Non-reciprocal microwave systems

The notion of reciprocity and non-reciprocity is a fundamental scientific concept, important in many branches of physics, chemistry and engineering. In a general sense, electromagnetic non-reciprocity implies that the fields created by a source at an observation point are not the same when the source and observation point are exchanged. Non-reciprocal systems are essential for applications where unidirectional propagation is required and needs to be controlled. Generally speaking, these non-reciprocal systems enable the flow of energy to be controlled, and directed in certain preferred directions. In this sense, they play a decisive role in many applications.

The concept of electromagnetic reciprocity is closely related to that of the time-reversal symmetry of Maxwell's equations. The same property of temporal symmetry in the field equations also leads to the temporal reversibility of physical processes in lossless systems.

Nevertheless, the concepts of reciprocity and time reversal of physical processes present fundamental differences and must therefore be distinguished, particularly in lossy systems, as we shall see in the first instance. Next, the first applications of non-reciprocity in passive and then active circuits are presented.