Owen Duffy wrote:
I suspect that it is a technique to try to maximise the current moment to
get the highest radiation resistance. They then build out the radation
resistance with loss to achieve a specification maximum VSWR for direct
feeding at the base with 50 ohm line.

It could also be a technique to move part of the
loading up to the top of the antenna. I once won
a CA shootout with a top-loaded junk box antenna
that, in EZNEC, looks something like this:

http://www.w5dxp.com/SHOOTOUT.EZ
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73, Cecil http://www.w5dxp.com