SWR is the dimensionless ratio of two voltages, a maximum and a
minimum, which may be measured along a transmission line, spaced apart
by 1/4-wavelength of line.

Or it can be the ratio of two similarly displaced currents.

It is fully described without reference to line impedance, source
impedance, terminating impedance, line length, or where the max and
min volts are located.

Incidentally, before it can be used for anything, extra information is
needed.

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"Richard Fry" wrote in message
...
Isn't it necessary to define an impedance against which the antenna
input
impedance and SWR is referenced? It may not always be 50 ohms or
some other
"standard" impedance.