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.

The margin is the difference between what arrived and the sensitivity of the receiver

Transmitter power	WdBm
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:

1. A margin below 10 dB is a fair-weather link. 20 dB is the working benchmark for a permanent one.
2. Antenna gain is usually cheaper than power: +6 dBi at both ends buys the same 12 dB as a transmitter four times the size.
3. Cable losses at 5.8 GHz are enormous: a short run to the antenna is worth more than an expensive transmitter.
4. Check the Fresnel zone separately — the budget does not include it.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator

UR3PKI. «Link Budget Calculator: EIRP, Path Loss, Fade Margin». 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.