[Scan-DC] FM Radio Question

Shawn [email protected]
Sun, 28 Jul 2002 13:59:19 -0400


Bill,
This is much easier to do with a piece of paper than it is to do over
the computer.
But here goes:
Inside almost all radios is at least one "mixer."  A mixer takes in two
frequencies and delivers the sum and difference on it's output. 
Specifically, a mixer has an LO port (Local Oscillator), a RF port
(Radio Frequency), and an IF port (Intermediate Frequency).  The RF port
is essentially) connected to the antenna.  The LO port is connected to
an oscillator running at 10.7 MHz higher (hence the term "high side
injection") than the desired RF frequency.
So, you're listening to Tim Virgin, Graham's World, or Dr. Drew or
whatever at 99.1 MHz (WHFS).  The local oscillator inside the radio is
actually tuned to 109.8 MHz.  The difference is the 10.7 MHz IF (or
Intermediate Frequency).  However, there are multiples of the IF at the
mixers output.  Multiples are refereed to as "images" and manifest
themselves by a radio's susceptibility to picking up transmissions at RF
+ IF and RF - IF frequencies.  This is undesired.  Most higher end
radios have spec sheets and this is referred to as Image Frequency
Rejection or Image Rejection or something to that effect.
Lower end (i.e. cheaper) radios without good front end and IF filtering
are prone to poor image frequency rejection.  Taking the 99.1 MHz
example, your radio will be susceptible to transmissions at 99.1 + 10.7
(120.5 MHz) and 99.1 - 10.7 (88.4 MHz).  88.4 Mhz is within the FM radio
passband but it isn't a problem since we in the US broadcast on odd
channel assignments (i.e. 88.3, 88.5, 88.7 etc, etc...).

In an email that Blair sent you with this header:

Subject: Re: [Scan-DC] Airplane Related
Date: Sat, 20 Apr 2002 15:56:54 -0400
From: [email protected]
To: [email protected]

On Sat, 20 Apr 2002 13:05:50 -0400 William Rossiter III
<[email protected]> writes:
> Does anyone know of any websites where I can find frequencies for all
> the
> Washington DC airplane frequencies?  I need a complete listing
> please.

he listed a few frequencies that could cause your radio problems:

113 120.100 Dulles tower
115 120.450 Dulles approach


My money is on #115 causing you problems.

My son wants me to spend some time with him so I can't go on.  But put
"image frequency rejection" or "radio fundamentals" into Google and see
what sites you get.
Good luck,
-Shawn