Parallel ring-line rat-race circuit with very loose coupling utilizing short-circuited coupled transmission lines

This paper describes a novel rat-race circuit with very loose coupling less than −10dB utilizing parallel ring-lines composed of a quarter wavelength transmission line and a short-circuited quarter wavelength coupled one. For a conventional rat-race circuit, the minimum coupling factor is limited by the highest impedance of the ring-line which can be manufactured by a general PCB technology. The proposed rat-race circuit with parallel ring-lines can be realized the circuit with a very loose coupling factor of less than −10dB. By using the proposed structure, we can successfully design the compact rat-race circuit with a coupling factor of −20dB. The validity of this design procedure is confirmed by carrying out electromagnetic simulation and experiment. The fabricated −20dB rat-race circuit is actually realized −33dB reflection, −40dB isolation, and −21dB coupling at a center frequency of 4GHz, respectively.