A method for measuring the shielding effectiveness of planar anisotropic materials using a rectangular waveguide cell is proposed. Computational models are used to verify the behaviour of the cell and validate its ability to measure shielding effectiveness. Results of measurements on a control sample and three carbon-fibre composite laminates in a GTEM cell are presented. A dynamic range of 70 dB with a capability to discriminate 20 dB of anisotropy is achieved in the frequency range 400-1600 MHz using cubic cells of side length 200 mm and 300 mm
