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" <[email protected]> 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
