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In the focus of this project at the Research Center Jülich was to develop a system, which can be used for non-invasive measurements of plant biomass with a high temporal resolution. The system we have developed is using a microwave resonator. A microwave cavity resonator, microwave electronic module and correlation libraries for different plant species are the main components of this system. The dielectric permittivity is a measure of the extent to which a substance concentrates the electrostatic lines of flux. Imposing a static electric field across a medium, the relative dielectric permittivity denotes the ratio of the amount of electric energy stored in the medium relative to vacuum. The theoretical background of microwaves and microwave resonators is described for example in the work of Zinke & Brunswig (2000). The dielectric properties of water molecules and its fast reorientation in a field were used. Transfer of plant biomass into the resonator results in a reduction of resonance frequency. The differences between empty resonator and measurement of sample material were calculated. Relative and not absolute values to estimate plant biomass development were used (Menzel et al. 2009).
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