Sounds like a bad case of mixing products (i.e. 2A-B) where A is frequency A
and B is frequency B. Several years ago, I helped cure a similar problem on
146.820 MHz. Turned out four transmitters were involved in that fracas and it
only took two to start the repeater howling.
152.480 is one of the common paging frequencies. There is another around 157
MHz that is usually a troublemaker for 2m amateur repeaters. A lot paging
transmitters are NOT properly filtered. I have run across some that were
connected directly to the feedline and antenna with no circulator, no isolator,
nothing. This is unacceptable practice. No transmitter or receiver should be
looking directly into a feedline at any fixed site where multiple transmitters
are in operation or, where interference could likely erupt.
I doubt the UHF paging transmitters has a whole lot to do with the problem
although there is a slim chance they might. Chances are, the VHF transmitter
is the one involved.
The next thing I'd be curious about, given the paging company's comments about
the ability to key the transmitters separately, is this: How are they getting
the data from their offices to the transmitters? By radio link? By wireline?
or both? If by radio, therein may lie the second transmitter in the mix. If
not, keep searching.
One thing to keep in mind with the spectrum analyzer is that it is not likely
as sensitive as the repeater's receiver. Some spec an's are pretty sensitive,
others not so sensitive. So you may want to visit the paging site with it and
see what you find. Especially around 304.960 MHz which is the double of
152.480 MHz.
KC5DBH Matt
________________________________
From: Mike <mwbese...@cox.net>
To: Repeater-Builder@yahoogroups.com
Sent: Wed, October 28, 2009 1:28:56 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. The repeater is on 146.850 (-600 KHz), with the antenna
system about 120 feet up a water tower. The repeater itself is an Advanced
Communications System KRP-5000 running 35 watts through a set of 4 WACOM cans.
The feedline is 7/8" hardline feeding a DB-224. All jumpers are RG-214 MILSPEC
with MILSPEC connectors. This system has been in service for years and has
never given us any problems, and we are the only ones at the site.
To be clear, the interference we are experiencing is clearly audible on the
repeater input. I have the capability to monitor via telephone and have heard
it on the receiver, and I've also traveled to the site and heard the
interference on my mobile radio hooked to the repeater antenna. The
interference is also audible on the input in various locations around town.
Also, the interference can be heard on the input regardless of whether or not
the repeater transmitter is on. It also continued to be present during several
days of continuous heavy rain.
The interference typically shows up at least once a day, although some days
(rarely) it does not show up at all and other days it will show up several
times. Lately, it's been making an appearance around 10 a.m. and hangs around
for an hour or two. As it begins to disappear, it sounds as though it is
moving off frequency.
This interference has also been heard on at least two other repeaters in the
area. One is about 22 air-miles from the 146.85 machine and is on 145.110
(-600 KHz). It has also been heard on or near the output frequency of that
machine, and one evening I tracked it from about 145.120 to 145.190 as it swept
through each transmission. The other repeater it has been heard on is 147.300
(+600 KHz). I also have reports from a neighboring county a ham/deputy sheriff
there has been hearing it on VHF public safety frequencies. As you can see,
it's all over the place.
I've been working with the owner of a local paging company and we can clearly
tell that the data we're hearing is coming from 152.480 and 462.775. He has
two sites (about 20 miles apart) that simulcasts on both frequencies and when
those transmitters are active it's easy to tell that the data is the same. He
also tells me that he can key each transmitter separately and the data from
each transmitter will be heard on our repeater. We also believe that there are
other systems on nearby frequencies that are being heard on our repeater,
specifically 462.850 and 462.925.
I've run IMD numbers on everything I can think of, but can't come up with a
common thread. For it to be moving all through the 2-meter band and for it to
be mixing with several different frequencies, it seems to me that it's got to
be very ugly and unstable. What am I missing here?
Mike
WM4B
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