![Aerial antenna](https://upload.wikimedia.org/wikipedia/commons/f/f9/Aerial_antenna.JPG)
A television antenna (TV aerial) is an
antenna
Antenna ( antennas or antennae) may refer to:
Science and engineering
* Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves
* Antennae Galaxies, the name of two collid ...
specifically designed for use with a
television receiver
A television set or television receiver, more commonly called the television, TV, TV set, telly, tele, or tube, is a device that combines a tuner, display, and loudspeakers, for the purpose of viewing and hearing television broadcasts, or using ...
(TV) to receive
over-the-air broadcast television signals from a
television station
A television station is a set of equipment managed by a business, organisation or other entity, such as an amateur television (ATV) operator, that transmits video content and audio content via radio waves directly from a transmitter on the earth ...
. Television reception is dependent upon the antenna as well as the
transmitter
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which i ...
.
Terrestrial television
Terrestrial television or over-the-air television (OTA) is a type of television broadcasting in which the signal transmission occurs via radio waves from the terrestrial (Earth-based) transmitter of a TV station to a TV receiver having an ant ...
is broadcast on frequencies from about 47 to 250
MHz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...
in the
very high frequency
Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves ( radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter.
Frequencies immediately below VHF ...
(VHF) band, and 470 to 960 MHz in the
ultra high frequency
Ultra high frequency (UHF) is the ITU designation for radio frequency, radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one ten ...
(UHF) band
in different countries. Television antennas are manufactured in two different types: "indoor" antennas, to be located on top of or next to the
television set
A television set or television receiver, more commonly called the television, TV, TV set, telly, tele, or tube, is a device that combines a tuner, display, and loudspeakers, for the purpose of viewing and hearing television broadcasts, or using ...
, and "outdoor" antennas, mounted on a
mast on top of the owner's house.
[Johnson 1993 ]
Antenna Engineering Handbook, 3rd Ed.
', p. 29.5-29.6 They can also be mounted in a loft or attic, where the dry conditions and increased elevation are advantageous for reception and antenna longevity. Outdoor antennas are more expensive and difficult to install, but are necessary for adequate reception in
fringe
Fringe may refer to:
Arts
* Edinburgh Festival Fringe, the world's largest arts festival, known as "the Fringe"
* Adelaide Fringe, the world's second-largest annual arts festival
* Fringe theatre, a name for alternative theatre
* The Fringe, the ...
areas far from television stations. The most common types of indoor antennas are the
dipole
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
*An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system ...
("rabbit ears") and
loop antenna
A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that is usually fed by a balanced source or feeding a balanced load. Within this physical description there are two (possibly three) d ...
s, and for outdoor antennas the
Yagi Yagi may refer to:
Places
*Yagi, Kyoto, in Japan
* Yagi (Kashihara), in Nara Prefecture, Japan
*Yagi-nishiguchi Station, in Kashihara, Nara, Japan
* Kami-Yagi Station, a JR-West Kabe Line station located in 3-chōme, Yagi, Asaminami-ku, Hiroshima, ...
,
log periodic,
and for UHF channels the multi-bay
reflective array antenna
In telecommunications and radar, a reflective array antenna is a class of directive antennas in which multiple driven elements are mounted in front of a flat surface designed to reflect the radio waves in a desired direction. They are a type o ...
.
Description
The purpose of the antenna is to intercept
radio waves from the desired
television station
A television station is a set of equipment managed by a business, organisation or other entity, such as an amateur television (ATV) operator, that transmits video content and audio content via radio waves directly from a transmitter on the earth ...
s and convert them to tiny
radio frequency
Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...
alternating current
Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
s which are applied to the television's
tuner, which extracts the television signal.
[Joy, Edward B. ''TV Receiving Antennas'' in ] The antenna is connected to the television with a specialized cable designed to carry radio current, called
transmission line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...
