Calibration methods for microwave free space measurements

In this article calibration methods for the precise, contact-less measurement of the permittivity, permeability or humidity of materials are presented. The free space measurement system principally consists of a pair of focusing horn-lens antennas connected to the ports of a vector network analyzer. Based on the measured scattering parameters, the dielectric material parameters are calculable. Due to systematic errors as e.g. transmission losses of the cables or mismatches of the antennas, a calibration of the measurement setup is necessary. For this purpose calibration methods with calibration standards of equal mechanical lengths are presented. They have the advantage, that the measurement setup can be kept in a fixed position, for example no displacement of the antennas is needed. The presented self-calibration methods have in common that the calibration structures consist of a so-called obstacle network which can be partly unknown. The obstacle can either be realized as a transmissive or a reflective network depending on the chosen method. An increase of the frequency bandwidth is achievable with the reflective realization. The theory of the calibration methods and some experimental results will be presented.