OK, I did the decent thing and finished building the 15 ele yagi, based on dimensions from K7MEM online calculator.
A quick field trial from the Long Mynd suggests that it has similar gain to the old J Beam 15 over 15, and a nice clean polar diagram. Better still, it weighs 280 grams with the feeder, compared to 880 grams. The feeder is about 1.5m RG223 (double screened, silver plated terminated with SMA plug) which I grabbed at the Norbrec rally. The balun is a short length of semi rigid, from the same source.
I cut 17 elements in readiness, but the piece of pipe I pulled from a skip was only long enough for 15. At just over a metre, its quite practical to carry, though.
It took me about 100 Hours of day dreaming about clever jigs etc, but once I set to, it took about 2 hours work with steel rule, scribe, hacksaw, vice and drill :o)
I just need to acquire a smaller tripod, and I’ll be attempting a few more challenging activations.