antenna temperature


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antenna temperature

A measure of the strength of signal received by a radio telescope from a radio source. It is defined as the noise power received per unit bandwidth, divided by the Boltzmann constant, k . Antenna temperature depends on the surface brightness of the sky weighted by the telescope's beam rather than physical temperature of the antenna. A radio source of flux density S with an angular diameter that is much smaller than the antenna's beam will give an antenna temperature of SA e /(2k ), where A e is the effective area (see array) of the antenna. If the beam is filled by a uniform source of brightness temperature T B, the antenna temperature will also be T B .

Synchrotron emission from cosmic rays in the Galaxy produces a diffuse radio emission centered broadly on the galactic plane. Any practical observation of a radio source has to be made against this background emission, which at low frequencies limits the sensitivity of the radio telescope. The antenna temperature of the diffuse emission is called the background temperature; the electrical noise it produces in radio receivers is often called cosmic noise.

antenna temperature

[an′ten·ə ‚tem·prə·chər]
(electromagnetism)
The temperature of a blackbody enclosure which would produce the same amount of noise as the antenna if it completely surrounded the antenna and was in thermal equilibrium with it.
References in periodicals archive ?
Because the upper plane of the cylinder mainly reflects the sky brightness temperature, the weighted radiation temperature near the signal peak decreases (while the antenna temperature contrast increases), so the signal peak of the cylinder is the highest.
It means that, the different parts of the solid target will produce the secondary-reflection radiation temperature through the ground in addition to direct reflection of the sky brightness temperature, because the solid target has a certain height; furthermore, the radiation temperature of the ground background and the sky brightness temperature of other incident angles are higher than the sky brightness temperature of the zenith angle; therefore, the antenna temperature contrast (absolute value) of the solid target will decrease, namely, the peak of the output signal is smaller than the signal peak of the planar target.
Additionally, the first part in Equation (11) reveals the specific influence on antenna temperature contrast resulted from the reverse radiation noise, and it is list as below:
So the influence of reverse radiation noise on antenna temperature contrast cannot be neglected, and it is mainly related to the half-power beamwidth of antenna 63dB, the equivalent integrated factor of LO [xi].sub.IF], and the LO-RF port isolation LOR, when the detection distance R is fixed.
TABLE I A COMPARISON OF LABORATORY AND DRIVE TEST MEASUREMENTS Type of system AMPS AMPS GSM CDMA Equipment noise figure improvement (dB) 2.7 4.0 3.0 3.5 Estimated coaxial cable loss (dB) 0.8 1.5 2.5 2.0 System-level noise figure improvement (dB) 3.5 5.5 5.5 5.5 System-level sensitivity improvement (dB) 4.0 6.0 7.5 7.0 The data plot shows that this sensitivity improvement can be significantly greater than the noise figure reduction that is measured with a noise figure meter as is frequently the case in rural cell sites, particularly when omnidirectional antennas are used.[1] The opposite is true when the effective antenna temperature is high relative to 290 K, as may be the case in dense urban environments where significant man-made noise and ground clutter exist.
In the example mentioned previously, a noise figure reduction of 6.3 dB yielded a sensitivity improvement of 7.3 dB with an antenna temperature of 200 K.
disturbed sun) and the frequency.[4] Due to the small angle subtended by the sun (0.5 [degrees]), the overall contribution to the antenna temperature is much smaller for terrestrial applications.
The greatest portion of the effective antenna temperature is contributed by emissions and reflections from the ground and other physical obstacles.
When a PMMW sensor looks at the terrain, sky or a metal object, the directly measurable parameter is the antenna temperature. The anticipated antenna temperature in each case can be obtained by introducing the apparent brightness temperature in Equation 1.
If it is assumed that an antenna observes only terrain material within its pattern solid angle [[Omega].sub.A] and that the terrain material emissivity is the same throughout the extent of the antenna pattern, then the terrain antenna temperature [T.sub.G] will have the same value as [T.sub.g].[4] [T.sub.G] is then expressed as
A high-gain antenna aimed directly at the sun will have a large antenna temperature. Many antennas, however, have a beamwidth that is much larger than 0.5 [Degrees], so the high temperature of the sun is averaged with the cool space that surrounds it.
For example, a 2-dB loss at room temperature would produce an antenna temperature of 107 K.