Television antenna
Encyclopedia
A television antenna, or TV aerial, is an antenna
specifically designed for the reception
of over the air broadcast television signals, which are transmitted at frequencies from about 41 to 250 MHz in the VHF band, and 470 to 960 MHz in the UHF band in different countries
. To cover this range antennas generally consist of multiple conductors of different lengths which correspond to the wavelength
range the antenna is intended to receive. The length of the elements of a TV antenna are usually half the wavelength of the signal they are intended to receive. The wavelength of a signal equals the speed of light (c)
divided by the frequency. The design of a television broadcast receiving antenna is the same for the older analog transmissions and the digital television
(DTV) transmissions which are replacing them. Sellers often claim to supply a special "digital" or "high-definition television
" (HDTV) antenna advised as a replacement for an existing analog television
antenna, even if satisfactory: this is misinformation to generate sales of unneeded equipment.
Television antennas are used in conjunction with a tuner (television) that are included with television set
s.
for VHF or UHF loop antenna
s that are made to be placed indoors
are often used for television (and VHF radio); these are often called "rabbit ears" or "bunny aerials". because of their appearance. The length of the telescopic "ears" can be adjusted by the user, and should be about one half of the wavelength of the signal for the desired channel. These are not as efficient as an aerial rooftop antenna since they are less directional and not always adjusted to the proper length for the desired channel. Dipole antennas are bi-directional, that is, they receive
evenly forward and backwards, and also cover a broader band than antennas with more elements. This makes them less efficient than antennas designed to maximise the signal from a narrower angle in one direction. Coupled with the poor placing, indoors and closer to the ground, they are much worse than multi-element rooftop antennas at receiving signals which are not very strong, although often adequate for nearby transmitters, in which case they may be adequate and cheap. These simple antennas are called set-top antennas because they were often placed on top of the television set
or receiver.
The actual length of the ears is optimally about 91% of half the wavelength of the desired channel in free space
. Quarter-wave television antennas are also used. These use a single element, and use the earth as a ground plane
; therefore, no ground is required in the feed line
. See also: Dipole antenna#Quarter-wave antenna
. The length of the elements is about one half of the signal wavelength. Therefore, the length of each element corresponds to a certain frequency.
In a combined VHF/UHF antenna the longer elements (for picking up VHF frequencies) are at the "back" of the antenna, relative to the device's directionality, and the much shorter UHF elements are in the "front", and the antenna works best when "pointing" to the source of the signal to be received. The smallest elements in this design, located in the "front", are UHF director elements, which are usually identical and give the antenna its directionality, as well as improving gain
. The longest elements, located in the "back" of the antenna form a VHF phased array
. Other long elements may be UHF reflectors
Another common aerial antenna element is the corner reflector
, a type of UHF reflector which increases gain and directionality for UHF frequencies.
An antenna can have a smaller or larger number of directors; the more directors it has (requiring a longer boom), and the more accurate their tuning, the higher its gain will be. For the commonly used Yagi antenna
this is not a linear relationship. Antenna gain
is the ratio of the signal received from the preferred direction to the signal from an ideal omnidirectional antenna. Gain is inversely proportional to the antenna's acceptance angle. The thickness of the rods on a Yagi antenna and its bandwidth are inversely proportional; thicker rods provide a wider band. Thinner rods are preferable to provide a narrower band, hence higher gain in the preferred direction; however, they must be thick enough to withstand wind.
Two or more directional rooftop antennas can be set up and connected to one receiver. Antennas designed for rooftop use are sometimes located in attics.
Sometimes television transmitters are organised such that all receivers in a given location need receive transmissions in only a relatively narrow band of the full UHF television spectrum and from the same direction, so that a single antenna provides reception from all stations.
s. Placing an antenna indoors significantly attenuates the signal available to it. Directional antennas must be pointed at the transmitter
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 space closely enough that a single antenna suffices for all. A single transmitter location may transmit signals for several channels.
Analog television
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 was 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 was different for different transmissions. Careful positioning of the antenna could produce a compromise position which minimized 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 were different lengths or the antennas were too close together. Analog television is being replaced by digital, which is not subject to ghosting.
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.
