CHAPTER 4 ANTENNAS

4-1 CHAPTER 4 ANTENNAS LEARNING OBJECTIVES Upon completion of this chapter you will be able to: 1. State the basic principles of antenna radiation and list the parts of an antenna. 2. Explain current and voltage distribution on an antenna. 3. Describe how electromagnetic energy is radiated from an antenna. 4. Explain polarization, gain, and radiation resistance characteristics of an antenna. 5. Describe the theory of operation of half- wave and quarter-wave antennas. 6. List the various array antennas. 7. Describe the directional array antennas presented and explain the basic operation of each. 8. Identify various special antennas presented, such as long-wire, V, rhombic, turnstile, ground-plane, and corner-reflector; describe the operation of each. 9. List safety precautions when working aloft and around antennas. INTRODUCTION If you had been around in the early days of electronics, you would have considered an ANTENNA (AERIAL) to be little more than a piece of wire strung between two trees or upright poles. In those days, technicians assumed that longer antennas automatically provided better reception than shorter antennas. They also believed that a mysterious MEDIUM filled all space, and that an antenna used this medium to send and receive its energy. These two assumptions have since been discarded. Modern antennas have evolved to the point that highly directional, specially designed antennas are used to relay worldwide communications in space through the use of satellites and Earth station antennas (fig. 4-1). Present transmission theories are based on the assumption that space itself is the only medium necessary to propagate (transmit) radio energy.