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Home , Electrical length, Skywave

AM Systems

W.C. Alexander Director of Engineering Crawford Company

Abstract Atmospheric noise, natural and manmade, Over the years, the collective affect the signal-to-noise ratio at the knowledge of AM antenna systems and the receiver, but it does not affect the level of principles behind them has faded somewhat. signal arriving from the transmit antenna. Most of the tomes used by the last AM antenna systems are vertically generation of AM consulting engineers date polarized. This is done for a number of back to the 1940s and are no longer in print. reasons, including superior groundwave It is the purpose of this paper to propagation and simplicity of antenna provide the interested reader with a general systems. The downside of vertical body of knowledge of AM antenna systems. is that most atmospheric noise is also vertically polarized. Still, vertical 1.0 AM Antenna Basics polarization is a better choice for AM The purpose of any AM antenna is to broadcast than horizontal and virtually all radiate the power generated by the AM radiators are vertical. Not only are . Some antennas do this better horizontal dipole antennas mechanically than others, and there are many ways to get impractical, their radiation on the horizon is a signal into the air. not nearly as good as that of a vertical Non-directional antennas radiate radiator. Since it is necessary to erect two equally in all directions, providing the relatively tall towers to support a horizontal simplest way to get a signal out in an dipole, why not just drive one of the towers efficient manner. Directional antennas are directly and forget about the other tower and used to concentrate signal in some directions dipole? (toward population centers, for instance) Earlier, we mentioned that several while suppressing signal in others (toward things influence the amount of signal other stations which must be protected from received at a particular point from a given interference). antenna system. The first of these was the The antenna system is the last point amount of radiation toward the receiver. The in a broadcast system where the broadcaster amount of radiation toward a particular has any control over the signal. After that point is influenced by the transmitter power, point, environmental factors, receiver system losses, and characteristics and other factors have sway antenna . Transmitter power is over what the listener hears. The amount of self-explanatory. System losses come in signal received at a given point is dependent several areas — resistive losses in on the amount of radiation toward that point conductors, system and tuning from the antenna, the distance to the components, and losses. receiver, the conductivity of the earth Antenna efficiency is really defined in two between the transmit and receive locations, ways: One has to do with the vertical the character of the terrain between antennas radiation characteristics of the antenna; the and, sometimes, the ionospheric conditions. other has to do with the . AM Antenna Systems Alexander/2

We’ll look at both of these in more detail used internationally, the reason for this later. The efficiency of a non-directional having to do with treaties that predate the AM antenna is expressed in millivolts per adoption of the current set of US meter at one kilometer (mV/m/km), and this groundwave curves. figure is referred to as the inverse distance We previously mentioned the field or IDF. efficiency expressed as the inverse distance Another factor that influences the field of a non-. You have amount of signal received at a particular probably already figured out that the point is attenuation. Over perfectly conductivity of the ground in the region conducting earth, the amount of signal between the antenna itself and the receive received at a distance would be inversely point 1 km away will cause the field proportional to the distance from the strength at that point to be attenuated below transmit antenna. This relationship is known what it would be over perfectly conductive as the inverse distance rule. For example, if earth. How, then, can one accurately at a distance of 1 km a field strength of 100 measure the efficiency of an antenna? The mV/m is present, at 2 km the field strength answer is with many measurements taken would be 50 mV/m. At 4 km, the field radially, beginning very close to the antenna strength would be 25 mV/m, and at 8 km it (usually at the point where the first on-scale would be 12.5 mV/m. If you were to graph reading can be taken). The very close-in this relationship as field strength versus measurements establish the unattenuated distance on log-log graph paper, it would IDF while measurements farther away from plot as a straight diagonal line. the antenna establish the conductivity of the In the real world, the earth is not ground between the antenna and the last perfectly conductive. Ground conductivity point measured. varies from very good (seawater) to very poor (rock and certain soils). The more 2.0 Non-Directional Antennas conductive the ground is, the less a signal Non-directional antennas can come from an AM antenna will be attenuated and in several forms, but by and large these are the more the field strength versus distance simple vertical radiators. One type is base plot will resemble the inverse distance line. insulated and series-fed; the other is Over ground that is less conductive, the grounded base and shunt fed. We’ll look at more a signal from an AM antenna will be both these types in detail later. attenuated and the more the field strength Ideally, the electrical length of an versus distance plot will curve away from AM antenna will be 90 electrical degrees the inverse distance line. (1/4 ) or more. Antennas of this A family of groundwave curves is length provide adequate efficiency and published by the FCC for each group of . Sometimes, though, aeronautical frequencies, showing the effects of different or structural considerations force ground conductivities. These curves are the broadcasters to use shorter towers. The basis for predicting distance to a field apparent electrical length of short towers strength and thus the entire US allocation can be increased through the use of top system. A slightly different set of curves is loading. Top loading increases the AM Antenna Systems Alexander/3

to ground, and is usually signal radiated above the horizon. While achieved through use of a top hat (a flat, such an antenna may appear to be more horizontal disk attached to the top of the efficient than a shorter, non-sectionalized tower) or using bonded guy wires. antenna, the spherical (total) radiation in Because of mechanical both antennas will be the same for a given considerations, the use of top hats is not as amount of input power, assuming all losses common as other methods. The top hat must are the same. Sectionalization simply puts usually support its own weight and the signal where it is needed, toward the withstand wind, ice and other environmental horizon, in much the same manner as a hazards unsupported, so top hat size (and multi-bay FM antenna achieves antenna thus effectiveness) is limited. “gain.” By far, the most common method of top loading is through use of bonded guy 3.0 Current Distribution wires. This method uses sections of guy An insulated, non-top loaded tower wires bonded to the top of the tower that are will typically have a current loop (or usually bonded above the first insulators to maximum) 90 electrical degrees down from adjacent guy wires. Often, other non- the top of the tower. If the tower is shorter structural guys are added and bonded to than 90° long, the current loop will occur at structural guys to increase the effectiveness the tower base. Current distribution on a of the top loading. It is not unusual to see six single, insulated, uniform cross-section or more guy wires bonded together in a radiator will be more or less sinusoidal in “spider web” fashion in a top loading nature and is approximately defined as arrangement. follows: The advantages of top loading are increased base resistance, reduced base iA = I A sin(G − y) capacitive reactance, lower Q and improved bandwidth. While all this sounds very where: iA = current in amperes at height y attractive, it is almost always better to IA = Maximum current in amperes achieve these qualities with increased tower G = Tower height in degrees height rather than top loading. y = height in degrees of current Sectionalization is a method of element increasing the groundwave efficiency of a vertical radiator, improving groundwave There is always a current (or performance and reducing skywave minimum) and voltage loop at the top of any radiation. In a sectionalized tower, an tower that does not employ top loading. As insulator is placed near the center of an we move down the tower, the voltage will electrically long radiator and a network is decrease and the current will increase in an placed between the sections. In simple approximately sinusoidal fashion until a terms, the current in the upper section can be current loop and voltage node occur at the adjusted to be in phase with the signal in the point 90° below the top of the tower, if the lower section, thus focusing the signal tower is greater than 90° tall, or at the tower radiated toward the horizon and reducing the base if it is shorter than 90°. At the point AM Antenna Systems Alexander/4

where the voltage or current nodes occur, insulated bracket/handle so that all the voltage or current does not pass through measurements can be made with the loop the zero but rather reach minimum values and same distance from the tower leg. shift approximately 180° in phase in traversing the node region. On a tall tower 4.0 Vertical Radiation Characteristics (greater than 180°), more than one node will Generally speaking groundwave occur along the tower’s length. radiation (and apparent efficiency) from a Many things influence current vertical radiator will increase as the current distribution on a tower. Cross-section, loop moves up from the base. The optimum uniformity, and nearby conductors are electrical length of a vertical radiator is 225° among a few of these. In some cases (which or 5/8 wavelength. At this electrical length, we will explore in detail in the future), there current loops is occur at 45 and 135 degrees are actually two currents flowing on a above the base. Radiation on the horizon is particular frequency on a tower — the maximized and radiation above the horizon current that contributes to radiation and the is minimized. current that is induced from another nearby Shorter towers have more radiation radiator. This is the norm in directional above the horizon and thus produce more arrays, where a tower will have current flow skywave radiation and less groundwave from its own excitation and current flow radiation. Towers considerably shorter than from radiation arriving from other elements 90° produce so much radiation above the in the array. The current distribution on horizon that much of the power is wasted towers such as these is sometimes hard to into space. Nighttime power is usually much predict using conventional methods. more limited when using an electrically For most purposes, we assume short radiator, although considerably more sinusoidal current distribution on a radiator, daytime power may be allowed as a result of and it usually works fairly well. Modern the reduced groundwave efficiency. computer modeling using moment method The vertical radiation characteristic analysis can, however, do an excellent job of of a vertical radiator that is not top loaded or predicting the current flow and distribution sectionalized, or the function of theta, is on a radiator and this gives the designer a defined as follows: much better picture of what is happening. Knowing the current distribution is cos(G sinθ ) − cosG f (θ ) = important to the vertical radiation (1− cosG)cosθ characteristics of an antenna. Knowing where the current loop is on an antenna is essential when detuning a radiator for the purpose of eliminating reradiation. Occasionally, it is beneficial to where: measure the current distribution on a tower. f(θ) = function of theta To do this, a small sample loop is G = height of the antenna in constructed out of copper tubing or degrees aluminum angle and fitted with some sort of θ = vertical angle AM Antenna Systems Alexander/5

determine how much skywave field a This equation returns a multiplier by particular station would produce at a given which the inverse distance field of an location. The design engineer would find the antenna is multiplied to find the radiation at appropriate vertical angle for the distance to a particular vertical angle (θ). For example, the receiver, find the function of theta using if a tower is 90 electrical degrees tall, has an this formula, find the skywave multiplier inverse distance field of 300 mV/m at one from the FCC formula and multiply that by km and you wish to find the radiation from E(0/,θ), or the radiation at the pertinent the antenna at a vertical angle of 20 degrees azimuth and vertical angle from the station. above the horizon, the above formula gives I mention all this now only to show the need us a function of theta or multiplier of 0.914. to know the function of theta in some Multiplying this by the antenna’s circumstances. groundwave inverse distance field of 300 One way an electrically short tower mV/m, we find that the radiation at 20 can be electrically lengthened is using top degrees above the horizon is 274.3 mV/m at loading. If a tower employs top loading, the one km. function of theta is computed as follows: This information would be used in engineering a nighttime allocation to

cos Bcos(Asinθ) − sinθ sin B sin (Asinθ ) − cos(A + B) f (θ ) = cosθ [cos B − cos(A + B)]

where: electrical height based on current f(θ) = function of theta distribution achieved through top loading, A = the physical height of the tower the function of theta is higher than the same in degrees electrical height achieved without top B = the difference, in degrees, loading. between the apparent electrical Another type of antenna we briefly height (based upon current discussed in Part 1 was the sectionalized distribution) and the actual tower. Sectionalization is used to increase physical height the groundwave efficiency (and reduce θ = vertical angle skywave radiation) by placing an insulator near the center of an electrically long If, for example, a particular tower is radiator and controlling the current flow on only 60 degrees tall (A) but employs top each section with a network between the loading that makes it seem 30 degrees taller sections. For sectionalized towers, there is (B) based upon current distribution, and we yet another formula to determine the wanted to find the function of theta for a function of theta: vertical angle of 20 degrees, the above formula yields 0.923. Note that for the same

AM Antenna Systems Alexander/6

sin ∆[cos B cos(Asinθ )−cosG]+sin B[cos D cos(C sinθ )−sinθ sin Dsin (C sinθ )− cos∆ cos(Asinθ )] f (θ ) = cosθ [sin ∆(cos B −cosG)+sin B(cos D −cos∆)]

where: A = the physical height, in electrical degrees, of the lower section of the tower B = the difference between the apparent electrical height (based on current distribution) of the lower section of the tower and the physical height of the lower section of the tower C = the physical height of the entire tower, in electrical degrees D = the difference between the apparent electrical height of the tower (based on current distribution of the upper section) and the physical height of the entire tower. D will be zero if the sectionalized tower is not top loaded. G = the sum of A and B (A + B) H = the sum of C and D (C+D) ∆ = the difference between H and A (H-A)

