Guide
Adding up a radio link budget to find the fade margin
Before installing a wireless bridge or a long-range access point, it is worth checking on paper whether the signal will arrive strongly enough. A link budget does this by adding every gain and subtracting every loss between the transmitter and the receiver. The result, the margin above the receiver’s sensitivity, tells you how much unexpected loss the link can survive.
The inputs
Transmit power in dBm comes from the radio’s settings. Antenna gains in dBi are on the antenna data sheet. Cable and connector losses are small for short pigtails and noticeable for long runs of thin coax. “Other losses” collects things you want to allow for: obstructions, rain, polarisation mismatch. Frequency in MHz and distance in kilometres give the free-space loss. Receiver sensitivity is the weakest signal the radio decodes at the data rate you need; it is listed per rate in the radio data sheet, and faster rates need stronger signals.
The sum
Transmit power plus transmit antenna gain minus the transmit cable loss gives the EIRP, the power leaving the antenna. Subtract the path loss, add the receiving antenna gain and subtract the receive cable loss and the other losses. The result is the signal at the radio input. The difference from sensitivity is the fade margin. The page also solves the other way: with the same gains and losses, it calculates the range at which the margin falls to zero.
Using the margin
Radio paths are not static. Rain and humidity, moving vehicles, tree growth, interference and multipath all subtract from the signal over time. A margin of 10 to 20 dB is a common target for outdoor links, with more for long hops or high-availability needs. The page rates the margin, as in “Adequate fade margin” for the sample. A link with a margin of 3 dB works on a calm day and fails when it rains.
A worked example
The first preset is a 5.8 GHz link over 5 km. The page reports EIRP 34.0 dBm (2.51 W), free-space path loss 121.7 dB (20·log10(5) + 20·log10(5800) + 32.44), a received signal of −73.7 dBm and a fade margin of 11.3 dB, rated adequate. At zero margin the same equipment would reach about 18.4 km. The margin depends on the sensitivity chosen: the preset works out at a receiver sensitivity of −85 dBm, and a faster data rate needs a higher (less negative) sensitivity, which leaves less margin.
Limits
The path loss is the free-space value, so the link also needs a clear line of sight and, for long links, a clear first Fresnel zone, which the page does not calculate. Rain attenuation, atmospheric absorption and ground reflections can be included in “other losses” only if you estimate them yourself. Regulatory limits on EIRP depend on country and band and are not checked here. For the free-space loss alone, see the Free-Space Path Loss Calculator.
Related tools
- Free-Space Path Loss Calculator isolates the distance and frequency part of the sum.
- Wi-Fi Signal Calculator converts the received level into quality and SNR.
- Fibre Loss Budget Calculator applies the same method to optical links.
- PoE Power Calculator checks that the remote radio can be powered over its cable.
Checked against the current version of the Link Budget Calculator on .