Isolator utilizing artificial magnetic gyrotropy

A novel isolator, consisting of a raised microstrip transmission line on a substrate supporting two periodic rows of resonant metal rings loaded by a unilateral semiconductor lumped element, namely a field-effect transistor (FET), is proposed, analyzed, and measured. The loaded rings are placed on either side of the strip, where the magnetic field is circularly polarized, in opposite senses, which depend on the direction of propagation. As a result, symmetrically configured FETs allow/prohibit power propagation in the pass/stop directions by allowing/prohibiting the corresponding currents along the rings. In contrast to ferrite-based isolators, this isolator does not require a biasing magnet, which is generally cumbersome and non-integrable, but only an electrostatic bias voltage for the FETs, and it is not limited in frequency by excessive loss or magnetic bias. The fabricated prototype exhibits more than 30 dB of isolation at 3 GHz, in close agreement with the numerical simulations.