Variable Directional Coupler Employing Microfluidics

This paper details a novel variable directional coupler based on a broadside coupled stripline structure employing microfluidics. The coupler has shown good bandwidth of 7.5-12.5 GHz with only a single quarter wavelength section. The variable coupling has been achieved by creating chambers in the dielectric material between the two coupled strips. The chambers have the same length and width as the coupled strips. Conductive fluid is pumped in and out of each chamber leading to a change in coupling between the broadside coupled strips. Conductive fluids like the solution of ammonium chloride or potassium chloride have been used to change the conductivity of the chamber which provides the coupling variation. The chambers are isolated from each other by thin membranes. The fabrication is compatible with MIC techniques. A coupling variation of 24 dB to 11 dB has been obtained. The return loss characteristics at all ports are better than 15 dB.