[TowerTalk] Narrow Band Filters

[Michael Tope]

> 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?
>
You may have a point, Jim, but remember the ANC4 and
likewise the MFJ unit (I think) have all the active gain in the
noise leg. This is the controlled path which you would
hardwire to the offending transmitter (station A) with a
directional coupler to siphon off a sample of the
broadband noise. The key is whether or not the noise
cancellor would have enough dynamic range to pass
the offending transmitters fundamental (fm station A) without blocking and
still
deliver a large enough sample of the co-channel
broadband noise to its internal combiner such that you
could cancel the co-channel noise coming from the
receive antenna.


> >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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