[ionburn]

I cannot instantly see the mechanical method but an expanding star in the centre would be adjustable to vary the circumference. If this were sprung the tension would be self adjusting. It would require each turn to be identical in tension.

I think perhaps a bit too complex but may spur better ideas.

My own frame aerial of similar size has four spokes and small blocks with grooves, as you suggest. It is a square. There is no method of adjustment for tension, but I have not found it and issue.