S-Parameters for Calculating the Maximum Efficiency of a MIMO-WPT System: Applicable to Near/Far Field Coupling, Capacitive/Magnetic Coupling

S-parameters are members of a family of circuit network parameters. Other members are Z-parameters, Y-parameters, T-parameters, and ABCD-parameters
[1]
. The first published description of S-parameters was in the thesis of Vitold Belevitch, in 1945
[2]
. Kaneyuki Kurokawa popularized the S-parameters referred to as
power waves

[3]
, making S-parameters much easier to apply to microwave engineering when port voltages and currents are difficult to define and measure.