By way of example, if we have a towers are fed across the base insulator. This sectionalized tower that has a lower section type of tower is said to be series fed, since that is 120 electrical degrees tall (A) and the the excitation is, in essence, fed in series current distribution makes the lower section with the tower base. seem as if it is 20 degrees taller than that Occasionally, it is advantageous to (B), the overall height is 220 electrical use a grounded base tower as an AM degrees tall (C) and top loading of the upper radiator. Such circumstances may include section results in current distribution that mounting of an FM or other antenna on the makes the tower seem as if it is 15 degrees tower, use of an existing grounded-base taller (D) and we want to find the function tower for AM or, from time to time, of theta for a vertical angle of 30 degrees, proximity of the tower base to a populated the above formula yields 0.593. This building or structure. In these cases, it is formula is, obviously, quite cumbersome possible to use a grounded base tower with and difficult to solve using a pocket good results as an AM radiator if the guy calculator and paper. However, it lends itself wires are all insulated. easily to codifying into computer language The base impedance of a grounded- or a programmable calculator. base tower is essentially zero, but that impedance rises with height above the base. 5.0 Insulated and Grounded Towers At some location up the tower (assuming it By and large, the vast majority of is of adequate height), a point will exist that AM antennas consist of insulated base will provide an acceptable feed impedance. towers. Almost all directional arrays use The easiest way to feed a grounded base insulated base towers. There are several tower is with a slant wire, which is attached advantages to using an insulated base tower, at the aforementioned point and returns to chief of which is control of the current the transmitter building at an angle distribution on the tower. Insulated base approaching 45 degrees. This forms what is AM Antenna Systems Alexander/7

essentially half of a “delta” match. The tower base, and this is where the excitation location of the best attachment point for the is applied. This type of grounded base slant wire is usually determined by a cut- antenna is called a folded monopole. and-try method, although experience on the The skirt wire conductors form the part of the field engineer and modern outer conductor of a transmission line. Were computer modeling techniques can point to the short located at the quarter-wave point, it a starting point that should be close to the would transform the short at that end to an desired impedance. In most situations, if the open at the other end, creating a virtual base feedpoint is properly selected, the only insulator at the bottom of the tower. By matching needed will be a series to adjusting the location of the short between cancel out the inductive reactance of the the skirt wires and the tower, the impedance slant wire. at the feed point can be adjusted to a Because the excitation in a slant wire favorable value. This arrangement forms fed grounded base tower is shunted across what is essentially a “gamma” match. the grounded tower base, this type of Current flow in a folded monopole is antenna is said to be shunt fed. The current up on the skirt wires and down on the tower flow is up the slant wire and then on up the structure. Such antennas perform much like tower to the top, where a current node and base insulated towers. Radiation current voltage loop will exist. Some current, flows up the skirt wires to the tower and however, also flows down the tower from then on up to the top of the tower, where a the slant wire feed point and contributes to current node and voltage loop will exist. radiation. Because there are two radiated fields below the feedpoint (one from the 6.0 Base Impedance slant wire and the other from the tower The base impedance of an below the feedpoint), some suppression of insulated-base tower is determined primarily radiation occurs, usually on the side of the by the electrical length of the antenna, cross- tower where the slant wire is located. section, the extent of the ground system and Seldom is this a problem, however, and the the elevation of the feed point above ground. antenna is still considered to be a non- Short towers have much lower base directional radiator. resistance, and the reactance becomes quite Another way to feed a grounded base capacitive. When base resistance is quite tower is to mount an insulated skirt on it low, the fixed ground loss of one to three consisting of three or more wires suspended ohms becomes a significant part of the off the tower on insulators and parallel to radiation resistance of the antenna. This, in the length of the tower. The skirt wires are part, is the reason that short towers are less bonded together at the top of the tower and efficient than tall towers. then bonded to the tower itself at a point that Taller towers have higher base produces a desired driving point resistance, and at some point (usually impedance, usually somewhere below the around 80 electrical degrees), the reactance 90 degree point (0.15 is a crosses over and becomes inductive. There common location for this bond). The skirt is some benefit to selecting a radiator with wires are also bonded together near the close to a zero reactive component in its AM Antenna Systems Alexander/8

base impedance when designing an antenna system as is commonly used in one form or system. Empirical tower base impedance another in all AM broadcast antenna data is available and published in many systems. places, both in tabular and graph format. While it is certainly possible to use Through the use of moment method materials other than copper in a ground computer modeling, the base impedance of system, copper is by far the best many different tower configurations can be compromise for both performance, accurately predicted. This is particularly durability and economy. Copper is used useful with tapered or free-standing towers, almost exclusively for this application, so where the cross-section is great and the for the remainder of this discussion, we will parallel capacitance of multiple base assume bare, soft-drawn copper wire to be insulators is significant. the material of choice. In grounded-base shunt-fed towers, the feed point impedance is more dependent 7.1 Ground Currents on feed point location. Since this is usually Did you ever wonder what was the at least to a degree within the control of the return path for all that current flowing in user, a desirable base impedance can often your AM tower? These currents leave the be obtained. This impedance often has a antenna as displacement currents, flow relatively large reactive component; the through space and finally into the ground, at resulting Q is usually higher and bandwidth which point they become conduction lower than that of an insulated tower of the currents. Due to skin effect, these currents same height. usually flow very close to the surface as they flow radially back to the antenna base. 7.0 Ground Systems With copper ground radials in place, Beneath every AM antenna system is the ground current is made up of two parts: a ground system (or at least there is One part of this current flows through the supposed to be). The ground system is every earth itself; the remainder flows in the bit as important to the operation of an AM buried wires. This can generally be viewed antenna as the tower that is the vertical as a parallel resistive circuit. As the ground radiator. current flows inward through the ground or Without a copper (or otherwise the wire toward the antenna base, it is conductive) ground system, the losses in a continually added to by additional vertical AM antenna are very high. These displacement currents from the antenna that losses are largely due to the conduction of are flowing into the ground. Because these currents through the earth, which at best has additional currents differ in phase from the a high resistance. By placing a number of components already flowing in the ground wires in the earth from near the tower base and buried wires, the ground currents do not to radial points some distance away, we necessarily increase. provide a relatively low resistance path for the ground currents to return to the ground 7.2 Ground Losses in the vicinity of the tower base. These It has been established through radial wires make up the classic ground studies and years of experience with such AM Antenna Systems Alexander/9

systems that the ground currents at a result of these experiments: distance are proportional to an antenna’s • The distribution of currents depends field strength at one kilometer, or the on the wire size in a logarithmic inverse distance field (IDF). In these studies, fashion, making the size of wire used power was fed to a vertical radiator and the relatively non-critical actual current flowing in the buried wires at • The losses in a ground system are a distance from the tower base was inversely proportional to the number measured. The results further established of radial wires used that the current flowing in the ground radials • The resistance of an antenna of a at distances greater than 0.3 wavelengths given height is reduced as the from the tower base remain relatively number of radial ground wires is constant for varying heights of antennas increased with a constant input power. This indicates • The reactance of an antenna varies that the power lost in the ground at distances only slightly with differences in the greater than 0.3 wavelengths will generally ground system be the same, regardless of the antenna • The efficiency of an antenna is height. increased with the number of ground On the other hand, close to the radial wires used, with 120 wires antenna, ground currents of electrically short being the optimum balance between antennas become quite large. Knowing that cost and efficiency the power lost in a conductor is directly • The optimum length of a ground proportional to the resistance of that system is 140 electrical degrees conductor, the importance of maintaining a • The presence of a ground screen in good array of ground conductors within a the vicinity of the antenna base quarter wavelength or so of a short antenna makes no difference in antenna is apparent. It may be surprising to you that resistance or efficiency the losses in a ground system for a 1/4 • A buried radial ground system wavelength tower can easily reach 3 dB or functions equally well as the same more if many radials are missing, cut or number and length of radials deteriorated. The shorter the tower, the more installed above the surface of the pronounced the ground system losses will ground. be. This is one reason that electrically longer antennas are more desirable than short ones. It was largely as a result of these

experiments that the standard non- 7.3 The “Standard” Ground System directional AM antenna ground system was Many years ago, experiments were defined as 120 radials, 90 electrical degrees conducted to determine the effects that the in length, composed of #10 soft-drawn bare number, size and length of radial ground copper wire buried 6" to 8" below the wires had on the antenna resistance and field surface. With performance being roughly strength at a distance (efficiency) of AM equal between surface and subterranean antennas of varying heights. Some systems, it is more desirable to bury the interesting conclusions were reached as a ground system in order to protect it from AM Antenna Systems Alexander/10

damage. element arrays often flow in a spiral fashion The conclusion regarding the toward the individual tower bases. In the ineffectiveness of the copper screen early 1990s, a complex rooftop ground notwithstanding, a copper screen 25' to 50' system for a five-tower directional array was square (or 120 interspersed 50' radials) in constructed in grid rather than radial the vicinity of the tower base is also part of fashion. The resulting efficiency and the standard ground system. The purpose of stability of this unusual and unconventional the screen or additional short radials is to ground system proved to be quite stabilize the resistance of the antenna and satisfactory. the capacitance across the base insulator with changing ground conditions due to 7.5 Lightning Protection weather, moisture content and the like. A buried copper radial ground Experience has shown that a screen or system, while very effective as part of a additional radials close in are very effective vertical antenna system, is seldom at stabilizing an antenna’s impedance. satisfactory as a mechanism to dissipate the For directional antennas with energy from lightning that strikes a tower. multiple towers, the same basic ground Individual radials can be burned in two at system elements are used, except that where the point of connection to the tower base radials from different towers intersect, they strap when large lightning currents flow are terminated into and bonded to a through them. A separate, large conductor (0 transverse copper strap. For example, in a AWG or larger) wire is needed to discharge two-tower array with 1/4 wavelength strike currents. This conductor should be spacing, a transverse copper strap would be connected to an array of at least four eight- installed halfway between the two towers foot copperweld ground rods near the tower and all the radials that would intersect the base pier. other tower’s radials would terminate onto Insulated-base towers need some the transverse strap. method of discharging static across the base insulator, such as a static drain choke that 7.4 Current Flow has a high impedance at the RF operating As was mentioned earlier, in a frequency but a very low impedance to DC. single-element AM (non-directional) Towers that use sample loops at tower antenna, ground currents return to the tower potential must use sample line iso-coils to base radially. In the case of multi-element isolate the loops from the sample line at the directional arrays, displacement currents operating RF frequency, and such iso-coils will arrive at every point on the surface of serve well as static drain chokes. the ground from each element of the array, Static drain chokes, while serving to and all these currents components will all keep dangerous static potentials from have different phases. The current flow in building on an insulated tower, are not at all such a multi-element array will not, then, be effective at dissipating lightning strike entirely radial in nature. currents. In fact, they present a very high It has been shown in recent impedance to the ultra-fast rise times of a experiments that ground currents in multi- typical lightning current. A spark gap of AM Antenna Systems Alexander/11

some sort is needed directly across the tower is cut and the wire end is pushed back into base insulator to provide a path for such the furrow. lightning currents. Gaps of this sort most In this manner, the actual installation often consist of two horizontally separated of a radial ground system can proceed quite galvanized steel balls, with an air gap rapidly. A typical crew can install a full 120- between them, located just below the base radial system for a single tower in about one plate of the tower. “Horn” type gaps are day, assuming a clean site and that all the used in other installations, and from time to surveying/marking of the radial endpoints time, a “needle” gap is used. Both the ball has been done. and horn type gaps are of a design where It is important for the engineer in any arc across the elements is self- charge of the construction and maintenance extinguishing. As the sustained arc climbs of a station to observe the installation of the farther and farther out on the gap, the ground system. As we have previously spacing gradually increases until a point is established, the performance of an AM reached where the voltage across the gap is antenna system is directly related to the less than the breakdown voltage of the air condition of the ground system. Mistakes between the points where the arc is made in the installation process will be occurring. When that point is reached, the around long after the installation crew is arc is extinguished. gone and the performance of the station will Gap spacing is usually set by trial suffer as a result. and error, but a starting point can be The selection of a ground system calculated. The installation, maintenance installation crew should be made carefully and adjustment of spark gaps will be dealt and not necessarily be based upon the lowest with in a later chapter. bid. Choose a reputable contractor with good (and recent) references. Carefully review the amount of materials needed and 7.6 Ground System Installation keep track of the material as it is used in the Ground radials are usually installed installation. Copper wire, screen and strap by use of a specially adapted plow. Such are costly and can disappear quickly from a implements are equipped with a spindle for site. Spot check individual radials by the spool of wire and a tube or conduit that digging into the furrow and locating the conveys the wire from the spool to a point wire. More than once, such spot checks have beneath the surface just behind the plow revealed an empty furrow. blade. The end of each radial is secured to At the tower base, the ends of the the copper strap or ring of copper tubing at ground radials are terminated into a copper the tower base and then the plow blade is strap or piece of heavy copper tubing. One lowered into the ground at a point ten feet or method is to build a square frame around the less from the tower base. It is then pulled tower base pier out of wooden two-by-fours, radially away from the tower to the desired then to lay a length of four-inch copper strap radial endpoint previously established by over the wood. A galvanized roofing nail is survey and marked with a stake. When that then partially driven into the wood through point is reached, the plow is raised, the wire the copper strap for each radial, and the end AM Antenna Systems Alexander/12

