Tracking down Cable TV leakage interference to public safety repeater radio systems

The times I get called for a wireless system problem are not easy problems, because you can get someone else to do the easy calls at half my price. This was the case for a VHF (150 MHz range) police/fire radio system that was suddenly suffering from intermittently poor signal from a part of their coverage area. In fact, it happened on rainy days when pressed to the issue.

On VHF, voter systems are more common due to the much poorer portable (handheld) radio performance versus UHF and 800/900 MHz systems.

Checking voting receiver levels

I have seen in the past where even without interference, due to neglected maintenance, even when the equipment is on a high-dollar maintenance contract, there are issues. So the first task was to in the voter sites nearest the complaints, check the line levels at the main repeater/voter. If voter levels are incorrect, the voter can see this receiver as having poor SNR and always pick other receivers. The remote voter receiver levels at the central voter were acceptable.

Checking interference at each site

I could not see anything at the voter site that seemed the likely culprit using the spectrum analyzer, or just listening with carrier squelch. There was over 5 dB of desense, but that is not at all uncommon for an lite-urban VHF receiver. I did some radio checks with a one-watt portable from the complaint locations, placing a dictation recorder near a receiver in my vehicle tuned to the repeater output. The dispatcher and my recorder heard full-quieting audio, even as I moved around.

But this was on a non-raining day, so we decided to wait for the next rainstorm. And, we told dispatch to call us on any time they noticed this issue. On a heavy misty/light sprinkle day, they called with the problem. Good, I didn’t have to get soaked immediately.

I still did not hear anything discernible in the repeater receiver, nor see anything with the spectrum analyzer on the receiver antenna. So I did the desense test again, and noted a time-varying 10-20 dB of desense. That’s enough for the user to notice certainly. I noticed something interesting about the time-varying desense pattern. It was not wildly varying, but rather it would spend several seconds at one level, then several seconds at another level. This rang a bell back to my middle school TV repair days as consistent with the VSB analog TV signal behavior.

Cable TV radio interference characteristics

Analog VSB TV is amplitude modulated signal, with a big giant wasteful carrier, and a few MHz of bandwidth above the carrier including chroma, luma, sound, etc. Cable TV signals are just like over the air TV signals, except frequency shifted for our purposes. But how would I track this signal down?

There is a big giant carrier in VSB, with order(s) of magnitude more power than a narrowband slice of its modulation. Depending on where you are, cable TV channels 19 and 20 are prime suspects, using a cable TV frequency chart. If your radio frequency is in the 151-157 MHz range, then tune your diagnostic radio to 151.25 MHz, which is the video carrier. Using my portable set to this frequency, I did not hear the signal at ground level at the site. This is because the cable TV source may be 20-30 feet above ground, but you’re 6 feet above ground, while the police repeater receiver is 200+ feet above ground.

Tracking cable TV leakage interfering with VHF radio

I didn’t want to bother with a VHF yagi, which would have a distorted radiation pattern from a vehicle anyway. I used a mag-mount 1/4 wavelength vertical ~ 18” and decided based on the signal strength the leakage was within about 2 miles. Fortunately, in the Midwest outside the city core, the streets are on a wide grid, so within 20 minutes I was at the source. Using a pair of compact binoculars, I could clearly see CATV line running, and there looked to be fresh work done near the interference site.

The cable company came with unprecedented speed, with a pair of senior technicians–definitely not the guys that bore holes through studs installing cable in your home. Within two hours of their work the issue was solved. Seems an old line(s) had been cut and left there, and they weren’t properly terminated. I suppose the metal shavings and moisture caused some resistive coupling to something else, giving a sharp rise in leakage.

I can help you with your CATV leak-commercial radio interference issue

Hopefully these notes have helped. If you need more guidance, the first telephone consultation is of course free, and I can provide local as well as remote support for your issues.

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