Suppression of Harmonics in Microstrip Filters With Stagger Tuning and Voltage Redistributions

The stopbands of microstrip filters were extended by additional stubs to alter the resonant frequencies of the 2nd and 3rd harmonics, making the resonators stagger tuned. The stubs also change the voltage distributions in the resonators, modifying the coupling coefficients. Equations for estimating the frequency shifts are given, and the operating principle of the voltage redistributions is discussed. Harmonics were suppressed to better than -34 dB in measured filters with 22%, 15%, and 8% bandwidth, with no measurable increase in passband attenuation.