Hepta-band bandpass filter based on folded cross-loaded stepped impedance resonator

A hepta-band bandpass filter (HB-BPF) based on folded cross-loaded stepped impedance resonator (SIR) was investigated. A cross-loaded SIR microstrip structure was arranged to produce several transmission zeros. In order to reduce the filter size, a folded cross-loaded SIR was proposed. The HB-BPF was designed on FR4 microstrip substrate with epsilon(r) = 4.4, thickness h = 0.8 mm, and tan delta = 0.0265. This HB-BPF achieves transmission coefficients of 0.15, 0.61, 0.45, 1.14, 1.57, 1.77, and 2.08 dB at 0.74, 1.49, 2.25, 3.19, 3.93, 4.68, and 5.50 GHz, respectively. The HB-BPF only occupies 0.1 x 0.22 lambda g, where lambda g is the guided wavelength at a first passband centre frequency. A good agreement between simulated and measured results validates the design method.