The tenna-topper used to be Orange styrofoam - free with a fillup at the
"Union-76" station on the corner. (late 60's)
I don't think it stops corona or helps or harms the antenna in any way -
unless by making it easier to blow down.
One interesting experiment to try is put a ohmmeter on it - and - If
infinity, then maybe borrow a megger.

"Bill Ogden" wrote in message
...
One ARRL stand (at Dayton) handed out small, black foam balls that are
intended to be stuck on the end of a whip antenna. Stops corona (maybe --
let us skip that discussion).

I do not normally have a transmitter in my truck, so I stuck the ball on
my
standard truck radio antenna. (It is a 2003 Ford Ranger.) Since doing
this
I have noticed that the antenna almost always vibrates a little -- even
after sitting stopped (engine off) in the garage for many minutes. The
ball
on the top moves at least 1/8 inch; it is easily seen if I focus on it.

It appears I have a good resonant antenna to sustain this motion for long
periods, across an almost 3:1 range (the AM broadcast band) and a bonus
VHF
range (the FM broadcast band). I assume that Hooke's law (if I remembered
the right name for it) applies instead of the
break-from-metal-fatigue-or-metal-work-hardening-law will apply such that
the antenna does not snap off sometime. Perhaps I should patent this
resonant antenna that is almost frequency independent!

Bill
W2WO