Compact UWB BPF with triple notched bands using spiral defected ground structure

A compact ultra-wideband (UWB) bandpass filter (BPF) with triple sharply notched bands and sharp rejection shirts using spiral defected ground structure (DGS) is proposed. The filter comprises two aperture-backed interdigital coupled lines and a modified compact multiple-mode resonator (MMR) which is formed by a new triple-mode resonator and a stepped-impedance open-end stub in the centre. The modified MMR can generate two odd modes and three even modes in the desired band. Besides, the stepped-impedance open-end stub creates two transmission zeros on both sides of the passband, thus improving skirt selectivity. Then, three spiral DGSs are introduced, achieving triple sharply notched bands and providing a high degree of freedom for adjusting. To validate the design theory, a microstrip UWB BPF with three notched bands respectively centred at 5.2, 5.8 and 8.0 GHz is designed and fabricated. The simulated and measured results are in good agreement.