Ultra-Wideband, Differential-Mode Bandpass Filters with Four Coupled Lines Embedded in Self-Complementary Antennas

A design method for an ultra-wideband bandpass filter (BPF) with four coupled lines has been developed. For demonstration purposes, 50Ω-matched self-complementary antennas integrated with the ultra-wideband, differential-mode BPF with four coupled lines, a notch filter, and a low-pass filter (LPF) were prepared and tested. An optimized structure for a single-stage, broadside-coupled and edge-coupled four-lines BPF was shown to exhibit up to 170% fractional bandwidth and an impedance transformation ratio of 1.2 with little bandwidth reduction, both analytically and experimentally. Using the optimized structure, 6stage BPFs were designed to transform the self-complementary antenna’s constant input impedance (60πε−1/2 e (Ω)) to 50Ω without degrading bandwidth. In addition, two types of filter variations — a LPF-embedded BPF and a notch filter-embedded BPF — were designed and fabricated. The measured insertion loss of both filter systems was less than 2.6 dB over the ultra-wideband (UWB) band from 3.1 GHz to 10.6 GHz. The filter systems were embedded in the wideband self-complementary antennas to reject unnecessary radiation over the next pass band and 5-GHz wireless LAN band. key words: differential mode, antennas, band-pass filters, notch filters, low-pass filters, impedance transformer