The rapid development of industrial automation processes has created a growing need for improved sensors for process monitoring and control. With the introduction of ISO 9000, fast and accurate measurements of dielectric properties of materials have grown to a great importance in different applications of industry and agricultural engineering. Microwave sensors, which allow determining the dielectric properties of materials, have found industrial and scientific applications in the non-destructive testing of materials, identification of surface defects, spectroscopy, medical diagnosis, etc. When an electric field is applied across a dielectric material, the atomic and molecular charges in the dielectric are displaced from their equilibrium positions and the material is said to be polarized. Microwave sensors are based on the fact that the interaction between microwaves and the medium of propagation is completely determined by the medium dielectric properties. Microwave sensors are not sensitive to common industrial ambient conditions (dust, vibration, etc.) or static charges.