How to use it
- Enter the distance between the antennas and pick m, km, ft or mi.
- Enter the frequency in kHz, MHz or GHz.
- Optionally enter the transmitter power in dBm and the antenna gains in dBi (0 dBi is an isotropic antenna; a half-wave dipole is about 2.15 dBi).
- Read the path loss, EIRP and received power in dBm and mW.
Formula
FSPL (dB) = 20·log₁₀(4π·d·f / c)
With d in kilometres and f in megahertz the same formula is 20·log₁₀ d + 20·log₁₀ f + 32.45. The link budget is:
- EIRP (dBm) = P_tx + G_tx
- P_rx (dBm) = P_tx + G_tx + G_rx − FSPL
Received power in milliwatts is 10^(P_rx / 10). The wavelength is λ = c / f.
Worked example
Wi-Fi over 1 km at 2.4 GHz: 4π × 1000 × 2.4 × 10⁹ / 299,792,458 = 100,600, and 20·log₁₀ 100,600 = 100.05 dB. With 20 dBm and two isotropic antennas, the receiver sees 20 − 100.05 = −80.05 dBm, about 9.9 × 10⁻⁹ mW. The wavelength is 0.1249 m.
10 km at 900 MHz with 30 dBm and 12 dBi antennas at each end: FSPL = 111.53 dB, EIRP = 42 dBm, received power 30 + 12 + 12 − 111.53 = −57.53 dBm.
Tips
- 6 dB rule: each doubling of distance adds 6 dB of loss; ten times the distance adds 20 dB.
- Leave a fade margin. Real links need the received level 10 to 20 dB above the receiver sensitivity to ride out fading and obstacles.
- Keep the Fresnel zone clear. Free space loss assumes nothing blocks the first Fresnel zone around the line of sight; trees and terrain inside it add loss.