satellite dish

A dish antenna used to receive and transmit signals relayed by satellite.

Background

A satellite dish is a parabolic television antenna that receives signals from communication satellites in orbit around the earth. Its sole function is to provide the television viewer with a wider variety of channels.

The first communications satellite—Echo I—was launched by the United States in 1960, transmitting telephone signals. In 1961 Relay began transmitting television signals, and in the same year Syncom established itself as the first geosynchronous satellite capable of transmitting signals to one particular section of the earth's surface continuously.

The rapid advances in communication satellite technology were not simultaneously matched by advances in satellite dish use and technology. Television broadcasting began with individual stations that could only serve a limited area. Television networks had to provide their affiliate stations with recordings of programs if they wished to provide nationwide service. Satellite television was not widely available until the 1970s, when cable television stations equipped with satellite dishes received signals that were then sent to subscribers by coaxial cable. By 1976, there were 130 satellite dishes owned by cable companies, and by 1980, every cable television station had at least one satellite dish.

About that time personal satellite dish earth stations were selling for approximately $35,000 per unit. Taylor Howard, an employee at Stanford University who was well-versed in the usefulness of satellites as relayers of data, is credited with designing the first satellite dish for personal use. Howard's dish, which was placed into operation on September 14, 1976, was made of aluminum mesh and was about 16 feet (5 meters) wide. By 1980,5,000 satellite dishes had been purchased for home use. In 1984 alone 500,000 were installed. Recent reports state that there are 3.7 million owners of home satellite dishes worldwide, and the number will continue to grow.

A typical commercial satellite dish of the 1970s was made of heavy fiberglass, and the dish itself, at its smallest size, had a diameter of about ten feet (three meters). Since then, satellite dish design has shifted toward light-weight, aluminum mesh dishes (similar to Howard's homemade dish), some of which are inexpensive and small (three feet, or one meter, in diameter is typical), with many sections (petals) that can be easily assembled. England, Japan, and Germany, have led the way with direct broadcast TV, which sends signals directly to the viewer's dish, but the United States has yet to do so. This trend would yield smaller, more affordable satellite dishes and regulated satellite programming.

Raw Materials

The basic satellite dish consists of the following materials:

• A parabolic reflector made of fiberglass or metal, usually aluminum, with a protruding steel feed horn and amplifier in its middle.
• A steel actuator that enables the dish to receive signals from more than one satellite.
• A metal (usually aluminum) shroud measuring about 6 to 18 inches (15 to 45 centimeters) in height. It is installed on the dish's circumference perpendicularly to reduce side interference.
• Cables, most likely made from vinyl tubing and copper wiring.

The Manufacturing
Process

• To make fiberglass suitable for dish manufacture, a sheet molding compound mixture that includes reflective metallic material and ultraviolet scattering compositions is mixed with resin, calcium carbonate, and a catalyst cure. This mixture forms a paste that is poured onto a sheet of polyethylene film that has fiberglass added in chopped form. The result is a sheet layered with the compound paste, fiberglass, and the polyethylene film.
• This sheet is then pressed at 89 degrees Fahrenheit (30 degrees Celsius) to mature. To shape the sheet into the desired parabolic shape, it is pressed at high pressure (of 1,400-2,200 metric tons). The dish is then trimmed, cooled, and painted. After the paint has dried, the dish is packed for shipment in sturdy boxes.
• For metallic dishes, the common metal of choice is aluminum. This type of dish can be assembled in sections called petals, or all at once. An aluminum plate is perforated with a punching die, creating tiny holes. The size of these holes are contingent on the manufacturer's preference. Larger holes mean greater loss of the signal, so fairly small holes are selected. Another factor in the selection of hole size is the power of the broadcasting satellite. Newer, more powerful satellites require a hole size that is approximately half that required for older, less powerful satellites. The newly perforated aluminum plate is then heated, stretched over a mold, cooled, and trimmed. A paint powder coating for protection is then applied using an electrostatic charge, in which the paint is given an opposite electrical charge from the plate. The dish or petal is then heated to melt the powder and seal the paint on. The petals are usually sealed together with ribs in the factory.
• Mesh petals are made from aluminum that is extruded—forced into a die of the proper shape. They are usually joined together on site by sliding them into aluminum ribs that attach to the hub and then securing them with metal pins.