of the radial is wrapped around the nail. Many installations use a copper strap Once all of the radials are plowed in, the from every tower to every other tower and radial ends are silver-soldered to the copper to the transmitter building ground system. strap and the nails are either removed or While there is nothing wrong with doing driven all the way in. The straps from the this, it is generally redundant and underside of the tower base insulator are unnecessary. The outer jacket of the also silver soldered to this strap. transmission line feeding each tower serves It is important to insure that silver well as a ground strap between the tower solder, not tin/lead solder, is used where and transmitter building or phasor. There is making bonds in a ground system. It will no real reason to connect the tower bases quickly deteriorate if buried, and the together with a strap, as the ground currents mechanical strength of tin/lead solder joints are carried adequately by the radials and the is inferior to that of silver soldered joints. ground itself. Another method calls for a piece of one-inch diameter copper tubing shaped into 7.61 Special Circumstances a circle around the tower base pier with the What happens when a special two ends silver soldered together. The circumstance exists at a site that does not individual radial ends are wrapped one time permit a “conventional” ground system to be each around the tubing and silver soldered installed? Where there is a will, there’s a into place. way, and there are just about as many If the installation uses short, variations on the basic ground system interspersed radials close in, the ends of scheme as there are engineers. Some of these radials are attached to the strap or these variations are quite creative. tubing in the same manner. Where a copper Many AM transmitter sites are screen is used, the individual pieces are laid located in flood planes or flood prone areas. out on top of the ground, cut to the proper In these cases, the transmitter building, size and shape and silver soldered together tuning houses and tower base insulators are and to the strap or tubing. Where a screen is often elevated so that the highest predicted used, it is best to cover it with large gravel level of flood water will not reach the to a depth of at least two inches to protect buildings or base insulators. In these and secure it. Pea gravel can be used, but a circumstances, the buried radial ground typical copperweld screen is likely to system is still used, but the ground screen or eventually work itself up through small interspersed short radials are mounted on a gravel and become exposed in places. counterpoise at base insulator level. A Typically, large gravel is less expensive frame of some sort is usually constructed in than pea gravel, so it is a better choice for a hexagon or octagon shape with a 20 - 25 more than one reason. foot radius, and the screen or short radials Where radials intersect a transverse are installed from a strap or tubing ring strap, they should be cut to length and below the base insulator to the edges of the bonded to the strap by silver soldering. counterpoise. The outer edges of the Strap intersections should be secured counterpoise are lined with copper strap, and mechanically and then silver soldered. the edges of the screen or the ends of the AM Antenna Systems Alexander/13

short radials are silver soldered to this strap. that go into the creation of a successful A copper strap is then run down each cathodic protection system are well outside support leg of the counterpoise to ground the realm of broadcast engineering. level, where it is bonded to the buried In other installations, it may be ground system, which has been installed in a necessary to extend a ground system across normal manner below the tower base pier or a creek, river or canal. Unless the waterway support pylon. A set of ground straps can be lined and capped, it is usually not (usually three or four) is run from the inner advisable to string ground radials across the ring of the counterpoise down the tower top. Plowing radials in through the bed of support pylon to join the inner ring of the the waterway is likewise inadvisable. buried ground system. In the same manner, a Flooding, dredging, erosion and the like are number of large gauge wires (0 AWG or prone to displace the radials and damage or larger) should be run from the bottom side destroy that part of the ground system. of the spark gap to the ground rod array A very effective way to cross a small around the base pier or pylon for lightning waterway is to install a strap, usually four protection. inches in width, along and parallel to either It is important to note that if the side of the waterway. All the radials that counterpoise support frame is constructed of intersect this strap should be trimmed to a conductive material, cathodic protection length and silver soldered to the strap in the must be used to protect it. Cathodic same manner as with a transverse strap. On protection is a method of inducing a the side of the waterway opposite from the negative DC current into a metal object that tower(s), the radials start at the parallel strap is in contact with the ground or another and continue to their full length. Then, in at conductive object, thereby preventing least three evenly spaced locations, a large corrosion. In the case of a steel frame strap is connected across the waterway to counterpoise without cathodic protection, each parallel strap, This strap, which is at corrosion of the frame will eventually take least six inches wide, is then buried a safe place, both below the ground and at the depth beneath the waterway invert (usually points where the copper ground material at least three feet). contacts the steel of the frame. If cathodic protection is used, it may be worthwhile to 7.7 Deterioration and Damage use insulated rather than bare copper wire in The ground system is often blamed the buried portion of the ground system. for signal problems. Since it is out of sight Were bare wire to be used, a good part of and somewhat difficult to observe in place, the cathodic protection current would flow it is easy for station personnel to assume that in the copper ground radials and not in the there must be something wrong with it when steel where it is needed, thus reducing the coverage is not what it should be. effectiveness of the cathodic protection. The truth of the matter is that a Should you find that the situation properly installed and buried ground system may call for cathodic protection, it is wise to will last many, many years. Except in the retain an engineering firm that specializes in most extreme circumstances, a buried such systems. The variables and calculations ground system will not deteriorate AM Antenna Systems Alexander/14

appreciably in place. Stations that have been careful inspection of the antenna site in place for fifty or more years have property should be made. The condition and operated with the original ground system integrity of the ground system should be without difficulty. Spot checks of radials checked by use of a field strength meter or have revealed them to be in excellent metal detector if damage is suspected. condition. It is a good idea to periodically The biggest dangers to buried ground inspect the ends of the radials where they systems are damage from construction or connect to the base strap or ring to be acts of God and vandalism/theft. A certain that they have not been burned open construction company installing a pipeline, by lightning. Periodically check the sewer, or underground cable across an AM lightning ground wires and connections as station’s site property can cut many radials well, particularly at the beginning and end and cause significant damage. When the of the thunderstorm season. ditch is filled back in, it may well become Vandalism and theft are perhaps the impossible to locate the cut ends of the most common dangers to a buried ground radials and the more economic fix may be to system. Copper is valuable, and a plow in new radials in those directions. It determined thief can rip an entire ground may be impossible to avoid having the system out in a matter of hours. He may not construction work take place across the get more than $500 from the scrap value of property, but with careful planning and the copper, but the cost to replace the system supervision, the cut radials can be spliced can exceed $50,000. before the ditch is filled in. The result will Perhaps the best way to protect a be no reduction in effectiveness of the ground system from thieves and vandals is system. to keep it properly buried. “Out of sight, out Acts of God present a more difficult of mind” is a maxim to remember in picture. Erosion, floods, earthquakes and the protecting your ground system. If thieves do like are usually beyond our ability to predict not know the buried copper is there, they accurately, and they can leave a ground will be unlikely to look underground for it. system uncovered and exposed. An ounce of Good overall site fencing is important, and prevention is the best medicine in cases nothing can substitute for good relations where such calamities are likely. Radials can with site neighbors. Keep that in mind when be plowed in deeper, for example, to prevent the farmer next door complains about station them from becoming unearthed in a flood or audio in his telephone. Fix or replace his from erosion. At some antenna sites where phone and enlist him to help you keep an erosion is a constant problem, a tractor with eye on the site. several implements is kept at the site and used to keep the ground system covered. 7.8 Shared Site Use Constant vigilance is necessary in such More and more, AM antenna site cases. Even a short length of radial that property is becoming too valuable to use as becomes exposed can easily be broken or only an AM antenna site. Open real estate is further unearthed by wildlife or livestock. scarce in many locations, and the big, open Following a flood or earthquake, a field with the tower on it is often very AM Antenna Systems Alexander/15

attractive to developers. While an in-depth the ground system is important, however, discourse on shared AM site use is beyond and the use of interconnected perimeter the scope of this discussion, the treatment of straps serves this purpose nicely. ground systems in such situations does bear looking at. 8.0 Directional Antenna Systems One common shared site use calls for In an ideal world, all AM antennas placement of a parking area or roadway would be non-directional (circular radiation across an AM station’s site property. There pattern). Transmitter site locations would be is nothing wrong with paving over a ground selected so that all the population to be system. In fact, this can serve to protect the served would be covered with a strong buried radials from damage and theft or signal with a minimum waste of signal into vandalism. The important things to unpopulated areas. Population centers would remember are to work carefully with the be evenly spaced, with medium and large contractor to insure that the radial wires are cities being a good distance apart. In this not broken during construction, and have a ideal world, the number of stations would be surveyor make an accurate set of as-built low enough that all the stations could live drawings. It is easy to break a radial during together on the band without causing the paving process, and to prevent this from interference to one another. happening, it is a good idea to place some In the world we live in, though, sand around each radial to give it a cushion . things just aren’t that way. Transmitter site The as-built drawings will allow you or locations are selected as a compromise others to accurately locate buried radials between cost (land is much cheaper away during future excavations. from the city), location and permissible use. Buildings can be placed on top of a As Mr. Murphy usually has it, seldom is a station’s ground system, but this may not be site available where it will work the best for a good idea if RF power densities are high the station. Population centers are often or the antenna system is a directional array. clumped together, as on the east coast, and If a building must be placed on the ground the number of stations is very high, over system, depending upon the size of the 4800 at last count. With only 107 channels building, it may be best to treat the building on the regular AM band, we all know the in the same manner as you would a canal or band is way overcrowded. Interference waterway. Run a copper strap around the between stations is common. All this trouble outside of the building’s foundation, started when the second AM station signed terminating intersecting radials onto the on the air! strap. Several straps can then be run under To help overcome some of these the building’s foundation, connecting problems, when the broadcasting industry between the straps along the building sides. was in its infancy, directional antennas were Very little in the way of displacement introduced. The very first directional currents will enter the ground through the antennas were used to direct the signal into building, so having radials in place beneath areas where coverage was desirable. Later, the building is not important. Providing a directional antennas were used both for radial path for currents already flowing in directing coverage and for protecting co- AM Antenna Systems Alexander/16

and adjacent-channel stations from and interference protection. interference. As the band became more and The problem gets even more more crowded, directional antennas were complex with respect to nighttime used to “shoehorn” new stations in wherever allocations. During the day, in most there was a hole. The result... well, the circumstances only the groundwave signal current AM band with all its overcrowding must be considered. At night, however, the and interference problems is the result. The skywave signals become a factor. Not only FCC terms this condition as a “mature” must radiation be limited in the direction of band. protected stations, but the vertical angle or Consider that a 20:1 groundwave theta (the angle above the horizon that will protection ratio (26 dB) is required between result in a reflection of the signal off the co-channel stations, and 6 dB is required so that it arrives in the protected between first adjacent channel stations. In station’s vicinity) must be considered as the middle of the AM band with average well. Limitations on nighttime radiation values of conductivity, the 0.5 mV/m (still called MPR) are almost always contour of a typical station transmitting with expressed in mV/m at 1 kilometer at a 1 kW will lie at a distance of about 45 miles specific value or range of values of theta. from the antenna site. The same or a similar For example, the MPR from a particular station’s 0.025 mV/m contour (1/20 of 0.5 nighttime facility toward another might be mV/m) will lie at a distance of about 140 35 mV/m at 1 km at a theta range of 16 to 24 miles from the antenna site. Assuming two degrees. That would indicate that the typical 1 kW co-channel stations both have radiation on that azimuth could not exceed non-directional radiation patterns, they 35 mV/m at 1 km for any vertical angle cannot, then, be located closer than 185 between 16 and 24 degrees. This is where miles to one another and still maintain the the function of theta of the antenna, required groundwave protection to one discussed in Part 2 of this series, comes into another. If one of these stations uses a play. directional antenna to reduce its radiation From all this, you can see that toward the other station, the stations can be determining what a directional pattern must located much closer together. The maximum look like is a complex affair. Seldom is this radiation permissible from one station process as simple as protecting a single toward another is expressed in mV/m at 1 station. Allocation pictures often look like kilometer at the appropriate azimuth, and jigsaw puzzles, and with different engineers call this value the MPR conductivities Having an effect along the (maximum permissible radiation). various radials, coming up with a pattern to Seldom is a groundwave allocation fit the situation can be very difficult. problem so simple as one station protecting just one other co-channel station — usually 8.1 A Simple Two-Tower Pattern there are many stations that are entitled to In a directional antenna, the radiation protection. By using a directional antenna pattern is created by controlling the and carefully selecting the operating power, amplitude and phase of the RF current in a balance can be achieved between coverage each element of the array. The resulting AM Antenna Systems Alexander/17

field at any point is the vector sum of the from the northern tower. This results in individual element radiation components. complete cancellation of the fields from the In locations where the fields from two elements. the various elements are in phase with one Now let’s consider the same array another, the fields add; where they are out of but move our observation point to a location phase, they subtract. In most locations, the due south of the array. At this observation fields are not perfectly in or out of phase, point, equal fields will arrive from each of and in the case of arrays the two elements, with more than two but because the elements, there is a southern element is combination of addition 90 degrees closer, and cancellation at most its field will arrive points. 90 degrees ahead of Let’s consider a the field from the simple two-tower northern tower (90 directional array. Let’s degree leading assume our array’s space phasing). elements are spaced 90 Since the current degrees apart on a north- Figure 1 fed to the southern south bearing, and that the elements are element is delayed by 90 degrees, that driven with equal currents (1.0 ratio) with cancels the 90 degree leading space phasing, the phase of the current to the southern and the resultant field from the southern tower lagging by 90 degrees. If we stand at element arrives in phase with the field from an observation point some distance from the the northern element. The fields completely array on a bearing of zero degrees True (due add, so the resultant field at the observation north), equal fields will arrive at the point is equal to F1 + F2. observation point from each point. Because At other points around the array, the elements are spaced 90 degrees apart, neither complete addition nor complete the field arriving from the southern element cancellation will occur, and vector addition will arrive 90 degrees later than the field of the arriving fields is used to determine the from the northern element (90 degree resultant field. Figure 1 shows the resulting lagging space phasing). Add to that the 90 directional pattern of our two tower array. degree phase delay in the southern element The theoretical parameters for such current and the field arriving at our an array would normally be listed as observation point from the southern element follows: is 180 degrees out of phase with the field

