Probably you convinced yourself for 100% that the antenna limits the bandwidth.

Yes, I had overestimated the effect of the matching system, and, as you say, the
antenna Q is far higher then the Q of the matching network. The fact that the
capacitance (corresponding to the capacitive reactance of the shortened D.E)
varies with frequency does not change things.





As other people said, the Q of a hairpin made of 5…10mm tubing is over
100. As the BW of your L match is far below that (also for the 200 Ohm
case), losses in the hairpin are that low, that they can practically
not be measured via field strength measurement. So 3.2 times higher
then in the 50 Ohms case is still negligible.

I agree with the conclusion





After all the calculations and simulations, I hope your 6m Yagi is still useful
for you.

Yes, after precisely tuning the D.E., it shows a good SWR all over the band of
my interest. Nevertheless, before mounting the antenna, I would not have
suspected such a narrow SWR response.

73

Tony I0JX