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. Television antennas are manufactured in two different types: "indoor" antennas, to be located on top of or next to the television set, and "outdoor" antennas, mounted on a mast on top of the owner's house. 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 areas far from television stations. The most common types of indoor antennas are the dipole ("rabbit ears") and loop antennas, and for outdoor antennas the yagi, log periodic, and for UHF channels the multi-bay reflective array antenna.
Video Television antenna
Description
In most countries, television broadcasting is allowed in the very high frequency (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 ultrahigh frequency (UHF) band from 470 to 698 MHz, called band IV and V in Europe. The boundaries of each band vary somewhat in different countries.
To cover this range, antennas generally consist of multiple conductors of different lengths depending on the wavelength of the radio waves they receive. The elements of most antennas are half-wave dipoles; metal rods 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 above range of frequencies is too wide to be covered by a single antenna, so often separate antennas for the VHF and UHF bands are used; or a combined antenna that has both VHF and UHF elements mounted on the same boom. During the last decade many countries in the world switched from broadcasting using an older analog television standard to newer digital television (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" (HDTV) antenna advised as a replacement for an existing analog television 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.
Maps Television antenna
Indoor
Indoor antennas may be mounted on the television itself or stand on a table next to it, connected to the television by a short feedline. Due to space constraints indoor antennas cannot be as large and elaborate as outdoor antennas, and they are not mounted at as high an elevation; 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 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 antenna used to receive the VHF television bands, consisting in the US of 52 to 88 MHz (band I) and 174 to 216 MHz (band III), with wavelengths of 5.5 to 1.4 m. It is constructed of two telescoping rods attached to a base, which extend out to about 1 meter length (approximately one quarter wavelength at 52 MHz), and can be collapsed when not in use. For best reception the rods should be adjusted to be a little less than 1/4 wavelength at the frequency of the television channel being received. However the dipole has a wide bandwidth, so often adequate reception is achieved without adjusting the length. The half wave dipole has a low gain of about 2.14 dBi; 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 lobes 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) 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. This consists of a single telescoping rod about a meter long attached to the television, which can be retracted when not in use. It functions as a quarter-wave monopole antenna. 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 reception pattern, with maximum sensitivity in directions perpendicular to the antenna axis, and gain similar to the half-wave dipole.
Loop antenna
The UHF channels are often received by a single turn loop antenna. 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.
Flat antenna
Soon after television broadcasting switched from analog to digital broadcasting, indoor antennas have evolved beyond the traditional "rabbit ears." RCA is one manufacturer which has commercially sold a flat antenna. Flat antennas are very lightweight, very thin, and square-shaped like a thin notebook. They connect to televisions, or to digital converter boxes, with a single coaxial cable, and may be sold with an optional signal amplifier. Internally, the thin, flat square is a loop antenna, with its circular metallic wiring embedded into conductive plastic. The amplifier must be plugged into a power source, but the flat antenna does not require a power source. The flat antenna may need some moving around to achieve an optimum reception, but it eliminates a lot of manual manipulation which is inherent in use of the "rabbit ears".
Outdoor
An outdoor TV antenna is a high-gain directional antenna often needed to achieve adequate reception in fringe reception areas, greater than 15 miles from the television station. Outdoor antennas have a unidirectional radiation pattern so the correct end of the antenna must be pointed at the TV station. The received television signal passes down a feed line (transmission line) into the house to the television. Older antennas used flat 300 ohm twin-lead cable. This had to be kept several inches away from metal objects such as the antenna tower or gutters, so it had to be mounted on standoff insulators. Modern antennas use 50 ohm RG-6 coaxial cable which attaches to the television with a type F connector.
Outdoor antenna designs are often based on the Yagi-Uda antenna or log-periodic dipole array (LPDA). These are composed of multiple half-wave dipole elements, consisting of metal rods approximately half of the wavelength of the television signal, mounted in a line on a support boom. These act as resonators; 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. Another design, used mainly for UHF reception, is the reflective array antenna, 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.
Outdoor antennas are highly directional. They have a narrow main lobe; that is, their maximum sensitivity (gain) is only achieved over a narrow angle along their axis, so they must be pointed at the transmitting antenna. This presents 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. 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 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 attics. Placing an antenna indoors significantly attenuates the level of the available signal. 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 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 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 but is far more prone to interference; the same reflected signal that causes ghosting in an analog signal would produce no viewable content at all in digital.
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 guide mast or mast. The wire connecting the antenna to indoors is referred to as the downlead 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 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 ordinances 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 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 antennas will 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 asphalt shingles, roof felt, and a plywood roof deck 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 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=?/2). The wavelength of 54 MHz (Channel 2) is 5.5 meters (? x f = c) so the antennas must be a minimum of 2.25 metres, or about 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.
Safety
- TV antennas are good conductors of electricity and attract lightning, acting as a lightning rod. 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 guyed 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. British entertainer Rod Hull died after falling from his roof where he had been trying to improve reception for a football match.
See also
- Radio masts and towers, sometimes called Radio and TV antennas
- Satellite dish
- Satellite television
- Terrestrial television
References
External links
- useful article on the installation of digital antennas
- Article on the basic theory of TV aerials and their use
- See Which TV Stations You Can Get on a Map
- 'Up on the roof' antenna page
- NEC Lab - A tool to design and test television antennas.
- Search for (cheap, home-made) 'DIY TV antenna' pages
- Images of 'DIY TV antennas'
- Search for 'DIY pie-pan TV antenna' pages
- Images of 'DIY beer/soda-can TV antennas'
- CEA Antenna Codes and flow chart for adjusting antenna for nest reception
Source of article : Wikipedia
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