Need Help With This Noise on Public Safety System

[70cutlass442]

This noise has been noted many times of the last year and we are chasing our tail on it. This is on a voted public safety system that is laid out as follows.

VHF CM200 Receiver at remote sites, cross band to UHF CM200 for uplink.
UHF CM200 Downlink into a Raytheon PTG-10 pilot tone generator (located at TX site with repater).
PTG-10 into a GE voter (black chassis and cards).
Voter to a series of relays and an Icom FR3000 repeater (exciter) into a 100 watt TPL VHF amp.

-System has two remote RX sites
-VHF CM200 at main repeater site, uses duplexor and shares antenna with repeater.

This issue is manifesting itself on a single remote site. We capture the below video and noise on camera (16 second mark). It is ONLY on this one site.

So far we have tried the following with no resolution of issue.

-Change voter site card.
-Change location of voter site card in cage.
-Change out UHF uplink radio to new radio.

We are slated to move to a SNV-12 and Quantar next week however it appears as if this issue is not with the repeater or voter at this point.

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Sometimes this issue resolves itself within a few seconds, sometimes it goes minute or more. Tonight dispatch noted it started all by itself. Units can "talk over" the sound.

Any and all suggestions are appreciated.

 

[Floridarailfanning]

This is not the first time a problem like this has been discussed on this forum and though I could be mistaken, from what I recall the issue was caused by a powerful repeater adjacent to the receiver, which was inadequately isolated. So an obvious question would be whether another repeater is installed near the VHF RX site.

 

[Tech21]

What you have going on is that somebodies system, somewhere is using the same frequency but in digital, not analog. Or like the previous poster said, somebodies repeater isn't properly isolated or grounded properly. Does it happen only when a specific field unit keys up or does it happen with multiple users? If it's the former, when's the last time a pm was done on those radios? This noise is happening while your RX side is open so most likely somebody a couple counties over is using the same frequency but is running a digital system which is causing this issue on yours. You can try adding attenuation on your rx multicoupler to see if that helps.

First thing I would do is an FCC frequency search and see if anybody around you is using the same frequency.

 

[TampaTyron]

I may not be correct and have little experience with analog voting..... but that sounded like a burst of intermodulation, where transmitters are mixing and creating a signal on the rx frequency. Can you connect a directional coupler or iso-tee on the rx line (and hook a spectrum analyzer in peak hold mode to the iso-tee isolation port)? Does it seem to happen more at a certain time of day or after/during specific weather events? What other transmitters are at the site? TT

 

[RFI-EMI-GUY]

This is likely involving your own system, not another licensee.

That howling is feedback. The hollow tone is group delay probably introduced by the CM200 radios. You are getting some sort of interference between your own ICOM repeater and one of the CM200 receivers. I would start by bypassing the 100 watt PA to see if the problem rectifies itself. Is the VHF amplifier creeating harmonics or IM products? If not, then I would look into filtering the UHF CM200 receiver .

 

[70cutlass442]

This is not the first time a problem like this has been discussed on this forum and though I could be mistaken, from what I recall the issue was caused by a powerful repeater adjacent to the receiver, which was inadequately isolated. So an obvious question would be whether another repeater is installed near the VHF RX site.

The nearest VHF transmitter to the RX site in question is our repeater at 3 miles away. I do have a preselector we can put on the VHF side to see if we are getting interference in through there.

What you have going on is that somebodies system, somewhere is using the same frequency but in digital, not analog. Or like the previous poster said, somebodies repeater isn't properly isolated or grounded properly. Does it happen only when a specific field unit keys up or does it happen with multiple users? If it's the former, when's the last time a pm was done on those radios? This noise is happening while your RX side is open so most likely somebody a couple counties over is using the same frequency but is running a digital system which is causing this issue on yours. You can try adding attenuation on your rx multicoupler to see if that helps.

First thing I would do is an FCC frequency search and see if anybody around you is using the same frequency.

Being that this only happens on a single site I assumed that I had interference on the "link". I found no other users on the UHF link for hundreds of miles. As of tonight I set the link channel on one of our unused business band channels to see if we have a close channel causing the issue.

