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How to Calculate an RF Link Budget
Heshan Fernando
Co-founder & COO
Planning a 4 km wireless link between two buildings. The radios are 20 dBm, the antennas are 16 dBi each, and the receiver sensitivity is -85 dBm. Will it work?
A link budget answers that in about two lines of arithmetic, because everything in dB adds and subtracts. The harder question is how much margin you need — and that’s where clear-path calculations start lying to you.
The arithmetic
Everything is decibels, so gains add and losses subtract.
Free-space path loss is the dominant term:
FSPL(dB) = 20·log₁₀(d_km) + 20·log₁₀(f_MHz) + 32.44
At 4 km and 5,800 MHz: 12.04 + 75.27 + 32.44 = 119.8 dB.
Then the budget:
RX = TX power + TX antenna gain + RX antenna gain − path loss − cable losses
20 + 16 + 16 − 119.8 = -67.8 dBm
Against a -85 dBm sensitivity, that’s 17 dB of margin. Workable.
One counter-intuitive detail: path loss rises with frequency, but not because air absorbs more. It’s geometry — the effective aperture of an isotropic receiver shrinks with wavelength. Higher-gain antennas recover it, which is why high-frequency links use dishes.
What margin you actually need
Ten decibels is a common minimum for a fixed outdoor link. Twenty is comfortable. The reasons are all things the free-space formula ignores:
Rain fade, which is negligible below about 6 GHz and increasingly serious above it.
Foliage, which grows. A link with clear sight lines in March can be marginal by July, and worse in five years.
Fresnel zone obstruction. Radio doesn’t travel in a pencil-thin line — it needs clearance in an ellipsoid around the direct path. An obstacle that doesn’t block line of sight can still cost several dB by intruding into the Fresnel zone, and this is the single most common reason a link that “should work” doesn’t.
Alignment drift, from wind, mast flex and thermal movement.
| Term | Typical Value | Notes |
|---|---|---|
| TX power | 20 dBm | Often regulated by band |
| Antenna gain | 10–24 dBi | Higher gain = narrower beam |
| Cable loss | 0.5–3 dB | Worse at higher frequency |
| Margin needed | 10–20 dB | For rain, foliage, drift |
Why people get stuck here
- Free-space treated as complete. It assumes clear line of sight and a clear Fresnel zone.
- Fresnel zone forgotten. Visual line of sight is not the same as radio clearance.
- Cable losses omitted. At 5.8 GHz, a few metres of the wrong coax can cost several dB.
- High-gain antennas as a free fix. More gain means a narrower beam, which makes alignment critical and drift more punishing.
Common mistakes to avoid
- Designing to zero margin because the maths says it reaches.
- Ignoring vegetation that will grow into the path.
- Using long coax runs at high frequency rather than mounting the radio at the antenna.
- Assuming both ends can use maximum power — regulatory limits vary by band and region, often as EIRP rather than transmit power.
- Aligning by eye and never checking received signal at both ends.
How to do it with RF Link Budget Calculator
The RF Link Budget Calculator computes path loss and reports the margin.
- Enter frequency and distance to get free-space path loss.
- Add transmit power, both antenna gains, and cable and connector losses.
- Compare the received level against receiver sensitivity.
- Check the margin — under about 10 dB and the link won’t survive rain, foliage or misalignment.
- Verify Fresnel clearance separately; the budget assumes it.
Other RF and network calculators are in the tools directory.
Frequently asked questions
How much margin is enough?
Ten decibels is a common minimum for a fixed outdoor link, twenty is comfortable. Rain, foliage growth and thermal drift all erode margin over a link’s life.
Does free-space loss cover everything?
No. It assumes clear line of sight with a clear Fresnel zone. Obstructions, ground reflections and vegetation add losses it doesn’t predict.
Why does higher frequency mean more loss?
It’s a geometry effect — the effective aperture of an isotropic receiver shrinks with wavelength. Higher-gain antennas recover it, which is why high-frequency links use dishes.
Final thought
The budget tells you whether the link closes on paper. Fresnel clearance and future foliage tell you whether it still closes in three years — and that’s the calculation people skip.