A VHF (150 MHz range) police/fire radio system was suddenly suffering from intermittently poor signal from a part of their coverage area, being noticably worse with rainy weather. On VHF, receiver voter systems are more common due to the much poorer portable (handheld) radio performance versus UHF and 800/900 MHz systems.
We first checked the voter sites nearest the complaints including 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.
There was no apparent problem intrinsic to 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 urban VHF receiver. 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 light rain day, the problem recurred. I still did not hear anything discernible in the repeater receiver, nor see anything with the spectrum analyzer on the receiver antenna. The desense test now 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 was consistent with the VSB analog TV signal behavior.
Cable TV radio interference characteristics: analog VSB TV is amplitude modulated signal with a powerful 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?
The powerful VSB carrier has order(s) of magnitude more power than a narrowband slice of its modulation. Cable TV channels 19 and 20 are prime suspects, using a cable TV frequency chart. For radio frequency is in the 151-157 MHz range tune diagnostic radio to 151.25 MHz, which is the video carrier. A portable set to this frequency did not hear the signal at ground level at the site. The cable TV source may be 20-30 feet above ground while the police repeater receiver is 200+ feet above ground.
Tracking cable TV leakage interfering with VHF radio: a VHF yagi would have a distorted radiation pattern from a vehicle anyway. We used a mag-mount 1/4 wavelength vertical ~ 18" and decided based on the signal strength the leakage was within about 2 miles. 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. 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.