"Roy Lewallen" wrote in message
news:z8WdnThgM7y9_5fVnZ2dnUVZ_gWdnZ2d@easystreeton line...
There are two quite separate ways which ground affects a vertical
antenna's performance.
The first is loss due to current returning to the antenna base when the
antenna is grounded, or induced in the ground under an elevated radial
system. To minimize loss, you want as much of the current to flow through
radial wires as you can. The power loss is I^2 * R. For a given power
input, I is much lower for a half wave bottom fed vertical than a quarter
wave bottom fed vertical. So the loss due to the conducted or induced
current is much less, and you can get by with a much simpler ground system
with the half wave vertical and still have low loss.
This ground loss is usually the chief determining factor of a vertical's
efficiency.
Here we are again forgetting that we are dealing with standing wave circuit
and cos/sin current distribution along the elements. Half wave vertical
might have low current at the base but quarter wave away it will be max
(assuming half wave elevated electrical radial). The radiation pattern is
formed between the radiator and radials (and how they are affected by ground
under).
Radials close to ground couple to it and depending on ground RF quality we
are dealing with decent reflecting mirror or "RF eating sponge".
The other effect of ground is that the field from the antenna reflects
from it some distance from the antenna. The reflected field adds to the
directly radiated field to form a net field which is different at each
elevation angle. This is a major factor in determining the antenna's
elevation pattern. The conductivity and permittivity (dielectric constant)
of the ground affect the magnitude and phase of the the reflected field,
so the pattern changes with ground quality. In general, the more
conductive the ground the better the low angle radiation. However, you
can't compensate for this factor when the ground is poor by improving the
ground system. The reason is that the reflection takes place much farther
from the antenna than nearly any ground system extends. And low angle
radiation, where the improvement is most needed, reflects the greatest
distance away. The only way to improve the situation is to move the
antenna to a location where the ground is better, which usually isn't
possible or practical.
Dense radial field with electrical length of radials around wavelength has
shown remarkable imrpovement in low angle performance over "regular"ground.
Because of the two separate effects, the overall field strength might be
better or worse as the ground conductivity improves, and it might even be
better at some elevation angles and worse at others.
Roy Lewallen, W7EL
Yuri, K3BU.us
Yuri Blanarovich wrote:
"Cecil Moore" wrote in message
...
Al Lorona wrote:
It's funny to think that really terrible ground can have an advantage
over pretty good ground.
Free space is just about the most terrible "ground"
that one can imagine. :-)
--
73, Cecil http://www.w5dxp.com
So much disinformation by W8JI School of DC circuitry :-)
Modeling various configurations shows benefits of good ground, especially
for taller than 1/4 wave radiators.
Myth that half wave radiators do not need ground is spreading like snake
oil wild fire. They need it but "looking" for it further out, not just at
the base.
I will anytime trade good ground (mirror) for lossy (RF sponge) ground.
Its just where the radiator is "looking" for the mirror, taller one -
further out, enhancing signals at lower angles.
3/8 vertical with some 3/8 physical length radials start morphing into
far field.
Yuri, K3BU.us