Further refining and validation of RF absorber approximation equations for anechoic chamber predictions

Abstract

Indoor antenna ranges must have walls, floor and ceiling treated with RF absorber. The normal incidence performance of the absorber is usually provided by the manufacturers of the materials; however, the bi-static or off angle performance must also be known. In a recent paper [1], a polynomial approximation was introduced that gave a prediction of the reflected energy from pyramidal absorber. In this paper, the approximations are used to predict the quiet zone (QZ) performance of several anechoic chambers. These predictions are compared with measurements performed per the free space VSWR method of four different chambers. Among the chambers analyzed are a 7.3m by 3.65m by 3.65m range with a 24-inch absorber, operating from 1 to 6 GHz with a 91 cm spherical QZ and a 5.18 m path length. Another chamber is 7 m long by 3.3 m wide with a 2.4 m height. 12-inch absorber is used to treat the internal surfaces and the QZ changes from 63 cm to 20 cm from 2 GHz to 18 GHz. The path length is 5.18m. While performing the comparison, changes are made to the calculations to further improve the predictions of the computations. A chamber previously analyzed is computed again after the changes to see whether there are improvements in the prediction. The results show that the polynomial approximations can be used to give a reasonably accurate and safe prediction of the QZ performance of anechoic chambers and improve some of the previous comparisons especially at lower frequencies where the ray tracing is not that accurate.