Bret, you might have your PA going in to oscillation creating the spurs due to 
a highly
reactive duplexer.

We had a similar problem here many years ago and fixed it with a simple tuner 
on the TX
similar ot the GE Z matcher . The one that we used was "Home Brew".

When the tuner was adjusted for minimum VSWR, the spurs went away.

73 John VE3AMZ


----- Original Message ----- 
From: "brett" <brett_daw...@yahoo.com>
To: <Repeater-Builder@yahoogroups.com>
Sent: Saturday, September 04, 2010 7:26 AM
Subject: [Repeater-Builder] Interference on VHF repeater


> Hi all,
>
> I have come across an interesting problem which you may be able to shed some 
> light on.  I have an intermod issue where my TX 
> sometimes opens up my RX.  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.
>
> I have also disconnected feeders from all other RF equipment on site - still 
> no change.
>
> The fact that the IM product frequency changes with time (drift rate is 
> roughly a few kHz's an hour) makes me think that there is 
> either another unknown source of RF on site which has poor freq stability 
> (pretty unlikley), or somehow my TX freq is involved in 
> producing this freq.
>
> I have inserted a 6dB pad in the antenna port of the duplexer and found that 
> the IM products drop 12dB, and also curiously, the 
> frequency of the products change.  Removing the pad reverses this effect.  I 
> have repeated this many times and the result was 
> always the same.  It appears that the frequency of the IM product is 
> dependent on the strength of the radiated field from my 
> antenna.
>
> This is my question:  I have read that it is possible for a strong EM field 
> to excite metal (eg tower member) such that 
> re-radiation will occur at a frequency which is different from that which 
> excited it.  Can anyone confirm they have seen this, or 
> can anyone point me to a reference that talks about this?
>
> I should also mention there are multiple solar panels and associated 
> regulators on site.  The regulators have been discounted as 
> possible sources, but the panels (given they may have bypass/blocking diodes) 
> may be a mixing location, however the source of the 
> drifting tone is still unclear.
>
> Thanks,
>
> Brett VK2CBD.
>
>
>
>
> ------------------------------------
>
>
>
> Yahoo! Groups Links
>
>
>

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