📡️ Friis Path Loss Calculator

The Friis transmission equation is the first tool any wireless engineer reaches for. It predicts how much power arrives at a receiver when you know the transmitter power, both antenna gains, the frequency and the distance. In its decibel form the free-space path loss is Lₔ = 20·log₁₀(4πd/λ), and the received power is Pₘ = Pₕ + Gₕ + Gₖ − Lₔ − (cable and connector losses).

Free-space path loss grows by 20 dB per decade of distance and 20 dB per decade of frequency — which is why a 5 GHz link loses far more than a 900 MHz link over the same distance, and why higher-gain antennas are mandatory at millimeter-wave frequencies. In the real world obstacles, reflections and rain add extra loss, but the Friis equation gives the ideal baseline every budget starts from.

The calculator converts frequency and distance to a common unit, computes the wavelength λ = c/f, then applies the decibel form of the Friis equation. The path loss output is always the free-space value; real-world rain and high humidity add extra per-kilometre loss, but this calculator does not model atmospheric attenuation. The received power is derived from TX power, antenna gains and total loss, and is also shown as a link margin relative to a user-set sensitivity.

The derived link margin (received power minus receiver sensitivity) tells you whether the link will close — anything above about 10 dB gives comfortable rain and fade headroom. Combine the path loss with the line-of-sight calculator to check whether the Fresnel zone is clear before you buy hardware.

1. Enter the operating frequency (MHz or GHz as selected).
2. Enter the link distance (km or miles).
3. Enter TX power in dBm and TX/RX antenna gains in dBi.
4. Optional: add cable loss per end and receiver sensitivity.
5. Click Calculate to see path loss, received power and link margin.

Why is free-space path loss 20 dB/decade of both distance and frequency?

In the Friis equation the spreading term (4πd/λ) is a ratio of distance to wavelength. Doubling the distance doubles the spreading (6 dB); doubling the frequency halves the wavelength, which doubles the number of wavelengths in the path and adds another 6 dB.

What is a good link margin for a reliable link?

For fixed point-to-point links, 10–20 dB of margin is standard to cover rain, foliage movement and equipment drift. For indoor WiFi, aim for at least 5–10 dB. The margin is the difference between received power and the receiver's sensitivity.

Does the Friis equation work indoors?

Only as an upper bound. Indoors, reflections, walls and furniture add 10–30 dB beyond free-space loss. The equation is most useful for outdoor line-of-sight links; for indoor planning use a model like ITU-R P.1238 or a site survey.

How do I include rain fade?

Rain fade mainly affects frequencies above 10–12 GHz. Add an extra loss of roughly 0.1–1 dB/km depending on rain rate and frequency. This calculator does not model atmospheric attenuation; for licensed links use the full ITU-R P.838 method.

Can I use this for satellite links?

Yes, but add the slant range (not the surface distance), and be aware that atmospheric absorption at low elevation angles and polarization effects matter. For GEO satellites the path loss alone is typically 190–210 dB at C/Ku band.

What if my antennas are not pointed at each other?

Antenna gain assumes boresight alignment. Off-axis, the gain drops according to the antenna pattern — typically 3 dB at the half-power beamwidth. Misalignment, especially with high-gain dishes, is a common reason real links fall short of the calculation.

Use the 60% Fresnel-zone rule before trusting any link: the first Fresnel zone must be clear of trees and rooftops or the real loss will exceed the free-space value. Remember that doubling the frequency costs 6 dB of link budget — this is why sub-GHz is preferred for long-range IoT. Cable loss matters more than most people think: 10 m of thin coaxial at 2.4 GHz can eat 3–4 dB before the antenna. Include connector and lightning-arrester losses as a fixed 0.5–1 dB per end. For a quick sanity check, the rule of thumb is that free-space path loss at 1 km is about 92 dB at 1 GHz.