Reg Edwards wrote:
Or they find a line which does exist but on which it is
impossible for the thing to measure anything because it is located in
the wrong place.

Reg, the SWR meter may be smarter than you think. Here's
an experiment for you. The system is lossless.

XMTR--a--1WL 50 ohm--b--1WL 75 ohm--c--1WL 92 ohm--d--load

An SWR meter calibrated for 50 ohms will read the SWR on
the 50 ohm feedline when installed at points a,b,c, or d.

An SWR meter calibrated for 75 ohms will read the SWR on
the 75 ohm feedline when installed at points a,b,c, or d.

An SWR meter calibrated for 92 ohms will read the SWR on
the 92 ohm feedline when installed at points a,b,c, or d.

Now Reg, you have to admit that an SWR meter that can read
the SWR on the 92 ohm feedline when installed at point 'a'
is a darned smart meter. :-)
--
73, Cecil, http://www.qsl.net/w5dxp