On Sat, 19 Apr 2008 14:20:33 -0700, Roy Lewallen
wrote:

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.

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.

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


Maybe I should change the subject line, but here goes.
First of all, i am fishing for information, not challenging anyone's
intelligence.

I understand from books I have read, that a ground mounted vertical
antenna needs many radials. IIRC, the point of diminishing returns on
adding radials falls somewhere between 64-128 radials. I imagine the
best radial-based ground I could have for 20 meters would be a solid
copper disk with about 16 feet radius, give or take. However, I
recall in the ARRL Antenna handbook, not the latest version, but one
prior to this one, there is no noticeable difference between a raised
ground plane antenna with 4 elements as opposed to 128. (From here,
or another antenna forum, I heard for the first time that it holds
true for two radials.)

I am still trying to figure out why so many radials are needed on the
ground and a few feet higher so few are needed.

Actually, more important than the why, is how high is high enough to
reduce the optimum number of radials? For example, i want to build a
20 meter vertical. I understand the best place for it is on top of a
100 foot+ tower, but somewhere in between, there has to be a place
where 4 radials above ground is noticeably better than the same 4
radials on the ground.

Another point I have heard in the forums, but not confirmed, is that a
reduced size vertical element doesn't gain much by adding radials
longer than the antenna is high.

73 for now,
N4PGW
Buck




--
73 for now
Buck, N4PGW

www.lumpuckeroo.com

"Small - broadband - efficient: pick any two."