Compact dual-band rat-race couplers

In this letter, a design of a compact, dual-band, rat-race coupler is presented. The dual-band characteristics are obtained by using two shunt open-stubs with the conventional coupler. The proposed design can cover a wide range of frequency ratio of the dual bands within the fabrication limit of a low and high impedance microstrip line. Frequency variations of the bandwidths of this coupler in terms of isolation, matching, amplitude, and phase imbalance are also discussed. Theoretical prediction has been verified in the microstrip line by fabricating a dual-band coupler at the 900/1900-MHz mobile bands. Experimental results agree well with the theoretical predictions. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 277–281, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23063