Dan wrote -
A 21MHz or so ground mounted monopole with a small radial field
would
be a minimal investment in materials to do a BLE style empirical
investigation, at least compared to that at 3MHz. It wouldn't be a
minimal investment in time but might be an interesting experiment.

Given that it would be an epic challenge to measure field strength
at a
grid of points on a hemisphere surrounding the monopole, I imagine a
single point field strength measurement would be an acceptable
metric?
Thoughts on doing the measurement? I guess BLE would be a good
guide
for the empiricist.

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The results of such experiments would probably generate far less
interest than BLE's original work and would not be of great use. But
somebody might possibly create a reputation out of it.

To repeat BLE's experiments at HF, and to be of use, would require
measurements to be made over a wide range of frequencies, over various
lengths of radials, over various numbers of radials, over a range of
soil resistivities and over a range of soil permittivities.

Complexity and cost would be enormous. Financial returns would be
relatively small. Who would invest in such a project? How many people
wishing to erect a 28 MHz vertical over a set of radials need more
knowledge than what already exists. Even CB-ers could do it!

It would be far more economic, with a guessed understanding of how
radials work, to write and dedicate a computer program to do the job.
With a little more tidying-up, program Radial_3 would do. And it's
FREE to USA citizens. No licence required! ;o)
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Reg.