Limitations on the absorbing performance of particular classes of materials

Integral relations play an important role in the theory of frequency-dependent properties of materials. In particular, several useful constraints on the high-frequency performance of materials and devices are derived from the classical Kramers-Kronig (KK) relations. The paper summarizes results available on constraints to the bandwidth to thickness ratio for particular classes of radar absorbing materials, namely, dielectric and magnetic absorbers. It is shown that the bandwidth to thickness ratio of dielectric absorbers is a function of the number of resonances, both dielectric and interference, incorporated in the frequency dependence of the reflectivity within the operating frequency band. The results obtained may be useful in other problems where the integral constraints are available, such as the problem of bandwidth of small antennas.