. Earlier antennas used a flat cable called 300
Ω twin-lead
Twin-lead cable is a two-conductor flat cable used as a balanced transmission line to carry radio frequency (RF) signals. It is constructed of two stranded or solid copper or copper-clad steel wires, held a precise distance apart by a plastic ...
. The standard today is 75 Ω
coaxial cable
Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a p ...
, which is less susceptible to interference, which plugs into an
F connector or
Belling-Lee connector (depending on region) on the back of the TV.
[Johnson 1993 ]
Antenna Engineering Handbook, 3rd Ed.
', p. 29.6 To convert the signal from antennas that use twin-lead line to the modern coaxial cable input, a small transformer called a
balun
A balun (from "balanced to unbalanced", originally, but now dated from "balancing unit") is an electrical device that allows balanced and unbalanced lines to be interfaced without disturbing the impedance arrangement of either line. A balun ...
is used in the line.
In most countries, television broadcasting is allowed in the
very high frequency
Very high frequency (VHF) is the ITU designation for the range of radio frequency electromagnetic waves ( radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter.
Frequencies immediately below VHF ...
(VHF) band from 47 to 68 MHz, called VHF low band or
band I in Europe; 174 to 216 MHz, called VHF high band or
band III in Europe, and in the
ultra high frequency
Ultra high frequency (UHF) is the ITU designation for radio frequency, radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one ten ...
(UHF) band from 470 to 698 MHz, called
band IV and
V in Europe.
[Johnson 1993 ]
Antenna Engineering Handbook, 3rd Ed.
', p. 29.2 The boundaries of each band vary somewhat in different countries. Radio waves in these bands travel by
line-of-sight; they are blocked by hills and the visual horizon, limiting a television station's reception area to , depending on terrain.
Analog ''vs''. digital
In the previous standard
analog television
Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, instantaneous phase and frequency, ...
, used before 2006, the VHF and UHF bands required separate
tuners in the television receiver, which had separate antenna inputs. The
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
of a
radio wave equals the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
(c), divided by the frequency. The above frequency bands cover a 15:1
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
ratio, or almost 4
octave
In music, an octave ( la, octavus: eighth) or perfect octave (sometimes called the diapason) is the interval between one musical pitch and another with double its frequency. The octave relationship is a natural phenomenon that has been refer ...
s. It is difficult to design a single antenna to receive such a wide wavelength range, and there is an octave gap from 216 to 470 MHz between the VHF and UHF frequencies. So traditionally separate antennas (or on outdoor antennas separate sets of elements on a single support boom) have been used to receive the VHF and UHF channels.
Starting in 2006, many countries in the world switched from broadcasting using an older
analog television
Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, instantaneous phase and frequency, ...
standard to newer
digital television
Digital television (DTV) is the transmission of television signals using digital encoding, in contrast to the earlier analog television technology which used analog signals. At the time of its development it was considered an innovative advanc ...
(DTV). However generally the same broadcast frequencies are used, so the same antennas used for the older analog television will also receive the new DTV broadcasts. Sellers often claim to supply a special "digital" or "
high-definition television
High-definition television (HD or HDTV) describes a television system which provides a substantially higher image resolution than the previous generation of technologies. The term has been used since 1936; in more recent times, it refers to the g ...
" (HDTV) antenna advised as a replacement for an existing
analog television
Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, instantaneous phase and frequency, ...
antenna; at best this is misinformation to generate sales of unneeded equipment, at worst it may leave the viewer with a UHF-only antenna in a local market (particularly in North America) where some digital stations remain on their original
high VHF frequencies.
Reception issues
![West german tv penetration](https://upload.wikimedia.org/wikipedia/commons/8/86/West_german_tv_penetration.svg)
Places unable to be reached by television broadcast transmitters are known as "black spots" in Australia. In
East Germany
East Germany, officially the German Democratic Republic (GDR; german: Deutsche Demokratische Republik, , DDR, ), was a country that existed from its creation on 7 October 1949 until its dissolution on 3 October 1990. In these years the state ...
