ANTENNA BASICS AND SINGLE‐BEAM ANTENNA

This chapter is on the payload unit which is the most eye‐catching: the antenna. It addresses antenna basics and single‐beam antennas. There are five sizes of antenna‐beam coverage, defined the same for transmit and receive and for all orbits. These include: global or earth‐coverage of the visible earth, zonal, regional, and spot, which covers an area about the size of metropolitan New York City. A reflector antenna has at least a main reflector and a feed. A reflector antenna is inherently wideband if the phase center of the feed or virtual feed is exactly at the focus for all intended frequencies. Horn antennas can provide two kinds of beam, a global beam and a spot beam. Antenna gain is a function of two‐dimensional antenna pointing error. The antenna will have an onboard autotrack system, which keeps the antenna's peak of beam pointed to a beacon or a communications signal.