Antennas do not broadcast their signals linearly, but within an angle that depends on the model in question. The spherical expansion of the signal waves produces amplification or interference of the effective power output at certain distances along the connection between the transmitter and receiver. The areas where the waves amplify or cancel themselves out are known as Fresnel zones.
The Fresnel zone 1 must remain free from obstruction in order to ensure that the maximum level of output from the transmitting antenna reaches the receiving antenna. Any obstructing element protruding into this zone will significantly impair the effective signal power. The object not only screens off a portion of the Fresnel zone, but the resulting reflections also lead to a significant reduction in signal reception.
The radius (R) of Fresnel zone 1 is calculated with the following formula assuming that the signal wavelength (λ) and the distance between transmitter and receiver (d) are known.
R = 0,5 * √ (λ * d)
The wavelength in the 2.4 GHz band is approx. 0.125 m, in the 5 GHz band approx. 0.05 m.
Example: With a separating distance of 4 km between the two antennae, the radius of Fresnel zone 1 in the 2.4-GHz band is 11 m, in the 5-GHz band 7 m.
To ensure that the Fresnel zone 1 remains unobstructed, the height of the antennas must exceed that of the highest obstruction by this radius. The full height of the antenna pole (M) should be as depicted:
M = R + 1m + H + E (earth's curvature)
The allowance for the curvature of the earth (E) can be calculated at a distance (d) as E = d² * 0.0147 – i.e. at a distance of 8 km this is almost 1 m.
Example: With a distance of 8 km between the antennae, the result in the 2.4-GHz band is a pole height above the level of the highest obstruction of approx. 13 m, in the 5-GHz band 9 m.
