Compact ultra-wideband bandstop filter using capacitive loads

A compact UWB band-stop filter (BSF) using capacitive loads is proposed. In contrast to its traditional counterparts, this proposed BSF only consists of two capacitors and a short-ended coupled line (C-section). By introducing capacitors inside the C-section and between two ports, respectively, cross-coupling and source/load coupling are introduced to create up to four transmission zeros. Meanwhile, shunt capacitive effect of loaded capacitors can further reduce the length of C-section. After working principle and design method of this proposed BSF are well-illustrated, a prototype with 20 dB fractional bandwidth of 110% at 4.24 GHz is finally designed and fabricated. It occupies only 0.45λg × 0.061λg, where λg is the guided wavelength at the centre frequency.