All antennas are two terminal devices.
There is no such thing as a single point feed antenna.

Changing a horizontal antenna to a vertical antenna at the same height
does not improve the radiation angle.
If it did everyone would have their horizontal antennas in the
vertical position.

AM radio stations depend on ground wave signals not sky wave.

A horizontal antenna theoretically does not have a ground wave so
verticals are used for AM stations as they produce a ground wave
signal.

For sky wave signals height is important in order to produce better
lower angle of radiation. Lower angle radiation provides longer hops.

Adjusting the height to adjust the impedance of the antenna is not to
be worried about. That's what matching systems are for.
With multielement antennas (beams) the feed point impedance can be
very low even if the antenna is high in the air. Some other means of
matching the antenna to the line is required.

Regards
Gary


On Sat, 03 Jan 2004 04:13:27 GMT, (Larry W4CSC) wrote:

On Fri, 02 Jan 2004 13:00:34 GMT, Shortwave Sportfishing
wrote:


Well, you kind of danced around the answer, but I'd still like to know
how the ground plane effects the radiation angle which logically would
also have an effect on reception of a signal. On page 3-9 of the ARRL
Antenna Handbook (16th addition - sorry, it's the latest I have at
hand at the moment) states:

On a vertical antenna that USES a ground plane, the radiation angle
increases towards straight up as the ground plane becomes "smaller",
electrically less efficient. On a half-wave, end-fed vertically
polarized antenna, where no ground plane is used as part of the
antenna design, I doubt you could measure any difference.

"The total current in the antenna consists of two components. The
amplitude of the first is determined by the power supplied by the
transmitter and the free-space radiation resistance of the antenna.
The second component is induced in the antenna by the wave reflected
by the ground. This second component, while considerably smaller than
the first at most usefull antenna heights, is by no means
insignificant."

Ham antennas, dipoles, beams, etc., used for HF communications are
HORIZONTALLY polarized antennas. This is a whole new ball game when
they are close to "ground" be it a sheet metal roof or the ground,
itself. Unlike the radiation pattern of the vertical halfwave in
question, the radiation pattern of a horizontal dipole, which is still
perpendicular to the dipole wire, INTERSECTS the ground plane below it
and the RF re-radiates or reflects off the ground plane. The
radiation pattern of a horizontal dipole very near ground is straight
up and has a hot-air-balloon shape straight up. As the antenna moves
away from ground, a dimple forms in the "balloon pattern" which forms
a null at zenith with the radiation now two "lobes", perpendicular to
the dipole whos angle of radiation drops from zenith out towards the
horizon as the dipole becomes 1/2 wavelength off ground. Beyond 1
wavelength off ground, the pattern becomes the familiar donut
perpendicular to the horizontal wire radiating upward and outward,
even down towards the ground plane, whos reflections and re-radiation
phase angles caused the odd pattern in the first place.

Vertically-polarized signals point the NULL in the radiation pattern
off the ends of the dipole. One of these nulls is towards the ground
plane so little re-radiation takes place. A 1/4 wavelength "ground
plane antenna" has a radiation pattern elevated only slightly towards
zenith, which isn't much of a problem at all.

So it would seem that the "ground plane/wave" is not an umimportant
consideration when considering antennas.

Ground effect is VERY important in a horizontally polarized dipole or
beam antenna. That's why we put the beams way up on towers so they
radiate towards the horizon, not at high radiation angles. The phase
shifted re-radiated patterns of the slightly longer reflector and
slightly shorter directors (lagging and leading, respectively) "pull"
the donut towards the directors and away from the reflector, pointing
the beam's radiation pattern in the desired, narrow direction....and
giving great gain.....if it's not too close to the ground, that is!

Further on that same page, is the following:

"Changing the height of the antenna above the ground will change the
current flow assuming that the power to the antenna is constant."

Again, this is for "ham antennas" which are generally horizontally
polarized. The reflected wave from the ground back to the horizontal
dipole GREATLY changes its impedance characteristics because that
reflected wave causes a phase shifted current in the radiating
element, itself. Close to ground, this creates a large REACTIVE
component, which shows up as reflected power at the transmitter
output, not good at all.

Again, it would appear that the "ground plane/wave" is not
insignificant.

Now, as I understand it, at VHF frequencies, the methodology of
providing energy to the antenna (loading/feed) is not as important to
the generation/reception of the signal as is height. In fact, if I
read the pattern charts correctly, the height of the antenna has more
to do with the lobe pattern (the donut you were discussing) than the
method of feeding the antenna.

Yes/No?

No, not on VERTICALLY polarized antennas.

On VHF there is no replacement for POWER and ALTITUDE. VHF is
line-of-sight communications. The higher the transmitter and receiver
antennas are, the longer your range and better your signal at the
longer range. It's why WCSC runs hundreds of kilowatts from a 2000'
tower.....RANGE. They pay very dearly for both. You should see the
WEEKLY electric bills and tower maintenance bills. On a skywave
system, like AM radio at night or ham radio HF, altitude is not very
important other than to make the antenna's radiation pattern and
impedance what we want because it's horizontal polarization.