Satellite terms and definitions

[grynner]

basic satellite terminology its c/p

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.
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Hope this clears up a few misconceptions