Microwave characterization of optically sensitive polymer and application to a tunable antenna

This paper investigates the use of a photoconductive polymer to optically tune a microwave device or antenna. The dielectric properties of a composite (Polymer + Cadmium Sulphide) photoconductive polymer are extracted using microwave reflection measurements on a coplanar waveguide test structure. The material under test was overlaid on an interdigitated capacitor that was fabricated on a transparent glass substrate. The material characterization was done with and without broadband optical illumination. The results obtained from the characterization show that the polymer was photoconductive. An extracted dielectric constant (ε_r) of 4 and loss tangent (tanδ) of 0.004 were obtained without illumination, while the loss tangent increased to 0.11 with illumination and the ε_r value had no measureable change. The properties of the novel photoconductive polymer were used in the theoretical design of an optically tunable dipole antenna for the GSM band (1900 to 2100 MHz). Tunability was achieved by applying the characterized polymer to a slotted segment of the dipole arm, and effectively changing the dipole length by optical illumination.