Understanding FM Systems: A New Series
     

With this article, we begin an in-depth look at FM transmission systems.

As we were wrapping up a previous series on the shared use of AM sites, we asked engineers to list RF-related topics they would like to see covered at length in a technical series. The answer overwhelmingly was FM transmission systems.

In this series, we will cover everything from the transmitter to the receive antenna. As we progress, we welcome reader input; there is little reason why we could not dwell on or revisit certain areas of high interest.

As we begin, we will focus first on antenna site considerations. While few of us have much control over the location of our antenna sites, perhaps there is room for change in some situations. For the rest, the information that we present here will help us evaluate the performance of our radio stations as a function of site location and antenna height.

Sufficient amplitude

Location, location, location. Those are the three most important factors in real estate, and they are equally important for radio transmission systems. This applies to AM, FM, TV, MMDS, cellular, PCS, two-way, paging and other RF-based services.

Ideally, the antenna for an FM broadcast station would be situated at a location that presents a clear line of sight to the entire desired service area. The antenna would have uniform horizontal- and vertical-plane radiation patterns, with no reflections from natural or manmade objects.

Unfortunately, the real world is different, full of obstructions, manmade and natural, that partially or fully obstruct the path from the transmitting to receiving antenna.

Real-world transmitting antennas exhibit some non-uniformity in the horizontal plane, and in the vertical plane, half of the energy is radiated above the horizon into space, wasted. Reflections from objects produce amplitude variations in the received signal that cause noise and signal dropouts.

The variables that go into the performance of a particular antenna site are numerous, and many of these factors are beyond the broadcaster's control. Many can be mitigated, however, with good site selection, and it is on those that we must focus when searching for an antenna site.

The goal of the broadcaster is to produce a signal of sufficient amplitude to overcome noise and provide at least 20 dB of signal-to-noise ratio, at as many of the receiver locations within the desired service area as possible.

How much signal is sufficient to meet this goal depends largely upon the receiver and its antenna. In the absence of interference, a signal level of as low as 2 uV/m may be sufficient for many of today's automobile receivers. Portables may require as much as 500 uV/m. Interference from co- and adjacent-channel stations usually increases the amount of signal required for acceptable reception.

Clear shot

There is no substitute for a clear line of sight between the transmit and receive antennas. This is one of the first rules in VHF transmission.

A transmitter site with a clear line of sight to virtually all the target service area thus is superior in most cases to one that is blocked by terrain or manmade obstructions to parts of the area. In some cases, simply having line of sight is not enough. In engineering our microwave and UHF STL paths, we always consider Fresnel zone clearance, knowing that a path with less than 60 percent first Fresnel zone clearance will be marginal. We often neglect this consideration in engineering our FM transmitting antenna locations.

Fresnel zone clearances are circular areas surrounding the direct line-of-sight path that vary with frequency and path length. The longer the path and lower the frequency, the larger the mid-path clearance required for clear-path reception. As mentioned, 60 percent first Fresnel zone clearance is all that is required to meet the clear-path reception objective; but that can be quite large at FM frequencies.

The first Fresnel zone radius can be computed using the formula R = 1140?d/f, where R is the radius in feet, d is the path length in miles and f is the frequency in MHz.

A quick example of 60 percent first Fresnel zone radius for typical broadcast situations are 267 feet for a Class A, 378 feet for a Class B and 463 feet for a Class C1. Keep in mind that we're talking about terrain clearance at the mid-point between the transmitting and receiving antennas required to produce clear-path reception.

These translate to antenna heights above ground of 534 feet, 756 feet and 925 feet respectively. With the exception of the Class C, the antenna heights are well above the maximum height above average terrain (HAAT) values for the classes.

This brings us to the conclusion that height is a significant factor in most antenna site situations. As a rule, greater height is more useful than higher power in producing higher receive signal strength, all other factors being equal.

Out of phase

Multipath is a nasty word in the vocabulary of most radio engineers and station managers.

