A 5 km, 2.4 GHz point-to-point Wi-Fi bridge
With typical 15 dBi antennas and modest cable loss, this link closes with a healthy positive margin under clear line-of-sight conditions.
RF link budget calculator
Result
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How it works
Free-space path loss (FSPL) is how much a radio signal weakens simply from spreading out over distance in an obstruction-free vacuum-like path — it grows with both distance and frequency, which is why higher-frequency links (like microwave backhaul) need a much clearer, shorter path than lower-frequency ones (like FM radio) to cover the same distance.
EIRP is the power that would need to radiate from an idealised isotropic antenna to produce the same signal strength as the actual transmitter and its directional antenna — it is transmit power plus antenna gain, minus whatever the feeder cable and connectors waste as heat before the signal reaches the antenna.
The link margin is the safety buffer between the signal the receiver actually gets and the weakest signal it can still decode reliably (its sensitivity). A comfortable positive margin, typically 10–20 dB for outdoor links, absorbs rain fade, seasonal foliage, minor misalignment and equipment ageing without the link dropping.
Formulas used
Free-space path loss
FSPL(dB) = 20·log₁₀(d_km) + 20·log₁₀(f_MHz) + 32.44
- d_km
- Distance, in kilometres
- f_MHz
- Frequency, in megahertz
EIRP and received power
EIRP = Ptx + Gtx − Ltx; Prx = EIRP − FSPL − other losses + Grx − Lrx
Link margin
Margin = Prx − receiver sensitivity
Worked examples
The same link at 10 GHz instead of 2.4 GHz
Raising the frequency alone increases FSPL by 20·log₁₀(10000/2400) ≈ 12.4 dB, eating directly into the margin unless gain or power is raised to compensate.
Assumptions and limits
- The path is treated as free space — a real terrestrial link has additional losses from terrain, weather and obstructions, which belong in "other losses."
- Antenna gains are with respect to an isotropic radiator (dBi); a link stated in dBd (relative to a dipole) needs about 2.15 dB added to each gain first.
- All powers, gains and losses are added algebraically in decibels, which only holds because decibels are already a logarithmic (ratio) unit.
Frequently asked questions
Why does doubling the distance not simply double the path loss in dB?
FSPL depends on the logarithm of distance, so doubling the distance adds a fixed 20·log₁₀(2) ≈ 6 dB regardless of the starting distance — the loss in decibels grows additively, not proportionally, as distance increases.
What should I put in "other losses"?
Anything not already captured: rain and atmospheric attenuation, foliage or partial obstructions, polarisation mismatch, or a deliberate fade-margin allowance for reliability. For a genuinely clear line-of-sight microwave link this can be small; for anything less ideal, be generous.
Updated