I tried 64.5 mm - the axial ratio is excellent, 1 dB, but the VSWR rises to 1.3:1.
Hi there Mike,
happy to see the two resonances in the SWR plot - and symmetrically about the CF of interest too, essential for good axial ratio in this type of design. And yes, at the minor cost of slightly increased SWR at the sum impedance spot frequency:
Cheers - Michael, oh2aue