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Very baffling Mototrbo DMR problem

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ZS6HZ

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Hello all, and thank you for reading this. It's a long post, but if you read it through, you might be able to solve my very vexing problem.

I am in the Congo, in a town called Goma near the Rwanda border. The repeater is mounted on a hill that overlooks the entire town, and in my twenty-odd years of radio experience I would have sworn that a repeater up there would cover the entire area with ease. The furthest comms distance is only 5 miles from the repeater.

I'm using a DR-3000 that is set to Rx on 151.600MHz and Tx on 157.600. We get coverage throughout town, except in one key area about 2 miles from the repeater. In that area we can access the repeater from a vehicle radio, but not with a portable.

THIS IS THE STRANGEST SYMPTOM. In this area, a portable radio can hear other repeater users, but can't be heard when it transmits, BUT when the PTT button is pressed, the radio doesn't give the long tone that it would if it was not accessing the repeater. The portable MUST be accessing the repeater but the repeater is either not receiving the portable properly, or not passing the audio from the portable.

My original diagnosis was that there was a lot of local noise at the repeater site and the repeater was being de-sensed. I installed a cavity bandpass filter on the receive frequency to eliminate any noise, but it hasn't helped at all.

I am now baffled, confused, and ready to tear my hair out..

I've swapped the repeater itself, the duplexer, the antenna, and the antenna cable. I've tried the newest DP-1400 portable in case the problem lay with the DP-4401's that we're using.I've swapped the repeater frequencies.

I am at my wits end now, and I HAVE to solve this problem before the weekend. ANY info or feedback will be highly appreciated.I can provide copies of codeplugs, GPS co-ordinates etc. ... anything that you might need to help diagnose the problem.

Thank you, in advance...

Marc ZS6HZ
 

JRayfield

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A MOTOTRBO portable can sometimes get a 'handshake' with a repeater, yet the signal into the repeater can be too low for reliable voice communications.

Have you checked the noise floor at the repeater site? Also, how high is the 'hill' over the town? What other equipment is at the repeater site?

John Rayfield, Jr. CETma
W0PM
Rayfield Communications
 
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If other radios are able to use the repeater properly, then the problem is not the repeater, antenna or cabling. If any of that was the problem, then no radio works. I agree that that the problem is probably RF noise and the mobiles have a stronger output are able to "overpower" the noise and get through, while a portable is not able to "overpower" the RF noise. Remember, anything could be producing just enough noise to block a portable.
You say the problem is only in 1 part of town about 2 miles away, have you only tried at only one location (in town) or does this happen within a certain diameter. IE try crossing the street and see it works there on the other side, or the other end of the block.
 

ZS6HZ

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A MOTOTRBO portable can sometimes get a 'handshake' with a repeater, yet the signal into the repeater can be too low for reliable voice communications.

Have you checked the noise floor at the repeater site? Also, how high is the 'hill' over the town? What other equipment is at the repeater site?
Good morning John, and thank you for your reply.

I really don't have any decent way to measure the noise floor, but I am certain that it's high. The high site is a typical Congolese mess. There's no control at all and there are literally dozens of assorted repeaters up there, with n attention to high site standards, safety issues etc. I'll post some pics of the high site shortly. It's even possible that there's other repeaters on the same frequencies!

I am going to the site this morning, and I will use my little Kenwood dual bander to listen around at the noise levels.

I was hoping that the band pass cavity I installed would kill the noise, but it hasn't made any difference at all.

The hill is about 80m (240 feet) above the town and my antenna is on a 12m mast.
 

ZS6HZ

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You say the problem is only in 1 part of town about 2 miles away, have you only tried at only one location (in town) or does this happen within a certain diameter. IE try crossing the street and see it works there on the other side, or the other end of the block.
Please see the picture that I posted. It shows the coverage problems.

Any idea of a simple way to find the noise? I was thinking of going up to the site and switching off all the other repeaters and seeing if that helps...
 

ZS6HZ

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is the antenna an Omni? Is it top or side mounted on the tower?
It's a folded dipole omni-directional antenna, mounted vertically on a 12m long steel pole. It is mounted about 1.5m from the top of the pole. (a roughly 45 degree angle from top of pole to top of dipole.) I can post a pic... but antennas are difficult to photograph properly...

Cable is unfortunately RG-213, but it is a reasonable quality cable.

Just a thought... could noise be coming in via the mains AC cable?
 

R8000

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Your repeater as far as I know is the same as our XPR series. They aren't meant to be used in a high RF environment like that unless you filter the crap out of it. A flat pack duplexer probably wont cut it. Looking at the mess there, other users haven't filters their stuff, lots of sideband noise I am sure.

Perhaps programming the repeater temp in analog to allow you to listen to what's going on. You will be able to hear the noise if it's there.

I can only recommend you use good filtering, and encourage other users to filter as well. I understand that there are no rules at the site, but maybe you can use this chance to be the one to start telling folks to filter their stuff out of courtesy.

