary meets the re- ately spaced 144-MHz dipoles works handle a moderate amount of transmit quirements of this phase detector. The from 130 to 165 MHz. You will get power. Philips ECG553, NTE-555, output is a positive or negative indica- greater tone amplitude with greater di- Motorola MPN3401 and similar types are tion, applied to meter M1 to indicate left pole spacing, making it easier to detect suitable. Ordinary 1N4148 switching di- or right. the null in the presence of modulation on odes are acceptable for receive-only use. the signal. But do not make the spacing Off the null, the polarity of the switch- ANTENNA CHOICES greater than one-half free-space wave- ing pulses in the receiver output changes Dipole antennas are best for long-dis- length on any frequency to be used.

13.28 Chapter 13

Chapter 13.pmd 28 7/28/2006, 9:15 AM Fig 13.46 — Schematic of the Simple Seeker. A capacitor from point T to ground will lower the tone frequency, if desired. A single SPDT center-off toggle switch can replace separate power and function switches.

Best bearing accuracy demands that pass filter (the series 4.7-kΩ resistor and indicator deflects left when the signal is signals reach the receiver only from the shunt 470-pF capacitor) to the receiver. to the left. Others prefer that a left meter switched antenna system. They should The electronic switch is on a 20-pin DIP indication indicates that the antenna is not arrive on the receiver wiring directly pad, with the phase detector on another rotated too far to the left. Whichever your (through an unshielded case) or enter on pad (see Fig 13.49). choice, you can select it with the DPDT wiring other than the antenna coax. The Because the phase detector may behave polarity switch. Polarity of audio output phase detecting system is less amplitude differently on weak and strong signals, varies between receivers, so test the unit sensitive than systems such as quads and the Simple Seeker incorporates an audio and receiver on a known signal source Yagis, but if you use small-aperture an- attenuator to allow either a full-strength and mark the proper switch position on tennas such as “rubber duckies,” a small audio or a lesser, adjustable received sig- the unit before going into the field. signal leak may have a big effect. A wrap nal to feed the phase detector. You can PIN diodes, when forward biased, of aluminum foil around the receiver case plug headphones into jack AF2 and con- exhibit low RF resistance and can pass up helps block unwanted signal pickup, but nect receiver audio to jack AF1 for no to approximately 1 W of VHF power tighter shielding may be needed. attenuation into the phase detector, or re- without damage. The transmit position on Fig 13.48 shows a “sniffer” version of verse the external connections, using the the function switch applies steady dc bias the unit with helix antennas. The added pad to control level to both the phones to one of the PIN diodes, allowing RDF circuits fit in a shielded box, with and the phase detector. communications from a hand-held RDF the switching pulses fed through a low- Convention is that the meter or other transceiver.

EMI/Direction Finding 13.29

Chapter 13.pmd 29 7/28/2006, 9:15 AM Fig 13.48 — Field version of the Simple Seeker with helix antennas.

Fig 13.49 — Interior view of the Simple Fig 13.47 — “H” frame for the dual dipole Simple Seeker antenna set, made from Seeker. The multivibrator and phase 1 /2-in. PVC tubing and tees. Glue the vertical dipole supports to the tees. Connect detector circuits are mounted at the vertical tees and handle to the cross piece by drilling both parts and inserting box ends. This version has a large cotter pins. Tape the dipole elements to the tubes. convenient built-in speaker.

