A spurious suppression of microstrip quarter-wavelength resonator BPF

A new spurious suppression technique is applied for a line-coupled microstrip quarter-wavelength BPF. By folding the adjacent resonators at a proper length, the coupling of 2^nd higher mode is suppressed while that of the fundamental mode is kept reasonably high. It makes the 1^st spurious band suppressed whose center frequency is 3 times of that for the fundamental pass band. As a result, the BPF could extend the range of spurious suppression up to 5 times of the fundamental band.