Fresnel Zone Calculator
Radius of the first Fresnel zone at any point along a radio path, and the 60% clearance normally required.
Results
What this tool does
Radio waves do not travel along a line, they travel through a cigar-shaped volume between the two antennas, and anything intruding into that volume steals signal even when the antennas are in plain sight of each other. The zone is fattest halfway along and pinches to nothing at each end, which is why a tree standing at the midpoint of a link matters far more than one standing beside the mast.
Formula
r = 17,31 √(d1 d2 / (f (d1 + d2)))
Variables
| Symbol | Meaning | Unit |
|---|---|---|
d1 | Distance to the obstacle | km |
d2 | Distance from the obstacle to the far end | km |
fr | Frequency | GHz |
RD | First Fresnel zone radius | m |
R6 | 60% clearance needed | m |
DT | Total path length | km |
RM | Radius at the midpoint | m |
Worked example
- Distance to the obstacle2 km
- Distance from the obstacle to the far end3 km
- Frequency2.4 GHz
- First Fresnel zone radius12.2400 m
- 60% clearance needed7.3440 m
- Total path length5.000 km
- Radius at the midpoint12.4924 m
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
Frequently asked questions
Why 60% and not 100%?
Keeping 60% of the first Fresnel zone clear gives essentially the same signal as a completely unobstructed path — the outer part of the zone contributes little, and some of it even interferes destructively. Below 60% the loss climbs quickly, and an obstacle on the direct line of sight itself costs about 6 dB even though you can still see the far antenna.