FM Antennas: The Silent Component

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If you were to consider for a moment the question: "What is the most important part of the transmission system?" likely the first answer you'd come up with is "the transmitter." That's not really true, though, is it? The STL, the transmitter, transmission lines, and the antenna are all equally important. With the failure of any one of those, your station will be off the air. I'll focus on the FM antenna in this installment, and in doing so I'll consider three separate scenarios that would prompt you to buy a new one: the installation of a new main antenna, the installation of a broad-band antenna as part of a back-up system, and a low-power booster (or translator) antenna.

Photo by Martyn Gregory, Shively Labs.

Photo by Martyn Gregory, Shively Labs.

There are many questions to be answered before purchasing an antenna; some are the same no matter which of the three scenarios are being considered, and some are of course specific to the scenario. Here are some common questions:
■ What is the required effective radiated power (ERP)?
■ What is the available transmitter power output (TPO)?
■ What are the necessary downward radiation characteristics?
■ What is the necessary polarity (i.e., circular, vertical, horizontal)?
■ What is the available space on the structure?
■ How high will the antenna be above the ground?
■ Will the structure have the strength to hold the antenna in all conditions?

So, as you can see, some amount of research needs to be done even before you issue any kind of a request for quotations. Let's consider these questions in more detail.

Replacing an old antenna

The ERP vs. TPO question is easy to answer in the event that you are simply replacing an old antenna, and not changing transmitters. As an example: If you needed 17.5kW TPO for 50kW of ERP (with the old antenna power gain of three, and accounting for transmission line loss) then likely you'll be getting a new antenna with the same amount of power gain and similar physical characteristics, such as the amount of space needed on the tower. In this event you would still want to make sure that the weight and wind-load characteristics of the new antenna are acceptable for the tower, and that the downward radiation characteristics are the same or better than the old antenna.

- continued on page 2

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