Digital Noise Interferance?

[k7bpg]

Good Morning,

Hope you all are well. I wanted to see if I could pick your guys brains on an issue I am having on an interference issue my shop is trying to figure out for our local fire department. We have been banging our heads against the wall for a couple weeks now trying to narrow down what exactly we are dealing with. This problem started at one hill top on a 154.xxx freq, now it is present at a different hill top, about 20 miles away, on a 158.xxx freq.

The equipment is VHF Analog Quantar repeaters. The issue is an intermittent noise that sounds, as other posts have described on the forum, as a "brarararararar" noise. It only happens after a field unit has finished talking, the repeater stays open and the noise is present. The repeater does not activate with this noise by itself, the repeater must be keyed either by dispatch over the wire line or a field unit. The noise is also not causing trouble with legitimate traffic, all users can hear each other clearly, its only after everyone is done talking that the noise then appears. The noise is also varying in how long it is present as well as strength. Sometimes choppy, some times it holds the repeater open till it times out.

Troubleshooting thus far. We isolated the Quantar in different ways, one by removing the wireline to see if the noise was coming over the 4 wires, no change, noise still coming over the repeater. We then removed the external antenna and put on an antenna from a portable inside the building, the noise is gone and the repeater functions normally from my portable inside the building. When we hooked the external antenna back up the, "brarara" noise came back right back. This has lead me to believe it is from an external source from the building. We however can not hear the noise when we hooked up our service monitor to the external antenna.

Next I put a T inline with the external antenna feeding the quantar, feeding the T to the service monitor. The noise is present and a small signal is seen possibly around -95dBm. It is hard to tell because that too is not always seen along with the noise, but it's never there without the noise. On our spectrum analyzer we also see the TX signal become a very fuzzy arc when the noise is present, normal TX signal is a smooth arc when users are talking. I am fairly certain this is an external source getting it as it only happens on the external antenna.

As for settings on the Quantar:
- Hardware Config; Hadware platform set to Quantar, system type set to conventional, station type set to analog only, rx and tx bands are VHF_r2, wire line is 4-wire, freq ref is internal, multi-coded squelch is disabled, scanning receiver is disabled, wildcard is disabled, simulcast is disabled, phone patch is disabled.
- Access Code Table; set RX and TX squelch is set to PL
- Channel Setting are; Modulation Type is set to analog, TX rated deviation is 2.5mhz, receive channel bw is narrow12.5/15khz, carrier squelch transition is normal, Analog RX activation is SC=Carrier and PL/DPL, Analog Rptr Activation is SC=Carrier and PL/DPL, Analog Rptr Hold-In is SC=Carrier and PL/DPL and Analog Rptr Access is None.
- RF Config settings; repeater operation is in repeater, Max deviation is 92%, low speed/pl deviation is 17%, antenna relay disabled, packet data is disabled, astro tx filter in narrow pulse.

We have swapped out Quantars with other known good ones on the shelf as well, no change. We also changed around RX settings such as squelch and PL but the noise is still there in all its glory. The issue that is making my head hurt is we can not see the interference or hear it when we try to listen over the air with our radios set to repeater input, our service monitor or spectrum analyzer. We thing we see something way down at -95dBm but its muddled in the noise floor so we cant be definite that is it.

Im sure I left out some information, been up since 11:45 last night troubleshooting the issue since it has appeared at the new site and is now more consistent there. Ask away about anything I didn't cover, hopefully somebody has dealt with this before. I found this post, http://batboard.batlabs.com/viewtopic.php?t=44414, but it didn't lend much help once I checked over the settings we have in the radio after getting some sleep this morning.

Thanks
Brian

[tuckerm]

Is it this: https://www.youtube.com/watch?v=SRfvh0UTFHw

[k7bpg]

Yes, that is the exact noise. What is that, and how the heck do I get rid of it?

I will post the noise I have when I get it uploaded as well.

