Effects of walls on coupling between coupled combline resonators

The effects of the open-side end-wall and ground walls (planes) on the coupling between two identical combline resonators coupled through slots are analysed. It is shown that the electric coupling, which is necessary to realise filters with transmission zeros, can be achieved only when the ground planes are sufficiently close to the resonator post while the open-side end-wall is sufficiently far from it. A criterion for choosing the wall distances from the post is presented. Also, it is shown that the tuning screw (which is mounted on the end-wall) significantly reduces the electric coupling only when it is very close to the post