24dBm-(189.3 +2 +2) dB = -169.3dBm and about 2dB more in the 

 Northern Hemisphere. (Note: In making signal level calculations 

 it is convenient to express power levels in decibel form. Since 

 a decibel represents a ratio of two power levels, it is necessary 

 to specify a reference level when absolute power is specified in 

 decibel form. By convention, one of two values is used as a 

 reference. When a power level is specified in dBm, the 

 reference is understood to be 1 milliwatt, 10"^ watts. When the 

 power is expressed in dBw, the reference is understood to be 1 watt. 

 Thus, +34dBm is a power level 34dB above 10 ^ watts or 2.5 watts. 

 Similarly, 4dBw is a power 4dB above 1 watt or 2.5 watts. A power 

 level given in dBw may be converted to dBm, or vice versa, by 

 adding or subtracting 30dB since the difference between 1 watt and 

 10"^ watts is 30dB). 



The natural noise against which the signal energy must compete is 

 composed of two components. The first is cosmic noise with an 

 effective noise temperature of about 75 degrees Kelvin ( K) for an 

 elevation angle of 5 degrees or more. This noise will be nearly 

 constant with increases occurring only when the antenna beam sweeps 

 past radio stars or the Sun. The user has no control over this 

 source of noise. 



The second major source of noise is the receiver. The noise 

 contribution of the receiver is measured by its noise temperature; 

 another term often used is noise figure. As the noise temperature 

 of a receiver is difficult to measure directly, it is standard 

 practice to measure the receiver noise figure and convert this to 

 noise temperature. The noise temperature and noise figure are 

 related by equation (1). 



NF = 10 log /Tree +l\ (1) 

 yTref j 



tit; = receiver noise figure m dB 



Tree = receiver noise temperature (degrees Kelvin) 



Tref = reference temperature (degrees Kelvin see 

 discussion below) 



log = is the logarithm to the base 10 



The reference temperature, Tref, is usually taken to be in the 

 vicinity of ambient room temperature, approximately 293 K. In the 

 material that follows, a reference temperature of 289.855 K will 

 be used. The reason for this choice is that when 289.855 K*-* is 



multiplied by Boltzmann's constant (1.38 x 10"^' joules per °K) 



53 



