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This paper presents a novel wideband microstrip bandpass filter with low insertion loss and harmonic suppression. The filter is based on a circuit model for an optimum distributed highpass filter consisting of only 3 short-circuited stubs separated by non-redundant connecting lines. In order to reduce the size and to suppress the harmonic passbands of the filter, the uniform connecting lines are replaced by stepped-impedance lines. The design was successfully realized in theory and verified by full-wave EM simulation and the experiment. Theoretical, simulated, and measured results are in excellent agreement. The measurements show low insertion loss of less than 0.5 dB at the center frequency and a flat group delay of less than 0.3 ns over the entire passband.