Installation

• All dishes, when complete, will have the necessary equipment (the feed horn, the amplifier, etc.) installed in the factory. When the dish has been set up at the local dealer, it is transported to the site location on a open trailer. Satellite dishes can be installed either by professionals or by the purchaser, with assistance from the retailer if necessary. The method selected depends upon the size of the dish and the mechanical expertise of the purchaser.
• An installation site reasonably clear of obstructions not more than 246 feet (75 meters) from the house is selected. Site selection is also contingent on local building codes and the possibility of microwave interference from radio and television towers in the vicinity. Once a site is selected, the base must be installed first. The base of most satellite dishes consists of a concrete foundation that extends below the frost line. A clayey soil is excellent, while sandy or rocky soil requires more concrete. A base tube filled with concrete is then set into the concrete foundation.

  Some satellite dishes require a slab mount installation, a method considered to be more stable than typical base construction. In some cases, slab mount installation is necessary since the site selected for the placement of the satellite dish is unstable. The slab is generally 1.6 feet (.5 meter) square and 3.2 feet (1 meter) deep. Soil is excavated to the proper depth and the concrete is poured. A triangular steel mount fixture is then embedded into the concrete.

• Next, the pedestal is attached to either the base tube or the triangular steel mount fixture. The elevation arm is then attached to the pedestal.

Alignment

• The mounted satellite dish must be aligned in order to point toward the satellite. The angle at which the dish is eventually situated will vary according to which satellite is selected and at what latitude the dish is located. Coaxial cables connect the satellite to the receiver that is located in the house near the television. A trench must be dug for these cables, which are placed into a pipe before being buried.

Quality Control

Satellite dishes for consumer use are not usually required to undergo rigorous tests with set standards, but some parameters are generally met. For example, so that the microwaves are received properly, the surface of the dish should be as smooth as possible and its parabolic shape should be exact. It must also be composed at least partially of metal, otherwise the microwaves will not reflect. If the dish is either mesh or perforated aluminum, the holes must be relatively tiny to minimize loss. Dish size is important; it should match that appropriate to the latitude. The mount should be sturdy, and the dish aligned properly for maximum reception.

Members and joints are tested and compared to the American Steel Construction Institute or the American Aluminum Association methods rules, whichever apply. The satellite dish should be built to withstand high winds, snow, ice, rain, and extreme temperatures.

After the dish is installed, the owner is generally responsible for cleaning it twice a year, more if necessary, tightening and lubricating all bolts once a year, and trimming obstructive weeds and trees from around it. In rare occasions, the owner must adjust the alignment to correct bad reception.

The Future

Satellite dishes will become ubiquitous in upcoming years. More communication satellites will certainly be launched, and the growth explosion in individual satellite dish ownership will continue. One factor that should affect home satellite dish ownership in the near future is the switchover to more powerful satellites that will transmit signals in the K band (12 GHz). Because most of the present satellite dishes accept signals in the C band (3.7 to 4.2 GHz), owners of C band satellite dishes will have to convert them to K band. Researchers and designers are contemplating even smaller dishes that could be placed on a rooftop or outside a window and still function as well as the larger satellite dishes of today.

Some experts see the growth of satellite television as a revolution that is less concerned with crystal clear images of old sitcoms than with the possibilities of two-way communication that universal dish ownership would promote. Satellite television will be used to pay bills, shop, and participate in game shows. It can also be used to communicate over long distances, perhaps to play interactive video games with someone halfway across the continent. Some visionaries see the revolution as the return of one-on-one communication like that of a town meeting. In any case, it is almost certain that satellite television will continue to proliferate in upcoming years.

