"Honus" ) writes:

What I meant was that 10000 KHz had an image 910 KHz below it, but a station
that I received at 10855 KHz didn't produce an image 910 KHz below that
frequency. I was wondering why that was; why some frequencies produced
images, but others didn't. Sorry if I wasn't clear.

Because there is some sort of filtering at the front end.

Thus the image frequency will always be attenuated to some extent. If the
incoming signal is weak enough, then the attenuation will be sufficient
to knock out the signal at the image frequency.

The problem of image rejection increases as the signal frequency increases.
At 1MHz, the image 910KHz away is quite a large percentage of the signal
frequency, and even fairly simple front end filtering can knock out the image
so that only very strong signals will ever appear. At 10MHz, the 910KHz
is a much smaller percentage of the signal frequency, so you either need
better front end filtering, or start to think of moving the IF frequency.
As the signal frequency approaches 30MHz, image rejection will become awful
with such simple receivers, because the 910MHz is an even smaller
percentage of signal frequency.

I've seen reviews of cheap receivers from the late sixties, with 455KHz IFs,
and comments like "image rejection was almost non-existent on the top band
[20 to 30MHz]".

But note that the issue is not just the low IF frequency. It's also a matter
of the front end filtering. A really cheap receiver would have minimal
filtering at the front end, so it would only reject the weaker signals as
the signal frequency went up. But the better receivers would have better
filtering, and often could get away with a 455KHz IF; those would go to
two stages of RF amplification before the mixer, with the extra tuned circuits
to go with them. It was the extra tuned circuits that helped the image
rejection on the higher bands.

Michael