[App_rpt-users] Desense (Echo) on RTCM

Hayden Honeywood haydenph91 at gmail.com
Tue Sep 12 00:35:09 EDT 2017


Yes. The radio in it's previous format had an audio delay in it, not as
much as the RTCM of course, but it was noticeable as a growl when CTCSS was
encoded on top of it.

I suspect I either have a dodgy receiver which did not present any issues
on the bench (while of course no duplexer/antenna was attached) or perhaps
an antenna problem which didn't show up as bad on the previous repeater
that was installed. It was also brought to my attention that perhaps some
patch cables between the duplexer and/or antenna feedline may have been
disturbed when I installed the RTCM repeater, perhaps one is noisy.

In any case, a test into a dummy load should show up the issue, and I know
I have a good known system working at home for the second site, as I found
a spare duplexer.



> It does sound like your repeater is hearing itself. The delayed echo is
> because of the jitter buffers required to stream audio. Normal repeater
> trouble shooting methods apply. You have to figure out how the Tx RF is
> getting into the Rx. I've never heard of the Rf coming down the GPS cable.
> I suppose that's possible. But even if it was your receiver should be
> shielded well enough to prevent the desense.
>
> On Mon, Sep 11, 2017 at 8:10 PM, Hayden Honeywood <haydenph91 at gmail.com <http://lists.allstarlink.org/cgi-bin/mailman/listinfo/app_rpt-users>>
> wrote:
>
> >* Hi All,
> *>* Gotten around to finally installing one of my VOTER boards (RTCM) to Site
> *>* 1.
> *>* All tested on the bench - the only thing I did not test was duplexing on
> *>* the bench as I had no spare duplexers.
> *>>* Installed to the site - nothing changed from the duplexer patch cables
> *>* onwards to the antenna. Now I have what I assume is a huge amount of
> *>* desense. The previous repeater installed was the same model radio and
> *>* worked fine.
> *>>* Disabled CTCSS encoding as the repeater would talk to itself echoing over
> *>* and over.
> *>* Attached is a sample audio recording, you can hear the growl of the audio
> *>* delay in the system.
> *>>* I've been waiting to get back to the site (snow and time permitting), but
> *>* in the meantime thought I'd post here to see if anyone has had any similar
> *>* experience with desense and the RTCM's talking to themselves....
> *>>* My second unit works on the bench, duplexing into a dummy load and antenna
> *>* fine. Measuring desense is tricky as the radios have been modified so they
> *>* don't have any external audio connections anymore, so the lid has to be
> *>* popped to connect the service monitor.
> *>>* Next stop is to test the system into a dummy load at the site, if that
> *>* fails, then remove the system and replace with Site 2 equipment and test.
> *>* If it duplexs fine into a dummy load and not into a antenna, has anyone had
> *>* issues with RF maybe coming back down the GPS antenna cable? That is the
> *>* only physical external connection near the antenna we've changed from the
> *>* previous system.
> *>>* Regards
> *>* Hayden VK7HH*
>
>
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