A stripline diplexer with stopband and isolation performances enhanced by metalized holes

A stripline diplexer with both stopband and isolation performances enhanced by metalized holes is presented in this letter. Two stripline filters are designed using folded stepped-impedance resonators (SIRs), and then a T junction is used to combine the two designed stripline filters so that a diplexer is constructed. A stripline to microstrip line transition is studied and used to guide signals from stripline to microstrip line. To suppress unwanted parallel plate mode (PPM) signals so that better stopband and isolation performances could be obtained, metalized holes connecting two ground planes are used between the two bandpass filters. Measured results show that almost larger than 20dB signal suppression enhancement could be obtained for both signal branches at upper stopband, larger than 80dB suppression could be obtained for both signal branches at frequency bands far away from passbands, and isolation is also enhanced due to suppression of unwanted parallel plate mode signals.