FWLec25 - A F Peterson Notes on Electromagnetic Fields...

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A. F. Peterson: Notes on Electromagnetic Fields & Waves 11/04 Fields & Waves Note #25 Introduction to Antennas Objectives: Introduce an idealized Hertzian dipole antenna and present its electromagnetic fields. Use the fields to illustrate some of the properties of antennas. Discuss the near fields and far fields, the radiated power, and the radiation pattern. Consider some of the issues associated with transmitting antennas and receive antennas. The role of antennas Antennas convert electrical signals in circuits or transmission lines into electromagnetic waves, or convert waves into electrical signals that can be filtered, amplified, or otherwise processed to extract useful information. Antennas play a fundamental role in wireless communications and radar systems. Their analysis and design is a major part of electromagnetic engineering. A wide variety of antennas have been developed since Hertz (during the 1880s) first used simple antennas to demonstrate that signals could be converted to waves, transmitted through the air, and received. Antennas might consist of straight wires, a single circular loop, or a coil of wire wrapped around a magnetic core. Other antennas are similar in structure to acoustical horns or optical reflectors. Antennas are often made in a way that enables them to conform to the surface of an aircraft or other vehicle. A number of simple antennas can be combined together and excited in a coherent manner to form an antenna array. Arrays allow a substantial amount of control over the direction into which electromagnetic energy is sent or received. Because arrays permit real-time changes in the antenna performance, they are important for a number of military applications. In practice, antennas used for transmitting need to be a significant fraction of the wavelength l at the frequency of transmission (typically l /3 or greater). Horn antennas may be several wavelengths in size, while reflector antennas may be hundreds of wavelengths in diameter. Antennas designed for use at low frequencies must therefore be relatively large physical structures. As the frequency of operation increases, the physical size of the antenna decreases. Array antennas are usually several wavelengths to several dozen wavelengths in dimension. Fields of a Hertzian dipole To illustrate some of the characteristics of antennas, we consider a simple antenna known as a Hertzian dipole. This is really an electrically short current element, as illustrated in Figure 1. It can also be thought of as two point charges of opposite sign, as illustrated in Figure 2.
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