Spherical dielectric lens and method of making same

Abstract

Abstract 1,117,885. Aerials. TOKYO KEIKI SEIZOSHO, KK. 15 July, 1966 [17 July, 1965], No. 31842/66. Heading H4A. [Also in Division B5] A spherical Luneberg type radio lens of radius r 2 (Fig. 7) comprises a central homogeneous core 6 having a radius r 1 which exceeds 0À4r 2 , and a number of outer layers, the dielectric constant of a layer with a mean radius r approximating to the value The dielectric constant of the core 6 is chosen so that there is minimum variation from equiphase over a tangent plane F with illumination at a point P which is diametrically opposite, and formulae are given for this purpose. A lens may be used as a retro-reflector by providing a metallic coating over part of its surface. A method of manufacture is described in which a quantity of plastics foam beads made, for example, from polyethylene or polystyrene, are foamed to a desired extent by heating and are then sorted into groups of different particle sizes by sieving. A batch having a required bulk density and a corresponding dielectric constant may be prepared by mixing suitable proportions of beads from different groups. Other groups may be prepared from quantities of beads which have been foamed to different desired extents and fine control of the dielectric constant of a batch may be effected by the addition of titanium dioxide. An adhesive binder such as vinyl acetate is mixed with each batch. The core of a lens is prepared by packing a batch of beads having the appropriate dielectric constant into a spherical mould. Moderate heating, for example to 70‹ C., may be applied to promote hardening. Outer layers may be prepared in two hemispherical halves which are placed together for final hardening. Thus, one half may be made by packing part of a batch of beads between a spherical core or partly-formed lens 20, Fig. 11, and a hemispherical mould 21, and the other half by packing another part of the same batch of beads between hemispherical moulds 22 and 23, whose diameters correspond respectively to those of the mould 21 and the core 20.

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