[Scan-DC] re: FM Radio Question

[David Matthews]

Almost all FM receivers have an IF (Intermediate frequency) of 10.7 Mhz.
When you tune to 99.1 MHz, you're actually adjusting the Local Oscillator in
the FM radio to109.8 MHz.  In the mixer stage, the 99.1 MHz signal from the
RF amplifier mixes with the 109.8 Local Oscillator signal to produce (among
other things) a difference signal of 10.7 MHz which is then amplified by the
IF amplifier stages and then demodulated to produce audio.

If the RF amplifier is broadbanded enough (or the incoming signal is strong
enough), a signal on 120.5 (in the aircraft band) can *also* mix with the
109.8 local oscillator to produce a 10.7 MHz IF frequency.  This is called
"Image Interference".

Aircraft band transmissions are AM and thus fairly narrow bandwidth.
However, the IF filters in your FM receiver can pass signals ranging from
10.5 MHz to 10.9 MHz.  This suggests that the aircraft signal you're
receiving may be between 120.3 and 120.7.  Given the fact that this AM
signal is being heard by an FM detector, its more likely than not that the
actual fequency is near the edge of the range.  If it were in the center of
the range, the upper and lower sidebands would probably cancel each other
out and the FM detector wouldn't hear anything.  I'd try using your scanner
to listen across the range 120.1 to 120.9 (in AM mode) when you experience
the interference.

There are a number of other possiblities that could explain what you're
seeing, including possible mixing in the RF stage caused by front-end
overload, but the possibility of image interference as described above is
the easiest to check out.

--------- original msg ---------
Date: Sun, 28 Jul 2002 09:36:29 -0400
From: "William D. Rossiter III" <RadioGuy101@comcast.net>
Subject: [Scan-DC] FM Radio Question

I notice that sometimes at night when I am listening to my FM radio (88-108
MHz)....  I hear commercial airplane pilots.... this... mostly happens when
my
radio is tuned to 99.1 WHFS.