"John Ferrell" wrote
I am attempting to compare the gain of a full size (1/4 wave) vertical
to various shorter verticals at 160 meters.

I am looking at the gain in dbi on the 2D plot. I see 1.39 dbi for the
full 1/4 wave and I see 1.52 dbi for the 28 foot version.

I don't believe the shorter antenna has less gain. Where am I going
wrong?
_____________

Assuming that your vertical is ground-mounted, getting the "right answer"
for this question depends on the quality of your wire models, and especially
your assumptions about the ground plane, and the way your vertical connects
to it.

A classic 1/4-wave vertical monopole working against a very good radial
ground system such as used in AM broadcast should produce a peak gain of
around 5 dBi. The theoretical max for such a system with perfect conditions
is 5.15 dBi.

Electrically short(er), base-insulated vertical monopoles, for a given
frequency, have lower radiation resistance at the feedpoint near the tower
base. So, given the same r-f resistance in the path to the ground plane,
the radiation efficiency of the shorter system will drop.

RF