Tom W8JI wrote: > If you test some popular loop antennas for pattern, which I have done, you > will find many have significant skewing in the nulls. The nulls often are > not 180 degrees apart. This proves the loop is in multiple modes, one of > which (electric field probe) would involve mast and feedline radiation. > This > is true even with an exceptionally clean mounting and feed arrangement. > > This occurs because the feedpoint design causes the loop to be sensitive > to > common mode on the element, the mast, and the feeder.
I tried to address this issue in the feed system I described in NCJ. Instead of a "balun" (meaning a bifilar transmission line transformer), this design uses a true flux coupled transformer. This gives extremely high feedline isolation. In fact, my main limiting factor tends to be direct leakage into the coax feed line, which remains the same if I replace the antenna with a 50 ohm load. In testing for nulls, I have never detected skews when testing away from conductive objects, like other antennas, power lines, buildings, etc. OTOH, the pattern degrades very noticeably when near these objects due to distortion of the EM fields around those objects. This is of course not the fault of the loop antenna. It should also be noted that the Pixel loop amplifier has to be mounted with nylon insulating washes so that it has no electrical connection besides the 1 foot jumper provided. Failure to do so could degrade the pattern. At least this is what the company rep told me. He made it sound like a lot of people ignored that recommendation. Rick N6RK All good topband ops know how to put up a beverage at night. _________________ Topband Reflector
