[BC] RF interference problem

[Kent Winrich, K9EZ]

Let us know how it turns out!

Mark Humphrey wrote:

>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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