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Topband: Loop-family antenna notes

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Subject: Topband: Loop-family antenna notes
From: (Gary Breed)
Date: Mon, 27 Nov 2000 19:00:48 -0500

After reading W7IUV's comments and other recent posts on various members
of the "loop-like" RX antenna family, some of my accumulated notes may be
useful.  First, remember that the EWE, K9AY loop, flag, pennant, delta, etc.
all operate on the same principle -- a terminated loop.  The differences
are in the way ground is used, how they are fed, and their physical shape.

Many K9AY loop users have e-mailed me directly with concerns and comments
(and kudos). Problems could be traced to three areas: incorrect construction
of antenna or transformer, coupling to nearby structures (including to the
feedline), and insufficient preamplification. Quite a few attributed their
problems to local ground conductivity, but I do not believe this is as big
a problem as it seems -- one or another of these antennas can provide
useful directivity in free space or over perfect ground!

Another, more subtle variation among these antennas (which may be what
W7IUV is experiencing), is in the shape of the directional pattern. All
of these antennas have basically a cardioid pattern, however, the depth of
the null and the vertical angle where the null is deepest vary with the
shape of the loop and the termination. The optimum termination value can
vary with local ground conductivity, or more often, with deviations from
the "standard" dimensions in the published descriptions (thus my earlier
note on incorrect construction).

 As published, all of these antennas use a purely-resistive termination.
It is not well-known that you can control the vertical angle and depth of
the null by using a reactive termination (R-L or R-C) to adjust the phase
and current relationship between termination and feepdoint. I have modeled
this behavior extensively, but only did one "sanity check" experiment to
verify that the effect was present outside the computer. It's something
for another experimenter to try; the K9AY loop geometry was intentionally
chosen to put the null where I want it with just a resistor.

NEC-2 modeling programs are sufficiently accurate for modeling these loops,
even for ground-connected antennas -- but do not try the usual technique
of increasing the number of segments until convergence is reached. When
designing the original K9AY loops, I found that the model closely matched
observed behavior with one segment for each 1-1/2 feet of wire (on 160).
Models for other loops behave similarly using this segmentation scheme.

I hope these notes help you build your own loop-type RX antennas.

73, Gary

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