A Parallel-Coupled Microstrip Bandpass Filter With Suppression of Both the 2nd and the 3rd Harmonic Responses

This paper presents a parallel-coupled microstrip bandpass filter with simultaneous suppression of both the 2nd and the 3rd harmonic passband responses. Half-wave open-ended resonators are used to provide transmission zeros for the 2nd and the 3rd harmonics so that the spurious passbands at the 2nd and the 3rd harmonic frequencies are reduced. In addition to placing the resonators, the filter is bent twice by 90 degrees to achieve more compact size and higher suppression of the 3rd harmonic passband. This method can be applied to the conventional parallel-coupled microstrip bandpass filter without any significant change of the conventional filter design