TABLE OF CONTENTS Antennas are used to launch signals to free space or to recover them. This chapter discusses the main characteristics of antennas, including:
Antenna gain and sidelobes for linear radiators, simple planar arrays, and circular arrays;
Antenna patterns with low sidelobes, Chebysh v, Zolotar v, Taylor, and Bayliss distributions;
Phased arrays with quantizing effects;
Antenna losses, including beam shape and scanning losses;
Synthesis of beams;
Equivalence of active and passive antenna systems;
Introduction to sidelobe cancellers and synthetic apertures.
Antennas radiate and bunch the transmitted energy in the correct direction and provide a gathering area for the returning echo energy. The most common form of antenna, currently, is a large parabolic reflector fed by a horn (or group of horns) shown diagrammatically in Figure 5.1. The larger reflector dimension is between tens and many hundreds of wavelengths long.
Here the horn is shown at the center of the reflector, which causes blockage. Offset feed systems have the horn placed outside the edge of the reflector so that little or no transmitted energy is reflected back to the horn. At the center of the beam, the electromagnetic waves are in the same phase as they would be if they came from the directrix. On transmission the energy from the horn is concentrated in one direction. During reception, the area of the reflector collects the echo signals and presents them in phase at the mouth of the horn.
The antennas are normally focused at infinity, that is, the horn is...
