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This paper presents the design and analysis of a high directivity directional coupler using synthetic coupled lines in 0.18-μm CMOS technology. Multi-conductor synthetic lines are used to increase coupling and miniaturize the circuit. The coupler is composed of two interwound spirals. The design is based on coupled-line models. Although the structure is perfect point-symmetric, the trends of directivity and difference of propagation constants are inconsistent with the symmetric coupled-line model due to non-uniformity. By applying a more general asymmetric coupled-line model, the circuit appears to be slightly asymmetric. The peak of directivity and the location of equal c- and π-mode propagation constants coincide. That is, the interwound spirals can be modeled more accurately as asymmetric coupled lines than symmetric ones in the design of high directivity directional couplers. The 24GHz coupler measures 100μm x 200μm (0.008λ0 × 0.016λ0) in chip area without pads. Maximum directivity achieves 28.5dB from simulation and 24.9dB from measurement.