|[AMRadio] Vertical antennas|
k4kyv at charter.net
Sat Nov 7 12:11:35 EST 2015
Rob K5UJ wrote:
> Class A AM bc stations use 190 degree towers, voltage fed at the base, but
with a ground system and copper mesh screen
> in the immediate vicinity of the tower base pier.
Plus a full complement of 120 or more radials. For a tower of that length,
the radials are usually longer than a quarter wave.
> You have to employ a ground system with a base fed ground mounted tower
regardless of its length, if you want
> to minimize ground loss.
You can think of the ground system from two different perspectives, which
both say the same thing, being merely a matter of semantics.
(1) The radials provide a lower resistance path than bare earth, to return
the ground current emanating from the vertical radiator back to the base of
the antenna. Ground currents result from displacement currents flowing
through free space from the radiating antenna to the earth, which then
return back to the base of the antenna to complete the circuit. The sum of
the resistances seen by the displacement currents is the radiation
resistance, and this "lost" energy is radiated into space as a radio wave.
The lost energy from ground resistance is lost as heat. About all it does
is to help keep the earthworms warm. The radials and ground screen act as a
low-resistance shunt across the return path through the lossy earth.
(2) You could think of the ground system as a conductive shield that
isolates the radiating element from the lossy earth. If the radials are
buried or laid on top of the ground, many radial wires are needed due to the
close proximity of the soil. As the shield (ground plane) is raised away
from the earth, fewer radials are needed, hence elevated radial systems are
said to be equally efficient while employing fewer radials. VHF and CB
ground planes, for example, may have radial systems one or more wavelengths
high, and a fully efficient ground plane at that height usually consists of
only three or four radials.
Hams often confuse the above function of the radial system with an equally
important function with a vertical that is considerably shorter than 180
degrees: to complete the missing portion of the half-wave radiator, allowing
it to be self-resonant. A half wave vertical is inherently self-resonant
and zepp feeders could be used, but still, without a ground system, much of
the rf is wasted as earth loss in the vicinity of the base of the antenna,
unless the whole thing is raised well above ground. OTOH, a quarter-wave
monopole cannot be self-resonant due to the missing other quarter wave. The
ground plane provides that missing quarter wave, something for the
quarter-wave monopole to be current-fed against. With a perfectly lossless
ground system, the theoretical base impedance is 36 ohms, exactly half the
feed point impedance of a dipole in space, 72 ohms.
Elevated radials are more effective when cut to resonant length, usually a
quarter wave. Radials on the ground or buried do not need to be any
particular resonant length. The optimum number and length of the radials in
the system are a whole other topic of study.
A half-wave vertical high in the air works almost the same as a quarter wave
ground plane at the same height. The bottom quarter wave (bottom half of
the antenna) functions just like quarter-wave ground plane radials as far as
resonance is concerned. But there is a difference. If the radials extend
horizontally, symmetrically in all directions, their radiations cancel out,
just like the wires in a capacity hat of a top-loaded vertical or vertical
tee. Therefore, the base impedance of the elevated ground plane is 36 ohms,
but if the radials were all bent down vertically into a single bundle, they
would become the other half of a radiating half-wave dipole, and the feed
point impedance would increase to 72 ohms. The radials can be sloped down
at an intermediate angle between 90 and 180 degrees to make a convenient
feed point impedance between the two extremes, 50 ohms for example.
Here are a couple of interesting pages on the topic (you may have to copy
and paste the full link into your browser, since the reflector software
often truncates the URL) :
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