The higher the antenna is placed, the better it will perform. An antenna of higher gain
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 the weather. There are also varying local ordinance
s which restrict and limit such things as the height of a structure without obtaining permits
. For example, in the USA, the Telecommunications Act of 1996
allows any homeowner to install "An antenna that is designed to receive local television broadcast signals", but that "masts higher than 12 feet above the roof-line may be subject to local permitting requirements."
will never be an option in weak signal areas.
shingles, roof felt, and a plywood
roof deck
is considered to attenuate the signal to about half.
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 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=λ/2). The wavelength of 54 MHz (Channel 2) is 5.5 meters (λ x f = c
) so the antennas must be a minimum of 2.25 meters, or ~89 inches apart. It is also important that the cables connecting the antennas to the signal splitter/merger be exactly the same length, to prevent phasing
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. Bandpass filters
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.
Antenna
Antenna may refer to:-Science and engineering:* Antenna , also known as an aerial, a transducer designed to transmit or receive electromagnetic Antenna (pl. antennas in radio/TV, antennae in biology) may refer to:-Science and engineering:* Antenna (radio), also known as an aerial, a transducer...
specifically designed for the reception
Reception
Reception is a noun form of receiving, or to receive something, such as information, art, experience, or people. It is often used in the following contexts:...
of over the air broadcast television signals, which are transmitted at frequencies from about 41 to 250 MHz in the VHF band, and 470 to 960 MHz in the UHF band in different countries
Television channel frequencies
The following tables show the frequencies assigned to broadcast television channels in various regions of the world, along with the ITU letter designator for the system used. The frequencies shown are for the video and audio carriers. The channel itself occupies several megahertz of bandwidth....
. To cover this range antennas generally consist of multiple conductors of different lengths which correspond to the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
range the antenna is intended to receive. The length of the elements of a TV antenna are usually half the wavelength of the signal they are intended to receive. The wavelength of a signal equals the speed of light (c)
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
divided by the frequency. The design of a television broadcast receiving antenna is the same for the older analog transmissions and the digital television
Digital television
Digital television is the transmission of audio and video by digital signals, in contrast to the analog signals used by analog TV...
(DTV) transmissions which are replacing them. Sellers often claim to supply a special "digital" or "high-definition television
High-definition television
High-definition television is video that has resolution substantially higher than that of traditional television systems . HDTV has one or two million pixels per frame, roughly five times that of SD...
" (HDTV) antenna advised as a replacement for an existing analog television
Analog television
Analog television is the analog transmission that involves the broadcasting of encoded analog audio and analog video signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency of the signal...
antenna, even if satisfactory: this is misinformation to generate sales of unneeded equipment.
Television antennas are used in conjunction with a tuner (television) that are included with television set
Television set
A television set is a device that combines a tuner, display, and speakers for the purpose of viewing television. Television sets became a popular consumer product after the Second World War, using vacuum tubes and cathode ray tube displays...
s.
Simple/indoor
Simple half-wave dipole antennaDipole antenna
A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven element. It consists of two metal conductors of rod or wire, oriented parallel and collinear with each other , with a small space between them. The radio frequency voltage is applied to the antenna at...
for VHF or UHF loop antenna
Loop antenna
A loop antenna is a radio antenna consisting of a loop of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line...
s that are made to be placed indoors
Indoor antenna
An Indoor antenna is a type of radio or TV antenna placed indoors, as opposed to being mounted on the roof. Indoor antennas are usually a simple and cheap solution that may work well when the receiver is relatively near to the broadcasting transmitter and the building walls do not shield the radio...
are often used for television (and VHF radio); these are often called "rabbit ears" or "bunny aerials". because of their appearance. The length of the telescopic "ears" can be adjusted by the user, and should be about one half of the wavelength of the signal for the desired channel. These are not as efficient as an aerial rooftop antenna since they are less directional and not always adjusted to the proper length for the desired channel. Dipole antennas are bi-directional, that is, they receive
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
evenly forward and backwards, and also cover a broader band than antennas with more elements. This makes them less efficient than antennas designed to maximise the signal from a narrower angle in one direction. Coupled with the poor placing, indoors and closer to the ground, they are much worse than multi-element rooftop antennas at receiving signals which are not very strong, although often adequate for nearby transmitters, in which case they may be adequate and cheap. These simple antennas are called set-top antennas because they were often placed on top of the television set
Television set
A television set is a device that combines a tuner, display, and speakers for the purpose of viewing television. Television sets became a popular consumer product after the Second World War, using vacuum tubes and cathode ray tube displays...
or receiver.
