I generally use them for signal enhancement or rejection.
They can also
be used for noise elimination >>IF<< the noise is coming
from a point
source. If noise is emanating from a power line or other
they won't do much good.>>
The noise canceller does two things:
1.) If you have a point source radiator at any distance it
can eliminate that source.
In this case you only need sample the noise source as close
as possible to the source and combine that signal with the
signals from the main antenna to null the noise.
2.) If you have a radiator or multiple source radiators of
any type in the far field it can also eliminate that source
as long as they are in the same general direction.
In this case you need fairly wide and sometimes planned
separation of noise and signal antennas. The noise canceller
really acts as a phasing system forming a pattern null in
the direction and at the angle of the noise source or
sources. The signal must not be from the direction and angle
of the noise.
In many cases a noise canceller will work perfectly fine on
power line noise.
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