A novel tri-band rat race coupler with T-shape step impedance transformers

A novel tri-band 3-dB rat race coupler operating at two arbitrary frequencies and their mean frequency is presented through replacing quarter-wavelength impedance transformers in a conventional rat race coupler by T-shape step impedance transformers, which can be equivalent to quarter-wavelength impedance transformers at the three frequencies. Closed-form design equations are derived. For verification, a microstrip tri-band 3-dB rat race coupler operating at 1.5/3.5/5.5 GHz is fabricated, aimed to cover the 1.5 GHz GPS band, and the 3.5/5.5 GHz WiMAX bands. Simulated and measured results are in good agreement.