CHEBYSHEV MULTILEVEL ABSORBER DESIGN CONCEPT

Pyramidal- and wedge-absorber materials are used extensively in anecho ic measurement chambers to attenuate stray signals. Typical absorber l ayouts result in large absorber walls in which the absorber tips and b ases are roughly aligned in the same plane. Such a quasi-periodic conf iguration produces a strong coherent specular reflection which dominat es the absorber scattered field. Based on the multisection impedance t ransformer concept, one can divide absorber elements into different le vels (layers) set that this coherence can be destroyed to reduce the s pecular-absorber scattering level. The synthesis of this desired behav ior can be implemented by the Chebyshev transformer technique, which p rovides the largest bandwidth given a passband ripple threshold. The r esulting reflected field is then the product of the original absorber response times the Chebyshev reduction factor, which is independent of polarization and absorber properties. Various measured results are us ed to show that more than a 10-dB improvement can be achieved at the c ritical low end of the frequency band using this approach. This improv ement cannot be achieved using conventional design concepts unless the absorber size is doubled.