Cecil, W5DXP wrote:
'I probably should have said rho^2 = Pref / Pfwd, rho can be plus or
minus 1."

Terman mentions a power ratio at the bottom of page 97 of his 1955
edition:
"This definition of standing-wave ratio is sometimes called voltage
standing-wave ratio (VSWR) to distinguish it from the standing-wave
ratio expressed as a power ratio, which is (Emax / Emin) squared.

Best regards, Richard Harrison, KB5WZI