Tower Ratio Phase Spacing Orient Height 1 1.000 0.0 0.0 0.0 90.0 2 1.000 -90.0 90.0 180.0 90.0

AM Antenna Systems Alexander/18

If the phase of the two elements is west side of the array. reversed (i.e. the phase to the southern If we move to an observation point element advanced 90 degrees rather than north of the array, equal fields will arrive retarded), the pattern will be reversed, with from the two elements, but the field from the complete cancellation to the south and southern element will arrive 180 degrees addition to the north. behind that of the field from the northern Now let’s consider the same simple element. Because the currents in the two two-tower array, but elements has the same let’s increase the spacing phase, the fields to 180 degrees and feed arriving at the the two elements in observation point will phase with one another be completely out of (0 degree phase shift).At phase and will thus an observation point due cancel. The same is east of the array, the true at an observation equal fields from the two point due south of the elements will arrive at array. Figure 2 shows the same time, since the the resulting observation point is Figure 2 directional pattern of equidistant from both elements. Since the this two-tower array. phase of the currents in the two elements is The theoretical parameters for such the same, the fields from the two elements an array would normally be listed as arrive in phase and thus completely add. The follows: same is true at an observation point on the

Tower Ratio Phase Spacing Orient Height 1 1.000 0.0 0.0 0.0 90.0 2 1.000 0.0 180.0 180.0 90.0

By altering the elements (current phase plus currents in the two elements space phase) will vary, and making them unequal, producing patterns of there will be neither complete different shapes. The same is cancellation no complete true of changing the spacing addition at any point around of the elements. the array. The result is that The design engineer nulls are somewhat filled and can vary all these parameters lobes are not as large. — current, phase and spacing By altering the phase — in addition to moving the of the current in the elements, bearing of the line of towers the resulting instantaneous to achieve the desired pattern. phase of the fields from the If more nulls or broader nulls Figure 3 AM Antenna Systems Alexander/19

are needed, additional elements of a distant station, where are added to the array. For each radiation toward that station additional element, another pair needs to be suppressed without of nulls is created. By placing necessarily placing a null in that nulls close together, a broad arc direction. where the radiation is We can combine these suppressed can be created. The two patterns — multiply them, possibilities are limitless. if you will — to achieve a four- tower parallelogram array that 8.3 Pattern Multiplication produces a pattern with nulls at In a more complex 280, 345, 350 and 105 degrees example, if we start with a (see Figure 5). The nulls at 345 simple two-tower array with a and 350 degrees combine to 315-degree tower line, 90- produce, in effect, a broad degree spacing and 106 degree Figure 4 single null, which is useful in phasing, this will result in nulls protecting a large contour or a at 280 and 350 degrees or 35 national border. degrees either side of the tower We multiply patterns line (see Figure 3). through the use of vector Now let’s suppose we arithmetic. Since we already need additional nulls at 345 and know what the parameters for 105 degrees. A two-tower both of the two-tower patterns pattern with a 45-degree tower are, we have the parameters for line, 180-degree spacing and towers 1, 2 and 3. All that 90-degree phasing will produce remains is to find tower 4's the pattern shown in Figure 4. parameters. As shown in Figure This pattern was 5, simply multiply the field selected to show that with a ratios of towers 2 and 3 and wide-spaced array, maximum add the phases of towers 2 and Figure 5 radiation does not occur on the 3 to find the parameters for azimuth of the tower line. This type of tower 4. The parameters for this pattern is often useful in daytime protection parallelogram array would then be:

Tower Ratio Phase Spacing Orient 1 1.000 0.0 0 0 2 1.000 106.0 90 315 3 1.000 90.0 180 45 4 1.000 196.0 201 18

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8.4 Pattern Size While scientific principles govern this art, So far, for simplicity of illustration, experience through trial and error is what all the patterns shown have equal radiated makes an engineer proficient at pattern fields, or field ratios of 1. We can fill the design. There are many tricks to the trade nulls of our patterns by making the currents that simplify the pattern design process. A unequal. As we fill the nulls, we also reduce novice designer, for example, may take the size of the pattern major lobe. The many times longer to produce a desired amount of energy in a pattern remains the pattern than an experienced designer using same, regardless of the depth of the nulls or the basic principles we have discussed here. the size of the lobes. The size of a pattern is Computer directional antenna design found by integrating the hemispherical programs are a big help and time saver, but energy flow (the power radiated on and it is entirely possible (and even easy) to above the horizon in all directions). For a design a terrible pattern with one. The given power input and loss, this will remain computer cannot substitute for knowledge of the same, regardless of pattern shape. We the craft. can liken this to a balloon filled with air. For those building and maintaining You can squeeze it in the middle, but the directional arrays, it helps to understand the ends bulge out. As you reduce pressure in basic principles of array design. Faced with the middle, the ends return to their normal the adjustment of an array, without a good size. Squeeze both ends and the middle understanding of the designer’s intentions, bulges out. Still, no matter where or how the adjusting engineer may well find himself hard you squeeze, the total size of the in an iterative trial-and-error situation that balloon does not change. may never produce the proper pattern. A As a pattern design progresses, it is good understanding of vector arithmetic and important to keep an eye on what is going antenna design principles will speed the on above the horizon. It is easy, particularly process by allowing the engineer charged with more complex patterns, to wind up with adjusting the array to make educated with a good bit of energy being radiated decisions as the tuneup proceeds. In the next above the horizon and into space. This part of this series, we will examine in detail power is, for most intents and purposes, lost, the use of vectors in DA adjustment. and in the case of most nighttime antennas, it is harmful as it causes skywave 8.5 Real World Parameters interference to other stations. Because the Anyone who has ever looked at the total hemispherical energy in a pattern never FCC license for an AM directional station changes, by keeping an eye on the size of has probably seen that there are two sets of the horizontal pattern, the designer can directional antenna parameters listed. One generally tell if he has a vertical radiation set is theoretical; the other is operating problem. If the horizontal pattern size or parameters. There are usually significant RMS begins to shrink, that power is going differences between them. somewhere, and that somewhere is up. The theoretical parameters indicate Directional pattern design is a the radiated field ratios and the phases of the complex art that takes years to learn well. radiated fields. If the towers are of equal AM Antenna Systems Alexander/21

heights, the theoretical loop current ratios radiation, and the current induced by mutual will be equal to the radiated field ratios; if coupling from other elements. The current the tower heights are different, they will be that contributes to radiation tends to be different. sinusoidal, but the effect of the induced Assuming sinusoidal current flow, a current tends to distort that sinusoidal current maximum or loop will occur 90 current distribution. The position of the electrical degrees below the top of a vertical current loop can be quite a distance from radiator. If that radiator is less than 90 where sinusoidal current distribution would electrical degrees tall, this loop will exist at place it. the tower base. If the tower heights are the There are many methods of same, the theoretical parameters which are determining the correct operating shown on the station license are those the parameters for a directional array, but all, to designer used to mathematically create the one degree or another, rely on trial-and-error pattern and they reference this current loop. adjustments and field measurements. When the towers are of unequal height, Knowledge of the design, experience, and these ratios reference the radiated fields. computer modeling can all help to make The operating parameters, on the each trial an educated trial (rather than a other hand, indicate the values shown on the random guess) and shorten the tuneup antenna monitor when the array is properly process. adjusted to produce the correct pattern. The operating parameters often deviate 8.6 Standard Patterns significantly from the theoretical In all the examples we have used so parameters, and this is due, in large part, far in this series, on certain azimuths, the sample system errors and to the mutual vectors from the array elements completely coupling between towers. cancel one another and the resultant No matter how careful the installer radiation on that azimuth is zero. This is is, there will be errors in the sample system. difficult to achieve in the real world. It is difficult to make all the runs of sample Reradiation, scatter and drift in the phasing, line exactly the same length, and each coupling and sampling systems limit how individual run will have a slightly different close to zero the radiation on a null radial . Differences in the can be adjusted and maintained. In January values of antenna monitor terminating of 1981, the FCC instituted the standard resistors, differences in the sample loops or pattern, which increases the size of the current transformers, even slight differences theoretical pattern (or calculated pattern) in the locations of the sample pickups all by a specific amount. In another part of this contribute to sample system error. series, we will look at how the standard Mutual coupling, on the other hand, pattern is calculated. distorts the assumed sinusoidal When the standard pattern was characteristics of the current flow on each instituted, the FCC calculated the standard tower. There are two on-frequency current pattern for all existing stations and components in each element of a directional authorized them by modification of each array — the current that contributes to station’s license. Today, when the FCC AM Antenna Systems Alexander/22

authorizes a directional pattern for a station, the voltage vector reference it is the standard (and not the theoretical) axis.) pattern that is authorized. The FCC proscribes radiation that exceeds the Si cos(Φi - Φ) is the space phasing standard pattern value on any azimuth. The portion of βi due to the location of th designer must use the standard pattern as he the i tower and Wi is the electrical designs a pattern to fit a particular phasing portion of βi.) application as the FCC requires use of the standard pattern in all calculations of Where: Si = electrical length of spacing of interference and coverage. the ith tower in the horizontal plane from 8.7 The Directional Antenna Formula the space reference point. The equation to calculate the pattern shape in the horizontal plane for a Φi = true horizontal azimuth directional array of n towers is: orientation of the ith tower with respect to the space reference i =n axis.

E =Σ Ei fi (θ ) βi Φ = true horizontal azimuth angle of i =1 the direction to the reference point (P) measured clockwise Where: E = total effective field strength from true north. vector at unit distance (P) for the with respect to the Wi = time phasing portion of βi due voltage vector reference axis. to

th the electrical phase angle of the i = i tower in the directional radiated field of the ith tower antenna taken with respect to the voltage array. vector reference axis.

n = total number of towers in the This equation, when applied to a array. directional array, will yield a complex number that represents the field strength and Ei = magnitude of the field strength phase of the signal arriving at a particular at unit distance in the horizontal th observation point from each element in the plane produced by the i tower array. This is no more than we were doing in acting alone. our heads in Part 5 of this series with simple two-tower arrays. With more complex βi = Si cos(Φi - Φ) + Wi (phase arrays, things get a bit harder, but the basic relation of the field strength at principle is the same. While this formula the observation point (P) for the th may not readily lend itself to working out i tower taken with respect to with pencil and paper, computer types will AM Antenna Systems Alexander/23

find that it is easily written into computer code. Φ = true horizontal azimuth angle for 8.8 Envision the Vectors which the vector is being To see the vectors for a particular calculated. azimuth, all we really need to know are the theoretical parameters and the following Φi = true horizontal azimuth formula: orientation of tower i from the reference tower.

βi =θi +[Si cos(Φ −Φi )] If you look closely, you will see that this is actually the rotational portion of the Where: βi = phase relation of the field from tower i on the specified azimuth. other formula. At first glance, it looks a lot easier, but what are all those variables? Let’s look at a typical set of directional θi = phase of tower i with respect to the reference tower. antenna theoretical parameters and see if things start to make sense. Look at Table 1. Si = electrical length spacing of tower i from the reference tower.

