:

The alloy is Type 301 (17% chromium, 7% nickel stainless steel) hard
drawn
wire, or even better is a 17/7PH (precipitation hardening) grade that our
firm once produced for tank aerials, that also may take a few knocks.

Both (especially the PH grade) need a final heat-treatment (420C for a
few
minutes IIRC) for ultimate spring properties, but to get such a tight
bend
you'd need quite a thin wire. It's a matter of the proof strain or limit
of
proportionality, how much the outer skin can stretch without taking a
permanent set, compared to the neutral central axis, on the bend.

There speaks a man who obviously knows what he's talking about. Yes, I
believe we can order 301 from our guy in N. London who's very
accommodating on such matters. Thanks for a valuable steer!
Just one point, though: is stainless steel a reasonable radiator of RF
energy?

Back years ago (30 or so) there was an ad in a magazine about either a
loaded CB or 5/8 two meter whip about 36 inches long bent in a circle made
of the 17/7 material. Not sure what the whips are actually made of but
most are some kind of steel .