Interference tracking on TRS

[otobmark]

What is best practices for running down interference into a TRS? I have seen scanners, KrakenRF, and even service monitors paired with a preamp or even BDA. Often the problem is an out of spec BDA but not always. The monitor approach does let you “see” the signal characteristic.

 

[KevinC]

Lots of variables, but several question...

Have you seen the interference on a spectrum analyzer? So you'll know it's what you're looking for in you find it on the ground.

Is this broadband or a narrow signal?

Is this a Motorola P25 system? If so have you checked the station logs for possible clues? Such as NAC/PL/DPL and other info.

Is this simulcast? If so does it affect multiple subsites? If not do you have access to multiple sites to use those to give you a general idea of where it it?

I'm extremely successful at locating interference and all I've ever used is tower sites, spectrum analyzer, omni and yagi antennas, appropriate filters and roof tops/parking garages. I've never not found what I was looking for, some took several weeks, but I always found the source (assuming it stayed long enough). I stopped counting after around 500 hunts.

 

[otobmark]

Simulcast Moto. Last issue was input signal coming through a BDA. I wasn’t involved but I think it was hitch hiking on a legit signal which was opening the voice channel. I’m not a TRS guru and usually only directly involved with looking for analog interference (I use SSB when possible and yagi/attenuator—kind of crude). Have a friend who has been dropped into doing this for city (BDA tech) and I’m trying to help out unofficially. He’s sharp so he will come up to speed quickly I’m sure. Analyzing system logs is out of my wheelhouse and I don’t really have that access. He will have access so I’ll quiz him on the subject. I have monitored off the air data before mainly watching affiliation Issues. I’ve heard rumors of a multisite installed tracking system for just this purpose being considered. Could just be software for existing TRS honed for time delay triangulation. We can track PLL signatures.
As you said process can take time which is irritating. If it’s a single channel I guess they take it down on TRS until resolved. I like the Kraken especially if you have more than one. I think newer apps let them collaborate in near real time over internet.
Main question I guess was is there a best practices or does everyone freelance it?

 

[KevinC]

Simulcast Moto. Last issue was input signal coming through a BDA. I wasn’t involved but I think it was hitch hiking on a legit signal which was opening the voice channel. I’m not a TRS guru and usually only directly involved with looking for analog interference (I use SSB when possible and yagi/attenuator—kind of crude). Have a friend who has been dropped into doing this for city (BDA tech) and I’m trying to help out unofficially. He’s sharp so he will come up to speed quickly I’m sure. Analyzing system logs is out of my wheelhouse and I don’t really have that access. He will have access so I’ll quiz him on the subject. I have monitored off the air data before mainly watching affiliation Issues. I’ve heard rumors of a multisite installed tracking system for just this purpose being considered. Could just be software for existing TRS honed for time delay triangulation. We can track PLL signatures.
As you said process can take time which is irritating. If it’s a single channel I guess they take it down on TRS until resolved. I like the Kraken especially if you have more than one. I think newer apps let them collaborate in near real time over internet.
Main question I guess was is there a best practices or does everyone freelance it?

Starting in 7.16 all systems included "Interference Locator". I've had limited success with it though.

Best practices are use whatever resources you have available to you. Use the subsites to see if it is stronger at any of them. But first you really need to "see" it on a spectrum analyzer so you know what you're looking for.

 

[KevinC]

And I'm not saying you, but a lot of people just want to run out and look for the interference without understanding what they are even looking for or using tools at the site first.

I've been doing this for 30+ years and I'm pretty darn good at it. I'll be more than happy to assist you remotely all I can if you want.

After I had to leave my last job they had a real bad interference issue and they called in a company to help them...at $18k a week with no guarantee of anything. I wish I could do this freelance.

 

[RFI-EMI-GUY]

And I'm not saying you, but a lot of people just want to run out and look for the interference without understanding what they are even looking for or using tools at the site first.

I've been doing this for 30+ years and I'm pretty darn good at it. I'll be more than happy to assist you remotely all I can if you want.

After I had to leave my last job they had a real bad interference issue and they called in a company to help them...at $18k a week with no guarantee of anything. I wish I could do this freelance.

I would do it for a living if folks didn't think it should be found and fixed for free.

