To determine Zo, start around 10 metres.

If very little happens to input impedance between 10 and and 15 metres
then you already have Zo = Zin = Ro + jXo.

Neither am I interested in efficiency or antenna input impedance..
The problem of Efficiency has already been sorted out.

All I wish to know is Zin = Zo of a single radial, at various lengths
greater than about 10 metres, of diameter = 1.64mm, depth = 25mm,
ground resistivity = 150 ohm-metres, permittivity = 16, at a frequency
of about 8.07 MHz.

That is the input impedance of one radial when the attenuation is
about 20dB or greater.

To summarise, I wish to know Zo = Ro + jXo for one radial.


Reg, the radial impedance rapidly converges to 101.6 + j 21.1.

10 m -- radial Z = 102 + j 20.99
12 m -- radial Z = 101.3 + j 21.1
14 m -- radial Z = 101.65 + j 21.32
16 m -- radial Z = 101.7 + j 21.1
18 m -- radial Z = 101.615 + j 21.1
20 m -- radial Z = 101.61 + j 21.11

Frank