It is a good descriptor of the destructive effect that occurs when the same radio signal arrives at a receive point by multiple paths.

When these signals arrive in phase, for the most part all is well and the incident field strength is greater than it would be in the case of a single signal path. When they arrive out of phase, however, at least some degree of cancellation takes place, resulting in a reduced incident field strength, with complete cancellation (zero incident signal) in the worst situations.

To make matters worse, sometimes complete cancellation can occur on frequencies close to carrier while less-than-complete cancellation takes place on sideband frequencies. In many cases, this results in a demodulated sound much more offensive to the listener than the quiet hiss of no signal. Motion in an automobile produces a constantly varying multipath situation, often causing picket-fencing (the effect of the slats in a picket fence alternately permitting and then blocking the signal), which is objectionable to the listener.

The worst-case multipath scenario occurs when the transmitting site is on one side of the service area and a range of mountains or high hills is located on the other. Receivers within the service area get the direct line-of-sight signal from the transmitting antenna, but they also get a reflected signal from the mountains or hills. In such a case, few locations within the service area are free of multipath effects.

Perhaps the best location for a transmitting antenna in such a geographic scenario, assuming that a mountaintop location is out of the question, is on a hill near the mountain range. A directional antenna would be used to reduce radiation toward the mountains and maximize it toward the service area.

This will result in greatly reduced reflections. While it would be impossible to eliminate reflections completely, they could be reduced so that the ratio of direct-to-reflected signal at most locations throughout the service area is sufficiently high to nullify the effects of multipath.

Grazing and tilting

Ground reflections play a part in the overall propagation of FM signals, particularly the vertically-polarized component.

Almost all FM signal coverage lies between the horizon and 10 degrees below the horizon. This is called the grazing angle, and it lies between the horizontal plane from the transmitting antenna and the earth's surface. Vertically-polarized energy is attenuated considerably more than horizontally-polarized energy at angles greater than about 2 degrees. As a result, circularly-polarized signals tend to be reflected more as elliptical rather than circular. It is important in site selection to avoid grazing angles that are greater than about 2 degrees (the Brewster angle). Simple geometry would suggest that sites close in to the service area would be more prone to produce such high grazing angles, indicating that a more-distant site may be preferable.

We mentioned the vertical-plane radiation characteristics of real-world transmitting antennas. Some of these characteristics come into play when selecting a transmitting antenna site.

If a transmitting antenna is at a considerable height above the target service area, the main elevation plane lobe may overshoot the target service area, with the energy being radiated out into space. The more bays an antenna has, the narrower the main elevation plane lobe will be. Antennas with a small number of bays (less than four) exhibit a broad elevation plane lobe, making such overshoot of the target service area less likely.

In those situations where a large number of bays is used and the antenna is high above the target service area, it may be desirable to employ beam tilt to lower the beam angle slightly. Typically, just enough beam tilt is used to center the main elevation plane lobe on the distant edge of the target service area or on the horizon, whichever is closer. We will discuss beam tilt in more detail, but we mention it here because it does affect site selection.

Antennas with many bays exhibit elevation plane nulls. The more bays, the farther away from the antenna site that these elevation plane nulls hit the ground. If the area within a few miles of the antenna site is populated and you wish to provide service to this area, it may be desirable to employ null fill. A very small amount of null fill is all that is necessary to provide adequate service in these close-in areas. We will discuss null fill later as well.

When searching for a new antenna site location, consider these factors in addition to the permissible area to locate determined by the allocation. In areas where the allocation picture is tight, you may have few choices for an antenna location and you may have to compromise on one or more of these criteria. In areas where you have some breathing room, you may well have several choices. In nearly every situation, some compromise is necessary; trade-offs are inherent in site selection.

If you are evaluating your existing site, chances are that you are well acquainted with existing signal problems. Perhaps looking at your site in light of these site selection factors will help you understand the site's shortcomings.

In the next installment, we will deal with the question of transmitter power vs. antenna gain.

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