FM Transmission Systems

Rockwell Media Services, LLC 158 West 1600 South, Suite 200, St. George, Utah 84770

Reprinted by permission from W.C. Alexander, Crawford Broadcasting, Director Engineering, [email protected]

FM Transmission Systems

W.C. Alexander Director of Engineering Crawford Broadcasting Company

IntroductionAbstract Unfortunately, the real world is very different from this ideal. The real world is The variables in any given FM full of obstructions, manmade and natural, transmission system are many. They include that partially or fully obstruct the path from factors such as antenna height versus ERP, the transmitting to receiving antenna. Real- antenna gain versus transmitter power, world transmitting antennas exhibit some vertical plane radiation patterns, Brewster non-uniformity in the horizontal plane, and angle, Fresnel zone, polarization, site in the vertical plane, half of the energy is location and topography among others. In radiated above the horizon into space, where this paper, we will examine each of these it is wasted. Reflections from objects also variables, the tradeoffs between cost and produce amplitude variations in the received performance, antenna and transmission line signal that cause noise and signal dropouts. types, installation and maintenance The number of variables that go into techniques and procedures. the performance of a particular antenna site is quite large, and many of these factors are 1.0 Antenna Site Considerations beyond the broadcaster=s control. Many can While few of us have much control be mitigated, however, with good site over the location of our antenna sites, selection, and it is on those that we must perhaps there is room for change in some focus when searching for an antenna site. situations. For the rest, the information The goal of the broadcaster is to which we present herein will help us produce a signal of sufficient amplitude to evaluate the performance of our radio overcome noise and provide at least 20 dB stations as a function of site location and of signal-to-noise ratio at as many of the antenna height. receiver locations within the desired service Location, location, location. Those area as possible. How much signal is are the three most important factors in real sufficient to meet this goal is largely estate, and they are equally important for dependent upon the receiver and its antenna. radio transmission systems. This applies In the absence of interference, a signal level equally to AM, FM, TV, MMDS, cellular, of as low as 2 uV/m may be sufficient for PCS, two-way, paging and other RF-based many of today=s automobile receivers. services. Portables may require as much as 500 uV/m. Ideally, the antenna for an FM Interference from co- and adjacent-channel broadcast station would be situated at a stations usually increases the amount of location that would present a clear line-of- signal required for acceptable reception. sight to the entirety of the desired service area. The antenna would have uniform 1.1 Fresnel Zone horizontal- and vertical-plane radiation There is no substitute for a clear line patterns, and there would be no reflections of sight between the transmit and receive from natural or manmade objects. antennas. This is one of the first rules in

2 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/2

VHF transmission. A transmitter site with a This brings us to the conclusion that clear line of sight to virtually all the target height is a very significant factor in most service area is thus superior in most cases to antenna site situations. As a rule, greater one that is blocked by terrain or manmade height is more useful than higher power in obstructions to parts of the area. In some producing higher receive signal strength, all cases, simply having line of sight is not other factors being equal. enough. In engineering our microwave and UHF STL paths, we always consider Fresnel 1.2 Multipath Considerations zone clearance, knowing that a path with Multipath is a nasty word in the less than 60% first Fresnel zone clearance vocabulary of most radio engineers and will be marginal. We often neglect this station managers. It is a good descriptor of consideration in engineering our FM the destructive effect of the same radio transmitting antenna locations. signal arriving at a receive point by multiple For those not familiar with Fresnel paths. When these signals arrive in phase, all zone clearances, they are circular areas is well and the incident field strength is surrounding the direct line-of-sight path that greater than it would be in the case of a vary with frequency and path length. The single signal path. When they arrive out of longer the path and lower the frequency, the phase, however, at least some degree of larger the mid-path clearance required for cancellation will take place, resulting in a clear-path reception. As mentioned above, reduced incident field strength, with 60% first Fresnel zone clearance is all that is complete cancellation (zero incident signal) required to meet the clear-path reception taking place in the worst case situations. objective, but that can be quite large at FM To make matters worse, sometimes, frequencies. The first Fresnel zone radius complete cancellation can take place on can be computed using the formula R = frequencies close to carrier while less than 1140 ••d/f, where R is the radius in feet, d is complete cancellation takes place on the path length in miles and f is the sideband frequencies. This in many case frequency in MHz. results in a demodulated sound much more A quick example of 60% first offensive to the listener than the quiet hiss Fresnel zone radius for a few typical of no signal. Motion in an automobile broadcast situations are 267 feet for a class produces a constantly varying multipath A, 378 feet for a class B and 463 feet for a situation, often causing picket-fencing (the class C1. Keep in mind that we=re talking effect of the slats in a picket fence about terrain clearance at the mid-point alternately permitting and then blocking the between the transmitting and receiving signal), which is quite objectionable to the antennas required to produce clear-path listener. reception. These translate to antenna heights The worst-case multipath scenario is above ground of 534 feet, 756 feet and 925 where the transmitting site is located on one feet respectively. With the exception of the side of the service area and a range of class C, the antenna heights are well above mountains or high hills is located on the the maximum height above average terrain other. Receivers within the service area get (HAAT) values for the classes. the direct line-of-sight signal from the

