How do you calculate an RF link budget?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

A link budget computes received power, P_r = P_t + G_t − L_t − L_path − L_misc + G_r − L_r (dB), and compares it with receiver sensitivity S = −174 + NF + 10·log10(B) + SNR_req. The difference is the link margin, which must cover fading, interference and uncertainty.^[1]^[2]

What goes into a link budget?

Transmitter output power and feed loss.
Antenna gains (realized, in the link direction) and polarization loss.
Path loss: free space or a model; atmospheric, rain, foliage and penetration loss.
Receiver noise figure, bandwidth and required SNR or E_b/N₀.
Fade margin for multipath (often 10–20 dB or more for Rayleigh fading at high availability).
Implementation losses.

What is a worked example?

Item	Value
Tx power	+14 dBm
Tx antenna gain	+2 dBi
Path loss (868 MHz, 2 km, free space)	−97.2 dB
Rx antenna gain	+2 dBi
Received power	−79.2 dBm
Noise floor (125 kHz, NF 6 dB)	−174 + 51 + 6 = −117 dBm
Required SNR (example)	+5 dB → sensitivity −112 dBm
Margin	32.8 dB

Spread-spectrum modulations can work at negative SNR, improving sensitivity.

How are satellite link budgets different?

They use EIRP and G/T: C/N0 = EIRP − L_path − L_atm + G/T − 10·log10(k), with 10·log10(k) = −228.6 dBW/K/Hz. Rain fade at Ku and Ka band dominates margin; see ITU-R P.838.^[3]

Frequently asked questions

What link margin is enough?

Depends on fading statistics and availability; 10–20 dB is common for terrestrial links.

Is sensitivity a fixed number?

It depends on data rate, modulation, coding and target error rate.

Should I use peak or realized antenna gain?

Realized gain in the actual link direction.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Wireless Communications: Principles and Practice, 2nd ed.”. T. S. Rappaport, Prentice Hall. 2002.
“Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
“Recommendation ITU-R P.838: Specific attenuation model for rain for use in prediction methods”. ITU-R. current version.

Categories: Antennas and propagation

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