Printed high-directivity metamaterial MS/NRI coupled-line coupler for signal monitoring applications

Using the general coupled-mode formulation, it is shown that the interaction between a regular transmission line and a negative-refractive-index (NRI) line leads to complex coupled modes. Specifically the coupling between a regular microstrip (MS) and a NRI line is analyzed and the theory reveals conditions giving rise to high directivity. Experimental results are presented for a fully printed MS/NRI coupled-line coupler exhibiting a coupling level of -27dB, isolation of -72dB, and directivity of 45dB at the design frequency of 2.04GHz. Such low-coupling levels but high-directivity couplers are well suited for signal monitoring and instrumentation applications.