Miniaturized Wide-Band Rat-Race Coupler

New designs of wide-band rat-race couplers are proposed. The wide-band operation is achieved with the use of the microstrip nonuniform transmission line sections for the branches of the conventional rat-race coupler. The design formulas are developed using ABCD matrix and the even- and odd-mode analysis. The theoretical analysis has been verified by measurements of the two manufactured wideband rat-race couplers, one operate within 0.85-1.92 GHz and other within 1.55-3.55 GHz frequency range with the equal normalized characteristic impedance functions. For both fabricated couplers, the isolation parameter is better than 15 dB over a 77% relative bandwidth. Also, it is shown that the designed wide-band rat-race coupler can be realized in higher frequency bands with the fixed fractional bandwidth. (c) 2012 Wiley Periodicals, Inc. Int J RF and Microwave CAE 23: 675-681, 2013.