The actual length of the ears is optimally about 91% of half the wavelength of the desired channel in free space
Free-space optical communication
Free-space optical communication is an optical communication technology that uses light propagating in free space to transmit data for telecommunications or computer networking."Free space" means air, outer space, vacuum, or something similar...
. Quarter-wave television antennas are also used. These use a single element, and use the earth as a ground plane
Ground plane
In electrical engineering, a ground plane is an electrically conductive surface.-Radio antenna theory :In telecommunication, a ground plane structure or relationship exists between the antenna and another object, where the only structure of the object is a structure which permits the antenna to...
; therefore, no ground is required in the feed line
Feed line
In a radio antenna, the feed line is the cable or other transmission line that connects the antenna with the radio transmitter or receiver. In a transmitter, it feeds the radio frequency current from the transmitter to the antenna, where it is radiated as radio waves. In a receiver it transfers...
. See also: Dipole antenna#Quarter-wave antenna
Outdoor
An aerial or rooftop antenna generally consists of multiple conductive elements that are arranged such that it is a directional antennaDirectional antenna
A directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....
. The length of the elements is about one half of the signal wavelength. Therefore, the length of each element corresponds to a certain frequency.
In a combined VHF/UHF antenna the longer elements (for picking up VHF frequencies) are at the "back" of the antenna, relative to the device's directionality, and the much shorter UHF elements are in the "front", and the antenna works best when "pointing" to the source of the signal to be received. The smallest elements in this design, located in the "front", are UHF director elements, which are usually identical and give the antenna its directionality, as well as improving gain
Antenna gain
In electromagnetics, an antenna's power gain or simply gain is a key performance figure which combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the figure describes how well the antenna converts input power into radio waves headed in a specified direction...
. The longest elements, located in the "back" of the antenna form a VHF phased array
Phased array
In wave theory, a phased array is an array of antennas in which the relative phases of the respective signals feeding the antennas are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions.An antenna array...
. Other long elements may be UHF reflectors
Reflector (antenna)
An antenna reflector is a device that reflects electromagnetic waves.It is often a part of an antenna assembly.The most common reflector types are...
Another common aerial antenna element is the corner reflector
Corner reflector
A corner reflector is a retroreflector consisting of three mutually perpendicular, intersecting flat surfaces, which reflects waves back directly towards the source, but shifted . Unlike a simple mirror, they work for a relatively wide-angle field of view. The three intersecting surfaces often have...
, a type of UHF reflector which increases gain and directionality for UHF frequencies.
An antenna can have a smaller or larger number of directors; the more directors it has (requiring a longer boom), and the more accurate their tuning, the higher its gain will be. For the commonly used Yagi antenna
Yagi antenna
A Yagi-Uda array, commonly known simply as a Yagi antenna, is a directional antenna consisting of a driven element and additional parasitic elements...
this is not a linear relationship. Antenna gain
Antenna gain
In electromagnetics, an antenna's power gain or simply gain is a key performance figure which combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the figure describes how well the antenna converts input power into radio waves headed in a specified direction...
is the ratio of the signal received from the preferred direction to the signal from an ideal omnidirectional antenna. Gain is inversely proportional to the antenna's acceptance angle. The thickness of the rods on a Yagi antenna and its bandwidth are inversely proportional; thicker rods provide a wider band. Thinner rods are preferable to provide a narrower band, hence higher gain in the preferred direction; however, they must be thick enough to withstand wind.
Two or more directional rooftop antennas can be set up and connected to one receiver. Antennas designed for rooftop use are sometimes located in attics.
Sometimes television transmitters are organised such that all receivers in a given location need receive transmissions in only a relatively narrow band of the full UHF television spectrum and from the same direction, so that a single antenna provides reception from all stations.
Installation
Antennas are commonly placed on rooftops, and sometimes in atticAttic
An attic is a space found directly below the pitched roof of a house or other building . Attic is generally the American/Canadian reference to it...
s. Placing an antenna indoors significantly attenuates the signal available to it. Directional antennas must be pointed at the transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
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 space closely enough that a single antenna suffices for all. A single transmitter location may transmit signals for several channels.
