Overview

The spacecraft antenna system, which is often viewed as part of the communications payload, has many special and important characteristics and therefore is deserving of its own chapter. Modern antenna systems provide a variety of communication coverage patterns such as domestic, regional, and spot beams, allowing the repeater to be utilized in an efficient and flexible manner. In the classical bent-pipe repeater design, the antenna coverage typically is fixed prior to launch. Changes on orbit are limited to what can be physically switched within the repeater. Onboard digital processing provides more flexibility because beams can be reshaped and even created dynamically in accordance with demand. The use of such processing is revolutionizing antenna system design because of the much wider range of performance options that can be obtained. Still, microwave antennas have not changed fundamentally since the bulk of the initial R&D was performed nearly 70 years ago.

This chapter explains how typical antennas are physically constructed and how they create the appropriate coverage patterns. An exact description of antenna operation would require use of electromagnetic theory, which (while mentioned in Chapter 4) is beyond the scope of this book. However, the somewhat simplified explanations that follow should provide a practical understanding of spacecraft antennas. Systems engineers need to develop requirements for payload antennas in cooperation with antenna experts who understand the details of the theory, the appropriate software tools, and the means to implement and test the physical hardware. Those are the stock and trade of the antenna...