Electromagnetic Field Analysis And Simulation Of Microwave Planar Transmission Line Discontinuities

Planar transmission lines are commonly used in microwave integrated circuit and high speeddigital circuits and their discontinuities are always complex problems in theoretical research andengineering applications. There are some conventional types of planar transmission lines:microstrip, coplanar waveguide (CPW), stripline, slotline and so on. The microstrip is actually themost widely employed. The transmission line discontinuities should be analyzed by means of thenumerical method due to their irregular shapes. The finite differential time domain (FDTD) is oneof the most effective electromagnetic numerical methods. Because of its simple, robust nature andits ability to incorporate a broad range of nonlinear materials and devices, FDTD is useful for awide range of applications such as antenna design, microwave circuits, bio/EM effects, EMC/EMIproblems and photonics.Typical discontinuities in various planar transmission lines, mainly in microstrip, are analyzedby using the electromagnetic simulation software which was developed especially for transmissionline discontinuities based on FDTD method. The electromagnetic characteristics for planartransmission line discontinuities and transitions are studied. The radiation and high order modescaused by discontinuities are preliminarily investigated. The main research contents andinnovations are as follows:(1) The FDTD three-dimensional full-wave electromagnetic simulation software based and itsalgorithm are presented. The software consists of two main modules: one is the objectelectromagnetic modeling and the other is the FDTD solver of Maxwell’s equations ofelectromagnetism. The advantages of the software include simple and robust numerical algorithm,applicable to nearly any geometries,3D visualization, good scalability and so on. The simulationresults, compared with those of commercial EM software and experiment measurements, showthat the software is efficient.(2) The frequency and dispersive characteristics of planar transmission lines’ discontinuities (e.g.bend, step in width, T-junction, cross-junction) are analyzed based on FDTD method. Thecompensation and optimization for typical microstrip discontinuities are discussed in detail. Thediscontinuities’ influence on the microstrip propagation characteristics decay exponentially awayfrom the discontinuities and the quantitative results of are obtained.(3) The transitions of microstrip to other transmission lines including microstrip-coaxial,microstrip-waveguide and microstrip-CPW are investigated. Simulative experiments andparameters’ optimization are made to improve the transmission characteristic of transitions. A newstructure for the microstrip-CPW transition is proposed and shows an improved performanceproperty.(4) By combining the FDTD method with Poynting vector theory, the radiation of microstripbend and stub are preliminarily studied. The results show that the radiation losses of surface waveand spatial wave are obviously increasing with frequency. The reverse method is first proposedto investigate the properties of excitation and decay of the high order modes introduced by themicrostrip step. Some results including the ratio of main modes and high order modes areobtained.