Analysis and Implementation of a Dual-Band Coupled-Line Trans-Directional Coupler

This brief presents, for the first time, the analysis and implementation of a dual-band coupled-line trans-directional (CL-TRD) coupler. It is realized by cascading three periodically capacitor loaded coupled line segments, which are equivalent to a symmetric stepped impedance coupled line with a 180° difference between the even- and odd-mode electrical lengths. Explicit design equations of the proposed coupler were derived using the equivalent approach and the even-odd mode decomposition analysis. For demonstration, a 3-dB CL-TRD coupler operating at 0.9/2.4 GHz was designed. Under the criterion of |S ₁₁ | <; -15 dB and |S ₂₁ | <; -15 dB, the measured relative bandwidths are 37.8% and 10%, respectively, for the lower and upper bands. While for amplitude imbalance <; 1 dB, the relative bandwidths are 35.6% and 8.3%, relatively, with the output ports phase differences of ± 90 ± 5°.