The so-called SWR meter is just a resistance (not impedance) bridge.

The bridge is at balance and indicates SWR = 1:1 when a resistance of
precisely 50 ohms is connected to its output terminals. It is
arranged within the meter that this 50-ohm resistance, or whatever is
connected to the output terminals, is the transmitter load.

With the meter in its normal location, the load is the input impedance
of the transmission line to the antenna. So when the input impedance
of the line, as determined by Zo of the line and the antenna input
impedance, is 50 ohms then the meter indicates SWR = 1:1 regardless of
Zo, line length and antenna impedance.

As Roy says, in the special case of line Zo being precisely 50 ohms it
so happens that the meter will correctly indicate SWR along the line.
For any other value of line Zo the meter will indicate varying degrees
of nonsense.

At HF, line Zo is frequently anywhere between 50 and 600 ohms and a
tuner is used to transform line input impedance, either up or down, to
the 50 ohms required by the transmitter. But Zo is not affected and
the SWR meter indications remain in error.

Whatever Zo and antenna impedance may be, the meter always indicates
whether or not the transmitter is correctly loaded with a resistive 50
ohms.

Note that the circuit operates independently of transmitter internal
impedance whatever that may be.
----
Reg.

If the meter is misapplied, it's misapplied. If my transmitter is
designed to operate optimally into a 300 ohm load and I use an "SWR"
meter calibrated to 50 ohms, it's not going to do any better job
indicating proper transmitter matching than indicating SWR on a 300 ohm
line.

I will continue happily to call my SWR meter an SWR meter, and know
enough about what's going on inside it to apply it
appropriately--whether it's to the task of giving me an indication of
SWR on a transmission line or the task of indicating proper loading on
a source.

I suppose there are many who will continue to happily call them SWR
meters and NOT understand how to properly apply them. I'd much rather
work on educating them to understand how the meter works and how to
apply it properly than to insist they call it by some other name. I've
been at the task since B.R.E.

But of course, not everyone sees it that way.

Cheers--and Merry Christmas,
Tom