The vector network analyzer readily lends itself to a variety of techniques for measuring material properties. The open-ended coaxial probe is one technique that allows swept frequency measurements of permittivity. The open-ended coaxial probe technique requires little or no sample preparation and can make non-destructive measurements on a variety of materials. It is especially suited for materials in liquid or semi-solid form. An understanding of network analyzer calibration and the coaxial probe model provides insight into calibration techniques that minimize systematic errors. This understanding also.facilitates the extension of measurement conditions beyond normal measurement constraints leading to a simple procedure which provides a first order correction of perturbations to the systematic errors.