Brian

[fineshot1]

Possibly an input frequency adjacent channel noise from distant Mototurbo or another digital noise

[k7bpg]

Here is the noise we recorded earlier this morning.

https://www.youtube.com/watch?v=zhQb6lPUkw0

Brian

[tuckerm]

http://batboard.batlabs.com/viewtopic.php?t=108436

[Bill_G]

Yep. Check your cables, duplexer, and antenna. Use superflex and hardline, not LMR or RG8. Use a good quality antenna, and have it feet, not inches away from the tower. Sweep the duplexer.

If this is an existing repeater that was working well for a long time, and then suddenly developed this problem, your antenna could be bad.

[Jim202]

As a comment to what has already been said, use of braided shield type coax is a no no. If your using LMR coax for the transmission line going to the antenna, get rid of it.

With that said, you need to walk through this one step at a time. If you can find out which other transmitters at the site are coming on when you see this signal on your service monitor, try and find the owner. See if you can obtain where their antenna is located in respect to the one on your system.

The other transmitter could have a problem that is getting into your antenna. Your pass cavity system might not be taking the source signal out enough so it doesn't bother you. The antennas could be too close. Your transmission line or their transmission line could be a source of the problem.

You and the other transmitter owner need to be good neighbors. You need to work together, so don't start by yelling or pointing fingers at who is causing the problem. You need to get the facts and let them bring you both to a conclusion.

There could be the possibility that your antenna or the other system antenna took a lightning hit. Have you shut your system down and measured the SWR on your antenna yet? Do you still have the same coverage. System coverage can fall off with a bad antenna, but still show a good SWR.

Have you checked all you transmission line connectors to see if they are tight? Have you checked to see if your rack and cavities are grounded to the shelter master ground bar? Is the other system grounded the same way?

There are just so many places and things that can cause this type of problem. It also could be some loose or rusted bracket or a coax cable that is rubbing it's shield some place on the tower. But the cable rubbing will only show up when it's windy. The tower guy wires might not be grounded and something on them is rusted and causing the problem.

Take a long deep breath and step back from the problem for a few minutes. It may even take someone else to come in and help you with this problem. Don't expect to solve it over night. This type of problem could take you weeks to locate.

Good luck on your hunt. But please let the group know how your doing. We all learn from issues like this and it's important that you let us know what you find.

Jim

[k7bpg]

Thanks for all the input fellas, I will see if I can answer the questions without forgetting anything.

Also, now that I have had some sleep, here is some more info on the issue from my end. This problem started at Site A and was on a freq of 154.xxx. The repeater is on a multiplexer with other fire repeaters, no other repeater has been affected by this issue. We have been trying to narrow the issue down at Site A for some weeks now. No matter what we would change settings wise, or even putting in different quantars, the problem persisted. Now this issue is at site B on a freq of 158.xxx. I listened to both sites yesterday in the shop, site A does not seem to have the issue any more, it seems to have magically jumped to site B. Both sites are either leased or owned by the agencies I work for, so I have good details of each site as far as users and equipment. This issue is also a new one as of the past few weeks, both repeaters have been operating at these sites for years without issues such as the one we are having now.

My hope by sharing the recording of the noise, is that some one else had heard it and knew what kind of equipment gives off that noise, hopefully pointing to a culprit of some sort.

I think that is it, I will let you know what else my brain bubbles it up as time goes on.

Here goes:

"Check your cables, duplexer, and antenna. Use superflex and hardline, not LMR or RG8. Use a good quality antenna, and have it feet, not inches away from the tower. Sweep the duplexer."

I have checked the connections from the quantar, to the filtering system to the multiplexer out to the bulkhead heading out the building at both sites. All coax is superflex and hardline from the antenna on down. The antenna is sweeped with an antenna analyzer every year during annual PM's of the site, still checks good as well. The antenna is mounted away from the tower by a two foot arm from what I can see on the ground looking up. It is a good quality antenna as well, this is used for the local city fire department so we use good equipment for them.

