Fresnel Zone Calculator

The Fresnel zone is a 3D elliptical region between a transmitter and receiver antenna in a wireless link. It defines the volume of space through which most of the signal energy travels. Obstructions within this zone — especially the first Fresnel zone — can cause signal diffraction and significantly degrade link performance.

Even if you have a clear line of sight between antennas, obstructions within the Fresnel zone can reflect, diffract, or absorb signal energy, causing multipath interference and signal loss. Ensuring sufficient Fresnel zone clearance is essential for reliable outdoor wireless links such as Wi-Fi bridges, microwave backhauls, and point-to-point radio systems.

The radius of the nth Fresnel zone at a point along the path is given by: r = sqrt(n × λ × d1 × d2 / D), where λ is the wavelength, d1 is the distance from the transmitter to the point, d2 is the distance from that point to the receiver, and D is the total link distance. For the first Fresnel zone (n=1), this gives the most critical clearance radius.

As a rule of thumb, at least 60% of the first Fresnel zone radius should be clear of all obstructions to avoid significant signal loss. Full clearance (100%) is ideal, but 60% is generally considered the minimum threshold for acceptable link performance.

The k-factor (or earth radius factor) accounts for atmospheric refraction of radio waves near the earth's surface. A standard value of 4/3 (≈1.333) is used for average atmospheric conditions. In super-refractive environments, a higher k-factor is used; in sub-refractive or ducting conditions, a lower value applies. This affects the effective earth bulge calculation.

The limit height of an obstruction is calculated as: H_limit = antenna_height_at_point − 0.6 × Fresnel_radius. If the actual obstruction height exceeds this value, the link will experience signal degradation. This calculator computes the maximum allowable obstruction height based on your antenna heights and Fresnel zone radius.

Yes, for longer wireless links (typically over 10 km), the curvature of the earth creates an effective bulge that can intrude into the Fresnel zone even when the terrain appears flat. The earth bulge at any point along the path is calculated as: h = d1 × d2 / (2 × k × Re), where Re is the earth's radius (~6371 km). This calculator includes earth bulge in the clearance analysis.

Common frequencies for point-to-point links range from 900 MHz (long range, penetrates obstacles better) to 5.8 GHz and 60 GHz (shorter range, higher bandwidth). Higher frequencies have smaller Fresnel zones, making clearance easier to achieve, but they are more susceptible to rain fade and atmospheric absorption over long distances.