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Author Topic: Repeater noise along with audio when TX enters the air ..!  (Read 1361 times)

PY2HR

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Repeater noise along with audio when TX enters the air ..!
« on: June 10, 2019, 11:27:45 AM »

A loud snore or hum sounds when the repeater comes on.
Hello, Good evening to the whole community, I'm new here, sorry, error.

I'm losing my hair trying to eliminate that intermodulation snore when the TX of the VHF repeater 145.390 mhz (- 600khz) comes on the air only when I plug in the antenna.
Note: at 50ohms load in place of the antenna does not show, except with signals of -125dbm,
I work with IC-F211 VHF on the RX and a Motorola M120 on TX, 06-cavity duplexer ¼ wave and 01 antenna comet gp9-m, tightly adjusted with a 110dbm rejection insulation and 1.8db insertion loss even with those numbers not I can understand why this snore appears when connected to the antenna.
Has anyone ever encountered this problem?
Abs. Moises PY2HR
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W9IQ

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RE: Repeater noise along with audio when TX enters the air ..!
« Reply #1 on: June 10, 2019, 11:31:23 AM »

I am not exactly sure what you mean by "snore" in this context. It is probably a bad translation.

In any case, most duplexers will not do very well if they are presented with a mismatched load. Have you measured the SWR of your antenna system at the coax that plugs into the duplexer? Are you using double shielded coax cable for all connections near the repeater and duplexer?

- Glenn W9IQ

« Last Edit: June 10, 2019, 11:44:38 AM by W9IQ »
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

PY2HR

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RE: Repeater noise along with audio when TX enters the air ..!
« Reply #2 on: June 10, 2019, 12:09:10 PM »

Thanks for the reply, sorry for some errors, besides being a beginner here, I use Google Translate.
The repeater operates with a Comet GP9N antenna with SWR of 1.4, with approximately 25 meters of RG58 cable and an elektril duplexer of 6 ¼ wave cans with a rejection of 110db, the VSWR of the duplexer with 1,2, the jumper between the cans of ¼ / VL (about 13cm in length) with RG214, the jumper between the can and the radio with ½ / VL, interaction zero with a load of 50 ohms, when I put in the antenna comes a snore along with the audio.
« Last Edit: June 10, 2019, 12:22:42 PM by PY2HR »
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WB8VLC

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RE: Repeater noise along with audio when TX enters the air ..!
« Reply #3 on: June 10, 2019, 12:36:05 PM »

The RG-58  is probably the biggest problem and it is not recommended for repeaters of any type.

You should only be using good quality hardline coax and most important, do not use anything like LMR400 either as these types of cables are know to cause passive intermod (PIM) issues.

On the transmitter output, prior to the duplexer, you really should have an isolator/circulator and a following low pass filter to clean up the transmitter.

I'd say the use of RG-58 coax is the major issue here, once you get good hardline feedline this should make a large improvement.

Again do not use any type of LMR style coax for repeater service, only Hradline should be used especially if you are experiencing PIM issues as this is what it appears to be in your case.
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N2AYM

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RE: Repeater noise along with audio when TX enters the air ..!
« Reply #4 on: June 10, 2019, 01:27:57 PM »

Abs. Moises PY2HR - It sounds like what you are talking
about is NOT RF related but in fact may be the CTCSS
level on the repeater audio output. This level may be
a little on the high side and your receiver is not filtering
it out and passing it on to your receiver speaker.
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W9IQ

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RE: Repeater noise along with audio when TX enters the air ..!
« Reply #5 on: June 10, 2019, 01:30:44 PM »

Abs. Moises PY2HR - It sounds like what you are talking
about is NOT RF related but in fact may be the CTCSS
level on the repeater audio output. This level may be
a little on the high side and your receiver is not filtering
it out and passing it on to your receiver speaker.

I cannot think of a scenario where that would be the case with the antenna connected but not so when a dummy load is connected to the repeater...

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

PY2HR

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RE: Repeater noise along with audio when TX enters the air ..!
« Reply #6 on: June 10, 2019, 01:57:08 PM »

I'm going to exchange the RG58 for the RGC213 since the line is not so long at the maximum 24 meters I think I will not have a very big loss, I thought of replacing the jamper with RG214 for RG400 of double silver mesh to try to improve some RF leak what the Do you think so?
« Last Edit: June 10, 2019, 02:00:50 PM by PY2HR »
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WB8VLC

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RE: Repeater noise along with audio when TX enters the air ..!
« Reply #7 on: June 10, 2019, 09:48:58 PM »

Yes anything but RG-58 should be a quantum improvement.

Remember these old wise ham sayings and you'll be ok.

Friends don't let friends use RG-58 anywhere near a repeater nor above 1.8 MHz.
Friends don't let friends use G5RV's for any serious HF DX.
friends don't let friends use Endfed half craps for serious HF DX.
Friends don't let friends start out in ham radio using 2meters.
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NA4IT

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RE: Repeater noise along with audio when TX enters the air ..!
« Reply #8 on: June 11, 2019, 03:34:39 AM »

Even some small hard line cable would be better.
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N8EKT

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RE: Repeater noise along with audio when TX enters the air ..!
« Reply #9 on: June 11, 2019, 05:42:17 PM »

The fact that this problem doesn't exist with a 50 ohm load
in place of the antenna shows that no issues exist with coax jumpers or the duplexer.
Yes you should use a better feedline to the antenna and something designed for repeater use like heliax.
And if you insist on using a GP9 i would solder all joints of the antenna before using it on a repeater or you will be plagued by static and desense at sone point.



