| SATELLITE TERMS AND FAQ
Terms and FAQ
Actuator
The motor arm that moves the dish on a polar mount from side to side. The motor usually runs on 36 volts DC.
Az/El mount
A manual satellite dish mount, where you have to adjust the elevation and azimuth manually independent of each other (up/down and sideways).
C-band
3700 to 4200 MHz frequency band (3.7 to 4.2 GHz)
DVB
Digital Video Broadcast, an international MPEG2 encoding and transmission standard. This system is not compatible with DCII transmission.
FEC
Forward Error Correction, a way of transmitting the same data twice in case some of the data was lost the first time. This is useful under weak signal conditions.
Feed horn
A circular opening located at the focus point of the dish. Radio waves enter the opening and is guided down the feed horn. The radio waves will be picked up by an LNB or LNA connected to the feed horn.
Footprint
The coverage area of a satellite transmission. It defines a region on the surface of the earth where the signal is receivable. A satellite can have several beams (footprints) with many different coverage areas.
Frequency
The number of waves per second in a radio wave.
kHz - 1000 waves per second
MHz - 1000000 waves per second
GHz - 1000000000 waves per second
Gain
Power of amplification, or amount of amplification, often measured in dB. Negative gain is loss of signal.
Geostationary
An orbit around the equator of the earth where objects (satellites) seems to be standing still in relationship with a fixed point on earth. The satellites seems to be standing still in the s**. They are of course moving at the same rate as earth\'s rotation, one revolution per 24 hours.
H-to-H mount
A special satellite dish mount that will track satellites down to the horizon, both in the east and west. The dish has a travel range of 180 degrees.
Ku-band
11.700 to 12.200 GHz (US)
10.600 to 12.900 GHz (Europe)
12.200 to 12.700 GHz (DBS)
LNB
Low Noise Block down converter, a device that amplifies and converts a block of frequencies to a lower block of frequencies. The LNB is an active device placed at the focus point of the dish.
LO
Local Oscillator, a circuit in the LNB(F) or LNC. It determines how much the frequency is downconvertet by in the LNB(F) or LNC. Standard LO for C-band is 5150 MHz, and 10.750 GHz for Ku-band.
MPEG
Motion Pictures Expert Group, a digital compression standard for audio and video.
Noise Figure
The amount of noise a device is generation on its own. Lower noise is better, and is often a measurement of the quality of a LNB, LNA or LNBF.
Offset dish
A dish where the focal point is not in the center of the dish. This is common on Ku dishes, and the offset angle is often 30 degrees. The reason for using the offset dish is that the LNB and feed is not creating a shadow on the dish, thereby giving higher efficient of the dish. When the dish edge is vertically, seen from the side of the dish, it is actually looking at an angle of 30 degrees upwards.
PID
Packet IDentification, a header in a packet of data telling what the data is for.
Polarity
Radio waves are polarized. They can be linear or circular. Linear polarity is Vertical or Horizontal, circular polarity is Righ-hand and Left-hand circular polarity.
Polarizer
A device that selects the received polarization of the radio waves. It can be a small motor turning the pickup probe, or done electronically in an LNBF.
Polarmount
A special satellite dish mount that setup correctly will track all the satellites in a geostationary arc in the s**, from east to west. Only movement sideways is necessary, the up/down adjustment is done by the polarmount itself.
Reed Sensor
Used in actuators and dish motors to report back the movements of the dish. A counter in the motor control unit can tell where the dish is pointing, and the information can be stored and retrieved when you want to move the dish to a specific satellite. Only two wires are required to connect the sensor.
Scrambling
A way of distorting the picture or sound so it is not viewable. A descrambler is needed to receive the correct signal. This is used to transmit audio and video securely, or to get payment for the service.
SR
Symbol Rate, the amount of data transmitted every second.
Transponder
A satellite channel with an assigned frequency and polarity.
