Dual Antipodal Vivaldi Antenna-Based Moisture Sensor for Industrial Process Control

Moisture measurement in industrial processes is of great importance for quality control and energy saving. One possible method for moisture measurement is microwave transmission using two antennas, where the material moisture is determined based on the permittivity. However, multiple reflections between the antennas can distort and misrepresent the measurements. In this context, this article deals with the design of tapered slot antennas (TSAs) specifically developed for moisture measurements in natural materials such as wood chips or paper. Design aspects of the antenna as a density-independent moisture sensor with respect to improved selectivity to water content in a composite natural material are explained and requirements concerning frequency range and bandwidth are given. In particular, the advantages of specially designed dual antipodal Vivaldi antennas are compared to those antenna types, which are usually used for such applications, namely horn antennas. Design aspects are discussed and new approaches are proposed to improve manufacturability by printing the antenna on a flexible substrate and wrapping it around a block of low-loss foam. A manufactured antenna is experimentally fully characterized by measurements. Low reflectivity at the free spaceports is verified, and its superiority in comparison to horn antennas is documented.