Tom Donaly wrote:
Cecil Moore wrote:
The characteristic
impedance of a horizontal wire above ground is
constant at 138*log(4D/d)

The characteristic impedance is not to be confused
with the voltage to current ratio existing on a
standing-wave antenna any more than the characteristic
impedance of a transmission line is to be confused
with the voltage to current radio existing along
its length when the SWR is not 1:1.

Have you verified this experimentally, Cecil? If you did,
how did you do it?

Here's a quote from "Antennas Theory" by Balanis: "The current
and voltage distributions on open-ended wire antennas are similar
to the standing wave patterns on open-ended transmission lines.
.... Standing wave antennas, such as the dipole, can be analyzed
as traveling wave antennas with waves propagating in opposite
directions (forward and backward) and represented by traveling
wave currents If and Ib ..."

As Balanis suggests, the body of technical knowledge
available for "open-ended transmission lines" is applicable
to "open-ended wire antennas", e.g. dipoles, which really
are nothing but lossy *single-wire* transmission lines.

That characteristic impedance equation for a single-wire
transmission lines can be found in numerous publications and
is close to a purely resistive value. A #14 horizontal wire
30 feet above ground is very close to a characteristic
impedance of 600 ohms. (One half of a 1/2 wavelength dipole
is simply a lossy 1/4 wavelength stub with Z0 = ~600 ohms.)

Before he passed, Reg Edwards had some earlier comments on
the characteristic impedance of a 1/2WL dipole above ground.

Like a normal transmission line open stub, a 1/2WL
dipole supports standing waves that can be analyzed. For the
purposes of a voltage and current analysis, I^2*R losses and
radiation losses can be lumped together into total losses
associated with some attenuation factor, similar to analyzing
a 1/4WL lossy normal stub.

In fact, the losses to radiation from
one half of a 1/2WL dipole can be simulated by EZNEC using
resistance wire in a 1/4WL open stub. Using EZNEC with a
resistivity of 2.3 uohm/m for a 1/4WL open stub gives a pretty
good model of what is happening with one half of a 1/2WL dipole
which is only a lossy single-wire transmission line above earth.
--
73, Cecil http://www.w5dxp.com