14/18 volt switch
A change in the voltage on the LNB coax cable. This change of voltage can control the polarity in a LNBF, or switch other devices on the coax cable.
22kHz switch
A tone that can be sent via the LNB coax. This tone can control the LNB or switches on the coax cable.
DiSEqC
digital satellite Equipment Control.
MPEG
Moving pictures Expert Group.
C/N
Carrier to Noice-ratio.
NTSC
National Television Standards Committe.
SECAM
SEquential Couleur A Memorie.
PAL
Phase Alternating Lines.
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LNB
Low Noise Block-downconvertor (so called because it converts a whole band or "block" of frequencies to a lower band).
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Is there actually a different LNB for prime focus dishes + offset dishes? Surely an LNB's innards are the same and the feedhorn or the C120 flange is the only difference?
In the old days, LNB noise figures were high, the gain (amplification) was low and satellite transponder power was typically 20 Watts. Imagine trying to see a 20 Watt light bulb 24,000 miles away! (You'd have trouble seeing a 20W bulb at the end of a 24 yard corridor).
So, an LNB and feedhorn had to be matched to the dish. The internal antenna of the LNB had to be at the exact focal point of the dish and the horn had to be flared in such a way that, with the LNB at the focal point, the horn could "see" the exact circular area of the dish - no more and no less. If it was less then it wasn't collecting signal from the full area of the dish. If it was more, it was also collecting unwanted "noise" from any warm object (wall) or from the sky behind the dish.
A good compromise was to take just part of a much larger paraboloid dish and mount the LNB in an "offset" position. The curvature of this partial dish is such that the focal point is now much lower so the LNB and feedhorn no longer obscure the signal path as they would with a "prime focus" dish.
Nowadays, satellite transponders can produce typically 50 or 60 Watts and LNBs have higher gain and lower noise figures. With these strong transmissions, you can get away with murder. People stick any old thing on the end of the boom arm - which rather explains why one man's 0.6dB LNB is another man's nightmare when the signal strength is not optimum! The Sky minidish, for example, is a compromise between size and performance. It's very important that the LNB matches the dish exactly. This is one good reason why the dish comes with its own LNB.
The manufacturers might "fudge" the issue if asked. After all, if they admit that their LNB works best with, say, an 80cm Lenson Heath dish and you just bought an 1 metre dish made by someone else, you might not be too happy.
If you "mix 'n' match" by picking a 60cm dish and a Universal LNB at random, the chances are that the performance could be no better than that of the Sky minidish.
As a general rule, any standard LNB will work with a circular (prime focus) dish or an offset focus dish which is taller than it is wide (which "looks" circular when viewed by the LNB).
However, a dish which is wider than it is tall will need a special LNB.
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Different kinds of Satellite Dishes
1) Offset antenna
2) Prime focus antenna
3) Flat antenna
4) Cassegrin antenna
5) Multi focus antenna
Offset antennas
These antennas represent just a part of a parabolic or prime focus antenna. Their focus is not in the geometrical centre of the dish but a bit lower. Since the LNB doesn’t stand in the way of the signal these antennas can be smaller than others.
Prime focus antenna
Prime focus antennas have a parabolic shape and characteristic is that the focus is in front of the centre of the parabola in the geometrical centre that is. That means if the LNB is attached to this antenna it can be found above the middle of the antenna and so the LNB blocks a part of the emitted signals and therefore these antennas have to be bigger than offset antennas. Offset antennas replaced these antennas nearly completely. Today they are mostly used for the reception of signals in the C belt, since these are the only antennas that are built in scopes up to 10m.
Cassegrin antennas
These antenna with two reflectors receive signals better. It looks like an offset antenna with the exception that in the place where the offset antenna has a LNB attached the cassegrin antenna has an additional reflector, which has the task to reflect signals coming from the main reflector onto the LNB, which is placed on the LNB-carrier in front of the small reflector. This means the signals are reflected twice before they get to the LNB.
