Double waveguides are suitable for the measurement-technical determination of the dielectric properties of films. The structure consists of a short rectangular waveguide terminated at both ends, which is fed on both sides with coaxial probe. A decrease of the construction height is implemented in the middle of the waveguide, with simultaneous increase of the width. As a result of the associated increase of the strength of the electrical field, the transmission property of the system reacts particularly sensitively to introduced dielectric materials. The test objects can be routed on the side over non-radiating slots in the waveguide. A change of the films with regard to composition or thickness can be evaluated with the change of the phase characteristic between the ports.