Design of LPF using suspended substrate microstrip lines as high-Z sections for stopband extension

This article introduces the use of suspended substrate microstrip lines (SSMLs) as high-Z sections in the design of lowpass filters (LPFs) to extend the stopband. The SSMLs are formed by etching slots on the backside of microstrip lines and placing the substrate on an aluminum base with grooves that cover these slots. The higher characteristic impedance property of SSMLs brings about not only shorter length for high-Z lines but also a larger high-to-low impedance ratio, thus deepening the stopband suppression and shifting the spurious passband away. As a demonstration, a wide stopband LPF combining this structure with hammer-shaped stubs is designed, fabricated, and measured. The 20 dB stopband is obtained from 1.45 to 11.77 GHz. Compared with reported LPFs, this shows the widest fractional bandwidth (FBW) of 156% without sacrificing other performances. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1204–1207, 2016