How does the insertion loss of a switch affect the overall system noise figure?

A switch placed in the RF signal path adds its insertion loss directly to the system noise figure when the switch is before the first amplifier (LNA). Using the Friis formula for cascaded noise figure: NF_system = NF_switch + NF_LNA + (NF_next - 1)/G_LNA + ... The NF of a passive component (switch, cable, filter) equals its insertion loss in dB: NF_switch = IL_switch (dB). For a switch with 0.5 dB insertion loss before the LNA: NF_system = 0.5 + NF_LNA + remainder. If NF_LNA = 1.0 dB: NF_system = 0.5 + 1.0 + small remainder = 1.5 dB (minimum). Without the switch: NF_system = 1.0 dB. The 0.5 dB switch loss directly degraded the system NF by 0.5 dB. Impact: (1) In a sensitive receiver (radio astronomy, deep-space communication): 0.5 dB NF degradation is unacceptable. Use the lowest-loss switch available (MEMS: 0.1 dB) or place the switch after the LNA. (2) In a cellular base station (NF ≈ 3-5 dB): 0.5 dB switch loss causes a 0.5 dB degradation in receiver sensitivity, equivalent to approximately 5% reduction in cell radius. This is significant across a network of thousands of base stations. (3) In a handset receiver (NF ≈ 5-8 dB): 0.5 dB switch loss is a smaller fractional degradation but still impacts the edge-of-cell performance. Mitigation: (a) Place the switch after the LNA (the LNA gain reduces the impact of downstream losses). (b) Use a bypass LNA before the switch (in some architectures). (c) Use the lowest-loss switch technology available for the specific frequency and requirements.