Evaluation of losses in microstrip antenna materials

A microstrip antenna consists of a conducting patch etched on a dielectric substrate over a ground plane. To evaluate the antenna efficiency, one must know the losses in the material. The dielectric loss is specified by its loss tangent and the copper by its conductivity (or equivalently by its skin-depth at a given frequency). Usually, they can be determined separately with well-known experimental procedures. However, for the copper-cladded material used in microstrip antennas, the surfaces of both the dielectric slab and copper (20) plates are deliberately roughened and filled with bonding agent in-between for mechanical strength. In this case, the separate measurements for the two materials become meaningless. Many measurements were conducted in this manner, and in most cases good consistent results were obtained. The loss parameters so determined resulted in theoretically computed antenna impedance characteristics in nearly perfect agreement with the measured results. However, the inconsistency in a few other cases suggests that there may be a significant amount of inhomogeneity in the manufacturing process in the material from one part to the other, or from batch to batch.