----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
> 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 (come to think of it the MFJ
may have gain in both legs). 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 in its noise sampling leg 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 station B's receive antenna.
If fundamental overload were a real problem you
could put a trap or narrow bandpass on the noise
input of the cancellor since insertion loss is not an
issue there. That might complicate tuning, however,
if you had to tweak a tuneable trap everytime the
other station QSYed.
And remember, this scheme would only be needed
to null out QRM from a station operating on the same
band (classic one station on CW, the other on
SSB problem), so you wouldn't really be dealing
with trying to null multiple sources of interference
(QRM from the adjacent bands could be filtered
with conventional bandpass filters and stubs).
I have wanted to try this sort of scheme for some
time. Perhaps I'll lash something together at the
local club station to see how it works. You have
pointed out some important pitfalls which I have
naively ignored, so it will be interesting to see
how readily they can be overcome. If the
off-the-shelf noise cancellor scheme turned out
to be a dud due to the dynamic range limitations
of the active phase shifter, you could probably
build a passive scheme pretty easily using relays
and pieces of coax (I remember W8JI describing
something like this that he put together).
>
>
> >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.
>
Yes, but 1000 feet is a lot better than 100 ft, especially if
you get the tx and rx antennas in mutual nulls. The key I
think is to pick the right rigs (start out with the lowest
broadband noise emitters available) so you are not at
a big deficit to begin with.
> 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.
>
Yes, you need very high Q to do same band filtering
effectively.
> 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
I agree. If you optimize every piece in the system (e.g. pick
rigs with the lowest available noise output), use maximum
antenna separation with mutual nulls between antennas, then
you might not need additional filtering. But if you did, then
you could probably get a little more attenuation with carefully
designed hi-q notch filters without sacrficing too much TX
power.
In any case, it all sounds like a lot of work :)
73 de Mike, W4EF......................
P.S. sorry for the premature reply sent earlier. I must
have hit alt-s by mistake while typing
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