Tower Field Phase Spacing Orientation

1 1.000 0 0 0

2 1.000 90 90 0

These are the theoretical parameters Si - This is the spacing of the tower, of a typical two-tower in electrical degrees, directional array, similar from the reference tower. to other examples we In this case, the spacing have used in this series. of the reference tower Let’s plug these with respect to itself is parameters into our zero, and the spacing of vector formula and see tower #2 with respect to what happens. the reference tower is θi - This is the 90°. phase of the tower with Figure 6 Φi - This is the respect to the reference tower. In this case, orientation of the tower, in electrical the phase of the reference tower with respect degrees, from the reference tower. In this to itself is zero, and the phase of tower #2 case, the orientation of the reference tower with respect to the reference tower is 90°. with respect to itself is zero, and the AM Antenna Systems Alexander/24

orientation of tower #2 with respect to the The conventional way to plot vectors has reference tower is 0°, or true north. zero azimuth at 3 o’clock and angles plotted Φ - This is the azimuth from the counterclockwise. For the sake of simplicity center of the array to the observation point. and those that have been taught that north is Another way to express this is as the always at the top of the page, the examples azimuth on which we wish to compute the here will place zero degrees at 12 o’clock vectors. and angles will be plotted clockwise. If you Let’s start with Φ = 0° true. When wish to do it the conventional way, do so. It we plug in all the parameters for tower 1 makes no difference in the length of the (the reference tower), we come up with a resultant which way you do it. value of 0°. This is the same as the phase of Starting near the center of the page, tower 1, and this will always be true of the make a dot to represent the starting point. reference tower. If the phase of the reference Using the protractor and ruler, draw a line tower had been other than 0°, the formula 1.0 inches long at an angle of 0°, or straight would have yielded up on the page. This whatever the phase of represents tower #1's the reference tower vector. Now, move the was. In other words, it protractor to the end of is not necessary to run this line (the north end) the reference tower and draw the vector for through the formula — tower #2. This line, 1.0 only the other towers in inches long at an angle the array. Just know the of 180° (or straight field and phase of the down), will lie back reference tower. The over and completely complex number cover the tower #1 representing the vector vector, with the end for tower #1 at an falling back at the azimuth of 0° true, starting point. The then, is 1.000∠0°. Figure 7 resultant vector is the Now, let’s run the parameters for distance and angle tower #2 through the formula. When I did between the starting point and the end of the this, I came up with a phase of 180°, so the tower #2 vector. Since these points coincide vector for tower #2 at an azimuth of 0° true in this case, the resultant vector is 0∠0°. is 1.000∠180°. This vector diagram for an azimuth of 0° With these numbers in hand, let’s true, shown in Figure 6, shows graphically plot them on a piece of paper. You’ll need how at that azimuth the signals from the two nothing more than a ruler and protractor. towers completely cancel one another. AM Antenna Systems Alexander/25

Now let’s try the same thing at an What if you have more than two azimuth of 180° true. Remember that the towers? It’s simple. Calculate the vector for vector for the reference tower is always the each tower and draw it onto the end of the same, in this case 1.000∠0°. Plugging tower preceding vector. The resultant vector will #2's parameters and a 180° azimuth into our still be from the starting point to the end of formula, I get a vector the last vector. When of 1.000∠0°. When I dealing with arrays draw the two vectors with more than two on paper, it is easy to towers, it is important see that the vector from to label the vectors as tower #2 completely you draw them. If you adds to the vector from dn’t, it is easy to lose tower #1, yielding a track of which vector resultant vector of belongs to which 2.000∠0° (see Figure tower. 7). Three-tower We’ve seen the vectors are fairly easy extreme cases of to draw, but when four or more towers are complete cancellation Figure 8 and complete addition. involved, it becomes Now let’s try something in between. Plug in more difficult. It should be understood that an azimuth of 90° true and tower #2's the angle of the resultant vector is not parameters and you should come up with a important. Only the length is important. As tower #2 vector for 90° true of 1.000∠90°. you experiment and plot different vectors, Draw tower #1's vector of 1.0" at an angle of don’t get hung up on the resultant angle. The field strength meter does not care what the 0°, then draw tower #2's 1", 90° vector onto incident phase of the resultant field strength the end of that. If you measure the resultant vector is. vector, you’ll find it is 1.414" long at an If you wish to plot the directional angle of 45° (see Figure 8). You can see pattern of the array on a piece of polar graph from this graphical representation that the paper, simply plot a dot representing the signals from towers 1 and 2 partially add to scaled length of the resultant vector on the produce a field that is greater than the field azimuth at which the vector was calculated. of either one, but less than the sum of the In the case of the 180-degree azimuth in the two combined. If you run the same problem example above, you would plot a point at for an azimuth of 270° true, you’ll find you distance from the center of 2 times the scale come up with the same vector. Go ahead and value at an azimuth of 180 degrees. For the try this at other in-between azimuths. You’ll 90-degree radial you would plot a point at a find that when you get closer to 0°, you get distance from the center of 1.414 times the more cancellation of the two signals; when scale value at an azimuth of 90 degrees. you get closer to 180°, you get more Calculating the length of the resultant and addition. plotting it at five-degree increments all the AM Antenna Systems Alexander/26

way around, you can then smoothly join the lines commonly used today to feed power points together in a graphical representation from the transmitter to the antenna system. of the pattern shape. One of these is the open-wire feeder, of which there are several variations. 8.81 Using the Vectors With the vectors in hand, we have at 9.1 Open-Wire Feeders our disposal a method of graphically Transmission lines or feeders created depicting what is happening on any given from open (unshielded) wires supported on azimuth. This is particularly helpful when glass or ceramic insulators are commonly setting up an array or working to correct a referred to as open-wire feeders. Amateur problem. By plotting the vectors on a given operators often use ladder line, which radial, you can see how the different towers consists of parallel conductors separated by interact to create the resultant field on that Plexiglass rods, to provide a balanced feed radial. Many monitor points are on null for high-frequency (HF) antennas. Twin- radials, where some degree of cancellation lead, which used to be the feedline of choice occurs. By plotting the vectors, you can for television receiving antennas, is another visualize how towers pair-up on the radial. variation of the balanced open-wire feeder In parallelogram arrays, pairs of towers theme. Neither of these has any practical use often cancel other pairs of towers. Using in broadcasting because of the relatively low vectors, you can get a mind picture of the power handling capability, but there are best way to go about adjusting such an some other types that were commonly used array. in radio’s early days. Three and six-wire unbalanced systems are the most common of 9.0 Transmission Lines these, and some vestiges of these systems Just as the electrician must choose remain today. No new systems have been the correct size and type wire for a particular constructed using open-wire feeders in many wiring job, and the plumber must select the years. appropriate pipe, as radio engineers we must Three-wire systems, where two choose the right size transmission line. If the ground conductors supported on telephone wrong line is chosen, the results can be with pole cross-arms bracket the center RF catastrophic. Proper transmission line conductor are one such type. This type of selection can make the difference between transmission line has a relatively high having a system that is reliable and capable impedance, can withstand high peak of continued operation under adverse voltages and is relatively easy to maintain. conditions, or having a borderline system Cracked and broken insulators as well as that is a time-bomb primed for failure. deteriorated or damaged support structures Sometimes the choice is clear and the are the biggest problems with this type of differences are obvious; other times the open-wire feeder. limitations are hidden. In this installment of The six-wire feeder consists of four our AM Antenna series, we'll look at feed relatively small ground conductors lines and explore these limitations. bracketing two parallel center RF There are three types of transmission conductors in a cubical fashion. In effect, AM Antenna Systems Alexander/27

this creates a of sorts. Six-wire Under the broad heading of coaxial cable are systems are typically supported on short several sub-categories. In broadcast creosote poles. The impedance is usually transmission applications, we primarily use lower than three-wire open feeders. semi-flexible cable, sometimes referred to Disadvantages are lower peak voltage by the trade names “Heliax®”or capacity, generally lower power handling “Flexwell®,” and rigid line, sometimes ability and greater mechanical complexity. referred to as "hard line." With so many more wires — usually 8- or 10-gauge soft-drawn copper — and six glass 9.21 Semi-Flexible Cable insulators on each rather complex support Semi-flexible cable is by far the structure, there are many points for potential most common transmission line in use in problems. Ice poses a particular hazard for AM stations. It is ideal for a wide range of this type of open-wire feeder. One low and medium power uses, and its cost is environmental problem that is often caused considerably lower than that of comparably by this type of feeder occurs when the rated rigid line. The term "semi-flexible" is spacing between ground and center used because the bending radius of the line conductors is sufficient to allow small birds is quite large. For a 1-5/8" line, the to perch on the center conductor wires. minimum bending radius is 20". Anyone When the birds take to flight, their wings who has wrestled line such as this into a can come into contact with the outer wires. tight space or through a conduit will attest The results of this inadvertent shunting of that “semi-flexible” is indeed an appropriate the transmission line are predictable. The moniker. feeder survives; the birds do not. Under the heading of semi-flexible Open-wire feeders were used in the cable are both foam- and air-dielectric lines. early days of broadcasting for a number of Foam lines are designed for applications that reasons. One was that they could be made do not require a pressurization path to the on-site out of readily available materials. antenna. Their average power handling Another was that they were capable of capability is lower, and loss is higher than withstanding large standing wave ratios. In the same size air-dielectric line, but their the days before the operating impedance peak power ratings are higher. This is bridge, driving point impedances in multi- primarily due to the higher losses of foam as element arrays were difficult or impossible a dielectric. Foam-dielectric cable is to measure with any accuracy. Transmission available in sizes up to 1-5/8". lines that could withstand the inevitable high Air-dielectric lines utilize a spiral VSWR were needed. polyethylene spacer to keep a constant spacing between the inner and outer 9.2 Coaxial Transmission Lines conductors. To maintain safe operation, this Coaxial cable is the de facto standard type of line must be kept under constant transmission line today and it has been for pressurization with dry air or nitrogen. quite some time. As its name implies, Usually, a dehydrator or nitrogen regulator coaxial cable is constructed using two is connected to the line at the transmitter concentric conductors on a single axis. building end of the run to provide AM Antenna Systems Alexander/28

pressurization. Weekly checks of the such an occasion. amount of pressure in the line should be part of the engineer's routine inspection. Leaks 9.3 Characteristics that develop are a sure sign of impending Several properties determine the trouble and should be investigated quickly. suitability of a given line for a particular Air is, by far, a superior dielectric to application. These properties are published foam. It does ionize more easily than foam, in charts and graphs provided by the however, and this results in the lower peak manufacturers of such lines. Every engineer power ratings. Because air does not heat up should keep several transmission line as foam does with power applied, air catalogs handy. In addition to line ratings, dielectric lines can handle higher average they contain all kinds of other useful power than foam lines of the same size. engineering information. Copper heating becomes the primary average power limiting factor in air 9.31 Impedance dielectric lines. Impedance is one of the most important properties of a transmission line. 9.22 Rigid Lines The value of a transmission line's impedance Rigid lines are available in sizes is determined by the size and spacing of the from 7/8" to 9-3/16". They have inherent inner and outer conductors as well as the low losses and high power handling dielectric constant of the dielectric material capability. Rigid lines are normally made in between the conductors. Most lines in use 20-foot lengths with flanges on each end. today are rated at 50Ω, although I have Inner conductors are made of high- measured the impedance of such lines to conductivity oxygen-free copper that are ±10% of the nominal rated value. 75Ω is a supported inside the outer conductor by peg common impedance value in older systems, or disk insulators with a low dielectric particularly AM systems. Occasionally, constant (usually ceramic or Teflon). transmission lines with characteristic If you have ever experienced a impedances of 51½Ω, 52Ω, 63Ω and even catastrophic failure of a transmission line, odder values can be found in older you will appreciate the repairability of rigid installations. For all practical purposes, line. Often, when a burnout occurs, it is unless you are replacing an older line and confined to a relatively small area. The there is some compelling reason to use an affected section(s) can be easily replaced odd-impedance line, 50Ω is likely to be the and the remainder of the line cleaned desired impedance of any line purchased thoroughly to remove soot particles. For a today. fraction of the cost of replacing the entire transmission line run, a station can often have its rigid line repaired and be back on 9.32 Power Rating the air in a very short period of time. Many The power handling capability of a stations using rigid lines keep a spare transmission line is absolutely critical to its section or two on hand along with spare proper selection and safe use. It is limited by connectors, flanges and hardware for just either the maximum peak power AM Antenna Systems Alexander/29

