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As far as FM interference, thats a very good possibility but why just 13?
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The FM band is between VHF-Low channel 6 and VHF-High channel 7. Strong FM transmitters are more likely to cause interference to channels 7-13 than UHF channels.
They can cause interference from fundamental overload or from a second harmonic of an FM transmitter.
The second harmonic of WYRD-FM on 106.3 MHz is 212.6 MHz. Channel 13 is 210 to 216 MHz, but WYRD-FM is not one of your strongest FM channels, so the interference would more likely be from fundamental overload from the stronger FM transmitters.
WLOS, on real channel 13, is your weakest VHF-High channel.
To test the theory, insert an FM filter/trap.
Another possible source of interference to WLOS is electrical interference, which is worse on VHF than UHF. It can be caused by LED and CFL lamps, switchmode power adapters, electrical motors, & etc. You can listen for noise on a clear frequency on an AM broadcast band portable radio.