NOVEL SYMMETRICAL COUPLED-LINE DIRECTIONAL COUPLER BASED ON RESONANT-TYPE COMPOSITE RIGHT-/LEFT-HANDED TRANSMISSION LINES

A novel kind of symmetrical backward-wave coupled-line coupler with arbitrary coupling level is proposed in this paper which is based on resonant-type composite right-/left-handed transmission lines (CRLH TLs). First, an equivalent circuit model and procedure for circuit parameters extraction are presented to reveal the inherent nature of the unit cell of the CRLH coupler. Then a CRLH TL composed of four cascaded unit cells is demonstrated to point out the way to achieve balanced condition. At last, even/odd modes analysis based on full-wave simulation is employed to explain the operating principle of the coupler. Both quasi 0-dB and 3-dB CRLH couplers are demonstrated experimentally. The quasi 0-dB backward coupling is achieved over the range from 1.69 GHz to 2.19 GHz (-3 dB bandwidth in measurement), which represents the fractional bandwidth 25.8%. The maximum coupling coefficient 0.52 dB is obtained at 1.96 GHz, where the directivity and isolation is 20.8 dB and 21.3 dB, respectively. The 3-dB couplers shows an amplitude balance of 2 dB and quadrature phase balance of 90±5 degree over the fractional bandwidth of around 11.4%, from 1.99 to 2.23 GHz.