This paper presents a novel design approach to design in-line microstrip bandstop filter with accurate design theory and sharp skirt selectivity. This kind of bandstop filter is based on a simple coupled-line structure, indicating compact and flexible circuit layout for microstrip implementation. For a single-section bandstop filter, the scattering parameters and their constrain conditions are achieved, which provides an effective design guide for multi-section bandstop filters. Theoretical analysis indicates that the even-mode and odd-mode characteristic impedances can be easily used to determine the desired bandstop performance while the total circuit layout keeps very compact. For demonstration, seven numerical examples are designed, calculated, and compared. Finally, both experimental and simulation results of a two-section two-cell microstrip bandstop filter operating at 1 GHz are presented to verify the theoretical predications.
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