"Richard Fry" wrote -
"Reg Edwards" wrote
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.
________________

Terman (Radio Engineer's Handbook, p 846) shows a chart for the
radiation
resistance and reactance ranges for five, guyed MW broadcast towers,
as
measured against a good radial ground system--an ohm or two above
true
earth. For electrically 1/2-wave radiators, R ranged from 300 ohms
to 450
ohms and Z ranged from about -100 ohms to -350 ohms.

RF

=====================================

As I said, 1/2-wave antenna input impedance is a function of conductor
diameter. It is not a relatively fixed value (around 36 ohms) as is a
1/4-wave vertical.

Impedance decreases as diameter increases. Or as the ratio of
height/diameter decreases.

Terman's LF and MF towers (not wires) have extrordinary thick
conductors (probably latttice masts) compared with a low antenna
height in terms of wavelengths. Hence the low input impedances in his
examples.

If I remember correctly, Terman makes no reference to height/diameter
ratio and is therefore misleading by using tower antennas as typical
examples. He tells only half the story. Terman should not always be
used as the Bible.

EZNEC can be used to estimate input impedances of the more common
1/2-wave wire antennas at HF when very near to ground. Which is what
amateurs are interested in. They usually fall in the range 1500 to
3500 ohms.

At VHF and UHF with short, fat, thick rods for conductors, (similar in
proportion to LF lattice towers) input impedances can fall to several
hundred ohms or smaller.

Input impedance values of isolated dipoles are theoretically
calculable but are impossible to measure. Nevertheless, calculated
values are very useful in antenna system design. Couldn't do without
them!

Maths takes precedence over measurements. Without maths, experimenters
flounder about in the dark.
----
Reg, G4FGQ