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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

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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 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.
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