Free-Space Friis Equation Calculator
Estimate received power for a line-of-sight RF link using the Friis transmission equation. Optionally add receiver sensitivity to calculate link margin and maximum theoretical range.
What is the Friis Equation?
The Friis transmission equation estimates how much power a receiving antenna gets from a transmitting antenna in an ideal free-space path. In wireless engineering, it is one of the foundational equations used for RF link budgets, range planning, and quick feasibility checks.
In plain language: it tells you how signal power decreases with distance and frequency, while also accounting for antenna gain.
Core formula
Where:
- Pr = received power
- Pt = transmit power
- Gt, Gr = transmitter and receiver antenna gains (linear scale)
- λ = wavelength
- d = separation distance
Logarithmic (dB) form used in practice
This dB form is easier for engineers because multiplication/division becomes addition/subtraction.
How to use this Friis equation calculator
- Enter transmit power in dBm or watts.
- Enter antenna gains in dBi.
- Set operating frequency and link distance with unit selectors.
- Add extra losses (cables, connectors, mismatch, polarization, radome, etc.).
- Optionally enter receiver sensitivity to calculate margin and theoretical max distance.
Worked example
Suppose you have:
- Pt = 20 dBm
- Gt = 2 dBi
- Gr = 2 dBi
- f = 2.4 GHz
- d = 1 km
- Losses = 0 dB
At these settings, free-space path loss is around 100 dB, so received power lands in the neighborhood of -76 dBm. Depending on your modulation and noise floor, this may be enough for lower data rates and may be marginal for higher-throughput modes.
Assumptions and limitations
The Friis equation is idealized. It assumes a clean free-space environment and does not inherently account for clutter or fading. Real deployments can differ significantly.
Friis works best when
- There is clear line-of-sight (LOS).
- Antennas are properly aligned and polarization-matched.
- You are in the antenna far-field region.
- Multipath and obstruction effects are small.
Friis can be overly optimistic when
- Indoor walls, foliage, rain, or urban clutter are present.
- Interference and noise are high.
- Cable/connector losses are underestimated.
- Antenna patterns or orientation are poorly controlled.
Practical RF planning tips
- Always include a fade margin (often 10-30 dB depending on reliability target).
- Use measured cable losses and connector counts, not guesses.
- Validate with field tests or propagation tools for non-ideal environments.
- Check regulatory EIRP limits before increasing power or gain.
FAQ
Is this calculator only for Wi-Fi?
No. It works for any RF system where free-space assumptions are acceptable: microwave links, telemetry, satellite downlinks (with additional link factors), and many lab setups.
Why does higher frequency usually reduce range?
For the same antenna gains and distance, free-space path loss increases with frequency. That means less received power unless compensated by higher gain antennas or power adjustments.
Can I use it for near-field coupling?
No. Friis is a far-field relation. Near-field behavior follows different electromagnetic coupling principles.