RF Link Budget & Fade Margin Calculator

How it works

EIRP = P_tx − L_tx + G_tx; P_rx = EIRP − FSPL − L_misc + G_rx − L_rx; fade margin = P_rx − sensitivity; FSPL = 20 log d(km) + 20 log f(MHz) + 32.45

A link budget adds up every gain and subtracts every loss between the transmitter’s output connector and the receiver’s input, then compares the result with the weakest signal the receiver can decode at the wanted data rate. The difference — fade margin — is the reserve against rain, multipath, foliage growth, misalignment and interference; 20 dB is the usual design target for carrier-grade availability, 10–15 dB for less critical links. Every item is in decibels so the budget is plain addition, and because free-space loss grows 6 dB per doubling of distance, the same margin also translates into a maximum range. EIRP, the power radiated in the antenna’s favoured direction, is what regulators limit, so the budget also shows whether the design is legal; the cheapest fixes are usually a bigger dish or shorter feeder, not a more powerful radio.

Worked example

A 5.8 GHz bridge: 27 dBm − 2 dB cable + 24 dBi dish = 49 dBm EIRP. Over 15 km the path loses 131.2 dB in free space plus 4 dB of rain and misalignment; the far dish adds 24 dBi and its cable takes 2 dB, so −64.2 dBm arrives — 15.8 dB above a −80 dBm sensitivity (≈ 99.9% availability). For a 20 dB margin the path could be at most 9.2 km, or one modulation step slower.

Frequently asked questions