I may not be correct and have little experience with analog voting..... but that sounded like a burst of intermodulation, where transmitters are mixing and creating a signal on the rx frequency. Can you connect a directional coupler or iso-tee on the rx line (and hook a spectrum analyzer in peak hold mode to the iso-tee isolation port)? Does it seem to happen more at a certain time of day or after/during specific weather events? What other transmitters are at the site? TT

Yes, I can. That will take some time but is the road we are looking to head down.

This is likely involving your own system, not another licensee.

That howling is feedback. The hollow tone is group delay probably introduced by the CM200 radios. You are getting some sort of interference between your own ICOM repeater and one of the CM200 receivers. I would start by bypassing the 100 watt PA to see if the problem rectifies itself. Is the VHF amplifier creeating harmonics or IM products? If not, then I would look into filtering the UHF CM200 receiver .

As noted above, the UHF downlink that this is happening on originates from the VHF receiver that's 3 miles away. I have a preselector that I can put on that site. I can lower power as well. I would suspect that 3 miles of horitontal isolation would offer in excess of 90db isolation... but at this point all ideas are being considered.

 

[TampaTyron]

I am sure you already know this, so I am tossing it out there for everyone else's benefit........ Just because no one is licensed, it doesn't mean no one is transmitting. I see all the time: expired licenses, misprogrammed radios, stuff bought online and just tossed up, intermodal, harmonics, etc.

I know it isn't common practice, but my approach is to assume everything is bad until I prove it is good. Then, go through each piece. Save the test results to compare to later testing. Once you can confirm it isn't anything in your system, then you can look externally. Often I find issues in the investigation process that once fixed, make the system operate better.

TT

 

[ramal121]

This is likely involving your own system, not another licensee.

That howling is feedback. The hollow tone is group delay probably introduced by the CM200 radios. You are getting some sort of interference between your own ICOM repeater and one of the CM200 receivers. I would start by bypassing the 100 watt PA to see if the problem rectifies itself. Is the VHF amplifier creeating harmonics or IM products? If not, then I would look into filtering the UHF CM200 receiver .

Yes that loud howl is classic analog IMD. You say it can last up to a minute or so? To hold the receiver open for any length of time you would need the receiver's PL/DPL component in the mix to do this. Although highly distorted it can be decoded for a time until the decoder gives up. What PL/DPL are in the VHF and UHF side of things? Usually with a stand alone repeater this is easy to do.

Now I would lean on the problem being at the repeater end, but don't discount the rceiver site. Stranger things have happened. I would monitor the UHF downlink freq from that site. If you hear it then that's where you should be headed. If all is quiet from there then stay with the repeater.

Take a hard look at your own equipment. Quality of the feedlines, connectors, etc. Maybe a little clean up is in order. CM radios are not the most robust thing for RF site applications (and for that fact neither is the Icom repeater). You may have to play with filtering, shielding and isolators. Also don't discount the IMD may be created outside your equipment which adds a whole new fun factor to your quest. Divide and conquer. Good luck.

 

[70cutlass442]

Yes that loud howl is classic analog IMD. You say it can last up to a minute or so? To hold the receiver open for any length of time you would need the receiver's PL/DPL component in the mix to do this. Although highly distorted it can be decoded for a time until the decoder gives up. What PL/DPL are in the VHF and UHF side of things? Usually with a stand alone repeater this is easy to do.

Now I would lean on the problem being at the repeater end, but don't discount the rceiver site. Stranger things have happened. I would monitor the UHF downlink freq from that site. If you hear it then that's where you should be headed. If all is quiet from there then stay with the repeater.

Take a hard look at your own equipment. Quality of the feedlines, connectors, etc. Maybe a little clean up is in order. CM radios are not the most robust thing for RF site applications (and for that fact neither is the Icom repeater). You may have to play with filtering, shielding and isolators. Also don't discount the IMD may be created outside your equipment which adds a whole new fun factor to your quest. Divide and conquer. Good luck.

Well I am glad you mention that.... TX and RX are both on 74.4. I bet if I were to run a split tone this would clean it up. I have a new frequency pair and are sourcing the necessary cables and s/w for the 9 different series radios being used. I bet after the reprogram (with a split tone) this would clean up. I would also suspect a preselector would clean this up if it is infact getting back in from the site that is three miles away which is the only one we seem to have this issue with. I have a DPL on the up/downlink.

