Link Margin Calculator
Work out the received signal power and link margin for a radio link from its budget of transmit power, antenna gains, path loss and receiver sensitivity. The margin tells you how much fade headroom the link has before it drops out.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the transmit power in dBm and the total path loss in dB (from a free-space or terrain model).
- Add the Tx and Rx antenna gains in dBi (leave at 0 for isotropic) and any other losses such as feeder, connector and mismatch losses in dB.
- Enter the receiver sensitivity in dBm (usually negative), then read off the received power, the link margin and the pass/marginal/fail verdict.
How it works
Everything is done in decibels, so gains and losses simply add and subtract: received power Rx (dBm) = Tx power + Tx gain − path loss + Rx gain − other losses. The link margin is the received power minus the receiver sensitivity, margin (dB) = Rx − sensitivity. A margin of roughly 10 dB or more is a healthy design target, 0–10 dB is marginal, and below 0 dB the received signal is under sensitivity and the link cannot close.
Worked example
Worked example. With Tx = 30 dBm, Tx gain = 10 dBi, path loss = 120 dB, Rx gain = 10 dBi and 3 dB of other losses: Rx = 30 + 10 − 120 + 10 − 3 = −73 dBm. Against a −90 dBm sensitivity the margin is −73 − (−90) = 17 dB, a comfortable, good link.
Common mistakes
- Sign errors: sensitivity and dBm powers are usually negative, so subtracting a −90 dBm sensitivity ADDS 90 dB to the margin.
- Forgetting feeder, connector and mismatch losses, which can add several dB on each side and eat into the margin.
- Treating a positive but small margin as safe — 0–10 dB leaves no headroom for rain, fading or foliage.
Frequently asked questions
What link margin should I aim for?
About 10 dB or more is a common target for a reliable fixed link; higher (20 dB+) is used where rain fade or heavy multipath is expected. Below 10 dB the link works only in good conditions.
Where does the path loss come from?
From a propagation model — free-space path loss for line-of-sight, or a terrain/clutter model such as two-ray or Longley-Rice for real paths. Use a separate path-loss or free-space calculator to get that figure first.
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Tip: Enter any known values to calculate the remaining results.
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