Gregorian dish
A subversion of satellite antenna that uses a concave hyperbolic
reflector that points signals to the converter and that is placed
opposite of the main reflector.
Flat antennas
This antenna is made of many units, which receive signals, afterwards this received signals are united and directed to the LNB. Technically speaking this antenna collects electromagnetic waves. Since this antenna receives through several units it receives a stronger signal. This antenna can be smaller than advised for several areas.
Multi focus antennas
These antennas are made not long ago. They are offset antennas, which have a specially built reflector, which reflects received signals from surrounding satellites to one focus. You can recognize such an antenna easily, since it quadratic and it’s longer than wider.
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Adjusting the polar mount
If the adjustments are done in the correct order, you can get a dish that tracks just perfect. You should have an unwarped satellite dish, and a straight ground pole, it will make things easier.
1- Start with checking the mounting of the feed horn. All the legs on the tripod should be of the same length. You should measure them, and do any adjustment you can if they are not the same length. Next, you have to check the distance from three different points on the edge of the dish, to the center of the feed horn. Remember, even if the tripod legs have the same length, that does not mean the feed is centered! You might have to "bend" the feed back into center of dish, or adjusting the tripod legs to get the feed centered in the dish.
2- Set the off-set angle on your polar mount (declination). This is an adjustment that tilts the dish *forwards* at an angle of about 4-6 degrees, depending on what latitude you live. You can find the exact angle for your location in charts, but if you set it for about 5 degrees, you'll be close enough to get going. This adjustment is usually done on one of the mounts connected directly to the dish.
3- You then move the dish to the highest point on your polar mount. You do this by using the actuator. You can do this by visually looking at the dish and the polar mount. You are basically centering the dish on the highest point on the polar mount. Now, you have to set the elevation angle of the dish. I like to use a meter for this, but it is also possible to do it without. The elevation angle is about 40 degrees, depending on your latitude. This is not very critical at this point because you will adjust this angle for best reception later. If you measure the angle on the mount, you might have to add the declination angle to get the true dish pointing angle.
4- You need to find a satellite that is located just south of your location. In most cases, there is a satellite close to the longitude you live. A few degrees off will not make much difference because the dish moves almost flat in the center of arc. Try a Ku band satellite because the accuracy is much higher. However, you might look for a C band satellite when you start. It will be easier to find than a Ku band satellite. Having the dish parked at the highest point of the arc, you have to turn the WHOLE polar mount on the ground pole to you hit the satellite. If your elevation was way off, you might not even get a signal. Adjust the elevation and turn the mount again until you find the satellite located "straight south".
5- Fine tune the elevation angle. Turn the mount sideways until max signal and then adjust the elevation angle until its maxed. At this point, you have set the off-set angle and the elevation angle for the satellite at the highest point in the arc.
6- Now, you have to get the dish to track on the sides of the arc. This is where most people fail. DO NOT adjust any elevation angles on the mount at this point! Move the dish using the actuator to a satellite on one side of your arc. You should hopefully see the signal from the satellite, if not, pick a satellite closer to the center of the arc. Peak the dish on the satellite using the actuator. Next, you have to push or pull upwards and downwards on the dish. You don't have to use much force, just a bit to see if the signal gets better or worse when you push/pull on the dish. What you are actually doing is to change the elevation angle a bit. If your dish is pointing at a satellite to the east of center and you have to push up on the dish to get a better signal, then the elevation angle must be adjusted higher. You adjust this by turning the WHOLE mount to the east! You have to use the actuator and move the dish a bit west to peak the signal. You go back and forth until the dish has the correct elevation. Next, you have to check a satellite on the other side of the arc. If you peaked the dish for center, and then for one side, the other side should be very close. This will depend on your ground pole, offset angle/elevation angle and quality of feed/dish.
7- If your dish is not hitting center on the other side, try the same adjustment as above. If the dish needs to be pushed up to get a better signal, then TURN the WHOLE mount in that direction. If the dish needs to be pulled down for a better signal, then turn the mount the opposite direction (towards the higher point on arc).
