Free RF Link Budget & Friis Path Loss Calculator
The 300th Tool Milestone! Calculate Free Space Path Loss (FSPL), EIRP, received signal power (Prx), fade margin, and line-of-sight range for wireless links.
📡 Transmitter, Channel & Receiver Parameters
✓ Excellent link reliability (≥ 15 dB fade margin)
Assumes clear line of sight with 60% first Fresnel zone clearance.
The Friis Path Loss Law: Every doubling of distance increases path loss by 6 dB (requiring 4× more power to maintain equal signal strength). Doubling the frequency also adds 6 dB of free space loss due to shrinking effective aperture area of receive antennas.
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The Classic Friis Transmission Equation
Formulated by Harald T. Friis at Bell Laboratories in 1946, the link budget equation accounts for every decibel of gain and loss between a radio transmitter and its receiver:
Free Space Loss: FSPL = 32.44 + 20 × log10( f_MHz ) + 20 × log10( d_km ) (dB)
Received Power: P_rx = EIRP - FSPL + G_rx - L_cable_rx - L_misc (dBm)
Fade Margin: Fade_Margin = P_rx - Sensitivity_rx (dB)
Why Fade Margin Determines Reliability
A wireless link that operates with 0 dB fade margin will drop connection whenever atmospheric moisture, rain fade, thermal ducting, or antenna swaying occurs. Industry standards mandate:
- 10 dB Fade Margin: Minimum acceptable for non-critical telemetry and IoT.
- 15 to 20 dB Fade Margin: Standard for 99.9% uptime WiFi bridges and cellular backhaul.
- 25 to 30 dB Fade Margin: Required for mission-critical aerospace, satellite downlinks, and public safety links.
Frequently Asked Questions
What is the difference between dBi and dBd?
dBi measures antenna gain relative to a theoretical isotropic point source radiator. dBd measures gain relative to a real half-wave dipole. Because a dipole has 2.15 dBi of natural gain, the conversion is: dBi = dBd + 2.15.
What is the Fresnel Zone and why does it matter?
Even with visual line-of-sight between two antennas, radio waves spread out in an elliptical football-shaped volume called the Fresnel Zone. If trees, buildings, or the earth intrude into the first 60% of the Fresnel radius, destructive multipath interference causes severe signal fading.
Why does a 5 GHz link have higher path loss than 2.4 GHz?
Radio waves themselves do not dissipate more energy in vacuum at higher frequencies. However, as frequency rises, the physical aperture size of an antenna with fixed gain shrinks, capturing fewer photons from the expanding wavefront.