Yuri Blanarovich wrote:
"Cecil Moore" wrote
The characteristic impedance of a horizontal dipole is
~constant. Since a dipole is a standing wave antenna,
the voltage to current ratio varies along its length.
That varying impedance (V/I) is NOT the characteristic
impedance which is relatively constant for a horizontal
wire.

How do we apply (calculate char. imp.) the above to say, full wave (quad)
loop or vertical monopole?

That's a good question. For a horizontal wire, its obvious
that the forward wave reflects from the open-circuit at the
end of the wire. We know there are standing waves on a loop
but exactly where are the reflections originating? I
suspect they are originating at the feedpoint, i.e. the
forward wave travels all the way around the loop and is
reflected from the impedance discontinuity at the feedpoint.
Note that the feedpoint impedance of a full-wave loop is
in between the feedpoint impedances of a 1/2WL dipole and
a 1.5WL dipole indicating that the forward wave travels
about 1WL before being reflected in the loop.

Every segment of a monopole is a different
distance from ground and therefore has a slightly different
characteristic impedance which probably doesn't change very
fast as it is a log function. For instance, for the sake
of discussions, it seems reasonable to assume that the Z0
of a vertical stinger is in the neighborhood of a few
hundred ohms and would be easy to measure. At whatever
frequency causes the stinger to be 1/8WL, measure the
impedance. That will be fairly close to the characteristic
impedance of the stinger at the measurement point.
--
73, Cecil http://www.w5dxp.com