Ed Cregger wrote:

Yes, BUT, the 5/8th wave radiator will put more of the signal toward the
horizon, instead of launching it at a 40 degree plus angle away from the
horizon. So while one configuration can have higher dbi ratings, it
doesn't count unless the signal goes where it will be most effective.
. . .

When mounted on a perfect ground plane of infinite extent, any ground
mounted vertical monopole higher than 1/2 wavelength will have one or
more high angle lobes. As the height increases above 1/2 wavelength, the
gain at the horizon increases even though a high lobe appears at around
60 degrees above the horizon. The gain at the horizon peaks out at about
5/8 wavelength, where the high lobe is about 9 dB weaker than the main
lobe. As the antenna gets longer than 5/8 wavelength, the power going
into the upper lobe starts reducing the gain at the horizon (and the
lobe's elevation angle slowly drops) until at one wavelength, all the
power goes to the upper lobe and there's no radiation at the horizon at all.

The gain increase of 1/2 or 5/8 wavelength antennas over shorter
monopoles comes about by a narrowing of the lobe pointing toward the
horizon. Unfortunately, though, radiation at the low angles is severely
attenuated by reflection from real ground. And this is just where most
of the power from longer verticals is going. So a 5/8 wave HF vertical
usually won't exhibit the gain over a shorter antenna you see with a
perfect ground simulation. Likewise, a finite ground plane like a car
roof impacts low angle radiation, so it has more of an effect on a 1/2
or 5/8 wave radiator than a shorter one, and once again you often won't
see the gain you might expect.

A few minutes with the demo version of EZNEC or a similar program shows
the effect of finite ground on various antenna heights very clearly. Use
MININEC-type ground to eliminate the separate effect of ground system
resistive loss. The full EZNEC program will let you model an antenna on
a car top (by using a wire grid to simulate the car top).

Roy Lewallen, W7EL