3 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/3

transmitting antenna, but they also get a indicating that a more distant site may be reflected signal from the mountains or hills. preferable. In such a case, there will be few locations within the service area where multipath 1.4 Vertical Radiation Pattern effects will not be a factor. We mentioned early on the vertical- Perhaps the best location for a plane radiation characteristics of real-world transmitting antenna in such a geographic transmitting antennas. Some of these scenario, assuming that a mountaintop characteristics come into play when location is out of the question, is on a hill selecting a transmitting antenna site. near the mountain range. A directional If a transmitting antenna is located at antenna would then be used to reduce a considerable height above the target radiation toward the mountains and service area, the main elevation plane lobe maximize it toward the service area. This may overshoot the target service area, with will result in greatly reduced reflections. the energy being radiated out into space. While it would be impossible to completely The more bays an antenna has, the narrower eliminate reflections, they could be reduced the main elevation plane will be. Antennas so that the ratio of direct-to-reflected signal with a small number of bays (less than four) at most locations throughout the service area exhibit a broad elevation plane lobe, making is sufficiently high to nullify the effects of such overshoot of the target service area less multipath. likely. In those situations where a large 1.3 Grazing Angle number of bays is used and the antenna is Ground reflections play a part in the high above the target service area, it may be overall propagation of FM signals, desirable to employ beam tilt to lower the particularly the vertically-polarized beam angle slightly. Typically, just enough component. Almost all FM signal coverage beam tilt is used to center the main elevation lies between the horizon and 10 degrees plane lobe on the distant edge of the target below the horizon. This is called the grazing service area or on the horizon, whichever is angle, and it lies between the horizontal closer. We will discuss beam tilt in more plane from the transmitting antenna and the detail in a later installment, but mention it earth=s surface. Vertically-polarized energy here because it does impact site selection. is attenuated considerably more than Antennas with a large number of horizontally-polarized energy at angles bays exhibit elevation plane nulls. The more greater than about 2 degrees. As a result, bays an antenna has, the farther away from circularly-polarized signals tend to be the antenna site that these elevation plane reflected more as elliptical rather than nulls hit the ground. If the area within a few circular. It is important in site selection to miles of the antenna site is populated and it avoid grazing angles which are greater than is desirable to provide service to this area, it about 2 degrees (the Brewster angle). may be desirable to employ null fill. A very Simple geometry would suggest that sites small amount of null fill is all that is close in to the service area would be more necessary to provide adequate service in prone to produce such high grazing angles, these close-in areas.

4 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/4

total cost of each possible TPO/gain 2.0 Antenna Gain vs. TPO combination must be made. Factors which How many bays do I need in my enter into this equation include the cost of antenna and how much transmitter power do the antenna, transmission line and I need? This question often does not have a transmitter, tower cost (keep in mind that hard and fast answer. Rather, the engineer larger antennas require taller towers to designing an FM transmission system must achieve a given HAAT), and long-term look at many factors to come up with the operating cost. Operating cost should be best possible combination of antenna gain factored in over a period of time, usually the and transmitter power for a given situation. useful life of the transmitter (say, 15 years), Assuming that for a given value of and added into the cost equation. For effective radiated power (ERP) we have a example, a high-gain antenna/low TPO choice of between one and twelve antenna transmitter combination for a class B or C bays. If we are dealing with a class A or station will often result in considerable up- other relatively low power station, we can front costs because of added tower height narrow the choice down to a fewer number and the cost of a larger antenna, but the of bays. Quite obviously one would not use power cost savings from the lower power a ten-bay antenna, which is over 100 feet in transmitter will offset this over a period of length, for a 6 kW ERP station. Any savings time. The converse is also true. The best in transmitter cost would be consumed many starting point is that range of gain/TPO times over by the added antenna cost and values that fits the budget. additional tower height needed to achieve Once we get past what we can and the required height above average terrain cannot afford, there are as many approaches (HAAT) with such a large antenna. to what is the “best” gain/TPO combination On the other end of the spectrum, a as there are engineers. Most any broadcast 100 kW station would likely not utilize a engineer who has been in the business one- or two-bay antenna. Any savings in awhile has an opinion on the topic. Most of tower height and antenna cost would be these are strong opinions, usually based on more than offset by the cost of the large personal experience in one or more combined-amplifier transmitter necessary to situations. As such, there is a degree of achieve the required ERP with a low-gain validity to most such views. How, after all, antenna. can one argue with empirical data? With the choices narrowed a bit, we The underlying truth, however, is still have a fairly wide range of choices for that each situation is unique with a whole set antenna gain and transmitter power output of variables that may or may not bear any (TPO) to achieve the required ERP. It is at similarity to experiences elsewhere. To get this point that other factors come into to the right answer for a given situation, one consideration. must examine each of these variables. 2.1 Economic Considerations Economics is probably the biggest 2.2 Terrain Effects factor with most stations. Without an The first of these, and perhaps the unlimited budget, a careful analysis of the one that has the biggest effect on

5 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/5

performance, is the lay of the land. What is transmitter site. Reality is usually the terrain like? The ideal situation is flat, somewhere in between. In these cases, unbroken terrain in all directions from the where the populated area is far enough away transmitter site. The equation becomes one from the transmitter site to be situated well of simple line-of-sight. Unless there is within the main vertical plane lobe of most significant population close to the tower any antenna, are we back to a simple site, there is virtually no performance economic decision? Usually not. difference between a high-gain/low TPO Multipath often exists within the and a low-gain/high TPO. In such cases, the coverage area of an FM station. Multipath best combination is the most economical. is, as its name implies, the result of the Find the combination that produces the signal arriving at the receiving antenna by desired ERP at the desired HAAT for the disparate paths. There is (usually) the direct, lowest cost. or line-of-sight path, and then there are any Add to the equation terrain features number of other paths resulting from such as hills or mountains and the picture is reflections and refractions. Instantaneous completely different. One of the most severe phase cancellations occur at many points, cases is one where the transmitter site itself producing the familiar fuzz and choppiness is located atop a high terrain feature with the of multipath. Site selection is the single populated area to be served at significant most important factor in preventing depression angles below. In that case, the objectionable multipath, but antenna vertical plane radiation pattern of the selection has something to do with it as well. antenna very much comes into play. The Consider the case where a object is no longer one of producing the transmitting antenna is located just outside maximum amount of signal on the horizon of a metropolitan area. The tall buildings but also (and often more importantly) downtown are usually pretty good reflectors includes producing good field strengths of RF and their tops tend to be close to the much closer in. same elevation as the transmitting antenna, Manufacturers provide graphs and thus putting them in the strongest part of the tabulations of the vertical plane radiation vertical plane pattern. Receivers located in patterns of their antennas. This data can be vehicles and homes below are at a used to locate the first and second vertical disadvantage because they are in a weaker plane nulls as well as the relative field at part of the vertical plane pattern and they are any angle below the horizon. With this data in an area where shadowing and attenuation in hand, one can use the free-space loss from trees, buildings and terrain features formula along with the ERP to calculate the further weakens the direct-path signal. Now expected best-case field strength at any factor in the relatively strong reflected given point. The performance of one signal from the downtown buildings and you antenna can be compared to another to find have a recipe for lousy reception in the very that combination that will best serve the heart of the desired coverage area. desired areas. One way to combat this is by So far, we have discussed the two increasing the amount of direct-path signal extremes, flat terrain and mountaintop by employing a much broader main vertical

