How this calculator works
Fresnel zones describe path-length differences around a direct radio path. The paraxial radius depends on wavelength and the distances to both ends, reaching its maximum at the midpoint. The clearance estimate reserves 60% of the first-zone radius even when another zone number is selected. It assumes equal antenna heights above a flat common datum and omits Earth curvature, terrain profiles and atmospheric refraction.
Formula
rn = sqrt(n lambda d1 d2/(d1+d2)), lambda = c/f. rmax = sqrt(n lambda D/4). Required height = obstruction height + 0.6 r1.
Worked example
For wavelength 1 m and a 100 m link, the first-zone midpoint radius is 5 m. With antenna height 10 m and an obstruction 5 m high, the clearance margin is 10 - 5 - 0.6(5) = 2 m.
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Frequently asked questions
Does the radius equal antenna height?
No. It is measured from the direct line between the antennas, so terrain and line height must be considered separately.
Why use first-zone clearance for a higher zone?
The common 60% clearance guide refers to the first zone. Higher-zone radius is reported separately.
Can this model a long link over curved Earth?
No. Add curvature and atmospheric refraction using a path-profile model for long or uneven routes.