(determined by the electric field strength a number of other factors. In choosing a and dielectric constant) or the maximum transmission line, always allow for 2:1 average power (determined by the allowed VSWR. This will provide an adequate safety temperature rise of the inner conductor). margin in most cases. Using the manufacturer's supplied The formula for derating a ratings, you can tell at a glance what the transmission line for VSWR and modulation average and peak power capabilities of a is as follows: given line are. What takes more than a glance to determine is whether a given line P P = PK size is suitable for a given application. A D (1+ M )2 VSWR look at just about any manufacturer's power rating graph will also show that the peak and Where: P = Transmission line derated average power ratings tend to converge at D power lower frequencies so that at AM frequencies, where skin effect is minimal, they are the P = Transmission line rated peak same. PK power

9.33 Derating M = Modulation percentage as a Consider an AM station that needs to decimal replace the transmission line to one of its directional array elements. Let's assume that Using this formula, you can compute there is normally 10 kW of power flowing to that our 7/8" foam line, rated at 44 kW peak that particular element. According to the power, is only good for a little over 4 kW! manufacturer's published ratings, 7/8" foam Take my word for it; at a station here in dielectric line is capable of safely handling Dallas, this very scenario played out with 44 kW. That should be plenty of safety the result being a burned up, brand new margin, shouldn't it? Let's look closer. transmission line. If I had my way, If we're going to modulate the power manufacturers would overlay this formula in being fed to the DA element 100% positive, red on their power rating tables and graphs! the peak power will be equal to 40 kW. That's getting pretty close to our 44 kW 9.34 Attenuation rating. If we allow for 125% positive Attenuation is another important line modulation, our peak power is over 50 kW, characteristic. It is caused by a combination well above the peak power handling 2 of the I R losses of the copper and the capability of the transmission line. dielectric losses of the dielectric material. Another variable that we need to The losses in the dielectric material tend to allow for is VSWR. In any real world be directly proportional to the frequency. situation, even in the best matched system, Conductor losses are related to the there will be times when the VSWR on a dimensions, permeability and conductivity transmission line will be higher than 1.0:1. of the material and tend to vary with the This can be due to ice, changing ground square root of the frequency. conductivity, defective ATU components or While the rated attenuation of a AM Antenna Systems Alexander/30

transmission line is very significant at FM types of fitting are available. and TV frequencies, it is seldom significant When choosing the fitting to use on a at AM frequencies. Line losses are so low in transmission line, the connection provided a typical AM system that they can be by the equipment manufacturer should be ignored altogether, provided that the considered. Obviously, the two have to transmission lines are otherwise adequately match. In high altitude situations, the end rated. terminal adaptor may be a better choice than the EIA flange because of the larger spacing 9.4 Fittings between the stud at the end of the terminal Many types of fittings and and the body of the connector. EIA flanged terminations are available to allow us to connections are prone to arc over in high make the transition between the coaxial power situations at high altitude, where the transmission line and RF sources and air ionized more easily. terminations. The end terminal adaptor is in common use at AM frequencies. This device 10.0 RF Ammeters creates an airtight seal to the transmission To maintain proper operating line and provides a brass stud connection for parameters in an antenna system, it is the center conductor. The body of the fitting necessary to have a means of accurately is equipped with threads and a nut for the measuring the antenna current. We have ground connection. On air-dielectric lines, a several tools available to us to measure this gas port is provided on the fitting to parameter, and in this part of our AM facilitate pressurization or purging. By far, antennas series, we will examine the this is one of the easiest means of different types and look at their advantages connecting the tubing in a phasor or antenna and limitations. tuning unit to the transmission line. LC-type connectors were once 10.1 The Thermocouple RF Ammeter popular at lower power levels for lines up to The most popular RF current 1" in diameter. This is a coaxial screw-on measuring device has long been the fitting very similar to the PL-259 connectors thermocouple ammeter. This device consists used on RG-8 cable. of a DC meter movement connected to a EIA flange connections are very thermocouple through which RF current popular means of line termination. The outer flows. When the thermocouple is heated, a conductor of the flange bolts onto the proportional current flows through the meter chassis of the equipment, and the center winding, causing deflection of the meter. conductor is attached with a “bullet”, which Thermocouple RF ammeters have is a spring-loaded expansion connector that the advantage of simplicity. They are fits inside the center conductor. EIA flange typically manufactured with stud-type terminations for air dielectric lines provide a connections by which they are connected gas port. None is needed on foam line into the RF circuit under test. To prevent terminations. Rigid transmission lines damage due to lightning currents or static almost always terminate in EIA flanges, discharges, a make-before-break switch is although adaptors to end terminals and other typically provided in permanent metering AM Antenna Systems Alexander/31

circuits to allow the meter to be placed into base current ratios, it may be desirable the circuit for measurement and removed during the construction and tuneup process from the circuit for normal operation, all to calibrate the RF ammeters against one without disturbing the continuity of the RF another. If one meter in the set is at current path. significant variance with the others, it can Some metering circuits are then be replaced with one that tracks the manufactured with a “hot jack,” which is a other meters or a calibration chart can be make-before-break receptacle into which a made. Likewise, if the base currents in an removable meter is plugged. The meter itself array are found to be out of tolerance but all is usually mounted on a phenolic handle to the other array parameters and monitor point permit it to be safely plugged into the RF field strengths are normal, it is a good idea circuit without causing RF burns to the user. to check the calibration of the thermocouple The advantage of this plug-in type of meter RF ammeters before doing anything else. is that the thermocouple meter itself is If the inaccuracy of a thermocouple stored in the transmitter building, out of the meter is believed to be caused by a elements that can affect its calibration. magnetized meter movement, a degaussing Thermocouple RF ammeters have coil can be used to demagnetize it. In most the advantages of relatively low cost, cases, the meter’s accuracy will return when simplicity, accuracy and broad frequency the residual magnetism has been removed. range. The disadvantages are susceptibility to damage. Lightning currents and static 10.2 The Toroidal RF Ammeter discharges through a thermocouple meter The other type of RF ammeter that is are usually fatal. Nearby lightning becoming very popular is the toroidal-type, discharges can cause magnetization of the which consists of a shielded, toroidal current meter movement, resulting in inaccurate transformer connected to a rectifier/filter readings. This type of damage is particularly that in turn drives a meter. This type of RF troublesome, since it may not be readily ammeter employs a separate, shielded apparent, as a catastrophic failure would be. current transformer which produces a fixed It is easy to check the calibration of a voltage per ampere of RF current flowing in thermocouple RF ammeter, because of its the circuit being measured. The meter is wide frequency range. Simply connect the housed in a separate unit and contains the meter being checked in series with a rectifier, filter and a switch that removes the calibrated AC ammeter across the secondary meter itself from the circuit. A relay that of a variac. By adjusting the amount of allows remote activation of the meter is current flowing with the variac, the optional on some units. The current indications of the two ammeters can be transformer is connected to the meter unit compared. If it is discovered that the RF with a piece of coaxial cable. At the factory, ammeter is in error, repairs can be attempted the meter, transformer and cable are or a calibration chart can be made to calibrated as a system. In the field, using a validate the meter’s indications. different coaxial cable or switching metering In a directional array, where base units between transformers will result in current RF ammeters are used to maintain inaccurate current readings. AM Antenna Systems Alexander/32

One of the biggest advantages of this calibration. type of RF ammeter is that there is no direct About the only disadvantage to the connection to the circuit under test. This has toroidal RF ammeter is initial cost. They are a twofold effect: one, it does not introduce a more expensive than their thermocouple new component into the circuit’s counterparts. Long term, however, their cost impedance; and two, the propensity for is lower than that of thermocouple meters. damage by lightning strikes and static Their better durability, stability and discharges is decreased by a large factor. accuracy will more than compensate for Another advantage is that with a their higher initial cost. In some cases where single pickup transformer, a meter with multiple scales are needed to accommodate multiple, switchable scales can be used. different patterns or current values, a single Many radio stations operate with switchable-scale toroidal meter may be less significantly different power day and night. expensive than two thermocouple meters Some of those that operate with similar with all the requisite switching hardware. powers operate different directional patterns where the current in a particular element 11.0 Sampling Systems may be large in one configuration and very At the heart of every directional small in another. It is difficult to measure antenna is a sampling system that provides such diverse values of current with a meter indication of the relative currents and phases using a single scale. Using a multiple, in the elements of the array. The key to a switchable scale toroidal RF ammeter, a successful sampling system is stability. A current of 25 amperes can be accurately stable sampling system will make true measured on one scale while a current of changes in array operating parameters less than 5 amperes can be measured on evident so that they can be compensated for another. Optional relay switching within the and the array kept in adjustment. A ammeter allows remote base current sampling system that is not stable will metering on the appropriate scale. This can provide false indications of array drift that be tied into the pattern switching logic of the the engineer will then “chase” with phasor array and the correct scale will always be controls, unknowingly cranking the array selected for the current mode of operation. out of proper adjustment with no clear way One manufacturer is now offering a back! toroidal RF ammeter package on an A directional antenna sampling insulated carry-around, plug-in frame. This system is made up of three elements: a package offers the same advantages as the sample loop or sample transformer for each plug-in thermocouple RF ammeter — it element, an antenna monitor that measures allows the meter to be stored in the the relative amplitude and phase of each controlled environment of the transmitter sample, and transmission lines that connect building, out of harms way. Another the samples to the antenna monitor. advantage is that base current ratios can be read much more accurately using a single 11.1 Sample Loops meter than they can using multiple meters A sample loop is just what its name with varying amounts of error in their implies — a one-turn metallic loop that is AM Antenna Systems Alexander/33

permanently attached to the tower. A typical 11.2 Sample Transformers loop is made of galvanized or stainless steel Sample transformers consist of a angle iron. Some loops are made of large- shielded toroidal loop through which a diameter copper tubing, but these are not as conductor carrying RF current to the tower durable as steel loops. As a rule, a sample is passed. Such transformers are essentially loop must be mounted 10 - 15 feet above the the same as those used in toroidal RF ground, except on tall towers, where the ammeters. A given RF voltage is produced loop is positioned 90 electrical degrees per RF ampere flowing through the below the top of the tower. If the towers in conductor being sampled, e.g. one or two the array are all the same height, the sample volts per amp. loops should all be located at the same Sample transformers are typically height above the base insulator. mounted inside the tuning house or ATU Sample loops can be either insulated enclosure and as such, have the advantage of from the tower and kept at ground potential, being out of the elements. Such pickups are or operated at tower potential. The latter very stable and sampling systems that method is more common. In such cases, the employ them seldom exhibit drift. In sample line itself is wound into an isolation addition, no iso-coil is needed. The coil that presents a high impedance at the disadvantage of sample transformers is that carrier frequency to carry the current sample they sample the current below the base across the base insulator without insulator as opposed to the actual radiation significantly affecting the impedance of the current. The sample taken at that point will tower or disturbing the sample itself. This include a small component that flows to iso-coil provides a convenient static drain ground through the base insulator for the tower, and a capacitor can be used capacitance. In addition, sample across part of the winding to achieve a transformers become unreliable is the parallel and “float” the tower for element being driven is more than 130 or so modes where that element is not used (such electrical degrees tall. as non-directional operation). When ordering a set of sample Sample loops have the advantage of transformers, the manufacturer can provide being mounted above the base insulator, on a set that is matched as closely as possible. the radiating element itself. As such, they Voltage output within 1% and phase tend to provide a superior sample and give a tracking within 0.5° are attainable, and when relatively good indication of the current and using sample transformers to set up an array, phase at the loop location. The disadvantage it is best to be as accurate as possible. is that they are exposed to the elements all By way of comparison, it is usually the time and subject to deterioration and easier to set up a new directional antenna damage. When the antenna monitor using sample loops than using sample suddenly shows array parameters at transformers. From an operational variance, particularly when only one standpoint, sample transformers are better to element seems to be affected, check the work with because of their stability and sample loops first. reliability. Ideally, one would use sample loops to set up an array and sample AM Antenna Systems Alexander/34

