chapter 13 - Chapter 13 ANTENNAS The Ten Commandments of...

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Chapter 13 ANTENNAS The Ten Commandments of Success 1. Hard Work: Hard work is the best investment a man can make. 2. Study Hard: Knowledge enables a man to work more intelligently and effec- tively. 3. Have Initiative: Ruts often deepen into graves. 4. Love Your Work: Then you will find pleasure in mastering it. 5. Be Exact: Slipshod methods bring slipshod results. 6. Have the Spirit of Conquest: Thus you can successfully battle and overcome difficulties. 7. Cultivate Personality: Personality is to a man what perfume is to the flower. 8. Help and Share with Others: The real test of business greatness lies in giving opportunity to others. 9. Be Democratic: Unless you feel right toward your fellow men, you can never be a successful leader of men. 10. In all Things Do Your Best: The man who has done his best has done every- thing. The man who has done less than his best has done nothing. —CHARLES M. SCHWAB 13.1 INTRODUCTION Up until now, we have not asked ourselves how EM waves are produced. Recall that elec- tric charges are the sources of EM fields. If the sources are time varying, EM waves prop- agate away from the sources and radiation is said to have taken place. Radiation may be thought of as the process of transmitting electric energy. The radiation or launching of the waves into space is efficiently accomplished with the aid of conducting or dielectric struc- tures called antennas. Theoretically, any structure can radiate EM waves but not all struc- tures can serve as efficient radiation mechanisms. An antenna may also be viewed as a transducer used in matching the transmission line or waveguide (used in guiding the wave to be launched) to the surrounding medium or vice versa. Figure 13.1 shows how an antenna is used to accomplish a match between the line or guide and the medium. The antenna is needed for two main reasons: efficient radiation and matching wave impedances in order to minimize reflection. The antenna uses voltage and current from the transmission line (or the EM fields from the waveguide) to launch an EM wave into the medium. An antenna may be used for either transmitting or receiving EM energy. 588
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13.1 INTRODUCTION 589 EM wave Generator Transmission line Antenna Surrounding medium Figure 13.1 Antenna as a matching device between the guiding struc- ture and the surrounding medium. Typical antennas are illustrated in Figure 13.2. The dipole antenna in Figure 13.2(a) consists of two straight wires lying along the same axis. The loop antenna in Figure 13.2(b) consists of one or more turns of wire. The helical antenna in Figure 13.2(c) consists of a wire in the form of a helix backed by a ground plane. Antennas in Figure 13.2(a-c) are called wire antennas; they are used in automobiles, buildings, aircraft, ships, and so on. The horn antenna in Figure 13.2(d), an example of an aperture antenna, is a tapered section of waveguide providing a transition between a waveguide and the surroundings.
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