A television antenna , or TV antenna , is an antenna designed specifically for reception of over-the-air television broadcast signals, transmitted at frequencies from about 41 to 250 MHz in tape VHF, and 470 to 960 MHz in UHF bands in various countries. Television antennas are produced in two different types: the "indoor" antenna, placed above or next to the television, and the "outside" antenna, mounted on a pole above the owner's house. They can also be installed in the attic or attic, where dry conditions and elevation elevations are advantageous for reception and antenna longevity. Outdoor antennas are more expensive and difficult to install, but are required for adequate reception in remote suburbs from television stations. The most common types of indoor antennas are the dipoles ("rabbit ears") and loop antennas, and for yagi outdoor antennas, periodic logs, and for UHF channels of reflective multi-bay array antennas.
Video Television antenna
Description
In most countries, television broadcasting is allowed in very high frequency (VHF) bands from 47 to 68 MHz, called low band VHF or band I in Europe; 174 to 216 MHz, called the high band VHF or band III in Europe, and in ultrahigh frequency (UHF) bands from 470 to 698 MHz, called IV and V bands in Europe. The boundaries of each band are somewhat different in different countries.
To cover this range, the antenna generally consists of several conductors of different lengths depending on the wavelength of the radio waves they receive. The elements of most half-wave dipole antennas; metal rods half the wavelength of the signal they want to receive. The signal wavelength is equal to the speed of light (c) divided by the frequency. The above frequency range is too wide to be covered by a single antenna, so often separate antennas for VHF and UHF bands are used; or a composite antenna that has a VHF and UHF element installed in the same boom. Over the past decade many countries in the world have shifted from broadcasting using older analogue television standards to newer digital television (DTV). But generally the same broadcast frequency is used, so the same antenna used for old analog television will also receive new DTV broadcasts. Sellers often claim to provide specially recommended "digital" or "high definition television" (HDTV) antennas in place of existing analogue television antennas; at best this is wrong information to generate unneeded equipment sales, at worst it can leave the viewer with a UHF-only antenna in the local market (especially in North America) where some digital stations remain at their original VHF frequency.
Maps Television antenna
Indoor
An indoor antenna can be mounted on the television itself or standing on a table next to it, connected to a television with a short bait line. Because of the limitations of antenna space in the room can not be as large and complicated as outdoor antennas, and they are not installed at high altitudes; for this reason indoor antennas generally do not provide good reception like outdoor antennas. They are often very adequate in urban and suburban areas that are typically in strong "radiation footprint" from local television stations, but in rural acceptance areas only external antennas can provide adequate reception. Some of the simplest indoor antennas are described below, but a wide variety of designs and types exist. Many have a dial on the antenna with a number of different settings to change the antenna receiving pattern. This should be played with settings when viewing the screen, until the best images are obtained.
Rabbit ears
The oldest and most widely used indoor antenna is rabbit ears or rabbit ears , which are often equipped with new television sets. This is a simple half-wave dipole antenna used for receiving VHF television bands, consisting 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 telescope rods attached to the base, which extends up to about 1 meter in length (about one quarter wavelength at 52 MHz), and can collapse when not in use. For best reception, the rods should be adjusted to be less than 1/4 wavelength on the frequency of received television channels. However the dipole has a wide bandwidth, so acceptance is quite often achieved without adjusting its length. Dipole half wave has a low gain of about 2.14 dBi; this means unidirectional and sensitive to the remote station as a large roof antenna, but the wide angle reception pattern allows it to receive multiple stations located in different directions without the need for re-adjustment when the channel is changed. The dipole antennas are two-way, that is, they have two main lobes in the opposite direction, 180 Â ° apart. Instead of staying in a position like other antennas, the elements are mounted on a sphere-and-socket connection and can be adjusted to various angles in the form of "V", allowing them to be moved out of the way in crowded places. Another reason for the form of V is that when receiving the channel at the top of the band with a fully extended rod, the antenna element will usually resonate at the 3rd harmonic. In this mode, the maximum gain direction (main lobe) is no longer perpendicular to the rod, but the radiation pattern will have the lobe at an angle to the rod, making it advantageous to be able to adjust it to different angles
Whip antenna
Some portable televisions use a whip antenna. It consists of one telescoping rod about a meter long that is attached to the television, which can be pulled when not in use. It functions as a quarter wave of monopole antennas. The other side of the feedline is connected to the ground plane on the TV circuit board, which serves as ground. The whip antenna generally has an omnidirectional reception pattern, with maximum sensitivity in the direction perpendicular to the axis of the antenna, and obtains similar to the half wave dipole.
