"John Popelish" wrote in message
...
David wrote:
Can someone provide a full description of how a quarterwave vertical
antenna
with radials works? Length of radials is also a quarterwave.

I find that many books give a good description of antennas like the
Yagi,
and then suddenly become very vague when describing the quarterwave
vertical. Books
refer to image theory where an image of the radiating element is
produced by
the radials, and show a spear shape going into the ground. Some say the
radials are the other half of a dipole.

What difference does it make if the radials are in free space or in the
ground? Some articles claim that the radials tend not radiate because
they
cancel out, while other other articles claim that the radials simulate a
ground plane and reflect the radio wave. Can you explain this
contradiction?

The vertical element is usually called the radiating element. How well
do
the radials radiate? The same magnitude of current flows into the
vertical
element as the radials, although the current into the radials is split.

A normal ground plane is a large sheet of metal that reflects the radio
wave
emitted by the radiating element. If there are four radials, each a
quarterwave long, do the radials form a ground plane? Or is there too
much
of a gap for them to form a ground plane?

If the radials are disconnected and taken away, with the vertical
quarterwave element still connected to centre conductor, do I still have
a
radiating element? What happens to the SWR?

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.