" What I am getting at is if the TX source Z is 50r and the antenna load Z
is 50r any length of coax will do.

Yes, of course, (neglecting any loss in the cable).

The effect of tuning the coax length
will be most evident if the system is reactive at either end. How much
that difference it really makes I dont have experience in.


It makes no difference to the vswr, all changing the cable length will do is
change its phase.
(again neglecting loss in the cable, and assuming that the cable impedance
matches the source).

73
Jeff