Loop Antenna
UHF channels are often received by single-lap antennas. Because the "rabbit ear" antenna only covers the VHF band, it is often combined with a UHF loop mounted on the same base to cover all TV channels.
Flat antenna
As soon as the television broadcast switched from analog to digital broadcast, the indoor antenna had evolved beyond the traditional "rabbit ears." RCA is one of the manufacturers that commercially sells flat antennas. The flat antenna is very light, very thin, and square-shaped like a thin notebook. They are connected to a television, or to a digital converter box, with a single coaxial cable, and can be sold with an optional signal booster. Internally, a thin, flat square is a loop antenna, with a circular metal cable embedded into a conductive plastic. The amplifier must be plugged into a power source, but a flat antenna does not require a power source. Flat antennas may require multiple moves to achieve optimal reception, but eliminate the many manipulations inherent in the use of "rabbit ears".
Outdoor
An outdoor TV antenna is a high-gain, directional antenna that is often required to achieve adequate reception in the fringe reception area, more than 15 miles from the television station. The outdoor antenna has an unidirectional radiation pattern so that the correct antenna tip should be directed to the TV station. Television signal received through the feed line (transmission line) into the house to the television. The older antenna uses a 300 ohm twin-lead flat cable. These should be stored a few inches away from metal objects such as antenna towers or gutters, so it must be mounted on a standoff stand. The modern antenna uses a 50 ohm RG-6 coaxial cable attached to a television with an F-type connector.
Outdoor antenna designs are often based on a Yagi-Uda antenna or log-periodic dipole array (LPDA). It consists of several half-wave dipole elements, consisting of a metal rod approximately half the wavelength of a television signal, which is mounted in a line in the support boom. It acts as a resonator; the electric field of the incoming radio waves pushes the electrons in the rods back and forth, creating standing waves of oscillating voltage in the rods. An antenna can have a smaller or larger number of trunk elements; in general the more elements the higher the gain. Another design, used primarily for UHF reception, is a reflective array antenna, consisting of a vertical metal screen with several dipole elements mounted in front of it.
The television broadcast tape has a frequency too wide to be covered by a single antenna, so a separate antenna is used for the VHF and UHF bands, or VHF/UHF combo antennas. VHF/UHF antennas are actually two antennas that feed the same feedline that is mounted on the same support boom. Longer elements that take VHF frequencies are located in the "back" of the boom and often function as log-periodic antennas. The shorter element that receives the UHF station is located in the "front" boom and often acts as a Yagi antenna.
The outdoor antenna is very purposeful. They have a narrow main lobe; ie, the maximum sensitivity (gain) is achieved only at a narrow angle along its axis, so it should be directed to the transmitting antenna. This presents a problem when the television station to be received is in a different direction. In this case two or more pointed roof antennas each pointing at different transmitters are often mounted on the same pole and connected to one receiver. The alternative is to use a single antenna mounted on the rotator ; a remote servo system that rotates the antenna in a new direction when a button next to the television is played.
Sometimes television transmitters are arranged in such a way that all receivers in a given location need to receive transmissions only in the relatively narrow spectrum of the full UHF television spectrum and from the same direction, so that a single antenna provides reception from all stations.
Installation
Antenna is usually placed on the roof, and sometimes in the attic. Placing the antenna indoors significantly weakens the level of available signal. Directional antennas should be directed to the transmitter they receive; in many cases great accuracy is not required. In certain areas it is sometimes arranged that all television transmitters are in the same direction and the frequency of use is so close that one antenna is enough for all. One transmitter location can send signals to multiple channels. CABD (broadcast distribution of communal antennas) is a system installed inside the building to receive free-to-air TV/FM signals transmitted over radio frequencies and distribute them to an audience.
Analog television signals are vulnerable to shadows in the image, some close-up images that give the impression of a blurry and repetitive image on the image. This is because the signal is reflected from nearby objects (buildings, trees, mountains); multiple copies of the signal, from different forces and subject to different delays, are taken. This is different for different transmissions. Careful antenna position can result in a compromise position that minimizes ghosts in different channels. Ghosting is also possible if multiple antennas connected to the same receiver pick up the same station, especially if the length of the cable connecting it to the splitter has a different length or the antenna is too close together. Analog television is replaced by digital, which is not exposed to ghosting but is much more susceptible to interference; the same bounce signals that cause ghosting in analog signals will not produce content that can be viewed at all in digital.
