Mathew,
After looking at the number of transmitters operated by the Fulton County
Sheriff and the Rochester Police Department, I can see that there are a number
of possibilities. For example, when you double the Rochester PD channel at
155.130 MHz and then subtract the nearby NOAA Weather Radio Station WXK74 at
162.475 MHz, you get a third-order IM product at 147.305 MHz- only 20 kHz away
from your 2m repeater's output. An Amateur-grade radio might be loose
enough to pick that up. That's why I asked about whether a
commercial-grade radio received the same interference.
My gut feeling is that there is a third-order IM product that involves
one of the police or sheriff frequencies, and the mixing may be occurring in a
transmitter that does not belong to Fulton County or the City of
Rochester. I believe that the best way to determine what carriers are
causing the interference is to set up a spectrum analyzer where the interference
is very strong, and monitor the spectrum to see what carriers are always present
when the interference occurs. Since the sheriff uses UHF control stations,
you might also monitor that band for clues. Steady carriers used by
broadcast station STL (studio-to-transmitter link) or remote-monitoring
equipment often are on the air 24/7 and can be just dead carriers most of the
time. High-power paging transmitters are also IM
candidates.
I'd collect a lot more data before contacting anyone on the
law-enforcement side. Use accurate equipment to determine exactly which
frequencies are key to the interference. Some of the channels are very few
kHz apart, so you need to be able to identify each
frequency.
When the time comes to make contact with the agency involved, don't call
the chief! Look up the point of contact for the station licensee in the
FCC license database. For Fulton County license KSB609, it's the
Communications Director at 574-223-2910. Just call and ask for the Radio
Shop. Tact and diplomacy goes a long way here, especially since you are
offering to help clear up a little problem that may cause interference to their
radio system.
73, Eric Lemmon WB6FLY
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