THE DIGITAL SATELLITE TV HANDBOOK: WE GIVE YOU THE WORLD FOR JUST $49.95!

Satellite Installations

Produced by Shelburne Films
Written and presented by Mark Long

Video production © copyright 1997 Shelburne Films. Text and Graphics © copyright 1997 MLE INC. All Rights Reserved

This web page contains selected excerpts from our new Satellite Installations videotape. Use the EXCERPT INDEX below to go directly to any topic of interest or click on the Shelburne Films icon right to review the entire table of contents for this new educational resource. In addition to producing the Satellite Series of videotapes in co-operation with MLESAT, Shelburne Films also produces a wide array of educational TV programs, including a highly acclaimed travel series. For further information, please visit their web site at http://www.shelburnefilms.com.

SATELLITE INSTALLATIONS EXCERPT INDEX

| Azimuth & Elevation | System Noise Performance and the LNB |

| Coaxial Cable & Connectors | Wiring the Feedhorn | Wiring the Actuator |

| Dual-Band Installations | Small Dish Installations | Tuning Digital IRDs |

Azimuth and Elevation

Satellite signals are microwaves which travel in a straight path along the line of sight and all geostationary satellites are located in an arc which goes across the sky. If we want to receive these satellites, we need to have an unobstructed view of this arc. You can conduct a preliminary survey at the site by facing south for locations north of the equator or north for locations in the southern hemisphere. Here we have a relatively clear and unobstructed view of the sky from the Southeast to the Southwest. Even at this site, however, there are limitations: the mountain blocks our view of the south-western horizon and trees block the south-eastern horizon. To determine which satellites may be blocked from any given location we will need to learn about the two basic co-ordinates for finding any satellite: the azimuth and the elevation.

The azimuth co-ordinate represents the bearing of the satellite from the site location, while the elevation is the tilt angle at which the dish looks up at the satellite. Every satellite within view from the site location has its own unique pair of azimuth and elevation co-ordinates which can be calculated using a computer program or a simple az/el chart. Once you know the azimuth co-ordinate for any satellite, you can use a compass to determine its direction from the site location. To achieve a high degree of accuracy, however, you must first correct the compass readings from magnetic north to true north. Magnetic correction maps are available for different regions from a variety of sources.

You can call up your local airport control tower to find out the magnetic correction factor for your area. If true north is east of magnetic north, the correction factor must be subtracted from the compass readings; when true north is west of magnetic north, the correction factor is added. All compass readings should be taken out in the open, well away from large metal objects or overhead a.c. power lines and transformers.

In the northern hemisphere, the satellite nearest to true south, that is 180 degrees from true north, will have the highest value of elevation. When we say, elevation, we mean the angle in degrees at which the dish must tilt up from the horizon in order to receive the satellite. For locations south of the equator, the satellite nearest to true north will be the highest in the sky. The further away the satellite's direction or 'azimuth bearing' is from the site's true north/south line, the lower its elevation in the sky.

An inclinometer is another tool that we use to determine the elevation angle that we need to see the satellite. Once we know the azimuth and elevation co-ordinates for every available satellite, we can point the inclinometer in the direction of the corrected compass bearing and then tilt it back until it reads out the satellite's elevation angle. We therefore can visually confirm whether or not any satellite will be available before we even begin constructing the dish. If trees, terrain or building block the view, then we can consider another, more advantageous location nearby.

System Noise Performance & the LNB

Molecular motion within all matter generates a noise background which permeates the entire electro-magnetic spectrum used to propagate communication signals, including the satellite frequency bands. Geostationary satellites, which uplink and downlink signals across a vast distance of more than 22,300 miles, generate enough power to overcome this natural phenomenon, but only when the correct antenna size and electronics are used to receive their signals. The temperature of all thermal noise is expressed in degrees Kelvin, a measurement scale that can be related to other more familiar temperature scales such as Celsius or Fahrenheit. The higher the noise temperature, the stronger the noise source.

 

Externally generated noise is received by the parabolic antenna along with the desired signal. When the satellite dish tilts up towards the 'cold' sky, its noise temperature is relatively low. If the antenna must tilt downward to receive a low elevation satellite, the antenna's noise temperature will increase dramatically because it is now able to see the 'hot noise temperature of the earth. The actual amount of noise increase in this case is a function of antenna diameter. Minimum antenna elevation angles of 5 degrees, for C-band, and 10 degrees for Ku-band, usually are recommended.

 

 

 

Thermal noise also is internally generated within the LNB's first stage of electronic amplification. C-band LNBs are graded according to their their noise temperature performance in degrees Kelvin, while Ku-band LNBs are rated according to an equivalent noise figure in dB. In either case, the lower the LNB's noise rating, the better its performance. The noise temperature of our system also will also increase if moisture is allowed to enter any of the system's connections. In some cases, moisture can cause irreparable damage if it is allowed to corrode the LNB's sensitive internal circuitry. Another potential source of damage to the LNB is lightning. Be sure that you connect the dish to a ground rod pounded into the earth next to the pad. Surge protectors also are available to protect the indoor electronics from damage.

