Design of Dual-Band Microstrip Rat Race Coupler with Circuit Miniaturization

Rat race ring coupler with stepped-impedance sections is designed to have a dual-band characteristic. This is realized by attaching open stubs to both ends of each stepped-impedance section to create a phase of 90deg at two designated frequencies. Transcendental equations are derived for solving impedance ratio R and lengths of the stepped-impedance sections. Circuit with miniaturized area can be obtained if R > 1 is chosen. Such microstrip ring hybrids operating at 2.45/5.2 GHz with R = 1 and R = 4 are designed and fabricated. Experiments are performed to confirm the theory.