Extended‐stopband microstrip lowpass filter using rat‐race directional couplers

A class of microstrip lowpass filter (LPF) with a sharp‐rejection, ultra‐broad stopband is reported. It employs rat‐race directional couplers properly arranged to operate as bandstop transversal filtering sections (TFSs). The theoretical foundations of this LPF approach are described, and the performance control of its building bandstop TFS is verified. Moreover, a proof‐of‐concept microstrip LPF prototype with a 3 dB cutoff frequency of 1 GHz and a 20 dB stopband up to 5.86 GHz has been built and tested.