Lecture 3, Earth Segment

Presentation / Lecture 3, Earth Segment

Date Submitted: 06 June 2001

Written by RPC Telecommunications. Website: http://www.rpctelecom.com

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This is the third in the series of general satcom tutorial lectures submitted by RPC Telecommunications.

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 Printable Version
-Section 1
Types and components of an Earth Station
-Section 2
General Construction
-Section 3
Antenna Theory
-Section 4
Radiation Patterns
-Section 5
Polarisation
-Section 6
Noise
-Section 7
Low-noise Amplifiers
-Section 8
Power Amplifiers
-Section 9
Tracking
Low-Noise Amplifiers
The information carrying capacity of any radio system is proportional to the ratio:

C / T = (carrier power / system noise temperature)

It is therefore necessary to make the system noise temperature as small as possible to maximise the information capacity.

The value of T on the downlink of a satellite system depends primarily on the noise temperature of the earth-station antenna and the amplifier following it.

A satellite antenna looks at the earth (at a temperature of around 290K) so there is little point in spending a lot of money to fit it with a low-noise amplifier. However, an earth-station antenna looks at the sky and its noise temperature is usually much lower than 290K.

As an example, the noise temperature of an earth-station antenna working at 4 GHz varies from about 20K at high elevation angles to around 45K at an elevation angle of 5 (when the sky is clear).

Earth terminals equipped with large antennas used to use cryogenic parametric amplifiers (paramps).

Cryogenic means "at a very low temperature" and cryogenic paramps were cooled to around 20K (i.e. -253 C) by using refrigerating plant circulating gaseous helium. Cryogenic paramps are expensive and require a lot of skilled maintenance effort.

Higher satellite powers have made them unnecessary for most satellite systems and they are rarely used nowadays.

Types of LNA in common use today include:
  • Uncooled Field-Effect Transistor (FET) amplifiers which have a noise temperature of 55 to 75K at 4 GHz or around 200K at 11 GHz
  • Amplifiers cooled by thermoelectric diodes which have a noise temperature of 35K to 45K at 4 GHz and around 120K at 11 GHz

In the above example, an antenna of gain 52 dBi and noise temperature 35K is connected to an LNA of gain 50 dB and noise temperature 80K via waveguide and hence to a receiver via co-axial cable. What is the system noise temperature Te?

First some definitions:
  Tr = (NF - 1) x 290 K (1)
  Tp = [(1 - G) / G] x 290 K (2)
  To = G . Ti K (3)

Where (all components assumed to be at 290K):
  Tr = noise temperature NT corresponding to a noise figure NF
  Tp = noise temperature of a passive network (e.g. waveguide) of gain G
  To = output noise temperature of a noiseless network of gain G

Thus:
  T1 = [(1 - 0.955) / 0.955] x 290 = 13.7K
  T2 = 80 
  T3 = [(1 - 0.25) / 0.25] x 290 = 870K
  T4 = (15.85 - 1) x 290 = 4307K

Now:
  Te = Ta.G1 + T1.G1 + T2 + T3/G2 + T4/(G2.G3)

Where:
  Te = noise temperature of the system (earth-station) referred to the input of the LNA
  Ta = the noise temperature of the antenna at its output terminals and G1, G2, G3 and T1, T2, T3, T4 are the gains and input noise temperatures of the corresponding networks as given in the previous figure

Hence:
  Te = 35 x 0.955 + 13.7 x 0.955 + 80 + 870 / 105 + 4307 / (105 x 0.25) = 33 + 13 + 80 + 0.17
i.e.
  Te = 126 K

Next: Section 8 - Power Amplifiers

 
 

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