Radar Absorbent Materials (RAM) have many applications ranging from the design of ultra wide band sinuous and spiral antennas to the design of measurement chambers and stealth vehicles. Following initial work by Rozanov and Brewitt-Taylor on the fundamental limitations of passive RAM, we have made significant contributions to the topic (mostly unpublished) for the UK government (DSTL) and their prime contractors. This included oblique incidence performance, inclusion of multiple FSS (metasurfaces), design methods for maximum theoretical bandwidth and the use of magnetic materials. The theory on fundamental limitations can be advanced to provide constructive design methods capable (in principal) of achieving the fundamental bandwidth limits, at least for non-magnetic materials. The use of magnetic materials requires the inclusion, within the theory, of realistic dispersion for the relative permeability.
Design of optimal RAM requires the recognition that the Rozanov Brewitt-Taylor ultimate performance limit implies that the reflection coefficient of a conductor backed absorber is minimum reflection phase. This implies that the input impedance , as a function of the Laplace variable for angular frequency , has special properties. Let be represented as a rational function to finite (but possibly arbitrarily large) order,
where and are real polynomial functions of subject to the requirements that is a realiseable passive function. The additional requirement that the reflection coefficient is minimum reflection phase is that the polynomial must be Hurwitz stable. At normal incidence, represents the characteristic impedance of free space. This permits the application of filter theory and control theory to devise equivalent circuits which meet the ultimate performance limits. We published one open-literature paper on the topic which provided some basic results (see fssram), also showing how FSS or metasurfaces can be included.
Since the ultimate performance limit features the (real valued) magnetic relative permeability at zero frequency it is clear that the theory must be modified if the relative permeability cannot maintain its zero frequency value over the range of frequencies required by the absorber. This complicates matters and requires us to consider realistic and realiseable forms for the magnetic dispersion of the relative permeability in order to derive modified fundamental limits when a magnetic material is present.