Compact ultra-wide stopband lowpass filter using transformed stepped impedance hairpin resonator

A compact microstrip lowpass filter (LPF) with ultra-wide stopband using transformed stepped impedance hairpin resonator is proposed. The transformed resonator consists of a stepped impedance hairpin resonator and an embedded hexagon stub loaded coupled-line structure. Without enlarging the size, the embedded structure is introduced to get a broad stopband. A prototype LPF has been simulated, fabricated and measured, and the measurements are in good agreement with simulations. The implemented lowpass filter exhibits an ultra-wide stopband up to 12.01f(c) with a rejection level of 14 dB. In addition, the proposed filter features a size of 0.071.gx 0.103 lambda(g), where lambda(g) is the waveguide length at the cutoff frequency 1.45 GHz.