Years ago some folks bought front end pass band filters for 40M. If I
remember correctly they were for notching out the foreign broadcasts on 40M
phone. I believe they each covered about 20KHZ of the band. I remember
seeing an array of these switchable for different segments for the CW
portion of 40M. Just curious if something like this would work. Don't
remember what the attenuation was like out of the passband. Are they still
available and can they be made for different bands
N2TK, Tony
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com]On Behalf Of Tom Rauch
Sent: Monday, July 05, 2004 2:04 PM
To: Jim Lux; Michael Tope; Jim Brown; Tower Talk List
Subject: Re: [TowerTalk] Narrow Band Filters
> I think the best bet is going to be some sort of fairly
high level nulling
> antenna scheme. A reference antenna, and a
phasing/amplitude network into
> a passive combiner in front of the receiver. The slickest
approach might be
> two reference antennas spaced a quarter wave apart and a
pair of goniometer
> type combiners to allow phase and amplitude adjustment
with fairly broad
> band.
The ONLY choice without good radios and wide spacing of
antennas is a nulling device of some kind. You also have to
be sure everyone sharing the same band uses transmitters
with low composite noise output.
It is also true that stubs won't work. Stubs have very low
Q. Practical components in tuned circuits are actually much
more selective.
Since source levels are high, the nulling device could be
entirely passive. However, this would absolutely rule out
moving the antennas. Even an antenna blowing in the breeze
or something around the antenna moving could destroy the
null. This is why it is imperitive people use low composite
noise radios.
By the way, if you use phase style nulling the null that
notches the transmitter fundemental to prevent overload will
NOT null the composite noise. The reason is the composite
noise is on the receive frequency, the transmitter overload
comes from a different frequency. The entire system must be
designed with matched SWR and time delay, rather than simple
phase rotation.
A much better solution is antenna planning, selective
tuners, and good equipment. Field day power levels are
usually only 100 watts or so. Composite noise of MOST radios
is down 100dB or more off when far frequency, so with
careful radio selection most problems can be reduced to
acceptable levels.
I can sucessfully null my 160 transmitters by 80dB allowing
useful duplex work within a few kHz of my own transmitter,
but whenever I change antennas or antenna directions it
requires total revison of level and time delay in the
nulling system. My system has 5 miles of coaxial cables and
hundreds of relays and is complex to program. Worse than
that, the slightest arc in anything around the antenna
systems shows up as noise that isn't nulled and can't be
nulled.
It is MUCH easier and more cost effective to just use good
radios, and keep the antenna far apart. People also have to
remember how antennas work. A horizontal antenna has strong
vertical component off the ends, so if you are using cross
polarization you often will find the deepest null is when
the horizontal is broadside to the vertical antenna. With
two dipoles the deepest null is often with the dipoles
slightly skewed from end-to-end, and you MUST use good
baluns.
The entire thing is complex, and most casually chosen
solutions won't work at all.
> A real question is what's the best way to cancel the noise
sidebands from
> the TX (I think you can knock the carrier down with more
conventional
> techniques). Since they are random, some adaptive
canceller with a sample
> from the TX is probably the best approach.
Random does not matter. The requirement is a tap on the
transmitter feedline, a time delay, and a phase inversion.
One it is nulled it is nulled unless the antennas or
something around the antennas changes. With correct time
delay, frequency changes of even 5% will not matter.
73 Tom
_______________________________________________
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Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
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