Hi Don,
Then, why do broadcast stations that use vertical towers at approximately
a half wavelength, purchase valuable real estate and spend thousands of
dollars for the copper to install from 120 to 240 or more radials, each
usually a half wave or more in length?
They almost always do not.
First, they would be fools or have idiots for engineers to have more than
120 radials. The only real reason they use 90 radials or so is it sometimes
gets them out of expensive proof-of-performance measurements.
The exact FCC text is down the page here:
http://www.w8ji.com/counterpoise_systems.htm
Second, nearly all stations except the old clear channels on the low end use
towers around 1/4 wave, often just around 0.2 WL.
See G. H. Brown: "Ground Systems as a Factor in Antenna Efficiency", IRE
Proceedings, June 1937 p. 753. Brown demonstrated that the distribution
of earth currents and ground losses is such that the region of maximum
current and loss occurs at a distance of about 0.35 wavelengths from the
base of a ground mounted half wave vertical antenna, which was verified
experimentally.
But the field is so weak at that point that there really isn't much to be
gained when the radiator is 1/2 wave tall. Plus broadcast stations have a
huge investment, and overkill is not an issue for them. That's why the
transmitters are so conservative.
There is zero loss at the base of the antenna itself, since there is no
base current because the antenna a fed at a current node. An rf ammeter
inserted in the ground lead, as well as one inserted in in the antenna
lead attached to the insulated base of the radiator will read zero. >
That is absolutely wrong.
A typical half-wave BC tower is in the several hundred ohm range because of
tower thickness. They are almost never over 1000 ohms. Halfwave BC towers
at exact resonance are typically about 1 ampere per kilowatt.
There isn't an end-fed half wave in the world with infinite impedance and
zero current, even the very thinnest lossless wire would not be zero.
<<The ground losses occur farther out from the base of the antenna. Low
effective earth resistance provided by a good ground system is ABSOLUTELY
NECESSARY for vertical antennas of ANY height if one expects good radiation
efficiency. The claim that no ground system is needed for a half wave
vertical is nothing more than a long-standing popular misconception.
Actually we do need a ground system, that much is correct, but requirements
are much relaxed from very short antennas. With a half wave end fed wire
element it is pretty tough to lose more than one or two dB in ground loss,
but it certainly needs something there to allow it to be fed. It cannot be
fed without some counterpoise.
Thick towers are another matter. The surge impedance of the tower is so low
that end impedance never gets all that high. This is why thick antennas are
wide bandwidth, and why extreme power SWBC stations use thick cage elements.
They do that to keep the impedance extremes, even at the open element end,
down.
<<
This topic prompted me to dig out and review an anecdote I recall reading in
my decades-old copy of CQ magazine's Vertical Antenna Handbook, by USNR
Capt. Paul H. Lee, K6TS (1974). He reported receiving mail from a ham who
had made the "discovery" that he could tune and operate a half wave vertical
without a ground system, feeding it by a parallel tuned tank circuit whose
lower end is grounded. Since an rf ammeter in the ground lead showed no
current, he could dispense with the ground system and its loss. He
suggested to the Capt. that he should "discover the new world of half
verticals with no ground system".
Current is not zero. It cannot be zero at the feedpoint of any end-fed
antenna. That is an absolute practical and theoretical impossibility. It is
a myth or a pretty gross misconception.
While Captain Lee had a lot of good stuff, he was also over the top with
some things.
It is quite easy, with a thin vertical half-wave element, to get into the
80% efficiency range with a very minimal ground screen.
People think of losses as current flow problems, but a significant portion
of losses in a half wave ground independent vertical are electric field
induced losses at the antenna base. The same would be true for a thin
element and no radials, and just a ground rod or several ground rods.
Thinner half wave verticals are pretty efficient with a minimal ground. When
they get extremely thick the base current increases, and conduction losses
can be a significant factor.
73 Tom
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Topband reflector - topband@contesting.com
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