A Compact Dual-Band Coupler With Coupled Microstrip–Slotlines

In this letter, a novel dual-band coupler made of only three coupled microstrip–slotline (CMSL) sections is proposed to achieve a tight coupling with a high band ratio in a small size. A desired dual-band operation can be obtained by proper design of the coupling levels of the CMSL sections. Explicit design equations of the proposed coupler were derived by the method of even- and odd-mode excitations, and the characteristics of the CMSL were studied. For demonstration, a 3-dB dual-band CMSL coupler operating at 1 and 3 GHz is designed, fabricated, and measured. The proposed coupler has the advantages of small size, tight coupling, and high isolation.