Kevin Kwan writes:
>Does anyone on list know if there's a such thing as a talking intermod
>filter? I asked around and people say that I have to see to use one!

        What?  Maybe they are talking about spectrum displays that one
might use to determine where the strong signals are that are mixing.
The usual way intermod happens is when two or more signals are very
strong at the front end of your receiver.  The RF stages begin to act
like mixers and produce sum and difference frequencies that you hear
when they happen to equal the frequency you are listening to.

        The only way to get rid of intermod in a receiver that has it
is to either notch out the frequencies that are saturating your
receiver front end or use a filter network that only passes the signal
you are wanting to listen to.  There is no neat little black box
called an intermod filter sitting on the shelf at Radio Shack.

        There are a number of ways to make notch filters such as tuned
cavities or shorting stubs.  You don't specifically have to see to use
them, but you do need to have some way to measure signal strength so
you can tell when your notch filter is exactly tuned.

        The notch filter might fit on to one side of a T connector
with your receiver on the other side of the T and the antenna feeding
the third connector.  If the notch filter was close to one of the
intermod sources, that signal will go down in strength and not
overload your receiver front end as much.  If the filter is in the
form of a cavity, you would tune the cavity until the unwanted signal
was at its lowest level.

        Other notch filters include shorting coax stubs, LC networks
and even special crystal filters for extremely deep notches with very
high Q.