Noise Interference on Repeater…

[ramal121]

some kind of interference issue when a radio stops transmitting on it.

The interference sounds like squelch but coming through a tube..

So a radio stops transmitting and the repeater hangs on for a time with a weird static noise that sounds like it's being sucked down a bathtub drain. Correct? This is a classic example of IMD. An image of your transmit signal is showing up on your receive frequency and it retains enough of the PL/DPL component to keep the receive open and the repeater active.

So where could the problem lie? Anywhere in the antenna system and beyond. Suspect any component that is not PIM rated. Duplexer, connectors, coax (beware LMR users), antenna (not a fan of Diamond/Comet for duplex service), Also site environment. Rusty bolts, dissimilar metal contact, active equipment without proper filtering and on and on.

You can try and sniff around with a a good spectrum analyzer with high dynamic range or some receiver with heavy pass filtering so you can weed out the lowest of signals floating back to your receiver. Barring that you have a slog ahead as you swap out components looking for a change. A dummy load is a worthy troubleshooting aid in cases loke this.

 

[drake1792]

So a radio stops transmitting and the repeater hangs on for a time with a weird static noise that sounds like it's being sucked down a bathtub drain. Correct? This is a classic example of IMD. An image of your transmit signal is showing up on your receive frequency and it retains enough of the PL/DPL component to keep the receive open and the repeater active.

So where could the problem lie? Anywhere in the antenna system and beyond. Suspect any component that is not PIM rated. Duplexer, connectors, coax (beware LMR users), antenna (not a fan of Diamond/Comet for duplex service), Also site environment. Rusty bolts, dissimilar metal contact, active equipment without proper filtering and on and on.

You can try and sniff around with a a good spectrum analyzer with high dynamic range or some receiver with heavy pass filtering so you can weed out the lowest of signals floating back to your receiver. Barring that you have a slog ahead as you swap out components looking for a change. A dummy load is a worthy troubleshooting aid in cases loke this.

I have a CommScope DB-404 that I purchased a while ago just haven’t had the time to put it up. Would this be a better antenna for duplex operations?? I’m using 1/2” heliax so I don’t think the issue could lie in the cabling or the jumpers since they are double shielded RG400. Looking into a new duplexer so switch out with this flatback Celwave.

 

[ramal121]

Heliax and RG-400 are just fine, wouldn't use anything less. The DB-404, although not perfect, since it's new shouldn't be an issue and many used for repeater service. You should also use quality connectors. Silver with a gold center pin. Forget those cheap plated ones.

As far as what's beyond the antenna do you have any other radios or antennas nearby? Anything electronic like routers, access points, satellite dishes, etc? If you are testing at the house is your antenna nimble enough to move it around to different locations just for giggles?

Be aware that IMD requires your transmitter plus one or more other signals to create the mixing products in a non linear junction needed to produce the phantom signals that is bringing you grief.

Desense, on the other hand, can be created by just your transmitter alone. One is a strong off channel signal that bum rushes your receiver and causes the limiter to reduce it's sensitivity. Usually from a maladjusted duplexer. The other cause is where the high transmit power reacts to a defect somewhere and makes a wide band noise component. Think of a spark gap transmitter and is usually more prevalent at lower frequencies. This creates a "RF fog" that makes it difficult for the receiver to pick out low level signals. Sub par transmitters will also emit an excess of phase noise and do the same thing.

What sets this apart from your problem is that desense in and by itself will not hold your repeater open with the sucky noise as long as it's PL protected. Regardless desense is a common problem and should be checked after site install and periodically afterwards.

To be honest with you, if I was to stumble across a repeater that exhibits your symptoms my face would blank, my shoulders fall and I'd call the office to tell them that this make take some time.

 

[drake1792]

Heliax and RG-400 are just fine, wouldn't use anything less. The DB-404, although not perfect, since it's new shouldn't be an issue and many used for repeater service. You should also use quality connectors. Silver with a gold center pin. Forget those cheap plated ones.

As far as what's beyond the antenna do you have any other radios or antennas nearby? Anything electronic like routers, access points, satellite dishes, etc? If you are testing at the house is your antenna nimble enough to move it around to different locations just for giggles?

