Another possibility is a radar reflector installed in the glider.
These things are much cheaper than a transponder, and would give at least
some info...

I'd love to see if my local boating supply shop has one that would fit
--

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Mark J. Boyd

I wasn't thinking about the big jets but the person in the PA28 etc. who
uses our NDB as a turning point at 2000 ft. The beacon is in the middle of
the airfield we use as our gliding club base. The area is marked as an area
of intense gliding activity and the airfield is marked with cables. We've
had a couple of near thing over the airfield.

Peter.