Stray field ring resonators and a novel trough guide resonator for precise microwave moisture and density measurements

The paper first describes the design and construction of two different kinds of quasi-planar stray field ring resonators for microwave moisture and density measurements, which are typical for the state of the art, and then the practical performance of such sensors. The changes in resonant frequency and resonator bandwidth are utilized for the determination of the moisture content or the material's density. Nonlinear temperature behaviour is identified as a critical property of conventional stray field resonators, which restricts their measurement accuracy. A novel trough guide resonator is then presented which is entirely manufactured from a ring of alumina oxide ceramics and overcomes the temperature problems. The ring is metallized at its surface except at the front face, which permits a stray field to come into contact with the material under test. The described resonator is capable of withstanding considerable abrasive forces without having a protective cover. At the same time it shows excellent linear and predictive temperature behaviour, which allows precise compensation of the inherent temperature dependence of the resonator. Under constant environmental conditions, the sensibility and accuracy of the old and new resonator designs are exactly the same. However, at varying temperature, an error can be removed to a much higher extent for the new resonators, restricting deviations in the moisture content caused by temperature to typically less than 0.1%, as compared to errors of up to several per cent for the previous designs over a temperature range of similar to 20 - 50 degrees C.