5.1 Introduction

Antennas employed in space communications are key components providing the vital links between the ground and the spacecraft.

The fundamental principles of antenna theory are based on classical electromagnetic field theory and are discussed in detail in many major text books.1,2 The basic starting point for the theory is Maxwell's equations for a homogeneous isotropic medium which relate electric and magnetic fields to source currents. From Maxwell's equations, the wave equation can be derived which describes the propagation of an electromagnetic wave in a nondispersive homogeneous isotropic medium. The performance of an antenna configuration is found from solution of the wave equation, generally using approximate techniques.

Antennas can be broadly classified by the frequency spectrum in which they are commonly applied, or by their basic mode of radiation. The most important type for space communications is the aperture antenna which includes horns, reflectors and lenses (although the latter are not usually employed in this particular application). The aperture antenna will form the basis of the present discussion. A further antenna type, namely the array antenna, is used in some space applications, for example mobile communications.

5.2 Basic Aperture Antenna Definitions and Relationships