A novel compact high-performance microstrip 26GHz ultra-wideband (UWB) bandpass filter for vehicle radar systems

A novel compact quasi-millimeter-wave UWB bandpass filter (BPF) with a midband frequency of 26 GHz and a fractional bandwidth of 20% is developed for automotive radar systems. Based on the even- and odd-mode method, analytical closed-form formulas are derived first for accurate design of two microstrip stubs-loaded multi-mode ring resonators with pre-designated bandwidths and multiple transmission zeros in their stopbands. Then a UWB BPF is configured by cascading these two ring resonators and a parallel-coupled step-impedance resonator. The filter exhibits desired wide passband with a minimum measured insertion loss of 1.8dB, sharp attenuations near the passband, and very wide stopband with measured attenuations larger than 30dB over 10-40GHz. The Federal Communications Commission's UWB indoor limit is satisfied quite well.