How to use it
- Enter the frequency in GHz (2.4 or 5.8 for Wi-Fi bridges, for example).
- Enter the link length in km and the distance from antenna A to the point you want to check.
- Enter the height of the obstacle there above the ground level of the antennas’ bases, if any.
- Keep the first zone, 60 % clearance and k = 1.3333, or change them.
Formulas
- Wavelength λ = c ÷ f, with c = 299,792,458 m/s
- Zone radius rₙ = √(n × λ × d₁ × d₂ ÷ (d₁ + d₂))
- Earth bulge h = d₁ × d₂ ÷ (2 × k × R), with R = 6371 km
- Antenna height (both ends equal) = obstacle + bulge + clearance × r
All distances in metres inside the formulas. The antenna height assumes flat ground at the same level at both ends and equal masts.
Worked example
A 10 km link at 5.8 GHz, checked at its midpoint (5 km from each end):
- λ = 299,792,458 ÷ 5.8 × 10⁹ = 5.1688 cm
- r₁ = √(0.051688 × 5000 × 5000 ÷ 10,000) = 11.37 m
- 60 % of it is 6.82 m
- Earth bulge with k = 1.3333: 5000 × 5000 ÷ (2 × 1.3333 × 6,371,000) = 1.47 m
- with no obstacle, both antennas need 8.29 m above the ground
Tips
- The midpoint is the worst place for the zone (it is widest there) and for the bulge.
- Trees grow. Leave extra height above vegetation for the years the link should work.
- Higher than the formula is better. Terrain and buildings rarely match a flat-earth profile; check the real profile along the path.