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Author Topic: Jamming or Desense or Both?  (Read 711 times)

Posts: 53

« on: November 29, 2016, 03:22:01 PM »

We have an unusual situation. We can't tell if we are being jammed or if we have an odd case of desense. We know that a trip to the site will tell us if it is the latter.

We appear to be free of desense. When listening to the input we can hear someone key up activating the repeater and we hear what sounds like a hetrodyne (squeal). The signal is strong enough that it rides in audible with a strong signal. We've even heard someone with a programmed DTMF button doing it repeatedly. We cannot find the direction that the apparent transmitted signal is coming from.

We believe that it is a remotely activated reverse repeater. We have someone that shares the vault that is not happy we are there who has a frequency agile remote base that we believe is being parked just off of our receive enough to be within the window. He can activate right through his controller. The interference continues until the timeout timer expires or the hang timer is cancelled. I can actually see the signal  on a service monitor  when using  an external antenna  and the 20dB preamp behind dual cavities more than 1 mile from the repeater site. The machine is a MSF 5000 with a 30khz IF in a 15khz band. Yes, we plan on either replacing the repeater or putting a crystal filter in line at the expense of signal.

But now we question whether or not our squelch is too low (even though PL'd) and we have desense that we have not been able to identify and conditions are just right to trip it. It seems to occur during rush hour times and usually frequently on the weekends. The repeater can be hit from 140 miles out one direction but not in another. There is a strong suggestion that it is in fact a reverse repeater being turned on and off at someone's will. As I've said we hear them keying up. Just as I cancel  the Hang timer  they quit their mic and it starts all over again. This is one reason we can't tell where it's coming from. If it makes a difference our antenna is above a Verizon installation. The spec analyzer looks clear. I can't live at the repeater site and even if I could I suspect the transmitter would desense the spec analyzer before I could prove it was an incoming signal on the input.

Besides going to the site and testing for desense and touching up the duplexer what are some other options available to us? Just posting this exposes us to misfired have nothing better to do then prove our situation accurate.

MSF 5000 VHF
Sinclair Q2330e 6-Cavity Duplexer
Dual RX Cavities
RG214 Cables


Posts: 442

« Reply #1 on: December 01, 2016, 05:19:03 PM »

You might eliminate the problem by removing the preamp

You already understand that you have a 30khz repeater

By adding a 20db preamp, you are making it even wider

Thus making it worse

Posts: 53

« Reply #2 on: December 02, 2016, 04:36:47 AM »

Thank you for taking the time to reply but you misread what I wrote. I wrote, "I can actually see the signal on a service monitor when using an external antenna and a 20dB preamp behind dual cavities ==> more than 1 mile from the repeater site <==". There is no preamp on the repeater.

Posts: 442

« Reply #3 on: December 02, 2016, 11:49:17 AM »

Sorry OM didn't see that

You can also narrow tune the receiver and gain some improvement in selectivity but it doesn't sound like there would be any help here

The squealing feedback sound indicates that either the repeater is hearing itself or hearing an upside down repeater or cross bander

I've also seen this symptom when the repeater tx signal mixes with another signal and becomes the input frequency or close to it

Don't know if yours has a circulator installed or not

My apologies if this has already been done

Posts: 53

« Reply #4 on: December 03, 2016, 08:27:48 PM »

Thank you again. The latter part of your response is what we are more interested in. I've asked help from local hands and they seem to think it's a reverse repeater. We can hear someone on the inside looking there mic to activate it and then not attempting to use the repeater that's making it clear it's deliberate. Finding it has proven itself to be difficult.
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