The two port method allows you measure impedance over a wide impedance range
using shunt/series connections. With the two port method, you can use either S11
or S21 to perform measurements; however, S11 has a more limited noise floor than
S21, and it is highly susceptible to VSWR and other factors that exist between the
fixture connector and substrate. Because of this, generally S21 is preferred. The S21
shunt connection is suitable for 50 Ω or lower impedances while the S21 series
connection is suitable for 50 Ω or higher impedances. Because EMC components
span a wide range of impedances depending on their purposes, their impedances
must be measured with an adequate combination of S21 shunt/series connections.