The first thing I would do is find out if it really is AM BCB stuff, or SW
BC, or what it actually is.
BCB filters won't help if it is a 3 MHz SWBC, or something just above 80
meters. Filters also will not help if it is rectification in the antenna
system, or a real signal from mixing or a parasitic somewhere else.
I've had broad spectrum noise like that from AM transmitter issues, some
from Georgia and some from other states, and some SWBC station WWCR.
I had one AM station in Georgia licensed to run just a few watts at night
that was running a kilowatt, and the transmitter also had a wide parasitic
that went from 1600 kHz up past 3 MHz. I had to drive halfway to Savannah to
locate it.
One thing happening now is AM stations share antennas, and SWBC stations
nest antennas close together.
Generally what I do is listen to the distorted QRM audio on one ear, and
start at the low end of the AM BC and work my way up through shortwave
looking for a signal match. Almost all of the time I find the match, and
some of the time it isn't even stuff from the AM band.
Usually, but not always, mixing is on spot frequencies. Usually, but not
always, real wide stuff is arcing or parasitics at the BC station.
Almost always a 10-30 dB filter cleans up a receiver. If the transmitter
were right next door, you might need 50-60 dB. If you are sharing a feedline
with them, then you need in the 100 dB range of filter. One port mixers with
signals all through the same port are not linear with padding. If you can't
hear a significant change with 10-20dB attenuation, the mixing is probably
not receiver side of the attenuation.
Before throwing too many bypasses or filters at things, I'd try to get some
idea what it is.
73 Tom
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