Topband: Low band antenna project questions
Richard (Rick) Karlquist
richard at karlquist.com
Mon Mar 7 22:07:40 EST 2016
On 3/7/2016 3:11 PM, Jim Brown wrote:
> pattern of any antenna is produced by the reinforcement of the direct
> wave by the first reflection from the earth. That reflection is MUCH
> stronger, and is strong at a lower angle, if the "earth" at the point of
> reflection is sea water. Sea water will NOT improve signal strength in
> directions where the first reflection is over land.
> 73, Jim K9YC
Not quite. The reinforcement of vertically polarized waves
from a perfect ground plane is 3 dB. For a real ground,
the worst that can happen is that you lose the whole 3 dB
and get down to the free space value ... as long as you are talking
about angles above the pseudo-Brewster angle. For angles below
the pseudo-Brewster angle, the reflection amplitude is the same
for both a perfect ground and a lousy ground. The difference
is that for lousy ground the PHASE of the reflection is inverted.
This results in near perfect cancellation at low angles.
As we all know, this effect is much greater than 3 dB.
For a ground mounted vertical, I don't think the idea of
doing ray tracing to see if the "point of reflection" is
over sea water is very useful, because the vertical does
not model as a source at a definable height, AFAIK. It's
probably more like the wave has to be over salt water not
just at a point of reflection, but continuously from the antenna
out to some critical radius. In this region, it is still
a ground wave. Possibly, this radius corresponds to the
point where the ray has reached a critical height
such that the wave has been "launched" and can be
considered to be a skywave. I don't know if this critical
height is a function of take off angle or ground conductivity.
For example, if the take off angle is 30 degrees, and
the critical height is a half-wavelength, the critical
radius (in terms of wavelength) is half of the square
root of 3 (0.866...). This is just trig. I don't
claim these are the actual numbers.
If there really were a single "point of reflection", then
I could put some half wave reflector wires on the ground
a long distance from the antenna and generate a huge
signal at a particular low angle. If this worked,
I suspect it would have been discovered long ago
The over the horizon radar people may have figured this
out. They use huge ground screens to actually try to
simulate the vertical on the beach paradigm.
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