Spajky wrote:

- if I connect to a 50ohm TX out a cable of 75ohms (w/o "perfect"
Z transform add_on there), I can aspect approx. 1: 1,5 SWR [50:75]
& because of Z miss-match I will loose no more than 0,5dB (that same
my book states for that SWR in some diagram); yes I know that is in
"ideal" conditions & TX is only 50mW ... could you confirm me
briefly that?

No, you would lose 0.18 dB if your source is something like a signal
generator with fixed 50 ohm output impedance. If you match the impedance
so the source sees 50 ohms, you'll regain the 0.18 dB.


- next : twin-lead/symetric cable of only half wavelenght (or 3x,5x
etc. half wavelenghts, shorted by a velocity factor sure) mirrors the
input Z to its end; does a coaksial cable do the same at the same
lenght or for that is a different simplified formula?

All transmission lines transform impedances the same, regardless of
construction. However, the impedance seen at the input of an integral
number of half wavelengths of real transmission line won't equal the
load impedance, because of line loss. The error will be small if the
load impedance is about the same as the line Z0. But if they're quite
different, even a small amount of loss will change the input impedance a
significant amount.

Roy Lewallen, W7EL