In this paper, a simple method based on coupling control theory to suppress multiple harmonics in open-loop bandpass filters is proposed. Coupled feeding is utilizing in this paper, and by choosing proper coupling length and position of the input/output port, multiple harmonics could be suppressed. The input impedance of microstrip line with its end open-circuited is firstly showed and the theory on harmonics suppression is then analyzed to choose a proper coupling position. A second-order open-loop bandpass filter is designed and manufactured, measured and manufactured results both have a stopband to 5f0 with rejection more than 20dB. On the base of the filter proposed, the stopband can be broaden to 10f0 by adding two stubs. It well validates the proposed solution. 

This paper presents a method of designing microstrip dual-band bandpass filter fed with lumped capacitors. Unlike the tapping or coupling feeding structures, the lumped capacitors could make the external quality factors have suitable values in two different pass bands. Thus no impedance transformers are needed when designing dual-band bandpass filter fed with lumped capacitors. An example of this kind of filter is designed, fabricated and measured. The measured results accord well with the simulated results, showing validity of the proposed method.