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A two-iris waveguide-probe technique is introduced for measuring the electromagnetic properties of a lossy conductor-backed material layer. A flanged open-ended rectangular waveguide is applied to the material under test, and the reflected signal is measured under two conditions. The reflection is first measured when the aperture of the waveguide is unobstructed; then, the reflection is measured with an iris placed in the aperture of the guide. These two measurements allow the extraction of both the permittivity and permeability of the material. The theoretical reflection coefficient necessary to perform the extraction is obtained using a rigorous full-wave approach combining a modal expansion in the waveguide and iris regions with a magnetic-field integral equation formed using equivalent currents at the waveguide aperture. The optimum iris size is determined by minimizing the propagated error due to instrumentation uncertainty and by comparing the extracted parameters to those found using a two-thickness method. Measurements of a commercially available magnetic radar-absorbing material demonstrate the feasibility of the two-iris approach.