Novel bandpass filter with size reduction and harmonic suppression

A novel microstrip bandpass filter with size reduction and harmonic suppression is proposed in this article. The filter is based on microstrip open loop resonators with coupled open stubs. By adding coupled open stubs to open loop resonator, extra controllable transmission zero can be obtained. The extra transmission zero can be tuned at the second harmonic frequency to suppress spurious response. When compared with the conventional microstrip open-loop bandpass filter, this filter could achieve size reduction and spurious responses suppression simultaneously. A demonstration filter was designed, fabricated, and measured. Theoretical and experimental results are presented and show good agreement. © 2007 Wiley Periodicals, Inc.