Abstract: A generator (6) directs microwave signals to a waveguide (1) having a linear array of slots (2), and the signals are received by measuring device (5). The local permittivity or moisture content of material adjacent the waveguide (1), such as a stack of timber, may be determined at each slot (2) via regression analysis. The total permittivity or moisture may be determined by averaging measurements obtained for both transmission directions of the waveguide (1). The apparatus may be used for controlling drying parameters, such as drying time, for the material.