6 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/6

plane lobe. This is done by using a lower vertical plane lobe is narrowed, focusing the gain antenna. The vertical plane main lobe main lobe energy on the horizon. Think of a from a lower gain antenna is much broader, single-element antenna as a floodlight which producing stronger signals at lower fairly well lights up an entire room with a depression angles that will often overcome relatively low light intensity. Increase the the reflected signals so that complete “gain” of the floodlight by focusing it with a cancellation does not occur. reflector, turning it into a spotlight. The I mentioned above that every same amount of light energy is emitted by engineer has his own opinion, usually based the bulb, but now it produces a much greater on some specific experience, as to the “best” intensity in a much smaller area. Continue to TPO/gain combination to produce a given focus the light until it is a brilliant pinpoint ERP. In my opinion, the most consistent and you have done in essence the same thing reception is produced by higher TPO/lower as stacking twelve or more elements in an gain combinations. More signal at lower FM transmitting antenna. There are several depression angles where the receivers live is places where this analogy falls apart, but it almost always a good thing. illustrates the general principle. Maximum coverage is achieved by 3.0 Vertical Plane Characteristics focusing the “beam” of the vertical-plane Consider that with an FM antenna, lobe on or just below the horizon. Increasing we achieve “gain” by increasing the number the antenna height naturally pushes the of elements in the antenna. Antenna “gain” horizon out by allowing the antenna to look is referenced to the field produced by a slightly beyond the curvature of the earth. If horizontally-polarized half-wave dipole in a tight vertical-plane “beam” of a multi- free space. A horizontally-polarized antenna element antenna mounted on a tall tower is with one bay, for example, would nominally focused on the horizon (i.e. perpendicular to exhibit a power gain of one; adding an the tower and antenna), most of the energy additional element to the antenna would radiated from the antenna will overshoot the increase its power gain to two. Additional target coverage area, being wasted out into bays would likewise continue to increase the space. This isn’t much of a problem in cases gain of the antenna. Circularly-polarized where the antenna has few elements or is (CP) antennas would exhibit half this mounted below 500 feet above average nominal gain, as half the power of the terrain. However, in cases where a multi- antenna is vertically polarized (i.e. a two- element antenna is mounted 1,000, 1,500 or element CP antenna has a nominal power even upwards of 2,000 feet above average gain of one). As a rule, only the terrain, steps must be taken to mitigate the horizontally-polarized mode is considered overshoot. when discussing antenna gain. The seemingly obvious solution A single-element antenna exhibits a would be to mechanically tilt the antenna so large vertical-plane lobe with the radiation that its vertical-plane “beam” is focused in being distributed evenly both below and the desired location, just below the horizon. above the horizon. As additional elements While this technique does work, it has a are added to increase antenna gain, the serious drawback in that it only lowers the

7 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/7

beam on the side toward which the antenna horizon) increases. As a result, the distance is tilted and it raises the beam an equal from the tower to the point on the ground amount on the opposite side, in effect where the first null lands increases. This null robbing Peter to pay Paul. In certain area, even though relatively close to the situations, this may be an acceptable course tower, is often an area of real signal of action, such as where an antenna is problems. With virtually no direct-path located with all the populated area to one signal, all the remaining signal comes from side, or where the antenna has a significant reflections and refractions and the net signal amount of terrain shielding on the side away is plagued with multipath effects. If there is from the populated area. In both such cases, significant population within the null area the loss of signal on the opposite side would (or if there is a major thoroughfare through not matter. the area), something must be done to mitigate these effects and make the station 3.1 Beam Tilt listenable there. A better way of achieving “beam The best solution is to take steps in tilt” is done electrically by delaying the the antenna design to prevent the signal in currents to the lower antenna elements. This the null from going all the way to zero. A is easily achieved by simply inserting a very small amount of null fill, usually 5% or “delay line” (or a short, additional length of so, is more than adequate to provide plenty transmission line, between the power divider of direct-path signal to overcome all the and the lower antenna elements. Electrical reflections and refractions. beam tilt has the advantage of lowering the Null fill does not come without a vertical-plane lobe equally in all directions. penalty, however. That power put back into Typical values of electrical beam tilt are 0.5 the vertical-plane null must come from to 1 degree. somewhere, and it mostly comes from the Another byproduct of increasing main vertical-plane lobe. As such, null fill antenna gain is that of elevation plane nulls. generally slightly lowers the gain of the Any antenna with two or more elements antenna will, in addition to the main vertical-plane How does one determine the distance lobe, have other secondary lobes both above to the first null? A quick method of and below the horizon. Along with these determining the approximate distance is: secondary lobes come vertical-plane nulls. Nulls above the horizon are of no consequence since they have no effect on Ant. Hgt .( ft ) xNo . ofBays Null Radius( mi .) = coverage. Nulls below the horizon are a 5280 different story as it is below the horizon where the desired coverage area lies.

