However, I
found I could reduce the size of a 40M antenna to nearly half by
surrounding the entire radiator with a water jacket only 40cM thick.
It has a good match, and loses roughly a quarter dB to the full size
antenna. It remains to be seen if this is just another aberration, or
is an actual solution.

73's
Richard Clark, KB7QHC

Surely it is your model that is an aberration. Did it include such things as
the loss tangent of the dielectric and the effects of the resistivity of the
liquid ?

Di-electric loading is a valid concept, but it has to be done sensibly, and
modelled even more carefully, taking ALL the effects of the di-electric into
account. GIGO.

Regards
Jeff