Directivity Enhancement of Tight Couplers

Abstract

An enhancement of 22 dB in directivity, and 15 dB in return loss was achieved in an improved wiggly-line coupler. The improvement was obtained by introducing two new degrees of freedom to the wiggly-line. Heat maps of coupling, directivity and return loss, as a function of the new degrees of freedom, show previously unavailable fine-tuning capabilities, from which optimal parameters were derived. The use of the improved wiggly-line allows a better even- and odd-mode phase velocity compensation, and an improvement of 42% of even- and odd-mode velocity difference is shown. The improved wiggly-line concept is used to design a tight, tandem, 1.6dB coupler as a center stage of 3-stage symmetrical wideband, 6-18 GHz, 90- degree hybrid coupler implemented in microstrip coupled line topology using bond wires for the tandem section. The loose stage is designed using periodic, quasi-lumped, capacitively-loaded coupled lines. A CST simulation of each design step is shown.