Link Budget Calculator: Free-Space Path Loss, Received Power and Maximum Range
Add up transmit power, antenna gain, cable loss and free-space path loss to get the received level and the margin left after fade. The same numbers give you the maximum range at which the link still closes — useful when someone asks for “a 20 km 5.8 GHz link” and nobody has checked the arithmetic.
Radio and antenna inputs
Results recalculate as you type.
Typical values to start from
Replace these with your own radio datasheet figures.
| Link type | Sensitivity | Fade margin |
|---|---|---|
| LoRa / sub-GHz telemetry | -137 dBm | 10–20 dB |
| Wi-Fi 2.4 GHz, 20 MHz | -85 dBm | 10–20 dB |
| Wi-Fi 5 GHz, 40 MHz | -78 dBm | 10–20 dB |
| LTE / 5G CPE | -100 dBm | 15–20 dB |
| Licensed point-to-point | -85 dBm | 20–30 dB |
Sensitivity figures are receiver-dependent and are quoted here only as a starting point, not as a specification of any product.
| EIRP at the transmit antenna | 52.5 dBm |
| Free-space path loss | 121.7 dB |
| Received power | -46.7 dBm |
| Total gain available (TX + RX) | +48.0 dB |
| Link margin to sensitivity | 23.3 dB |
| Margin after required fade allowance | 13.3 dB |
| Maximum range at that margin | 23.2 km |
| 6 dB rule: each doubling of distance costs | 6.0 dB |
Free-space loss assumes a clear first Fresnel zone and no rain. Check the path geometry with the Fresnel zone calculator before treating these numbers as final.
The arithmetic
EIRP = PT + GT − LT
PRX = EIRP − FSPL + GR − LR − Lextra
margin = PRX − SRX − FM
dmax = 10^[ ( EIRP + GR − LR − Lextra − SRX − FM − 32.44 − 20·log10f ) / 20 ]
The 32.44 constant is the free-space loss at 1 km and 1 MHz for isotropic antennas. Everything else is bookkeeping in decibels: gains add, losses subtract.
How to read the result
The received power on its own tells you very little. What matters is the distance between that number and the level at which the receiver stops working — and then a further allowance for everything that will change after installation: rain, foliage growth, a wet radome, multipath, and the fact that the antenna pattern is never exactly the datasheet curve.
| Margin after fade allowance | What it means in practice |
|---|---|
| More than 20 dB | Very comfortable. You can trade gain for a wider beam, a smaller antenna, or lower transmit power. |
| 10–20 dB | Healthy for a fixed link with a stable path profile. |
| 3–10 dB | It will work on a clear day and show intermittent drops in rain or heat. Improve alignment, gain or cable first. |
| Below 3 dB | Not a design, a hope. Reduce distance, raise antenna height, or use a higher-gain format. |
| Negative | The link does not close at all, regardless of alignment. |
The 6 dB rule, and why it matters when sizing antennas
Free-space loss rises 6 dB every time the distance doubles, and 6 dB every time the frequency doubles. That single line explains most link surprises: a 2.4 GHz design moved to 5.8 GHz loses about 7.6 dB for the same distance, which is roughly the gain of a good panel antenna thrown away. It also explains why 3 dB of extra gain in total buys only 41% more range, and why doubling the range needs 6 dB — the step from a 24 dBi panel to a much larger dish.
+3 dB → ×1.41 | +6 dB → ×2.0 | +10 dB → ×3.16
Beamwidth is not in the equation, but it decides the outcome
A link budget assumes the two antennas point at each other. On a 20 km path, a 3 m mast sway of 0.5° moves the beam by 175 m at the far end — trivial for a 10° panel and fatal for a 2° dish. If the budget says the link closes but the field results disagree, the usual cause is alignment, not arithmetic: wind load on the mount, a pole that was never plumbed, or a pattern that is narrower than the datasheet suggests.
Where cable loss quietly eats the margin
Every metre of RG-58 at 5.8 GHz costs roughly 1 dB. A 10 m run therefore gives away about 10 dB — more than doubling the antenna gain would recover. Moving the radio to the mast and running Ethernet or fibre instead of coax is often the cheapest 6 to 10 dB available on an existing link.
Frequently asked questions
What fade margin should I plan for?
Why does doubling the distance cost 6 dB?
Should I enter dBi or dBd for antenna gain?
Does antenna beamwidth change the link budget?
How accurate is a free-space calculation in the real world?
Related tools and pages
Need gain that actually matches the budget?
Send us the band, the distance and the mounting point. We will tell you which format closes the link with margin to spare — and which one only looks good in a datasheet.
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