On 2014-03-17 21:29:35 +0000, Michael Black said:

On Mon, 17 Mar 2014, Jon Danniken wrote:

Hi all, I picked up an SDR radio dongle, and have been playing around
with receiving. As I am playing around with it today, I am noticing
something odd (to me).

We have a college station locally that broadcasts at 88.1MHz FM. As
with many college radio stations, it has a very weak signal, but I can
just pull it in with rabbit ear antennas.

As I was poking around at ~24MHz, and I was able to pull this same
station in at 23.645MHz. Even more odd, I was getting a much better
signal at 23.645MHz than at the 88.1MHz "official" frequency.

Additionally, as I scanned around, I also found all of the other
stations doing this as well; 91.9MHz, which is too weak to tune in, I
can hear perfectly at 28.035MHz. I can also tune in to 96.1MHz at
30.240MHz, 105.5MHz at 30.325, and so forth.

So here's my question, is this something that is "normal" in radio, or
does this instead point to some peculiarity with my radio/software setup.

Thanks for any suggestions,

Jon

FM stations apparently can use something around 26 MHz for remote
feeds, but I don't know the details other than it has come up in one of
the newsgroups before, someonehearing FM broadcasting in the wrong
place.

Do those things do any conversion, or is it straight to baseband? If
it converts to some other frequency first, then maybe you are seeing an
image. And since those things aren't so well tuned, so image rejection
might not be so great. That doesn't explain why the wrong frequency
would have a stronger signal.

Find out if it converts to an IF first, then do some math.


MIchael

It is also common to find FM broadcast feeds up around 460 MHz.