Improved low-frequency performance of pyramid-cone absorbers for application in semi-anechoic chambers

A mathematical model, developed by the authors to describe the low-frequency reflection properties of arrays of pyramid-cone absorbers, is used to improve the design of these cones in the range of 30 to 300 MHz. The model shows that at frequencies such that the transverse spacing of the cones is small compared to a wavelength, a reflecting wave is not influenced by the individual cones, but rather the wave is acted upon as if only a one-dimensionally layered, anisotropic equivalent medium exists, whose parameters depend on the geometry and electrical composition of the cones. It is thus possible using elementary means to calculate the reflection coefficient of an arbitrarily polarized and incident plane wave from an array of these cones and hence to assess their performance in an anechoic chamber. Results of the application of this design to semianechoic measurement chambers are presented. These are found to provide closer correlation to the site attenuation curves for an ideal open-field test site