Roy, W7EL wrote:
"I don`t think EZNEC or other NEC-based programs will give accurate
field strength any closer to a wire than a few wire diameters."
The Antenna Section in Keith Henney`s "Radio Engineering Handbook" was
written by Edmund A. Laport, Chief Engineer of RCA`s International
Division at the time (around 1950).
On page 637 Ed writes:
"Where high power is to be transmitted, or at high altitudes, antenna
insulation and conductor designs require care to details. For h-f use,
only radial potential gradients need to be considered. At high
altitudes, pluming may occur with consequent damage to the system.
Fortunately in practice, high power is generally used with directive
antennas, and the power is divided among several dipole sections thus
tending to minimize the problem. A thin-wire dipole gives an end
potential of about 3,900 volts rms for 1000 watts input for a height of
0.25 wavelength. It will be higher for smaller heights, and falls to a
minimum of about 1,700 volts as height increases to 0.75 wavelength;
beyond this point it settles down to the free-space value of about 3,000
volts. Potentials vary as the square root of the power ratio and as the
inverse square root of the capacitance per unit length. For a potential
of 3,900 volts on a wire 0.101 in. in diameter (No, 10 B&S), the radial
gradient is of the order of 31 kv per cm. As a rough approximation for a
cage, the gradient for one wire is divided by the number of wires in the
cage."
The multiwire observation is important because, if the potential
gradient at any point in air becomes greater than 30,000 volts/cm., the
air becomes ionized and sparking or corona discharge will occur.
On page 645, Ed writes:
"With 100-kw carrier input (to an 8-dipole array), the end potential on
each dipole is 7,500 volts rms. A 3000-ft. 580-ohm two-wire balanced
feeder used with this antenna had an efficiency of 67 per cent.
I am comfortable with Ed`s experience which can be scaled for the power
and configuration of the antenna.
Best regards, Richard Harrison, KB5WZI
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