, the areas that could not receive western TV signals were referred to as the ''
Tal der Ahnungslosen'', or "Valley of the Clueless".
Indoor
![Rabbit-ears dipole antenna with UHF loop 20090204](https://upload.wikimedia.org/wikipedia/commons/2/22/Rabbit-ears_dipole_antenna_with_UHF_loop_20090204.jpg)
Indoor antennas may be mounted on the television itself or stand on a table next to it, connected to the television by a short
feed line
In a radio antenna, the feed line (feedline), or feeder, is the cable or other transmission line that connects the antenna with the radio transmitter or receiver. In a transmitting antenna, it feeds the radio frequency (RF) current from the ...
. Due to space constraints, indoor antennas cannot be as large and elaborate as outdoor antennas, they are not mounted at as high an elevation, and the building walls block some of the radio waves; for these reasons, indoor antennas generally do not give as good reception as outdoor antennas.
They are often perfectly adequate in urban and suburban areas which are usually within the strong radiation "footprint" of local television stations, but in rural
fringe reception
A broadcast range (also listening range or listening area for radio, or viewing range or viewing area for television) is the service area that a broadcast station or other transmission covers via radio waves (or possibly infrared light, which ...
areas only an outdoor antenna may give adequate reception. A few of the simplest indoor antennas are described below, but a great variety of designs and types exist. Many have a dial on the antenna with a number of different settings to alter the antenna's reception pattern. This should be rotated with the set on while looking at the screen until the best picture is obtained.
Rabbit ears
The oldest and most widely used indoor antenna is the ''rabbit ears'' or ''bunny ears'', which are often provided with new television sets.
It is a simple
half-wave dipole
In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole wi ...
antenna used to receive the VHF television bands, consisting in the US of 54 to 88 MHz (
band I) and 174 to 216 MHz (
band III), with wavelengths of . It is constructed of two telescoping rods attached to a base, which extend out to about length (approximately one quarter wavelength at 54 MHz), and can be collapsed when not in use. For best reception, the rods should be adjusted to be a little less than
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
at the frequency of the television channel being received. However the dipole has a wide
bandwidth
Bandwidth commonly refers to:
* Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range
* Bandwidth (computing), the rate of data transfer, bit rate or thr ...
, so often adequate reception is achieved without adjusting the length.
The measured
gain
Gain or GAIN may refer to:
Science and technology
* Gain (electronics), an electronics and signal processing term
* Antenna gain
* Gain (laser), the amplification involved in laser emission
* Gain (projection screens)
* Information gain in de ...
of rabbit ears is low, about ―2 dBi, or ―4 dB with respect to a half wave dipole.
This means it is not as directional and sensitive to distant stations as a large rooftop antenna, but its wide angle reception pattern may allow it to receive several stations located in different directions without requiring readjustment when the channel is changed. Dipole antennas are bi-directional, that is, they have two
main lobe
In a radio antenna's radiation pattern, the main lobe, or main beam, is the lobe containing the higher power. This is the lobe that exhibits the greater field strength.
The radiation pattern of most antennas shows a pattern of "''lobes''" at v ...
s in opposite directions, 180° apart. Instead of being fixed in position like other antennas, the elements are mounted on ball-and-socket joints and can be adjusted to various angles in a "V" shape, allowing them to be moved out of the way in crowded quarters. Another reason for the V shape is that when receiving channels at the top of the band with the rods fully extended, the antenna elements will typically resonate at their 3rd
harmonic. In this mode, the direction of maximum gain (the
main lobe
In a radio antenna's radiation pattern, the main lobe, or main beam, is the lobe containing the higher power. This is the lobe that exhibits the greater field strength.
The radiation pattern of most antennas shows a pattern of "''lobes''" at v ...
) is no longer perpendicular to the rods, but the radiation pattern will have lobes at an angle to the rods, making it advantageous to be able to adjust them to various angles.
Whip antenna
Some portable televisions use a
whip antenna
A whip antenna is an antenna consisting of a straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter. A whip antenna is a form of monopole antenna. The antenna is designed to be flexible so ...