Ok, now to Jims questions;

The site I was at last night is the counties building, to which I have a good relation ship with. I work for the Cities communications shop, so we all work together to help share resources. We are on good relationships with each other as well. This problem however is a head scratcher for us. Plus it isnt making us look very good that we havent found it yet, feeling the pressure to get it fixed. Trying to find a source to help lead me to what the problem may be, not one to point a finger, but I cant point at anything at the moment as well.

During my several visits I have tried to narrow the interference down to possible equipment in the building. I have done some signal hunting and when I look at the building and/or tower, the signal is not present. We find a faint bump around -95 when we try and sweep outside of the building but still not much is seen. It is intriguing that this issue is not at our site 20 miles away, an is no longer at the original site. We have wondered if it could be an itinerate user in the local area.

As far as lighting, we have arresters on all feedlines and non of them are damaged. The coverage is still what is has always been too, no change or reports from users in the remote service areas that they cant get into the repeater. Its for Fire, and they will report very quickly if there equipment isn't working right.

All connections have been checked, grounds checked as well. These are Motorola R-56 compliant sites and are inspected annually for anything wrong with the grounding system.

Both sites have self supporting towers, feedlines are all installed with standoffs and rubber donuts to keep coax from rubbing as well.

I will now go practice my breathing exercises, then keep plugging away. Thanks for the input so far, gives me stuff to look over and look back at the steps I have already done as well to see if I have missed anything.

Brian

[Bill_G]

Well, so far our experience is that specific noise you are hearing is generated by the Quantar xmitter interacting with a failure in the transmission system, and not by an external co-channel offender. It's a well known, but not well understood problem with VHF Quantars. If the noise is only occurring during the squelch tail, then turn on the front panel speaker, set the station for CSQ with the front panel button, and listen in as traffic passes through. If you can hear it in the Quantar speaker, then place a mobile on the same inbound freq on an open port of the rcvr multicoupler. If that hears it as well, then it's real. Now turn down the xmit power in 3db steps to see if it is being generated by the xmitter banging something in the transmission system.

[k7bpg]

Ok, gave it a try and included a video as well. When in csq all i hear is normal static noise, nothing else. Even had a call come over when in csq and it was clear as well. I put a portable tadio on the multicoupler as well, no noise present when in stby or receiving traffic. As we were running tests the noise came back at the other site we were having this issue with initialy. So now two sites are experiancing the same issue, 20 miles apart. On lunch now, will let you know what else we find.

https://youtu.be/uWwPGAQOqww

Brian

[k7bpg]

Ok, tried this and maybe found something. I keyed up the transmitter only and listened to the front speaker. I heard intermittant static through the front psnel soeaker. I then put a hand held on the same rx system and was recieving a carrier on the rx freq whith only the transmitter keyed. Mind you there was no filtering between the rx antenna and the portable radio. So it could have just desensed the portable.

Otherthan that i still havent found much. FCC was out today and they didnt fond any spurious emmissions or interferance out in the area. So kind of back at square one.

One final thing i want to try to either point the finger at the Quantar, or rule it out, is reprogram a GTR repeater we have at our site and see if the problem follows the frequency, or stays with the Quantar on the swapped freq with the GTR.

Long day again, off to rest my brain.

Brian

[k7bpg]

So a question on the topic of TX and RX systems regarding the howling sound, since it sounds like that is where most of you have found the issue when it has happened. I noticed yesterday at site B that there was a splitter on the RX antenna in the building that wend to a dangling coax. I will be removing this any way after talking to the user of that coax, he doesn't need it any more. I'm wondering if that could introduce some of this issue. It would allow for leaky RF to bounce back into the building. Be interesting to see when I get it removed when I go back up.

Had one of the Quantars that we swapped out on the bench today, due to this noise issue. I cant get it to make the noise on the bench, even having the TX and RX antennas sitting next to each other in the shop.

Brian