How a link budget works
Start with the transmitter power, add antenna gains, subtract the free-space path loss and any cable or connector losses. The result is the received signal. The difference between that and the receiver's sensitivity is the fade margin, which protects the link against rain, interference and alignment drift.
Example: 5 km link at 5.8 GHz
FSPL = 13.98 + 75.27 + 32.44 = 121.7 dB. Received = 20 + 23 + 23 − 121.7 − 2 = −57.7 dBm, a 22.3 dB fade margin against −80 dBm sensitivity. Keep at least 60% of the 8 m Fresnel radius clear of obstacles.
Fade margin guide
Under 10 dB: unreliable. 10–20 dB: works in good conditions. 20–30 dB: solid outdoor link. Over 30 dB: very robust, or you can reduce power.
Frequently asked questions
What is free-space path loss?
The loss of signal strength as a radio wave spreads out over distance in open space. It increases 6 dB every time the distance doubles.
Why does the Fresnel zone matter?
Obstacles inside the Fresnel zone reflect and diffract the signal even with visual line of sight. Keep at least 60% of it clear.
Does higher frequency always lose more?
Yes in free space, for the same antennas. But higher-frequency antennas of the same size have more gain, which partly offsets it.