.
This consists of a single telescoping rod about long attached to the television, which can be retracted when not in use. It functions as a
quarter-wave monopole antenna
A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane.
The driving signal from the transmitter is applied, o ...
. The other side of the feedline is connected to the
ground plane on the TV's circuit board, which acts as
ground. The whip antenna generally has an
omnidirectional
Omnidirectional refers to the notion of existing in every direction. Omnidirectional devices include:
* Omnidirectional antenna, an antenna that radiates equally in all directions
* VHF omnidirectional range, a type of radio navigation system for ...
reception pattern, with maximum sensitivity in directions perpendicular to the antenna axis, and gain similar to rabbit ears.
Loop antenna
The UHF channels are often received by a single turn
loop antenna
A loop antenna is a radio antenna consisting of a loop or coil of wire, tubing, or other electrical conductor, that is usually fed by a balanced source or feeding a balanced load. Within this physical description there are two (possibly three) d ...
.
Since a "rabbit ears" antenna only covers the VHF bands, it is often combined with a UHF loop mounted on the same base to cover all the TV channels. Also, it is used in some antennas notably Antennas Direct (ClearStream).
Flat antenna
Soon after television broadcasting switched from analog to digital broadcasting, indoor antenna marketing evolved beyond the traditional "rabbit ears." Flat antennas are lightweight, thin, and usually square-shaped with the claim of having more omnidirectional reception. They connect to televisions only with a coaxial cable; they may also be sold with a signal amplifier requiring a power source. Internally, the thin, flat square is a loop antenna, with its circular metallic wiring embedded into conductive plastic.
Bar antenna
As a co-existed with flat antenna, bar antenna is an antenna with the elements inside . It was sold with a signal amplifier requiring a power source. Antop is an example, because they invented the bar antenna. Also bar antenna is also use outdoors as well, thus having excellent reception than flat antenna.
Outdoor
When a
higher-gain antenna is needed to achieve adequate reception in suburban or
fringe reception
A broadcast range (also listening range or listening area for radio, or viewing range or viewing area for television) is the service area that a broadcast station or other transmission covers via radio waves (or possibly infrared light, which ...
areas, an outdoor
directional antenna
A directional antenna or beam antenna is an antenna which radiates or receives greater power in specific directions allowing increased performance and reduced interference from unwanted sources. Directional antennas provide increased performance ...
is usually used. Although most simple antennas have
null
Null may refer to:
Science, technology, and mathematics Computing
* Null (SQL) (or NULL), a special marker and keyword in SQL indicating that something has no value
* Null character, the zero-valued ASCII character, also designated by , often use ...
directions where they have zero response, the directions of useful gain are very broad. In contrast, directional antennas can have an almost unidirectional
radiation pattern
In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the ''directional'' (angular) dependence of the strength of the radio waves from the antenna or other source.Constantine A. Balanis: “A ...
so the correct end of the antenna must be pointed at the TV station. As an antenna design provides higher gain (compared to a dipole), the
main lobe
In a radio antenna's radiation pattern, the main lobe, or main beam, is the lobe containing the higher power. This is the lobe that exhibits the greater field strength.
The radiation pattern of most antennas shows a pattern of "''lobes''" at v ...
of the radiation pattern becomes narrower. Outdoor antennas provide up to a 15 dB gain in signal strength and 15-20 dB greater rejection of
ghost signals in analog TV.
Combined with a signal increase of 14 dB due to height, and 11 dB due to lack of attenuating building walls, an outdoor antenna can result in a signal strength increase of up to 40 dB at the TV receiver.
Outdoor antenna designs are often based on the
Yagi–Uda antenna
A Yagi–Uda antenna or simply Yagi antenna, is a directional antenna consisting of two or more parallel resonant antenna elements in an end-fire array; these elements are most often metal rods acting as half-wave dipoles. Yagi–Ud ...
