Novel Corrugated Coupled-Line Stage with Ideal Frequency Response and Its Application to Bandpass Filter Design with Multi-Harmonic Suppression

Coupled-line stages are designed with corrugated periods to accurately allocate inherent transmission zeros at three leading even harmonics of design frequency, f_o. The corrugation pattern is finely trimmed so that phase velocities of the two eigenmodes can be simultaneously equalized at these frequencies. By decreasing number of coupled periods in a stage, frequencies of the zeros can be arbitrarily scaled up to a certain extent. In this way, a parallel-coupled line bandpass filter with a cascade of several such stages can be synthesized to be free of spurious harmonics up to at least 10 f_o, with aid of tapped input/output coupling scheme. The measured data of the fabricated bandpass filter show a rejection level of better than 30 dB in the upper stopband up to 18 f_o.