tailieunhanh - Giai đoạn mảng anten P5

Array Elements Fixed beam broadside arrays may employ low- or moderate-gain elements, but most arrays employ low-gain elements owing to the effects of grating and quantization lobes (see Chapter 3). | Phased Array Antennas. Robert C. Hansen Copyright 1998 by John Wiley Sons Inc. ISBNs 0-471-53076-X Hardback 0-471-22421-9 Electronic CHAPTER FIVE Array Elements Fixed beam broadside arrays may employ low- or moderate-gain elements but most arrays employ low-gain elements owing to the effects of grating and quantization lobes see Chapter 3 . This chapter is concerned with these low-gain elements. Moderate-gain elements such as the spiral helix log-periodic Yagi-Uda horn and backfire generally do not have unique array properties and are covered in numerous other books. DIPOLES Thin Dipoles The dipole with length L approximately a half-wavelength is widely used because of its simple construction and good performance. Dipoles are made of two collinear and contiguous metallic rods or tubes with the feed between or of conical conductors typically hollow or of strips or triangles printed on a thin dielectric support. Microstrip dipoles are discussed later. A strip dipole where strip thickness is small compared to strip width is equivalent to a cylindrical dipole of radius a equal to one-fourth the strip width w Lo 1953 . A half-wave dipole has a pattern symmetric about the dipole axis with 6 measured from the axis it is cosWZcos sin 7 The half-power beamwidth is deg and the directivity is . As the dipole length shortens the pattern approaches that of a short dipole sin 0 with half-power beamwidth of 90 deg and directivity of dB. For lengths longer than a half-wavelength the pattern sharpens then breaks up. At L the main lobe is broadside and the two sidelobes are small. But at L the sidelobes at 45 deg are larger than the main beam. Finally the full-wave dipole with sinusoidal current distribution has a null at broad 127 128 ARRAY ELEMENTS side. Pattern bandwidth then goes from short dipole length to length of roughly . The impedance bandwidth is usually limiting. It was shown by Abraham in 1898 that a vanishingly thin .