Amen Bob, Tom really helps a lot of guys and gals with his expertise. Quite a guy.
73 Price W0RI 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" <w...@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 _________________ Topband Reflector
