Miniaturisation method for coupled-line bandpass filters with identical and minimal number of reactive elements

This study presents an improved method of reactive loading for miniaturisation of coupled-line filters. Compared to the previous method, the proposed method requires a reduced number of reactive elements. Moreover, miniaturisation is accomplished with equal reactance. Effectively, this further decreases the number of reactive elements. Therefore apart from improved loss performance because of the reduced number of low-Q lumped elements, low-cost fabrication and simpler design procedure are also achieved while maintaining a similar rate of miniaturisation. A complete set of design equations is provided for both shunt-capacitive and series-inductive loading. The ideal simulation results, which are in excellent agreement with those of conventional counterparts based on λ/4 lines, theoretically validate the method, whereas the experimental results that match the ideal results better than the previous method demonstrate the practicality of the proposed method.