Friis Path Loss Calculator

Compute Free Space Path Loss (FSPL), received power, and link margin using the Friis transmission equation. Adjust frequency, distance, antenna gains, system losses, and transmitter power.

dBm
MHz
km
dBi
dBi
dB
dBm
for fade margin
m
Classic Friis formula: FSPL (dB) = 20·log₁₀(d) + 20·log₁₀(f) + 32.44 (d in km, f in MHz).
? Wi-Fi 2.4 GHz (100m)
? LTE 1800 MHz (2 km)
⚡ 5G mmWave 28 GHz (500m)
?️ Satellite C-band (36,000 km)
? Ham Radio 144 MHz (50 km)
Privacy first: All calculations are performed locally in your browser – no data is uploaded.
Verified against ITU-R P.525 & IEEE Friis reference – accuracy ±0.01 dB

Understanding the Friis Transmission Equation

Developed by Harald T. Friis at Bell Labs in 1946, the Friis transmission formula quantifies the power received by an antenna under ideal free‑space conditions. It is fundamental for wireless communication, radar, and radio astronomy. The equation in logarithmic form:

\[ \text{FSPL (dB)} = 20\log_{10}(d) + 20\log_{10}(f) + 32.44 \]

where \(d\) is distance (km), \(f\) is frequency (MHz). The received power: \(P_r(\text{dBm}) = P_t + G_t + G_r - \text{FSPL} - L_{sys}\).

The Friis equation assumes line-of-sight propagation, isotropic antennas in the far-field, and no multipath or atmospheric absorption. While real-world links experience additional fading, the Friis model provides the baseline path loss essential for any link budget. The calculator also evaluates the Fraunhofer distance (far-field limit) \(d_f = \frac{2D^2}{\lambda}\); you may adjust the antenna aperture D to match your actual antenna. For accurate far-field assessment, use your antenna's physical diameter.

Applications & Real-World Relevance

  • 5G/6G Link Planning: Estimate path loss for mmWave and sub‑6 GHz deployments.
  • Satellite Communications: Evaluate Earth-to-space link budgets.
  • Drone & UAV Telemetry: Predict control range and video downlink strength.
  • Amateur Radio (Ham): Validate VHF/UHF/SHF propagation expectations.
  • IoT & LoRaWAN: Determine maximum cell radius under free-space assumption.

Step‑by‑Step Usage

  1. Enter transmit power (dBm), frequency (MHz) and distance (km).
  2. Provide antenna gains (dBi) and system losses (cable/connector loss).
  3. Optionally add receiver sensitivity to compute fade margin.
  4. Click Compute Link Budget — FSPL, received power, and margin appear instantly.
  5. Visualize path loss vs distance using the interactive chart.

Example Scenarios & Verified Data

Scenario Pt (dBm) Gt (dBi) Gr (dBi) L (dB) Freq (MHz) Dist (km) FSPL (dB) Pr (dBm) Typical reliability
Wi‑Fi 2.4 GHz (indoor) 20 2 2 1 2450 0.1 80.2 -57.2 Excellent
LTE 1800 MHz (rural) 33 15 0 2 1800 2.0 103.6 -57.6 Good
5G mmWave 28 GHz (urban) 30 24 5 3 28000 0.5 115.4 -59.4 Marginal possible
Satellite C-band 40 42 38 1.5 4000 36000 195.6 -77.1 Requires high gain & LNA
Ham Radio 144 MHz 50 6 6 1 144 50 109.6 -48.6 Very reliable
Case Study: 5G mmWave Street Deployment

An operator deploys a small cell at 28 GHz with Pt = 30 dBm, Gt = 24 dBi (phased array), Gr = 5 dBi, distance 200 m (0.2 km). Our calculator yields FSPL ≈ 97.4 dB, Pr ≈ -38.4 dBm, and a fade margin > 40 dB assuming standard sensitivity. This confirms feasibility for gigabit links, but atmospheric oxygen absorption (~15 dB/km at 60 GHz) would degrade higher bands; our tool highlights the baseline.

Far-field & Validity Conditions

The Friis equation holds when the distance d > d_f (Fraunhofer distance). We compute far-field using \(d_f = 2D^2 / \lambda\) with user‑defined antenna aperture D. If distance is less than far-field distance, near-field effects dominate – the calculation remains indicative but actual losses may differ. A red warning appears when this occurs.

Common Misconceptions

  • FSPL is frequency dependent – Yes, due to the effective aperture concept; higher frequencies suffer higher free-space loss over same distance.
  • Antenna gains always improve link – Gains focus energy, but also affect beamwidth; misalignment reduces effective gain.
  • Friis model works everywhere – It ignores reflections, diffraction, and multipath. For indoor/urban environments, add empirical models (ITU-R P.1238).

Grounding in RF engineering – This implementation follows the exact IEEE definition of the Friis transmission equation. References include "Friis, H.T. (1946). A Note on a Simple Transmission Formula", IRE Proc.; Balanis, C.A. "Antenna Theory" (4th ed.). Verified against ITU‑R P.525‑4.

Frequently Asked Questions

Free Space Path Loss (FSPL) is the theoretical loss under ideal isotropic conditions with no obstacles. Total path loss includes additional fading, atmospheric loss, and polarization mismatch.

Indoor propagation includes reflections and wall attenuations. Friis gives an optimistic upper bound. For realistic indoor use, add 10–30 dB margin.

Higher antenna gains concentrate radiated power toward the receiver, effectively increasing the signal strength at the receive antenna terminals.

The far-field distance is calculated based on the antenna aperture D you provide. For precise engineering applications, enter the actual aperture size of your antenna. The indicator then accurately tells you if you are in far-field.

Link margin is the difference between received power and receiver sensitivity. A positive margin ensures reliable communication under fading, interference, and weather.
References: Friis Original Paper (IRE 1946) | Friis Transmission Equation | ITU-R P.525 (Free-space attenuation)