This sounds like a problem I traced about 20 years ago on a VHF
paging system. The PA was tube and there was some issue the tech had
with the PA not firing on rf drive .. so he locked the PTT to the PA
on all the time and then just turned the exciter on and off....The
problem was the PA was self resonant +- a sweep of our freq and
would go there whenever the transmitter exciter was NOT present...
and it drifted with temperature.... so was a 300 w moving target....
I guess he never looked at it with a watt meter while the exciter was
off.... How we finally identified it was we could hear the tone blips
at the end of the interference cycle ( when the station was really
being asked to tx) bleed through from the PURC tones just before it
went away.. We then watched the TX carrier of the real tx come on
perfectly synchronously....to the absence on our end...By watching
what came on when it went off we figured out the freq... as there was
no modulation during interference...They toggled perfectly on a wide
sweep spectrum analyzer...
During the day it was gone as the system was busy.. but at night when
it got quiet.... we got hammered...Then I DF'ed the particular
tx.....as it was a simulcast....And the tech shut it down until it
could be fixed correctly and our problem went away....
The tone sequencing leaked through from the link system which pointed
us to paging... in this case it was a 152 .xxxx to protect the guilty
:-) and moved to 151.xxxx area blocking some control stations inputs
at a nearby dispatch center...
Doug
KD8B
At 06:28 PM 10/28/2009, you wrote:
Mike,
If it moves around based on time of day, my first guess is a PA
that's gone bad, and has a parasitic that's temperature-related.
If you've tracked an individual spur drifting 70 kHz up the band
during a single transmission, this is not some (intentional)
oscillator drifting, but some combination of failed components or
tuning which has produced a parasitic.
Sorry to say, but a paging transmitter owner swearing his stuff is
clean is pretty meaningless. The assumption in his industry is the
professionals who maintain his stuff are not the problem, it's
"those damn hams." Sadly, it may more often be the other way around
these days, as companies maintaining paging equipment have
transitioned to underpaid, under-trained card-swappers instead of
component-level technicians with a clue about RF systems.
73,
Paul, AE4KR
----- Original Message -----
From: <mailto:mwbese...@cox.net>Mike
To: <mailto:Repeater-Builder@yahoogroups.com>Repeater-Builder@yahoogroups.com
Sent: Wednesday, October 28, 2009 12:28 PM
Subject: [Repeater-Builder] Pager Interference to 2-meter & VHF
Public Service Band
A couple of weeks ago, our repeater system started to experience
interference from a paging system...
...one evening I tracked it from about 145.120 to 145.190 as it
swept through each transmission...
.
