TABLE OF CONTENTS Radar absorbing materials are based on the fact that some substances absorb energy from electromagnetic fields passing through them. Such materials have indices of refraction that are complex numbers. In the index of refraction, which includes magnetic as well as electric effects, the imaginary component accounts for the loss in a material. The term loss refers to the dissipation of power or energy, quite analogous to the way energy is consumed by a resistor when electrical current passes through it. The loss is actually the conversion of electrical energy into heat, and although most absorbers do not dissipate enough energy to get detectably warm when illuminated by a radar, this is nevertheless the mechanism by which they operate. At microwave frequencies, the loss is due to a number of effects on the atomic and molecular level. However, for most practical electric absorbers a majority of the loss is due to the finite conductivity of the material, whereas for most magnetic absorbers at microwave frequencies, magnetization rotation within the domains is the principal loss mechanism. In any event, it is customary to group the effects of all loss mechanisms into the permittivity 
and permeability ( ?) of the material because the engineer is usually interested only in the cumulative effect.
Several common usages exist for expressing the complex permittivity and permeability. Generally, we shall deal with the relative permittivity, 
, and relative permeability, ? r , which are normalized by the free-space...
