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The link budget sums all gains and subtracts all losses in the signal path from transmitter to receiver. The result must exceed receiver sensitivity plus required fade margin for reliable communication.
P_rx(dBm) = P_tx + G_tx - L_tx - FSPL - L_misc + G_rx - L_rxFSPL is the dominant loss in most links. It increases with both frequency and distance according to the Friis transmission equation. At higher frequencies, more directional antennas are needed to compensate.
FSPL(dB) = 20log₁₀(d) + 20log₁₀(f) + 32.44 (d in km, f in MHz)Fade margin is the excess signal above receiver sensitivity to maintain link availability during atmospheric fading, rain attenuation, or multipath. Typical targets: 10-15 dB for 99.9% availability, 20-25 dB for 99.99%.
Updated: July 2026
Outdoor WiFi bridge using directional antennas for building-to-building connectivity.
→ Received power: ~-47 dBm (well above -80 dBm sensitivity)
Long-range IoT sensor communication in rural area.
→ Received power: ~-102 dBm (within LoRa -137 dBm sensitivity)
Coaxial cable loss at 5 GHz can be 0.3-1.0 dB per meter. A 3m cable run with 2 connectors adds 2-4 dB of loss. Use low-loss cable (LMR-400 or better) and minimize connector count.
Rain attenuation becomes significant above 10 GHz. At 60 GHz, heavy rain can add 20+ dB/km of loss. Include ITU-R P.838 rain attenuation models for frequencies above Ku-band.
Calculate the power budget for an RF communication link by summing transmitter power, antenna gains, and subtracting all path losses. Determines whether the received signal exceeds the receiver sensitivity threshold with adequate fade margin for reliable operation.