[Johnson 1993 ]
Antenna Engineering Handbook, 3rd Ed.
', p. 29.16-29.21 or
log-periodic dipole array (LPDA).
[Johnson 1993 ]
Antenna Engineering Handbook, 3rd Ed.
', p. 29.14-29.16 These are composed of multiple
half-wave dipole
In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna. The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole wi ...
elements, consisting of metal rods approximately half of the
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
of the television signal, mounted in a line on a support boom. These act as
resonator
A resonator is a device or system that exhibits resonance or resonant behavior. That is, it naturally oscillates with greater amplitude at some frequencies, called resonant frequencies, than at other frequencies. The oscillations in a resonator ...
s; the electric field of the incoming radio wave pushes the electrons in the rods back and forth, creating
standing waves of oscillating voltage in the rods. The antenna can have a smaller or larger number of rod elements; in general, the more elements the higher the gain and the more directional. Another design, used mainly for UHF reception, is the
reflective array antenna
In telecommunications and radar, a reflective array antenna is a class of directive antennas in which multiple driven elements are mounted in front of a flat surface designed to reflect the radio waves in a desired direction. They are a type o ...
, consisting of a vertical metal screen with multiple dipole elements mounted in front of it.
The television broadcast bands are too wide in frequency to be covered by a single antenna, so either separate antennas are used for the VHF and UHF bands, or a combination (combo) VHF/UHF antenna.
A VHF/UHF antenna is really two antennas feeding the same feedline mounted on the same support boom. Longer elements which pick up
VHF frequencies are located at the "back" of the boom and often function as a
log-periodic antenna. Shorter elements which receive the
UHF stations are located at the "front" of the boom and often function as a
Yagi antenna Yagi may refer to:
Places
* Yagi, Kyoto, in Japan
* Yagi (Kashihara), in Nara Prefecture, Japan
* Yagi-nishiguchi Station, in Kashihara, Nara, Japan
* Kami-Yagi Station, a JR-West Kabe Line station located in 3-chōme, Yagi, Asaminami-ku, Hiroshima ...
.
Since directional antennas must be pointed at the transmitting antenna, this is a problem when the television stations to be received are located in different directions. In this case, two or more directional rooftop antennas each pointed at a different transmitter are often mounted on the same mast and connected to one receiver; for best performance
filter
Filter, filtering or filters may refer to:
Science and technology
Computing
* Filter (higher-order function), in functional programming
* Filter (software), a computer program to process a data stream
* Filter (video), a software component tha ...
or
matching circuits are used to keep each antenna from degrading the performance of the others connected to the same transmission line. An alternative is to use a single antenna mounted on a ''rotator'', a remote
servo system that rotates the antenna to a new direction when a dial next to the television is turned.
Sometimes television transmitters are deliberately located such that receivers in a given region need only receive transmissions in a relatively narrow band of the full UHF television spectrum and from the same direction hence allowing the use of a higher gain grouped aerial.
File:Mast 1-2.jpg
File:UHF TV Antenna 001.JPG, A UHF television antenna
File:Rabbit ear dipole antenna rooftop elevated.jpg, A rabbit ear indoor antenna weatherproofed and installed outdoors
Installation
![Antenna](https://upload.wikimedia.org/wikipedia/commons/7/76/Antenna.jpg)
Antennas are commonly placed on rooftops, and sometimes in attics. Placing an antenna indoors significantly attenuates the level of the available signal.
Directional antennas must be pointed at the
transmitter
In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which i ...
they are receiving; in most cases great accuracy is not needed. In a given region it is sometimes arranged that all television transmitters are located in roughly the same direction and use frequencies spaced closely enough that a single antenna suffices for all. A single transmitter location may transmit signals for several channels. CABD (communal antenna broadcast distribution) is a system installed inside a building to receive free-to-air TV/FM signals transmitted via radio frequencies and distribute them to the audience.
