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

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Frank,

Excellent results! The radial has already converged on Zo = 102 + j21
at a distance of 10 metres. Just where Radial_3 predicts it should.

The magnitude of Zo is within 20 percent of NEC4 and the impedance
angle is in the right ball-park with the correct sign.

Now work downwards from 10 metres, to about 7.5 metres, the 3/4-wave
resonant point, to find the point where Zin has truly diverged from
Zo. You will have to work in gradually smaller increments.

Could you go down to the 1/2-wave resonant point at about 4 metres?

You will now be able to see what I'm heading for.

By the way, how much hard labour is all this causing you? Don't try
to tell me what you are actually doing because I havn't the foggiest
idea.
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Reg,