Television antenna


A television antenna is an antenna specifically designed for use with a television receiver to receive 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" 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 and loop antennas, and for outdoor antennas the yagi, log periodic, and for UHF channels 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.
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.
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 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" 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 frequencies.

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 54 to 88 MHz and 174 to 216 MHz, 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, 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 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 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.

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 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 either separate antennas are used for the VHF and UHF bands, or a combination 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.
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.

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 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 ; 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 guyed mast 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 will be able to receive weaker signals from its preferred direction. Intervening buildings, topographical features, 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. 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 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. The wavelength of 54 MHz is 5.5 meters 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.

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