Hi Laurence and LF,

I have used ~ 40 cm helium balloons a couple of times, to get on air with 60 to 100 m of thin 0.4 mm copper wire. The balloon was closed off by a small upside-down plastic bottle (ex body lotion, or the like), with a hole in the base coverd by a piece of tesa tape as a filling valve. The wire end was simply wrapped around the neck of this bottle, with no extra length of nylon filament or insulators. Ran up to abt. 3 Amps RF into it without any problems. "Ground-based" experience with my 200 pF topload showed that corona near the wire ends can be reduced further by paralleling two (or more) thin wires for the last couple of meters, with a U-turn at the end and two 5 cm lightweight spacers.

Actually I was never very much worried about the balloon coming off the wire, but more concerned about not letting it off near the bottom end, with the risk of a string of copper shorting some high voltage powerline, possibly causing an arc after meltdown.