Here are the steps you should follow to calculate the limit height of the obstruction:
First, you have to get the radius of the first Fresnel zone at the point to be checked: r.
You also need to know the antenna's height: H.
Subtract 60% of the Fresnel zone radius from the antenna's height: H - 0.6 * r.
The result is the maximum obstruction height you can get at the selected point of the 1st Fresnel zone.
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