To lay out an ultra-wide band (uwb) microwave sensor system for inline determination of moisture and feed rate in forage harvesters during harvest process, engineers need to identify the optimal specification and configuration of the system, e. g. regarding spectral range and spectral resolution. For a better understanding of the complex interactions of electromagnetic wave propagation in the crop channel of harvesters, a simplified model for estimating the permittivity of the crop material was derived from literature capable to model the effects of disturbing parameters like density and temperature, mainly affected by water. Utilizing the transfer-matrix method, the sensor system was modeled in a transmission configuration and validated based on lab test data. A sufficient match of the results is demonstrated. That enables to investigate, understand, and consider sensor setups and influences of varied main parameters with that model.