Conclusion
Non-reciprocal microwave systems

This article presented the principle of time reversal in Maxwell's equations, as well as the Casimir principle which defines it more broadly in particle systems. The experimental time reversal proposed by M. Fink in acoustics and electromagnetism was also discussed, along with its limitations. The differences between reciprocity and time reversal were then reviewed. Non-reciprocity due to the Faraday effect in magneto-optical materials led to the first non-reciprocal microwave devices proposed by Hogan in 1952.

Although widely used in microwaves, these devices had two major shortcomings: their volume and the frequency limit due to the ferrite core. The use of active components has made it possible to overcome this difficulty, all the more easily since their frequency limit is constantly increasing with advances in microelectronics. It has also made it easier to...