Effect of film thickness on the radiation efficiency of a 4.5 GHz polypyrrole conducting polymer patch antenna

Microstrip antenna performance is affected by the conductivity and thickness of the patch. Generally the patch thickness is expected to be at least a couple of skin depths thick for reasonable antenna performance. However, planar microwave antennas based on conducting polymers (CP) can exhibit reasonable antenna performance despite a relatively low conductivity and a thickness below a skin depth. In this paper, a detailed study on the effect of CP patch thickness on overall antenna performance is presented. Microstrip antennas for 4.5 GHz operation were fabricated on a 3.2 mm thick FR-4 substrate using polypyrrole patches with a constant electrical conductivity of 2000 S/m. The antennas were identical apart from using four different patch thicknesses ranging from 40 to 140 μm. In all cases, the thickness of the patches used in this study is less than one skin depth (168 μm). The gain and radiation efficiency of the antennas was measured. The lowest gain observed was 2.42 dB for 40 μm thickness with a radiation efficiency of 38%, and the highest gain observed was 4.63 dB for 140 μm thickness with a radiation efficiency of 65%. Results presented in this paper clearly indicate that it is possible to obtain reasonable antenna performance, even if the patch thickness is a fraction of skin depth.