The figure shows a dual-beam, dual- polarization Ku-band antenna, the reflector of which comprises an assembly of small reflectarrays arranged in a piecewise-planar approximation of a parabolic reflector surface. The specific antenna design is intended to satisfy requirements for a wide-swath spaceborne radar altimeter, but the general principle of piecewise-planar reflectarray approximation of a parabolic reflector also offers advantages for other applications in which there are requirements for wide-swath antennas that can be stowed compactly and that perform equally in both horizontal and vertical polarizations.
The main advantages of using flat (e.g., reflectarray) antenna surfaces instead of paraboloidal or parabolic surfaces is that the flat ones can be fabricated at lower cost and can be stowed and deployed more easily. Heretofore, reflectarray antennas have typically been designed to reside on single planar surfaces and to emulate the focusing properties of, variously, paraboloidal (dish) or parabolic antennas. In the present case, one approximates the nominal parabolic shape by concatenating several flat pieces, while still exploiting the principles of the planar reflectarray for each piece.
Prior to the conception of the present design, the use of a single large reflectarray was considered, but then abandoned when it was found that the directional and gain properties of the antenna would be noticeably different for the horizontal and vertical polarizations. The reason for this difference in performance is related to strong spatial variations in phase, including phase wraps (phase variations in excess of 360°).
By arranging small reflectarrays in a piecewise-planar approximation of a parabola, instead of constructing one large reflectarray on a single planar surface, one minimizes the number of phase wraps per panel and reduces the angle of incidence at each reflectarray patch. This makes it possible to simultaneously maximize the vertical- and horizontal-polarization gains, to improve the radiation pattern, and reduce sensitivity to fabrication and adjustment errors.