MOTOROLA MTR2000 feedback/interference

[garthjonesfa]

Dear Alex/forum,

I have an ongoing problem with a repeater based at the top of a 14 story
building in a large city. I have a heavy military presence in the surrounding area and I am experiencing an 'echo' like feedback on the frequencies TX 158-.XXX and RX 153.XXX. 12.5 Khz WZ PL reverse burst.

I have accounted for ECM (electronic counter measure) from Military sources and this is not the cause. It sounds like the repeater is not disengaging after activation and the echo FB occurs at this point.

My main problem is that it is very intermittent and on some days happens only once or twice for 10-15 seconds and that is it. The repeater is getting heavy usage and I am using GP and GM Motorola sets.
The AE is co-located with a lot of other AE (Microwave/UHF/other VHF) but I have other repeaters located elsewhere in the country with similar circumstances and this problem has not occured.
Could it be other frequency breakthrough or possible coax emission? (there are a lot of coax in the comms room and roof feed channel)
I am sure the MTR is adequately suppressed but it sounds and looks to me like interference from another repeater on a close frequency.
There are 2 x GR500 located near my repeater, but AE's are well spaced.

[libuff]

i know exactly what you are talking about... don't know the cause... it was happening on the Queens West FDNY EMS Dispatch channel... the Communications coordinator reset the voter (as far as i know), and the system got better.

[jim]

One of our local multi-community dispatch centers gets this maybe 1-2 times a day on their VHF system. Most of their components are MTR2Ks. The "radio shop" never did repair this, although I'm convinced it is also a voter problem.


Are you operating a single point or do you have satellite receivers and a voter?

[garthjonesfa]

Guys,

Thanks for the replies, I am a little ignorant as to what a voter actually is, i am a Brit so perhaps it has another name? Also it is single point no sat bolted on.

I am at the end of my tether to be honest guys, short of massive relocation (which I am sure will nto cure it) I am at a loss.

Garth

[VA3XDJ]

This also happens a few times a week to a local EMS repeater.

I suspect the duplexer/filter is not rejecting the TX side enough so that because of the delay created by the repeater controller, it feedbacks and then goes into an endless loop.

I'm no pro, but I'd have the filters/duplexer checked.

[VA3XDJ]

Voter = Receiver that submits S-Meter reading and audio plus tones to a controller for the controller to then decide which receiver has best lock on signal and pass audio thru repeater.

My city police (Hull, Quebec) has several receivers that kick in at once resulting in distorted audio and loud PL tone along with the crackle of the dirty speakermics/batteries on their fleet of HT1000.

Believe it or not, they don't have a single mobile!

Also, the tech's adjusted the audio 'by ear' apparently. I'll submit a sample when I get proper audio into my soundcard.

[garthjonesfa]

Guys,

I only discovered this site by accident researching the problem on the
web. I am very glad I registered! I thank you all for your input and I will action the points you have made. I will keep you all posted. Many thanks to you VE2XDJ, I will be sat in front of said repeater today, laptop in hand, tweaking and pondering..

once again many thanx.

[mark102]

What does your "noise" sound like? We have been experiencing occasional noise at the end of our transmissions on a UHF MTR2000. It sounds like digital in a "hole" similar to an echo. This doesn't key the repeater but keeps the repeater up after a transmission. It comes and goes with no rhyme or reason. I have wav files of this noise.

[FMROB]

As my recent and very limited experience with repeaters installed in high RF enviromnets, it can be hell. Sometimes no matter what you do things get hosed up. The thing to start with is have an RF survey done of the sight. Ths data will allow you to better determine what filtering to install.
Sometimes it not your stuff. You could have installed this unit, without a problem for a year or so, but something else goes into service and causes grief, now your stuck.


Most likely some reciever filtering suchs as the units from angle linear will freatly help. http://www.anglelinear.com/ This guy is awesome, he will help you out, but you need to call him with all of the information. Once you determine your noise floor of the site, and the offensive frequency he can get you the stuff that you need.

This stuff is just on the RX side, but you should also address the output side with some filtering, circulator, etc.

You may also want to cehck out http://www.repeater-builder.com/rbtip/

Good Luck, Rob

[Dan562]

Hello Garth,

Some things you haven’t provided in this forum in your MTR2000 repeater hardware/software configuration:

Do you know the age of this station: Less than 1 year old, 2-4 years old, 5+ years old? The Serial # 512 ABC 1234 would help determine when it was built.

Was this unit tuned on frequency and shipped from the factory?

Does the repeater have X182 factory tuned duplexers or were the duplexers tuned out in the field?

Or

Is this VHF repeater using an external RF combiner system on the site?
Using separate Tx and Rx antennas on this site.

Is this repeater supplied with the X265 High Performancen Receiver Preselector?

Is the repeater using an OEM Receiver Pre-Amplifier to boost the sensitivity?

Are you aware of the other frequencies being used on this site or the surrounding area sites 10 to 20 air miles away? Since you have only provided the first 3 digits for Tx and Rx frequencies, I will provide some examples:

Tx 158.2125 MHz Rx 153.2125 MHz
Tx 158.5125 MHz Rx 153.5125 MHz
Tx 158.8125 MHz Rx 153.8125 MHz

Consider the second combination of frequencies as your pair and note the difference between the 1 & 2 and 2 & 3 are separated by 300 KHz. What you are looking for is an exact frequency separation of 100, 200, 300, 400, 500, 600, etc between lowest and the middle and the middle and the highest frequency pair.

In the MTR software there’s a repeater parameter known as “drop out time.” What is this set up for?

Since you are using PL, What is this parameter set up as PL Only or PL / Carrier Squelch? (Known as “And Squelch”).

Dan

[Will]

Since you are using PL, What is this parameter set up as PL Only or PL / Carrier Squelch? (Known as “And Squelch”).

This is a very important setting, PL AND carrier to activate the repeater, and loss of PL OR carrier to drop out the repeater.
Reffered to as AND/OR squelch. These are set in the RSS.

[radio-link]

In case there are several repeaters with the same shift very near to your repeater, or even at the same site, just do some intermodulation calculations, and you will see, whatever frequency pair you are using, IM products on the inputs show up! It only can be cured with good antennas producing not too much IM, 100% shielded cables (no RG-something, but 1/2" or better), good duplexers (preferrably with bandpass characteristics, no LP/HP ones), circulators, maybe reduced output power, RX attenuators, and similar measures. It really can be a PITA to get rid of such problems, and things get not easier when other repeater owners do not want to cooperate.