There is another possibility. Here in Ohio, some agencies like sheriff departments use multiple transmitters running in simulcast mode. This is done by having a GPS receiver at each transmitter site and using the satellite transmissions as a frequency reference for the public safety transmitters. The result is that while all of the transmitters are locked to the same frequency reference, there are slight variations in the phase of the signals from the different sites as they hit your antenna. This will result in a fading in and out of the received signal as the signals from multiple sites go in and out of phase with each other. I have several agencies on my feed that use this technique and it can be distracting until you get used to hearing it.
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