Dave Platt wrote:
The hardware stores around here sell special unions, with insulated
threaded fittings, for mating iron/steel pipes to brass. I've
pondered whether these might be useful for making certain forms of
(non-DC-connected) copper-tubing J-pole antennas.

Those are specifically mating up old iron piping with new copper.
For the prevention of galvanic action.

Jeff

--
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." Benjamin Franklin
"A life lived in fear is a life half lived."
Tara Morice as Fran, from the movie "Strictly Ballroom"