General Satellite Technology information

T24

T24

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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 LN:cool:.

However, a dish which is wider than it is tall will need a special LNB.
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Types of LNB

  • Universal
  • Twin
  • Dual
  • Monobloc
  • Octo
  • Quad
  • quattro
  • C Band
  • Unicable
  • Ka Band
  • Ku Band
  • C120
  • WR75
 
T24

T24

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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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T24

T24

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#3
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EIRP & DISH SIZE


LNB 0.6dB

EIRP in dB~~~~Dish size in cm.

64----------------22

63----------------24

62----------------26

61----------------28

60----------------30

59----------------32

58----------------34

57----------------36

56----------------38

55----------------40

54----------------45

53----------------50

52----------------50

51----------------55

50----------------60

49----------------60

48----------------70

47----------------75

46----------------80

45----------------90

44----------------90

43----------------100

42----------------110

41----------------120

40----------------120

39----------------135

38----------------150

37----------------180

36----------------240

35----------------300

34--------------- 360

32----------------450

30----------------570
 
T24

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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.
 
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