Precise dielectric measurements at 35 GHz using an open microwave resonator

The use of an open microwave resonator for dielectric measurements is studied. The resonator is of the hemispherical type and consists of one plane and one concave copper mirror. The sample is a plane parallel sheet of lateral dimensions exceeding 50 mm and is placed on the plane mirror. Measurements on unsintered polytetrafluoroethylene, high-density polyethylene and TPX show that for materials with loss angles in the range 50-500 micron rad the loss can be measured with standard deviation of plus or minus 2% + 1 micron rad. The standard deviation for permittivity measurements is about plus or minus 0.1%. A full discussion of the possible sources of error is given.