Free RF Link Budget & Friis Path Loss Tool
Calculate Free Space Path Loss (FSPL), received RF power ($P_{rx}$), receiver thermal noise floor, and fade margin (dB) for wireless IoT, cellular, and satellite links.
📡 Frequency & Distance
📊 Link Budget & Fade Margin
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1. The Friis Transmission Equation & Path Loss
Formulated by Danish-American radio engineer Harald T. Friis at Bell Laboratories in 1946, the Friis transmission equation calculates the ratio of received power ($P_{rx}$) to transmitted power ($P_{tx}$) between two antennas in free space: $$ rac{P_{rx}}{P_{tx}} = G_{tx} cdot G_{rx} cdot left( rac{lambda}{4 pi d} ight)^2$$ Expressed in practical logarithmic decibels (dB), Free Space Path Loss (FSPL) between isotropic antennas is: $$ ext{FSPL (dB)} = 20 log_{10}(d_{ ext{km}}) + 20 log_{10}(f_{ ext{MHz}}) + 32.44$$ Every doubling of distance ($2 imes$) adds $+6 ext{ dB}$ of path loss (the inverse-square law). Likewise, doubling the carrier frequency adds $+6 ext{ dB}$ of path loss because higher-frequency antennas have physically smaller effective aperture areas ($sim lambda^2$).
2. Receiver Noise Floor & Sensitivity Threshold
At room temperature ($T_0 = 290 ext{ K}$), Johnson-Nyquist thermal noise generates an inescapable noise power density: $$N_0 = k cdot T_0 = -174 ext{ dBm/Hz}$$ Across a communication channel of bandwidth $BW$ with a receiver front-end Noise Figure $NF$, the total noise floor is: $$P_{ ext{noise}} = -174 ext{ dBm/Hz} + 10 log_{10}(BW_{ ext{Hz}}) + NF$$ To successfully demodulate the incoming signal, the received power must exceed the noise floor by the modulation's minimum Signal-to-Noise Ratio ($SNR_{min}$): $$P_{sens} = P_{ ext{noise}} + SNR_{min}$$ While high-speed Wi-Fi (64-QAM) demands $SNR ge +22 ext{ dB}$, spread-spectrum protocols like LoRa (Chirp Spread Spectrum) can decode signals deep below the thermal noise floor ($SNR = -7.5 ext{ dB}$ to $-20 ext{ dB}$), delivering miraculous multi-kilometer range with tiny battery-powered transmitters.
3. Link Fade Margin & Reliability
The Fade Margin is the buffer of excess signal above receiver sensitivity: $$ ext{Margin (dB)} = P_{rx} - P_{sens}$$
- $< 10 ext{ dB}$ Margin: Unreliable; signal will drop during rain, fog, or vehicle movement.
- $15 - 20 ext{ dB}$ Margin: Standard commercial grade ($99.9%$ link availability).
- $> 25 ext{ dB}$ Margin: High-reliability mission-critical / public safety grade ($99.999%$ uptime).
Frequently Asked Questions
Why does a 900 MHz LoRa signal travel farther than 2.4 GHz Wi-Fi?
Two reasons: (1) Free space path loss is ~8.5 dB lower at 900 MHz than 2.4 GHz due to larger antenna aperture, and (2) LoRa uses ultra-narrow bandwidth (125 kHz vs 20 MHz), which lowers the receiver thermal noise floor by 22 dB, dramatically improving sensitivity.
What is the Fresnel zone in wireless link budgets?
The first Fresnel zone is an elliptical volume between antennas that must remain clear of obstacles (trees, buildings, terrain). If 60% or more of the first Fresnel zone is obstructed, the signal experiences severe diffraction losses (often 10 to 30 dB) even if there is visual line-of-sight.
What is EIRP (Effective Isotropic Radiated Power)?
EIRP is the actual RF power radiated in the direction of maximum antenna gain: EIRP (dBm) = P_tx (dBm) + G_tx (dBi) - Cable_loss (dB). Telecommunications regulators (FCC, CE) set strict legal limits on maximum EIRP (e.g. +30 dBm / 1W or +36 dBm / 4W) to prevent interference.
How does rain affect microwave links?
At frequencies below 6 GHz, rain attenuation is negligible (< 0.1 dB/km). Above 10 GHz (e.g. 24 GHz, 60 GHz, 80 GHz E-band), raindrops absorb and scatter electromagnetic wavelengths, introducing 5 to 20 dB/km of rain fade during heavy downpours, requiring large fade margins.