Miniaturized spurious passband suppression microstrip filter using meandered parallel coupled lines

A novel meandered parallel coupled-line structure is proposed to suppress the first spurious passband of a microstrip bandpass filter. The meandered parallel coupled line can equalize the modal transmission phases at any interested frequency such as first spurious passband frequency due to its dispersive response. Several filters are designed and fabricated on a substrate with a dielectric constant of 10.2 and a thickness of 50 mil (1.27 mm) to demonstrate the validity of the proposed method. The example filters are chosen to be of large fractional bandwidth and with a relatively high dielectric-constant substrate where, in these cases, the first spurious passband near twice of the passband frequency (2f_o) seriously degrades the conventional parallel coupled filter´s upper stopband performance. The upper stopband performance of the proposed filters drastically improves that the measured first spurious passband are suppressed to better than -50 dB for all example filters. Besides, a meandered coupled line not only suppresses the filter´s first passband, but also largely shrinks its length. The characteristics of the proposed structure and the design procedures of the filter are described in detail