Microwave Transmission Line Discontinuity Analysis: A Foundation for a Fast Microwave Computer Aided Design Program

Abstract

Modern communication systems, in particular those using wireless technology, usually employ a large number of microwave circuits operating at microwave frequencies (0.3 to 30 GHz). Analog cellular phones, for example, contain two microwave filters that operate in the 800 MHz frequency range. Traditionally, microwave circuits were designed using empirical equations to model the circuit elements and the resulting circuits would require hand tuning. However, with the growing demand for wireless communication, accurate design of microwave circuits that no longer require hand tuning became a necessity. Today, advanced numerical methods provide accurate complete electromagnetic, or full-wave, solutions to complicated electromagnetic field theory problems. These methods are available due to the increase of computing power available at low cost and to the extensive research on numerical methods for the design of microwave circuits. However, many of these advanced numerical methods still require significant computational power, so even with today's powerful desktop computers, analysis can still take several hours to complete. Consequently, these methods are not suited for use during the iterative design process. Since computer aided design (CAD) of microwave integrated circuits relies on accurate characterization of discontinuities, analyzing them is a logical first step towards the generation of a microwave computer aided design program. The objective of this research is to develop a set of routines to accurately, quickly and efficiently analyze steps, right angle bends and T-junctions, which are the fundamental building blocks for microwave circuits. The routines are developed to handle both rectangular waveguide H-plane and microstrip discontinuities. The discontinuity analysis method described in this thesis extensively utilizes the mode matching method, which is a numerical method that efficiently and accurately characterizes discontinuities in structures with well defined boundary conditions. The results obtained are verified with published results. As an example of how the method is used for a practical real world problem, a rectangular waveguide H-plane diplexer is designed and analyzed.