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W1BR

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RE: Repeater noise along with audio when TX enters the air ..!
« Reply #10 on: June 11, 2019, 06:44:41 PM »

What are you seeing on the receiver limiter and discriminator when the transmitter is keyed? This sounds like an externally generated IMD product.  Can you try a temporary antenna on the system? 
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KB8VUL

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RE: Repeater noise along with audio when TX enters the air ..!
« Reply #11 on: June 24, 2019, 09:52:50 PM »

OK, need more info.
First eliminate the obvious, no RG-58 or LMR anything for cable.
If you are working on the cheap and are running PL-259 connectors and not N connectors, then locate some FSJ1-50 cable and get the Amphenol stlye connector and RG-8X reducers and solder the sheild to the reducer and assemble the end and solder the center pin.  This is still hard line and for VHF it will be plenty good for jumpers.

Now tell us about the environment that the repeater is in.  I assume it's at a tower site, but what else is there?  AM/FM broadcast, TV, commerical and public safety?  Or are you the only one on the site?
Do you have an actual service monitor to take readings with and a spectrum analyzer or are you just winging it?

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PY2HR

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Re: RE: Repeater noise along with audio when TX enters the air ..!
« Reply #12 on: April 19, 2020, 11:49:50 AM »

The RG-58  is probably the biggest problem and it is not recommended for repeaters of any type.

You should only be using good quality hardline coax and most important, do not use anything like LMR400 either as these types of cables are know to cause passive intermod (PIM) issues.

On the transmitter output, prior to the duplexer, you really should have an isolator/circulator and a following low pass filter to clean up the transmitter.

I'd say the use of RG-58 coax is the major issue here, once you get good hardline feedline this should make a large improvement.

Again do not use any type of LMR style coax for repeater service, only Hradline should be used especially if you are experiencing PIM issues as this is what it appears to be in your case.


Hello dear WB8VLC colleague,
I'm losing all my hair with this problem,
I made all the changes in the VHF repeater station, according to the recommendation I changed the descent cable to the hardline, I changed the jumpers between the cans to RG400 double silver mesh, I removed the GP9 and put the cushcraft ARX-2B ranger in the first perfect moment the noise completely disappeared and the receiver was 100% zero interaction, opening a squelch with -120dbm, a perfect, clean signal. As soon as the day was falling, at dusk the noise came back, it's like an ugly snore along with the audio and the receiver was desensitized as if it were an interaction, when I turn the transmitter off, the audio clears, the next morning without touching anything cleaning has regained its sensitivity even so there are periods of the day and night that the tinnitus bothers again. I moved the RPT and it was still the same. I believe I am having the passive intermod (PIM). Tell me more about isolator / circulator and PIM, if there is anything else I can do to solve it?

Att, Moises C.
PY2HR
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WB8VLC

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Re: Repeater noise along with audio when TX enters the air ..!
« Reply #13 on: April 19, 2020, 12:55:52 PM »

During a time when the repeater is working fine during the day, apply some cold spray to the duplexer, connectors, the isolator to see if it is temperature related.

It sounds like by replacing either poor cable or a poor PIM antenna has lowered the noise floor but it has also opened up a 2nd or even a 3rd temperature issue when night time produces cooler temperatures.

Come to think of it, It may be easier when nighttime comes and the noise appears to simply apply heat with a heat gun to the repeater components to see if the noise goes away. 

Repeater noise fixes are like peeling away layers on an onion, each layer produces a fix while lowering noise floor but this also exposes other issues.

Once you peel the layers away and fix each issue you should eventually get the repeater working.

I had a 6 meter noise issue that was actually several small issues and the individual fixes took months to resolve.
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PY2HR

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Re: Repeater noise along with audio when TX enters the air ..!
« Reply #14 on: April 20, 2020, 09:24:24 AM »

During a time when the repeater is working fine during the day, apply some cold spray to the duplexer, connectors, the isolator to see if it is temperature related.

It sounds like by replacing either poor cable or a poor PIM antenna has lowered the noise floor but it has also opened up a 2nd or even a 3rd temperature issue when night time produces cooler temperatures.

Come to think of it, It may be easier when nighttime comes and the noise appears to simply apply heat with a heat gun to the repeater components to see if the noise goes away. 

Repeater noise fixes are like peeling away layers on an onion, each layer produces a fix while lowering noise floor but this also exposes other issues.

Once you peel the layers away and fix each issue you should eventually get the repeater working.

I had a 6 meter noise issue that was actually several small issues and the individual fixes took months to resolve.

Ok dear WB8VLC, Thanks for the reply, it was very useful, I will follow your guidelines, heating or freezing the moment you have the defect. By the way adding a few more questions below,
- Can the distance from the antenna element to the tower generate PIM ?
- Lack of grounding in the tower can generate PIM ?
- The distance or the position of the repeater in relation to the antenna must be observed ?

Att.
PY2HR
Moises C.
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