Hasan et al,

Tom says current can be detected in radials well beyond the 20dB
attenuation limit. This is easily explained.

The total current flowing in the system at a distance is in the soil
due to its far greater cross-sectional area. Especially when soil
resistivity is low. Nothing in particular happens in the soil at the
end of the 20dB limit.

The small current in a radial is INDUCED in it by the relatively
larger total current flowing in the soil in parallel with it. The
radial current is NOT generated by the voltage at its input. Its high
attenuation isolates it from its input.

What current flows in a radial has a progressively less effect on the
total current (which is what matters) as distance increases.
Eventually, it doesn't matter whether the radial is there or not.

The limit is reached when the radial input impedance converges on Zo,
the radial's characteristic impedance. This occurs when radial
attenuation is around 18 or 20dB. Beyond that distance the current
flowing in the ground carries on, as usual, unaffected whether the
radial is there or not.

Resonant effects, small peaks and troughs in the impedance-frequency
curve, also die away at the 20dB or even lower limit. There's not much
left even at 14dB.

Radial attenuation increases rapidly with frequency. So shorter
radials can be used at 14 MHz than at 1.9 MHz. When 30 MHz is the
lowest frequency of use, and soil resistivity is high, a dipole,
without radials, is more likely to be used than a vertical.

(Comment: I guessed correctly I would be accused of trolling when I
introduced the subject of radials as transmission lines.)
----
Reg, G4FGQ.