Chasing interference for Motorola (as an FTR) was challenging and exciting. However to everyone else I was taking too long and finding too much "other" stuff. Remember 800 MHz its clear up here? Well they forgot about Channel 69 TV translators and poor design of commercial 800 MHz mobiles, spewing LO into lower channels of 808 MHz Public Safety range, and poor design of voted receivers reacting to low levels of undesired CSQ activity (Early Smartnet 1 "feature".) Yeah I finally did find it! Cost Motorola a lot to fix their own system and product problems. Yeah and a Staff Engineer wrote up my findings in a textbook.

Garry C. Hess : Land Mobile Radio System Engineering; Chapter 2.7 Example Problem: Spurious Power Emission: page 13.

 

[MTS2000des]

Ironically one of my main RFSS sits on the very mast that used to have an actual channel 69 here in Atlanta on the Westin Peachtree Plaza. Today the station is long gone, but the Westin is my "go to" site to start Yagi pointing whenever some Chuck and a Truck loser tosses up a SureCall (The Bowelfeng of BDAs) turd box and it starts oscillating.

 

[KevinC]

Ironically one of my main RFSS sits on the very mast that used to have an actual channel 69 here in Atlanta on the Westin Peachtree Plaza. Today the station is long gone, but the Westin is my "go to" site to start Yagi pointing whenever some Chuck and a Truck loser tosses up a SureCall (The Bowelfeng of BDAs) turd box and it starts oscillating.

For me it was Wilson BDA's. I'd always look for that blue box.

 

[kayn1n32008]

For me it was Wilson BDA's. I'd always look for that blue box.

Ohhh the old 800MHz, direct connect only ones that were designed to be connected to a mounted phone like the M800... such garbage.

 

[mrsvensven]

Like KevinC, I've only used a spectrum analyzer and a yagi. I can't match his 500+ hunts, but I've probably done about 50. These fancy DF tools on the market look cool but I've never found the need to buy one. I hear they are very susceptible to being confused by reflections.

I've always used a spectrum analyzer and looked visually at what the signal looks like rather than decoding it and listening to audio. I suppose the attenuator technique could work as long as you know what the offending signal sounds like and you can be sure you don't get sidetracked chasing some other signal that isn't your problem. You would be surprised how many random signals exist that you can waste time chasing but aren't actually what you are looking for.

If you're using any wideband spectrum analyzer equipment like an SDR or even a high end $30,000 unit, you absolutely must have a window filter connected that will block cell and other frequencies from overloading the test equipment. Otherwise you will get nowhere.

While a lot of interference comes from BDAs, even more common is cheap knockoff imported amplified TV antennas purchased from online marketplaces. They are not part 15 certified and therefore not legal to sell, but the market is flooded with them anyway.

 

[mikewazowski]

We asked about putting booster stickers on the side of our trucks for every booster we shut down. Management said no.

 

[KevinC]

While a lot of interference comes from BDAs, even more common is cheap knockoff imported amplified TV antennas purchased from online marketplaces. They are not part 15 certified and therefore not legal to sell, but the market is flooded with them anyway.

I found my first one of those in 2002. NO ONE believed that could wreak havoc like it did.

 

[mrsvensven]

One other tip- 90% of the time, the offending signal moves up and/or down on frequency over time. If a signal is stable and not moving, the chance that it would ever line up with your legitimate frequency is slim. It's the moving left to right on a spectrum analyzer that you should be looking for. That's how you can identify a signal as being the likely culprit even if it isn't exactly on your own frequency at the moment that you visit your trunked radio site to begin your hunt. Even if a signal seems to only be harming you intermittently, it's still possible to hunt for it if it's bouncing around on different frequencies a few hundred kHz away from yours.

 

[KevinC]

One other tip- 90% of the time, the offending signal moves up and/or down on frequency over time. If a signal is stable and not moving, the chance that it would ever line up with your legitimate frequency is slim. It's the moving left to right on a spectrum analyzer that you should be looking for. That's how you can identify a signal as being the likely culprit even if it isn't exactly on your own frequency at the moment that you visit your trunked radio site to begin your hunt. Even if a signal seems to only be harming you intermittently, it's still possible to hunt for it if it's bouncing around on different frequencies a few hundred kHz away from yours.

Excellent point! One the hardest things I tried to get guys to do was not set their span at 25 kilohertz and say “I don’t see anything”.