3.2 Null Fill The antenna manufacturer can With an increasing number of provide a complete vertical-plane pattern for elements, the elevation angle of the first null the antenna being considered, and with this (i.e. the first vertical-plane null below the you can find not only the location of the null

8 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/8

but its width. Overlaying that on a map can sidebands, the perfect center-of-channel load help you determine whether or not null fill is presented by the antenna system becomes a even needed. much-less-than-perfect load at the sideband frequencies. Final amplifier tuning is no 4.0 VSWR Bandwidth longer optimal and a significant amount of The bandwidth of a commercial FM power is reflected from the antenna system broadcast signal, depending on whether or back to the final amplifier. The result is a not subcarriers are used, is about 260 kHz. change in the otherwise constant amplitude This figure is representative of a carrier of the FM carrier with modulation, or modulated 110% (or 82.5 kHz deviation), amplitude modulation. The phenomenon is the maximum allowed under 47 C.F.R. called synchronous AM because the §73.1570(b)(2). amplitude modulation occurs synchronous The term VSWR Bandwidth is with the FM modulation of the carrier. generally defined as that bandwidth over which the antenna system has a VSWR of 4.2 Intermodulation/Crosstalk 1.1:1 or less (5% reflection coefficient). The Other signal problems caused by antenna system includes the radiating narrow VSWR bandwidth or asymmetrical elements, the interbay transmission line, passband are reduction in power amplifier matching devices and the transmission line. efficiency, intermodulation distortion and VSWR bandwidth is generally measured at stereo crosstalk. The greater the deviation, the input to the transmission line (or the the worse the resulting signal degradation. output of the transmitter). Sometimes, such signal anomalies are Too often, FM antenna systems are incorrectly identified as multipath when in tuned with the engineer watching the reality they originate in the transmitter and transmitter reflectometer while the tower antenna system. Needless to say, it is highly worker adjusts the matching device on the desirable to maintain good VSWR tower. While this should result in the lowest bandwidth. attainable VSWR at the carrier frequency, it does not address the VSWR at the 4.3 Transmission Line Effects sidebands. Higher values of VSWR at the As mentioned above, the antenna sideband frequencies or an asymmetrical and transmission line must be viewed and passband will result in all sorts of treated as a system rather than discrete undesirable effects. elements, in much the same way as the phasing and coupling system along with the 4.1 Synchronous AM towers themselves are treated as a system in Synchronous AM is one such AM directional work. Each has an effect on undesirable effect of narrow VSWR the other. The longer the transmission line, bandwidth and/or asymmetrical passband. the greater the effect it will have on the Consider a system which exhibits a very low performance of the antenna system, and not VSWR on carrier and higher VSWR values only in the area of losses. at the sideband frequencies. As the carrier is Long transmission lines introduce modulated and power shifts to the considerable phase delay in the transmitted

9 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/9

signals. While that is not a problem with for minimum synchronous AM. respect to the power traveling from the transmitter to the antenna, it is a problem 5.0 Antenna Designs when power is reflected back to the There are numerous FM antenna transmitter from the antenna. The reflected designs commercially available with power is delayed en route back to the power differing characteristics. Some amplifier, where it recombines with the commercially-available FM antenna types energy coming out of the power amplifier. It are the panel with crossed dipole, ring, ring- doesn’t take a lot of imagination to see how stub, slanted dipole and “rototiller.” an on-channel signal with an unknown phase relationship and instantaneous 5.1 Ring Radiator deviation can muck up an FM signal. In the early days of FM Because of this delay effect, with broadcasting, several variations of the ring long transmission lines it is even more radiator were designed for both horizontal important to maintain good VSWR and vertical polarization. It was found that bandwidth. In fact, some engineers insist on by adding vertical stubs to the ends of the sideband VSWR of 1.08:1 or less out to 130 radiator, it was possible to produce elliptical kHz either side of the carrier in situations polarization. It is difficult to produce true where long transmission lines are employed. circular polarization with a ring or ring stub I mentioned above the reflectometer radiator because the axial ratio varies with method of tuning an FM antenna. A much azimuth. Over the years, the design has been better method employs a signal generator improved by adding a second ring. One and impedance bridge. An even better advantage of this type of radiator is its method makes use of a network analyzer. It excellent circularity in free space. A is only by making an impedance sweep of disadvantage is that it is a high-Q design and the entire passband and plotting the results is more susceptible to detuning because of on a Smith chart can one accurately evaluate icing. the overall VSWR bandwidth and performance of an antenna system. 5.2 Ring-Stub Radiator Without specialized test equipment A variation on the ring radiator is the available, it is sometimes possible to get at ring-stub. The horizontally-polarized least a rough idea of the VSWR bandwidth radiation is produced by the ring portion of of an antenna by observing the screen the radiating element. The popular Shively current in the power amplifier. Screen 6814 is an example of a commercially- current in some amplifier designs is a direct available ring-stub design. indicator of amplifier loading. Modulation activity in the screen current is often an 5.2 Slanted Dipole indicator that VSWR bandwidth is less than The slanted dipole antenna was optimal. Checking synchronous AM noise is developed in the 1970s. As its name implies, another good indicator, provided that it is it consists of two half-wavelength slanted known that the transmitter tuning (especially dipoles bent 90 degrees. The two dipoles are in the IPA and PA grid) has been adjusted fed in-phase. The ratio of V-pol and H-pol

