The Research Of Three-dimensional Electromagnetic Coupling Filter

With the rapid development of communication, radar technology, the system with miniaturization and lightweight has been put forward with higher requirements. As the key component for frequency combination and separation, the filter has been required not only the miniaturization and lightweight, but also better selectivity and out-of-band rejection. Based on Generalized Chebyshev filter theory and mixed electromagnetic(EM) coupling theory, this paper provided a series of novel filters with more compact and enhanced out-of-band rejection, such as planar mixed EM coupling filter, multilayer mixed EM coupling filter, and multi-band mixed EM coupling filter.First of all, the basic mathematical model, polynomial and traditional cross coupled matrix of the general Chebyshev filter are analyzed. The frequency-dependent coupling parameters are introduced in the traditional coupling matrix, and then the frequency dependent coupling matrix is obtained, which has been carried out with relevant examples. Owing to the frequency-dependent coupling parameters, more transmission zeros can be realized. The proposed frequency-dependent coupling theory extend the structure type of Generalized Chebyshev filterSecondly, the distributed capacitance denoted for electric-coupling and a short stub denoted for magnetic-coupling, and mixed EM coupling were introduced by adjusting the strength ratios of the electric-coupling and magnetic-coupling. Based on the novel structure, a series of Chebyshev filters with more transmission zeros have been designed. A four-order mixed coupling bandpass filter without cross-coupling and source-/load-coupling were designed, in which three transmission zeros were generated near the passband because of the proposed frequency-dependent parameters; By introducing mixed EM coupling in the conventional cross-coupling filter and box structure filter, more transmission zeros have been obtained near the passband of proposed filter for enhanced frequency selectivity.Thirdly, multi-band bandpass filters using modified SIRs were presented. A novel multi-mode modified SIR was proposed by loading an open stub in the conventional quarter-wavelength SIR. Based on the proposed multi-mode SIR, a variety of mixed EM coupling dual-/tri-band bandpass filters with compact size and high frequency selectivity was designed, fabricated and measured; besides, a modified SIR by using the closed loop line instead of the low-impedance line of conventional SIR was proposed, and a dual-band bandpass filter with compact size and enhanced selectivity was designed and measured. Then a more compact mixed coupled tri-band bandpass filter was presented using the combination structure of two structures mentioned above.Finally, multi-layer mixed EM coupling resonance units were proposed to achieve compact size in further. The proposed multi-layer resonators have a more compact size than the planar resonators. A dual-layer closed loop half-wavelength SIR was designed. Owing to the inside electric-coupling, the proposed multi-layer SIR has a more compact size. Based on the proposed multi-layer SIR, cross-coupling and mixed EM coupling filters have the advantages of compact structure, good frequency selectivity, wide stopband bandwidth, and this kind of filter could use ordinary printed circuit board manufacturing process which is convenient, simple and easy to manufacture, and is beneficial to popularizing and applying filters.