On Tue, 28 Oct 2014, dave wrote:

On 10/27/2014 01:48 PM, whit3rd wrote:
On Monday, October 27, 2014 12:01:45 PM UTC-7, David Platt wrote:
In article ,
micky wrote:
How can the same FM station appear at two different spots on the dial?

(3) Intermodulation. If you have two strong stations nearby, their
signals can mix (either in the receiver front end, or elsewhere)
and create "spurious" signals located on either side of their true
locations on the dial. These spur signals will often be noisy and
distorted.

This sounds very likely; if it is due to front-end nonlinearity, it's
possible to test/treat it by inserting an attenuator between the FM antenna
and the receiver (assuming the receiver has a plug-in antenna).
Lower the signal level, and the spurious response should go away.

Alternately, one can attenuate (filter) either the interfering FM station
or the (presumably
AM) difference-frequency station: this can be done with a lossy
antenna+load
placed near your radio, so can apply without access to antenna terminals.


I used to work on Radio Row in Houston. One day the FCC came to visit KILT FM
100.1 because they were causing squeals on the aeronautical band. It wasn't
any of the station's pro gear making the interference; it was an old console
FM receiver in the station lobby. 100.1 + 21.4 = 121.5. Radio row was on the
direct approach to Hobby Airport or this old mis-aligned radio would have
never been busted.

I thought that was some of the basis of the ban on electronic devices on
airplanes.

Certainly there is folklore that when AM/FM transistor portables became
cheap and available, suddenly people were using them on airplanes, and
that did or could have caused interference, precisely because the local
oscillator radiated and in the aircraft band.

It's murky whether that was the specific cause of the rule or not, and
probably made murkier since it's been forty years since I read about this.

Michael