In this paper, a technique for the characterization of the complex permittivity of flat material panels is presented. Using broadband horn antennas and a simple measurement process, the permittivity and dielectric loss tangent of the material under test can be extracted from measured S-Parameters. Measurement data for three different material panels up to 67 GHz is given. Emphasis is put on the characterization of concrete as common building material, which is usually not easily characterizable due to mechanical properties of the material itself (i.e. brittleness) or the measurement techniques available.