Designing a microstrip bandpass filter with multicoupling stepped impedance resonators

A compact multicoupling microstrip bandpass filter based on microstrip stepped impedance resonator (SIR) characteristics was presented. The principle of second harmonic suppression with a proper impedance ratio was introduced, which improved the rejection at the stopband of the bandpass filter. A microstrip multicoupling SIR bandpass filter with 3.65 GHz center frequency and 3.5% fractional bandwidth was designed and fabricated. The result of simulating the filter showed -10 dB attenuation at the first harmonic center frequency, making the upper skirt deeper and the bandpass response more symmetric than filters without microstrip SIR. The experimental filter was measured and had -3.2 dB insert loss at the central frequency and a bandwidth of approximately 120 MHz. The measured result was consistent with the simulated one.

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