Recalling Jim Brown's posting yesterday of Rudy Severn's excellent
recent work, the current maximum in a radial occurs at 0.25 wavelength
from it's open end & loss will be minimized when that current maximum
is at the feedpoint.
For consideration: The first link below is from a recent paper of Valentino
Trainotti, a highly-regarded antenna engineering professor at the University
of Buenos Aires. He shows that it is the quality of the ground plane within
1/2 of a free-space wavelength surrounding the base of a monopole
(regardless of its electrical height) that is important in its operation.
He states that the conductivity of the ground plane within a 1/2-wave radius
of a monopole is part of the "wave generator," and its r-f loss must be low
in order to maximize the radiation efficiency of the antenna system. No
requirement or effect is stated for the v.p. along any buried radial wires
used in that boundary area.
RF currents entering the earth from radiation by the monopole out to a
radius of 1/2-wavelength need to travel back to the "ground" terminal of the
antenna system in order for the monopole to radiate. The conductivity of
the ground plane in that region needs to be minimized, no matter what the
presence of the earth around buried radial wires does to their v.p. So the
wires ideally should extend out to 1/2 of a free space wavelength, no matter
what electrical length they may have due to their v.p. when buried.
Here is a quote from the 1937 BL&E paper on ground systems. Note especially
the last sentence there.
\\ Brown, Lewis, and Epstein: Ground Systems (p. 757)
These losses are due to conduction of earth currents through a high
resistance earth and to dielectric losses in the base insulator of the
antenna. We shall next consider the earth currents flowing toward the
The earth currents are set up in the following manner. Displace-
ment currents leave the antenna, flow through space, and finally flow
into the earth where they become conduction currents. If the earth is
homogeneous, the skin effect phenomena keep the current concentrated
near the surface of the earth as it flows back to the antenna along radial
lines. Where there are radial ground wires present, the earth current
consists of two components, part of which flows in the earth itself and
the remainder of which flows in the buried wires. As the current flows
in toward the antenna, it is continually added to by more displacement
currents flowing into the earth. It is not necessarily true that the earth
currents will increase because of this additional displacement current,
since all the various components differ in phase. //
Below are two clips from the BL&E paper showing the distribution of r-f
current in the earth around the base of monopoles of heights from 22 degrees
to 99 degrees, and the current distribution along buried radial wires of
0.41 wavelength (in free space).
Note that current is not maximized at a distance of 1/4-wavelength (free
space) from the open ends of those buried radials. It is maximized near the
common point of those radials, adjacent to the base of the monopole.
R. Fry, CPBE
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