It's pretty rare for modern solid-state transmitters to emit on other than the assigned frequency, even if one wants it to. And the only time in my recent memory was when another local broadcast engineer replaced the final tube in an FM transmitter. While it fit the socket, it wasn't the right tube exactly, and there were spurs across the FM band. In that case, I had notify the offending station, and the problem was soon rectified. know of no other recent case. My stations not only are using recent model solid-state transmitters, but our primary site has extensive filtering that keeps every transmitter operating well within FCC limits, keeps each station from reflecting into other transmitters. The more common problem is the one discussed in this thread, where an inexpensive antenna and receiver are relied upon to work forever without any regular inspection, by an absentee licensee without any staff. With one exception, all of my FM translators are fed directly via 950 MHz studio-transmitter links. The one exception uses a high-end rebroadcast receiver and a commercial-grade rugged Scala receive antenna. Occasionally in the field there can be sum and difference frequencies that can cause problems, but we haven't ever run into that in our market. I'm sure that others have had issues however.
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