transformers to monitor it. The expense of between the two lines would be expected. using both kinds of sample is too great, This is unacceptable in a directional antenna however, so this is seldom (if ever) done. sampling system. In choosing a sampling system 11.3 Sample Lines transmission line, there is both normal and The transmission lines used in a phase-stabilized line. Phase-stabilized sampling system are a critical element. In a transmission line has been temperature typical system, all lines are of the same treated to achieve a repetitive phase- electrical length, exhibiting the same temperature characteristic for reliable amount of phase shift. Typically, _" or ½ " tracking. This is achieved through heat foam coaxial cable is used for sample lines. cycling at the factory that relieves the When installing the system, the line to the mechanical stresses that are part of the farthest element in the array is laid out and manufacturing process. Phase-stabilized cut. Then, the remaining lines are cut to transmission line is considerably more match the longest line. To achieve equal expensive than normal line, and its value is phase shifts in all the lines, it is then questionable. After a few weeks or months necessary to measure the electrical length of in service, a normal transmission line the lines, either with a time domain becomes naturally phase-stabilized, and if reflectometer (TDR) or by measuring the the lines are all installed in the same resonant frequency of the transmission line. environmental conditions, the phase shifts of Through careful measurement and trimming, all the sample lines in the system will it should be possible to achieve phase shifts change by the same amount with within 0.1° from line to line. temperature variations anyway. When installing sampling transmission lines, it is important to install 11.4 Antenna Monitors all of each of the lines in similar The final element in a directional environmental conditions. For example, if antenna sampling system is the antenna one line is buried underground, all the lines monitor. This device terminates the sample should be buried underground. If the last 20 lines and electronically compares the feet of one line is above ground in the relative currents and phases of the samples. transmitter building, the last 20 feet of all One tower for each mode of operation is the lines should be similarly installed. selected as the reference tower, or the tower Excess lengths of line, such as that of lines to which the other tower samples are that run to the nearer towers in the array, referenced. The indicated phase of the should be stored in the same conditions as reference tower is usually zero, and the the rest of the lines. Typically, if the sample current ratio is 1.000. lines are buried underground, the excess line The phases of the other towers in the is also buried. The purpose of this is to array are displayed as leading or lagging maintain a uniform phase shift from line to that of the reference tower. For example, if line. Were all of one line buried and another one tower in the array indicates -80° on the line half buried and half in direct sunlight, antenna monitor, that means that the current considerable variation in phase shift in that sample is lagging 80° behind the AM Antenna Systems Alexander/35

current in the reference tower sample. calibrated. With modern moment method The currents in the other towers in computer modeling, however, it is possible the array are displayed as a ratio, or a to closely predict what the actual driving percentage of the current in the reference point ratios and phases should be. With an tower. For example, if one tower in the array accurate sampling system and this computer indicates a ratio of 0.650 on the antenna model in hand, it is much easier to set up an monitor, the current in that element is 65% array. of that in the reference tower. More important than accuracy, Older antenna monitors could only however, is stability. Since a directional select and monitor one tower at a time. The array is set up and the proper indicated loop reference, or calibration of the phases and ratios are determined by field reference tower current sample, had to be strength measurements, it is more important adjusted each time before the remaining that a sampling system indicate changes ratios were read in order to insure accuracy. from the properly adjusted indications than This type of antenna monitor was difficult it is that it accurately resemble the actual (but not impossible) to accommodate with a currents and phases in the system. remote control system. Modern antenna monitors 12.0 Control Systems simultaneously monitor all tower samples at An important part of most directional once. The loop reference is continuously and antenna systems is the control circuitry. The automatically set. An analog voltage sample majority of stations operating with a representing each tower’s ratio and phase is directional antenna have more than one continuously available, making remote mode of operation. Some stations operate monitoring of the antenna monitor easy. non-directional daytime and directional at These modern antenna monitors also have night. There are a few that operate just the an amplitude mode, where the actual RMS opposite — directional day and ND at night. voltage of the sample is displayed. If the Others operate directional day and night, but loss of the transmission line and the volts with different patterns. Some stations have per amp output of the sample transformer (if yet a third mode or pattern for critical hours. used) are known, this display can be used to Some means of switching between patterns remotely monitor base currents, a very and modes is needed. handy feature indeed! 12.1 Why switch? 11.5 Accuracy A station that operates non- Because of distortions in the directional part of the time and directional sinusoidal current distribution in the towers during other times must have a means of of a directional array that are caused by detuning the unused towers. The same is mutual coupling between the elements, it is true of multiple pattern directional arrays just about impossible to obtain a sample that where some towers in the array are not used resembles the theoretical phases and ratios in some modes. The method used to detune of the properly adjusted array — no matter the unused towers differs with the situation, how carefully the sampling system is and some engineers prefer one method over AM Antenna Systems Alexander/36

others. Whatever method is used, however, Another type of RF switch that is to implement the detuning it is necessary to sometimes seen in situations where lower switch out that tower’s directional network power levels are involved is the vacuum and switch in a detuning network at the relay. Depending upon the impedance of the tower. RF circuit where the relay is inserted, this When operating with different type of switch can be a very economical directional modes, it is often necessary to alternative. Vacuum relay coils draw very switch in different components in the little current, their operation is very fast, and antenna tuning unit (ATU) for the different there is generally only one moving part. At modes. For example, the daytime directional higher power levels or in circuits where RF pattern may require a 90-degree lagging voltages or currents are high, a vacuum network at one tower, while the nighttime relay may not work. pattern may require an 80-degree leading While RF contactors are generally network. Two separate tee-networks would ruggedly designed, they can easily be be required, and a means of switching damaged by lightning, arcing contacts and between them would be needed. the like. Lightning, if not properly shunted, may jump from the RF conductors over to 12.2 RF Contactors the motor or solenoid as it seeks ground Switching of components and through the AC neutral. The results are networks in an AM antenna system is predictable — a burned out solenoid or usually accomplished using different motor. Limit switches are frequent variations of the RF contactor. This device casualties of lightning as well. uses sturdy, plated finger-stock and plated RF arcs can occur if the excitation is shorting bars to create a high-current not completely removed before the contactor capacity switch. Several individual SPST goes into motion. An improperly designed switches are typically arranged on an or operating control system may allow this insulated frame with a rocker-type armature condition to occur. When it does happen, to create SPDT and DPDT switch arcs develop between the finger stock and arrangements. 110/220-volt AC solenoids or the shorting bar. The plating is destroyed at electric motors are used to actuate the this point, and eventually the shorting bar armature. Solenoid-driven switches are very may become spot-welded to the finger stock fast and provide almost instantaneous or the supporting frame, preventing the transition between switch states, but the switch from moving. Another effect is that current draw of the solenoids is high and the the points which have arced tend to be poor rather violent mechanical action produces connections and heat builds up there as significant wear and tear. Motorized RF current flows. This heat can destroy the contactors are much gentler and they switch in a short period of time. typically require very little current to operate, but they are mechanically more 12.3 A Proper Control System complex and are quite slow to operate, At the heart of any multi-mode AM taking a full second or more to transition antenna system is a control system to between states. manage the operation of all the RF switches AM Antenna Systems Alexander/37

and . The job of this system is to 50 kW daytime transmitter be allowed to properly sequence all the events that must come on feeding a system with one ATU take place as a mode change occurs and to still configured for the 1 kW nighttime prevent excitation of the system without all pattern, in all likelihood the in the the switches in the proper position. ATU will be destroyed, and the transmission As an example, let’s consider a line is at serious risk of being damaged as multi-tower directional array that must well. switch between DA-day and DA-night modes. When the command is received from 12.4 Safety Functions the local pushbutton or the remote control to Operator safety functions should also switch patterns, first the controller must be incorporated into an antenna control mute the excitation. This is achieved either system. In the systems I design, I always by opening an interlock, disabling the drive insist in fully interlocked phasor cabinet or inhibiting the plate circuit of the doors so that opening any door or cover transmitter in use at the time. After a preset plate on the phasor will remove the RF amount of time has passed (usually a tenth excitation from the system. I also to a quarter of a second or so to allow the incorporate a key-switch operated interlock transmitter excitation to shut down bypass circuit which will allow operation of completely), a command to move all the RF either transmitter into the dummy load while switches in each of the antenna tuning units the main is on the air. We have all been in and phasor is initiated. Once a tally is situations, some stressful, where we get in a received from all the switches showing that hurry and perhaps forget to turn the they have successfully moved to the new transmitter off before opening an equipment position and another preset amount of time access door. It is important that we protect has passed, the transmitter excitation is ourselves and other workers from these reenabled. From pattern selection to situations with safety features. successful completion of the switch, if everything is properly adjusted, should be 12.5 Relays vs. Logic less than a half second — well within the Over the years, antenna control recovery time of most AM receiver AGCs. systems have traditionally been designed In a properly designed, properly adjusted using relay logic. Throughout my career, I system, listeners will notice little more than have often thought that these same functions a pop when the switch is made. could be achieved easily and more If something goes awry during the economically using logic circuits instead of switch, say a contactor does not move, the mechanically latching relays and time-delay control system should sense this and prevent relays. That is true, but with the control the excitation from coming back on. Phasor system indirectly connected to all that steel and ATU components, including sticking up into the air, the mean time transmission lines, can be seriously between failures could be measured as the damaged or destroyed if excitation is time span between the last repair and the allowed to come on feeding an incorrectly next thunderstorm! Relays are fairly configured system. Should, for example, a immune to the large surges that are bound to AM Antenna Systems Alexander/38

come in when lightning strikes, and they can that the system power supply must be on handle large amounts of current. and all the relays/switches in the proper Only recently, however, one position for the selected mode before the manufacturer has come out with a PCL transmitter will be allowed to come on. (Programmable Logic Controller) based Redundant power supplies are common, antenna switching controller. This controller since the failure of the controller supply will is completely programmable to operate a set prevent the transmitter from coming on, of RF switches and transmitter interfaces in even if all the RF circuitry is correctly any combination that can be imagined. The configured! interface to the outside world has been Because the control system is so ruggedized to give the system good central to the transmitter site’s operation, surge/lightning immunity. periodic checks of the power supply voltage Whether relay controlled or logic are important. Also, it has been said that controlled, the fact remains that either cleanliness is next to godliness. This is motors or solenoids that run on either 110 or especially true when it comes to relay-based 220 volts AC must be actuated by the control systems. A dirty relay here can keep control system. There are two ways to do the station off the air. this: One involves running a constant AC The limit and travel switches on power feed to each tower and using low contactor motors or solenoids are especially voltage slave relays to actually apply the AC prone to give trouble. Because of the rather power to the solenoids or motors. The violent action of solenoid operated advantage with this method is that a single contactors, these switches really get run of AC power cable can be run to each slammed open and shut. Proper adjustment tower and other AC powered loads — tower of limit switches and actuating hardware is lights, ATU lighting, utility outlets, etc. — critical. can be operated from that same source. The For some reason, in those areas that down side is the added components (relays, are plagued with them, fire ants seem to be sockets and wiring) necessary. attracted to AC power (you guys in the The other method involves switching north, get ready — they’re coming!).I have the AC current to the motors or solenoids seen fire ants completely pack solenoid directly from the controller. If solenoids are mechanisms to the point where the contactor used, this involves running a rather large could not move. Periodic cleaning out of (#10 or better) conductor to each side of these areas with a vacuum or high pressure each solenoid plus a neutral. If motors are air is necessary to keep the mechanisms used, smaller conductors can usually be moving freely. Also, a few moth balls in a used. A separate source of AC power must cup left in the ATU will go a long way then be run to each tower for the ancillary toward keeping ants, wasps and other pests loads. away. Finally, it is important to 12.6 Care and Feeding periodically test all the interlock and safety Most modern antenna control circuits in the system. If the transmitter systems are built to be fail safe. That means excitation is not being completely killed AM Antenna Systems Alexander/39

before any contactor starts to move, arcing of the mutual coupling between the array and arc damage will occur. An improperly elements. working interlock can get you killed. Test You have probably noticed that if these features at least once a year. you crank the phase of one tower out a The antenna control system should couple of degrees and touch nothing else, be invisible if it is working properly. By there is a change in all the other phases, starting with a good design and continuing ratios and base currents in the system in with good maintenance, you can achieve addition to a change in the common point trouble-free operation for many years. resistance/reactance. The same thing generally holds true no matter what the 13.0 Troubleshooting cause of the original change, be it a phasor From time to time, things go wrong control being adjusted or a component with all antenna systems. Things are bad changing value. enough when it happens with a simple non- When an engineer calls me saying directional antenna. When a multi-element that something is wrong with his array, I directional array goes haywire, it can be always start by asking him if the common maddening! point impedance has changed. If it hasn't, we Rule number one in this situation is start with the antenna monitor. DON'T PANIC! The tendency is to grab A good next step is to check the phasor controls and try to correct the antenna monitor by swapping its inputs situation immediately; this is the wrong around. A common failure mode in some thing to do! Don't touch anything until you antenna monitors is a stuck or open relay. have enough information to make an The mercury-wetted relays in these units educated decision of what to do. should be trouble-free and long-lived but they tend to wear out over time. I have 13.1 Incorrect Antenna Monitor replaced scores of them over the years. Parameters When a relay sticks, it may cause all the Let's assume that one or more of the tower readings other than the reference parameters as shown on the antenna monitor tower readings to be incorrect. To check for are at variance with their proper values. this, disconnect all inputs but the reference Start by making a note of the proper values tower, then connect one of the other tower on a piece of paper with the values as read sample lines to each of the other inputs in on the monitor alongside. In this way you turn. You should see the normal indication can quickly see what parameters are at of phase and ratio for the sample line being variance. If only one or two values are out used as it is moved from input to input. of whack while all the others are normal, When you come to a channel where you do there is a good chance that the problem is not get the correct readings, that is the one with the sampling system or monitor and the with the bad relay. If there is a stuck relay, it array is functioning normally. As a rule, tends to load the other channels so while the when something changes in an array due to phase readings may be normal during this a component malfunction, all the parameters procedure, the ratio readings will often be are affected to some degree. This is because low for all but the channel with the defective AM Antenna Systems Alexander/40