Analog television
Analog television is the original television technology that uses analog signals to transmit video and audio. In an analog television broadcast, the brightness, colors and sound are represented by amplitude, instantaneous phase and frequency, ...
signals are susceptible to
ghosting in the image, multiple closely spaced images giving the impression of blurred and repeated images of edges in the picture. This is due to the signal being reflected from nearby objects (buildings, tree, mountains); several copies of the signal, of different strengths and subject to different delays, are picked up. This is different for different transmissions. Careful positioning of the antenna can produce a compromise position which minimizes the ghosts on different channels. Ghosting is also possible if multiple antennas connected to the same receiver pick up the same station, especially if the lengths of the cables connecting them to the splitter/merger are different lengths or the antennas are too close together.
Analog television is being replaced by digital, which is not subject to ghosting; the same reflected signal that causes ghosting in an analog signal would produce no viewable content at all in digital. However, in this case, interference causes significantly greater image quality degradation.
Rooftop and other outdoor antennas
Aerials are attached to roofs in various ways, usually on a pole to elevate it above the roof. This is generally sufficient in most areas. In some places, however, such as a deep valley or near taller structures, the antenna may need to be placed significantly higher, using a
guyed mast
A guyed mast or guyed tower is a tall thin vertical structure that depends on guy lines (diagonal tensioned cables attached to the ground) for stability. The mast itself has the compressive strength to support its own weight, but does not ha ...
or
mast. The wire connecting the antenna to indoors is referred to as the ' or ''
drop'', and the longer the downlead is, the greater the signal degradation in the wire. Certain cables may help reduce this tendency.
The higher the antenna is placed, the better it will perform. An antenna of higher
gain
Gain or GAIN may refer to:
Science and technology
* Gain (electronics), an electronics and signal processing term
* Antenna gain
* Gain (laser), the amplification involved in laser emission
* Gain (projection screens)
* Information gain in de ...
will be able to receive weaker signals from its preferred direction. Intervening buildings, topographical features (mountains), and dense forest will weaken the signal; in many cases, the signal will be reflected such that a usable signal is still available. There are physical dangers inherent to high or complex antennas, such as the structure falling or being destroyed by weather. There are also varying
local ordinance
A local ordinance is a law issued by a local government. such as a municipality, county, parish, prefecture, or the like.
China
In Hong Kong, all laws enacted by the territory's Legislative Council remain to be known as ''Ordinances'' () af ...
s which restrict and limit such things as the height of a structure without obtaining
permits. For example, in the United States, the
Telecommunications Act of 1996
The Telecommunications Act of 1996 is a United States federal law enacted by the 104th United States Congress on January 3, 1996, and signed into law on February 8, 1996, by President Bill Clinton. It primarily amended Chapter 5 of Title 47 of ...
allows any homeowner to install "An antenna that is designed to receive local television broadcast signals", but that "masts higher than above the roof-line may be subject to local permitting requirements."
Indoor antennas
As discussed previously, antennas may be placed indoors where signals are strong enough to overcome antenna shortcomings. The antenna is simply plugged into the television receiver and placed conveniently, often on the top of the receiver ("set-top"). Sometimes the position needs to be experimented with to get the best picture. Indoor antennas can also benefit from RF amplification, commonly called a TV booster. Reception form indoor antennas can be problematic in weak signal areas.
Attic installation
Sometimes it is desired not to put an antenna on the roof; in these cases, antennas designed for outdoor use are often mounted in the attic or loft, although antennas designed for attic use are also available. Putting an antenna indoors significantly decreases its performance due to lower elevation above ground level and intervening walls; however, in strong signal areas reception may be satisfactory.
One layer of
asphalt
Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term a ...
shingles,
roof felt
Bituminous waterproofing systems are designed to protect residential and commercial buildings. Bitumen (asphalt or coal-tar pitch) is a mixed substance made up of organic liquids that are highly sticky, viscous, and waterproof. These systems ar ...
