A novel compact directional coupler with high directivity based on CRLH transmission lines

Designing and fabricating a directional coupler with composite right/left-hand (CRLH) transmission lines with a coupling level of 15 dB is presented in this paper. Compared to conventional couplers, this asymmetric structure has achieved this coupling level compared to the simple transmission line. This structure is comprised of interdigital capacitors and short connection stubs. The distance between two coupled lines is equal to 0.4 mm, and the stubs are spirally designed to compress the structure. The subs’ length is approximately equal to λ/4, and the impedance of the ports is equal to 50 Ω. After designing and simulating, the coupler was fabricated and measured. The directional coupler at the frequency of 1.9 GHz has an isolation level of 44 dB and a directivity of 67.1 dB. Overall, construction and simulation results are in good agreement.