10 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/10

radiated power is determined by the slant controllable azimuth pattern and better angle. Shunt feeding provides for equal bandwidth. Cavity-backed panel antennas currents in all four dipole arms, which provide a lot of control where directivity is produces excellent circularity. The Jampro important. PENETRATOR is an example of a slanted dipole antenna. 5.4 Gain With all these designs, antenna gain 5.3 Rototiller is achieved by vertically stacking multiple Another popular design, often called antenna elements or “bays” at full- or half- the “rototiller” because the radiating wavelength spacing on a common feed. A elements have a shape similar to that of the single circularly-polarized bay operating by blades of a rototiller, consists of two 2 itself usually exhibits a negative gain over a series-fed bent dipole elements, forming a half-wave dipole in free space, while two space phased, circularly polarized radiator. bays usually provide near unity gain. More This configuration in combination with the than two bays will exhibit a positive gain, relatively large diameter of the radiating and gains of up to seven are possible with an elements tends to produce good bandwidth antenna with 12 bays. and inhibit corona discharge. The Electronics Research (ERI) SHPX is an 5.5 Impedance Matching example of this type of antenna. Matching is an important consideration in FM antenna arrays. The 5.4 Panel Antennas simplest and perhaps most common method Panel antennas were developed in uses a VSWR tuner. This matching section the 1950s specifically for co-located is inserted between the transmission line and stations. They come in both flat and cavity- the antenna inter-bay line and consists of backed configurations. Both the flat frame variable capacitors at 1/8 wavelength and cavity are made of a large-diameter wire spacing along the main feeder at the antenna mesh with 4- to 12-inch openings, which input. This type of matching section is used electrically appears as a solid surface (but on many side-mounted antennas. It is produces very little wind loading). Two adjusted for minimum reflected power to the crossed dipoles are used as the illuminating transmitter. The net effect of this matching source for each panel or cavity. The two scheme is to place an impedance in parallel dipoles are fed 90-degrees out of phase to with the antenna impedance to match it to produce circular polarization. This type of the 50-ohm line. The disadvantages of this antenna is typically side-mounted on a method are that it is bandwidth limited and large-aperture tower. Because they are it produces standing waves within the inter- directional, panel antennas typically require bay line. three or four panels to be mounted around Another matching scheme involves the tower to produce a circular horizontal the use of dielectric or metal slugs on the radiation pattern; circularity on the order of main transmission line inner conductor. This 2 dB is achievable. The cavity-backed type of matching scheme produces better design offers better axial ratios with more bandwidth, although it is more difficult to

11 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/11

adjust. Air-dielectric rigid line is, as its Other more complex schemes utilize name implies, a fixed shape that is not phase quaderature compensation to achieve intended to bend. It generally comes in 20- extremely wide bandwidth. This type of foot flanged or unflanged sections. Sections matching scheme is more typically used on that are intended to go on the tower are master antennas fed by multiple co-located generally equipped with factory-installed stations. EIA flanges. Sections that are used for inside RF plumbing, such as between the 5.5 Deicing transmitter, RF switch or patch panel, Deicing equipment, either in the combiner and gas barrier are often supplied form of electric heat or radomes, tends to without flanges. Field flanges are available increase the complexity, cost, required which are attached by silver-soldering or maintenance and weight of an antenna. with hose clamps. A wide variety of When ice forms on an antenna, the resonant adaptors, including reducers, elbows and 45- frequency of the antenna tends to go down. degree sections are available to allow A narrowband antenna with even a small routing of rigid transmission line to virtually amount of ice will present an unacceptably any location within a transmitter building. high VSWR to the transmitter, possibly Semi-flexible line, aptly named leading to damage not only to the transmitter because of its somewhat limited bending but also to the transmission line the antenna capability, comes in either air-dielectric or itself. Deicing equipment keeps ice from foam-dielectric types. In the case of air- forming on the radiating elements, thus dielectric line, the inner conductor is held in preventing this detuning. Broadband place with a Teflon spiral. With foam- antennas, while detuned by ice just as more dielectric semi-flexible line (available in narrowband designs are, have sufficient sizes up to and including 2-1/4”), a foam bandwidth that the detuning has little effect dielectric material completely fills the space on the load presented to the transmitter. between the inner and outer conductors, Keep in mind that high initial cost holding a more-or-less constant spacing may be offset by many years of very low between the two. Semi-flexible line comes maintenance costs. The converse is also on a spool in a continuous run and is true. When selecting an antenna, the best generally ordered to length. An EIA flange approach is to select the very best antenna gas-barrier or gas-pass connector is ordered that your budget can stand. After all, what for either end to complete the transmission other part of your transmission system has line run. more effect on the signal you present to your audience? 5.1 Pressurization Either type of air-dielectric line 6.0 Transmission Line Types requires a supply of nitrogen or dehydrated In the case of FM antenna systems, air to keep the line pressurized to a few p.s.i. there are generally two choices for above ambient. The purpose of the dry air is transmission line types: rigid and semi- to keep out moisture. Pressurization of the flexible. line implies that the line must be airtight.

