On Thu, Jan 8, 2009 at 10:02 PM, Ed Richardson <ed_richardson@shaw.ca>wrote:
> Just wondering if anyone has experienced intermodulation caused by some
> component in an antenna system? I am getting severe broadcast band
> intermodulation across the 80m band and have ruled out the receiver.
>
>
Yes, I get very bad mixing of a local 1500AM station with CHU 3330 after
dark; it shows up on 1830. At times before I added a BCB filter to my
system, I could easily copy both stations simultaneously in AM mode to
identify them. It didn't go away 100% but I knocked it down to a not very
problematic level.
However, last week I started getting BURSTS of IMD on the same frequency
that appeared to be the same mixing product (though I couldn't really
identify it). It was a windy night.
I tracked the problem down to the (probably corroding) bearings in my very
old Radio Shack junker rotator that I'm using to rotate the aluminum mast
that supports my Moxon.
I grounded around the rotator with some strap and that seems to have fixed
the problem.
This wasn't broadband noise from a charge/discharge across the junction or
anything, this was bursts of noise in a small bandwidth around 1830.
Look for dissimilar metal joints that might be a little corroded and bond
the heck out of 'em ;-)
Also check *actual diodes* in the system. K1TTT posted about subharmonic
generation that he tracked down to some system component (the page doesn't
seem to work in Firefox somehow)
http://www.k1ttt.net/subharmonics/subharmonic_files/subharmonic_frames.htm
73
Dan
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