Specific interference on the VHF Business High band

[videobruce]

This has been going on for some time now, probably 9 months or so and it beyond annoying. This is using more than one 'scanner receivers', one being having 'digital' capabilities (UnidenBCD996P2) which is less susceptible to intermod than the RS Pro 2004, the other scanner). This happens on the SAME exact frequencies on both receivers, considering they are completely different designs, it's not intermod as far as I can tell.

Hard to explain, but here goes. I have been getting 'bombed' with what I assume is some type of 'digital' RF interference that only seems to hit selective VHF high band frequencies in the assigned Railroad band centered around 161MHz which happens to be my most important service I have monitored for decades.

I tried doing numerous Spectrum Analyzer scans for the frequency source of the interference to no avail. I tried to narrow the source down based on the frequency and hours that it was/is present and my 1st guess was ATV since it is not typical 2-way voice communications, it's somewhat different than most 'digital' UHF business channels (NXDN for example) with the 'buzz like sounding audio (for lack of a better term). It was more like what I remember hearing the sidebands of analog NTSC transmission when close to a xmitter sounder like.

It slacked off for a couple of months, I thought it was maybe gone, but it returned in the past few weeks, but a different 'sound' to it, but the same exact frequencies are affected. This time the audio has a 'motorboat' sound to it. Definitely different than before, but the exact same frequencies are affected. About 5 different channels, again of both receivers at the same time. Each receiver has it's own roof antenna.

My question is;
Since this is the VHF high band, NOT the overly crowded UHF business band where most all the 'digital business channels are, where on earth can this be coming from? A sub-harmonic of UHF on the high band? What and where below the high band would harmonic into that band??

Any other specific questions please ask.

 

[mmckenna]

Is this happening in one location, or is it over a wide area?

I found a digital video adapter that was transmitting a nice big spike right on our Fire VHF repeater output. Took a long time to track down, but eventually found it. I could hear it within about 250 feet of the device.

Cable TV coax cable can leak signals on very specific frequencies, also. Out in front of my inlaws house is a cable TV pedestal that has a strong carrier in the 2 meter band, right around 146.415MHz.

 

[videobruce]

Initially it was a somewhat large area, maybe a 3 or 4 mile radius. Now, I haven't track it.

Where (frequency range) would I look for possibilities?

 

[mmckenna]

Initially it was a somewhat large area, maybe a 3 or 4 mile radius. Now, I haven't track it.

If it is that big an area, then its something pretty strong, not cable TV interference. Might be intermod from a strong nearby transmitter.

Where (frequency range) would I look for possibilities?

A spectrum analyzer or something with a spectrum display would be what you need. Quickest way to find it is with something that shows you the spectrum.

 

[paulears]

Before you go too far, find a frequency that has the interference and then stick a proper radio on that frequency and see if it hears it? Transmitters, even cheaper ones have better filtering than scanners, which by their nature are wide open on every frequency known to man, because their wide band designs are meant to do that. If your two way radio reveals no interference, and normal operation on the troublesome frequency, the next thing is to check if it’s getting in via the antenna, or the case. Metal food tin, with a small hole for the cable, and pop it in. Sometimes screening the case like this removes the problem. Difficult to use a radio wrapped in tin foil though. If it’s getting in via the antenna connection and the other radio proves the actual real frequency is interference free, then it’s probably a combination of two or more strong signals your radio is combining internally. the solution is to find the frequencies that cause the combination, and filter them out. It’s sort of intentional bad design. You need sensitivity everywhere, but pure bad luck of geography wrecks your system when two string signals combine like this. Find them, filter them. Repeater filters can be tuned very sharp, so could be a cheaper way to remove the unwanted frequencies, once you locate them.

 

[videobruce]

A spectrum analyzer or something with a spectrum display would be what you need. Quickest way to find it is with something that shows you the spectrum.

Per my OP, I do have a SA that I was using to search adjacent VHF 'bands' with no luck.
These 'bursts' don't show on the SA on the frequency the two 'swcanners' receive them on (which is little surprise to me). I did state the same 'burst' shows on two different receivers on the same frequency which surprised me since the receivers are light years apart from one another (analog vs digital and 30 or so difference in age).

 

[paulears]

As Per my reply, hoping to help - my question was about discounting filter-free receivers, so do you have a proper tranceiver with decent filtering to try? Your spectrum analyser will show you the spectrum but it's pretty useless and intermodulation with the strangeness of summ and difference interaction. Two separate transmitters could be creating in your receivers products which appear to be on your indicated frequency - if they do this on a radio with much beter selection and rejection, then the SA should also show it. The fact the SA doesn't show it, indicates to me that your issue is oversaturation and then receiver products creating phantom signals that simply back up the SA - they are not there.

