Ultra-Miniature and Circularly Polarized Ferrite Patch Antenna

This article describes the development, the design and the measurement of a circularly polarized ultra-miniature antenna. The modeled device takes advantage of the magnetic properties of ferrite materials to achieve the desired specifications. Indeed, the antenna is operating at a frequency where the effective permeability is greater than one and the permeability has a negative real part, i.e., in the zone between the mu and mu eff resonances of the ferrite. The dimensions of the designed antenna are about (lambda(0)/23.6) x (lambda(0)/20.7) x (lambda(0)/29.4) at 2.42 GHz and a good circular polarization is achieved at the central operating frequency of the device, with a boresight axial ratio (AR) of 1.6 dB. This article is based on the modal study of a ferrite cavity which will be presented and detailed in a first step. The physical properties of the antenna and related material (dimensions, ferrite internal magnetic field, the saturation magnetization, etc.) will be deduced from this modal study and from the resolution of the wave equation inside a cylindrical ferrite cavity. All the antenna performances in terms of impedance matching, axialratio, and radiation pattern are reported and commented. This concept is finally validated by the measurement of a prototype.