How does temperature affect the low-noise amplifier (LNA) noise figure and the system G/T in satellite communications?

• A. Higher temperatures improve the LNA noise figure and increase G/T.
• B. Higher temperatures degrade the LNA NF and reduce G/T.
• C. Temperature changes have no effect on LNA NF or G/T.
• D. Cooling the system worsens sensitivity by increasing NF.

Answer: (B)

Temperature sets the thermal noise that every amplifier adds to a signal. As the LNA heats up, its internal noise sources grow, so the LNA’s noise figure increases. That means more of the signal’s potential SNR is lost right at the front end. In a satellite link, system G/T is the antenna gain divided by the total noise temperature of the receiver system. If the front-end noise temperature goes up while the gain stays about the same, the overall T_sys rises and G/T falls. So higher temperatures degrade the LNA noise figure and reduce G/T.

Cooling the system lowers the thermal noise, improving the NF and increasing G/T. The other options contradict the basic link between temperature and thermal noise, or claim cooling would worsen sensitivity, which isn’t the case.