An Asymmetric 2.4 GHz Directional Coupler Using Electrical Balance

The concept of electrical balance is used to achieve wideband isolation in an asymmetric 2.4 GHz directional coupler. An intuitive approach using impedance transformation property of transmission lines is used to develop the theory for designing arbitrary-coupling-ratio directional couplers. A coupler based on this theory is designed and fabricated on a two-layered RO4003C PCB. The coupler achieves a return loss of better than 10 dB over a 1.7-2.7 GHz range. The transmission coefficients of the strongly and weakly coupled ports are around -3.5 dB and -5 dB respectively, at 2.4 GHz. The isolation is better than 25 dB over a 1.4-3 GHz range.