Sorry Leo - missed the point...
A transmission line is length independant only if correctly matched . The antenna at it's tuned freqency should be OK - but the input impedance of a super-regen probably isn't. I think they're awful - apart from their easy detuning , poor sensitivity and terrible blocking performance they radiate at somewhere near their nominal receive frequency and upset other receivers in the area. A gain stage in front improves things, but I'd still pay a bit extra for a better architecture...
You can't even measure their input impedance with a VNA, because the emitted signal interferes with the VNA sampling signal! And even a gain stage's S12 isn't zero, so you're still confused.
Without knowing the input impedance you can't determine the correct length of transmission line except by using a trombone and an on-air sensitivity measurement - preferably in an anechoic chamber. It's been done. You're using the poor VSWR as a matching component.
horrible things . I designed a couple once under protest because the client was too mean to pay for a better design..
regards
Allan