Radio link budget calculator
What “free-space loss” really is
It is not absorption in the air — in a vacuum it is exactly the same. The energy spreads over a sphere whose area grows with the square of distance, and the receiving antenna intercepts only its share. So the loss depends on both distance and frequency: at a higher frequency the same antenna has a smaller effective area.
| Transmitter power | |
| Transmit antenna gain (dBi) | |
| Transmit path loss (dB) | |
| Receive antenna gain (dBi) | |
| Receive path loss (dB) | |
| Frequency (MHz) | |
| Distance (km) | |
| Extra loss: rain, foliage, obstacles (dB) | |
| Receiver sensitivity (dBm) | |
| Result | |
| Transmitter power | |
| EIRP | |
| Free-space loss | |
| Level at the receiver input | |
| Margin above sensitivity | |
📐 The formulas
Free-space path loss:
FSPL[dB] = 32.45 + 20·log10(f[MHz]) + 20·log10(d[km])
= 92.45 + 20·log10(f[GHz]) + 20·log10(d[km])
The budget:
EIRP = P_tx + G_tx − L_tx
P_rx = EIRP − FSPL − L_extra + G_rx − L_rx
Margin = P_rx − sensitivity
Check points:
| Frequency | Distance | FSPL |
|---|---|---|
| 145 MHz | 10 km | 95.68 dB |
| 2450 MHz | 5 km | 114.21 dB |
| 10 GHz | 1 km | 112.45 dB |
Where this calculator stops:
- The calculation assumes free space: a clear path with no obstacles and no ground reflection
- For urban areas use the Hata model, and for low antennas the two-ray model: both give considerably higher loss
- Extra losses (rain, foliage, walls) are entered by hand — there is no universal formula for them
- The Fresnel zone is not accounted for: even in line of sight, an obstacle beside the path eats several decibels
🎯 In practice
- A Wi-Fi link — is there enough power for this distance.
- Receiving a satellite: compare the calculated level with your sensitivity.
- Sizing up a VHF path to a repeater.
Practical advice:
- A margin below 10 dB is a fair-weather link. 20 dB is the working benchmark for a permanent one.
- Antenna gain is usually cheaper than power: +6 dBi at both ends buys the same 12 dB as a transmitter four times the size.
- Cable losses at 5.8 GHz are enormous: a short run to the antenna is worth more than an expensive transmitter.
- Check the Fresnel zone separately — the budget does not include it.
© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.
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UR3PKI. «Radio link budget calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/link_budget (licence CC BY-NC-SA 4.0).The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.