2. Microwave measurement systems

2.1 Notion of S parameters

At microwave frequencies (300 MHz to 30 GHz) and millimeter-wave frequencies (30 GHz to 300 GHz), propagation phenomena result in varying voltages and currents along transmission lines. Impedance (Z), admittance (Y) and hybrid (H) matrices, which relate to the notions of voltage and current, are difficult to describe. We therefore prefer to use pseudo-waves for power, which is easily measurable in the microwave and millimeter frequency ranges, allowing us to use the [S] matrix of scattering parameters.

Pseudowaves are purely mathematical quantities that can be defined in relation to a reference impedance Z _ref arbitrary for each circuit port, typically 50 Ω for conventional coaxial lines....