The non-linear test will help you immediately determine if the issue is inside
or outside of the radio(s). If inside, dB steps make a difference quickly and
"not linearly", meaning a 1 or 3 dB step makes a larger impact.
If it's linear, 10 dB steps make 10 dB improvements, and it's outside of the
radio. This could still be in shack wiring or other components, or on premiss,
the other side of town, or the state, as the one Tom tracked down.
tnx
Mike
> On Nov 3, 2015, at 3:51 PM, Tom W8JI <w8ji@w8ji.com> wrote:
>
> 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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