Spurious response suppression in wideband bandpass filters using chirality

Abstract

A simple chiral medium is used to enhance the response of a conventional microstrip coupled line bandpass filter (BPF). The approach is applicable to both narrowband and wideband BPFs using the presented chiral topology. The topology is based on an ultra-subwavelength planar chiral particle that is utilized in the interface between two slabs of substrate. More specifically the medium realizes the suppression of the second spurious response of the filter at a large bandwidth, which enables its practical application in wideband coupled-line based BPFs. The effect of the chiral particles on the substrate's effective permittivity and on the phase velocities of the even and odd mode of two coupled transmission lines on top of the substrate structure is explicitly used in the design of the filter. The methodology is applicable to any coupled-line BPF order. Two wideband microstrip coupled-line prototype filters are designed, fabricated, and measured, one involving a substrate without particles and one involving the chiral substrate. Measurement results are in good agreement with the simulations. The chiral medium based filter results in a wide fractional bandwidth (FBW) of 81% for a 1 GHz center frequency and a measured suppression level of −60 dB.