 

Coaxial Cable and Connectors

Three cables are needed to connect the dish to the indoor receiver. The first cable supplies a dc voltage and a pulse count to the receiver so that it knows how far to travel from satellite to satellite. The second cable provides the five volts dc pulse and ground required by the feedhorn's servo motor. All of these wires are contained in a single high quality direct buriable cable. Make sure that you purchase enough to complete the run all the way from the feedhorn, down the pole, into the ground and from the pole to the house, up the wall, through the attic, and down an interior wall while leaving enough left over to easily reach to the back of the receiver. If at all possible, avoid splices in your cable run as they are potential trouble spots which can allow permit moisture to enter the system.

The third essential connection is a shielded wire cable called coax that is used to connect the LNB to the receiver. Coax is made up of an inner wire covered with a plastic or foam sheath, and an outer mesh that is in turn surrounded by an outer plastic covering. An 'F' connector is attached to each end of the coaxial cable. Make sure that the center conductor does not short out to the outer ground sheath as this will blow the receiver's fuse and possibly damage its internal power supply.

When screwing an F connector onto mating connectors on the back of the receiver or LNB, you should take care to avoid bending or breaking the cable's inner conductor, thereby shorting out the connection. The receiver should be unplugged before making any coaxial connections to avoid shorting out the receiver's power supply. Once you are satisfied with the total system operation you will need to go back and waterproof the F connection at the LNB. This can be done by wrapping the connection with a sticky waterproof compound such as Coax-Seal.

For locations with a high incidence of rainfall, we recommend that you unplug the receiver, flood the inside of coaxial connector with silicon seal, and then seal the outer portion of the connector with Coax-seal. Do not plug the receiver back in until the silicon sealer has had ample time to dry or you will short out the receiver's fuse. The type of cable commonly used in satellite TV installations has a characteristic impedance rating of 75 ohms. There are several different kinds of 75 ohm coaxial cable available. RG-59U coax can be used to span distances of up to 100 feet. For longer lengths, lower loss RG-6 or RG-11 are used. To span distances of several hundred feet, special UHF line amplifiers with +10 or +20 dB gain also may be necessary to compensate for the high amount of signal loss or attenuation. The amplifier receives its power from the center conductor of the coaxial able just like the LNB does. Above ground pedestals are available to protect the amplifier from the environment and permit easy servicing should the amplifier ever malfunction.

Wiring the Feedhorn

The feedhorn cable is comprised of three color-coded 22 gauge (or larger) stranded wires. The three wires provide power, pulse, and ground connections for the feedhorn. Each of these wires connects to corresponding terminals on the back of the receiver. The wires are color-coded to help identify them when connecting to the three servo motor wires at the feed, usually red for power, white for pulse, and black for ground. The receiver uses pulses to keep track of the position of the feedhorn's pick-up probe. You therefore can adjust, or 'skew', the position of the probe and program the optimum polarization for any given satellite transponder into the receiver's internal memory circuitry. The feedhorn's servo motor rotates the pick-up probe, which swings back and forth while switching between the horizontally and vertically polarized transponders (odd and even channels).

 

Some satellites use an alternate form of polarization known as circular polarization, where the incoming wave front rotates in either a clockwise or a counterclockwise direction. To receive these satellites, a special feedhorn is required. Keep in mind that there are limits to the pick-up probe's clockwise and counter-clockwise movements. The feedhorn must be aligned on the antenna so that the probe can swing the 90 degrees from horizontal to vertical polarization without reaching the limits of its travel. If you find that you cannot skew the probe beyond a good picture on all channels, you will need to loosen the clamp that holds the feed onto its support and physically rotate the feedhorn until it is possible to do so. In extremely cold climates, where a servo motor may freeze up during the winter months, an alternate polarization device called a ferrite feed may be used instead. The ferrite feed, which electromagnetically rotates the polarization of the incoming satellite signal, requires a two-wire connection. Consult the receiver manual to determine the recommended way to connect the ferrite feed to the receiver.

Wiring The Actuator

The direct burial cable's actuator line is comprised of five stranded wires. These actuator wires should be connected to the appropriate terminals on the mount's motorized actuator and the back of the receiver. Like the feedhorn servo motor wires, the three shielded motor sensor wires also provide power, pulse, and ground. Reed and Hall-Effect sensors will only require two wires, one for pulse and the other for ground. The two large stranded wires connect to the large wire terminals at the actuator motor and to the motor wire '1' and '2' terminals on the back of the receiver. These connections supply 24 to 36 volts d.c. to the antenna actuator's d.c. motor. Now try to move the dish to the east or west; if the dish moves in the direction opposite to the one intended, reverse the wires connected to these two terminals.

