Low insertion loss microstrip dualband lowpass‐bandpass filter with controllable passband frequencies

This letter proposes a novel dualband filter consisting of one lowpass passband and one bandpass passband. The dualband filter is realized using stub‐loaded transmission lines and stepped‐impedance stubs. The key merit of the filter configuration is that the lowpass and bandpass passband frequencies can be flexibly controlled by tuning the impedances and lengths of the transmission lines. Without coupling gap between the transmission lines, the new filter shows low insertion loss and easy for fabrication. To illustrate the concepts, a dualband lowpass‐bandpass filter is designed, fabricated, and measured. The cut‐off frequency of the lowpass filter is selected to be 1 GHz. The central frequency of the bandpass filter is chosen to be 5.2 GHz. Experimental measurements are in good agreement with both simulations and theoretical expectations.