Richard,
You have just upset the apple cart for me.
For the sake of argument, assume a monopole with a single radial in free
space. I would assume that this would be a somewhat efficient antenna with
a skewed pattern. If I add an imperfect ground with the radial buried just
below the ground, I would expect that the efficiency of the antenna would
drop. NEC4 shows it loses about 10 dB vs. free space, with about 6 dB of
directivity in the direction of the radial wire using average ground. If I
use dry sandy ground, then we gain back about 3 dB with a very similar
pattern. I'm not claiming accuracy with NEC4 and ground, but the effect is
as I would expect. Therefore, if as you indicate that ground currents flow
into the radial wire then the efficiency should increase with a buried
radial due to additional currents flowing into the radial.
If you are correct, then we should all be using uninsulated wires for
radials. ??
Jim - KR9U
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Richard
Fry
Sent: Saturday, February 28, 2015 8:34 AM
To: topband@contesting.com
Subject: Re: Topband: Modeling "Ground" and losses
Comments to two earlier posts by separate posters (clips below):
> But if indeed a less lossy ground means that fewer radials are needed
> to be placed in the field, then the coupling to the less lossy ground
> is greater which I would expect to mean more loss in the radial field
> which would then require more radials to reduce the effect. I agree
> that radials shield the field from the earth; however it seems that it
> is not quite as simple as it first seems.
> I agree with Tom's analysis -- a good radial system SHIELDS the field
> from the earth, returning the field and the IN PLACE OF the lossy earth.
> Studying N6LF's excellent work lit up the light bulb for me in several
> ways. First, by noting that current in a radial inductively couples
> to the lossy earth underneath it, which dissipates power.
_____________
The source of the r-f current flowing on buried radials is the r-f current
flowing in the earth as a result of radiation from the vertical monopole.
Current is not "lost" to the earth from the buried radials. Instead,
current _ enters_ the radials from the earth around them, because the radial
wires provide a lower resistance path back to the 2nd terminal of the
antenna system than does the earth.
The r-f resistance of a set of buried radials is a circuit element in series
with the r-f current flowing on a monopole. That is why it is important to
system radiation efficiency for that r-f resistance to be as low as
possible.
The concepts in my statements above are not original to me. They are based
on the publications of Dr. George H. Brown, Dr. Frederick E. Terman, Edmund
Laport, and other authors of antenna engineering textbooks and papers.
Below are some supporting clips from the textbooks of G. Brown, F. Terman
and E. Laport.
Please note that none of these authors writes that the function of buried
radial wires is to act as a shield.
>From G. Brown et al, "Ground Systems as a Factor in Antenna Efficiency,"
Proceedings of the Institute of Radio Engineers, Volume 25, Number 6 -- June
1937, page 757:
\\ The earth currents are set up in the following manner. Displacement
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. //
>From F. Terman, "Radio Engineers' Handbook," First Edition (1943), page
842:
\\ Loss Resistance-Ground Systems ... Ground losses arise from the fact
that the current charging the capacity between the antenna and ground ?ows
through the capacity from the antenna to the earth and then back through the
earth to the grounding point at the transmitter. The earth is a relatively
poor conductor, so special provision must be made for returning these
currents to the grounding point on the transmitter with a minimum of loss.
One way of accomplishing this is to bury wires near the surface of the earth
for the purpose of providing a low resistance path through the ground back
to the transmitter. In order to be effective, these buried wires must be so
arranged that the charging currents entering the earth have only a small or
moderate distance to travel through the earth to reach a wire. //
>From E. Laport, "Radio Antenna Engineering," McGraw-Hill (1952), pages
115-118:
\\ 2.5. Ground Systems for Broadcast Antennas
Antenna performance is standardized with reference to the ground being a
perfectly conducting flat plane. Such an assumption serves a very useful
purpose in revealing the ultimate possibilities of a certain radiator in
terms of its dimensions and longitudinal and sectional geometry at a given
frequency. All practical deviations from this norm are due to a number of
empirical circumstances, of which one is the earth itself.
A line of electric force (displacement current) extends from the top of the
antenna through surrounding space to the earth. Upon entering a perfectly
conducting earth it becomes a conduction current which returns to the base
of the antenna and becomes a portion of the antenna current. The electric
lines of force of the antenna field are thus seen to be the continuation
current of a closed circuit through surrounding space. With a perfectly
conducting earth, the electric line of force is always normal to the
surface. When the earth is imperfectly conducting, the line of force tilts
forward in the direction of propagation. This means that the Poynting vector
at the surface of the earth is tilted downward and has a component that
points into the earth where it is dissipated. The component parallel to the
earth represents the power propagated onward in the half space above the
ground.
A vertical radiator above natural earth without any sort of ground system,
energized by an electromotive force between the antenna and the earth, would
require all earth currents to return to the antenna through a very imperfect
conductor. When a plane electromagnetic wave with its electric field normal
to the direction of propagation impinges upon the surface of an imperfect
dielectric, the power propagated into the dielectric sets up conduction
currents and displacement currents, both in quadrature to each other. The
ratio of the two is dependent upon the frequency, the conductivity, and the
inductivity. At the lowest radio frequencies, conduction currents are very
large with respect to the displacement currents, permitting the latter to be
neglected. With increasing frequency, displacement currents become more
important relatively, and eventually a frequency is reached where
displacement currents predominate over conduction currents.
The earth currents return to the base of a vertical antenna along radial
lines. At the base of the antenna, all the ground currents add together to
enter the antenna as the antenna current. The total ground loss is the
integrated losses at all points due to all the returning ground currents.
In ordinary soils this loss is considerable, and measures have to be taken
to minimize ground loss by the use of systems of buried radial wires that
conduct the returning ground currents to the base of the antenna through
high-conductivity circuits.
The distance from the antenna at which returning ground currents are of such
a low value as to be negligible is of the order of 0.5 wavelength. Beyond
about 0.4 wavelength, the gain in efficiency with increased length is seldom
a good economic investment, when a sufficiently large number of radials is
used.
Systematic measurements have shown that the effective length of a buried
wire decreases as the number of radial wires is decreased. Ground resistance
decreases as both the length and the number of buried radial wires are
increased. However, when the number of radials exceeds 120 and their length
exceeds 0.4 wavelength, one reaches the region of diminishing returns. With
such a ground system, the circuital and radiational characteristics of a
vertical radiator of the type used for broadcasting approach very nearly
those computed from theory for a perfectly conducting earth. //
Sorry for the length of this post, but the information from these authors is
worth reading and study (IMO).
R. Fry
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