Good question about how the data was getting to both transmitter
sites. I've been meaning to ask that question and keep forgetting.
I'll report back.
Mike
WM4B
On Wed, Oct 28, 2009 at 3:16 PM, Matt Harker wrote:
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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