If the problem was generated in your transmitter from mixing with the
paging transmitters it will usually only occur when your repeater
transmitter is on. It sounds like the interference signal is being
generated elsewhere. You should first determine if the problem is in
your installation or from the pagers. I would use a spectrum analyzer or
service monitor at the receive port of your repeater tuned to your
receive frequency to see what it looks like and measure the level. It
may look like a comb generator every time the pagers fire up! You can
then try another antenna into the analyzer to see if the interference
persists or use a directional antenna to see if it’s emanating from the
pager site or yours. Since you mention other repeaters in your area are
suffering from the same problem I would suspect at least one of the
paging transmitters does not have an isolator, which is required at all
sites I have ever dealt with. If you can pick up the interfering signal
on a separate antenna with test equipment and it points to the paging
site, you can demand the pager owners address the problem with isolators
or what ever it takes to minimize the interference.

Photoman wrote:
We have 146.04/.64 repeater on a nearby mountain top. It worked great for
years with a range of 100 miles or more. Since the phone company and a pager
company installed their high power transmitters near the site of the
repeater (within 100 yards) the repeater is virtually useless. After much
head scratching I believe that the difference in frequency of the pager
transmitter of 600 khz is the problem but have no idea how to solve the
problem without going to an odd split. The repeater coordinator for this
area of Virginia won't even consider that as an option.

The equipment that we are using is excellent. The transmitter and receiver
on the repeater are both Motorola Micor and were modified with to the
repeater frequencies by FCC 1st class licensed hams using Motorola parts.
This is not an equipment problem. We are running a set of Wacom cavities
which were bought new and are correctly tuned and the antenna is a
Phelps-Dodge Stationmaster. When the intermod occurs it is dependant on BOTH
pagers transmitting at the same time. If only one pager is transmitting
there is no problem. This may at first sound unusual but the pagers are in
the 150 mhz band and they are exactly 600 kc apart. These transmitter are
both 250 watts or more output.

My theory is that the 600 kc (difference of the 2 pagers) is mixing with the
output of the repeater 146.64 and producing the 146.04 signal, the repeater
input frequency. We are using sub-audible tone for repeater access and as
soon as a station working the repeater drops carrier the repeater drops. The
intermod cannot hold up the machine once the tone is removed. This may be
happening in the antenna or hardline connectors prior to the cavities. Every
test I have run, and there have been many, supports this conclusion.

We are not the only 2 meter repeater that has fallen victim of this problem
and in every case we have found two pager transmitters situated 600 kc apart
near the repeater. Most of the other machines have been taken off the air,
others just put up with it. No one has been able to solve the problem and
many technicians have studied it.

Moving the repeater far enough away is not an option since the peak of the
mountain is so small. Also we are using an existing tower which we would not
have access to at other locations. The searches I have done on Google has
turned up the stock answer of helical resonators which would apply to 2
meter radios but not repeaters. If you are familiar with the Micor equipment
you know that the receiver has excellent helical resonators built in.

Tonight I have considered the possibility of splitting the receiver and the
transmitter of the repeater and linking the rx signal by a 220 mhz link. I
am hoping that by reducing the level of the 146.64 signal by 50-60 db would
alleviate the problem. Maybe not, but I'm out of ideas. This split would be
only about 100 yards but could that be sufficient to relieve the problem?

Have any of you ever had this problem and solved it? Any input (pun
intended) on this matter would be appreciated.

Ken Sturgill, KC4IH
Marion VA
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