The most frequent problem (by far) with good noise cancellers is operator
related.
The second most common problem is antenna choice.
Any suggestion on how to use the unit best? Set up another single RX
Element? Use 'the tower'?
When you mix antennas to make a null, the signal levels from the antennas
have to be equal in the null direction. You really are adding two signals
from the null direction 180 out-of-phase together.
Logically, if one antenna has significant response in an undesired direction
with problem signals and the other does not, you can reduce signal-to-noise
of the good antenna when you add in the poor antenna to form a null. For
example, using a small vertical loop to further null a vertical array with
no overhead response will add overhead response and high angle horizontal
response even while increasing the null. The loop also has a 180 shift for
signals from the opposing directions, while a vertical does not. This can
create phase problems when adding the two together. You might have increased
back null and decreased front signal at the same time.
Another issue is antenna level and phase response with signal angle and
direction. A dipole, for example, changes polarization as the signal moves
off broadside. It is only perfectly horizontal directly broadside, and has
an increasingly tilted pattern as the signal moves toward the ends, where
the signal response is vertically polarized at high angles. The tilt is a
different rotation direction, depending on which way the signal moves from
broadside.
All of this factors in. We have to be careful what we mix together if we are
dealing with signals.
If we are dealing with noise alone and not looking for a pattern change,
then the noise antenna just has to have much stronger response to the noise
than to any signal.
Either way can remove noise, but the functions behind removing noise are
different.
If I had a local noise from one source, I would put a small antenna very
close to that noise source or next to something conducting a strong,
dominant, signal from that noise source. An insulator arc or arcs from one
point on a power line that was otherwise pretty clean could be picked up
anywhere along that line. Multiple insulator arcs from multiple locations,
all radiating to the receive antenna from different directions, are a
different story. Getting near the line would not work.
You can null an infinite number of sources if they come from one point, or
if they come from multiple points all in the same general direction and that
general direction is different than the desired signals.
It is pretty difficult to explain every possible case, but those are a few
of the most common situations.
The bottom line is:
Nulling noise from multiple sources in one basic direction, or nulling
signals, or changing patterns....you want similar antennas or similar
pattern responses (but far from the closest noise source). It is generally
easier if we do not mix antennas with grossly different responses.
Nulling a single noise source or multiple noise sources at a single
point....you want a local sense antenna near the source or near something
coupled to all the sources so the noise antenna hears way more noise than
signal. It doesn't matter what the antennas are.
73 Tom
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Topband Reflector
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