A Second Harmonic Separation Symmetric Ports 180° Coupler with Arbitrary Coupling Ratio and Transparent Terminations
In this paper, a novel microstrip line symmetric ports 180° coupler is presented to feature arbitrary coupling ratio, transparent termination impedance and second harmonic frequency separation. The proposed coupler is composed of two quadrature hybrid couplers and two phase shifters. The analytical design equations are derived to determine the electrical length of two delay lines which control the coupling ratio and absolute phase delay of proposed coupler. To verify our design concept, six different types of couplers with 1/2 and 1/3 coupling ratio and termination impedance of 25 Ω, 50 Ω and 100 Ω are designed, fabricated and measured. Both simulation and measurement results are in good agreement to further demonstrated proposed design theory.