Friday, January 13, 2012

Antenna



Antenna
Types of Antennas
Receiving antennas come in many different shapes, depending on the frequency and wavelength of the intended signal. A portable FM radio uses a half-dipole antenna to receive radio signals. The other half of the dipole is attached to the radio casing and acts as a ground. VHF television antennas use multiple elements to receive a broader range of broadcast signals. Many TV antennas include directors and reflectors, which are extra pieces of metal that reflect and focus TV waves into the dipole elements. TV satellite dishes are also reflectors. They focus high-frequency microwaves from satellites into the receiving element mounted in front of the dish.

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
Television Tower
Television antennas are built on tall towers so that high-frequency signals (which only travel in a straight line) can reach viewers without being blocked by nearby hills or buildings. Small dishes on this tower send and receive microwave signals from other stations or from reporters broadcasting live from nearby locations.

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.
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
Microwave Tower
Microwave transmissions are beamed from point to point using tall antennas. The antennas must be within sight of each other, since the microwave signals travel in straight, narrow paths.

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.
B
Shape
Antennas come in a wide variety of shapes. One of the simplest types of antennas is called a dipole. A dipole is made of two lengths of metal, each of which is attached to one of two wires leading to a radio or other communications device. The two lengths of metal are usually arranged end to end, with the cable from the transmitter or receiver feeding each length of the dipole in the middle. The dipoles can be adjusted to form a straight line or a V-shape to enhance reception. Each length of metal in the dipole is usually a quarter-wavelength long, so that the combined length of the dipole from end to end is a half-wavelength. The familiar “rabbit-ear” antenna on top of a television set is a dipole antenna.
Another common antenna shape is the half-dipole or monopole antenna, which uses a single quarter-wavelength piece of metal connected to one of the twin wires from the transmitter or receiver. The other wire is connected to a ground, or a point that is not connected to the rest of the circuit. The casing of a radio or cellular telephone is often used as a ground. The telescoping antenna in a portable FM radio is a monopole. This arrangement is not as efficient as using both ends of a dipole, but a monopole is usually sufficient to pick up nearby FM signals.
Satellites and radar telescopes use microwave signals. Microwaves have extremely high frequencies and, thus, very short wavelengths (less than 30 cm). Microwaves travel in straight lines, much like light waves do. Dish antennas are often used to collect and focus microwave signals. The dish focuses the microwaves and aims them at a receiver antenna in the middle of the dish. Horn antennas are also used to focus microwaves for transmission and reception.
C
Directivity
Directivity is an important quality of an antenna. It describes how well an antenna concentrates, or bunches, radio waves in a given direction. A dipole transmits or receives most of its energy at right angles to the lengths of metal, while little energy is transferred along them. If the dipole is mounted vertically, as is common, it will radiate waves away from the center of the antenna in all directions. However, for a commercial radio or television station, a transmitting antenna is often designed to concentrate the radiated energy in certain directions and suppress it in others. For instance, several dipoles can be used together if placed close to one another. Such an arrangement is called a multiple-element antenna, which is also known as an array. By properly arranging the separate elements and by properly feeding signals to the elements, the broadcast waves can be more efficiently concentrated toward an intended audience, without, for example, wasting broadcast signals over uninhabited areas.
The elements used in an array are usually all of the same type. Some arrays have the ability to move, or scan, the main beam in different directions. Such arrays are usually referred to as scanning arrays.
Arrays are usually electrically large and have better directivity than single element antennas. Since their directivity is large, arrays can capture and deliver to the receiver a larger amount of power. Two common arrays used for rooftop television reception are the Yagi-Uda array and the log-periodic array.
A Yagi-Uda consists of one or more dipoles mounted on a crossbar. The dipoles are of different lengths, corresponding to the different frequencies used in broadcast television transmission. Additional pieces of metal, which are called directors and reflectors, are placed on the crossbar in front of and behind the dipoles. Directors and reflectors are not wired into the feed element of the antenna at all but merely reflect and concentrate radio waves toward the the directors. Yagi-Uda antennas are highly directive, and receiving antennas of this type are often mounted on rotating towers or bases, so that these antennas can be turned toward the source of the desired transmission. Log-periodic arrays look similar to Yagi-Uda arrays, but all of the elements in a log-periodic array are active dipole elements of different lengths. The dipoles are carefully spaced to provide signal reception over a wide range of frequencies.
While the dipole, monopole, microwave dish, horn, Yagi-Uda, and log-periodic are among the most common types of antennas, many other designs also exist for communicating at different frequencies. Submarines traveling underwater can receive coded radio commands from shore by using extremely low frequency (ELF) radio waves. In order to receive these signals, a submarine unravels a very long wire antenna behind as it travels underwater. Television camera crews broadcasting from locations outside the studio use powerful microwave transmitter antennas, which can send signals to satellites or directly to the television station. Amateur, or “ham,” radio enthusiasts, who generally use frequencies between those of AM and FM radio, often construct their own antennas, customizing them for sending and receiving signals at desired frequencies.




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