"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?

Take a look at the phase angle between the feedpoint voltage
and the feedpoint current for the 28 foot version and try to
figure out an efficient matching network. That phase angle
is ~89.95 degrees. The feedpoint voltage is 1400 volts for
only 1.2 watts input. The feedpoint impedance is about
1.2-j1400 ohms.

Try using the helix feature to install a loading coil in the center
of the 28 foot antenna. The gain will drop as expected but the
feedpoint impedance will be easier to match.
--
73, Cecil, W5DXP