Mike,

I assume you will have ground surface or shallow buried radials.

There are hardly any resonant effects in shallow-buried radials so no need
to concern yourself with particular lenghts.

You will be far better off with more and shorter wires. The attenuation
along radial wires is quite high. There's no current flowing in them beyond
a small fraction of a wavelength from the focal point. They may as well not
be there.

Wire diameter selected solely on your ideas about durability. Enamelled
copper wire is fine and may be less expensive, easier to obtain than the
bare stuff.

Not much point in having radials longer than 1/10th or 1/15th wavelengths in
moderately good soil at the lowest frequency of interest. A little longer
in poor soil. Shorter in very good soil. Moisture content and fertility is
the guide.

Begin with one radial and increase the number by 50% until the RECEIVED
signal strength from lower frequency, stable, reliable transmitters stops
increasing. Then add a few more for luck. You will probably end up with
fewer radials than recommended in the handbooks.
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Reg, G4FGQ