Be aware that IMD requires your transmitter plus one or more other signals to create the mixing products in a non linear junction needed to produce the phantom signals that is bringing you grief.

Desense, on the other hand, can be created by just your transmitter alone. One is a strong off channel signal that bum rushes your receiver and causes the limiter to reduce it's sensitivity. Usually from a maladjusted duplexer. The other cause is where the high transmit power reacts to a defect somewhere and makes a wide band noise component. Think of a spark gap transmitter and is usually more prevalent at lower frequencies. This creates a "RF fog" that makes it difficult for the receiver to pick out low level signals. Sub par transmitters will also emit an excess of phase noise and do the same thing.

What sets this apart from your problem is that desense in and by itself will not hold your repeater open with the sucky noise as long as it's PL protected. Regardless desense is a common problem and should be checked after site install and periodically afterwards.

To be honest with you, if I was to stumble across a repeater that exhibits your symptoms my face would blank, my shoulders fall and I'd call the office to tell them that this make take some time.

The fiberglass antenna is mounted on a 10ft mast along with a discone scanner antenna right next to it along with an uhf yagi antenna for my base station. Also, where can I purchase some good quality RG400 jumpers from?

 

[ramal121]

Oooh yeah. Disconnect the antennas from the scanner and base and then test the repeater. Easy peasy. Unfiltered and unprotected radios are a definite possibility for clams.

We order our jumpers from Tessco or Talley but any good supplier should help you. There's a bunch.

 

[paulears]

Have we actually established a really good description of the problem?
The interference sounds like squelch but coming through a tube
I cannot imagine what you mean, so a recording would be good. Squelch closing, the favourite noise of movie radios - I can picture but interference that sounds like squelch in a tube, is mystifying it.

The advice you got - to turn off the TX and listen to a weak signal and then see if it changes in noise level when the TX is turned on is the best advice I feel.
.
I also suspect that the other comments about things like the power level make sense too. 50W and the small cavities is probably going to mean the filters won't be able to notch enough to prevent desense, so often increasing the power is counter productive.

I have a UHF repeater on my house - I'm on a hill, but my office is in the harbour. The repeater filters were tuned in the office, put in my van and driven 2 miles. That was enough to shift the tuning enough that I had to bring the gear home to do it again. They are that sensitive.
Ramal pointed out the 'hanging' - perfectly valid, but we're guessing what you mean as the description is a bit 'empty' - we're sort of reading into your explanation things we suspect might be going on, but we cannot be sure. We're making educated guesses.

I'm concerned that the filter was 'tuned' for 15 frequency pairs. I have a Retevis multi-channel repeater, and the filter tuning is OK across a couple of next door channels, but then drops off. If your filter was tuned on a different radio, then put in the post - I'd bet the tuning is way off!

 

[drake1792]

Have we actually established a really good description of the problem?
The interference sounds like squelch but coming through a tube
I cannot imagine what you mean, so a recording would be good. Squelch closing, the favourite noise of movie radios - I can picture but interference that sounds like squelch in a tube, is mystifying it.

The advice you got - to turn off the TX and listen to a weak signal and then see if it changes in noise level when the TX is turned on is the best advice I feel.
.
I also suspect that the other comments about things like the power level make sense too. 50W and the small cavities is probably going to mean the filters won't be able to notch enough to prevent desense, so often increasing the power is counter productive.

I have a UHF repeater on my house - I'm on a hill, but my office is in the harbour. The repeater filters were tuned in the office, put in my van and driven 2 miles. That was enough to shift the tuning enough that I had to bring the gear home to do it again. They are that sensitive.
Ramal pointed out the 'hanging' - perfectly valid, but we're guessing what you mean as the description is a bit 'empty' - we're sort of reading into your explanation things we suspect might be going on, but we cannot be sure. We're making educated guesses.

I'm concerned that the filter was 'tuned' for 15 frequency pairs. I have a Retevis multi-channel repeater, and the filter tuning is OK across a couple of next door channels, but then drops off. If your filter was tuned on a different radio, then put in the post - I'd bet the tuning is way off!

