Odd Quantar "howl" issue

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NavyBOFH
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Odd Quantar "howl" issue

Postby NavyBOFH » Fri Oct 06, 2017 12:12 am

To preface this topic, watch this video here (not mine), and hear that loud noise at the end of the second transmission... that will be the exact sound I am referencing in the following troubleshooting.
https://www.youtube.com/watch?v=SRfvh0UTFHw
or
https://www.youtube.com/watch?v=QGoCjw7cJ_g

Amateur radio linked repeater seems to make this howl noise at night, which before it begins people were complaining the repeater was "deaf" and couldn't pick up even a 50w transmitter just some handful of miles away. When it finally locks up it makes this howl noise and does it for easily 15-20 minutes which means it must have happened enough to not max out the TOT in RSS, or some other issue.

All I know is my coworker took the repeater out of the shack and brought it back to my office. It was then physically checked (all connections) and they double-checked RSS programming and the pre-selector and such before calling it "good". They also disabled the TOT and let it sit an entire day transmitting into a dummy load apparently with no issue.

Now I get the repeater back and verify it before reinstall. The repeater is on 146.250 + PL 91.5, with the PA putting out right at 57w, and receive sensitivity was solid at -117dBm. Any lower and receive was intermittent. This was strange since I was told it was breaking at -124dBm just earlier that morning... I let it run through the TOT a handful of times and get it "warm" to the rest of the equipment in the shack and its still at -117dBm. Whatever - good enough on a 4-bay folded dipole up 340'.

My issue is the howl - which to me I think had nothing to do with a loose cable or anything else. And at this point I am sitting here waiting for a phone call to hear "it's happening again" which will be even more suspicious since they seem to only notice at night - prompting me to think its a temperature issue inside the shack (night time so AC won't run as often), or the EMS repeater on the same tower (and EMS building 150 yards away) both on VHF might be broadcasting something at that time causing intermod. Duplexer is a Commprod that's factory tuned to the frequency and still dead-on.

I've heard this sound before - but I can't for the life of me remember what I did to fix it! I swore it had something to do with the repeater's frequency reference but the last time I experienced this issue was in ~2010-2011 onboard a ship that had a Type II system set up and were having that noise across all voice channels - and eventually figured our external frequency reference I think was absolutely out-of-whack.

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Fatboy
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Re: Odd Quantar "howl" issue

Postby Fatboy » Fri Oct 06, 2017 1:49 am

Sounds like intermod..... Possibly intermod to something that transmits continuously. Darkness may be a lighting system or something contracting enough to cause the noise. Just remember: if you can hear it, you can see it (on the spectrum analyzer). If you can see it, you can track it (with a spectrum analyzer). FB

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Falcon
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Re: Odd Quantar "howl" issue

Postby Falcon » Fri Oct 06, 2017 3:29 am

Fatboy wrote:Sounds like intermod..... Possibly intermod to something that transmits continuously. Darkness may be a lighting system or something contracting enough to cause the noise. Just remember: if you can hear it, you can see it (on the spectrum analyzer). If you can see it, you can track it (with a spectrum analyzer). FB


Or issues with the duplexer and not isolating properly.
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Notarola
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Re: Odd Quantar "howl" issue

Postby Notarola » Fri Oct 06, 2017 3:21 pm

Its some type of packet info. It is looping where the rx is picking up the txr. It also isnt intermod as the noise does not appear to drop out mid 'burst'. It is interesting that the noise is heard even with the PL decode enabled.

If your in the area near the repeater I would look at the input freq and near it to see what is heard. I have seen packet systems set up with way too much deviation causing splatter.

kj4chv
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Re: Odd Quantar "howl" issue

Postby kj4chv » Fri Oct 06, 2017 11:27 pm

I have that happen on 2 site simulcast conventional system with 8 RX sites before. It ended up being one of the RX sites. The digitac would properly vote a site, during the transmission the particular RX site would pick up the transmitter. Once the proper voted site dropped the digitac voted that particular RX site that was receiving the TX side. The ATAC receiver had to be replaced.

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motorola_otaku
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Re: Odd Quantar "howl" issue

Postby motorola_otaku » Tue Oct 10, 2017 8:38 pm

Yep, I've encountered that many times. It is IMD, it's not Quantar-specific, and anything from poor duplexer tuning to a bad coax jumper can cause it. I would start with the duplexer, then check it with the repeater on the bench as a system, and work your way up the feedline to the antenna eliminating points of failure as you go.

One thing that will for sure cause it: running two or more repeaters into a single multiband antenna with a ham-grade (Comet, Diamond, etc.) diplexer. Using LMR400 or other braided coax as feed line or anywhere else on the full duplex side can also cause it.


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