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
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
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
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
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
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.
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
fig.2 - AM and FM
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