WISP Wireless Link Budget
Estimate free-space path loss, received signal, fade margin, and the radius of the first Fresnel zone before you install a point-to-point or backhaul link.
1. Link inputs
Use the actual conducted power after radio output limits.
Use the modulation's published sensitivity, not a generic number.
Compare this with the recommended 60% first-Fresnel clearance.
How the calculation works
The free-space path loss is estimated with FSPL = 92.45 + 20 log10(distance km) + 20 log10(frequency GHz). The received level is then Tx power + Tx gain + Rx gain − cable losses − FSPL. Fade margin is the received level minus the required sensitivity. A positive margin is necessary but does not guarantee stable service.
Fresnel clearance
The first Fresnel zone is an ellipsoid around the direct radio path. At the midpoint, its approximate radius in metres is 8.66 × √(distance km / frequency GHz). A common planning target is to keep at least 60% of that radius clear of terrain, trees, buildings, and the tower structure. The midpoint is not always the worst obstruction; inspect the full path profile.
Reading the result
| Result | Meaning | Practical action |
|---|---|---|
| Received level | Estimated signal at the receive antenna input. | Compare it with the radio's sensitivity at the intended modulation. |
| Fade margin | Distance between estimated signal and required sensitivity. | More is better; choose a target based on rain, interference, and availability requirements. |
| 60% Fresnel clearance | Recommended clear radius around the direct path. | Use a terrain profile and confirm trees will not grow into the path. |
What this tool cannot know
It cannot see channel occupancy, adjacent-channel interference, multipath, antenna misalignment, polarization errors, rain attenuation, regulatory EIRP limits, or the exact radio chain. Treat a high number as permission to investigate further, not as a promise of throughput.