S-parameter is basically a means of characterizing n-port networks. By reviewing some traditional network analysis methods we'll understand why an additional method of characterization is necessary at high frequency.
Figure 1: A two-port network
A two-port device in Figure 1 can be described by a number of parameter sets. All of these network parameters relate total voltages and total currents at each of the two ports.
To see how parameter sets of this type can be determined through measurement, let's focus
on the H-parameter. is determined by setting equal to zero - applying a short circuit to the
output port to the network. is then the ratio of to - the input impedance of the network.
is determined by measuring the ratio of to - the reverse voltage gain - with input port open circuited.
Moving to higher and higher frequencies. some problems arise: