tailieunhanh - Radio Propagation and Remote Sensing of the Environment - Chapter 5

Chúng tôi đã được coi là radiowave tuyên truyền trên các phương tiện truyền thông không đồng nhất, tuy nhiên, tỷ lệ thay đổi các thông số phương tiện truyền thông rất nhỏ trong các ví dụ của chúng tôi rằng các kỹ thuật quang học hình học khá khả thi trong xấp xỉ đầu tiên. Trong chương này, chúng tôi sẽ xem xét tình huống khi sự biến đổi không gian của môi trường có thể được sắc nét trên phạm vi bước sóng bao gồm cả những thay đổi nhảy. Ví dụ như thả hình thành của mây. | 5 Radiowave Scattering CROSS SECTION OF SCATTERING We have already considered radiowave propagation in inhomogeneous media however the rate of change of media parameters was so small in our examples that the geometrical optics technique was rather workable in the first approximation. In this chapter we will consider situations when the spatial variation of the medium can be sharp on the wavelength scale including the jump changes. Examples include drop formation of clouds and rain areas of vegetation hailstones etc. The primary phenomena with regard to radiowave interaction with such inhomogeneities are diffraction and the associated wave scattering. Thus certainly wave absorption in the inhomogeneities does take place. Both processes scattering and absorption lead to attenuation of the incident wave power flow and a phenomenon known as extinction occurs. The waves field can be represented in the cases discussed here as the sum of the incident and scattered waves that is E Ei Es and H Hi Hs. The field of scattered waves is described by V X Es ikHs V X Hs -ikEs - ik e r -1 E. Here e r is the permittivity of the scattering waves of the homogeneities. It is assumed then that the permittivity of the medium is equal to unity in the absence of inhomogeneities. We may consider the second term in Equation to be a current with density je r -k e r -1 E -in e r -1 E. Then the scattered field can be expressed in the form of an integral from the inserted equivalent current. The integral must be extended in this case to the volume taken up by all of the scattering particles inhomogeneities . We shall first pay attention to the simple case of one particle and will consider the field at the wave zone. We must use Equations and to obtain Es . B X 4nr r V L EX r e-1 exp I -ik - - d V H s r X E s r Here integration is performed over the volume of the scattering inhomogeneities. 111 2005 by CRC Press 112 Radio Propagation and Remote Sensing of the .