RF Link Budget Calculator
Work out received signal power and link margin from transmit power, antenna gains, path loss, other losses, and receiver sensitivity.
Link budget
A link budget adds up every gain and loss between transmitter and receiver to check whether the signal arrives strongly enough to be decoded. The spare headroom above the receiver's sensitivity is the link margin.
P_rx = P_tx + G_tx + G_rx − path loss − other losses
Link margin = P_rx − sensitivity
A positive margin means the link should close; designers typically want 10 dB or more to cover fading and real-world obstacles. The maximum allowable path loss is the loss at which the margin reaches zero — a handy figure for estimating range.
How a link budget works
Everything in a link budget is done in decibels, which turns the messy multiplication of gains and losses into simple addition and subtraction. Start at the transmitter's output power, add the antenna gains (which focus energy in a direction), then subtract every loss along the way — the big one is path loss as the signal spreads over distance, plus cable, connector, and polarisation losses. Whatever power is left is what the receiver actually sees. Compare that to the receiver's sensitivity — the weakest signal it can still decode — and the difference is your margin, the safety buffer the link runs on.
Worked example
A LoRa node transmits at 14 dBm into a 2 dBi antenna, the gateway has a 6 dBi antenna, path loss is 120 dB, and cable and misc losses total 3 dB. Received power is 14 + 2 + 6 − 120 − 3 = −101 dBm. With a receiver sensitivity of −137 dBm, the link margin is −101 − (−137) = 36 dB — very comfortable, which is exactly why LoRa reaches kilometres. Halve the margin requirement and you can trade it for range, a smaller antenna, or lower transmit power to save battery.
What eats your margin
Real links lose margin to obstacles and walls, multipath fading, rain (at higher frequencies), antenna misalignment, and the Fresnel-zone clearance a ground-level link rarely gets. That's why the 10 dB-plus cushion matters — a link with zero calculated margin will not survive contact with the real world.
Related
Estimate the path loss first with thefree-space path loss calculator, or convert power units with thedBm to watts converter.
Common questions
- What link margin should I design for?
- As a rule of thumb, at least 10 dB for a fixed outdoor link with clear line of sight, and 20 dB or more where there is movement, weather, or anything obstructing the path. A margin under about 6 dB will work on the bench and fail in the field, because rain, a passing vehicle or a slightly rotated antenna can each cost several dB on their own.
- Which numbers go in as positive and which as negative?
- Transmit power and antenna gains add; path loss and cable or connector losses subtract. So Prx = Ptx + Gtx − Lpath − Lcable + Grx, all in dB or dBm. The result is the power arriving at the receiver, and the margin is how far that sits above the receiver's stated sensitivity.
- Where do I find my receiver's sensitivity figure?
- On the radio module's datasheet, usually quoted per data rate — and that qualification matters. Sensitivity improves markedly at lower rates, which is exactly why LoRa can reach so far: dropping the data rate buys back sensitivity, and therefore range. A module might be −95 dBm at high rate and −137 dBm at its slowest, a 42 dB difference worth more than any antenna change.
- Does more transmit power always fix a marginal link?
- It helps, but it is usually the wrong lever and it is legally capped. Doubling power buys only 3 dB, whereas a better antenna can buy 6–10 dB on both ends of the link, and lowering the data rate can buy far more than that. Regional rules also limit radiated power — see the ISM band table — so raising it may not even be lawful where you are deploying.