tailieunhanh - Electromagnetic Waves and Antennas combined - Chapter 17

Radiation from Apertures Các lĩnh vực bức xạ từ ăng-ten khẩu độ, chẳng hạn như khe cắm, mở ống dẫn sóng, sừng, phản xạ và ăng-ten ống kính, được xác định từ các kiến thức về các lĩnh vực so với độ mở ống kính của ăng ten. Các trường độ mở ống kính trở thành nguồn của các lĩnh vực bức xạ ở những khoảng cách lớn. Đây là một biến thể của nguyên tắc Huygens-Fresnel, nói rằng các điểm trên mặt đầu sóng mỗi trở thành nguồn của sóng thứ cấp hình cầu tuyên truyền ra bên. | 17 Radiation from Apertures Field Equivalence Principle The radiation fields from aperture antennas such as slots open-ended waveguides horns reflector and lens antennas are determined from the knowledge of the fields over the aperture of the antenna. The aperture fields become the sources of the radiated fields at large distances. This is a variation of the Huygens-Fresnel principle which states that the points on each wavefront become the sources of secondary spherical waves propagating outwards and whose superposition generates the next wavefront. Let Ea Ha be the tangential fields over an aperture A as shown in Fig. . These fields are assumed to be known and are produced by the sources to the left of the screen. The problem is to determine the radiated fields r H r at some far observation point. The radiated fields can be computed with the help of the field equivalence principle 1136-1142 1193 which states that the aperture fields may be replaced by equivalent electric and magnetic surface currents whose radiated fields can then be calculated using the techniques of Sec. . The equivalent surface currents are electric surface current magnetic surface current where n is a unit vector normal to the surface and on the side of the radiated fields. Thus it becomes necessary to consider Maxwell s equations in the presence of magnetic currents and derive the radiation fields from such currents. The screen in Fig. is an arbitrary infinite surface over which the tangential fields are assumed to be zero. This assumption is not necessarily consistent with the radiated field solutions that is Eqs. . A consistent calculation of the fields to the right of the aperture plane requires knowledge of the fields over the entire aperture plane screen plus aperture. However for large apertures with typical dimension much greater than a wavelength the approximation of using the fields Ea Ha only over the aperture to calculate the radiation patterns