(from rec.amateur.radio.antenna)
From: Reg Edwards <[email protected]>
Subject: Ah wuz rung !
Date: den 26 augusti 1999 22:33
I agree with Mr Dagaas's reason for efficiency of the 6-mast system
being 6 times that of an individual mast.
I would like to describe the operation in a little more detail.
(1) Each mast has its own set of ground radials. Whether the radial
wires of adjacent masts are in contact with each other is of no
consequence because no current flows across the junction.
(2) The masts are separated by a distance three times their height.
This ensures the ground system under each mast provides a ground
connection for that mast only. There is no sharing of ground
connections between masts.
(3) The radiation resistance of an individual mast when considered in
isolation is dependent on mast height in wavelengths as affected by
its share of the capacitance hat. When F = 16.67 KHz, wavelength =
18 kilo-metres. (11 miles). I estimate the radiation resistance to be
approximately 0.05 ohms.
(4) If the feedpoint resistance of one mast is 2.50 ohms then its
radiating efficiency is 1.96 percent. The input (loss) resistance of
one set of radials must be 2.45 ohms.
(5) When all 6 masts are parallelled at the top via the
top-capacitance wires, each mast draws 1/6th of the total current. For
the same total power input the loss in an individual ground system
falls to 1/36th of the previously value. So the total loss in all six
ground systems is only 1/6th of the previous total loss.
(6) The masts are spaced apart by only 0.0217 wavelengths. The
distance between the two outermost masts is 0.108 wavelengths. So for
practical purposes the masts are all in phase with each other and
behave as one vertical mast. So the radiation resistance remains as
before, 0.05 ohms. System efficiency is 6 times greater = 12
percent approx.
(7) As a bonus, the theoretical feedpoint resistance of one of the
masts is increased by 36 times = 88 ohms. But nothing is perfect. The
masts are not all exactly in phase with each other. The ground
systems are not entirely independent of each other. And so the actual
feedpoint resistance is 20 ohms it seems.
Question :- Is a conjugate match to the generator appropriate in this
case ?
So my original statement that multiple antenna conductors do not
increase efficiency was wrong. Nevertheless, efficiency increases only
when each conductor has its own completely independent ground system
by spacing them well apart. But the conductors must not further apart
than a small fraction of a wavelength.
These two simultaneous conditions cannot be achieved unless efficiency
is very low even after the improvement has been obtained. As it is at
SAQ.
But the design engineer of SAQ deserves our admiration. He certainly
understood exactly what he was doing 77 years back. And all he had was
a slide rule.
Reg, G4FGQ.
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