The antenna noise temperature is a measure of the noise entering the receiver via the antenna. The antenna picks up noise radiated by various man-made and natural sources within its directional pattern.
Various sources of noise, include noise generated by electrical equipment, noise emanating from natural sources including solar radiation, sky noise, background noise contributed by Earth, galactic noise due to electromagnetic waves emanating from radio stars in the galaxy and atmospheric noise caused by lightening flashes and absorption by oxygen and water vapour molecules followed by the re-emission of radiation.
Noise from these sources could enter the receiver both through the main lobe as well as through the side lobes of the directional pattern of the receiving antenna. Thus, the noise output from a receiving antenna is a function of the direction in which it is pointing, its directional pattern and the state of its environment. The noise performance of an antenna can be expressed in terms of a noise temperature called the antenna noise temperature.
If the antenna noise temperature is TA K, it implies that the noise power output of the antenna is equal to the thermal noise power generated in a resistor at a temperature of TA K. The noise temperature of the antenna can be computed by integrating the contributions of all the radiating bodies whose radiation lies within its directional pattern. It is given by
$T_A = \bigg( \frac{1}{4\pi} \bigg) \int \int G(\theta, \phi) T_b(\theta, \phi) sin \theta \space d \theta \space d \phi
$
where,
θ = azimuth angle
φ= elevation angle
G(θ, φ)= antenna gain in the θ and φ directions
Tb(θ, φ)= brightness temperature in the θ and φ directions
Figure summarizes some representative values for the increase in antenna temperature due to atmospheric constituents in the path, for both the downlink and the uplink.
Antenna Gain-to-Noise Temperature (G/T) Ratio
The antenna gain-to-noise temperature (G/T) ratio, usually defined with respect to the Earth station receiving antenna, is an indicator of the sensitivity of the antenna to the downlink carrier signal from the satellite. It is a figure-of-merit used to indicate the combined performance of the Earth station antenna and low noise amplifier combined in receiving weak carrier signals.
Significance of Earth stations antenna gain to noise temperature ratio:
The transmitter EIRP and receiver G/T together dictate the performance of the communication system and therefore one can be traded off against the other during the design optimization process
Possible trade-offs can be best understood by resorting to expression for Earth station G/T .
The generalized expression for G/T is given by
$G/T = C/No − EIRP +(L_p + L_m ) + k$
Where C/No, EIRP, Lp, Lm and k are carrier-to-total noise power spectral density, satellite’s effective isotropic radiated power, path loss, link margin and Boltzmann constant (in dBs) respectively. For a minimal cost Earth station, G/T should be minimized. This can be possible by either using relatively higher EIRP in the satellite or being able to afford a lower carrier-to noise ratio or both. For desired base band quality at the receiver, this can be achieved by using modulation schemes that are more immune to noise.
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