----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
> 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.
>
Yes, trying to match delay overy frequency would be a nightmare
in a field day situation (we are lucky to get all are stations up
and running in time for the beginning without the burden of fancy
nulling equipment). The approach I was suggesting would be to
notch the tx fundamental to an acceptable level at the receiver
where you can afford to take the insertion loss hit, and then null
the "on frequency" noise sidebands with a phase rotator
setup.
> 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.
Has anyone compiled composite noise test data on various
radios at larger frequency offsets? I think ARRL test reports
go to +20KHz maximum. You really need to see what the
noise spectrum look like at 50 to 500 KHz to make intelligent
decisions about which radios are good for this sort of
environment.
>
> 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.
>
Wow, that's pretty cool, Tom!
> 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.
>
Good point. I didn't think about that. We had our tribanders
setup "tip-to-tip" at Field Day (oops). We had "rope rotators",
so it might have been interesting to play with the orientation
to see about minimizing interference. Unfortunately, at our
Field Day everyone is so focused on getting points, that
there isn't much time to learn anything new. I get caught up
in it too. Its fun to get a big score, but of course the pay is
the same either way :)
> The entire thing is complex, and most casually chosen
> solutions won't work at all.
>
Yep, the only complex things that I have seen work really
well at Field Day were all tested before hand :):) Its really
very hard to anticipate all the problems that might occur
and get them nailed on the first shot.
> 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.
>
I am not convinced that manual nulling is all that hard even
if you don't have delay matching. I used to null a local
noise source on 160 meters with my ANC-4. It required
readjustment when moving frequency, but it was a fairly
simple adjustment (a few seconds). If both stations are
CQing on fixed frequencies, you set it and forget it. Still
you are right, Tom, a robust system based on good
radios and smart antenna placement which doesn't
require phasing techniques is the best. The KISS
principle is very important for a successful Field Day.
73 de Mike, W4EF............................
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