Chapter 2 Antennas

CHAPTER 2 ANTENNAS As an Electronics Technician, you are responsible for maintaining systems that both radiate and receive electromagnetic energy. Each of these systems requires some type of antenna to make use of this electromag- netic energy. In this chapter we will discuss antenna characteristics, different antenna types, antenna tuning, and antenna safety. ANTENNA CHARACTERISTICS An antenna may be defined as a conductor or group of conductors used either for radiating electromagnetic energy into space or for collecting it from space. Electrical energy from the transmitter is converted into electromagnetic energy by the antenna and radiated into space. On the receiving end, electromagnetic energy is converted into electrical energy by the antenna and fed into the receiver. The electromagnetic radiation from an antenna is made up of two components, the E field and the H field. The total energy in the radiated wave remains constant in space except for some absorption of energy by the earth. However, as the wave advances, the energy spreads out over a greater area. This causes the amount of energy in a given area to decrease as distance from the source increases. The design of the antenna system is very important in a transmitting station. The antenna must be able to radiate efficiently so the power supplied by the transmitter is not wasted. An efficient transmitting antenna must have exact dimensions, determined by the frequency being transmitted. The dimensions of the receiving antenna are not critical for relatively low frequencies, but their importance increases drastically as the transmitted frequency increases. Most practical transmitting antennas are divided into two basic classifications, HERTZ ANTENNAS (half-wave) and MARCONI (quarter-wave) ANTEN- NAS. Hertz antennas are generally installed some distance above the ground and are positioned to radiate either vertically or horizontally. Marconi antennas operate with one end grounded and are mounted perpendicular to the earth or a surface acting as a ground. The Hertz antenna, also referred to as a dipole, is the basis for some of the more complex antenna systems used today. Hertz antennas are generally used for operating frequencies of 2 MHz and above, while Marconi antennas are used for operating frequencies below 2 MHz. All antennas, regardless of their shape or size, have four basic characteristics: reciprocity, directivity, gain, and polarization. RECIPROCITY RECIPROCITY is the ability to use the same antenna for both transmitting and receiving. The electrical characteristics of an antenna apply equally, regardless of whether you use the antenna for transmitting or receiving. The more efficient an antenna is for transmitting a certain frequency, the more efficient it will be as a receiving antenna for the same frequency. This is illustrated by figure 2-1, view A. When the antenna is used for transmitting, maximum radiation occurs at right angles to its axis. When the same antenna is used for receiving (view B), its best reception is along the same path; that is, at right angles to the axis of the antenna. DIRECTIVITY The DIRECTIVITY of an antenna or array is a measure of the antenna’s ability to focus the energy in one or more specific directions. You can determine an antenna’s directivity by looking at its radiation pattern. In an array propagating a given amount of energy, more radiation takes place in certain directions than in others. The elements in the array can be arranged so they change the pattern and distribute the energy more evenly in all directions. The opposite is also possible. The elements can be arranged so the radiated energy is focused in one direction. The 2-1