Parasitic spiral arrays

The radiation pattern of an array of circularly polarized elements can be scanned by a mechanical rotation of the elements since the element phase is a function of orientation. This principle has been used in the design of two different arrays of Archimedean spirals, one a "parasitic lens array" and the other a "parasitic reflector array." The lens consists of two arrays of spirals mounted on opposite sides of a conducting plane, each spiral on one side being connected by a transmission line to a corresponding spiral on the opposite side. A feed horn is used to illuminate one of the arrays; the energy received by this array passes through the ground plane via the transmission lines and is reradiated by the spirals on the far side. If the elements are rotated to particular orientations, the incident spherical wave can be focussed, and further, the beam can be scanned if the spirals are rotated properly. The spiral reflector is a single array (using no transmission lines) mounted above a ground plane and, like the lens, is illuminated by a feed horn. It has the same focussing and scanning properties as the lens. Neither array is limited to circular polarization. With a combination of right- and left-hand elements, linear polarization can be used. Twenty-element lenses (2 by 10) and one hundred element reflectors (10 by 10) have been built and the results have verified the predicted performance.