Electronics > RF, Microwave, Ham Radio

RF interference on the VHF Business High band

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This is in the US market.
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 receiver', one having 'digital' capabilities (Uniden BCD996P2) which is less susceptible to interference than the Radio Shack Pro 2004 & Pro 2006 (made back by GRE in the late 80's and early 90's both very popular here.). This happens on the SAME exact frequencies on all 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 (Amateur TV) 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 sounded 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 all receivers at the same time. Each receiver has it's own roof antenna.

My question is;
Since this is the VHF high band (150-170 MHz), 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 this band??

Any other specific questions please ask.

Attached are a audio sample and screen shots of my Siglent SSA 3021X.
The spike w/ the marker is the interference. It bounces around within the bandwidth shown on the scope. It's anything but stable, either in signal strength or frequency. There no real pattern to it. It appears and disappears through the day and evening. It's definitely not in the immediate neighborhood, I'm guessing less than a mile away, but that is just an initial guess.

I also have a RF Explorer, but I have found inconsistent readings between the two SA's. Signals showing on one, but not the other.

Have you tried FM demod on it yet?  A narrow peak "bouncing" around implies stable frequency within an acquisition period (that's a DSA isn't it, i.e. acquire a huge gulp of samples, FFT it, repeat), but changing from period to period, implying frequency modulation at a rate comparable to the acquisition period; or there are large gaps in time between acquisitions.

So, perhaps you see something different if you change the sweep rate / display refresh / buffer size (number of samples) / etc.  If the modulation is periodic, aliasing it against the acq rate should be possible.

I suppose also possible, it's FM over such a range that you're not seeing the sidebands (or they're too low to receive), or more than one (as in the second spectrum).  But that seems unlikely, if it's 60Hz or thereabouts.

It could well be switching noise, perhaps from a newer GaN based device that has higher harmonics than ever, but 160MHz is still within the range of silicon devices as well.  The 60Hz-ish modulation would be typical for mains-powered equipment.

You haven't seen anything by sweeping a dipole/loop (or better) and looking for nulls?  Or check for polarization as well, perhaps it's mixed by multipath.


I'm afraid that most of that is somewhat over my head (above my pay grade).  :-[
But I did use the demod function. With nothing to compare it with, I didn't know what to expect. I'm not good at making home made antennas.  :-\

All of your scanners and other NBFM demodulator radios probably have a 10.7MHz I.F. strip so places to look would be 10.7MHz higher or lower than the channel frequency you are trying to receive. There are no 'subharmonics' however you may be hearing a 2nd or 3rd harmonic perhaps looking at anything in your area transmitting in the 80.5MHz or 53.6MHz range. The old varactor tripler units have gone out of favor with the F.C.C. for commercial products and we had to scrap a bunch of Moseley Microwave (900MHz) studio transmitter link stuff because it was outlawed. Truthfully the stuff sucked to begin with and would break into spurs that looked like a forest on a spectrum analyzer. 161-10.7 is 150.3 and 161 +10.7 is 171.7 perhaps pocsag pager interference or digital voice data for public services. Doubt very much it is amateur radio related as the bands and harmonics don't line up at all. Another possibility is leakage from the CATV neigborhood network. They may be polling water meters or electrical meters on oddball frequencies unique to the cable backhaul networking. If it interferes with your 'train channels' why aren't the railroads troubled by it?? Seems they would be having interference problems also and be gunning for bear to find the culprit? B.T.W., I retired from the G.E. locomotive factory so I have lots of train radio experience. Also worked for a short line that got gobbled up by G&W.


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