Roy Lewallen wrote:
It's not clear to me what's meant by the voltage (presumably relative to
ground) at the tip of a dipole. Suppose it's a quarter wavelength above
ground. How would you measure it? Or, how would you measure the voltage
at the top of a quarter wavelength vertical?

Who said anything about measuring it? We know it exists and can cause
corona in moist/salty circumstances. But I assume it could be measured
using something like an artificial ground at the tip of the monopole.

A dipole is akin to a leaky unterminated transmission line. The forward
wave travels out to the ends of the dipole where it is reflected by the
open circuit. Just as there is a large voltage at the end of an unterminated
transmission line, there is a large voltage at the ends of an unterminated
dipole (or at the end of a monopole). And just as we can make some assumptions
and estimate the magnitude of the voltage at the end of an unterminated
transmission line, we can make some assumptions and estimate the magnitude of
the voltage at the end of an unterminated dipole. The voltage anywhere along
a center-fed dipole is (Vfwd+Vref). The current anywhere along a
center-fed dipole is (Ifwd+Iref). The feedpoint impedance of a dipole is
(Vfwd+Vref)/(Ifwd+Iref) at the feedpoint. A CF dipole is a standing-wave
antenna with the voltages in phase and maximum at the tips. The voltages
are out of phase and minimum at the center feedpoint.

All we need is an estimate of the feedpoint impedance if the dipole was
terminated at each end thus turning it into a traveling-wave antenna.
I estimated about 600 ohms which put the tip voltage in the same ballpark
as Reg's estimate based on 'Q'.
--
73, Cecil http://www.qsl.net/w5dxp



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