Bootlace lenses for curved arrays

Abstract

The use of multifocal bootlace lenses as beamforming networks for multibeam array antennas has been well established. The angular coverage of these antennas depends on the shape and size of the arrays used. When the multifocal point principles are applied to bootlace lens fed curved arrays, the maximum angular coverage is less compared with similar size linear arrays. In fact, it becomes physically impossible to design large angle multifocal lenses for large size curved arrays.A bootlace lens consists of a feed array, an innersurface array and a radiating array, as shown by Fig.1. The elements of the radiating array are connected to the elements of the innersurface array by transmission lines.A new lens design method is introduced here for curved radiating arrays taking into account that some of the elements of the curved radiating arrays do not have any significant contribution in the directions of some of the main beams, due to shadowing and directional patterns of the radiating antenna elements. With this method, instead of having a number of perfect phase fronts in certain directions (i.e. perfect focal points), the degrees of freedom available in designing the lenses are used to get correct phase fronts for a number of chosen directions for each of the points on the radiating array curve. For a chosen radiating array, the lengths of the transmission lines and the positions of the antenna elements on the innersurface are calculated such that, the path lengths from a certain feed point to the phase fronts are equal to a pre-determined constant,for a number of chosen phase fronts. These phase fronts are different for each of the elements and are chosen to be in the directions that the radiating elements 'illuminate'.The design principles of this lens are introduced and the results of a computer study showing the feasibility of these lenses are presented. The full paper will present results of a study investigating the performance and design parameters of the lens. Lens Design Principles:The geometry of the lens and the nomeclature are introduced by Fig.2. Q(U,w) is a point on the radiating array curve and point P(X,Y) is the corresponding point on the innersurface connected to Q with a transmission line of length L. Feed array curve is an arc of radius f , and centre at 0. (0 is the origin of the X-Y coordinate system.)The path length from three points, F,(