First, get some kind of portable receiver out and put it on 1820 and
verify you are hearing the harmonic. I doubt it is being generated in
your gear. You will probably hear it because you are only 2 miles
from the tx site and if the station is running 5 or more KW they're
likely suppressing the harmonic adequately -- you're simply too close.
What we often hear in our own stuff like preamps is products generated
by a pair of broadcast stations mixing. This doesn't mean that a
broadcast station is never at fault. Investigation shows your problem
station is a 5 KW directional with 3 or 4 towers depending on the
pattern. https://radio-locator.com/info/KRIO-AM
There's more to go wrong there. You should look at the night pattern
and determine if your station is in a lobe or null. You can observe
if the harmonic strength varies from day to night. It's true that AM
stations aren't getting maintained as well as they once were.
Problems can crop up in the station's phasor, and tower matching
networks. The pattern may not be in specification or they may not
even be changing to the night pattern.
A lot of high pass filters do not attenuate well near the top of the
broadcast band so as to not compromise 160 m. performance. So fellows
with stations nearby that are on 1500 or higher: Choose your filter
with care. In the case of 910 you have a wider selection from which
to choose because just about all of them will knock a low frequency
like that down. Even if the problem is a harmonic, putting in one or
more filters is worthwhile.
What about the harmonic? If it's coming from the station, all is not
lost because it is a point source. You can receive with small loop
antennas oriented to put the harmonic in the loop's null, or better,
employ a phasing network using inputs from two separated receive
antennas that can throw a null on 1820 at the offending broadcast
station. Or you can use both methods. If you are happy with your
current receive antennas, I'd try a phasing network if all else fails.
73
Rob
K5UJ
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