On Thu, 01 Jan 2004 18:06:55 GMT, Shortwave Sportfishing
wrote:


How can an antenna work without a ground plane? At the frequencies
we're discussing, the ground effect in FM is about the same as it is
in AM if I understood your discussion points (deleted from this post)
correctly.

Modulation has no effect on antenna physics. The object is to fit at
least 1/2 wavelength onto a conductor. Where it is fed is of no
consequence to radiation, but does effect the feedpoint impedance.

As to the "ground plane".....

In an HF antenna, we are always dealing with an antenna where the
practical length is far shorter than the ideal length. At 7 Mhz, a
1/2 wave antenna is about 65' long. Any 1/2 wave dipole antenna is
complete and there is no ground plane. A ground plane is only
required if your antenna design includes an image antenna. Case in
point:

Look at 90% of the AM broadcast antennas in your area. Some AM
stations actually DO have 1/2 wavelength antennas, but a 1000 Khz this
is 468' long and is expensive to erect and keep erected, so they only
do it when they have no other choice. A 1/2 wavelength AM tower
requires no extensive ground radial system, either, so it would be
located in a dense city where you cannot lay out long radials to bury.
Most AM transmit antennas are near 1/4 wavelength in length (a 108" CB
stainless whip on Bubba's pickup is a 1/4 wavelength antenna on 27
Mhz). To get this 1/4 wavelength to "tune" (resonate), we have to
bend the other half wavelength and lay it out sideways in an L
pattern. However, erecting just an L 1/2 wave antenna fed at the
corner of the L creates a radiation pattern in the direction of the
horizontal of the L. To counter this effect, more horizontal elements
are laid out around the base of the 1/4 wave vertical part to make the
pattern omnidirectional, like the 1/2 wave resonant antenna. The end
result is like a CB "ground plane" antenna for 27 Mhz. A 1/4 wave
vertical over a set of "ground plane" radials, which don't have to be
buried to work.

On VHF marine at 160 Mhz, a whole half-wave antenna needs only be 34"
long, so there is no need to resort to shortened whips working against
"ground plane". The Metz antenna is a 34" stainless whip with an
autotransformer on the end of it that raises the feedpoint from 52
ohms, the cable impedance to the transmitter, up to several hundred
ohms, which excites the already-resonant element. No ground of any
kind is required, or desired.

Like you say, to each his own.