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The DMR Interference Case
By Robert W. Meister WA1MIK
I am the trustee of the 444.450 MHz (+5 MHz input) voice repeater located in northern Hamden, CT. The repeater has been on the air since 2001 and is fully coordinated. It runs PL encode and PL decode. The antenna is at about 1,100 feet Above Sea Level (ASL) and the repeater covers about a 45 mile radius.
In 2014 I had to change the repeater to "closed" status because of the sudden influx of unlicensed teenagers purchasing inexpensive Asian portable radios and using the repeater without having an amateur license. I also shut the repeater off for a while and changed the PL code.
In August 2016 Jim N1GTL asked me to turn the repeater back on and make it "open", so I reprogrammed the PL code and we chatted for a while. The next morning the repeater was being keyed up perhaps a dozen times per hour, sometimes with no audio, sometimes with some sort of digital-sounding noise burst that was breaking through the PL decoder. It was worst in the early morning: 3am to 10am, and occasionally in the evening around sunset. This time of year and these times of the day are particularly susceptible to a phenomenon called "ducting" where distant signals can go great distances while conditions exist.
In an attempt to figure out what signals were actually being received, I disabled the PL decoder on the repeater receiver. Solid digital data was coming through. Jim recognized it as a MOTOTRBO DMR signal. At first I thought it was a user's digital radio, but DMR radios alternately transmit and receive, due to the two time-slot nature of the MOTOTRBO protocol, and this was a continuous data stream that sounds quite different from what a user radio outputs. That meant the repeater was picking up the output of a DMR repeater (which transmits continuously while being used) that had popped up sometime earlier in the year.
Frequency coordinator's band plans from New England through the mid-Atlantic region specify that UHF repeaters on frequencies ending in X00 and X50 should receive high, transmit low, just as my repeater does (it receives 449.450 MHz and transmits 444.450 MHz), while repeaters on frequencies ending in X25 and X75 should receive low, transmit high. There's an upside-down repeater up in Deerfield New Hampshire 150 miles away from my repeater and I suspect it was put into service before the coordination guidelines standardized on in-hi, out-lo. As far as I know we peacefully co-exist on our frequencies with different PL codes. There's also a large linked repeater system in Rockland County NY run by Adam N2ACF on the same frequencies as my repeater, and we peacefully co-exist as well.
So, back to the DMR signal. In an attempt to identify the signal, Jim N1GTL configured a CDM mobile radio in carrier squelch with a VOX-operated voice recorder and we connected it to the receiver port of the repeater's duplexer. Over one weekend we had in excess of 1,000 recorded keyups; none revealed any CW ID.
I brought my Agilent spectrum analyzer up to the site one morning at 6:30am and viewed the receiver port of the repeater's duplexer. The minimum signal I was able to detect was -120dBm, or 0.224 microvolts, yet I saw nothing even when the repeater receiver was decoding the DMR signal. So that didn't do much good either. I did notice a lot of signals in the 450-455 MHz band and thought perhaps there was some sort of inter-modulation product that was producing a signal that the repeater was picking up.
Jim and I were considering temporarily installing a Motorola XPR 8300 MOTOTRBO repeater at the site, thinking it had a better receiver than what I was using at the repeater. But before we got to that point, I remembered that I had a 4-cavity band-pass filter in the basement that I had used on a previous repeater. It was tuned to pass 449.450 MHz and had a 4 MHz bandwidth. I connected it to my spectrum analyzer to check the tuning and also retune it a bit lower, so it would cut off anything above 450 MHz. In the process of tuning the filter, I figured that, since it's a band-pass filter, I should use a Return Loss Bridge (RLB) to tune it, so I tried, but that was a complete failure and I got different results depending on which jack on the filter I connected the RLB to, so I retuned it with just the spectrum analyzer. Jim and I installed the filter and left it sitting outside the repeater cabinet (the site is very secure).
As it turns out, the band-pass filter was completely ineffective because the signal I was trying to filter out was coming in on the repeater's input frequency. But I didn't know that when I put it in, thinking it was some out-of-band signal. It did absolutely nothing to the on-channel interference but it did reduce the sensitivity of the receiver by 1.9dB. The filter will be removed very soon.
Wondering why I couldn't use the RLB to tune the filter, I asked Kevin W3KKC about it, because he manufactures and sells RLBs and ought to know how to use them to tune filters. Kevin provided great information and suggested I ask Jeff WN3A for additional information, because Jeff is the local RF guru. So I fired off an email to Jeff. He replied that the RLB would only be useful to fine-tune the input and output match of the filter and a Vector Network Analyzer (VNA) is really the preferred tool to use to tune a multi-section filter like the one I had. Lacking that piece of equipment, the spectrum analyzer was the best I could use.
