|Wednesday, 18 October 2006|
Television has become an almost indispensable system for the dissemination of information and entertainment throughout the world. Cable systems are currently used for delivering signals to television receivers. This system is highly successful for delivering picture perfect signals to a television receiver over a large range of frequencies. Despite the fact that cable television installations are on the rise and more and more televisions are receiving television signals from cable, cable TV may not be available in sparsely populated or remote areas because of the high cost of providing cable TV to a small number of customers, there is still a significant number of televisions, including an increasing number of portable televisions, which receive television signals "from the air" via antennas. Antennas for transmitting television signals typically transmit in the VHF (175 to 250 MHz) frequency range or the UHF (470 to 860 MHz) frequency range. The antenna is responsible for directing the transmitted signals that carry the television pictures and sound and/or other signals to an audience. Television broadcast antennas typically radiate horizontally polarized signals. When an elliptically polarized signal is desired, the horizontally polarized signal is modified or supplemented to generate a vertically polarized signal component which, when combined with the horizontally polarized signal, results in an elliptically polarized signal. Antennas which operate in circular polarization are generally satisfactory for use in the UHF frequency range where the wavelength of the translated signal is relatively small. Directional antennas are utilized in many applications and are often capable of being pointed in a desired direction. There are many types and variations of directional antennas, including phased array, mechanically steerable, turntable mounted tiltable and non-tiltable flat plate, turntable mounted Lumberg lens, and other such antennas. Since signals are transmitted over terrestrial networks and received through the use of antennas. Signal strength and quality are often dependent upon the geography of the land between the transmitting antenna and the receiving antenna. Although such transmission methods are still in use today, the use of satellites to transmit television signals is becoming more prevalent.
Satellite television systems have emerged as an alternative to cable television systems. In digital satellite communications systems, a ground-based transmitter beams a forward error coded uplink signal to a satellite positioned in a geosynchronous orbit. The satellite relays the signal back to a ground-based receiver antenna in a separate location. Direct broadcast satellite (DBS) systems allow households to receive television, audio, data and video directly from the DBS satellite. The satellite television system employs a dish antenna to collect satellite signals, and the signals are then reflected to low noise block down converters (LNB) equipped at the focus of the dish antenna for amplifying the signals and reducing its frequency down to around 1 GHz, the signals of radio frequency are transformed to be of intermediate frequency. The adjusted signals are transmitted to an indoor TV channel selector by a cable to select the signals of a desired channel, and the selected signals are then amplified and modulated to recover into video and audio signals for TV watching. Dish antennas and receivers for audio/video transmission signals allow home viewers to receive television programming directly from satellite transmissions. The satellite dish antenna is typically secured to a mounting and must be aligned. A typical dish antenna constructed to receive satellite signals comprises a dish-shaped reflector that has a support arm protruding outward from the front surface of the reflector. Satellite antenna gain is proportional to the aperture area of the reflector. Vehicle mounted satellite antennas which are operable while the vehicle is in motion, can provide one-way or two-way satellite communications. The application of television reception may be advantageously applied in common carrier transportation such as long distance buses, and in recreational vehicles including boats. Satellite antennae designed for use on a moving vehicle have a low profile. These low profile antennas are short but wide so that the overall aperture area is kept high.
The antennas used for receiving analogue television signals consist of a "rake" antenna or Yagi type antenna which is traditionally placed on the roof of the house. Digital terrestrial televisions have been gradually replacing analogue television. Digital television is a new type of broadcasting technology. DTV antenna systems transmit the information used to make television pictures and sounds by data bits, rather than by waveforms, as performed by NTSC systems. With digital television, broadcasters will be able to provide television programming of a higher resolution and better picture quality than what can be provided under the current analog antenna systems. High definition television (HDTV) is an emerging technology that is capable of providing service either in an analog or digital format. A high definition television (HDTV) system processes a television signal having approximately double the horizontal and vertical image resolution of a standard television signal or greater resolution. An HDTV signal also may exhibit a greater image aspect ratio compared to a standard television image.