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Radio Propagation and Remote Sensing of the Environment - Chapter 2

Trong chương trước, chúng ta định nghĩa một làn sóng máy bay như một làn sóng có đặc tính phụ thuộc vào chỉ có một phối hợp Cartesian. Chúng tôi cũng lưu ý rằng làn sóng máy bay được kích thích bởi một hệ thống nguồn phân phối thống nhất trên một mặt phẳng vô hạn. Bởi vì một nguồn có kích thước vô hạn là một trừu tượng, khái niệm về một làn sóng máy bay cũng là trừu tượng. Chúng tôi cũng thành lập trong chương trước, đến nay từ các nguồn thực (nguồn kích thước giới hạn),. | 2 Plane Wave Propagation 2.1 PLANE WAVE DEFINITION In the previous chapter we defined a plane wave as a wave whose characteristics depend on only one Cartesian coordinate. We also noted that the plane wave is excited by a system of sources distributed uniformly on an infinite plane. Because a source of infinite size is an abstraction the notion of a plane wave is also abstract. We also established in the previous chapter that far from real sources sources of limited sizes radiated waves can be considered to be spherical and that the phase of radiated waves close to the pattern maximum is constant on a spherical surface of radius R where R is the distance from the source. Locally a spherical surface of large radius differs little from a plane and may be supposed to be a plane in the defined frames of space. Let us now refine the bounds of these frames. Ignoring unimportant details the spherical wave may be described by the following expression e 1 E T ẹ R where the vector T does not depend on R and describes the wave polarization amplitude and distribution angle. Here R x2 y2 z2 where x y and z are coordinates of the observation point. Accordingly the start of the coordinate system is situated at the point where the radiator is located. Let us imagine that the pattern maximum is close to the direction defined by the condition r .ựx2 y2 0 . If we consider that in some of the space area r2 z2 then approximately exp ikz ikr 2 2z E T---------------- z 2.1 It is now easy to set up the conditions such that the field depends on only coordinate z. In this case the front of the spherical wave may be considered to be locally plane thus the wave is also thought to be locally plane. The wave phase of Equation 2.1 changes in plane z const due to the law 0 kz . 2z 2.2 31 2005 by CRC Press 32 Radio Propagation and Remote Sensing of the Environment By convention the wave phase is considered to be constant in the stated plane if coordinates x and y change within such limits so that