Side Wall Diffraction & Optimum Back Wall Design in Far-field Antenna Measurement Chambers at Vhf/uhf Frequencies

is a member of URSI Commission B, a senior member of the IEEE, and a member of the Eta Kappa Nu and Sigma Xi honor societies. He has approximately 100 journal and conference publications in the field of electromagnetics. His current research interests include electromagnetic and guided-wave theory, electromagnetic propagation and radiation in complex media and structures and electromagnetic materials characterization. ABSTRACT 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 cross section dimensions of far field chambers at VHF/UHF frequencies can be electrically small, often as little as 3λ. In this paper the side wall reflections at VHF/UHF bands are studied in more details for elongated rectangular and tapered chambers. In particular, the reflectivity is evaluated in rectangular chambers as a function of electrical dimensions of the chamber cross – section and of the ratio W (width of the chamber) or H (height of the chamber) to L (length – separation between antennas) for values ranging from 0.5 to 2. The methods of reflectivity improvement are presented and compared. In particular, the conventional chamber design is compared with a " Two Level GTD " approach [4,5,7] and the latter one shows significant reflectivity improvement in the test zone, even at longer source antenna AUT separations. The side wall reflections are examined in tapered chambers as well. The back wall reflection mechanism, which assumes multiple incident waves – direct from the source antenna and reflected from the side walls, floor and ceiling, is offered and confirmed by the simulation, which, in turn, yields an optimized back wall chamber design (see also [6]). Anechoic chambers utilized for far-field antenna measurements at VHF/UHF frequencies comprise primarily rectangular and tapered designs. They typically consist of a shielded enclosure of significant dimensions treated by efficient absorbing materials, and consequently are very costly, with the cost increasing as the lowest operating frequency is reduced. Errors introduced during the construction of such chambers are difficult, expensive or sometimes impossible to correct. As a result, rigorous and accurate 3D electromagnetic (EM) analysis is an important procedure that may be utilized to increase confidence, reduce the risk …