Manufacturers will continue to make smaller and less costly satellite dishes. Recently, for instance, 18-inch (45.7-centimeter) diameter dishes have been introduced into the market in Japan, Europe, and the United States. These dishes are small enough to fit on a windowsill or a porch railing. Manufacturers are also working on producing a flat-plate dish for satellite signal reception.

Where To Learn More

Books

Baylin, Frank, and Amy Toner. Satellites Today. ConSol Network, Inc., 1984.

Clifford, Martin. The Complete Guide to Satellite TV. Tab Books, 1984.

Easton, Anthony T. The Home Satellite TV Book. Wideview Books, 1982.

Prentiss, Stan. Satellite Communications. Tab Books, 1987.

Sutphin, S. E. Understanding Satellite Television Reception. Prentice-Hall, 1986.

Traister, John E. Guide to Satellite Television Installation. Prentice-Hall, 1987.

Traister, Robert J. Build a Personal Earth Station for Worldwide Satellite TV Reception. Tab Books, 1985.

Periodicals

Booth, Stephen A. "Signals from Space," Popular Mechanics. April, 1992, p. 60.

Elrich, David. "Satellite TV: It's Worth a Closer Look," Home Mechanix. September, 1990, p. 78.

[Article by: Rose Secrest]

A C band satellite dish

A satellite dish is a type of parabolic antenna designed to receive microwaves from communications satellites, which transmit data transmissions or broadcasts, such as satellite television.

Contents 1 Principle of operation 2 Europe 3 Systems design 4 Types 4.1 Motor-driven dishes 4.2 Multi-satellite 4.3 VSAT 4.4 Ad hoc 4.5 Others 5 In numismatics 6 See also 7 References 8 External links

Principle of operation

The parabolic shape of a dish reflects the signal to the dish’s focal point. Mounted on brackets at the dish's focal point is a device called a feedhorn. This feedhorn is essentially the front-end of a waveguide that gathers the signals at or near the focal point and 'conducts' them to a low-noise block downconverter or LNB. The LNB converts the signals from electromagnetic or radio waves to electrical signals and shifts the signals from the downlinked C-band and/or K_u-band to the L-band range. Direct broadcast satellite dishes use an LNBF, which integrates the feedhorn with the LNB. (A new form of omnidirectional satellite antenna, which does not use a directed parabolic dish and can be used on a mobile platform such as a vehicle was announced by the University of Waterloo in 2004.^[1]

The theoretical gain (directive gain) of a dish increases as the frequency increases. The actual gain depends on many factors including surface finish, accuracy of shape, feedhorn matching. A typical value for a consumer type 60 cm satellite dish at 11.75 GHz is 37.50 dB.

With lower frequencies, C-band for example, dish designers have a wider choice of materials. The large size of dish required for lower frequencies led to the dishes being constructed from metal mesh on a metal framework. At higher frequencies, mesh type designs are rarer though some designs have used a solid dish with perforations.

A common misconception is that the LNBF (low-noise block/feedhorn), the device at the front of the dish, receives the signal directly from the atmosphere. For instance, one BBC News countdown shows a "red data stream" being received by the LNBF directly instead of being beamed to the dish, which because of its parabolic shape will collect the signal into a smaller area and deliver it to the LNBF.^[2]

Modern dishes intended for home television use are generally 43 cm (18 in) to 80 cm (31 in) in diameter, and are fixed in one position, for Ku-band reception from one orbital position. Prior to the existence of direct broadcast satellite services, home users would generally have a motorised C-band satellite dish of up to 3 metres in diameter for reception of channels from different satellites. Overly small dishes can still cause problems, however, including rain fade and interference from adjacent satellites.