 

[RFI-EMI-GUY]

My basic kit is a Yagi, pre-amplifier, bandpass filter and some sort of receiver from an ICOM IC-R7000 to a system analyzer. Then you need high ground which can be schlepping all that gear to a rooftop or using parking garages. When I went after the leaky local oscillator problem, the source was weak, pulsing and unknown. We had several clues, none of which made any sense until the first Motorola MOSTAR was caught. After 6 weeks of finding all kinds of potential problems, but not THE one, I was working from a helicopter which was a wild ride in MIA airspace and had a helper on the ground. I got a fix on the signal at an intersection and the other FTR swooped in and found an HVAC company van with an antenna. Armed with that information, the next day I parked at a very busy intersection two counties north (The county where the SMR used by the HVAC company was based) and sure enough every time I got a hit, there was commercial truck, an 800 MHz NMO antenna, and name on the commercial vehicle was noted. In the company database, all were MOSTAR radios operating on that same SMR. When these companies travelled into Miami, they would start scanning for the home SMR and we would get chirps on a couple channels of the City of Miami Smartnet system. During a football game at the Orange Bowl the problem intensified because apparently a bunch of commercial vehicles were used for tailgate parties and showed up.

 

[RFI-EMI-GUY]

We asked about putting booster stickers on the side of our trucks for every booster we shut down. Management said no.

Who ya gonna call? Booster Busters!

 

[RFI-EMI-GUY]

Excellent point! One the hardest things I tried to get guys to do was not set their span at 25 kilohertz and say “I don’t see anything”.

Yeah a doppler won't do any good either for the sweeping signals. It can get frustrating looking at then sweep by on the analyser and you find yourself at the end, when the signal is strong, looking at every building for signs of a donor antenna. And sure enough.... A NEXTEL dealer.....

 

[kayn1n32008]

We asked about putting booster stickers on the side of our trucks for every booster we shut down. Management said no.

Oh man, you wouldn't be able to see paint. Same for my friend that worked for a western Canada headquartered cell carrier.

 

[K5TXC]

Like KevinC, I've only used a spectrum analyzer and a yagi. I can't match his 500+ hunts, but I've probably done about 50. These fancy DF tools on the market look cool but I've never found the need to buy one. I hear they are very susceptible to being confused by reflections.

I've always used a spectrum analyzer and looked visually at what the signal looks like rather than decoding it and listening to audio. I suppose the attenuator technique could work as long as you know what the offending signal sounds like and you can be sure you don't get sidetracked chasing some other signal that isn't your problem. You would be surprised how many random signals exist that you can waste time chasing but aren't actually what you are looking for.

If you're using any wideband spectrum analyzer equipment like an SDR or even a high end $30,000 unit, you absolutely must have a window filter connected that will block cell and other frequencies from overloading the test equipment. Otherwise you will get nowhere.

While a lot of interference comes from BDAs, even more common is cheap knockoff imported amplified TV antennas purchased from online marketplaces. They are not part 15 certified and therefore not legal to sell, but the market is flooded with them anyway.

A little off topic. In the early 80's our local Ham repeater started locking up in transmit due to a weak unmodulated signal on the input. This occurred every afternoon around a certain time of day. I went out trying to triangulate the signal with a beam connected to a variable attenuator hooked to a 02AT Icom handheld. I finally narrowed it down to one neighborhood, and a travel trailer parked in the backyard. It turned out to be the amplifier connected to the TV antenna mounted on the RV. The weather had been cool for the week that the problem occurred. It turned out that as the sun warmed the interior of the RV, the frequency of the amplifier would change causing it to match the repeater input frequency. Another factor was the battery voltage on the RV. Apparently the battery hadn't been charged in a while causing the voltage to be low, this seemed to have an effect on the frequency output of the amplifier too.

 

[speedway_navigator]

It's been 10 years since I was an RFI contractor for Verizon when they turned on their 700 MHz LTE system but a Yagi and spectrum analyzer was all I used. I took an R+S class on the PR100 from Dane Brockmiller, nice tool but it's dang heavy and the menu system was hard to learn.

Dane said he took students out for some testing during class and they stopped at a building with some antennas on the roof. Not long after that a security guard walked across the street to find out what they were doing. The guard said it would be a really nice idea if they moved on from the federal facility. They did.

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