Design of a novel tri-band rat race coupler

As the conventional rat race coupler can only operate at the fundamental frequency and may cause odd harmonics, the design method of a novel tri-band rat race coupler is presented. In this method, the quarter-wavelength impedance transformers in the conventional rat race coupler are replaced by the Π-shaped stepped impedance ones (ΠSITs) with four circuit topologies, which enables the proposed coupler to operate at two arbitrary frequencies and their center frequency. Then, by taking into consideration the practical realization range of transmission line impedance (20~150 Ω), the ratio of the third operating frequency to the first one of the proposed coupler is selected from 1.70 to 4.10 and from 6.60 to 12.30. Moreover, the corresponding analytic design equations are deduced, and two microstrip 3-dB tri-band rat race couplers based on different ΠSIT circuit topologies, which respectively operate at 2.0/3.9/5.8 GHz and 0.9/3.5/6.1 GHz, are designed and fabricated. Simulated results accord well with the measured ones, meaning that the proposed design method is feasible.