Just some ideas for ya.
 

ZS6HZ

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but maybe you can use this chance to be the one to start telling folks to filter their stuff out of courtesy. .
LOL. That's never going to happen. These clowns can't even comply to the simplest of electrical safety standards... there's extension cables running through the mud, generator electrical connections insulated with plastic shopping bags, exposed wiring, etc.

And there's no band planning AT ALL. you pay the guy in charge of the high site $100 a month, and he lets you do whatever you want to do up there.

Just an example of how much of a mess it is up there, the only fresh water for the people to drink, is collected from the evaporator of an air conditioner dribbling into a coke can, the line voltage I measured varies between about 100V to about 270V, and there's a television station studio up there that uses a $200 compact camcorder as it's primary camera. :D
 

902

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Hello, Marc. Very interesting situation. I have limited ability, but based on a 12m antenna at 1° 40' 55" S/ 29° 13' 32" E do show that your signal should work to the west in the area of concern. Now, you said the antenna is 45 degree angle from the top of the pole. Just so I know we are using the same terminology, the antenna itself is not vertical with respect to the ground, it has a 45 degree mechanical tilt??? If so, what direction is this tilt with respect to the area where you coverage is deficient? Is the tilt away from the deficient coverage or toward it, or diagonal to it?

My thought at the moment - based on that 45 degree statement - is that the major lobe of radiation is either pointing up and the signal is overshooting the region of interest, or down and is undershooting it. Ideally, the antenna should be vertical, perpendicular to flat earth. If you can, please do attach a pic. Your site pics are fascinating, but I've seen similar here in the U.S.

I don't think this is an isolation issue, although under ideal conditions I wouldn't use RG-213 for duplex service and feel compact duplexers are the scourge of VHF. Your 6 MHz of separation should be adequate isolation, nonetheless. Especially considering your external filtering.

73!
 

ZS6HZ

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Thanks for your response. You misunderstand what I meant about the 45 degrees. The antenna is perfectly vertical and perpendicular to the ground.

It is common practice here to allow for a 45 degree "umbrella of protection" from the top of a pole to help protect against lightning strikes on the antenna.

Do you think that reorienting the antenna will make much difference? I guess it's possible that there's a null in the direction were experiencing difficulty in.
 

902

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Thanks for your response. You misunderstand what I meant about the 45 degrees. The antenna is perfectly vertical and perpendicular to the ground.

It is common practice here to allow for a 45 degree "umbrella of protection" from the top of a pole to help protect against lightning strikes on the antenna.

Do you think that reorienting the antenna will make much difference? I guess it's possible that there's a null in the direction were experiencing difficulty in.
I'm limited in that I don't have good terrain or GIS data from your part of the world. Google does have good images though, and I don't see anything that would have excessive clutter attenuation to the west. You should be clear to about 14 km in that direction and about 17 km to the north. If anything, you should have some spotty coverage to the east about 4.5 km and be clear to the horizon over the lake. But that is probably based on unreliable data and no basemap to outline the region.

Okay on vertical. I understand now. I'm guessing you have DB-224 or similar omnidirectional folded dipole with an exposed phasing harness and somewhat movable elements. Please do send a photo of the antenna, how it's attached to the mast, and what the orientation is. I'm curious to see how it's mounted (if you are using standoffs, the directions of the elements, etc.).
 

ZS6HZ

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Howdy,

I'm on my phone right now, so typing is a hassle. My apologies for being a bit short on my reply. ;)

I'll get some pics tomorrow morning and post them here.

The dipole is not adjustable. It's a fairly standard commercially available antenna from South Africa with a sealed balun and aluminium element.

Would I be correct I'm guessing that only transmitters on frequencies equal to, or below mine would cause descending problems? There's a 250 foot tower about 60 feet away from our site, bristling with dishes, antennas, and a rats nest of cables running up it. I'm guessing that the chances if microwave systems causing the trouble is quite small?

I suspect there is a fairly high powered FM broadcast transmitter up there somewhere too.
 

902

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Howdy,

I'm on my phone right now, so typing is a hassle. My apologies for being a bit short on my reply. ;)

I'll get some pics tomorrow morning and post them here.

The dipole is not adjustable. It's a fairly standard commercially available antenna from South Africa with a sealed balun and aluminium element.

Would I be correct I'm guessing that only transmitters on frequencies equal to, or below mine would cause descending problems? There's a 250 foot tower about 60 feet away from our site, bristling with dishes, antennas, and a rats nest of cables running up it. I'm guessing that the chances if microwave systems causing the trouble is quite small?

