Microstrip "wiggly-line" bandpass filters with multispurious rejection

A method to achieve the rejection of multiple spurious passbands in parallel-coupled-line microstrip bandpass filters is proposed. As it was previously demonstrated by the authors, using a continuous perturbation of the width of the coupled-lines following a sinusoidal law, the wave impedance can be modulated so that the first undesired harmonic passband of the filter is rejected, while the desired passband is maintained virtually unaltered. In this letter, the scope of the method is extended to reject multiple spurious passbands by employing different periods in each coupled-line section tuned to the different bands to be rejected. Simulated and measured data show that for an order-seven bandpass filter prototype, a rejection level exceeding 30 dB is obtained in the first four spurious passbands, while the desired pass-band is kept almost unaltered.