Frank, I am not interested is what happens at very short lengths or
what Radial_3 makes of it.

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.
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Reg.