A stepped waveguide technique for the characterization of conductor-backed absorbing materials

A variety of techniques have been developed for measuring the electromagnetic properties of homogeneous, isotropic materials. Commonly, a sample is excised and placed into a waveguide cross section, and the S-parameters of the system are measured. The well-known Nicolson-Ross-Weir technique is then used to find both the permeability and permittivity of the material in closed form. Certain conditions, however, prevent this approach from being used. Many radar absorbing materials are manufactured with a conductor backing that cannot be removed without potentially altering their electromagnetic properties. The materials are typically quite thin, and cannot be easily increased in thickness. Because of the conductor backing, they cannot be used to fill a waveguide cross section since no transmission would be available. Similarly, since their thickness cannot be increased, a two-thickness reflection technique cannot be employed.