A rather popular concept in amateur radio is that the physical lengths* of buried radials used with a vertical monopole need not be any longer than the physical length/height of the monopole they are used with. The clip below has a good explanation of the purpose and function of such buried radials. It was written in 1943, however the Physics of such systems has not changed since then. The clip includes a sentence about the lengths of buried radials (outlined in red), that should be of interest here. * Or alternately, that the velocity of propagation along buried radial wires needs to be accounted for in determining their effective lengths.