Dielectric resonators and filters for cellular base-stations
The investigation into three types of dielectric resonators for use in base-station filtering applications is presented. The triple-mode cubic TE016 resonator is shown to have good performance for high Q applications. Its suitability for realising conventional narrowband selective bandpass filter responses is proved. The effect on the response of spurious intercavity couplings through irises is studied. The triple-mode cubic TE016 resonator is also used for the realisation of a novel type of filter, the even-odd hybrid mode reflection filter, which eliminates the need for cross-couplings for any symmetrical frequency response. The insensitivity of the new type of filter to most spurious couplings is shown in the case of a sixth degree elliptic filter. The drawbacks of this type of filter are also described. The new dual-mode conductor-loaded dielectric resonator is presented. An exact model of the resonator is necessary for an accurate study of this resonator. The axial mode-matching technique is used. The relative numbers of modes to use in each section of the model for optimum convergence primarily depend on the mode type and the relative diameters of the dielectric cylinder and the metal disc. The convergence of the resonant frequencies is good. That of the quality factors is slower but still provide useful approximate results. These convergences are affected respectively by large electric and magnetic field amplitudes in singularity regions. The resonant frequency and Q. of the fundamental mode are primarily dependent on the diameter and height of the dielectric cylinder respectively. The resonator geometry is optimised for Qu and spurious separation at 900 MHz and trade-offs between the two criteria are quantified. The resonator is found to be particularly suited for medium Q applications, i. e. between 4000 and 7000. The third resonator, the dielectric-loaded TEO,, resonator, is shown to be well suited for applications around 2 GHz and requiring Qu's of a few thousands. Theoptimum cavity cross-section dimensions for maximum Qu volume are almost constant over a wide range of cavity diameters. The trade-off between Qu and spurious separation is explained. Coupling bandwidth limitations in the case of off-line cavities are found to be solvable by using off-centred resonators. 80 MHz wide filters, optimised for ease of manufacturing, are built.