FM Antennas: The Silent Component

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Another scenario

Let's look at our second hypothetical scenario. In this case our project is to come up with an antenna that will be used to combine four FM stations in to a common antenna, for purposes of a backup transmitter site. This is a low-to-mid power application; all four stations will have an ERP of around 7kW.

Shively 6832. Photo: Mike Fitzpatrick/NECRAT.US

Shively 6832. Photo: Mike Fitzpatrick/NECRAT.US

In this application I'll use the Shively 6832 series. The specs from the website show that the six-bay array has a power gain of 3.024 at 98MHz. (I spoke with Shively regarding this application and was told that the power needs to be limited to 2.5kW per station.) The antenna input is an EIA 1-5/8" flange. A quick check of Andrew HJ7-50A (1-5/8" coax) indicates that it would handle this power at a single frequency. I would seek advice from the antenna manufacturer in this case to be sure that the connector size and the coax can handle the four combined RF signals. Research indicates that the coax length in this case is 150', so again referring to the HJ7-50 data, the efficiency of this line is 93.3 percent at 98MHz. So, after running the same set of calculations we did earlier, we find that we'll need about 2.5kW of TPO. (Since this is a wide-band system, the power we need will vary slightly with frequency.) Again, using data supplied on the website, you can see the necessary aperture is 40.8' and that the recommended space needed is 60.8'. You can also take the weight and wind-load figures to help your structural engineering consultant determine whether or not the tower will safely hold this antenna.

Clearly if you were to plan a system such as this you would need a four-port combiner. One possibility is the Shively 2640-04-1/1. 5kW per input (max) and 15kW through the output (max) seems appropriate in this scenario.

Jampro JHPC

Jampro JHPC

I've highlighted Shively in this hypothetical scenario, but there are other options, of course. As one example, you could opt for the Jampro JPCB in a multiple-element array; or (although it would be overkill for this power level) you could consider a Dielectric DCR-S series wide-band antenna array, or the ERI Rototiller Axiom series.

Jampro makes combiners as well, such as the RCCC-X9X for this power level. ERI is also very well known for its combiners. Again, at this power level, you could opt for the 955-6 (if stations are spaced by at least 1MHz) or the 955-8 if the spacing is down to 800kHz. Options with Dielectric include the DFC14005CIF (for 800kHz spacing). Myat makes combiners as well; at this power level and with 800kHz spacing, an option would be the CSFMBP8400CZ, which is a four-cavity filter (per station) constant-impedance combiner, for 8kW per station.

- continued on page 5

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