Compact parallel-coupled microstrip diplexers with good stopband rejection

In this work, compact microstrip diplexer designs based on two parallel-coupled bandpass filters are presented. By the proper adjustment of line lengths of the coupled-line sections, the phase of the input reflection coefficient for parallel-coupled bandpass filter can be adjusted to the desired value so as to minimize the interaction between the two bandpass filters. The diplexer may then be easily realized by the direct connection of two bandpass filters in parallel with no further fine-tuning required. This leads to a simpler design procedure as well as more compact circuit size. In addition, good stopband rejection can be obtained. Specifically, two microstrip diplexer design examples with passband center frequencies (F₀₁, F₀₂) = (2 GHz, 2.8 GHz) and (2 GHz, 2.32 GHz) are presented to validate the effectiveness of proposed design method.