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Metamaterials often have sharp resonances in permittivity or permeability at microwave frequencies. The sizes of the inclusions are of the order of millimeters, and this means that it is more convenient to carry out the measurement in free space. Time gating is often used in the free-space method to remove multiple scattering from the antennas and the surrounding objects. However, this lowers the resolution in the frequency domain, making it difficult to resolve the resonances reliably. Diffraction around the sample could also reduce measurement accuracy. A calibration procedure, based on the 16-term error model, which removes the need for time gating by correcting for both multiple scattering and diffraction, is developed. This procedure is tested on carbonyl iron composite and split-ring resonators, and the results are presented.