re can be related to noise temperature by the following formula: NFdB =10 log(1+Te/290) (9.20) where Te is the effective noise temperature measured in kelvins. Note that the kelvin temperature scale is based on absolute zero. Example. If the noise figure of a device is 1.2 dB, what is its equivalent noise tempera- ture? 1.2 dB = 10 log(1 Te/290) 0.12 = log(1 + Te/290) 1.318 = Te/290) 0.318 = Te/290 Te = 0.318 x 290 Te = 92.22 K Antenna noise (Tant) is calculated by the following formula: (9.21) 9.3 SATELLITE COMMUNICATIONS where la is the numeric equivalent of the sum of the ohmic losses up to the reference plane and is calculated by (9.22) where La is the sum of the losses in decibels. Sky noise varies directly with frequency and inversely with elevation angle. Some typical sky noise values are given in Table 9.5. Example. An earth station operating at 12 GHz with a 10° elevation angle has a 47-dB gain and a 2.5-dB loss from the antenna feed to the input of the LNA. The sky noise is 25 K developing an antenna noise temperature of 240 K. The noise figure of the LNA is 1.5 dB. Calculate the G/T. Convert the 1.5-dB LNA noise figure value to its equivalent noise temperature. Use formula (9.20). For this sample problem Te = Trecvr. 1.5 dB = 10 log(1 + Te/290) = 119.6 K Trecvr Tsys = Tant + Trecvr = 240 + 119.6 = 359.6 K. Now we can calculate the G/T. Derive the net antenna gain (up to the reference plane-at the input of the LNA). Gnet = 47 - 2.5 = 44.5 dB G/T = 44.5 dB - 10 log Tsys = 44.5 - 10 log 359.6 = +18.94 dB/K or just + 18.94 dB/K. For earth stations operating below 10 GHz, it is advisable to have a link margin of 4 dB to compensate for propagation anomalies and deterioration of components due to aging. Such a margin gives a comfortable safety factor, but every decibel above and beyond Table 9.5 Sky Noise Values for Several Frequencies and Elevation Angles Frequency Elevation Angle Sky Noise (GHz) CONCEPTS IN TRANSMISSION TRANSPORT what actually needed to close the link costs money. Here is where good system 