Description

fig.1 - How antennas do look

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.
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.

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.
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.

Antenna properties

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.

Example:

AM antennas

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.
FM antennas
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.

fig.2 - AM and FM waves

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.

Useful links

Electromagnetic induction