Anechoic chamber profiles synthesis

Abstract

An universal method is proposed and profiles of the inner surface of anechoic chamber in the longitudinal and transversal sections are designed. The necessary number n of the rays transmitted from the radiation zone and reflected from the chamber walls and entered the anechoic zone (reception zone) to obtain a given anechoicness has been determined by the method. The level of die anechoicness (reflection) in the reception zone is estimated by the value K ≤ pⁿ, where p is the power reflection coefficient of anechoic chamber covering material. The method is based on Geometrical Optics. The shape, dimensions and location of zones are given preliminarily. The core of the method is the synthesis of critical rays congruencies that pass from the transmitting antenna to the receiving one after a minimal (necessary) number n of reflections from chamber walls. Fig. 1-3 show the transversal sections of the inner surface of anechoic chambers with longitudinal goffers. Figs. 4-5 show the longitudinal sections of chambers with/without the longitudinal goffers.

In curvilinear profiles (Figs. 1, 2) the anechoic zone boundary (cylinder with radius a) is a caustic of the congruence of rays i which go out from the radiation zone (coaxial cylinder with radius 2a) along a tangent. This guarantees that all indirect rays from the transmitting antenna to the receiving one have n ≥ 2 reflections from chamber walls. The broken line profile in Fig.l satisfies the requirement of n ≥ 2 reflections of indirect rays 2,3 from the corresponding sections AB, BC at the points near the break. The profiles in Fig.3 satisfy the requirement of n ≥ 3,4 reflections of indirect rays. The rays going out from the radiation zone (radius a) along a tangent after the (n - i)th reflection should be parallel with each other and not enter the reception zone (radius 2a).

The longitudinal semi-profile in Fig.4 is composed of two conjugated curves: the quasi-parabola MN and the quasi-ellipse KNP. Rays 1,2,3 go out from the radiation zone (radius 2a) along a tangent. Rays 1,2 are reflected from KNP and touch the reception zone (radius a). A ray 3 is reflected from MN in parallel to a ray 2. In Fig. 5 the aperture F1F2 is the radiation zone, the space ABF3F4 is the reception zone. The chamber profile is composed by segments of ellipses FIN (with focuses F2, F4), NP (F2, F3), F2M (FI, F3) MP (FI, F4). There are n ≥ 2 reflections of indirect rays from the transmitting antenna to the receiving one in the chambers (Fig. 4,5). Broken line profiles may be designed for zones in Figs. 4,5. These profiles are simpler for manufacturing but they have overall dimensions greater than curvilinear ones in two or three times.

Thus, the curvilinear profiles designed according to the suggested method enable one to decrease the size of chambers comparatively with the broken line profiles.