[BC] RF interference problem

[Mark Humphrey]

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.
>
>