Selecting an FM Antenna


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Selecting an FM antenna for use at a facility is much like choosing any other component: Some cost more than others, some are of higher quality than others, and some have different technical characteristics that lend themselves to different applications better than others. Regardless of which antenna you select, the best quality antenna you can afford should be chosen as this is the last item in your transmission chain, and the last point over which you have control of your signal.

Factors to be considered before making a purchase include local climate, effective radiated power, transmitter power, tower loading, population concentrations in your coverage area, directional patterns, and non-ionizing radiation protection. Your consulting engineer together with your sales representative are valuable resources to aid in the decision.

Installing an FM antenna

The accumulation of ice on an antenna will affect the match presented to the transmitter. It does not take much ice to present a load that will cause foldback or even VSWR trips. Icing tends to be most frequent in areas such as the plains and Ohio valley states. De-icers or radomes will limit the effect of ice accumulation. De-icers, which do not really de-ice but only aid in preventing ice from accumulating, are both costly and maintenance intensive. Radomes on the other hand do not fail unless physically broken, but can add a substantial amount of loading to a tower. Southern coastal areas do not require either for ice protection, but the addition of radomes may be desirable in some cases to limit element corrosion from salty air.

Power determination

Stations are of course licensed by their effective radiated power. For a given ERP there are numerous different combinations of transmitter power and number of antenna bays that will achieve a specified effective radiated power. Larger antennas wind up costing more initially, but the long-term backcharge is less due to lower electricity consumption by the transmitter. While it may seem like a good idea to go with a large antenna in order to reduce the transmitter power, such a course of action can have detrimental consequences.

For instance, a 100kW station may wish to select a 12-bay full-wave antenna in order to keep the transmitter power around 11-12kW or so. As the number of bays increases on an antenna, the beamwidth in the vertical plane drops drastically. A 12-bay antenna will have a half-power beamwidth of a little more than 2 degrees with much of the vertical plane relative field below the beam at less than 0.2. The result is that if your site is close to your population center you may overshoot the desired area resulting in spotty coverage. On the other hand, if your site is roughly centered between several cities located 30-40 miles out, as is the case with one group in the plain states, then a large number of bays is perfectly acceptable since the local fauna most likely is not listening to your station.

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