Experimental validation of the “Two - Level GTD” method for design of anechoic chambers

Anechoic chambers utilized for far-field antenna measurements at VHF/UHF frequencies typically comprise rectangular and tapered designs. The primary purpose of conventional far-field chambers is to illuminate a test zone surrounding the Antenna Under Test (AUT) with an electric field that is as uniform as possible, while multiple reflections from the side wall absorber assemblies are kept to a minimum. The “Two - Level GTD” method has been offered in to reduce the reflectivity and improve overall performance for anechoic chambers operating at frequencies >;1 GHz, where the characteristic dimensions of the side-walls are normally more than 30λ. It includes the shaping of the side walls of the anechoic chamber along with application of a “fish - bone” absorber layout - both are designed based on GTD principles. The cross section dimensions of far field chambers at VHF/UHF frequencies can be electrically small, often as little as 3λ. In this paper the “Two - Level GTD” method is applied to improve the reflectivity in an anechoic chamber with external dimensions as small as 18'(H) × 18'(W) × 33'(L) operating from as low a frequency as 100 MHz. The simulations are performed first to compare the reflectivity results achieved with a “Two-Level GTD” with the reflectivity achieved in a conventional anechoic chamber. The anechoic chamber has been built and tested. The reflectivity results are obtained thru test zone probing and confirm the reflectivity improvement and overall outstanding performance of the chamber.