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First Fresnel zone
First Fresnel-zone radius at a point along a path.
HB3YAQ · Ajdin HadzicLearning to take awayhttps://hb3yaq.ch/en/tools/fresnel
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Formula & context
r₁ = √[λ · d₁d₂ / (d₁ + d₂)]
The zone describes space around the direct path where obstacles affect the phase of combining waves. The radius is greatest at the path midpoint.
A worked example
At 435 MHz, 5 km from each station, r₁ is about 41.51 m; 60% is 24.91 m.
Assumptions & limits
60% of the radius is a common planning allowance, not a guaranteed clearance threshold. Check curvature, terrain and vegetation separately.
Fresnel zones: more than line of sightUnderstand the reasoning →From reading to experimenting
First Fresnel zone
At 435 MHz, 5 km from each station, r₁ is about 41.51 m; 60% is 24.91 m.
Open example with starting values →Frequency: 435 MHz · Distance to station A: 5 km · Distance to station B: 5 kmBack to the learning article →Then change exactly one value. Compare the result and use the formula to explain why it changes. The model limits below the calculator also apply here.