12 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/12

EIA flange connectors come equipped with dielectric line, quite often a large section of rubber O-rings to provide an airtight flange- the line must be removed and a replacement to-flange seal. Connectors fitted to semi- piece spliced in. The cutting process allows flexible air-dielectric line are equipped with copper and Teflon shavings to fall into the special rubber seals that provide an airtight line below the cut, and these shavings can fit between the brass connector and the accumulate in one location and produce an copper line. arc, causing further damage. Quite often, the A method of getting the pressurizing only recourse in the case of a damaged air or nitrogen into the transmission line semi-flexible line is to replace it in its must be provided, and this generally comes entirety. in the form of a gas-barrier. The gas-barrier Semi-flexible line has the advantage of can either be integral in a semi-flexible line lower cost, sometimes as little as half the connector or a stand-alone unit with an EIA cost of a comparable rigid line. It also has flange on either end. A fitting is provided the advantage of being in a continuous run for connecting the pressurization tubing. as opposed to being joined with expansion It is absolutely critical that moisture connectors (“bullets”) every twenty feet. be kept out of air-dielectric transmission Expansion, contraction and vibration of a line. Moisture will cause oxidation in the rigid line cause chafing where the expansion copper, which increases attenuation at connectors join the inner conductors and this higher frequencies. Attenuation on the order often eventually results in a less than of 4 dB per 100 feet has been measured in 3- optimal connection. The I2R loss at the and 3-1/8” semi-flexible and rigid air connection causes heat, which further dielectric lines that have not been degrades the connection and eventually pressurized. This represents a tremendous produces a burnout. power loss, which translates directly to signal loss. 7.0 Transmission Line Selection A broadcast engineer designing an 5.2 Rigid vs. Semi-Flexible Line FM transmission system has a number of There are advantages and choices for transmission line size and type. disadvantages to both types of transmission The factors that enter into this decision are lines. Rigid line has the advantage of lower transmitter power, line length and antenna losses. This usually amounts to a few gain. All three factors are somewhat hundredths of a dB per 100 feet over a variable. It is not uncommon for a given comparable semi-flexible line. This can add ERP and antenna gain to have the up to a significant amount of power in the transmitter power level determined by the case of a long transmission line run. transmission line size and type. Changing to Another advantage that rigid line has is a larger or lower-loss line may permit the that it is more repairable than semi-flexible. use of a smaller transmitter, somewhat It can be removed from the tower in 20-foot offsetting the added cost of the better line. sections, the inner conductor can be The other side of that coin is that the larger removed and both inner and outer can be line may require a beefier tower to support cleaned. In the case of semi-flexible air- it, adding cost to the project. Usually,

13 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/13

however, the choice is a little clearer. 7.2 Line Loss 7.1 Power Rating With a likely candidate selected, find The first step in finding a suitable the loss of the line at the operating transmission line for a system is to frequency and line length. Manufacturers determine the approximate power the line typically provide this information in dB per will have to carry. This is calculated by 100 feet or 100 meters. Divide the line dividing the effective radiated power (ERP) length in feet or meters by 100 and multiply in kW by the antenna power gain, or by by the per unit loss to get the total line loss subtracting the antenna gain in dB from the in dB. ERP in dBk and converting back to kilowatts. This calculation will yield the 7.3 System Calculation antenna input power, or the transmitter To find the transmission line input power output less the line loss, which is as power (or transmitter power output, TPO), yet unknown. While the antenna input add the line loss in dB to the antenna input power does not represent the total power in power in dBk. Convert back to kilowatts by the line, it is a good starting point. Keep in dividing by ten and taking the antilog. The mind that in combined systems, the power in loss of the line in kilowatts can be the line is the sum of the power of each determined by subtracting the antenna input station. power from the transmitter power output. Next, go to the manufacturer’s Divide the antenna input power by the TPO catalog and find a line that exceeds the and multiply by 100 to calculate the calculated line output power by a good efficiency of the line in percent. margin, keeping in mind that the input Table 1 below shows an example of power (which is the maximum power the a system calculation. The system specifies line will have to carry) will be somewhat an ERP of 50 kW, a four-bay antenna, and higher. While any of a number of 500 feet of line at 100 MHz. transmission lines may have an adequate power rating, there will be few choices that offer both acceptable performance and Table 1 economy. A good rule to follow when selecting a transmission line is to use the ERP 50.00 16.99 smallest line possible consistent with the kW dBk required power handling capability. There Antenna Gain 2.133 3.290 dB are certainly exceptions, such as in the case Antenna Input Power 23.44 13.70 of a very long transmission line, where it kW dBk may be more important to keep losses (and Line Loss 0.851 0.700 dB thus long-term operating costs) down than Transmitter Power 27.54 13.40 the initial investment. As a rule, however, Output kW dBk the goal should be to insure adequate performance while minimizing cost. Rounding in accordance with 47 C.F.R. §73.212, the operating TPO becomes

14 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/14

27.5 kW. transmission line installation in many cases. A bag of cable ties or a box of Wraplock in 7.4 Safety Factors the tower rigger’s pouch is deemed The next question becomes one of sufficient to secure the line to the tower. headroom. Is a 3-inch line with a 42 kW Most radio engineers have seen transmission power rating adequate? To find the answer, line installations where little more than we must derate the line for VSWR. To do electrical tape or cable ties was used to this, simply divide the power rating of the secure the line. line by the worst-case VSWR. A good While such inexpensive measures will work number to use to account for possible icing in the short term, over a period of time, of the antenna is 2:1. In our example, the trouble will likely come as a result. Cable derated average power capability of the line ties deteriorate with exposure to sunlight for 2:1 VSWR is 21 kW. Is this reasonable and extreme temperatures, becoming brittle, for a 27.5 kW input power? breaking and falling off. Thermal The answer to that question is expansion/contraction as well as other determined by estimating the risk of the differential motion between tower and line VSWR ever getting to that point. Are the can cause Wraplock to chafe against the protective circuits in the transmitter outer jacket, often resulting in the outer reliable? Are there deicers or radomes on the jacket and eventually the outer conductor antenna, and are they effective and reliable? being cut through. Once this happens the In most cases, you can count on your inside of the line is exposed to the elements. protective and deicing systems to do the job. It is not wise, however, to put your 8.1 Installation Hardware eggs all in one basket. An outboard VSWR Transmission line manufacturers or reflected power monitoring device is a offer a wide array of mounting hangers, good investment to protect your very brackets and hardware that are designed to expensive transmission line. The trip point protect lines from differential motion of the device should be set below the level problems and keep them working properly that would exceed the rated average power for many years. In addition to the hardware, of the line with full transmitter power line manufacturers publish specifications for output. In our example, the line is rated at 42 mounting hardware, including recommended kW. Divide that by the TPO of 27.5 kW to spacing between hangers for different wind find the maximum VSWR that will derate load and radial ice values. The published the transmission line to the TPO. In our values are not simply theoretical numbers or example, this VSWR is 1.53:1. The idea is designed to promote the sale of hardware. to make sure that in the worst case  with They have been derived from extensive the transmitter running at full power and empirical data  including wind tunnel some sort of antenna problem  the line testing  and are based on the EIA RS-222 will be protected. standard (Structural Standards for Steel Antenna Towers and Antenna Supporting 8.0 Transmission Line Installation Structures). Little thought is given to proper

