At microwave frequencies a ground plane cannot always be defined as circuits are large enough that a “ground” in one part of a circuit is not the same as a “ground” in another spatially separated part of the circuit. If they were the same “ground” then charges would need to be able to instantaneously redistribute and this is not possible because of the finite speed of causality (that is, the speed of light). One of the consequences of this is that voltage and current occur as forward- and backward-traveling voltage and current waves on transmission lines, or in microwave networks as incident and reflected voltage and current waves at interfaces between different parts of a circuit. At any point the sum of the forward-traveling (or incident) voltage wave and the backward-traveling (or reflected) voltage wave is the total voltage used with low-frequency circuits. A similar description applies to currents. A voltage wave directly relates to the movement of power or what are called power waves. Because of this, the conventional circuit parameters, the \(y\) and \(z\) parameters defined when there is a single ground, prove to be inadequate to describe signals in microwave circuits. Scattering parameters, or \(S\) parameters, are the most convenient network parameters to use with microwave circuits as they neatly describe the properties of traveling waves of voltage and current. A further attribute is that the \(S\) parameters relate directly to power flow.
In the early days of electrical engineering all circuits were called networks. That usage remains with microwave circuits but it is more commonly used with circuits that operate at microwave frequencies but only the external terminals are presented and internal details often hidden. The use of the network term also conveys a subtle reminder that there is not necessarily a universal ground. \(S\) parameters are defined even when there is not a universal ground.
Most RF and microwave design is concerned with the movement of signal power and minimizing noise power to maintain a high signal-to-noise ratio in a circuit. This maximizes the performance of communication, radar and sensor systems. Thus maximizing power transfer is critical and the design technique that does this is called matching.