Abstract

A technique for a single parabolic reflector antenna to generate both the high-gain pencil beam pattern and the needed surface mapping fan beam pattern is presented. A simple 2-D physical optics analysis has demonstrated that when a parabolic cylindrical reflector is fed by an array, a pulselike wide-beam far-field pattern can indeed be generated. The calculated pattern of a 3.65 m 2-D reflector antenna with f/D ratio of 0.333 is shown. The reflector is center-fed by a 13.6 GHz array of microstrip elements with 0.6 wavelength element spacing and uniform amplitude distribution. A ripple is formed on top of the beam because the reflector is in the near field of the array and thus the surface current formed on the reflector is not exactly sin x/x. In addition, not all the sidelobes of the array illuminate the reflector. Experimental results are reported for the Titan radar mapper, which requires a dual-fan-beam system with one beam on each side of the antenna axis. In this way, when the spacecraft flies by the Titan, the planet surface can be effectively mapped with a single radar by fast switching between the two fan beams. Due to the need of radar signatures at different encounter distances between the spacecraft and Titan surface, several fan beam scenarios are required, such as different beam widths and beam scan angles. Results for two cases are shown to demonstrate and verify the concept. >

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