Tom is correct in that a half wave vertical wouldn't have zero current at the
base feed point; I should have said "near-zero", since if it actually had zero
current there would by definition be zero power transfer, the radiator would be
an infinitely thin wire and the SWR would have to be infinite with no loss
(including any loss from radiation). I think that's what Capt. Lee had in mind
when he touched on this topic in his Vertical Antenna Handbook, but he was
citing his response to a correspondent who had proposed that the current at the
base of the 1/2 WL vertical was zero. Lee should have stated his case more
precisely for the benefit of the readers of his book.
The same is true with tuned feeders. At voltage and current nodes along the
line the current or voltage is never actually zero, since in every case all
lines have some loss. A tuned feeder looking into an open or short circuit
would have near-zero current and voltage points, but never actually zero, since
the VSWR can never reach infinity, and if that were possible, the current and
voltage at the loops would have to be infinite.
The finite physical width of the vertical radiator, plus the energy loss due to
the radiation resistance assures us that the base impedance would never be
infinitely high nor would the base current ever actually reach zero, and the
VSWR along the radiator would never reach infinity.
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.
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.
I have a nearly complete set of the 1930s IRE Proceedings, and this has
prompted me to dig them out and review some of the articles. I would be
interested in looking at any actual experimental data compiled to quantify
ground losses, using physical antennas fed with physical rf watts with data
collected using a variety of physical ground planes.
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.
Don k4kyv
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It is undesirable to believe a proposition when there is no ground whatsoever
for supposing it is true. — Bertrand Russell
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