Propagation of Short Radio Waves
This book treats the phenomena associated with the propagation of short radio waves between terminal points.
Inspec keywords: radiowave propagation; electromagnetic wave refraction; electromagnetic wave reflection; echo; atmospheric electromagnetic wave propagation; meteorology; radar
Other keywords: horizontally stratified atmospheric propagation; short radio wave propagation; atmospheric attenuation; radar targets; Earth surface radiowave reflection; meteorology; radiowave refraction; meteorological echoes; radar echoes
Subjects: Radar equipment, systems and applications; Electromagnetic wave propagation and interactions in the lower atmosphere; Electromagnetic compatibility and interference; Ionospheric electromagnetic wave propagation; Radiowave propagation
 Book DOI: 10.1049/PBEW024E
 Chapter DOI: 10.1049/PBEW024E
 ISBN: 9780863410994
 eISBN: 9781849193825
 Page count: 756
 Format: PDF

Front Matter
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1 Elements of the problem
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The following sections are discussed: ionosphere; long wave transmission; tropospheric refraction; refractive index; meteorological element; fieldstrength distribution; atmospheric scattering; attenuation; radar echoes; and absorption.

2 Theory of propagation in a horizontally stratified atmosphere
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This chapter presents a development of the theory of propagation of very short electromagnetic waves in free space, over a plane earth, and over a spherical earth, including the effects of the electromagnetic properties of the earth and refraction by the atmosphere. In a sense it is two chapters, for it presents detailed specific results and methods for computation of field strength in the absence of refraction or with a linear modified index profile, as well as theory and a few numerical examples for certain types of nonlinear profiles. In order to prevent duplication and to preserve generality as much as possible, however, the presentation has been designed to adhere closely to fundamental electromagnetic theory, from which specific results are developed as special cases. Because of the mathematical complexity of even the simplest cases of nonstandard refraction of practical interest the numerical results for these cases are few.

3 Meteorology of the refraction problem
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The purpose of this chapter is to describe the types of distribution of refractive index in the atmosphere and to discuss them in terms of the processes producing them and of the circumstances in which they occur. This chapter begins with a discussion of certain meteorological variables and processes and their relation to refractive index. A discussion of the vertical variation of refractive index follows. As secondary material the horizontal and time variations of this quantity are also considered. The chapter concludes with a discussion of new observational instruments and techniques.

4 Experimental studies of refraction
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This chapter summarizes the principal experiments that have been performed to investigate the effects of atmospheric refraction on microwave transmission. Many of these experiments (particularly the earlier ones) were highly varied in nature and were not performed or analyzed by methods now in general use. They often revealed transmission properties that now appear to fit into a broad general pattern, although at the time this unity was not apparent. Consequently we do not follow a chronological approach, but instead we have arranged the material in a way that seems most appropriate in the light of present information. The point of view adopted here is that the modifiedindex distribution is the property of the transmission medium that is of major interest and usefulness. The marked dependence of the transmission phenomena on meteorological conditions makes it imperative that radio and meteorological observations be made simultaneously and that the weather regime of the experimental location be considered. Because the radio results must be interpreted in terms of meteorological conditions, statistical studies of even large amounts of radio data have little general significance and will be treated only briefly in this chapter. Our main concern will be the qualitative aspects of transmission as influenced by tropospheric refraction.

5 Reflections from the Earth's surface
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The effects of reflection of waves from the earth's surface were discussed qualitatively in Sec. 22. In Sec. 211 formulas were given for the reflection coefficient and divergence factor in terms of the earth's electromagnetic properties and of the transmissionpath geometry. In this chapter these quantities and their effect on the coverage diagram will be discussed further. Surface roughness will be shown to cause marked departures from the results predicted theoretically for a smooth surface; the extent of this departure is related to the size of the surfaceroughness elements relative to wavelength and to the grazing angle. Reflections from the surface will be shown to limit accuracy of radar height measure ments at angles of elevation so small that the antenna illuminates the surface strongly and specular reflection from the surface is large.

6 Radar targets and echoes
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In the earlier parts of this book it has been assumed that radar targets could be characterized by the radar cross section σ, which has been treated primarily from the phenomenological point of view. In this chapter we shall examine the properties of σ and its relation to the properties of the target. It will be found that in only a negligible number of extremely simple cases is it feasible to calculate σ from the geometry; the remaining cases, which for the most part are those of greatest practical interest, are beyond the scope of existing mathematical methods.

7 Meteorological echoes
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This chapter presents a discussion on microwave radar echoes under certain meteorological conditions. The origin and intensity of meteorological echoes including the properties and meteorological structure of precipitation echoes are tackled.

8 Atmospheric attenuation
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In studying the propagation of microwaves through the atmosphere one is interested both in the absorption and in the dispersion, that is, how the index of refraction varies with frequency. A very general relation exists between the refractive index and the absorption coefficient that enables one to determine the absorption if the dependence of the refractive index on the frequency is known throughout the spectrum from v = 0 to v = °°. Vice versa, the refraction can be computed (apart from an additive constant O if the absorption is known for all wavelengths. In other words, assumptions cannot be made regarding how the refractive index varies with frequency without implications as to the amount of absorption. This fact has a bearing on the theory of nonstandard refraction. For instance, various investigators have noted that a variation of 1 per cent in the refractive index between λ = 1 and λ = 10 cm would lead to interesting differences in the trapping or duct phenomena at the two wavelengths. Such a variation, however, would necessarily lead to an unreasonably high absorption, in contradiction with experiment.

Appendix A: Application of the Lorentz reciprocity theorem to scattering
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There are in current use several reciprocity theorems, all of which are often loosely referred to as “the” reciprocity theorem. We apply here the version developed by H. A. Lorentz.

Appendix B: Coherent and incoherent scattering from assemblies of scatterers
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This appendix derives the upper bound of the ratio of the coherent to incoherent scattering. In comparison with a trapezoidal pulse of approximately the same “sharpness” it is seen that the smooth pulse has a much smaller coherent scattering.

Back Matter
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