 for frequencies from MHz to 100GHz. (The units on the diagonal lines are in dB) National Bureau of Standards radio station WWV transmits ionos- Equation 27.9, where P is the total power in dBW, F is the antenna pheric data information hourly. noise figure in dB, B = 10log bandwidth (in hertz), and 204 is 10log The signal to noise ratio (S/N) is often the determining factor in kTo, assuming T° = 290K. establishing communications, and it depends upon the absolute level of signal, the external noise in the propagation medium and the P n = F a + B - 204 (27.9) internal noise in the transmitting and receiving equipment. At h.f., communications are normally externally noise limited SO that it is the noise in the propagation medium which predominates, whilst at Atmospheric and man made noise information is given in CCIR v.h.f. and above the noise generated within the first stages of the publications and typical values are shown in Figure 27.4. Galactic receiver is usually the determining factor. Therefore there may be noise originates from sources outside the Earth's atmosphere, such no benefit in trying to increase the received strength of h.f. signals as the sun and the stars, and extends from about 15MHz to 100MHz. by using larger and higher gain antennas as noise may increase It is limited by ionospheric absorption below this frequency range proportionally with no improvement in signal to noise ratio. and atmospheric absorption above it. Atmospheric noise is the major source of noise in the m.f. and h.f. bands and is primarily due External noise is from three sources; galactic noise, atmospheric to lightning discharges SO it is particularly severe in the rainy season noise and man made noise. Noise power in a communication link is in tropical regions such as equatorial Africa, and at its lowest value given by Equation 27.8, where N is the noise power in watts, T is the temperature in degrees absolute, often taken as 290° as this in high latitudes at night. It is transmitted over long distances by skywave paths. Man