Design of branch line coupler loaded capacitively with interdigitated fingers by using microstrip fed coplanar structures (L-shaped conductor backed asymmetric CPS and U-shaped conductor backed CPW)

Abstract

In this letter, L-shaped conductor backed asymmetric coplanar stripline (LSCBACPS) and U-shaped conductor backed coplanar waveguide (USCBCPW) branch line coupler fed by microstrip transmission line have been investigated for the miniaturization. The number of interdigital finger unit cells located at the branch and through lines of the conventional branch line coupler is used to provide a desired miniaturization. The proposed branch line couplers in which all ports are formed by microstrip line have been constructed in two different configurations depending on the placement of the capacitively loaded fingers. The effects of the change of the cell number located at the branch and through lines on the center frequency and the propagation velocity are investigated. These effects are expressed by a simple formula depends on the cell number to use in the theoretical model obtained from the even-odd mode analysis of LSCBACPS and USCBCPW configurations. Proposed couplers are designed, fabricated, and tested for the experimental verification to compare with the simulation and theoretical results. Measured, theoretical, and simulation results are in a very good agreement. © 2012 Wiley Periodicals, Inc.