RF Field Guide · reviewed reference
Fresnel zones
Fresnel zones describe the volume around the direct path where reflected or diffracted energy can materially change the received signal.
Question
How much space around a radio path needs to be clear?
Inputs
- Frequency
- Distance from each endpoint to the potential obstruction
- Path height above terrain, buildings, and vegetation
Answer
Working
First-zone radius r1 = sqrt(λ d1 d2 / (d1 + d2)), with λ, d1, and d2 in consistent units. The radius is greatest near the middle of a symmetric path and shrinks toward either endpoint.
Visual explanation
- Endpoint A
- d1
- Clearance volume
- d2
- Endpoint B
Interpretation
Use a terrain profile to locate clearance bottlenecks, then check whether antenna-height changes, a different path, or a different band materially improve the result.