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Antenna temperature and equivalent noise temperature

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iVenky

iVenky

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I just couldn't understand these terms-

What do you mean by "antenna temperature" and "equivalent noise temperature"?


Thanks in advance.
 

A simple explanation would be :
Antenna noise temperature is the output noise power of a directional antenna; it depends on where the antenna "looks". If pointed to a wall or ground, the output noise temperature will be close to the ambient temperature. If pointed to the clear sky, it will be between ~ 30 to ~100 Kelvins, depending mainly on side lobes and where they point. Google "radiometry" for more.
Equivalent noise temperature is the noise temperature value at the output of an "equivalent" noise source. Noise output from noise sources can be adjusted to a desired value.

At microwaves, for low-noise work, noise temperature is used preferably instead of noise power. They are linked by the known formula P= kTB, where P is the power in Watts, k is the Boltzmann constant, k= 1.38 E(-23) J/K, T is the temperature in Kelvins, and B is the bandwidth in Hertz. So instead of pico- or nanowatts we have tens to thousands of Kelvins.
 

jiripolivka said:
A simple explanation would be :
Antenna noise temperature is the output noise power of a directional antenna; it depends on where the antenna "looks". If pointed to a wall or ground, the output noise temperature will be close to the ambient temperature. If pointed to the clear sky, it will be between ~ 30 to ~100 Kelvins, depending mainly on side lobes and where they point. Google "radiometry" for more.
Equivalent noise temperature is the noise temperature value at the output of an "equivalent" noise source. Noise output from noise sources can be adjusted to a desired value.

At microwaves, for low-noise work, noise temperature is used preferably instead of noise power. They are linked by the known formula P= kTB, where P is the power in Watts, k is the Boltzmann constant, k= 1.38 E(-23) J/K, T is the temperature in Kelvins, and B is the bandwidth in Hertz. So instead of pico- or nanowatts we have tens to thousands of Kelvins.
Click to expand...


Is this the noise due to thermal agitation of electrons that we study in kinetic theory?
 

iVenky said:
Equivalent noise temperature is the noise temperature value at the output of an "equivalent" noise source. Noise output from noise sources can be adjusted to a desired value.
Click to expand...


I think I am getting it. I just couldn't find any difference between equivalent noise temperature and antenna temperature. I mean when you have an equivalent noise source how would you get a different temperature? I should be same as antenna temperature, isn't it?


Thanks in advance.
 

All discussed noise temperatures , from an antenna or the equivalent noise source, come from the Dicke system of microwave radiometers. In a Dicke radiometer, an input switch alternatively connects to receiver input the antenna output, and, the output from the equivalent noise source (exactly known). If both noise temperatures are equal, the instrument is "balanced", or, its output is close to zero. By this, the equality of both input noise temperatures is confirmed.
In practice, antenna noise output varies; the radiometer is therefore calibrated by replacing the antenna by a variable (or several) noise source(s) with different noise-temperature outputs chosen over a desired range.
Typically, microwave radiometers can linearly respond to input noise temperature over 0 to 500 Kelvins, or, a selected narrower range.
 

jiripolivka said:
All discussed noise temperatures , from an antenna or the equivalent noise source, come from the Dicke system of microwave radiometers. In a Dicke radiometer, an input switch alternatively connects to receiver input the antenna output, and, the output from the equivalent noise source (exactly known). If both noise temperatures are equal, the instrument is "balanced", or, its output is close to zero. By this, the equality of both input noise temperatures is confirmed.
In practice, antenna noise output varies; the radiometer is therefore calibrated by replacing the antenna by a variable (or several) noise source(s) with different noise-temperature outputs chosen over a desired range.
Typically, microwave radiometers can linearly respond to input noise temperature over 0 to 500 Kelvins, or, a selected narrower range.
Click to expand...

So the reason why we have equivalent noise is to reduce the error in the output. This looks to me to be somewhat similar to the offset drift in amplifiers.

Am I right?
 

I do not think so. The concept of Dicke switching and the equivalent noise source is to reduce the error in determining the INPUT noise temperature introduced to a radiometer from an antenna. It has nothing common with offsets or drifts in amplifiers.
 

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