"bbnn" wrote -
..
I heard that a 1/2 wave vertical has a Z of about 2000 ohms at the
base.
Is this the Z between the base of the vertical and ground that RF at
the
resonant frequency
'sees' ?

Doesn't the height that the vertical base is above ground affect
this
2000 ohm valve?
Or does then metal car roof nullify any height above ground affects
on
the Z?

I've noticed that measuring the return loss of a 1/4 vertical is
very low.
But measuring the return loss of a 1/2 wave or 5/8 wave antenna is
higher.??????????????
===============================

It is important to ask the right questions. The right questions are
those to which there are answers and practical measurements can be
made to confirm them.

For example, there is no answer to the question what is the impedance
between the lower end of a half-wave dipole and ground when the lower
end is at a height above ground.

It is impossible to make such a measurement directly because the
impedance meter MUST be associated with a length of wire which reaches
between the bottom end of the antenna and ground. This length of wire
MUST form part of the antenna and the antenna length is then no longer
a 1/2-wave dipole.

If the impedance meter is connected directly to the bottom of the
antenna then the measurement is no longer with respect to ground but
with respect to ground PLUS the impedance of that length of wire
whatever impedance it may have.

I hope, but I doubt it, whether I have made myself clear.

To understand what is going on it is necessary to resort to
theoretical arguments. Only the arguments are real - not the
measurements.

Thre is a 1/2-wave dipole isolated in space. It doesn't matter whether
it is horizontal or vertical. It is possible theoretically to
calculate from its length, diameter and frequency its input impedance
at one end.

But relative to what? To measure it is impossible. To where is the
other side of the meter to be connected? Nevertheless we mustn't
allow this stumbling block to get in our way. The theoretical, perhaps
approximate, calculated value can be used in further calculations to
answer real and very practical questions.

The input resistance of a half-wave vertical near to ground is 2 or 3
thousand ohms. It is uncertain because it depends on wire/conductor
diameter and on the conductivity and permittivity of the soil.
Because of its high value it is also sensitive to the antenna's
environment, trees, etc.

When fed at its base, near to ground, its input resistance can be
considered to be its radiation resistance. Because of its high value
relative to the ground connection resistance, very high power
efficiency can be expected.

Incidentally, half-wave or 5/8ths-wave verticals (not critical) have
the lowest radiation angles and are therefore good DX antennas
preferable to 1/2-wave horizontal dipoles at HF.
----
Reg, G4FGQ