Reflection-Mode Bandstop Filters With Minimum Through-Line Length

This paper presents a technique that enables the design of high-order microwave bandstop filters with a total through-line length of significantly less than one quarter-wavelength at the filter center frequency. The total through-line length using this technique can be zero regardless of the filter order in theory, but it is limited in practice by the finite line length required to obtain a desired coupling magnitude between the through line and a resonator. A fifth-order quasi-elliptic reflection-mode bandstop filter design is shown with total through-line length of less than 1/15th of a wavelength in suspended-stripline technology. The measured response has a 42-MHz 30-dB bandwidth at a center frequency of 3 GHz and four reflection zeros. The bandstop filter design technique described in this paper shows great promise for reducing the size and weight of microwave bandstop filters.