On Mon, 5 Jul 2004 04:25:41 -0700, Michael Tope wrote:
>source, the noise cancellors like the MFJ unit are very
>easy to adjust. If fundamental overload from the
>co-band/cross-mode station is a problem
I have no experience with the MFJ unit, but my ANC4 is a dog for overload from
broadcast signals that aren't even very strong at my QTH (nothing within 7
miles of
me). I understand that newer production units have a BC filter, but that won't
solve
the basic overload problem on in-band signals. The ANC4 can be effective on
some bands with stable local sources, but can be a PITA to keep tuned with
different/multiple sources. And I think that fundamental overload would
severely limit
its usefullness on FD. I would like to find a better solution. Maybe the MFJ is
better?
>One the other hand, long pieces of coax are even simpler.
>We do our field day on a mountain top, so we could
>conceivable put separate receive antennas down the
>hill (towards the northeast of course) to improve isolation.
Yes, but you're still stuck with that 1,000 ft diameter circle.
>The co-band/adjacent-mode stations could share a single
>receive antenna (and coax) located at maximum distance
>from the TX antennas (a small tribander with a power
>divider for instance).
BTW, I've measured quarter wave open stubs made from very low loss RG-8, cut
for
both 80 meters and 6 meters, and they have far too low Q to be effective. K4GLM
tried quarter wave stubs on FD, with the same result.
I'm a big believer in the value of adding multiple small improvements to solve
a
problem. I agree that the design of these filters is non-trivial, and the
comments re:
circulating currents are probably dead on. But how about this -- a multistage
bandpass filter for 80 intentionally designed to be as narrow as possible,
tuned to
3525, and a bandpass filter for 75 with the sharpest possible cutoff at 3750.
If you
could pick up 6 dB from each filter, that's a 12 dB reduction in the trash that
each
receiver sees. Even 3 dB from each filter is a 6 dB reduction. Add that to the
physical separation of antennas and the choice of rigs with less trash and
you're on
your way to solving the problem.
75/80 are the easiest, of course, based on percentage bandwidth. But once
you've
found a solution there, it might be possible to translate it to 40 and maybe
20.
Jim Brown K9YC
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