Thanks, Reg. It certainly didn't 'further confuse' the issue for me,
though I can't speak for others.

Never one to let things rest without doing a bit of thinking about
them, I had a look at some info I have from R.W.P. King, and also did a
bit of NEC2 simulating. What I find is that for half-wave and 3/2-wave
dipoles, the shortening effect is nearly the same length (constant
frequency and wire diameter), but for full wave and 2-wave long
antennas, the shortening is greater. King and NEC2 agree pretty
closely for the half and 3/2 case but differ noticably for the full and
2-wave case, though again, the shortening is (somewhat more roughly)
the same for a given model when comparing the full and 2 wave cases.

I suppose the difference between the models, and the difference between
the resonant (odd-half-waves) versus anti-resonant (even-half-waves)
cases, can be accounted for by the terminal conditions. After all, the
electric field is quite high at the center feedpoint of the
even-half-waves antennas, so details of the terminal conditions (wire
diameter and spacing) are important there, much more so than with the
relatively low electric fields in that region for the odd-half-waves
antennas. The terminal conditions act roughly like a capacitor across
the feedpoint, and that has little effect with the low feedpoint
impedance of the odd-half-waves antennas, but a much larger effect with
the higher feedpoint impedance of the even-half-waves antennas.

I also used NEC2 to simulate the effects of a small top-hat: it was 4
radial wires at the top of a vertical, 0.001 wavelengths long. The
vertical diameter was .000001 wavelengths (as were the radials forming
the top hat). I found that adding that top had reduced the length for
resonance by exactly the same length in each case, for 1/4, 2/4, 3/4
and 4/4 wave tall antennas, probably within the accuracy of the
computing engine. (The differences among the shortenings was less than
0.01% of a wavelength; the shortening effect of the top hat was 0.4%.)

I suppose there are some higher-order effects going on too, but this is
close enough to satisfy my curiosity--for now. Thanks to Pierre for
posting an interesting question that has nothing to do with "velocity
factor."

Cheers,
Tom