> The EWE and K9AY both involve connection to ground, which isn't
> allowed with high accuracy ground in the standard NEC2 version of
> EZNEC. This can be approximated by using 2 or more 1/4 wl radials
> 0.001 wl above ground at right angles to the antenna wire. In fact,
> W7EL includes a K9AY model with EZNEC (not sure which version it came
> with).
>
> I've found that the K9AY is the hardest to model. Even small changes
> can make a big difference in the null structure. It makes it
> difficult to know what is "real". I think that the actual antenna is
> similarly "jiggery-pokery", leading to the common use of vactrols for
> variable terminations.
>
> Good luck,
> Terry N6RY
___________
Terry and all,
1. While I agree that modeling the K9AY Loop is not simple, the antenna is
definitely not "jiggery-pokery." The model came first, including
discussions with my good friend W7EL on whether the model was valid, and if
so, within what limits. Although the model I've settled on does not follow
all NEC-2 recommendations, it gives consistent results that are verified by
actual performance. The same can be said about EWE and Beverage antenna
models with connections to real, high-accuracy ground.
2. The use of Vactrols is a combination of real need and false perception --
Adjustment of K9AY Loop termination is useful when: a) the loop is used near
the upper limit of its useful range (total wire length about 1/4
wavelength); b) with extreme local ground conditions (seashore, desert,
large seasonal wet-to-dry changes); and c) when the loop is built with
different sizes and shapes. A pot, Vactrol or switched resistors are all
useful ways to adjust the termination.
However, Vactrols drift a lot with temperature and time. When Vactrol users
think they are "tuning" the antenna, they are simply returning the Vactrol
to the correct value after it drifted off.
73, Gary
K9AY
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