Dual Band Installations

In many part of the world, dual-band satellite TV installations provide the best of the C- and Ku-band worlds. Special dual-band feedhorns are available that place both the C and Ku-band feed openings at the focus of the dish. You can install an electronic switch out at the antenna which will connect the main coaxial cable to either a C or Ku-band feed. The receiver supplies the switching voltage up the center conductor of the coaxial cable. Universal Ku-band LNBs also are available that switch internally between the 10.7 to 11.7 Gigahertz and the 11.7 to 12.75 Gigahertz frequency spectrums.

 

 

 

The dual-band feed is a technical compromise that sacrifices a small amount of C-band signal in order to place both the C- and Ku-band feed openings at the focus of the dish. For those situations where the installer can't afford to sacrifice ANY Ku-band signal, a second C-band feed and LNB can be attached to one side of the Ku-band feed along the plane of the dish that is perpendicular to the mount's polar axis. The reverse is also possible if the C-band signals in your area are marginal. Another option is to install a second dish that is permanently pointed at a single satellite, or satellites collocated at a single orbital position.

Small Dish Installations

Most small Ku-band dishes are 'offset-fed' antennas which position the feedhorn away from the center of the dish so that it doesn't block any of the incoming signal. These 'asymmetrical' dishes actually use a oval shaped subsection of the same parabolic curve that their larger prime focus counterparts use. Another advantage is that the offset feedhorn looks up at the 'cold' noise temperature of the sky rather than the 'hot' noise temperature of the earth below.

Offset fed antennas use a different type of feedhorn than their prime focus counterparts. This special feed, which is often supplied together with the dish, is designed to effectively illuminate the oval shape of the offset-fed antenna. Most small dish systems these days provide a combination of feed and LNB called an LNF. Many of these units are designed to switch polarization's by means of a DC voltage which the receiver sends up the coaxial cable. This simplifies the hook-up between dish and indoor unit by limiting connections to a single coaxial cable. Simply hand rotate the LNF to peak one polarization, and the switching voltage will automatically reset the feed to the opposite sense of polarization.

Offset fed antennas usually come with a feedhorn support bracket that automatically sets the focal distance between the dish and feedhorn opening. There will, however, be some room for fine tuning the distance--usually about an eighth of an inch in or about. You should manually make this adjustment while monitoring the signal with a tuning meter or spectrum analyzer to ensure that you have reached peak level. The satellite elevation co-ordinates supplied by computer programs, manuals and other reference materials are almost always for prime focus antennas.

When adjusting the elevation angle of an offset dish, subtract the manufacturer's offset angle from the elevation angle provided by your reference. Many offset dish manufacturers supply a gauge on the antenna mount that automatically makes this correction for you. If not, place the inclinometer onto any section of the mount parallel with the rim of the dish. In this case, don't forget to subtract the antenna's offset angle from your readings. The fixed antenna mount that comes with the offset antenna usually offers a variety of installation options: on the ground, up a pole, attached to an outside wall, or under the eve of a house. Rooftop mounts are available that either penetrate the roof and attach to the building's rafters or mount directly on a flat roof or peak. Above all, select the type of mount which will give the antenna a clear, unobstructed view of the satellite.

 

Tuning Digital IRDs

Manufacturers of digital IRDs typically set up default values at the factory for the frequency, polarization, symbol rate and Forward Error Correction rate of a single digital DTH 'bouquet.' To receive a bouquet which uses different settings, new values must be programmed into the IRD. What's more, the IRD must be fully compatible with the conditional access format which the service provider uses to control subscription authorization. Digital DTH services use a form of modulation called QPSK, short for Quaternary Phase Shift Keying. With QPSK, two data bits are combined to produce a representative 'symbol' which is expressed as one of four distinct states or 'phases'. This phase shift typically occurs at rates of millions of symbols, or 'Megasymbols,' per second.

Symbol rates may vary widely from one digital DTH service to the next. It is important that the digital IRD be able to tune through the entire range of symbol rates that correspond to the services which the customer wishes to receive.

The FEC rate is the ratio of 'k' bits entering the digital encoder to 'n' bits exiting the encoder. FEC rates of 1 to 2, 2 to 3, 3 to 4, 5 to 6, and 7 to 8 are now in use worldwide. The uplink encoder inserts error correcting bits into the original signal's bit stream. The IRD uses this extra information to detect and compensate for signaling errors which may occur in the link between program source and destination. With some digital IRDs, the LNB local oscillator (LO) frequency must be entered into the unit's set-up menu along with the bouquet's default transponder frequency.

The L. O. frequency, which is printed on the LNB product label, is typically 5150 MHz for C-band LNBs and either 10.75 or 11.75 for Ku-band LNBs. Keep in mind, however, that a few LNB manufacturers use other L. O. values. To switch between digital DTH bouquets on the same satellite, enter the new transponder frequency and polarization, change symbol rates and switch to the new FEC setting. Keep in mind that it will take a few seconds for the IRD to load the new data. Select the manufacturer's default settings to go back to the original bouquet.

Last Updated: 09/26/03

THE DIGITAL SATELLITE TV HANDBOOK: WE GIVE YOU THE WORLD FOR JUST $49.95!