"John Popelish" wrote in message
...

\

Picture a half wave dipole, with a balanced feed. Two elements
perform the radiation and there is zero voltage swing at the exact
center of the dipole (though there is peak resonant current passing
through the center).

Now, cut that dipole exactly in half, and place a mirror at the half
way point. Half of the balanced feed line can be replaced by an
unbalanced (coaxial) feed line of half the impedance, since two of
those, with their shields connected and the center conductors out of
phase, would make a balanced feed line.

The radiation from the quarter wave half of the dipole is reflected by
the mirror to produce an an image of the missing half of the dipole.
The radials at the end of the quarter wave dipole act as the mirror.
This effect is pretty efficient as long as the radials are at least
1/4 wavelength long.

My experience with Navy UHF (225 - 400 MHz) antennas bears this out. There
are two vertically polarized omni antennas that appear in great numbers:
AT-150, which is a true dipole, fed with coax through an internal balun, and
the AS-390, which is a quarter-wave whip with eight "spider-leg" radials.
It is fed directly. They perform equally well and the system designer's
choice is generally based on mounting considerations.

There are over a dozen UHF antennas, some in stacked combinations called
"stovepipes", but of the single-unit antennas, the AT-150 and the AS-390 are
among the most common.