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.
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HOW ANTENNAS WORK
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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.
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PROPERTIES OF ANTENNAS
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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.
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Size
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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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Shape
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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.
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Directivity
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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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