Compact bandstop filter using signal interference technique

A signal interference technique is proposed to design a compact wideband bandstop filter (BSF). Two parallel transmission lines of different lengths and characteristic impedances are used. The signal propagates through these two paths and interferes at the other end with different phases and magnitudes. By varying these phases and magnitudes two finite transmission zeros can be obtained. To improve the stopband rejection level another zero is obtained using an open-circuited quarter-wavelength stub at the stopband centre frequency (f_c). One single section prototype fabricated BSF of f_c =0.449 GHz has 3 and 40 dB fractional bandwidths of 50.6 and 22.3%, respectively. The filter implementation area is 0.116lambda_gtimes0.163lambda_g, lambda_g being the guided wavelength at f_c