15 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/15

tabs to which hangers can be directly bolted. 8.2 Environmental Factors This is the simplest means of attachment, Where a tower is located has an and provides for very secure mounting. impact on the installation of a transmission Another means of mounting utilizes line on the tower. Values of design basic hose clamps or some other means of leg wind speed, the maximum wind speed at a attachment. The hose clamp, sometimes height of 10 meters over open terrain, are called a “round member adapter,” clamps to published for counties and states in EIA the tower leg and to the hanger itself TIA-222-E. This is a good place to start through a slot in the hanger. When utilizing when considering what hardware to use in a this attachment method, especially with particular installation. larger diameter transmission lines, some The maximum amount of radial ice means must be provided to get around the accumulation is another factor that must be flanges where tower sections mate. carefully considered when designing a Specially made standoff kits are available transmission line installation. Some from line manufacturers to provide a means locations, particularly those in the southern of getting the line securely past flanges tier of states, are especially prone to ice without allowing the line to chafe against storms and large accumulations of ice on the flanges. tower structures and attachments. Warm, Hoisting grips are “Chinese moisture-laden air rides up and over cold handcuff” devices that are designed to surface air and falls as rain. When the securely attach to a transmission line and supercooled raindrops impact the surface pull it up the tower without stretching or and objects on the surface, they instantly distorting the line. Typically, one hoisting freeze. It is not uncommon to have 1” or grip should be used every 200 feet to spread more of radial ice build up on a tower out the load. This helps keep the weight of structure, its antennas and lines in such the line on the load line and off the line circumstances, greatly increasing the dead itself. Once the line is in place on the tower, weight and cross-section (and thus wind the hoisting grips are secured to the tower to loading) of the tower, antennas and lines. provide permanent vertical support. With the design basic wind speed and maximum expected amount of radial ice 8.4 Grounding in hand, the manufacturer’s installation It is very important to “ground” a charts can be consulted to determine the transmission line to the tower at both top type of hanger which should be used and the and bottom and in some cases at several recommended maximum hanger spacing for locations along the line’s length. When a particular line. The quantity of hangers can lightning hits the tower, high-level currents then be calculated. will flow down the tower in all the available parallel paths, including in the outer 8.3 Installation Methods conductors of transmission lines. Because The next step is to determine how copper has a lower DC resistance than the the hangers will be attached to the tower steel of the tower, greater currents are prone structure. Many towers provide mounting to flow in the transmission line outer

16 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/16

conductors than in the tower steel. This will provided on each spring hanger to prevent often result in significant potential horizontal motion. A spring connects the developed between the transmission line collar to a clamp that is affixed to the line outer conductor and the tower itself, below the collar. The manufacturer provides resulting in arc-through of the transmission recommended settings for the springs to line outer jacket and pitting of the insure that the correct amount of tension is transmission line outer conductor. In applied to the line at each location. The extreme cases, the pitting will actually purpose of the springs is to allow differential penetrate the outer conductor, opening the motion between the line and tower structure line to the elements and eventually because of thermal expansion. A rigid destroying the line. Grounding or “bonding” copper transmission line exhibits the transmission line to the tower at frequent considerably more thermal expansion than a intervals is a good means of keeping the steel tower. If this is not allowed for, the line potential between the line and tower low and will buckle. preventing such damage. 9.0 Maintenance 8.5 AM Tower Installation In most cases, once a transmission AM towers are a special case. line and antenna have been installed on a Depending on the feed system, there are tower, little thought is given them until several methods of installing a transmission something goes wrong. That is unfortunate, line on an AM tower. In grounded-base because they do require a certain amount of (skirted or shunt-fed) towers, the method is maintenance to keep them reliable and essentially the same as with any other tower, operating at peak efficiency. except that more frequent bonding of the line to the tower should be done to minimize 9.1 Visual Inspections RF arcs through the line jacket. Regular mechanical maintenance of There are two basic means of an FM antenna system should include a installing transmission lines on insulated- complete visual inspection at least annually. base AM towers. If an isocoupler or isocoil This can be done in conjunction with is used, installation above the base insulator relamping or other tower maintenance to is the same as for a grounded-base tower. If minimize costs. Excitation must be removed a quarter-wave stub is used, the line is from the antenna and other steps taken to installed using insulated hangers. In that insure that the tower is safe for workers to case, the line is not bonded to the tower at climb and work in the vicinity of the any location below the shorting stub. antenna. The tower worker making the visual 8.6 Rigid Line Installation inspection should look at each element in Rigid transmission lines are another the array for proper installation; for burns or special case. Rigid lines are fixed to the pitting on the element ends; for cracks, tower at the top of the run and mounted in carbon traces or defects in insulators; and spring hangers for the remainder of the for any evidence of abnormal mechanical vertical run. A nylon-jacketed collar is stresses. Weep holes in the antenna