I suspect there is a fairly high powered FM broadcast transmitter up there somewhere too.
If you're talking about having a problem with a mixing product (algebraic mixing of strong signals in a non-linear junction... anywhere... rusty hardware, dissimilar metals, in the RX front end, etc.) systems can be susceptible from either higher or lower frequencies. A responsibly managed site would require new entrants to perform an "intermod study" which looks at every RF emitter, its frequency and level, and receiver frequency. An "IM suppression panel" could alleviate that, but just one on yours would only work if your transmitter is part of the problem. Otherwise it only makes you the good neighbor in a bad neighborhood.

Ideally I would be looking for a spectrum analyzer to look at my repeater input, looking for some interference-limited situation. Otherwise, I'd be listening in analog for any possible noise or intermittent degradation that can correspond to signals received elsewhere in the band (coming up and dropping simultaneously). Likewise, listening to the output side could determine any impairment to subscriber units in your affected area. Troubleshooting interference in a digital environment without instrumentation is entirely foreign for me, though. And you don't even know if you are looking for interference, per se. Are you able to drop to analog to listen for anomalies and intermittent signals? I take it the system is "live" and loaded with users, which would make the task a little more challenging (but would actually help if you knew where they were and could see them on a spectrum analyzer).

If this turns out to be a protocol issue rather than an RF issue, I wouldn't be able to help much more. My experience in having used digital modulation is that it actually performs as well or slightly better than analog because of error correction. So, if this is still something that's not yet loaded with users, and you are able to test in analog, you might hear what is happening.

I doubt microwave systems would be affecting your system - except maybe for intermediate frequencies. But without an audit (or a S/A), you really wouldn't know what else is on that tower. I also doubt that this is a "DMR problem" insofar as another waveform would probably be similarly deficient in performance. But I could be wrong.

What do you have with you, and what can you get access to?
 
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Just thought of something to try... leave 1 portable with a friend ground level in the problem area and you take another portable up to the tower site, and see if the 2 radios will work SIMPLEX, analog AND digital. If they do work simplex either analog or digital, then its some type of programming to access the repeater, if they don't work then its something else to rule out.
Also, program an analog channel with your RX and TX channels (in your radio to take up there) with NO PL and Open squelch, that way you will be able to hear if anything else is on your RX and TX freq, even if they have an unknown PL.

Gotta tell you, if I was in your neck of the woods, I would for sure, come help you figure out this problem, I always like a good challenge. Make for good education.
 
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ZS6HZ

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Gotta tell you, if I was in your neck of the woods, I would for sure, come help you figure out this problem, I always like a good challenge. Make for good education.
There's daily flights from New York to Johannesburg, South Africa. and then from Johannesburg to Kigali, Rwanda where I can pick you up and we can make the three and a half hour trip by road to the Congolese border... If you pack and leave now, you could be here by Wednesday I guess. :D

I have done all the simplex tests I could do, except for trying a simplex test using the repeater's own antenna because I had no way of attaching the repeater antenna to my portable, so I dragged a DM series mobile up to the high site, and "Voila"... no simplex coverage using the DM...

Then I started thinking... I took a portable in analogue simplex mode and walked all over the top of the hill, trying to contact my guy on the ground. At the very edge of the mountain (where we can't mount an antenna or install the repeater because the Egyptian UN troops have a lookout post there) I could talk to my guy 59. As I walked back towards the repeater room, he abruptly disappeared. I discovered that whenever I got near a specific mast, the signal would disappear. The mast has two phased four-stack dipoles on it, and I later determined that it is the antenna for the local FM broadcast station. They must be pumping several kilowatts I guess, and they're completely blocking the incomming signals from that direction.

BUT, I was still confused because they information I had been given was that the police, the UN, and another security company had city-wide comms from that site. So I spent Friday afternoon and Saturday finding out exactly what the REAL situation was, instead of hearing it from people who might not know the whole situation.

It turns out that the UN are using a TETRA system, obviously with multiple sites (I knew this, as I've been looking at masts and high sites around town, and I've been into the UN camps a few times) so that doesn't count.

The police and army, in typical Congolese disorganized fashion, are actually all on simplex. They have several guys walking around on top of the hill, and they rely on manual relays to get comms... a bit like a glorified CB network. You can imagine the garbled messages that must get passed on...

When I finally managed to get hold of the owner of the security company, he told me that they DON'T actually have great coverage, and the area where we were struggling to cover is also dead for them. If I'd known that from the start, I wouldn't have put our repeater there to begin with.

It just goes to show what value there is in getting GOOD local intelligence.

So, come Monday we're relocating the repeater. Will be scouting for sites today (with a slight hangover... I had a barbeque with the South African Airforce detachment of the UN Monusco force last night, and there was quite a lot of beer and whisky around).

The one plus factor of my hangover is that I got to get a close-up look at the SA attack helicopter, the Rooivalk, and the Oryx support helicopter. The Rooivalk is unbelievable!!! It is a flying uber-hunter, that is so well designed and capable. It was first battle tested here in the Congo recently, and it exceeded all expectations.

I'll let you know what happens on Monday. Thanks for all the great advice.
 

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