Reg Edwards wrote:

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 (|Vfor|+|Vref|)/(|Vfor|-|Vref|)

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

Or (|Ifor|+|Iref|)/(|Ifor|-|Iref|)

Or at an (antenna feedpoint) impedance of (R +/- jX) and a resistive Z0

SQRT[(R+Z0)^2+X^2] + SQRT[(R-Z0)^2+X^2] divided by
SQRT[(R+Z0)^2+X^2] - SQRT[(R-Z0)^2+X^2]

Or (1+rho)/(1-rho) if the reflection coefficient is known.

Or it can be [SQRT(Pfor)+SQRT(Pref)]/[SQRT(Pfor)-SQRT(Pref)]
which is the indirect one a typical MFJ dual-needle SWR meter
uses.
--
73, Cecil http://www.qsl.net/w5dxp