17 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/17

elements, if any, must be oriented so that tightened and any corrosion should be they face down. removed. In the case of rigid transmission 9.2 Deicer Maintenance lines, the length of each hanger spring If the antenna is equipped with should be checked against the deicers, they should be turned on during the manufacturer’s recommendation. If inspection. The tower worker should then necessary, the springs should be adjusted. verify that the deicer in each element is Nylon buttons or bushings in the spring working by feeling of the element. hanger collars should be checked. Should One of the most dangerous icing these fall out or become damaged, the situations for an FM antenna occurs when copper transmission line will chafe against all the elements’ deicers are working except the metal of the hanger. This will eventually one. All of the elements except that one wear through the copper of the line, leading remain clear of ice, and a serious mismatch to line failure. occurs in the interbay line. Because the Rigid and semi-flexible lines should mismatch is limited to that one element, the be checked for “hot spots” near flanges and transmitter’s VSWR foldback circuit may connectors. In the case of rigid lines, this is not see sufficient reflected power to cause it every twenty feet. An infrared thermometer to activate. With full power being fed to the can be used for this, or the tower worker can antenna, an arc can occur and sustain itself simply feel of the joint and compare it to the until serious damage has been done to the adjacent portions of the line. Localized antenna. From a protection standpoint, it is discoloration in rigid lines is a good better to operate with no deicers than with a indicator of abnormal heating. The cause of partially operating system. any significant heating should be A good deicer maintenance tool is a investigated, as heating at a joint is a permanently-installed AC ammeter on the precursor to a line failure. Such heating is deicer circuit. With the deicers operating often caused by a “split bullet” or chafing in normally (and after a ten-minute warm-up the inner conductor where it makes contact period), mark on the meter face the nominal with the “bullet.” The heating is evidence of current. From time to time, the deicers I2R loss in the joint, which is power not should be turned on and allowed to warm being radiated. In many cases, heating in the up. The current can then be checked against joints can quickly produce a thermal the mark. If there is a deviation, a tower runaway situation leading to catastrophic worker should be dispatched to investigate. failure of the line. One tool available today that is of 9.3 Transmission Line Inspection great help in locating hot spots in a Transmission lines should likewise transmission line or antenna is infrared be visually inspected annually. The photography. Hot spots will show up as a connections at the top and bottom should be lighter color. The advantages of this method inspected for security, and any evidence of are that it can be done from the ground and movement should be investigated. Ground it can detect subtler temperature rises. While connections should be checked and IR photography has several advantages, it is

18 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/18

not a substitute for a good visual inspection. handling capability) of the line. If moisture is allowed to enter the line, corrosion will 9.4 FM on AM Inspection result, producing higher losses, hot-spots In some situations where an FM and eventual line failure. The pressure antenna is mounted on an AM tower, the integrity of the line and antenna should be transmission line will be mounted using checked by closing the air valve feeding the insulated hangers. Each insulator should be line at the bottom of the run and observing inspected for cracks and carbon traces. The the line pressure over a period of several shorting stub, usually located somewhere hours. If the pressure bleeds off, there is a close to 90 electrical degrees (at the AM leak and it should be located and repaired. frequency) above the tower base, should be An adequate nitrogen supply should checked for a good electrical connection at be maintained at the site to insure that the the line and the tower. supply is not exhausted before another Other FM on AM situations employ delivery can be made. If the pressure an isocoupler to cross the base insulator integrity of the line is maintained, very little with the FM transmission line. In these nitrogen should be used. Pressurization cases, it is important that the transmission equipment should be regularly inspected, at line be electrically bonded to the tower least weekly, and maintained in good frequently along its length. Each bond working order. Desiccant should be rotated should be checked for good electrical out and dried or replaced whenever it begins connection. In addition, the entire outer to turn from blue to pink. A dehydrator with jacket of should be checked for evidence of a saturated desiccant pumping moist air into arc-through to the tower. If such evidence is a line is almost as bad as having no present, it likely indicates that more frequent pressurization at all. The use of a self- bonding is needed. Such arc-through, if recharging dehydrator will prevent this from permitted to continue, can eventually ever happening. penetrate the outer conductor of the line and open the line to the elements. Grounded-base AM towers, such as 9.6 System VSWR Monitoring folded unipoles and shunt-fed antennas, Finally, a check of the system likewise require the FM transmission line to reflected power should be made during be frequently bonded to the tower steel every site visit. A significant change, up or along its length. The same checks of the down from the nominal value, should be bonding and outer jacket should be made on investigated. A more sensitive indicator of grounded-base AM towers. load impedance in some power amplifier designs is the screen current. Higher screen 9.5 Pressurization Maintenance current generally indicates a lighter (higher Proper pressurization of air- Z) load. If PA tuning is otherwise correct, an dielectric lines is critical to their increase in screen current may be an performance. Pressurization with dry air or indicator of a developing antenna or line nitrogen has a direct effect on the problem. breakdown voltage (and thus the power In addition to checking the reflected power,

19 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS FM Transmission Systems Alexander/19

the VSWR foldback and trip circuits in the connections. A reflected power meter that transmitter or external protection device reads zero all the time may be an indication should be checked from time to time to that the connection, either between the slug insure that they are working. Transmitter and the coupler or between the coupler and manufacturers usually provide a procedure the transmitter, is bad. If that is the case, for checking and adjusting the internal chances are there is no VSWR protection in protective circuits. place, which can lead to a catastrophic If the directional coupler(s) use failure of the line/antenna and damage to the removable slugs to detect forward and transmitter. reflected power, they should be checked regularly for proper operation and good

20 ROCKWELL MEDIA SERVICES, LLC • FM TRANSMISSION SYSTEMS