8- Then, go back and check the other side. Hopefully, you're not far off. You might have to go from side to side before your dish tracks perfectly.
9- If, and ONLY if you can not get both sides to peak, both sides would be too low or too high. You can then do a small adjustment of the declination (elevation) angle to get the two sides into peak. BUT, only do this if you can confirm that both sides are low or high. If the dish is to high on the sides, but fine in the center, the declination angle is to low. Increase the declination and the elevation angle the same amount. They will cancel each other in the center of arc, but track lower on the sides.
10- You should now have a perfectly peaked dish If you used Ku band satellites for the peaking, it will be as good as it can get. If you used C band satellites, you might want to do the same thing using Ku band satellites.
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Satellite Tv.....
The person most widely given credit for the concept of using this orbit for communications is Arthur C. Clarke. In an article he published in Wireless World in October 1945 titled \"Extra-Terrestrial Relays: Can Rocket Stations Give World-wide Radio Coverage?Clarke extrapolates from the German rocket research of the time to a day when communications around the world would be possible via a network of three geostationary satellites spaced at equal intervals around the earth\'s equator.
It wasn\'t until 1963 that NASA set out to test Clarke\'s concept with the Synchronous Communications Satellite program. Unfortunately, Syncom 1—launched 1963 February 14—while successfully reaching geosynchronous orbit in an inclined, eccentric orbit was unsuccessful due to an electronics failure. Syncom 2—launched 1963 July 26—became the first operational geosynchronous communications satellite. Syncom 3—launched 1964 August 19—became the first geostationary satellite, finally fulfilling the prediction made by Clarke almost twenty years earlier.
Satellite broadcasting is made possible by the fact that communications satellites are fixed in geostationary orbit 22,300 miles above the equator, staying in the same position above the ground at all times. This allows satellite antennas that transmit and receive signals to be aimed at an orbiting satellite and left in a fixed position.
Satellite programming:
Satellite programmers broadcast, or uplink, signals to a satellite which they either own or lease channel space from. The signals are often scrambled, or encrypted, to prevent unauthorized reception before they are retransmitted to a home antenna. The uplinked signals are received by a transponder located on the satellite, a device that receives the signals and transmits them back to the earth after converting them to a frequency that can be received by a ground-based antenna. Typically there are 24 to 32 transponders on each satellite. In order to minimize interference between the transponders, the signals are transmitted with alternately polarized antennas. Each satellite occupies a particular location in orbit, and operates at a particular frequency assigned by the FCC.
Satellite signals:
The signals received at the satellite from a ground-based antenna are extremely weak in amplitude – much less than one watt. As a result, they must employ amplifiers that boost the signals to a level that can successfully be processed and retransmitted to the earth. After traveling 22,000 miles to a ground-based antenna, the signals are again very weak and must be amplified. Therefore, satellite “dishes” focus the signals onto the actual antenna. The signals from the antenna are then fed to a “low-noise block,” or LNB, amplifier which amplifies signal and converts them to a lower frequency. The lower the power of the satellite, the larger the antenna required to focus the signals. A C-Band satellite, with power ranging between 10 and 17 watts per transponder, typically has an antenna between 5 and 10 feet in diameter; whereas a high-powered Ku-Band satellite, with a range of 100 to 200 watts per transponder, only requires an antenna 18 inches in diameter. The signals from the antenna are fed to an integrated receiver/decoder (IRD), which converts them to a form that can be tuned by a TV set. Every IRD contains a unique address number, which is activated by a satellite programmer to allow it to receive subscription services. In addition, the IRDs modem port is connected to a telephone line, in order to access pay-per-view ordering services and transmit other data. A single IRD can supply one channel choice to one or more TV sets. In order to view two different programs at the same time on two different TV sets, two IRDs are required—one for each TV, and the antenna must be a dual-LNB type |
Linear Mode
