Microstrip open-loop resonator with multispurious suppression

A compact open-loop resonator with multispurious suppression is proposed. When excited symmetrically, the resonator shows two tunable transmission zeros. By adjusting the open-end gap capacitance, one of the zeros is placed near the passband and the other is tuned to collocate with the leading two degenerated higher order resonances, so that the circuit has a sharp transition as well as a wide upper stopband. The experimental circuit shows the first spurious peak occurs at four times the passband frequency (4 f _o), and the measurement shows good agreement with the theoretic prediction.