S-parameters are used in microwaves :

• The H, Y, Z and ABCD parameters are difficult at  microwave frequencies due to following reasons:

1. Equipment is not readily available to measure total voltage and total current at the ports of the network .
2. Short-circuit and open-circuit are difficult to achieve over a wide range of frequencies .
3. Presence of active devices makes the circuit unstable for short (or) open-circuit.
4. Therefore, microwave circuits are analyzed using scattering (or) S-parameters which linearly relate the reflected wave’s amplitude with those of incident waves.