AN ACTIVE ATTENUATOR FOR VHF-FM

During a VHF transmitter hunt, the received signal with a signal from a greater than 100 dB. strength of the received signal can vary 500-kHz oscillator. This process creates Varying the level of the oscillator sig- from roughly a microvolt at the starting mixing products above and below the in- nal provides the extra advantage of con- point to nearly a volt when you are within put frequency. The spacing of the closest trolling the strength of the input signal as an inch of the transmitter, a 120-dB range. products from the input frequency is it passes through the mixer. So as you If you use a beam or other directional ar- equal to the local oscillator (LO) frequency. close in on the target, you have the choice ray, your receiver must provide accurate For example, if the input signal is at of monitoring and controlling the level of signal-strength readings throughout the 146.52 MHz, the closest mixing products the input signal or the product signals, hunt. Zero to full scale range of S-meters will appear at 147.02 and 146.02 MHz. whichever provides the best results. on most hand-held transceivers is only 20 The strength of the mixing products The LO circuit (Fig 13.50) uses the to 30 dB, which is fine for normal operat- varies with increasing or decreasing LO easy-to-find 2N2222A transistor. Trim- ing, but totally inadequate for transmitter signal level. By DFing on the mixing mer capacitor C1 adjusts the oscillator’s hunting. Inserting a passive attenuator product frequencies, you can obtain ac- frequency. Frequency stability is only a between the antenna and the receiver re- curate headings even in the presence of a minor concern; a few kilohertz of drift is duces the receiver input signal. However, very strong received signal. As a result, tolerable. Q1’s output feeds an emitter- the usefulness of an external attenuator is any hand-held transceiver, regardless of follower buffer using a 2N3904 transistor, limited by how well the receiver can be how poor it’s shielding may be, is usable Q2. A linear-taper potentiometer (R6) shielded. for transmitter hunting, up to the point controls the oscillator signal level present Anjo Eenhoorn, PAØZR, has designed a where complete blocking of the receiver at the cathode of the mixing diode, D1. simple add-on unit that achieves continu- front end occurs. At the mixing product The diode and coupling capacitor C7 are ously variable attenuation by mixing the frequencies, the attenuator’s range is in series with the signal path from antenna

13.30 Chapter 13

Chapter 13.pmd 30 7/28/2006, 9:15 AM the Templates section of the Handbook CD-ROM. A circuit board is available from FAR Circuits. The prototype (Fig 13.51) uses a plated enclosure with female BNC connectors for RF input and output. C7, D1, L2 and R5 are installed with point-to-point wiring between the BNC connectors and the potentiometer. S1 mounts on the rear wall of the enclosure. Most hams will find the 500-kHz fre- quency offset convenient, but the oscilla- tor can be tuned to other frequencies. If VHF/UHF activity is high in your area, choose an oscillator frequency that cre- ates mixing products in clear portions of the band. The attenuator was designed for 144-MHz RDF, but will work elsewhere in the VHF/UHF range. You can tune the oscillator with a fre- quency counter or with a strong signal of known frequency. It helps to enlist the aid of a friend with a hand-held transceiver a short distance away for initial tests. Con- nect a short piece of wire to J1, and cable your hand-held transceiver to J2. Select a simplex receive frequency and have your assistant key the test transmitter at its low-

1 est power setting. (Better yet, attach the Fig 13.50 — Schematic of the active attenuator. Resistors are /4-W, 5%-tolerance carbon composition or film. transmitter to a dummy antenna.) With attenuator power on, adjust R6 for μ BT1 — Alkaline hearing-aid battery, L2 — 3.3- H RF choke. mid-scale S-meter reading. Now retune the Duracell SP675 or equivalent. R6 — 1-kΩ, 1-W linear taper (slide or C1 — 75-pF miniature foil trimmer. rotary). hand-held to receive one of the mixing J1, J2 — BNC female connectors. S1 — SPST toggle. products. Carefully tune C1 and R6 until L1 — 470-μH RF choke. you hear the mixing product. Watch the S-meter and tune C1 for maximum reading. If your receiver features memory chan- input to attenuator output. tiny 1.4-V hearing-aid battery with a nels, enter the hidden transmitter fre- This frequency converter design is un- homemade battery clip. If your enclosure quency along with both mixing product orthodox; it does not use the conventional permits, you can substitute a standard frequencies before the hunt starts. This configuration of a doubly balanced mixer, AAA-size battery and holder. allows you to jump from one to the other matching pads, filters and so on. Such at the press of a button. sophistication is unnecessary here. This CONSTRUCTION AND TUNING When the hunt begins, listen to the fox’s approach gives an easy to build circuit that For a template for this project, including frequency with the attenuator switched on. consumes very little power. PAØZR uses a the PC board layout and parts overlay, see Adjust R6 until you get a peak reading. If the signal is too weak, connect your quad or other RDF antenna directly to your transceiver and hunt without the attenua- tor until the signal becomes stronger. As you get closer to the fox, the attenu- ator will not be able to reduce the on- frequency signal enough to get good bear- ings. At this point, switch to one of the mixing product frequencies, set R6 for on- scale reading and continue. As you make your final approach, stop frequently to adjust R6 and take new bearings. At very close range, remove the RDF antenna al- together and replace it with a short piece of wire. It’s a good idea to make up a short length of wire attached to a BNC fitting in advance, so you do not damage J1 by stick- ing random pieces of wire into the center Fig 13.51 — Interior view of the active attenuator. Note that C7, D1 and L2 are contact. mounted between the BNC connectors. R5 (not visible in this photograph) is While it is most convenient to use this connected to the wiper of slide pot R6. system with receivers having S-meters,

