In radio frequency (RF) applications such as radio, radar and telecommunications, noise temperature of an antenna is a measure of the noise power density contributed by the antenna to the overall RF receiver system.
It is defined as "the temperature of a resistor having an available thermal noise power per unit bandwidth equal to that at the antenna's output at a specified frequency".
Moreover, an antenna does not have an intrinsic "antenna temperature" associated with it; rather the temperature depends on its gain pattern, pointing direction, and the thermal environment that it is placed in.
At around 150 MHz, it is approximately 1000 K. At 2500 MHz, it has leveled off to around 10 K. Earth has an accepted standard temperature of 288 K. The level of the Sun's contribution depends on the solar flux.
Newington: American Radio Relay League.