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FM Transmission Systems

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FM Transmission Systems 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).
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