Friis Transmission Equation Calculator
Estimate received power in free space using RF link budget inputs. Enter values in dB-based units where indicated.
What the Friis Transmission Equation Does
The Friis transmission equation predicts how much RF power is received by an antenna when a signal propagates through unobstructed free space. It is one of the foundational equations in wireless engineering, satellite links, radar basics, and line-of-sight system design.
This calculator gives you the most common practical outputs: free-space path loss (FSPL), wavelength, received power, and optional link margin if you provide a receiver sensitivity target.
Core Formula (Linear and dB Forms)
Linear Form
Where Pr is received power, Pt is transmit power, Gt/Gr are antenna gains (linear), λ is wavelength, R is distance, and L is system loss.
dB Link Budget Form
The dB version is easier for real-world engineering because gains and losses combine through simple addition/subtraction.
How to Use This Friis Calculator
- Pt: Enter transmitter output power in dBm.
- Gt / Gr: Enter antenna gains in dBi.
- Frequency: Use MHz (for example 915, 2400, 5800).
- Distance: Enter path length in kilometers.
- Losses: Include cable, connector, mismatch, polarization, or implementation losses in dB.
- Sensitivity (optional): Add receiver threshold to get link margin and maximum free-space distance estimate.
Example
Suppose you have a 2.4 GHz point-to-point link with:
- Transmit power: 20 dBm
- Transmit gain: 2 dBi
- Receive gain: 2 dBi
- Distance: 1 km
- Extra loss: 0 dB
The calculator will show FSPL around 100 dB and received power around -76 dBm. If your receiver sensitivity is -90 dBm, your link margin is about 14 dB, which is generally usable in good conditions.
Important Assumptions and Limitations
When Friis is valid
- Clear line-of-sight path
- Far-field antenna region
- No major reflections or diffraction dominating the channel
What Friis does not include
- Fading due to multipath
- Rain, foliage, and wall penetration losses
- Interference and noise floor variation
- Regulatory limits (EIRP constraints)
Practical RF Design Tips
- Keep at least 10 to 20 dB of link margin for reliability, depending on environment.
- Use directional antennas to increase gain and reduce interference pickup.
- At higher frequencies, path loss increases quickly, so distance and alignment matter more.
- Include real cable/connector losses in your budget; they are easy to underestimate.
- Validate calculations with field measurements and packet error performance tests.
Quick FAQ
Is higher frequency always better?
Not always. Higher frequency can support wider bandwidth, but FSPL rises with frequency, reducing range for the same power and antenna gains.
What is a good received power target?
It depends on modulation and receiver design. Compare your calculated received power against the device sensitivity for your intended data rate.
Can I use this for Wi-Fi, telemetry, and LoRa planning?
Yes, as a first-order free-space estimate. Then add environment-specific losses and fading margins for realistic planning.