to-noise ratio (SNR) is often the determining factor in establishing com- munications, and it depends on the absolute level of signal, the external noise in the propagation medium, and the internal noise in the transmitting and receiving equipment. At HF, communications are normally externally noise limited SO that it is the noise in the propagation medium that predominates, while at VHF and above the noise generated within the first stages of the receiver is usually the determining factor. Therefore, there might be no benefit in trying to increase the received strength of HF signals by using larger and higher-gain antennas because noise can increase proportionally with no improvement in SNR. External noise is from three sources: galactic noise, atmospheric noise, and man-made noise. Noise power in a communication link is given by equation 8.8, where N is the noise power in watts, T is the temperature in degrees absolute, often taken as 290 K because this corresponds to the normal room temperature of 17°C, and k is Boltzmann's constant (1.38 J.K-1. N=kTB (8.8) In communications links, noise is measured in terms of the noise power available from a lossless antenna and can be expressed as in equation 8.9, where Pn is the total power in dBW, is the antenna noise figure in dB, B = 10log bandwidth (in Hertz), and 204 is 10logkT°, assuming T° = 290 Pn=Fa+B-204 (8.9) Atmospheric and man-made noise information is given in ITU-R publications, and typical values are shown in Figure 8.4. Galactic noise originates from sources outside the Earth's atmosphere, such as the Sun and the stars, and extends from about 15 to 100 MHz. It is limited by ionospheric absorption below this frequency range and atmospheric absorp- tion above it. Atmospheric noise is the major source of noise in the MF and HF bands and is primarily due to lightning discharges, SO it is particularly severe in the rainy season in tropical regions, such as equatorial Africa, and at its lowest value in high latitudes at night. It is transmitted over long distanc