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Wireless Link Clearance Calculator

Check the clearance a point-to-point wireless link needs over trees and roofs, and its path loss.

Wireless Link Clearance Calculator

Check the clearance a point-to-point wireless link needs over trees and roofs, and its path loss.

Link distance (km)

Frequency

Fresnel zone to keep clear (%)

60% of the first Fresnel zone is the usual minimum.

Transmit power (dBm)

Antenna gain at each end (dBi)

Results

First Fresnel zone radius at the midpoint: 5.09 m

Clearance for 60% of it: 3.05 m

Earth curvature at the midpoint: 0.06 m

Height above obstacles at the midpoint: 3.11 m

Free-space path loss: 113.7 dB

Signal at the receiver (before cable loss): -47.7 dBm

Earth curvature uses the standard 4/3 earth radius for radio. Keep trees, roofs and terrain below the clearance line along the whole path, not just at the midpoint. Check the received signal against the radio's sensitivity and leave a fade margin.

Building a bigger job?Add these figures to a project, then add results from any of the other calculators. The project becomes one report you can save as a PDF and import again later to make changes.

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How it works

The first Fresnel zone radius at the middle of the link is 17.32 x the square root of the distance in km divided by 4 x the frequency in GHz, in metres, and at least 60% of it should be clear. The earth bulges by the distance squared divided by 8 x 4/3 x 6,371 km at the midpoint. Free-space path loss is 20 log(distance in km) + 20 log(frequency in GHz) + 92.45 dB.

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