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FM Antennas: The Silent Component
Our third scenario involves a different (though still common) type of antenna -- the log-periodic array. This type of antenna is an option in (Part 74) booster (or translator) applications, not only because of the gain, but the directivity as well. Often times in booster applications, you will seek to pinpoint the RF power in the desired direction, while keeping it from going where you don't want it to go (to the extent possible). Probably the most well known manufacturer of this antenna type is Scala (a division of Kathrein) and its version is known as the CL-FM. You can mount the CL-FM in either polarity easily enough, and perhaps even more importantly, you can design multiple CL-FM antenna arrays (with the help of Scala Engineering) for custom antenna patterns (varying the power between elements, as well as the phase angle between elements). The CL-FM also comes in a 1kW input version. Scala will also provide power dividers for multi-element arrays.
Just as with the other antennas we've talked about, you'll need to perform the same set of calculations before using the CL-FM. Power gain is five, and Scala's specifications give the weight and wind-load.
As you might expect, Scala is not the only manufacturer of log-periodic antennas: Shively offers its 6025 which can actually handle up to 5kW (7/8" EIA flange on the input); and, Jampro makes a log-periodic that it calls JAVA, also with 7/8" EIA flange, rated at 5kW max.
I've pointed out example products from the big four antenna manufacturers but it's important to remember that there are even more makes, especially in the lower-power categories. While pricing antennas, take a look at these other manufacturers as well: Propagation Systems Inc., Systems With Reliability, Armstrong, OMB, and Scala's parent company, Kathrein.
It's difficult to over-emphasize the importance of the passive elements of a transmission system. There can be many variables that need to be considered when deciding upon the correct one, as we have seen. The antenna is the final, vital link between you and your audience and as such, you should put as much attention and financial resources that you can towards its selection, purchase and installation.
Irwin is RF engineer/project manager for Clear Channel Los Angeles. Contact him at email@example.com.
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