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Re: [TowerTalk] Nasty RF Problem. Advice Please

To: "CARY GOOCH" <>,"'Daron J. Wilson'" <>, <>
Subject: Re: [TowerTalk] Nasty RF Problem. Advice Please
From: Jim Lux <>
Date: Sun, 10 Sep 2006 11:59:48 -0700
List-post: <>
At 09:35 AM 9/10/2006, CARY GOOCH wrote:
>It is too difficult for me to describe the FM antenna, I do know they
>are the nasty ones you don't want to sit on.
>I will find a picture so I can describe it better.
>But anyway, if they are circular polarized, (thinking point light source
>in my head) then I will not have any quiet spots.

CP doesn't have anything to do with omnidirectionality. It's fairly 
straightforward to come up with a dish antenna that has a beamwidth 
much narrower than a degree, but is CP. For example, the 70 meter 
Deep Space Network Antenna at Goldstone has a beamwidth of a few tens 
of millidegrees at X band, and is circularly polarized.

Many broadcast antennas are CP (or elliptical) because it helps with 
multipath, but the pattern is still "disk" shaped (not much point in 
radiating straight up or down)

>I tried shielding the antenna with the steel top off a 55 gallon drum
>while a co worker watched the signal, no difference.

The 55 gallon drum top is much less than a wavelength at FM broadcast 
band frequencies (3m, give or take), and essentially is 
invisible.  The signal will propagate right around and through it.

>I still think this was not too stupid an approach, since fm will not
>bend too well around a corner.

You'd need to have something that is many wavelengths on a side to 
effectively block the FM.

You *might* be able to work out something that is resonant and 
happens to create a "quiet spot", much like the resonant line 
breakers used to kill an interaction with a power line in the near 
field of an HF antenna.

>It has been suggested that I mount the antenna/radio inside a parabolic
>dish. Lots of problems with that.....

Aside from the fact that it doesn't help, if your problem is 
interference from the FM BC transmitter.


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