Ferromagnetic Resonance-Enhanced Electrically Small Antennas

It is proposed that ferromagnetic resonance (FMR) of thin-film ferrites can be utilized to simultaneously improve the radiation efficiency and input impedance matching of electrically small antennas (ESAs). To validate the concept, the role of FMR in radiation is first derived analytically with an ideal thin-film ferrite radiator. It was concluded that the Gilbert damping of the ferrite, determining the quality factor of FMR, directly impacts on the radiation efficiency of the antenna. A practical example is proposed in the form of an electrically small single-loop antenna loaded with a thin-film yttrium-iron-garnet (YIG) core. The prototype has been designed, fabricated, and evaluated through both full-wave simulations and experiments. The simulation results match well to the experimental results, demonstrating the efficacy and significance of the idea. In addition, broadband equivalent circuit models are derived to model both the electrically small loops with and without the FMR enhancement and to provide additional insights into the design. The circuit models prove to be effective in predicting the input impedance and radiation efficiency of FMR-enhanced ESAs at a precision comparable with full-wave simulations.