This chapter covers the following topics: characteristics of electromagnetic waves; radiowave optical principles; radiation and reception; and environmental wave-propagation effects.

This book is primarily concerned with the electromagnetic design and performance of antennas. Often of almost equal practical importance are the mechanical design factors, some of which will be mentioned throughout the book in connection with related discussions of electromagnetic behavior. It is desirable, however, for the reader to have some general feeling for the structural factors in the design of antennas, as an aid in relating the somewhat abstruse electromagnetic concepts to real physical structures. A thumbnail sketch of some of these aspects of antenna design will be given here, before the definition of electromagnetic parameters is taken up.

This chapter discusses array antennas that are designed so that the major lobe (the main beam) is pointed in one fixed direction or more. Commonly, array antennas are also designed to electronically, and thus almost instantly, change beam pointing direction. Such a beam pointing antenna is called a phased array, electronic scanning array, or electronically steered array (ESA). The general principles of arrays are discussed in the present chapter, and chapter 8, titled Electronically Steered Arrays, focuses on concepts and techniques more directly applicable to electronic beam steering.

Chapter 7 discusses properties of antennas and antenna analysis techniques not addressed in other chapters. Antennas are used in many different applications, and consequently there are numerous special properties needed to satisfy their requirements. The range of topics addressed in this chapter is wide, and includes techniques for providing wide bandwidths, multiple polarizations, low receiver noise, and extremely low side lobes. A specific application may, of course, employ more than one of these techniques. There is also a section solely on antennas for direction finding and another on antennas that accomplish beam scanning mechanically. The concept of synthetic-aperture antennas, although perhaps more of a signal processing than an antenna topic, is also discussed here, because it is a uniquely effective way of attaining very narrow antenna beamwidths through use of a relatively small antenna on a moving platform. Finally, the chapter closes with a section on seemingly unrelated subjects, namely, geometrical theory of diffraction (GTD), method of moments (MoM), and fractals. These topics are, however, connected by the subject of radiation from sharp edges (curvatures small compared to a wavelength), which currently cannot be analyzed by exact theoretical methods.

The majority of antenna measurements lie within two basic categories: impedance measurements and pattern measurements. The first category deals with one of the most important antenna parameters-the input impedance. The second category is a very broad and equally important one, with many subcategories, such as measurements of beamwidth, minor lobe level, gain, and polarization characteristics. Measurements of efficiency and noise may also be desired in some instances. As discussed in sec. 7.8, the antenna may affect the noise level of a receiving system in special cases. Consequently, the technique of antenna noise measurements is especially important in the lower atmospheric noise region of about 1 to 10 GHz. Therefore, the basic technique of antenna noise measurement and the effect of antenna noise on overall receiving system noise are described in the present chapter.

The most general form of a wave polarization is elliptical. The B-vector of an elliptically polarized wave can be regarded from three viewpoints. Namely, it can be considered to be (1) a rotating vector, the end point of which traces out an elliptical helix whose axis lies in the direction of propagation; (2) the resultant of the B-vectors of two linearly polarized waves of the same frequency; or (3) the resultant of the oppositely rotating Bvectors of two circularly polarized waves of the same frequency. Figure B-1 illustrates the B-vector of an elliptically polarized wave at various positions in space, at a fixed instant of time. An ellipse within an x-y plane is shown at the left end of this spiraling wave. The ellipse is created by tracing the end-point (terminus) of the B-vector onto the x-y plane, during the duration of one rf cycle.

Chapter 1 (sec. 1.4.5) includes an introduction to the polarization dependencies of the reflection coefficient for a smooth, flat surface and to the effects of multipath propagation. The present appendix includes equations and example calculations for reflection coefficients of smooth and rough earth surfaces. Furthermore, equations are given for calculating the pattern propagation factor F for both flat earth and spherical earth geometries. In addition, the SM that accompanies this book provides examples using Mathcad for calculating reflection coefficients and the pattern propagation factor F. Therefore, by changing the initial assumptions, the user may readily attain computer calculated solutions to a wide variety of problems involving reflections applicable to either flat earth or spherical earth geometries.

This book section discusses the far-field region, that is, the far zone, the region of an antenna field where the antenna radiation pattern is essentially independent of the distance from the antenna.

Appendix H includes calculations on the differences in distances from points on a planar surface to a distant point P. The results are used in Appendix G for determining antenna patterns versus range.