Precise dielectric measurements at 35 GHz using an open microwave resonator

An investigation has been carried out into the use of an open microwave resonator for dielectric measurements. 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 ¿ 50 mm and is placed on the plane mirror. The theory has been extended to cover this type of resonator. Measurements on unsintered polytetrafluoroethylene, high-density polyethylene and TPX show that for materials with loss angles in the range 50¿500 ¿rad the loss can be measured with standard deviation of ± 2% + 1¿rad. The standard deviation for permittivity measurements is about ± 0.1%. A full discussion of the possible sources of error is given.