A new band-stop filter design with triple electric paths for passband extension

In this paper, a band-stop filter (BSF) is presented with triple electric paths formed by the transformed radial stub (TRS) and phase inverter line (PIL). The filter topology is theoretically investigated for bandwidth extension with proposed equivalent circuit model. To validate the design concept, a BSF prototype with central frequency f_c of 1.6 GHz is designed and implemented. The measured results show that the filter features a fractional bandwidth of 94.3%. Also it demonstrates ultra-wide passband up to 19.24 GHz (12fc), insertion loss of 1.69 dB including the I/O connectors, and a compact size of 0.3λg × 0.3λg (λg is the guide wavelength at central frequency) with zero lumped elements applied.