I thanked him and casually mentioned that I had to retune the filter to put it on my repeater input to try to eliminate some interference I was getting that sounded like DMR data. Jeff responded almost immediately on Tuesday August 23, 2016, with the following:
"Well, I can tell you EXACTLY what your interference source is, and I'm not even looking at a spectrum analyzer! It's an uncoordinated DMR repeater in northern New Jersey on 449.450-. Aside from bothering you 100-ish miles away, Adam N2ACF in Rockland County, NY is also on 444.450+ and only 35 miles away and is being blown completely out of the water. Adam has already filed an interference complaint [with ARCC (Area Repeater Coordination Council)]."
As Adam Savage of Mythbusters is fond of saying, "Well, there's your problem!"
As a result of Jeff's email, Adam N2ACF and I started chatting via email, and on Wednesday August 24, 2016, Adam sent an email to the trustee of the DMR repeater informing him of the interference that has been traced to him, and I quote:
"My name is Adam and my call sign is N2ACF. I'm the owner of the 444.450, coordinated repeater, in Rockland County, NY. My input has been getting crushed by the output of your DMR repeater due to the fact that you chose to reverse the repeater pair from what the standard band plan calls for.
I'm curious; is there any particular reason why you did that?
Copied on this email is Bob WA1MIK. He owns the 444.450 repeater in Connecticut. While he too is getting hit on his input from DMR (we're not sure if it's also from you) he and I have lived peacefully for MANY years on the same pair, AND we often hear each other's machines too! However, what little we hear from each other is not nearly the harmful interference that I'm getting from you as your output frequency is on my input frequency!
In my 26 years of playing with repeaters, typically when one guy interferes with another, the guy being interfered with wants the other guy thrown off the pair. In our case, I would appeal to you to stay on the pair but REVERSE it to be within the band plan. I'm sure the three of us can live happily ever after, so long as we're not on each other's input frequency.
So again, was there any particular reason why you decided to reverse the pair? Are you willing to reverse it?
Any help would be greatly appreciated.
This elicited a reply from the trustee of the DMR repeater on Thursday, August 25, 2016, which I again quote in its entirety:
"This is the first email I have received from anyone about this issue.
My apologies. I was not aware that I was causing interference with my repeater in Hopatcong (449.450). I had been using this pair for a long time now, and this is the first report of interference. I also have been a repeater owner for many years now and switched my repeaters to MOTOTRBO (DMR) during the last two years. Of course I do not want to cause interference and will do whatever I can to correct this issue. I will make arrangements to change the frequency ASAP in order to eliminate the interference."
On Friday August 26, 2016, the trustee disconnected his MOTOTRBO repeater from the DMR network, but the repeater itself was still usable by local amateurs, and both Adam and I continued to receive interference from it. A couple of days later, the trustee and Adam spoke on the telephone, at which time Adam asked the trustee why he chose this pair. Adam reports the trustee's response, which is interesting:
"He advises that he's been seeking coordination from ARCC for OVER 15 years now, and they've been illusive at best. He monitored the frequency like I do, from a mountain top receiver, and heard nothing. DUH!! He's listening on our INPUT!!!!"
After another email from Adam to the trustee imploring that he shut the repeater off, finally, at 3pm Monday August 29, 2016, the trustee reported that he went to the repeater site and pulled the plug. His repeater is finally off the air. Not surprisingly, the DMR keyups and interference to my repeater ceased. It's been 100% quiet except for legitimate voice traffic for over two days now.
I don't know exactly where the DMR repeater was located, however the majority of the land in the town where he lives is between 900 and 1,100 feet ASL. I can only assume that his repeater was in a higher location. Add a tower to that elevation and you soon discover there's nothing in the way between the DMR repeater and mine, about 100 miles away.
Adam apparently wasn't hearing the digital data bursts because his repeater receivers are using mechanical PL reed decoders. In my repeater the receiver radio's microprocessor decodes the PL tone, and it's a bit more susceptible to false decoding.
Credits and Acknowledgements:
MOTOTRBO, XPR 8300, DMR, PL and a host of other radio terms are copyright by Motorola Inc.
Bob can be contacted at: his-callsign [ at ] comcast [ dot ] net.
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This article created 30-Aug-2016.
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