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.
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Topband Reflector
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