The Spacecraft Antennas

The function of the spacecraft antennas is to receive and transmit two distinct classes of signals: broadband microwave frequencies for communication service, such as television; and narrow-band V H F for beacon, command, and telemetry. The requirements on these antennas resulted from a thorough systems analysis which led to an intricate balance of many factors involving not only the spacecraft, but the ground station as well. These factors include such vital questions as: the modulation method, the location of frequencies, the choice of polarizations, the degree of attitude stabilization, etc., and are discussed in companion papers. 1 , 2 ' 3 l.i Require?ne?its One of the most important and difficult objectives was to make the patterns provided by both the microwave and V H F antennas as nearly 869 870 TITE B E L L SYSTEM T E C H N I C A L J O U R N A L , J U L Y 1963 isotropic as possible. This is necessary because the spacecraft is spin stabilized; consequently the received and transmitted signal amplitudes will fluctuate by an amount depending on the anisotropy of the antenna pattern. In addition, the microwave antenna must receive right circularly polarized FM signals centered at 6390 mc and transmit left circularly polarized FM signals centered at 4170 plus a beacon signal at 4080 mc. The wide frequency separation and the opposite sense of circular polarization for the 4- and 6-gc signals suggests two microwave antennas. A single VHF antenna is used to receive command signals at 123 mc and to transmit a 136-mc beacon which can be amplitude modulated with telemetry information.