I’ll try my best to get an audio clip but the repeater hasn’t been doing it recently since I programmed it to only open the receiver to Carrier and CTCSS and turned the power down to 25watts.

 

[paulears]

I’d call it a win then! I suppose that the only thing to do now is to listen to the traffic and work out where they are and their power levels. I’d try to build up a map. Colour coded marks green yellow or red, that kind of thing. Where you get people struggle, work out if that’s on higher powers or portables. It will then make a match with landscape hopefully, as in red in one direction is fine if there’s a hill in the way. Green is less impressive if they’re high etc etc.

 

[prcguy]

I’ll try my best to get an audio clip but the repeater hasn’t been doing it recently since I programmed it to only open the receiver to Carrier and CTCSS and turned the power down to 25watts.

That's fairly typical with mis tuned duplexers where the isolation is inadequate at some power level then you lower the power and its ok. In this case you reduced power by 3dB reducing the needed isolation by 3dB. You could still have desense and the test described above turning the transmitter on and off while receiving a weak signal will reveal that.

With a high quality repeater with good receiver and minimal transmitter noise/junk you should be able to operate just fine on a flat pack duplexer at 25w if the duplexer is tuned and meeting specs. Maybe more but only if its meeting specs and you need test equipment to verify or tune the duplexer.

 

[drake1792]

That's fairly typical with mis tuned duplexers where the isolation is inadequate at some power level then you lower the power and its ok. In this case you reduced power by 3dB reducing the needed isolation by 3dB. You could still have desense and the test described above turning the transmitter on and off while receiving a weak signal will reveal that.

With a high quality repeater with good receiver and minimal transmitter noise/junk you should be able to operate just fine on a flat pack duplexer at 25w if the duplexer is tuned and meeting specs. Maybe more but only if its meeting specs and you need test equipment to verify or tune the duplexer.

Understood. My plans are to replace the X50C with the DB-404 and then try and get the highest isolation I can with a different duplexer (not a flatback) and hopefully that will give me a pretty solid and reliable system.

 

[vagrant]

With a high quality repeater with good receiver and minimal transmitter noise/junk you should be able to operate just fine on a flat pack duplexer at 25w if the duplexer is tuned and meeting specs. Maybe more but only if its meeting specs and you need test equipment to verify or tune the duplexer.

Quite true indeed. I was able to run 50W out of a Motorola UHF Quantar into a flatpack notch filter...after tuning using an HP 8935...and it worked at most 20 miles away with my low level setup. Still, not all flat pack notch filters are the same and I agree on the 25W to avoid problems, or get a regular BpBr duplexer and then "tune it onsite". Ahh...I have learned much from this forum.

 

[prcguy]

Quite true indeed. I was able to run 50W out of a Motorola UHF Quantar into a flatpack notch filter...after tuning using an HP 8935...and it worked at most 20 miles away with my low level setup. Still, not all flat pack notch filters are the same and I agree on the 25W to avoid problems, or get a regular BpBr duplexer and then "tune it onsite". Ahh...I have learned much from this forum.

I've had repeaters everywhere on high mountains with some impressive range but right now I have two at my house, a UHF amateur and a GMRS both running about 50w out of the duplexer. The antenna is a DB Products, don't know the model but its two 4-bay DB411s on a single mast with separate feeds. Top antenna to the amateur and bottom to the GMRS and the base of the antenna is about 15ft above ground. Just the other day I was about 75mi away using the amateur repeater just fine. Duplexers are a PD-526 on the GMRS and a newer Motorola 1500 series, maybe a T-4085A on the amateur repeater with an extra pass can on the transmitter and a pass/reject can on the receiver feeding a preamp. All duplexers and filters were tuned on site in the rack.

I used to have two amateur repeaters on the same frequency using a transmitter combiner with one providing analog FM/P25 and the other in Yaesu Fusion mode. There was a splitter after the preamp to feed both receivers. Anyway, no desense, very good performance and the DB Products antenna works much better than a Comet GP-9N that I used previously in the same tower location.

