[Scan-DC] FM Radio Question

[Gary Chatters]

>
>I notice that sometimes at night when I am listening to my FM radio (88-108
>MHz), very short interuptions occur during the regular programing, and in
>their place I hear commercial airplane pilots (at least thats what I think I
>am hearing because I always hear the words "Delta, USAir, American West").
>For me, this always happens during the night and mostly happens when my
>radio is tuned to 99.1 WHFS.
>
>1.  Does anybody know what is happening?  (My theory is that it has
>something to do with planes flying over my house, and their cockpit radios
>overload my stereo.  I also think it could have something to do with the
>"harmonics" phenomona.)
>

The first thing that I would think of is reception at the image
frequency.  If your FM receiver has high side injection and the 
typical 10.7MHz IF, then the image would be at 
99.1 + 2 * 10.7 = 120.5 MHz.  This is right in the aviation comm range.

The IF on an FM BC receiver is quite wide.  It will probably pick
up anything from 120.4 to 120.6MHz.  If you look through a list of
aviation comm channels for this area you might find some assignments
it that range.  I don't have a list.

This still leaves a couple of questions: Why did your receiver demodulate
AM?  On a receiver with a good limiter, AM would be barely intelligible.
But I don't know what is used in current FM BC receivers.

The other question is why only at night?  The phenomenon should not
generally be affected by time of day.  Perhaps it has to do with
variations in flight patterns or maybe you just don't listen during
the day that much.  The signal would have to be quite strong to
override the BC station you are listening to.

gc