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A bispherical constrained lens having two surfaces of equal radii has been known to provide multiple beams with true time delay and no feed blockage. A generalized bispherical constrained lens with the radii of its two surfaces being allowed to vary is described. This additional degree of freedom can be used in limited coverage applications to make the lens more compact, in terms of total lens surface, compared to the equal radii case.
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J. Rao (1982) studied this question.
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