, and a
plywood roof deck
In architecture, a deck is a flat surface capable of supporting weight, similar to a floor, but typically constructed outdoors, often elevated from the ground, and usually connected to a building. The term is a generalization from the deck ...
is considered to attenuate the signal to about half.
Multiple antennas, rotators
![Multiple Antenna](https://upload.wikimedia.org/wikipedia/commons/d/db/Multiple_Antenna.JPG)
It is sometimes desired to receive signals from transmitters which are not in the same direction. This can be achieved, for one station at a time, by using a
rotator operated by an electric motor to turn the antenna as desired. Alternatively, two or more antennas, each pointing at a desired transmitter and coupled by appropriate circuitry, can be used. To prevent the antennas from interfering with each other, the vertical spacing between the booms must be at least half the wavelength of the lowest frequency to be received (Distance = ).
The wavelength of 54 MHz (Channel 2) is (λ × f =
c) so the antennas must be a minimum of apart. It is also important that the cables connecting the antennas to the signal splitter/merger be exactly the same length, to prevent
phasing
A phaser is an electronic sound processor used to filter a signal, and it has a series of troughs in its frequency-attenutation graph. The position (in Hz) of the peaks and troughs are typically modulated by an internal low-frequency oscil ...
issues, which cause ghosting with analog reception. That is, the antennas might both pick up the same station; the signal from the one with the shorter cable will reach the receiver slightly sooner, supplying the receiver with two pictures slightly offset. There may be phasing issues even with the same length of down-lead cable.
Band-pass filter
A band-pass filter or bandpass filter (BPF) is a device that passes frequencies within a certain range and rejects (attenuates) frequencies outside that range.
Description
In electronics and signal processing, a filter is usually a two-por ...
s or "signal traps" may help to reduce this problem.
For side-by-side placement of multiple antennas, as is common in a space of limited height such as an attic, they should be separated by at least one full wavelength of the lowest frequency to be received at their closest point.
Often when multiple antennas are used, one is for a range of co-located stations and the other is for a single transmitter in a different direction.
Safety
* TV antennas are good conductors of electricity and attract lightning, acting as a
lightning rod
A lightning rod or lightning conductor (British English) is a metal rod mounted on a structure and intended to protect the structure from a lightning strike. If lightning hits the structure, it will preferentially strike the rod and be conducte ...
. The use of a
lightning arrestor
A lightning arrester (alternative spelling lightning arrestor) (also called lightning isolator) is a device, essentially an air gap between an electric wire and ground, used on electric power transmission and telecommunication systems to protect ...
is usual to protect against this. A large grounding rod connected to both the antenna and the mast or pole is required.
* Properly installed masts, especially tall ones, are
guyed with
galvanized
Galvanization or galvanizing ( also spelled galvanisation or galvanising) is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are submerged ...
cable; no insulators are needed. They are designed to withstand worst-case weather conditions in the area and positioned so that they do not interfere with
power lines
Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines that facilitate this movement form a ''transmission network''. This is d ...
if they fall.
* There is an inherent danger in being on the rooftop of a house, required for installing or adjusting a television antenna.
See also
*
Radio masts and towers, sometimes called Radio and TV antennas
*
Satellite dish
*
Satellite television
Satellite television is a service that delivers television programming to viewers by relaying it from a communications satellite orbiting the Earth directly to the viewer's location. The signals are received via an outdoor parabolic antenna comm ...
*
Terrestrial television
Terrestrial television or over-the-air television (OTA) is a type of television broadcasting in which the signal transmission occurs via radio waves from the terrestrial (Earth-based) transmitter of a TV station to a TV receiver having an ant ...
References
External links
Article on the basic theory of TV aerials and their useSee Which TV Stations You Can Get on a Map
{{DEFAULTSORT:Television Antenna
Antennas (radio)
Radio electronics
Radio frequency antenna types
Radio frequency propagation
Radio technology
Antenna
Antenna ( antennas or antennae) may refer to:
Science and engineering
* Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves
* Antennae Galaxies, the name of two collid ...