Efficient methods for novel passive structures in waveguide and shielded transmission line technology

Abstract

With the rapid development of microwave and millimetre-wave systems, the performance requirements for passive band-pass filters and diplexers, as essential parts in these systems, are steadily increasing. Both rectangular waveguide and substrate-integrated waveguide technologies help to satisfy various high-performance requirements. Rectangular waveguides offer the advantages of low loss and high power handling capabilities, while substrate-integrated waveguides have the advantages of low cost and easy integration into planar circuit technology. Besides, the miniaturisation of electronic devices is of great importance, especially for microwave or millimetre-wave systems whose volume is limited by system considerations. Hence, the two main aims of this thesis are firstly to develop efficient methods which can improve the design reliability and reduce the design cycle of such passive devices, and secondly to present novel structures of band-pass filters and diplexers whose dimensions are reduced. In the first part of the thesis, a method based on the mode-matching technique is developed to rigorously and efficiently analyse the negative influence introduced by micromachining errors on the performance of band-pass H-plane iris filter. This analysis includes the effect on the centre frequency and 3 dB bandwidth caused by the round angles between waveguide walls and H-plane irises, or by the bevel angles on the H-plane irises. To remove these undesired influences, three approaches are proposed and verified with simulations performed with the finite-element method. In the next part, efficient approximation approaches are investigated in the framework of the mode-matching method to analyse the characteristics of cylindrical posts placed in the cross-section of a rectangular waveguide or substrate-integrated waveguide. Compared with the H-plane irises in rectangular waveguides, cylindrical posts are more promising for realising band-pass rectangular waveguide filters, because the geometries are easier to manufacture and less prone to machining errors. Thus, a general design procedure for band-pass post filters in rectangular waveguides and substrate-integrated waveguides is developed and verified with finite-element simulations and measurements on prototypes. The tolerance analysis for the band-pass filters is also explored quickly and accurately with the developed method, while the influence of realistic material losses on the insertion loss of various structures, is also quantitatively analysed with a full-wave simulation solver. Next, the characteristics of a shielded microstrip line for single-mode operation is investigated rigorously based on the mode-matching method. The research focuses on the influence of the metal enclosure dimensions on the fundamental mode, and the relationships between the cutoff frequency of the 2nd-order mode and the geometrical variables of the cross-section of the shielded transmission line. A similar method is then applied to an E-type folded substrate-integrated waveguide. The analysis demonstrates that the propagation characteristics for the first twenty modes in the E-type folded substrate-integrated waveguide and its corresponding equivalent rectangular waveguide are almost identical if the width of the middle metal vane in the E-type folded substrate-integrated waveguide is chosen reasonably. Exploiting this similarity property, a novel concept of band-pass post filter in E-type folded substrate-integrated waveguide technology is developed to reduce the band-pass filter dimension further, together with an efficient specific design procedure. The validity of the approach is verified via finite-element simulations and measurements on a fabricated prototype. Finally, to reduce the sizes of common diplexers, four types of novel three-port junctions are proposed, including two improved Y-junctions in substrate-integrated waveguide technology, a double-layer junction in substrate-integrated waveguide technology, a Y-junction in T-type folded substrate-integrated waveguide technology, and a junction with stairs in T-type and E-type folded substrate-integrated waveguide technology. Exploiting the flexibility of the in-house developed mode-matching code or a commercial finite-element simulation solver, the characteristics for all presented junctions are shown to satisfy the constraints for optimum performance of diplexers when adjusting the relevant variables in the corresponding structures. Three types of these junctions are then utilised in realising diplexers whose performance is verified over the required operation bands with either numerical simulations or measurements on fabricated prototypes. In summary, this thesis has introduced novel concepts and realisations of compact band-pass filters and diplexers in unfolded or folded substrate-integrated waveguide technology, as well as related structures. One of the crucial aspects emphasised throughout the research is the need for efficient and accurate modelling methods specifically tailored to support such developments. This has been demonstrated throughout the thesis with the combined use of powerful numerical methods and equivalent models based on symmetries or unfolded geometries.