Chris,
AO is mininec, and the mininec ground only starts to become reliable at
about 0.25 wl up. Hence, your modeling results will be of borderline
accuracy.
In general, the antenna properties do change in rather large ways as many
antennas are brouight closer to ground. The greatest change is likely to
be in the feedpoint impedance, but it is unlikely that you would have to
change any parasitical elements to get the best performance possible at
the low height. For a given design, a horizontal antenna will show gain
peaks at 5/8 and 1 1/8 wl heights, with gain dips at 3/8 and 7/8 wl
heights. Above 1 wl, the peaks and valleys get much shallower and and
disappear above 2 wl. However, for practical purposes, expect there to be
a feed Z difference between low and high mountings.
Did a study for Communication Quarterly several years ago on the affect of
height on the properties of a number of different types of antennas.
However, since I was using MININEC at the time, I started from 1/2 wl on
upward.
Hope this is useful.
-73-
LB, W4RNL
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