Jim,
Here is an NCJ article where Tom N6BT discusses the results of his drone
measurements:
https://ncjweb.com/features/mayjun19feat.pdf
In the article, Shiller alludes to energy at low angles being depleted
over distance and the connection between the surface-wave (ground wave)
radiation and attenuation of the low-angle sky-wave radiation:
"Energy in the lowest angles from a vertical antenna over ground
(assuming flat terrain) will be depleted by the ground as the energy
extends from the antenna for a particular distance. If this distance
is not limited, the NEC2 model calculates the resulting take-off
angle at an infinite distance. Earth’s surface, of course, is not
infinite, so the energy will be depleted over a distance that’s much
less than infinity. What might a realistic boundary be for the limit
of this energy depletion? A suggestion during conversations on this
subject led to considering that it might be when the surface wave
(ground wave) ends. In our empirical testing, we have seen this wave
over basically flat ground and noted it on drawings as “the spike.”
On one occasion, we were able to measure at a distance far enough
that we did not see the spike."
In this article on the psuedo-Brewster angle and vertical antenna
pattern formation, Bob Zavrel W7SX makes a similar connection:
http://www.arrl.org/files/file/QEX_Next_Issue/2016/March-April2016/Zavrel.pdf
"Notice that the E-field amplitude at low angles is simply
attenuated faster than in free space, unlike the nearly
discontinuous function the NEC models imply. Of course in the very
far field, the NEC pattern becomes a good approximation, but the
assumption of phase cancellation appears incorrect. The more
advanced versions of EZNEC do permit modeling of the ground wave,
but only at designated distances."
"The Figure 7 simulation is the same as in Figure 6, except the
ground surface field plot is removed to reveal the E-field
underground. It appears from this simulation that the
pseudo-Brewster angle—actually its counterpart — is formed by the
attenuation of the ground-surface wave. As the radiation angle
increases, its distance to the ground increases faster for a given
distance from the antenna. In other words as an E-field propagates
tangentially to a lossy dielectric, it is attenuated greater than in
free space."
I've often wondered about the connection between ground-wave and
low-angle sky-wave from a vertical antenna. These two articles seem to
hint at the relationship.
73, Mike W4EF.................
On 7/24/2024 12:41 PM, Jim Brown wrote:
On 7/24/2024 11:31 AM, kq2m@kq2m.com wrote:
> One antenna that does work well even over poor ground is a 4-square
with elevated radials, preferably above .05 wavelength in height.
Poor soil degrades the efficiency of vertically polarized antennas
both under the antenna AND in the far field, where the ground
reflection that reinforces the direct wave is created. Radial systems
affect ONLY what happens under the radials -- they screen the direct
field of the antenna from the lossy earth, and give the antenna a low
resistance path for its return current. That's ALL that they do. They
cannot compensate for lossy ground in the far field.
But ground characteristics DO vary with location, even with lousy
ground under the antenna, good ground in the far field can support the
needed reflection, making the antenna work well. That's the classic
case of a vertical near salt water. And N6BT, well-known for his "Team
Vertical" setups, has recently done field measurements with a drone to
show that a vertical very close to a drop-off (his testing was on a
mesa in AZ) produces a very strong signal in the direction of the
dropoff. He first showed this work as part of the Pacificon Antenna
Forum about five years ago, to which we both often contributed.
73, Jim K9YC
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