Synthesis And Design Of Microwave Dualband Filters

Modern communications systems, especially satellite communcations systems, have a complex frequency and spatial coverage plan. Noncontinuous channels might need to be transmitted through one beam. Therefore, dualband or multiband filters are needed to simplify the whole communcations system.The research work presented in this dissertation focuses on the synthesis theory of microwave dualband filters, including both symmetric and asymmetric cases. Firstly, the basic synthesis theory of cross coupled filters with generalized Chebyshev function is introduced in detail. Secondly, the synthesis methods of dualband filters based on two kinds of frequency transformation techniques are discussed, and an improved method is given. Thirdly, two different methods on asymmetric dualband filters are analysed. The former one makes two bandpass filters have the common input and output ports and introduces a special structure to increase the coupling between source and load. It can improve the attenuation characteristic in the stopband. The latter one employs a frequency transformation technique and then find out the coupling matrix. Finally, the synthesis theory of singleband and dualband filters using extracted-pole technique is presented. Different from the conventional extracted-pole technique, nonresonating nodes are introduced here. The synthesis process is essentially a Darlington approach, requiring to construct the input and output admittances to evaluate the elements of the network.In order to verify the work in this dissertation, some design examples are given and simulated with some electromagnetic softwares. Good simulation results state the validity of these methods.