"End effect" looks to me like a description of a result, not an
explanation of a cause. Its "proof" consists of the observation that
fatter dipoles have a shorter resonant length than thin ones. I'm afraid
a real explanation of why the "end effect" occurs requires much deeper
physics and math.

I don't believe that Tom's result with the top hat is a demonstration of
the same phenomenon that makes resonant fat dipoles shorter than thin
ones. Here's what the top hat experiment means:

Suppose you have a thin antenna of any length. Look at the current
distribution on the last few degrees of the antenna. I believe you'll
find that it's the same regardless of the antenna length. Then replace
the wire with a top hat. Again you'll find that the current distribution
on the top hat is the same regardless of the length of the antenna below
it. So it shouldn't be surprising that you can substitute one for the
other and get the same result regardless of the antenna length.

This proves that you can replace a part of an antenna with a capacitive
hat, and that the relationship between the length of wire and size of
top hat is, at least to first order, independent of the antenna length.
It's not clear to me what else it proves.

Incidentally, does this have anything to do with the "true length" of an
antenna? No one has stepped forward yet with an explanation.

Roy Lewallen, W7EL

Reg Edwards wrote:
Tom wrote -
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 "End Effect" is thereby proved.

Marvellous things are computing engines!
----
Reg.