TABLE OF CONTENTS The degree to which the antenna concentrates the radiated energy in a desired direction broadly referred to here as directivity is a key characteristic of virtually every airborne radar. Besides determining the radar's ability to locate targets in angle, directivity can vitally affect the ability to deal with ground clutter and is a major factor governing detection range.
In this chapter, we will learn how the energy radiated by an antenna is distributed in angle and examine the salient characteristics of the radiation pattern beamwidth, gain, and sidelobes. We will then see how the sidelobes may be reduced; how fast, versatile beam positioning may be accomplished with electronic scanning; and how high angular resolution and angular measurement accuracy may be achieved. Finally, we will learn how the beam may be optimized for ground mapping.
From common simplistic illustrations, it might be supposed that a radar antenna concentrates all of the transmitted energy into a narrow beam within which the power is uniformly distributed; that if a pencil beam were trained like a flashlight on an imaginary screen in the sky, it would illuminate a single round spot with uniform intensity. While this might be desirable, it is even less true of an antenna than of a flashlight.
Like all antennas, a pencil beam antenna radiates some energy in almost every direction. As illustrated in the three-dimensional plot of Fig. 1, most of the energy is concentrated in a more or less conical region surrounding...
