Discriminating coupling and its application to bandpass filters

This paper presents the discriminating coupling and its application to bandpass filter. According to the voltage current characteristic of the transmission lines, the fundamental mode or high-order mode can be suppressed by selecting suitable coupling region, whereas the coupling coefficient at desired frequency can be tuned to desired values. Therefore, it is easy to design wide-stopband or oversize bandpass filters. Furthermore, no additional circuit is needed. For demonstration purpose, two planar microstrip filters, and an oversize millimeter-wave filter are presented. The results are presented to verify the proposed method.