Analysis of shielded rectangular dielectric rod waveguide using mode matching

Analysis of shielded rectangular dielectric rod waveguide using mode matching

The limit of current technology for mobile base station filters is the multimode filter, in which each cavity supports two (or possibly three) independent degenerate resonances. Shielded dielectric resonators with a rectangular cross-section are useful in this application. In the design of these filters, manufacturers are using software packages employing finite element or finite difference time domain techniques. However, for sufficient accuracy these procedures require large numbers of points or elements and can be very time consuming. Over the last decade research using the mode matching technique has been used to solve this kind of difficulty for various types of filter design and waveguide problems. In this thesis a mode matching method and computer program is developed to calculate the propagation coefficients and field patterns of the modes in a shielded rectangular dielectric rod waveguide. Propagating, complex, evanescent and backward wave modes are included and the work shows the presence of a dominant mode, and other fundamental modes, not previously identified. The effect of the shield proximity on the propagation characteristics and mode spectrum is investigated, together with the limitations on the accuracy of the mode matching method. In addition, the fields within the shielded rectangular dielectric rod waveguide, are used to calculate the attenuation coefficient of the dominant and fundamental modes. The influence on the attenuation coefficient of the proximity of the shield to the rod is also evaluated for these modes and limitations on accuracy are discussed. The calculated numerical results for the propagation and attenuation coefficient values are verified by measurement. The propagation coefficients results are typically within 2% of those measured. Verification of the attenuation coefficient results is achieved by comparing calculated and measured Q at the resonant frequencies of a number of shielded rectangular dielectric rod resonators. The difference between calculated and measured Q values is on average less than 4%. In the absence of a full solution of the shielded rectangular dielectric rod resonator, these results provide useful design information for this structure. In addition, the work reported in this thesis provides a basis for a full electromagnetic solution of this type of resonator. This would encompass the cubic dielectric resonator in a cubical cavity.