[Antennaware] K9AY Loop modeling in NEC-2

[Pete Smith]

I have been working with NEC-2 modeling arrays of K9AY receiving loops, 
with the thought that if end-fire or broadside arrays of these things could 
be made to work well, it would be a neat alternative to the Beverages I 
don't have room for.

In the process, I've run into the basic issue of how to model the K9AY 
loop.  On his web site, at <www.w8ji.com/k9ayloop_mod.EZ>, Tom has posted a 
modeling file for this antenna which he attributes to Roy Lewallen, 
W7EL.  It appears to be an effort to work around the NEC-2 problem with 
wires connected to ground, by connecting the two ends of the K9AY loop 
instead to counterpoise wires that run perpendicular to the plane of the 
loop for 100 feet in each direction, 6 inches off the ground.

The trouble is that if I run this file in NEC-2, I do not get anything like 
the cardioid pattern K9AY reports, with a steep null to the rear.  At best 
there is a slight concavity in the 3D pattern at the mirror image to the 
maximum forward signal, but the F/B is no more than about 11 dB, and the 
RDF is about 8 (not too bad, by the way).

If I model the antenna with the loops connected to ground, while still 
using the Sommerfeld-Norton ground, I get results that are much like 
K9AY's, but MultiNEC warns me that the results cannot be trusted.  If I use 
the recommended alternative technique of substituting MiniNEC ground and 
adding loads to simulate the ground connection resistance, the pattern 
reverts to one that looks very much like that resulting from Lewallen's 
counterpoise approach.

I'd appreciate advice on:

1.  the validity of the various approaches.

2.  what would be an appropriate load resistance to use for ground 
conductivity in the miniNEC ground case.

3.  In the miniNEC ground approach, if both the source and the load on that 
leg of the antenna are specified as being located at the same place, I 
believe they will appear in parallel.  For better accuracy, should I place 
the source one segment further "up" the leg from ground, so that the load 
will be in series betweren the source and the ground?


73, Pete N4ZR

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