"Cecil Moore" wrote
Try the ARRL Antenna Book. In general, symetrical elevated
radials don't radiate. In general, ground mounted radials
are lossy.
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What everybody forgets about is that the velocity factor along
ground-mounted radial wires is about half of the free-space value.

Consequently, the 1/4-wave resonant length is crudely only half of the
elevated value.

On the other hand, the resonant length is very non-critical because Q
is very small - Q is only 2 or 3 and is even smaller at the high end
of the HF band.
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Reg, G4FGQ