The inter mod could be originating in your receiver or in one of the
paging transmitters. Part of it could even be originating in your own
transmitter.
The first thing I would try is a couple of notch cavities in your
antenna line tuned to one or both of the paging transmitter
frequencies.

First tune both to the same paging transmitter frequency. Then try the
other one. Then try one on each paging frequency.

If they make no difference then it is a good bet that the inter mod is
originating in some other transmitter. Your repeater signal may be
mixing in their transmitter. The only way to cure that is a notch
cavity in their antenna line tuned to your transmitter frequency.

A pass cavity or two in your receiver line may help. But you will be
able to get much more rejection with a notch than a pass.

If you don't have a notch cavity but do have a pass cavity you can use
the pass cavity as a notch by placing a T in the antenna line and
connecting only one loop of the pass cavity. That will function as a
notch.

73
Gary K4FMX



On Mon, 20 Dec 2004 20:48:54 -0500, "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
please reply to )
REMOVE THE "S" FROM netS in email address to respond.