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Temperatures
The power
received by an antenna in a bandwidth
is
conveniently expressed in term of antenna temperature,
,
which is the temperature for which an equivalent resistor would give
the same power
, following the Nyquist noise formula:
|
(2) |
As the quantity
is affected by the atmospheric absorption
and the antenna foward coupling factor
, we
define the quantity
through:
|
(3) |
The
, where
is the image band rejection factor,
accounts for a single sideband signal.
Brightness temperature,
, is the Rayleigh-Jeans temperature
of an equivalent blackbody which would give the same power per
unit area per unit frequency and per unit solid angle
as the
celestial source:
|
(4) |
For a resolved source, the antenna temperature is equal to the sky
brightness temperature. For an unresolved source, the coupling between
antenna temperature
and source flux density
is:
|
(5) |
where
is the antenna efficiency.
The noise temperature is the sum of the various noise
contributions:
|
(6) |
where
is the cosmic background,
is the sky noise,
is the ground noise pickup and
is the receiver
noise temperature. The forward efficiency
is a property of
the antenna that indicates how much coupling there is in the forward
hemisphere with respect to the full
sphere. With an atmosphere
at a physical tempeture
and with an opacity
at
the observed frequency, the sky noise and ground pickup temperature
are expressed as:
is the system temperature, the noise equivalent
temperature
of the receiving chain refered to a perfect
antenna located outside the atmosphere and for a single sideband
signal:
|
(9) |
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Gildas manager
2024-03-29