Can you identify this noise on a voted radio system?

[70cutlass442]

I have been chasing this sound for some time. I am using Moto gear for uplink/downlink and a GE voter with an Icom exciter through a TPL amp. Noise begins around 16 seconds and continues for awhile. You can hear the other units successfully step over the noise but it returns.

Any ideas?

 

[Thunderknight]

There was nothing linked/attached.

 

[70cutlass442]

There was nothing linked/attached.

I just realized that the supported format (.wav) is not allowed. It appears to let me upload it after it's zipped.

 

[mmckenna]

Where are you pulling the audio from?

 

[70cutlass442]

Where are you pulling the audio from?

The audio comes from the Evintide in dispatch. They operate a fixed base interfaced to a mindshare console so this audio is taken between the tone adapter (ITA2000) and the console.

 

[Thunderknight]

Reminds me of the FDNY 9/11 tapes. Heard the same squeal like sound on them.

 

[mmckenna]

is it heard on the RF side, or just the fixed base -> tone remote -> console side?

 

[nd5y]

I have heard similar feedback type noises:
1. During band openings when two VHF repeaters with reversed input and output frequencies with the same tone lock each other up.
2. Two repeaters locked up because of intermod mixing allowing each repeater's receiver to hear the other's transmitter.

 

[rescue161]

Sounds like RF from another transmitter on a combined antenna system. I had the same thing happen on my setup. Realigned the circulators, transmit combiner cans and the RX multi-coupler pass cans and the problem went away. It was one of the digital repeater transmitters getting into the analog repeaters receiver.

Find out who else is on or near your receive antenna(s) and listen to their TX frequencies. If the noise appears when they are transmitting, then hunt down the affected site and realign and or add filtering.

 

[12dbsinad]

You my friend have a classic case of intermod. A few things. What band? What is you're antenna setup? Are you running full duplex thru 1 antenna?

Intermod can be generated by literally anything, including rusted antenna hardware to other RF mixing at a congested site. Sometimes it isn't easily solvable. Mote details would help.

 

[AlexC]

I think as others suggested you need to narrow down the source of this. This isn't the typical intermod that I hear, and, suspect this could be leased line related - but there is not enough information in your post about how the system is constructed. Instead - i'll give you how I'd troubleshoot the issue.

With the issue happening:
What happens when you disconnect your console and listen with a buttset? Do you hear the noise?

What happens when you disconnect the tone remote adapter and listen on the end of the copper going to the console? Do you hear the noise?

What happens when you listen from the pins on the back of your repeater / voter? Do you hear the noise?

Does the voter indicate what site the audio comes in on?

Can you isolate this to a site by turning them off/on via the boter while the noise is happening? With bringing the sites up/down can you make it go away?

Can you listen to the audio from each site while it is happening using a buttset?

Does it happen on every site?

What happens if you bypass the voter and run in-cabinet repeat? Does the sound continue to happen?

Have you been up to each site with a spectrum analyzer to look for on-channel nose?

Have you looked at your MUX's (if you are running Fiber, T1, etc,. to see if you can isolate it to a port? Try listening to the DS0's using a T1 tester and see if you hear the noise on the DS0's if it's a T1.

Can you change out the ITA and the power supply?

Reseat cards in the Voter (kind of spectratac thing to do - but almost like every now and again you have to turn the pots to keep them happy, this might be similar in a GE voter?

Have you done an intermod study at all your sites?

Do you have any BDA's or Emergency Responder Radio Coverage systems in place? (oh you'll see these on a spec an at your master transmit site pretty clearly - and if you find noise external to your system there is a REALLY high chance these are to blame...)

Have you gone through your duplexers with a service monitor lately?

Check any RX Filters or Multicouplers? If you are using those scanner multicouplers that one of our local vendors LOVES to sell w/out filtering........ blame that.

How about checking antennas? VSWR, Return Loss, and DTF? Compare it to last PM.

Do you have Triple Circulators on everything?


Alex

 

[12dbsinad]

In listening to the file again, it sounds more like it's "hearing itself" or "mixing". Def need to camp out at the prime site to figure out whats going on. In my experience, mixing can be local like shielding issues or mixing in other people's equipment. I've even had a system (VHF) mix with a 2.4 Cadmium radio before that was used for the linking. The possibility's are endless.

Sometimes, if it's not an issue you can directly fix, turning down the TX power slightly "could" make it stop.

 

[com501]

You could call a competent radio shop with the correct test equipment. - just sayin'.

 

[NVAGVUP]

I suspect the system TX/RX CTCSS (or DCS) tones are identical. It sounds like this issue happens after someone "talk's into" system. The intermittent noise bursts AFTER dekey are symptomatic of intermod. Spurious emissions among other things can be a cause.

A red flag for me is the use of an exciter with a 3rd party PA. A transmitter that is rated for 85 db+ of spurious and harmonic attenuation is a start. If you are in a RF dense environment, I would recommend also recommend TX isolator. (# of stages depends upon RF/Site environment)

Drop your repeater/transmit drop out delay to zero. This does not "cure" IM. It only masks it as it minimizes the amount of time the transmitter and undesired system RX/interference coexist.