|To a communication or radar system, an antenna contributes noise from two sources. The antenna receives noise power because it looks out on the sky and ground. The ground generates noise because it is about 290K and a portion of the antenna pattern falls on it. Similarly, the sky adds noise dependent on the elevation angle and the operating frequency. Figure 1 gives the sky temperature versus frequency and elevation angle. The frequency range of lowest noise occurs in the middle of microwave frequencies of 1 to 12 GHz. The graphs show a large variation between the dashed curves, which occurs because of antenna direction and the pointing relative to the galactic center. In the middle of microwaves the sky noise temperatures are around 50 K, whereas near zenith the temperature is under 10 K. Near the horizon it raises because of the noise from oxygen and water vapor. The exact value must be determined for each application. As frequency decreases below 400 MHz, the sky temperature rises rapidly and becomes independent of antenna pointing. The curve continues the rapid rise at the same slope for lower frequencies. Low-frequency sky temperatures are often given as decibels relative to 290 K.
FIGURE 1 Antenna sky temperature. Noise temperature of an idealized antenna (lossless, no Earth-directed sidelobes) located at the Earth’s surface, as a function of frequency, for a number of beam elevation angles. Solid curves are for geometric-mean galactic temperature, sun noise 10 times quiet level, sun in unity-gain sidelobe, cool temperate-zone troposphere, 2.7K cosmic blackbody radiation, zero ground noise. The upper dashed curve is for maximum galactic noise (center of galaxy, narrow-beam antenna). Sun noise 100 times quiet level, zero elevation, other factors the same as solid curves. The lower dashed curve is for minimum galactic noise, zero sun noise, 90â—¦ elevation angle. (The bump in the curves at about 500MHz is due to the sun-noise characteristic. The curves for low elevation angles lie below those for high angles at frequencies below 400MHz because of reduction of galactic noise by atmospheric absorption. The maxima at 22.2 and 60 GHz are due to the water-vapor and oxygen absorption resonance.) (From L. V. Blake, A guide to basic pulse-radar maximum-range calculation, Naval Research Laboratory Report 5868, December 1962.)