Question: I have heard that it is possible to receive cellular frequencies (or
any frequencies, for that matter) on non-cellular channels by entering in an "image"
frequency. If this is really possible, can someone post some details about this?
Answer:
Image reception is usually an undesired function of a receiver but it some
cases is useful. Images are common in scanners because their receiver
front ends are not very selective.
To calculate an image frequency you need a couple of pieces of information.
Mainly, the frequency you want to receive and the first intermediate frequency
of your scanner. Multiply the IF by two and add or subract that number
to/from the frequency you wish to hear. If the result is frequency you can
program into your scanner, use it. Here's an example used to listen to
aircraft.
My scanner is not designed to be air band programable.
I want to hear 135.4 MHz.
The first IF of my receiver (in the specs) is 10.7 MHz.
2 x 10.7 = 21.4
21.4 + 135.4 = 156.8
135.4 - 21.4 = 114.0 (won't work in my receiver)
I program 156.8 into the scanner and I should be able to hear air traffic on
135.4. Of course, I also will hear a whoever is using 156.8.
Of course, your scanner must be able to receive AM on 156.8, which
would be a little unusual. Aircraft radios from 118-135.975 use AM in
the US. Most scanners that don't receive aircraft don't have an "AM"
position and aren't able to detect AM signals.
