Antenna

I

Introduction

Antenna, also referred to as an aerial, device used to radiate and receive radio waves through the air or through space. Antennas are used to send radio waves to distant sites and to receive radio waves from distant sources. Many wireless communications devices, such as radios, broadcast television sets, radar, and cellular radio telephones, use antennas.

II

How Antennas Work

A transmitting antenna takes waves that are generated by electrical signals inside a device such as a radio and converts them to waves that travel in an open space. The waves that are generated by the electrical signals inside radios and other devices are known as guided waves, since they travel through transmission lines such as wires or cables. The waves that travel in an open space are usually referred to as free-space waves, since they travel through the air or outer space without the need for a transmission line. A receiving antenna takes free-space waves and converts them to guided waves.

Radio waves are a type of electromagnetic radiation, a form of rapidly changing, or oscillating, energy. Radio waves have two related properties known as frequency and wavelength. Frequency refers to the number of times per second that a wave oscillates, or varies in strength. The wavelength is equal to the speed of a wave (the speed of light, or 300 million m/sec) divided by the frequency. Low-frequency radio waves have long wavelengths (measured in hundreds of meters), whereas high-frequency radio waves have short wavelengths (measured in centimeters).

An antenna can radiate radio waves into free space from a transmitter, or it can receive radio waves and guide them to a receiver, where they are reconstructed into the original message. For example, in sending an AM radio transmission, the radio first generates a carrier wave of energy at a particular frequency. The carrier wave is modified to carry a message, such as music or a person’s voice. The modified radio waves then travel along a transmission line within the radio, such as a wire or cable, to the antenna. The transmission line is often known as a feed element. When the waves reach the antenna, they oscillate along the length of the antenna and back. Each oscillation pushes electromagnetic energy from the antenna, emitting the energy through free space as radio waves.

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The antenna on a radio receiver behaves in much the same way. As radio waves traveling through free space reach the receiver’s antenna, they set up, or induce, a weak electric current within the antenna. The current pushes the oscillating energy of the radio waves along the antenna, which is connected to the radio receiver by a transmission line. The radio receiver amplifies the radio waves and sends them to a loudspeaker, reproducing the original message.

III

Properties of Antennas

An antenna’s size and shape depend on the intended frequency or wavelength of the radio waves being sent or received. The design of a transmitting antenna is usually not different from that of a receiving antenna. Some devices use the same antenna for both purposes.

A

Size

An antenna works best when its physical size corresponds to a quantity known as the antenna’s electrical size. The electrical size of an antenna depends on the wavelength of the radio waves being sent or received. An antenna radiates energy most efficiently when its length is a particular fraction of the intended wavelength. When the length of an antenna is a major fraction of the corresponding wavelength (a quarter-wavelength or half-wavelength is often used), the radio waves oscillating back and forth along the antenna will encounter each other in such a way that the wave crests do not interfere with one another. The waves will resonate, or be in harmony, and will then radiate from the antenna with the greatest efficiency.

If an antenna is not long enough or is too long for the intended radio frequency, the wave crests will encounter and interfere with one another as they travel back and forth along the antenna, thus reducing the efficiency. The antenna then acts like a capacitor or an inductor (depending on the shape of the antenna) and stores, rather than radiates, energy. The electrical length of an antenna can be altered by adding a metal loop of wire known as a loading coil to one end of the antenna, thus increasing the amount of wire in the antenna. Loading coils are used when the practical length of an antenna would be too long. Adding a coil to a short antenna increases the antenna’s electrical length, improves its resonance at the desired frequency, and increases the antenna’s efficiency.

The radio waves used by AM radio have wavelengths of about 300 m (about 1,000 ft). Most AM transmitter antennas are built to a height of about 75 m (about 250 ft), which, in this case, is the length of a quarter-wavelength. With a tower of this height, an AM radio antenna will radiate radio waves most efficiently. Since an antenna that is 75 meters tall would be impractical for a portable AM radio receiver, AM radios use a special coil of wire inside the radio for an antenna. The coil of wire is wrapped around an iron-like magnetic material called a ferrite. When radio waves come into contact with the coil of wire, they induce an electric charge within the coil. The magnetic ferrite helps confine and concentrate the electrical energy in the coil and aids in reception.

Television and FM radio use tall broadcast towers as well but use much shorter wavelengths, corresponding to much higher frequencies, than AM radio. Therefore, television and FM radio waves have wavelengths of only about 3 m (about 10 ft). As a result, the corresponding antennas are much shorter. Buildings and other obstructions close to the ground can block these high-frequency radio waves. Thus the towers are used to raise the antennas above these obstructions in order to provide a greater broadcasting range. Receiving antennas for television sets and FM radios are small enough to be installed on these devices themselves, but the antennas are often mounted high on rooftops for better reception.

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