Compact tri-band bandpass filter based on short stubs and crossed open stubs

A compact tri-band bandpass filter (BPF) based on two short stubs and crossed open stubs is analyzed and designed in this letter. The crossed open stubs are used to replace the quarter-wavelength connection line of the conventional quarter-wavelength short stub bandpass filter (BPF), two transmission zeros can be controlled easily by appropriately adjusting the lengths and impedances of the crossed open stubs. Three passbands are designed to operate at 1.8 GHz, 3.5 GHz and 5.45 GHz, respectively. The passband can be tuned effectively by changing the locations of the transmission zeros created by the crossed open stubs. A tri-band bandpass filter is designed and fabricated for demonstration. Good agreement can be observed between the measured and simulated results, indicating the validity of the design strategies.