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
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