TV Aerials... The science
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 75 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 VHFfrequencies 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.
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.
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.
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.