TABLE OF CONTENTS Satisfactory broadband RAM performance is predicated on getting the RF energy into the RAM and then providing sufficient loss to absorb the necessary energy within the allowed RAM thickness. These two requirements often conflict, because high-loss materials often have intrinsic impedances much different from that of free space, and thus suffer high front-face reflections.
There are two basic solutions to this dilemma, but each has its drawbacks. The first solution is to taper the loss from the front to the back of the absorber; this method is employed in Jaumann, graded dielectric, and geometric transition absorbers. However, the increased performance obtained by tapering the admittance is accompanied by increased thickness. Typically, such absorbers will be near a half-wavelength in thickness at the lower end of the frequency band over which they provide a 20 dB or greater level of RCSR. Some reduction in thickness can be obtained through use of circuit analog sheets to replace resistive sheets, or through hybrid (e.g., CA and graded dielectric) techniques. Again, a penalty must be paid, this time in terms of design complexity and cost.
The second solution is to employ materials with both a high loss and an instrinsic impedance near that of free space, which implies a material with a high value for both ? and 
. Many practical magnetic materials come close to meeting those requirements, but only over a very limited frequency range because of the highly resonant nature of the permeability. Again, multilayer techniques utilizing different...
