Compact Microwave Dual-band Bandpass Filter Design

The modern wireless communication systems require dual-band bandpass filters to support the standards that work at multiple frequency bands. This thesis demonstrates two design approaches for the development of compact microwave dual-band bandpass filters. The first approach is based on synthesising a dual-passband filter response utilising only one resonant frequency of the resonators. The second approach employs dual-band resonators that have tuneable the first and the second resonant frequencies to form the dual-passbands filter response.
The dual-passband response synthesis method synthesises a response with dual passbands that is generated by a frequency transformation that places a finite frequency zero within the single- passband of a filter to split it into dual passbands. The transformed dual-passband response is characterised by the synthesised coupling matrix that consists of the coupling coefficients between coupled resonators. Two filters have been designed and fabricated using microstrip square open-loop and TE01δ mode quarter cylindrical dielectric resonators. The investigation based on simulation studies and measured results revealed that unloaded quality factor of the resonator is required to be ten times greater than the quality factor of each passband in order to realise the narrow passbands.
The dual-band resonator methods employ multiple resonant modes of the resonator operating at different frequencies to implement the multiple passbands, respectively. Stepped impedance resonators in stripline and coaxial configurations have been presented and analysed for the realisation of dual-band bandpass filters. A second order dual-band bandpass filter formed by coaxial stepped impedance resonators has been designed, fabricated and tested. The measured frequency response agree well with the simulated response. The estimated breakdown power shows that the filter is capable of high power applications. Non-uniform pitch helical resonators are also proposed for the implementation of dual-band bandpass filters. Two non-uniform pitch helical resonator structures have been analytically modelled. The theoretical models of the non- uniform pitch helical resonators have been developed for accurate prediction of its dual-band characteristics. It is also employed in the general design process of the non-uniform pitch helical resonators. Resonator examples have been presented to show the applicability and validity of the analysis and simulation. Three dual-band bandpass filters have been designed and implemented using non-uniform pitch helical resonators. Their measured frequency responses agree reasonably with the ideal responses. Additionally, the simulation shows that the designed dual-band bandpass filters have relatively high power handling capability.