Coaxial line coupling to and reactive obstacles in circular waveguide

Abstract

Elementary circular waveguide devices, coaxial line-to-circular waveguide transitions, metal posts and irises, are investigated in this thesis. Coaxial line-to-waveguide transitions are fundamental for the excitation and coupling of the waveguide, whilst metal posts and irises function as reactive obstacles in a waveguide; together they form basic building blocks for a waveguide system. These problems are considered under the condition where the TE11 mode is the only propagating mode in the waveguide. The research methodology employs a judicious combination of numerical modelling and experiment; the approach is validated through cross-verification between numerical modelling and experimental results. Models and design data are obtained and represented in a normalised form to permit easy scaling to any waveguide size. In addition to characterising the devices, the focus is on compiling a set of design data and information to assist microwave engineers in designing circular waveguide applications involving these devices. Two types of coaxial line-to-circular waveguide transitions are considered: the probe-type and the end launcher-type. For each transition type, the normalised input impedance characteristics with respect to various parameters (such as transition dimensions and frequency) are investigated. The key outcome from the investigation of the probe-type transition is the identification of optimum· parameter combinations for achieving the lowest possible VSWR across the operating frequency band of the waveguide, for which the transition can be regarded as broadband matched. In the case of the end launcher-type, the transition's VSWR performance is considerably poorer than the probe-type, and a strategy for narrowband matching the transition is proposed. For the reactive obstacles, their reactive effects are modelled by using a T-equivalent circuit. The normalised reactances as a function of various parameters are characterised. An in-depth investigation is carried out for variable-height reactive posts, for which a comprehensive data set is compiled for the normalised reactances, together with a study of current distribution on the post. A limited amount of data is also gathered for the normalised reactances of irises. Finally, an enhanced design for the probe-type coaxial line-to-circular waveguide transition incorporating two reactive posts is proposed. This design has the benefit of being adjustable, which not only allows the transition performance to be adjusted after manufacture but also permits any additional small reflections to be effectively tuned out. The simplicity and practicability of the strategy proposed makes the design suitable for both one-off and mass production purposes. The design data and strategies presented in this thesis may form a useful resource for microwave engineers in designing circular waveguide applications.