Reg Edwards wrote:
You then include in the calculation the measurement or assumption of
the Zo of the 50-ohm coax, and the measurement or assumption of Zo of
the twin-line, and the forward and reverse powers, and the SWR on the
twin line can be deduced or assumed.

Actually, nowadays I use my MFJ-259B to read the resistance at
the choke-balun where I have adjusted the ladder-line length to
guarantee the existence of a current maximum point. It's actually
easier to do than to write about it. An assumption that Z0=50 ohms
is not necessary.

But if you think you are measuring SWR on anything you are cheating
and fooling yourself.

I actually have an SWR meter calibrated for balanced 380 ohms but it's
in a box somewhere in my garage. I found my indirect measurements to
be entirely accurate enough. In general, if one can isolate the problem
to 10% of the Smith Chart, one can solve any problem by tweaking.

Speaking of indirect measurements - let's say the feedline Z0 is 380
ohms with a VF of 0.9 and a length of 90 ft. The measured resistance
at the current maximum point is 30 ohms on 7.15 MHz. The SWR on the
ladder-line is 380/30 = 12.7:1. The feedline is 0.727 wavelengths
long. Plot the point 30/380 = 0.079 + j0 on a Smith Chart. Draw an
SWR circle through that point. Backtrack from that point around the
circle for 0.727 wavelengths and there's your antenna feedpoint
impedance (neglecting losses). Losses can be taken into account by
using SWR spirals instead of SWR circles. And of course, all of this
is done by a computer program after just a few seconds of data entry.
--
73, Cecil http://www.qsl.net/w5dxp