SCANTER 4000/4100: synthesis, design and manufacture of an artificial lens for an air surveillance antenna

The synthesis and design of a metallic lens in the aperture of a large flared horn antenna is reported. The lens provides sufficient correction of the quadratic phase error in the aperture of the horn to obtain ample antenna gain, while it simultaneously shapes the phase in the aperture such that a modified cosecant squared far-field pattern is obtained in the cross-sectional plane of the flared horn. The reflection from the lens due to the incident cylindrical wave (propagating between the flares of the horn) shall be weaker than -25 dB to maintain a stable antenna pattern within the desired bandwidth. Otherwise, the amplitude and of phase distribution in the aperture of the horn will be pertubed by the reflected field (returning subsequent to reflections from the inner surfaces of the horn). The aperture distribution would then be strongly dependent upon the exact location of the lens (located in the vicinity of the aperture). As the depth of the horn is 44 wavelengths, the perturbations due to the reflected field from the metallic lens would also become highly frequency dependent. A low reflection off the lens thus relaxes the mechanical requirements of its exact location, and is as such a prerequisite for a successful electrical design and for the series production of the antenna. (4 pages)