[BC] RF interference problem

[RON DOT'O SR.]

Very interesting!  Live and learn.  Let us know how it turns out.  I would 
also like to know the types and values of the components that cure the 
problem.

I had a recent problem where a rural neighbor about 500' from our 1KW AM 
stick started getting the station in their stereo, TV and computer when they 
moved things from the AC outlets on one wall to another wall.  I advised him 
to check all the connections from the breaker and all the outlet connections 
on that circuit and make sure they're all clean and tight.  Still waiting to 
hear back from him.

Ron Dot'o, Salem, OR.


----- Original Message ----- 
From: "Mark Humphrey" <mark3xy at gmail.com>
To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
Sent: Sunday, January 22, 2006 07:57
Subject: Re: [BC] RF interference problem


In theory, the modulation of an FM station can't be detected with a diode
(or transistor junction), but if something else introduces an AM component
on top of the FM signal, then it becomes an AM station.

Here's what I think is happening:   The FM station has a multibay antenna
mounted near the top of the big TV tower.  Shelby's studio is about a mile
away, so it is probably between one of the nulls and minor lobes of the FM
antenna's elevation pattern.  The downward angle of that null/lobe will
shift slightly with FM modulation (if the antenna is fed from the bottom
with bays tapped off the hardline, as most are) which makes the level of
signal at the studio vary in strength with modulation -- so there's the AM.



In fact, I'm dealing with a similar situation today at a newly-built talk
studio about a mile from the Roxborough antenna farm, home of most of
Philadelphia's TV and FM transmitters.  One FM station has been identified
as the cause of the problem (you can easily hear their audio being
demodulated in the mic preamps) and sure enough, it's the station with the
bottom fed six-bay antenna.   So I'm headed over there in a few minutes with
some big ferrite beads and we will try to cleam this up.

Mark



On 1/22/06, RON DOT'O SR. <ron_doto at msn.com> wrote:
>
> FM stations won't cause this problem in audio equipment as there is no
> "natural" detector in the components as there is for AM (transistor
> junctions to rectify and detect the RF).  A very strong VHF FM signal
> MIGHT
> cause buzz or hiss but is very unlikely.
>
>

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