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We propose a temporal coupled-mode theory for the design of microstrip bandpass filters with multi-resonant modes and reveals the mechanism of the filters based on multi-coupling and energy conservation theorem. As an example, the combined effect of the two orthogonal modes in dual-mode filters is analyzed in detail. By simply tuning the coupling efficiencies of the two resonant modes, two K-band integrated microstrip bandpass filters have been designed and demonstrate opposite asymmetric transmission responses. Both simulation and experiment results agree well with the theoretical analysis, which provides a new approach for filter design and optimization.Index Terms-Coupled-mode theory (CMT), dual resonant modes, microstrip bandpass filter.

In this article, the design and test of both a K-band dual-mode bandpass filter (BPF) pair and a K-band triple-mode BPF are presented based on N-mode temporal coupledmode theory (CMT). The expressions of transmission and reflection responses are analytically derived. All the parameters in the expressions have clear physical meanings and are easily optimized to reach the required filter performance. Aided by eigenmode simulations, concrete structures of the three integrated BPFs are designed and optimized to approach the calculated physical parameters. After the fabrications and measurements of the three BPFs, extended upper/lower stopband with high stopband rejections are achieved, and by increasing the number of resonant modes, improved frequency selectivity and better passband flatness are obtained. The analytical analysis well predicts the simulation and measurement results, which provides an efficient way for BPF designs. V C 2016 Wiley Periodicals, Inc. Int J RF and Microwave CAE 00:000-000, 2016.