I read in the ARRL Antenna Book (rel. 1980) that the mesh constituting the
ground screen has to be cut at 1/2-lamda (so displaying a radius of 20m to
work on 7 MHz) to effectively establish the height of the antenna instead of
the actual earth beneath the antenna.

I am not agree with the sentence... but if I am wrong, can someone explain
me were my objection is false...

IMHO, this mesh buried in the ground has not to be at all in resonance to
work properly as our objective is to make the ground as conductive as
possible for RF signals. So far, when you work with a vertical cut at 1/4l
standing directly over a metallic plate (your car hardtop or even using a
base like the MFJ-1904 RF ground-coupled base) you do not worry to know if
this surface is in resonance or not. As states Bencher, the manufacturer of
Butternut antennas, "for mobile operations, according to the counterpoise
principle, the metal body of the vehicle provides the capacitive coupling to
the earth itself". MFJ Enterprises confirms also that his "ground-coupled
portable antenna base, a 2x2 foot stainless steel square, provides an
effective RF ground[...]". The same principle applies operating from a boat
at sea where you do not expect that the "length" of the underneath open-sea
surface is at resonance with your antenna... but rather that it constitutes
a perfect conductive "ground", without losses. This "ground screen" plays in
fact the role of an electrical mirror, not the one of the node of a string
at which standing waves reflect back and radiate !

It seems that in fact many people confuse the ground with the antenna
itself, and grouding with coupling. To be resonant, a vertical must be cut
at 1/4-lambda and will use its mirror-image to reconstitute its electrical
"missing-half". It will display an omnidirectional pattern in the horizontal
plane (the current or the radiation pattern increasing from the top to the
ground connection, the RF voltage being the highest on top, at the open end
of the vertical). This pattern in sine curve has to reflect in the ground,
whatever its horizontal extent, to display the same radiation pattern as a
1/2-lambda vertical in free space. But in shortened vertical system for
example, the electric equivalent circuit presents much ground losses. It
must thus be balanced by a system offering to the antenna a low capacitance
to the earth while keeping the capacitance to the radials. This RF ground
coupling is got either in installing at ground level more radials around the
antenna base or raising the antenna and its radials above ground. On the
lower bands on the contrary this is achieved in using loading coils for
matching impedances. Therefore, in the first case, I shouldn't be surprised
if some hams tell mz that they buried hundreds of cans in their backyard
just to give to their antenna an earth more conductive.

On the other hand, the extent of this artificial ground is, on the contrary,
very important when you will emit as the distance up to 10-lambda around
your vertical influences much the efficiency of its angles of takeoff. This
both aspects are two thing completely different.

Thanks for your explanations (if am wrong).

Thierry LOMBRY

ON4SKY

http://www.astrosurf.com/lombry/menu-qsl.htm

A buried radial wire behaves as a very lossy transmission line.


The soil surrounding the wire has a very low insulation resistance.


Because of soil permittivity and water content, the line capacitance is also
much higher than the free space value.


The velocity of propagation along a buried radial wire can be as small as
1/12 of the free space value.


This makes nonsence of cutting radial wires to particular resonant lengths.


Also, due to the high loss in the soil, the attenuation of the current along
a buried radial wire is very high. There is little point in having a radial
wire longer than about 8 dB because its input impedance, as a transmission
line, ceases to decrease for longer lengths. The input impedance simply
converges on the line impedance Zo.


For average soil characteristics, at the lower HF frequencies the optimum
length of a buried radial is of the order of 1/10th of the free-space
wavelength. But it varies widely.


Hence the old adage - if you have any wire to spare then increase the
number of radials rather than increase their length. But in any case forget
all about the magic number of 120. Stop at 8, 16 or 32, or when the
strength of RECEIVED signals stops increasing.


For a crude estimate of the performance of a single buried radial wire,
download program RADIOETH.


For the effect of multiple rods, plates and radials, download program
EARTHRES.


For the performance of an inverted-L antenna above a system of ground
radials, download program ENDFEED.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

If by increasing the number of radials from 10 to 20 the improvement in
signal strength is only 1/10th of an S-unit then it is a waste of time and
copper in doing so.
---
Reg, G4FGQ

"Reg Edwards" wrote in message
...
If by increasing the number of radials from 10 to 20 the improvement in
signal strength is only 1/10th of an S-unit then it is a waste of time and
copper in doing so.
---
Reg, G4FGQ



Thanks for your replies that confirm what I stated in my page
http://www.astrosurf.com/lombry/qsl-radials.htm
So can we conclude that the assertion stated in the Antenna Book about the
Ground screen is false or at least subject to more comments ?
I have sent the question to "QST's doctor is in ? column". I hope that they
will answer dirtectly to me or via the magazine a next month.

73's
Thierry
ON4SKY