no fluro lights on site.  Actually no lighting at all - the site is solar 
powered so little capacity for lighting.

No repeater ocntroller - the RX is directly connected to TX via a very simple 
(passive) audio and PTT circuit.

I believe it is an IM product because the RX requires a CTCSS tone to open and 
the only TX on site that has the correct tone is my TX.  Also the interference 
is only present when my TX is active, and the hollow pipe sound on the audio 
does sound like audio feed back you'd expect if your own TX was part of the mix.

With a spectrum analyser in the RX signal path I can see the intermod products 
appear when the TX is active.  They are at approx 30kHz intervals across the RX 
half of the duplexer band pass.



--- In, "Paul Plack" <pl...@...> wrote:
> Brett,
> How did you determine it's an IM product?
> What repeater/controller combination are you using? I'd try powering down the 
> controller and manually keying the transmitter. If that solves it, it could 
> be the controller's reference oscillator or divider outputs leaking onto the 
> PTT line or elsewhere.
> Any compact fluorescent lights nearby?
> 73,
> Paul, AE4KR
> ----- Original Message ----- 
>   From: brett 
>   To: 
>   Sent: Saturday, September 04, 2010 5:26 AM
>   Subject: [Repeater-Builder] Interference on VHF repeater
>   I have the distinctive hollow pipe sound. Both TX and RX have the same 
> CTCSS tone. The intermod product is however not always present, and after 
> looking at the RX output from the duplexer with a SA I see a comb of products 
> that move slowly in time. When one of the products in the comb falls within 
> the RX bandwidth the RX opens, until it moves on.
>   This is not a busy site, and I have been able to power down everything on 
> site except my repeater. Problem remains unchanged.

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