EMI/Direction Finding 13.31

Chapter 13.pmd 31 7/28/2006, 9:15 AM the meter is not indispensable. The active p 28. Hill. (This book, available from ARRL, attenuator will reduce signal level to a Flanagan and Calabrese, “An Automated includes plans for the Roanoke Doppler point where receiver noise becomes au- Mobile Radio Direction Finding Sys- RDF unit and in-line air attenuator, plus dible. You can then obtain accurate fixes tem,” QST, Dec 1993, p 51. VHF quads and other RDF antennas.) with null-seeking antennas or the “body Geiser, “A Simple Seeker Direction Moell, “Transmitter Hunting — Tracking fade” technique by simply listening for Finder,” ARRL Antenna Compendium, Down the Fun,” QST, Apr 1993, p 48 and maximum noise at the null. Volume 3, p 126. May 1993, p 58. Gilette, “A Fox-Hunting DF Twin’Tenna,” Moell, “Build the Handy Tracker,” 73 Maga- RDF BIBLIOGRAPHY QST, Oct 1998, pp 41-44. zine, Sep 1989, p 58 and Nov 1989, p 52. Bohrer, “Foxhunt Radio Direction Finder,” Johnson and Jasik, Antenna Engineering O’Dell, “Simple Antenna and S-Meter 73 Amateur Radio, Jul 1990, p 9. Handbook, Second Edition, New York: Modification for 2-Meter FM Direction Bonaguide, “HF DF — A Technique for McGraw-Hill. Finding,” QST, Mar 1981, p 43. Volunteer Monitoring,” QST, Mar 1984, Kossor, “A Doppler Radio-Direction O’Dell, “Knock-It-Down and Lock-It-Out p 34. Finder,” QST, Part 1: May 1999, pp 35- Boxes for DF,” QST, Apr 1981, p 41. DeMaw, “Maverick Trackdown,” QST, Jul 40; Part 2: June 1999, pp 37-40. Ostapchuk, “Fox Hunting is Practical and 1980, p 22. McCoy, “A Linear Field-Strength Meter,” Fun!” QST, Oct 1998, pp 68-69. Dorbuck, “Radio Direction Finding Tech- QST, Jan 1973, p 18. Rickerd, “A Cheap Way to Hunt Transmit- niques,” QST, Aug 1975, p 30. Moell and Curlee, Transmitter Hunting: ters,” QST, Jan 1994, p 65. Eenhoorn, “An Active Attenuator for Radio Direction Finding Simplified, The “Searcher” (SDF-1) Direction Finder, Transmitter Hunting,” QST, Nov 1992, Blue Ridge Summit, PA: TAB/McGraw- Rainbow Kits.

13.32 Chapter 13

Chapter 13.pmd 32 7/28/2006, 9:15 AM