Abstract:This paper proposes a new method to implement change of polarity of cross-coupling in microstrip cross-coupling filter and cavity cross-coupling filter by the use of transmission line. To realize the cross-coupling between the source and a nonadjacent resonator by using a transmission line with an electrical length of 180°in microstrip cross-coupling filter, can make the polarity of cross-coupling change from capacitive to inductive. Using a transmission line with an electrical length about 180°in cavity cross-coupling filter, transmission zero can be shifted from upper stopband to lower stopband. A discussion of the effects of different electrical length transmission line on cross-coupling is presented. The positions of transmission zeros can be controlled by adjusting the lengths of transmission lines to improve the rejection level. With this proposed method, transmission zeros can be positioned arbitrarily using the same coupling structure, which makes the filter design more flexible.