 

[vagrant]

That Motorola/Celwave/Phelps Dodge PD526-4-2 six cavity UHF duplexer has some fantastic numbers for isolation and insertion loss. I like what I see much more than actual implementation of a Motorola 1500, but we work with what we have. Also, that is interesting about the GP-9N compared to the DB411 when using the lower two. I'm figuring the pattern is improved increasing the gain where you want it.

I sold my UHF Quantar to a buddy and it now has a home 3000' above me, so I still get to enjoy it and it runs in mixed mode replacing an analog repeater. I still use two Yaesu repeaters with a flat pack on UHF and a Telewave TPRD-1484 for VHF. I'm thinking of removing one though and releasing the pair.

@drake1792 - After I acquired that HP 8935 I was able to do things right and configure my duplexers onsite. Hopefully you have someone nearby that can assist you like a radio business, or even an amateur radio club...if they have the right device...and have a clue.

 

[drake1792]

That Motorola/Celwave/Phelps Dodge PD526-4-2 six cavity UHF duplexer has some fantastic numbers for isolation and insertion loss. I like what I see much more than actual implementation of a Motorola 1500, but we work with what we have. Also, that is interesting about the GP-9N compared to the DB411 when using the lower two. I'm figuring the pattern is improved increasing the gain where you want it.

I sold my UHF Quantar to a buddy and it now has a home 3000' above me, so I still get to enjoy it and it runs in mixed mode replacing an analog repeater. I still use two Yaesu repeaters with a flat pack on UHF and a Telewave TPRD-1484 for VHF. I'm thinking of removing one though and releasing the pair.

@drake1792 - After I acquired that HP 8935 I was able to do things right and configure my duplexers onsite. Hopefully you have someone nearby that can assist you like a radio business, or even an amateur radio club...if they have the right device...and have a clue.

I have a couple guys in a local amateur group that might be able to help me out. Just gonna keep upgrading equipment and seeing how good I can make this system…

 

[drake1792]

Just for reference, out of these three duplexers which would benefit me most with what I want to accomplish… any better than others? More cavities, better isolation…? Still learning about duplexers..

 

[prcguy]

Just for reference, out of these three duplexers which would benefit me most with what I want to accomplish… any better than others? More cavities, better isolation…? Still learning about duplexers..

The bandpass only has no notch and uses an extra cavity in each leg to get reasonable isolation. That ads at least another half dB of loss in each leg. The Other Telewave is configured as a four cavity window filter and not a duplexer. The last one is a really good duplexer but looks like someone has been diddling with it as the T connector doesn't look stock and that would add unnecessary length to the cables. You also have to inspect the cables, old versions have single shield RG-8 coax while newer version will have dual shield RG-214 or smaller double shield RG-142, which is mo betta.

 

[drake1792]

The bandpass only has no notch and uses an extra cavity in each leg to get reasonable isolation. That ads at least another half dB of loss in each leg. The Other Telewave is configured as a four cavity window filter and not a duplexer. The last one is a really good duplexer but looks like someone has been diddling with it as the T connector doesn't look stock and that would add unnecessary length to the cables. You also have to inspect the cables, old versions have single shield RG-8 coax while newer version will have dual shield RG-214 or smaller double shield RG-142, which is mo betta.

I guess my overall question is what kind of duplexer should I be looking for if my repeater will be living on its own site with no other transmitters or receivers nearby.

 

[prcguy]

I guess my overall question is what kind of duplexer should I be looking for if my repeater will be living on its own site with no other transmitters or receivers nearby.

If you want to run full 100w or whatever your repeater will put out a good 4 cavity pass/notch with 5" cans should be fine. Motorola 1500 is ok, Telewave, DB Products, Wacom, EMR, etc.

 

[drake1792]

If you want to run full 100w or whatever your repeater will put out a good 4 cavity pass/notch with 5" cans should be fine. Motorola 1500 is ok, Telewave, DB Products, Wacom, EMR, etc.

Sounds good. I appreciate all the help from everyone. Lots of knowledge gained!

 

[drake1792]

I’ve think I found something that looks like it’ll work good…idk (take a look at attached) Says Motorola brand. Is this similar to the 1500 you were talking about..?