Design Of Microwave Multi-Mode Cavity Filters

Microwave cavity filters are widely used in base stations of wireless communication systems,because of their low insertion losses,high quality factors and high power handling capabilities.The cavity filters with smaller size and larger fractional bandwidth,are much demanded to meet the stringent criterion of advanced wireless communication systems.However,it is hard to meet all these requirements by using conventional methods to realize the filter by employing a number of single-mode cavities.On the other hand,multi-mode resonators(MMRs)are used in a single waveguide cavity to miniaturize the filters.In this thesis,analysis and design of multi-mode wideband bandpass cavity filters is presented.The proposed cavity filters can be divided into two types:the filters loaded with off-centered metallic cylinders and the filter perturbed with rectangular split ring resonators(SRRs).The proposed filters employing off-centered metallic cylinders are coupled with perpendicularly-fed coaxial lines to excite the multiple modes in a single waveguide cavity.Unlike simple metallic cylinders,mushroom-shaped cylinders are used to reduce the size of the waveguide cavity.The height of the cavity is exploited to achieve the wider out-of-band rejection.Triple-,quadruple-and quintuple-mode wideband bandpass filters are designed by employing one,two and three off-centered mushroom shaped cylinders,respectively in a rectangular waveguide cavity which is perpendicularly-fed with coaxial lines.The triple-mode filter excites a transverse magnetic(TM)mode along with a pair of hybrid magnetic(HM)degenerate modes HM+and HM-at the center frequency of 2.4GHz with 58%wide fractional bandwidth.The quadruple-mode filter excites four quasi-TM modes at 2.93GHz with fractional bandwidth of 38%and out-of-band rejection extending to 2.3 times the center frequency.Similarly,the quintuple-mode filter excites five quasi-TM modes at 2.68GHz with fractional bandwidth of 43%and out-of-band rejection extending to 2.4 times the center frequency.On the similar lines,the computer aided design of another triple-mode filter perturbed with an off-centered mushroom-shaped cylinder is also performed by exploiting an equilateral triangular waveguide cavity.In this case,a TM mode along with a pair of degenerate HM+and HM-modes are the three resonant modes excited to realize the bandpass filter,which operates at 2.94GHz with 22%wide fractional bandwidth.Another proposed technique for designing a multi-mode wideband bandpass filter is based on SRR excitation in a rectangular waveguide cavity.Two rectangular SRRs are connected with the ends of coaxial line probes extended from two opposite sides of the cavity.Two hybrid modes(HM,HE)are excited to realize the bandpass filter at 2.5GHz with 53%wide fractional bandwidth.The design has also been extended to fourth order filter by cascading two filters.The S-parameters and mode charts of the proposed filters are explained by studying parametric variations in geometrical dimensions.The coupling scheme and coupling matrices are also utilized to analyze the filter.These studies play a role of guidelines for designing wideband bandpass cavity filters.The design examples of the aforementioned multimode scenarios are presented.The structures have been fabricated and measured results show an agreement with the simulated ones.The thesis has been concluded by highlighting a few limitations of the proposed multi-mode cavity filters.Since the performance of the multi-mode cavity filters is mainly based on the perturbation cylinders and coupling coaxial probes,it is advised to minimize the material losses and tolerance in the fabrication to obtain the desired results.With future research work in this field,it is anticipated to overcome these limitations,and the proposed cavity filters will find applications in wide dimensions of the microwave engineering.