> As for your symptoms, what you describe is more than one
problem. The level
> of noise changes because of multiple sources. One source
don't change in
> level.
I'm not disagreeing that he might have multiple sources,
but.....
I've seen many single sources that changed in level from
zero to very strong. There aren't gain leveled signal
generators, they are arcs and the intensity can change.
It is not accurate to say it takes one canceller for every
source, nor can multiple cancellers be used for multiple
sources as a general rule.
The basic principle of a canceller is very simple. The noise
is sampled, phase inverted, and combined with the noise from
the main antenna at the same level as that noise. What you
are really doing is setting the sensitivity of the sense
antenna and main antenna to the same level at the distance
and angle of the undesired signal, and inverting the noise
exactly 180 degrees. There is nothing requiring the
undesired signal to be a single source or a stable amplitude
source in order to have a deep null.
As long as the noise comes from the same general direction
it can be one source or a dozen and they can vary in level
without requiring readjustment. It's only when the noise
source or sources move in azimuth or are so close that they
surround the location that nulling can become impossible.
Multiple cancellers cannot be used, as a general rule,
because the addition of a second new device simply undoes
what the first device accomplished. The exception would be
if you had small sense antennas located right at each
source, and none of the other sense antennas could hear
significant noise from other noise sources. In a case where
multiple cancellers were used dynamic range of the system
would seriously decline. Existing cancellers are not high
dynamic range devices, they were designed to be inexpensive.
As a general rule the only way to know for sure if a
canceller will work is to get one and try a few different
sense antenna combinations on it. I have multiple groundwave
sources a few miles west of me, and I can null them totally.
They can come, they can go, they can change level, and I
never have to readjust the nulling because they are all from
the same direction and not in the near field.
73 Tom
_______________________________________________
RFI mailing list
RFI@contesting.com
http://lists.contesting.com/mailman/listinfo/rfi
|