An Investigation of Open- and Short-Ended Resonators and Their Applications to Bandpass Filters

In this paper, we investigate both open- and short-ended half-wavelength microstrip resonators and their applications to bandpass filter designs. Using voltage-wave analyses, we examine the resonance and coupling properties of these resonators at their fundamental and harmonic frequencies. Loading them with a lumped-circuit element at the locations of zero-voltage point of the second harmonic allows control of fundamental frequency response without altering that of the second harmonic. A resistive loading can suppress the second harmonic while a p-i-n diode loading yield a switchable bandpass filter. The coupling regions between an open-ended resonator and a short-ended resonator of higher order bandpass filters can be chosen such that second-, third-, and fourth-harmonic suppression, as well as dual passband, can be achieved. Simulation and experimental results agree well, validating the proposed bandpass filter design methodology.