Small-size planar tunable combline filter using decoupling walls

A class of planar tunable combline filter with compact dimensions is reported. The size decrease is achieved through an original planar coupling-reduction technique, useful for parallel-coupled transmission-line resonator filters. It consists of inserting planar decoupling walls, made up of a transmission-line segment shortened to ground at both extremes, between the adjacent filter resonators. Thus, the spacings between these resonators for a given coupling configuration can be significantly reduced, hence decreasing overall filter size. To prove the concept, simulation results for both stripline and microstrip technologies and measurements of a varactor-tuned microstrip combline filter prototype are given. Other applications of the proposed planar coupling-reduction method are discussed