Television antenna
A television antenna, also called a television aerial, is an antenna specifically designed for use with a television receiver to receive terrestrial over-the-air broadcast television signals from a television station. Terrestrial television is broadcast on frequencies from about 47 to 250 MHz in the very high frequency band, and 470 to 960 MHz in the ultra high frequency band in different countries.
Television antennas are manufactured in two different types: indoor and outdoor antennas. Indoor antennas are designed to be located on top of or next to the television set, but are ideally placed near a window in a room and as high up as possible for the best reception. The most common types of indoor antennas are the dipole, which work best for VHF channels, and loop antennas, which work best for UHF. Outdoor antennas on the other hand are designed to be mounted on a mast on top of the owner's house, or 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 these are the Yagi, log periodic, and the multi-bay reflective array antenna.
Description
The purpose of the antenna is to intercept radio waves from the desired television stations and convert them to tiny radio frequency alternating currents which are applied to the television's tuner, which extracts the television signal. The antenna is connected to the television with a specialized cable designed to carry radio current, called transmission line. Earlier antennas used a flat cable called 300 ohm twin-lead. The standard today is 75 ohm coaxial cable, which is less susceptible to interference which plugs into an F connector or Belling-Lee connector on the back of the TV. To convert the signal from antennas that use a twin-lead line to the modern coaxial cable input, a small transformer called a balun is used in the line.In most countries, television broadcasting is allowed in the very high frequency 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 band from 470 to 698 MHz, called band IV and V in Europe. 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, used before 2006, the VHF and UHF bands required separate tuners in the television receiver, which had separate antenna inputs. The wavelength of a radio wave equals the speed of light, divided by the frequency. The above frequency bands cover a 15:1 wavelength ratio, or almost 4 octaves. 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 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 standard to newer digital television. However, the same broadcast frequencies are generally used, so the 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 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 where some digital stations remain on their original high VHF or low VHF frequencies.
Reception issues
Places unable to be reached by television broadcast transmitters are known as black spots in Australia. In East Germany, the areas that could not receive western TV signals were referred to as the Tal der Ahnungslosen, or Valley of the Clueless.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 feed line. 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. Still, 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 54 to 88 MHz and 174 to 216 MHz, with wavelengths of. It is constructed of two telescoping rods attached to a base, which extend out to about length and can be collapsed when not in use. For best reception, the rods should be adjusted to be a little less than 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 measured gain 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. Still, 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. They 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 is no longer perpendicular to the rods. Still, 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 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 rabbit ears.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. This of course also depends by country and region: for example in the UK and Ireland, terrestrial TV broadcasts are only on the UHF band, meaning that a loop antenna is necessary and the rabbit ears would only be useful for FM radio reception.Flat antenna
A more recent phenomenon for indoor antennas are flat antennas, which are lightweight, thin, and usually square-shaped with the claim of having more omnidirectional reception. They are also marketed as being more in line with modern minimalistic home designs. Flat antennas may have a stand or could be hung on a wall or a window. Internally, the thin, flat square is a loop antenna with its circular metallic wiring embedded into conductive plastic.Outdoor
When a higher-gain antenna is needed to achieve adequate reception in suburban or fringe reception areas, an outdoor directional antenna is usually used. Although most simple antennas have null 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, so the correct end of the antenna must be pointed at the TV station. As an antenna design provides higher gain, the main lobe 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 or log-periodic dipole array. 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 and the more directional. 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 the two options are separate antennas used for the VHF and UHF bands or a combination VHF/UHF antenna. A VHF/UHF antenna combines two antennas feeding the same feedline mounted on the same support boom. More extended elements that pick up VHF frequencies are located at the back of the boom and often function as a log-periodic antenna. Shorter elements that receive the UHF stations are located at the front of the boom and often function as a Yagi antenna.
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 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.