 

[KevinC]

As Per my reply, hoping to help - my question was about discounting filter-free receivers, so do you have a proper tranceiver with decent filtering to try? Your spectrum analyser will show you the spectrum but it's pretty useless and intermodulation with the strangeness of summ and difference interaction. Two separate transmitters could be creating in your receivers products which appear to be on your indicated frequency - if they do this on a radio with much beter selection and rejection, then the SA should also show it. The fact the SA doesn't show it, indicates to me that your issue is oversaturation and then receiver products creating phantom signals that simply back up the SA - they are not there.

Or the reference level is set too high on the SA and the interference is below that.

 

[videobruce]

find a frequency that has the interference and then stick a proper radio on that frequency and see if it hears it?

Way ahead of you. I did that with one frequency months ago during the 1st 'wave' of interference and no, the interference was NOT there as I already expected. If it was the RR carrier (company) would of reported it. (I'm retired from them ). The other problem channels I do not have programmed in that transceiver (Moto hand held P1225 which is a real pain to program).

AFA intermod, I'm more than aware of that 'Covid' like disease that is across all consumer receivers (scanners). I had to install a tuneable trap to block FM broadcast interference that affects these Radio Skack scanners made by GRE (as good as they were). The effect is, it would 'blanket' the 160-162 MHz band, severly decreasing signal levels on those channels while not bothering the rest of the high band. If you can believe that.

 

[ladn]

I have been getting 'bombed' with what I assume is some type of 'digital' RF interference that only seems to hit selective VHF high band frequencies in the assigned Railroad band centered around 161MHz which happens to be my most important service I have monitored for decades.

Could be NXDN (NEXEDGE) digital. Many RRs are transitioning from conventional FM to NXDN ditigal. You'll probably need the proper extra cost entitlement to monitor it with a scanner.

 

[videobruce]

Negative, non have switched over here. In Toronto some have.
Besides I have the NXDN plugin.

One of the current audio signatures (my term) is a 'motor-boating like sound. Listen to the attachment (rename the extension to .mp3)

 

[KevinC]

I'd vote for router, switch or cable modem. Unplug any and all of those and see what happens.

 

[videobruce]

Then is must really be bad, I was still receiving it almost 2 miles away in the car.

 

[madrabbitt]

Then its not anything in your house or immediate area. I was thinking the same thing as Kevin when i heard the sound. It sounds electrical.

 

[videobruce]

It almost behaves as it is intentional as one is 'keying' the transmission, switching between 5 or 6 specific frequencies. The 'sound' is almost the same, thou surely not the same from many months ago before the 'lull' of a few months. It's constant anywhere between 10-20 seconds to a few minutes. Thou I'm not a night owl, I have been up after midnight and I don't remember hearing it then.
I have been driving around within a mile of my location and somewhat narrowed it down, problem is it doesn't always last long enough.

 

[mmckenna]

It's not triggering anything in my brain, but I'm thinking its either digital voice or some data burst thing. Rotating between frequencies would suggest a trunked system, or some sort of data acquisition system.

Finding emissions like this can take a huge amount of work. I had to track down a system that was interfering with a repeater input. Took weeks of careful work and a whole lot of driving.

 

[AM909]

Before you go too far, find a frequency that has the interference and then stick a proper radio on that frequency and see if it hears it? ...

Way ahead of you. I did that with one frequency months ago during the 1st 'wave' of interference and no, the interference was NOT there as I already expected. ...

So, just to clarify, you're sure that the signal was not heard by a "real" land-mobile radio at the same time as it was being heard by your scanners with the same or similar antenna? Did you happen to try opening the squelch on the radio? A signal wider than the IF filtering is set to can sound like noise to the squelch circuitry.

I know they're vastly different scanners, but exactly what are they, and what are the exact frequencies?

 

[madrabbitt]

only because i'm REALLY distracted from what i SHOULD be doing (updating a 6 year old resume) i'm going to run that sound clip thru some audio filtering.

I REALLY want to know what the spacing of the sounds are and if they're a harmonic of 60hz.

I'm stuck on it being electrical.

 

[videobruce]

160.80, 160.95, 160.98, 160.995, 161.07, 161.085, 161.19, 161.25
The bold ones seem to be the ones most affected. This is definitely NOT RR related.

My question is; what frequency bands (range) would this come from? There is no way it could be the UHF business band (450-470MHz) or in the new 420's, correct??

As to receiving this on a commercial radio, I only have four of those seven programed in and I only checked it once on 160.80 and the interference wasn't there. I will try to check again.

 

[videobruce]

AM909;​

Are you referring to the Moto P1225 or the actual scanners regarding the squelch? The squelch on the 1225 is fixed.
The three scanners are;
RS/GRE Pro 2004, 2006
Uniden BCD996P2

There is a difference between the GRE scanners and the current Uniden 'digital' scanner as far as intermod. It is noticeable better rejecting intermod, but still not fully immune. But, I don't think it is really intermod as I don't (or can't) detect two different signals together as I can usually with voice and some digital intermod interference. I admit, I'm not that knowledgeable with digital transmissions, so I'm not sure if one can really tell if these are really intermod or not.