Broadband asymmetric high isolation directional coupler using quadrature mode suppression

In this paper, the design of a novel asymmetric directional coupler is proposed. This coupler is designed to obtain uniformly tight coupling and high isolation. The coupler uses a divider-combiner configuration employing quadrature mode suppression to obtain high isolation. The design makes use of five section branch line couplers and two-section power combiners to obtain broad bandwidth for coupling, return loss and isolation. The proposed five-branch coupler is a new configuration which offers unequal power split to overcome the problem of large branch impedances associated with multiple branch couplers. The proposed asymmetric coupler has coupling better than 6 dB over the frequency range from 1.08 to 2.96 GHz, and isolation better than 25dB from 1.03 to 3.27GHz. Its directivity is above 20 dB from 1.05 to 3.17 GHz. For load sensing, this coupler offers a dynamic range better than 20 dB, from 1.26 to 2.52 GHz. This coupler is fabricated in single layer microstrip technique and on low cost FR4 substrate. High directivity over a broad bandwidth and low cost makes it suitable for several small signal transmit/receive applications.