Rooftop and other outdoor antennas
Aerials attached to the roof in various ways, usually on a pole to lift it on the roof. This is generally enough in most areas. But in some places, such as deep valleys or higher structures, antennas may need to be placed significantly higher, using a guide pole or mast. The wires that connect the antenna into the room are referred to as downlead or drop , and the longer the downlead, the greater the signal degradation in the wire. Certain cables can help reduce this trend.
The higher the antenna is placed, the better it will perform. The higher gain antenna will be able to receive a weaker signal from the desired direction. Building interventions, topographic features (mountains), and dense forests will weaken signals; in many cases the signal will be reflected in such a way that a usable signal is still available. There is a physical danger attached to high or complex antennas, such as falling structures or being destroyed by the weather. There are also various local rules that limit and limit things like the height of the structure without obtaining permission. For example, in the United States, the 1996 Telecommunications Act enables homeowners to install "Antenna designed to receive local television broadcasting signals", but "poles higher than 12 feet above the roofline may be worn to the local area." allow requirements. "
Indoor antenna
As discussed earlier, the antenna can be placed indoors where the signal is strong enough to overcome the lack of antennas. The antenna is simply plugged into the television receiver and placed comfortably, often at the top of the receiver ("set-top"). Sometimes a position needs to be tested to get the best shot. An indoor antenna can also take advantage of RF amplification, commonly called a TV booster. An indoor antenna will never be an option in the weak signal area.
Attic installation
Sometimes it is desirable not to install the antenna on the roof; in this case, antennas designed for outdoor use are often installed in attics or attics, although antennas designed for attic use are also available. Placing the antenna indoors significantly decreases its performance due to lower altitudes above ground level and intervening walls; however, strong signal area reception may be satisfactory. One layer of asphalt, a felt roof, and a plywood roof deck are considered to weaken the signal to about half of it.
Some antennas, player
It is sometimes desirable to receive signals from transmitters that are not in the same direction. This can be achieved, for one station at a time, using a rotator operated by an electric motor to rotate the antenna as desired. Alternatively, two or more antennas, each pointing to the desired transmitter and coupled with the appropriate circuit, may be used. To prevent antennas interfering with each other, the vertical distance between the booms should be at least half the wavelength of the lowest frequency to be received (Distance =?/2). The 54 MHz wavelength (Channel 2) is 5.5 meters (? Xf = c) so the antenna must be at least 2.25 meters, or about 89 inches apart. It is also important that the cable connecting the antenna to the signal/merger separator be the same length, to prevent the phasing problem, which causes ghosting with analog reception. That is, the antenna may both take the same station; signals from one with a shorter cable will reach the receiver a little faster, providing the receiver with two images slightly offset. There may be a phasing problem even with the same down-lead cable length. Bandpass filters or "signal traps" can help reduce this problem.
For the side-by-side placement of some antennas, as is common in limited height spaces such as attics, they must be separated by at least one full wavelength of the lowest frequency to be received at their nearest point.
Often when multiple antennas are used, one for the various co-located stations and the other is for a single transmitter in different directions.
Security
- The TV antenna is a good electrical conductor and draws lightning, acting as a lightning rod. Use of ordinary lightning arrestors to protect against this. A large grounding rod connected to an antenna and a pole or pole is required.
- Properly installed poles, especially high ones, are equipped with galvanized cables; no isolator required. They are designed to withstand the worst weather conditions in the area, and are positioned so they do not interrupt the power lines if they fall.
- There is a danger inherent in the roof of the house, which is required to install or set up a television antenna. British entertainer Rod Hull died after falling from his roof where he tried to increase his reception for a football match.
See also
- Towers and radio towers, sometimes called Radio and TV antennas
- Satellite plates
- Satellite television
- Terrestrial Television
References
External links
- useful article on digital antenna installation
- An article on basic theory of TV antennas and their usage
- See Which TV Stations You Can Get on the Map
- Antenna page 'Climb on the roof'
- NEC Lab - A tool for designing and testing television antennas.
- Search ("Inexpensive, Homemade DIY TV" page)
- Image from 'DIY TV antenna'
- Search the 'DIY PI-DI TV antenna' page
- Image from 'beer/soda cans of DIY TV'
- CEA Antenna Codes and flow chart to adjust the antenna for nest reception
Source of the article : Wikipedia
0 Comments