Tom,
Thank you for providing this information.
Your time is valuable and we all appreciate your input on these (and countless
other matter)!
73,
Bob AA6VB
Sent from my iPhone
> On Nov 17, 2013, at 10:55 AM, "Tom W8JI" <w8ji@w8ji.com> wrote:
>
> 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
> _________________
> Topband Reflector
_________________
Topband Reflector
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