NASA acts autotrack antenna feed system

This paper presents the spacecraft monopulse tracking feed system of the multibeam antenna (MBA) for the NASA Advanced Communication Technology Satellite (ACTS). The MBA has approximately 0.3O half-power beamwidth; the spacecraft alignment requirement is 0.025O. The feed system is designed to receive linearly polarized communication signals from 28.9 to 30.0 GHz and to provide the elevation and azimuth error tracking signals at 29.975 GHz within 0.01 O tracking accuracy. The feed system (Figure 1) utilizes a single multiflare conical horn and a multimode coupler (MMC). To provide a symmetric primary pattern for communication signals, a three-flaresection conical horn’ is utilized due to the need for simple configuration, light weight, and small size for the spacecraft. The MMC2 utilizes the lowest three circular waveguide modes, i.e., TE,,, TM,,,, and TE, to achieve the sum and two difference channels, respectively. The two difference channels are time multiplexed in the autotrack biphase modulator and then coupled to the sum channel. Theory, physical description, and experimental data of the feed system will be discussed.