<<But doesn't this merely confirm the notion that the half-wave vertical
needs some kind of ground system to work against, and in addition to
minimise ground loss when the base of the radiator is in the vicinity of the
lossy earth? A 2m or CB ground plane, with 3 or 4 radials, 20 ft or more in
the air, has very little ground loss. At the opposite extreme, a
ground-mounted broadcast tower with buried radials or radials lying on the
surface, needs a large number of radial wires to divert the rf return paths
away from the soil and thus keep ground loss to a minimum. In the case of
elevated radials, you are moving between those two extremes; it logically
follows that as the ground plane is raised above the earth from the surface
to a significant fraction of a wavelength, proportionally fewer radials are
needed to maintain ground losses at an acceptable value.>>>
Think about this, Don. I think this is one of the things Carl is missing.
The reason we have losses is charge movement in a lossy media. Anything that
moves charges in the lossy media creates heat.
Charge movement is related to field intensity in that media per unit volume
of lossy material.
When we make a resistor have larger cross sectional area of the same
resistive material exposed to the same terminal potential, the loss
decreases.
When we have a half wave vertical near the earth, field distribution in the
earth around that antenna comes from two things:
1.) The "push-against" force we have to use to push and pull charges into
and out of the antenna end.
2.) Fields (charge forces) that surround the element and extend out for some
distance.
Center feeding, if we can isolate the feedline, gets rid of number 1.
Number 2 always exists the same for a given element no matter how we feed
it.
So where are fields concentrated, and what is the density per volume of
space? While magnetic fields from charge movement are weaker at the base in
a half-wave, the electric field (charge different or electrical potential)
is much more intense at the base. This would be true for a vertical dipole
or for end feed.
We probably need a smallish screen there to distribute charge over a
somewhat wider area of earth. Current there is weak, and once we get any
area at all losses will go way down.
What about losses some distance out? Most of the charge difference is at the
base to earth, and base to antenna tip. Not much is a concern out some
distance. The magnetic field is more important some distance out, but it is
spread over a large volume of space and a large area of lossy media. Because
of that, there isn't much loss at all.
We can expect a half wave vertical with just a small screen at the base to
be well up in the high double digits of efficiency, and going from zero
radials to 100 radials only be a fraction of dB difference. A model over 5
mS/M soil e=13 shows adding 100 radials 200 feet long, on 2 MHz, increases
groundwave field strength just 0.38 dB, or about 10%, over no radials at
all.
<<<The old WWV site on the east coast used vertical centre-fed dipoles
mounted on wooden utility poles. I never read whether or not they deployed
radial systems at the bases of the poles, but I suspect not. However, the
lower ends of those vertical radiators were some distance above the ground
surface.>>>
The fellow who installed that, or designed it, was a friend of mine. They
just used vertical halfwaves. He said they did that to avoid radials.
<<<Every engineering text I ever read on the topic of broadcast antennas
always emphasised the point that a substantial ground plane was essential,
regardless of the height of the tower, and I don't recall ever seeing
anywhere that this was done merely to satisfy the FCC.>>>
When the FCC sets a standard requirement, few people say "this is only to
satisfy the FCC". They just usually say "this is the minimum".
If you read the FCC rules, the rules say:
"
(4) At the present development of the art, it is considered that where a
vertical radiator is employed with its base on the ground, the ground system
should consist of buried radial wires at least one-fourth wave length long.
There should be as many of these radials evenly spaced as practicable and in
no event less than 90. (120 radials of 0.35 to 0.4 of a wave length in
length and spaced 3° is considered an excellent ground system and in case of
high base voltage, a base screen of suitable dimensions should be employed.)
(5) In case it is contended that the required antenna efficiency can be
obtained with an antenna of height or ground system less than the minimum
specified, a complete field strength survey must be supplied to the
Commission showing that the field strength at a mile without absorption
fulfills the minimum requirements. (See §73.186.) This field survey must be
made by a qualified engineer using equipment of acceptable accuracy.
"
The choice is:
#4.) do the minimum they say at a cost of several thousand dollars and be
done.
#5.) measure and prove you need less at a cost of a few months delay and a
few ten's of thousands of dollars. This does not mean you run out to a mile
and measure. It means you plot FS on radials out for a long distance and
find the soil conductivity. Then, knowing conductivity, you estimate the FS
at a mile and compare FS of the multiple radial measurements to the expected
value normalized for that soil, and send them the paperwork.
Which one would you choose?
Also, BC stations are (for some reason) concerned with even 0.1 dB. Only
an illogical Ham would worry about 0.2 dB when QSB is 20 dB. The BC station
also wants excellent lightning protection, which involves more than rods.
73 Tom
_______________________________________________
It is undesirable to believe a proposition when there is no ground whatsoever for
supposing it is true. — Bertrand Russell
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