The LNA boosts the antenna signal to overcome feed line losses in between the antenna plus the receiver. To simplify the sounds attain calculation, the noise sources in the simple amplifier circuit in Figure one is usually decreased to just one whole RTI noise source (Vni, TOTAL), as proven in https://low-noiseamplifier90000.blogstival.com/58773133/the-5-second-trick-for-low-noise-amplifier