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220 ANTENNA BASICS

For example, the resistor = 1 at = 20℃ = 293°K generates in the bandwidth ∆ =

10MHz the noise signal of 6.4 μV. On the other hand, measuring the noise power emitted
by an antenna may be associated with antenna noise temperature and found from (5.35) and
(5.36)

= ⁄ (∆) [°K] (5.37)

Typically, noise temperature is normalized to ∆ = 1 Hz and falls below several tens degrees

for antennas of high quality. Equation (5.37) tells us that the higher temperature means more
noise disguising the signal that comes to receiver.

The environment surrounding noise that a receiving antenna can collect from such natural
sources as atmospheric and sky radiation, earth heating, the sun, lightning and endless human-
made intentional or unexpected sources such as jamming, car engines, power lines, electric
motors and generators, microwave ovens, radars, broadcast stations, and communication
system. The plots in Figure 5.2.12 illustrates the combined atmospheric and sky noise
13
temperature calculated through (5.35) – (5.37) for standard atmosphere conditions at sea
0
level. In practice, rain, snow, or fog can significantly increase this noise level. As we have found
in Chapter 4, antennas are usually directive devices receiving more or less power |(, )|
2
depending on azimuth φ and elevation θ angles. In common case (see Figure 5.2.5 as
exemplary) and especially for almost erratic human-made sources, we must expect that noise

signal distribution (, ) in space also depends on the same angles. Subsequently, the
total noise power received by an antenna from external sources can be written as the integral

Figure 5.2.12 Combine atmospheric and sky noise temperature for standard
atmosphere

summing the active power from all non-correlated sources

2
2
= 1 ∫ ∫ |(, )| (, )sin (5.38)

4 0 0
13 Public Domain Image, source: modified from https://www.st-
andrews.ac.uk/~www_pa/Scots_Guide/RadCom/part8/page3.html