Summary of design of a short loaded vertical antenna.

There is a 90-degree phase shift between the feedpoint and the tip of
the antenna.

With a short antenna most of the 90 degree phase shift is due to the
loading coil which is, in effect, a transmission line.

The transmission line has Zo, velocity, phase-shift and some
attenuation due to radiation resistance plus wire loss resistance.

Radiation resistance and phase-shift are directly proportion to coil
length, as are L and C and wire resistance.

Q = Omega*Total Inductance / Total R as is usual. Bandwidth =
Resonant Freq / Q.

Radiating efficiency takes into account wire resistance below the
coil, coil resistance, resistance of the wire above the coil, plus
ground loss resistance, plus the sum of the three radiation
resistances. All resistances are referred to the antenna feedpoint.

With short coils, radiation resistance of the coil is usually much
less than that due to the length of the wires above and below it. With
a helical antenna, wire loss resistance is always greater than its
radiation resistance.

At low HF, with a good set of ground radials, the loss resistance of
coil wire usually predominates. At high HF, the ground loss usually
predominates but the radiation resistance becomes important.

The whole business is calculable. There are few rules of thumb. The
only thing which is missing is the radiation pattern. If you don't
already know what the radiation pattern is in the vertical plane then
Eznec will attend to that. Actually, in the vertical plane, radiation
is approximately proportional to the cosine of the elevation angle.
The horizontal groundwave is stongest.

Some of the parameters need not be explicitly calculated.

EXAMPLE:

Starting data:
Height of antenna below coil = 2 metres = 79 inches.
Diameter of antenna below coil = 25mm = 1 inch.
Length of antenna above coil = 1 metre = 39 inches.
Diameter of antenna above coil = 10mm = 0.4 inches.
Length of coil = 152mm = 6 inches.
Diameter of coil = 76mm = 3 inches.
Overall antenna height = 3.152 metres = 124 inches = 10.3 feet.
Ground electrode loss resistance = 5 ohms.
Frequency = 3.8 MHz.

Calculated data:
Number of turns on coil = 53
Wire gauge = 13 or 14 awg.
Radiating efficiency = 10 percent.
Loss relative to full-size 1/4-wave vertical = 9.4 dB.
Self-resonant frequency of the coil = 8.2 MHz.
For maximum efficiency the coil is located at 50% of the overall
antenna height.
But there's not much extra loss by using base loading.

For other calculated data use program LOADCOIL which is about 6 years
old and I think I have lost the source code.
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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