[BC] Signal/Transmitter question... spur/harmonic or other issue?

[lists at loudandclean.com]

I have actually experienced AM-FM intermod on my car radio when parked in
the parking lot of WWZN (AM, 1510 KHz, 50 kW), which placed audio from
WERS (FM, 88.9 MHz, 4 kW, more than 7 miles away) on top of WUMB-FM (91.9
MHz, 660 Watts, more than 12 miles away).  I was totally freaked out, did
not think before I called, and wasted an hour or more of Rick Levy's time
at Broadcast Signal Lab chasing this thing.  Finally, I cranked up my car
and drove away from the 1510 site, and the IM faded out within a mile.

I'm pretty sure it wasn't "real" 'cause I haven't heard it on other
radios.  The mechanism by which it could happen -- WERS (88.9 MHz) mixing
with the second harmonic of WWZN (1.51 MHz) producing a sum frequency of
91.92 MHz -- isn't likely to have been happening in the WWZN facility,
because of the very low signal level of WERS by the time their 4 kW has
traveled 7+ miles, and the (assumed) extremely high turnaround losses of a
signal two orders of magnitude higher in frequency than WWZN.  It was more
likely happening in the radio.

Grady

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