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FM Interference to Cell Tower Systems

R

rfry

Someone on another board posted a problem about his 90 kW ERP FM interfering with a ~900 MHz cell system about a mile away.

After a bit of of investigation I posted the response below which might be of interest here, also...
_______________

The high field on the FM fundamental could have generated harmonics including the 9th in/near the cellular equipment itself.

In any case, the elevation pattern of your 6-bay, 1-wave spaced FM antenna on its ninth harmonic is not well-behaved. Any r-f on the 9th harmonic that reached the antenna would be radiated in many, very narrow lobes any one of which would take a lot of (bad) luck to hit a specific target.

That pattern is shown in the link below. The jagginess of the waveform is due to my calculation at only 0.1 degree intervals.

http://i62.photobucket.com/albums/h85/rfry-100/6-bayOn9thHarmonic.gif

RF
 
Most interesting. Given the amount of grief cell systems give our microwaves, I'd be hard pressed to rush myself to rermoving a harmonic - even if it exceeded -80 dB. Someplace I have the sripcharts and plots of a G-5 6bay on a tall tower we had measured using an FIM-71 and one of the VHF antennas on the aircraft - a JetRanger. What we got looked just like what E.R.I. said we would get. So I've pretty much takent heir word for it since then. The best part of the exercise was the look I got from the pilot when I asked him to do me a constant rate descent over a point to do the vertical pattern. (I knew zilch about helicoper dynamics at the time, I'm a fixed - wing pilot).
We compromised, and did the vertical using a constant rate ascent over a point instead!

We did measure a couple of existing FMs the same way as a part of due diligence, which left me feeling like a couple of the antenna manufacturers didn't quite have the idea yet; or the array needed looking at. The one we wound up buying got taken down posthaste in favor of something newer and better.
 
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