Europe

In Europe the frequencies used by DBS services are 10.7 - 12.75 GHz on two polarisations H (horizontal) and V (vertical). This range is divided into a "low band" with 10.7 - 11.7 GHz, and a "high band" with 11.7 - 12.75 GHz. This results in two frequency bands, each with a bandwidth of about 1 GHz, each with two possible polarizations. In the LNB they become down converted to 950 - 2150 MHz, which is the frequency range allocated for the satellite service on the coaxial cable between LNBF and receiver. Lower frequencies are allocated to cable and terrestrial TV, FM radio, etc. Only one of these frequency bands fits on the coaxial cable, so each of these bands needs a separate cable from the LNBF to a switching matrix or the receiver needs to select one of the 4 possibilities at a time.

Systems design

In a single receiver residential installation there is a single cable from receiver to LNB and the receiver uses different power supply voltages (14/18V) to select polarization and pilot tones (22 kHz) to instruct the LNB to select one of the two frequency bands. In larger installations each band and polarization is given its own cable, so there are 4 cables from the LNB to a switching matrix, which allows the connection of multiple receivers in a star topology using the same signalling method as in a single receiver installation.

Types

Motor-driven dishes

A satellite dish that is mounted on a pole and driven by a stepper motor can be controlled and rotated to face any satellite position in the sky. Motor-driven dishes are popular with enthusiasts. There are three competing standards: DiSEqC, USALS, and 36v positioners. Many receivers support all of these standards.

Multi-satellite

Special dish for up to 16 satellite positions (K_u-band).

Some designs enable simultaneous reception from multiple different satellite positions without re-positioning the dish. The vertical axis operates as an off-axis concave parabolic concave hyperbolic Cassegrain reflector, while the horizontal axis operates as a concave convex Cassegrain. The spot from the main dish wanders across the secondary, which corrects astigmatism by its varying curvature. The elliptic aperture of the primary is designed to fit the deformed illumination by the horns. Due to double spill-over, this makes more sense for a large dish.

VSAT

A common type of satellite dish is the very small aperture terminal (VSAT). This provides two way satellite internet communications for both consumers and private networks for organisations. Today most VSATs operate in Ku band, C band is restricted to less populated regions of the world. There is a move which started in 2005 towards new Ka band satellites operating at higher frequencies, offering greater performance at lower cost. These antennas vary from 74cm to 120cm in most applications though C-band VSATs may be as large as 4.0m.

Ad hoc

The dish is a reflector antenna and almost anything that reflects radio frequencies can be used as a reflector antenna. This has led to dustbin lids, woks and other items being used as "dishes". Coupled with low noise LNBs and the higher transmission power of DTH satellites, it is easier to get a usable signal on some of these "dishes".

Others

• Individual dishes serving one dwelling: Direct to Home (DTH).
• Collective dishes, shared by several dwellings: satellite master antenna television (SMATV) or communal antenna broadcast distribution (CABD).
• Automatic Tracking Satellite Dish
• Big ugly dish

In numismatics

The 50 Years of Television commemorative coin

Television has had such an impact in today's life, that it has been the main motif for numerous collectors' coins and medals. One of the most recent ones is the Austrian 50 Years of Television commemorative coin minted in March 9, 2005. The obverse of the coin shows a "test pattern", while the reverse shows several milestones in the history of television, including satellite dishes.

See also

• DiSEqC
• Satellite television
• Set-top box

References

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (April 2008)

1. ^ "Team develops in-car satellite TV". University of Waterloo. 2006-05-06. http://newsrelease.uwaterloo.ca/news.php?id=4072. Retrieved on 2008-04-29. 
2. ^ "News 24 STILL gets an F for physics". http://bnb.bpweb.net/N24/news24.htm. Retrieved on 2008-04-29. 

External links

Wikimedia Commons has media related to: Satellite dish (Television)

• European Commission: The right to use a satellite dish.
• DishPointer - Satellite dish alignment with Google Maps.
• Guide explaining how to align a satellite dish.
• Online Satellite Finder Based on Google Maps

General Electric satellite dish	Sky Digital "mini-dish"	Astro's "mini-dish"	Satellite Dishes installed on an apartment complex
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