relay. Sample loops can cause trouble, with Occasionally, antenna monitor welds and insulators breaking. Sometimes sample line terminating resistors can high winds can blow loops around so that become damaged by arcs or lightning they are no longer properly oriented. A strikes. The symptom will be a very high sample loop should be positioned so that it ratio on one of the antenna monitor is perpendicular to the tower face behind it. channels. Check these resistors with an A good way to check loop alignment is to ohmmeter while the sample lines are stand at the tower base and look up at the disconnected. They should all be very close loop — if positioned properly, the loop to the same value. should line up with the guy wire. Inspect the The detector diode in your antenna loops up close, looking for corrosion, loose monitor is one of those "future failure connections and hardware, etc. Most loops components" to watch out for. Usually this attach to the sample line with an N- or UHF- diode is a germanium type, very prone to connector of some sort. Check these damage from lightning. If this happens, connectors for water, corrosion, etc. There hopefully it will open completely. I have may be a copper strap or braid used to seen them become non-linear, however, jumper from the connector to the open end giving incorrect readings on all towers. The of the loop. This strap can easily break loose symptom of this condition will be a from the loop. Be sure that it is well bonded significantly changed loop reference setting to the metal of the loop. on the reference tower. If you have to crank Toroidal transformers can, from time that control more than half a turn to get to time, cause problems when used in a 100% on the loop meter, suspect the sampling system. Some are prone to arc detector diode. Another failure mode occurs internally when their output is unloaded. when a resistance develops in the detector While sample lines should always be diode. The symptom in this case is excessive terminated in the load resistors in the ratio meter wiggle with modulation, and all antenna monitor, it is possible that enough the ratios tend to be incorrect. voltage could develop at the tower end of a Another possibility is the faulty long sample line to allow an arc to occur. If sample line. You can check your sample a transformer is suspect, swap it with one lines by running an open circuit/short circuit from another tower (any but the reference impedance test on them. This will give you tower) and see if the problem disappears. the characteristic impedance and approximate electrical length of each of the 13.2 Incorrect Base Currents lines. You can also bridge the sample lines The most suspect indicating open-circuit at an odd quarter-wavelength instrument in any radio station is the base resonant frequency to determine exactly the current ammeter. They lie like a slick electrical length. The best way to check politician. From the moment they leave the sample lines is with a time-domain factory their calibration becomes suspect. reflectometer (TDR). These devices are Vibration, magnetic anomalies, temperature, generally available for rent, and many tower humidity, insects, moisture — everything — riggers now have TDRs in their shops. affects their accuracy. Toroidal current AM Antenna Systems Alexander/41

meters can also lie, but they are more failure component is the mica capacitor. I reliable than thermocouple meters. If always suspect them first, but they aren't everything else is okay (antenna monitor always the problem. parameters and monitor points), suspect one A symptom common to component of the meters. The very best way to keep failure is heat. Shut the system down and base current meters accurate is to use one immediately but carefully feel of all the meter at all the towers, carrying it to the components in the phasor and ATUs. Some towers and plugging it in when needed and components may be warm but none should storing it in the controlled environment of run hot. Suspect any hot component. Look at the transmitter building when not in use. all the coils for discoloration caused by This can be done with either type of meter. heating. Loose hardware can get red hot under current, causing oxidation and an 13.3 High Monitor Point intermittent connection. Look for leaking A likely cause of a high monitor capacitors as well. point reading is an anomaly at or in the If you have an RF bridge on hand, vicinity of the point itself. Reradiators and use it to measure the reactance of any other factors beyond the station's control can suspect components. If you aren't fortunate influence the field strength at a monitor enough to have a bridge at your disposal, point. If you find a point high, don't adjust you can use a capacitor checker. Many anything. Measure five or six points on the stations have the old-style "magic eye" radial and see how they compare to the last capacitor checkers lying about. Use a full or partial proof. If they are in, you can capacitor of known value to test the checker assume the array is in adjustment and the before testing the suspect cap. Take lead monitor point itself has become unusable. into consideration when reading §73.158 specifies the procedure for capacitor value on the checker. If you don't changing the monitor point on a radial. have a bridge or capacitor checker, many of If the entire radial is high, the array the better digital multimeters (some in the may be out of adjustment, even though the under-$100 range) have a built-in capacitor array parameters are all within tolerance. check function. It might be worthwhile to This can easily occur in arrays with very invest in one. tight nulls. Before you start cranking, There are many things that can go though, it is a good idea to put the array in wrong with a directional array and there is the non-directional mode and look at five or usually a ready fix for each type of problem. six points along the radial both ND and DA. Seldom, however, will cranking on the Compare the ratios with those in the last full phasor repair a problem. You may be proof. It could be that a conductivity change chasing a bad antenna monitor or sample is responsible for the high readings and the line, or compensating for a component with array is in adjustment. a changing value. When trouble comes, stop, think and troubleshoot the problem through 13.4 Faulty Component thorough investigation and logical thinking. From time to time, parts do fail in Crank on the array only after the problem directional antennas. The most common has been diagnosed and fixed. AM Antenna Systems Alexander/42

authority as to the proper input power), so 14.0 Regulatory Requirements be sure you use the common point current The FCC regulates broadcast value specified there rather than one stations, and in the past, there were many calculated for the station’s nominal power. and more specific regulations than there are §73.1215 lists the requirements for now. In the age of deregulation, the rules are indicating instruments, including common full of the seemingly harmless phrase, “as point and base current ammeters. Their often as necessary to insure compliance”. accuracy must be within 2% of the full scale This phrase took the place of specific reading, and the full scale value must not be intervals for various readings, calibrations more than five times the nominal reading. and the like that were once in the rules. Further, the meter must not read off-scale Now, although we have the freedom to make during modulation. There are certain other these measurements on our own schedule, requirements pertaining to scale divisions the monkey is clearly on our backs to keep and the like contained in that section. If you all the parameters within the terms of our are employing a meter manufactured or station licenses. If you are caught with a supplied by one of the mainstream broadcast parameter out of tolerance, you are likely to equipment manufacturers, you can be not only receive a Notice of Apparent reasonably sure that these minor Liability (NAL) for the parameter violation requirements have been met. but for not having measured it as often as You should be sure, however, that necessary to insure that it is within the normal reading is greater than one-fifth compliance as well! That may not seem fair, full scale, and that the meter does not exceed but it goes to show that deregulation may full scale under modulation. Many times, not be such a good thing after all. following a lightning strike or some other We will begin in the transmitter incident, an RF ammeter is replaced with building and take a walk through the whatever is currently available. Particularly regulatory requirements pertaining to every in the case of low power operation, part of an AM antenna system. sometimes the replacement meter will not be of the proper scale. 14.1 Antenna Current If you find yourself in this position An often misunderstood provision of and you are using a toroidal RF ammeter the FCC’s rules pertaining to operating (such as those manufactured by Delta power in directional antenna systems says Electronics), you can increase the sensitivity that the authorized input power shall exceed of the meter by running multiple turns of the the nominal power by 8% for stations RF feed through the toroid pickup. The authorized 5 kW or less, and by 5.3% for reading will be proportional to the number stations authorized more than 5 kW (see of turns through the donut (i.e. two turns §73.51). That means a 5 kW station will double the reading, three turns will employing a directional antenna will employ triple it, etc.). For example, suppose your an antenna input power of 5.4 kW. The low power nighttime operation has a station license should reflect the correct licensed common point current of 0.2 input power (and is, in any case, the final amperes and the lowest scale meter you can AM Antenna Systems Alexander/43

find is five amperes. By wrapping five turns water in the soil. Similar (and sometimes through the toroid, the 0.2 ampere reading more dramatic) changes can occur when the will appear as a 1.0 amp indication, which is ground freezes. If you have a common point in compliance with the rules. If you do this, bridge built into your phasor, you are all set however, you will need to file a form 302 to make a resistance measurement every (license application) with the FCC to modify time you visit the site. Otherwise you will the station license to show the new indicated have to use an operating impedance bridge current. to make the measurement from time to time The requirements for remote reading as required. If the resistance has changed ammeters are contained in §73.57. Remote more than 2% from the value specified in reading ammeters must be accurate to within the license, you will need to file an 2% of the reading on the regular (local) RF application for modification of license. ammeter. The sensors or pickups for remote reading RF ammeters must be located on the 14.3 Sample Systems transmitter side of the local RF ammeter. If In directional systems, next to RF your site employs remote reading RF ammeters, the sampling system is the most ammeters, it should be part of your regular likely element to produce false indications. routine to compare the readings between the The FCC rules contain specifications for local and remote meters. sampling systems, and specifically contain RF ammeters, particularly the requirements for “approved” sampling thermocouple meters, are the most likely systems. The basic requirement for an indicating instruments in an AM antenna “approved” sampling system is that the system to give incorrect readings or trouble. transmission lines exhibit uniform phase It is an excellent idea to keep a new, factory- shift (less than 0.5° difference from the calibrated meter (preferably a plug-in type shortest line to the longest line with normal meter) in the transmitter building. From time temperature variations). to time (at least annually), all the meters in Should some component in the the system can be calibrated against this sample system become damaged or reference meter. malfunction, it is permissible to operate for up to 120 days without notifying the FCC as 14.2 Antenna Resistance long as all other parameters (base current Periodic checks should be made of ratios, monitor points and common point the resistance at the common point or non- current) are maintained. Should the outage directional antenna base. The frequency of exceed 120 days, the FCC must be notified this resistance check will vary from station and special temporary authority requested. If to station, and will probably be determined portions of the system above the base by the nature of the environment around the insulator are replaced, a partial proof of antenna. Areas with poor ground performance will be required. Sample conductivity or sites with poor or system rules are contained in §73.68. deteriorated ground systems may see a considerable shift in base or common point 14.4 Monitor Points resistance with changes in the amount of Field strength measurements at the AM Antenna Systems Alexander/44

monitor points (MPs) of directional stations §73.62. must be made “as often as necessary to ensure that the field at those points does not exceed the values specified in the station 14.6 Fencing authorization”(§73.61). This is one of those §73.49 requires AM sites using instances where it doesn’t matter if you read series-fed, folded unipole and insulated base the MPs yesterday and they were correct, if antennas to enclose the base of each tower the FCC inspector finds one or more of them within a locked fence. The size of the higher than the licensed limit today – you enclosing fence is determined by the E- and are likely to get dinged both for the high MP H-field present in the vicinity of each tower and for not measuring the points often base. The safe thing to do is fence each enough. If you do not have an approved element in the array at the radius specified sample system, the same requirement in the FCC’s OET Bulletin No. 65 for the applies, but the interval between nominal power of the station. By doing this, measurements must not exceed 120 days. you can be assured that if something should go wrong and all the transmitter power is 14.5 Directional Operating Parameters fed to any one element, no person can enter On the antenna monitor, the into a field that exceeds the ANSI limit. Of indicated ratios must be maintained within course, you can fence the entire perimeter of 5% of the values specified in the station the property and forego fencing of the license. Phases must likewise be maintained individual elements, but in that case you within 3°of their licensed values. At the must otherwise delineate the areas where the tower bases, base current ratios must be RF radiation exceeds the ANSI limit. maintained within 5% of their licensed values. Note that it is not the base current 14.7 Proof Documents values themselves that are licensed, it is the Finally, there is a requirement in ratios. §73.154 that the results of the most recent Should an instance arise where the set of directional partial proof of parameters cannot be maintained within the performance measurements be kept in the prescribed values, you must measure the station records and available to the FCC for field strength at each of the MPs. If they are inspection. Many stations, however, do not all below the licensed values, you may have a copy of their last proof on hand. If continue to operate at full power for up to 30 you find that you do not have this document, days without further authority from the FCC you can order it from one of the copy while the problem is being corrected. If one services in Washington. Not only will or more of the MP field strengths exceeds having this document on hand comply with the licensed maximum, you must reduce the rules, it will provide you with a power to a level that brings the high MP benchmark for your directional system. field strength below the maximum. The Present readings can be compared to those rules pertaining to directional antenna in the proof to determine if a perceived system tolerances and procedures for problem actually exists. operating at variance are contained in