Antenna Temperature

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Antenna Temperature (antenna temperature) is a parameter that describes how much noise an antenna produces in a given environment. This temperature is not the physical temperature of the antenna. Moreover, an antenna does not have an intrinsic "antenna temperature" associated with it; rather the temperature depends on its gain pattern and the thermal environment that it is placed in. Antenna temperature is also sometimes referred to as Antenna Noise Temperature.

To define the environment (and hence give the full definition of antenna temperature), we will introduce a temperature distribution - this is the temperature in every direction away from the antenna in spherical coordinates. For instance, the night sky is roughly 4 Kelvin; the value of the temperature pattern in the direction of the Earth's ground is the physical temperature of the Earth's ground. This temperature distribution will be written as temperature distribution. Hence, an antenna's temperature will vary depending on whether it is directional and pointed into space or staring into the sun.

For an antenna with a radiation pattern given by radiation pattern, the noise temperature is mathematically defined as:

antenna temperature

This states that the temperature surrounding the antenna is integrated over the entire sphere, and weighted by the antenna's radiation pattern. Hence, an isotropic antenna would have a noise temperature that is the average of all temperatures around the antenna; for a perfectly directional antenna (with a pencil beam), the antenna temperature will only depend on the temperature in which the antenna is "looking".

The noise power received from an antenna at temperature antenna can be expressed in terms of the bandwidth (B) the antenna (and its receiver) are operating over:

noise power

In the above, K is Boltzmann's constant (1.38 * 10^-23 [Joules/Kelvin = J/K]). The receiver also has a temperature associated with it (receiver temperature), and the total system temperature (antenna plus receiver) has a combined temperature given by system temperature. This temperature can be used in the above equation to find the total noise power of the system. These concepts begin to illustrate how antenna engineers must understand receivers and the associated electronics, because the resulting systems very much depend on each other.

A parameter often encountered in specification sheets for antennas that operate in certain environments is the ratio of gain of the antenna divided by the antenna temperature (or system temperature if a receiver is specified). This parameter is written as G/T, and has units of dB/Kelvin [dB/K].

Finally, note that many RF engineers like to use the term Noise Figure (or Noise Factor, NF) to describe systems. This is the ratio of the input SNR (signal to noise ratio) to the output SNR. Basically, all RF devices (like mixers and amplifiers) add some noise. Antenna temperature doesn't really relate to a Noise Figure, as the signal level power input varies greatly with the desired signal's direction of arrival, while the noise added is a constant.


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