Topband: Loop-family antenna notes
Wed, 29 Nov 2000 00:05:27 -0100
After reading Gary's comments on the loop family antennas, I realized that
I had left out one bit of information regarding the "Delta" performance. It
seems that most of the delta performance degradation was caused by the 80
and 160 elevated radials and by the wire fence around the property.
Horizontal stuff! While the degradation caused by the towers was still more
that what was experienced with the flag and pennant, the horizontal stuff
were the real killers.
Those wishing to do modeling work might consider this aspect. At this time,
I cannot do any modeling myself(long boring computer story) so I will have
to rely on others to explain the results I see.
Larry - W7IUV
P.S. Guess I'll have top try a K9AY version some day, just to compare.
At 07:00 PM 11/27/00 -0500, Gary Breed wrote:
>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.
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