 

[RFI-EMI-GUY]

The split tone is only going to mask what is probably reducing the sensitivity of your remote receiver.

"I would suspect that 3 miles of horitontal isolation would offer in excess of 90db isolation... "

That 90 dB may not be enough as your receiver internal noise floor is probably in the order of -125 dBm. So some noise or dpurious or IM signal at -35 dBm (-85 dBc @ 100 watts) can be picked up by that RX.

Questions,

Do you have a co located receiver with your Icom transmitter operating duplex? If not why not?

What filtering is on the transmitter? Band pass? Reject?

 

[jeepsandradios]

The CM200 is not an ideal radio for a receiver especially in VHF. Ran into similar issues in my SAR system. If your on a good site/hill it can be worse. These were designed as mobiles in a car/truck. I resolved my stuff by installing preselectors inline with the RX line on both sites. If you can find some used ones from a Quantar or MTR they work well and have mini uhf connectors on them. Other option is a real filter tuned for your frequency only.

 

[jim202]

No one has mentioned what kind of coax your using at the sites and links. I hope your not going to tell us the coax cables are LMR type.

I will leave the rest of my comments to others.

 

[wgbecks]

You have good ideas and have been given some good advise. What's missing is that it appears (unless I missed it) that you haven't isolated
the noise, IM, or whatever to the remote VHF RX site, or to the UHF Link RX site. We know the noise is present going into the voter so you should go to the remote VHF RX site to determine what end of the link contains the noise source. You should be able to quickly accomplish this by listening to the local speaker or by bridging the audio line to the Link TX. In other words, you need to first identify the point of ingress before you can diagnose the cause.

 

[Tech21]

No one has mentioned what kind of coax your using at the sites and links. I hope your not going to tell us the coax cables are LMR type.

I will leave the rest of my comments to others.

LMR-400 up to the antenna on a 400 foot tower probably.

 

[70cutlass442]

You have good ideas and have been given some good advise. What's missing is that it appears (unless I missed it) that you haven't isolated
the noise, IM, or whatever to the remote VHF RX site, or to the UHF Link RX site. We know the noise is present going into the voter so you should go to the remote VHF RX site to determine what end of the link contains the noise source. You should be able to quickly accomplish this by listening to the local speaker or by bridging the audio line to the Link TX. In other words, you need to first identify the point of ingress before you can diagnose the cause.

I've been able to isolate the issue to a single RX site. What we did is put a camera in the shelter and watched the voter and the repeater. We observed that the voter card associated with a certain site would open up and allow the interference to enter the system. We replaced the voter card and moved to a different slot on the cage to eliminate the possibility that the voter was an issue. Now that some think the howl is suggestive of interference within our own system I will attempt a preselector at the problem site.

LMR-400 up to the antenna on a 400 foot tower probably.

lol. TX is 140' monopole with 1" Andrews LDF, RX site is 165' self suporter w/ Andrews 7/8" LDF.

 

[RFI-EMI-GUY]

"lol. TX is 140' monopole with 1" Andrews LDF, RX site is 165' self suporter w/ Andrews 7/8" LDF. "

Get a preselector on the RX.

 

[70cutlass442]

"lol. TX is 140' monopole with 1" Andrews LDF, RX site is 165' self suporter w/ Andrews 7/8" LDF. "

Get a preselector on the RX.

The TX site has a duplexor with a CM200VHF as a receiver. This site has been no problems. The interference seems to be on the one site 3 miles away.

 

[RFI-EMI-GUY]

The TX site has a duplexor with a CM200VHF as a receiver. This site has been no problems. The interference seems to be on the one site 3 miles away.

Yup, you need a preselector or cavity at the site 3 miles away that will filter out your TX

 

[70cutlass442]

Yup, you need a preselector or cavity at the site 3 miles away that will filter out your TX

Update as of 6/25/2020. Added preselectors to the two remote sites on 6/15. We have been free of the noise for 10 days so I feel confident this was the issue. Thank you all to who contributed!