Resonant cavity for the measurement of microwave magnetic permeability using the small perturbation theory

The design of a rectangular cavity resonator implies to solve the Maxwell equations inside that cavity, respecting the boundary conditions. The resonance frequencies appear as conditions in the solutions of those equations. When a small piece of a magnetic material is introduced in the cavity, the resonance frequency and the quality factor changes. These effects can be used in the measurement of the permeability of the material. The relations are derived from the perturbation theory of resonant cavities and are simple when we consider only the first-order perturbation in the magnetic field caused by the sample. This is guaranteed when linearity exists between the measured perturbation and the volume of the inserted sample. In this work, a resonant cavity to measure the magnetic permeability of a material, at 2.16 GHz, was developed and characterized. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 399–402, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23098