Analog television
Analog television
Analog television is the analog transmission that involves the broadcasting of encoded analog audio and analog video signal: one in which the message conveyed by the broadcast signal is a function of deliberate variations in the amplitude and/or frequency of the signal...
signals are susceptible to ghosting
Ghosting (television)
In television, a ghost is a replica of the transmitted image, offset in position, that is super-imposed on top of the main image on an analogue broadcast.-Common causes:Common causes of ghosts are:...
in the image, multiple closely spaced images giving the impression of blurred and repeated images of edges in the picture. This was 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 was different for different transmissions. Careful positioning of the antenna could produce a compromise position which minimized 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 were different lengths or the antennas were too close together. Analog television is being replaced by digital, which is not subject to ghosting.
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 lattice towerLattice tower
A lattice tower or truss tower is a freestanding framework tower. They can be used as electricity pylons especially for voltages above 100 kilovolts, as a radio tower or as an observation tower....
or mast
Radio masts and towers
Radio masts and towers are, typically, tall structures designed to support antennas for telecommunications and broadcasting, including television. They are among the tallest man-made structures...
. The wire connecting the antenna to indoors is referred to as the or drop
Drop (telecommunication)
Drop, in telecommunication, has several meanings.* In a communications network, a drop is the portion of a device directly connected to the internal station facilities, such as toward a telephone switchboard, toward a switching center, or toward a telephone exchange. A drop can also be a wire or...
, and the longer the downlead is, the greater the signal degradation in the wire.
The higher the antenna is placed, the better it will perform. An antenna of higher gain
Antenna gain
In electromagnetics, an antenna's power gain or simply gain is a key performance figure which combines the antenna's directivity and electrical efficiency. As a transmitting antenna, the figure describes how well the antenna converts input power into radio waves headed in a specified direction...
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 the weather. There are also varying local ordinance
Local ordinance
A local ordinance is a law usually found in a municipal code.-United States:In the United States, these laws are enforced locally in addition to state law and federal law.-Japan:...
s which restrict and limit such things as the height of a structure without obtaining permits
Construction permit
A construction permit or building permit is a permit required in most jurisdictions for new construction, or adding on to pre-existing structures, and in some cases for major renovations. Generally, the new construction must be inspected during construction and after completion to ensure compliance...
. For example, in the USA, the Telecommunications Act of 1996
Telecommunications Act of 1996
The Telecommunications Act of 1996 was the first major overhaul of United States telecommunications law in nearly 62 years, amending the Communications Act of 1934. This Act, signed by President Bill Clinton, was a major stepping stone towards the future of telecommunications, since this was the...
allows any homeowner to install "An antenna that is designed to receive local television broadcast signals", but that "masts higher than 12 feet 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. Indoor antennaswill never be an option 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 asphaltAsphalt
Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...
shingles, roof felt, and a plywood
Plywood
Plywood is a type of manufactured timber made from thin sheets of wood veneer. It is one of the most widely used wood products. It is flexible, inexpensive, workable, re-usable, and can usually be locally manufactured...
roof deck
Roof Deck
The roof deck is the roofing material layer between the primary structural components and either insulative layers or weatherproofing layers in a typical roof system.-Structure:...
is considered to attenuate the signal to about half.
Multiple antennas, rotators
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 rotatorAntenna rotator
An antenna rotator is a device which is used to change the orientation of a directional antenna. Most antenna rotators have two parts, the rotator unit and the controller...
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 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=λ/2). The wavelength of 54 MHz (Channel 2) is 5.5 meters (λ x f = c
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
) so the antennas must be a minimum of 2.25 meters, or ~89 inches apart. It is also important that the cables connecting the antennas to the signal splitter/merger be exactly the same length, to prevent phasing
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
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. Bandpass filters
Electronic filter
Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
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 rodLightning rodA lightning rod or lightning conductor is a metal rod or conductor mounted on top of a building and electrically connected to the ground through a wire, to protect the building in the event of lightning...
. The use of a lightning arrestor 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 guyedGuy-wireA guy-wire or guy-rope, also known as simply a guy, is a tensioned cable designed to add stability to structures . One end of the cable is attached to the structure, and the other is anchored to the ground at a distance from the structure's base...
with galvanized 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 if they fall.
- There is inherent danger in being on the rooftop of a house, required for installing or adjusting a television antenna.
External links
- Article on the basic theory of TV aerials and their use
- Consumer Electronics Association antenna mapping page
- http://www.siber-sonic.com/FM_reception/rooftop.html
- NEC Lab - A tool to design and test television antennas.