>[!abstract] >The Fresnel zone is a concept from wave propagation theory that describes a series of elliptical regions between a transmitter and a receiver where radio waves can travel. Even though the direct radio line-of-sight (RLOS) path carries most of the signal energy, waves can also reach the receiver by slightly longer paths due to diffraction or reflection. These longer paths fall within the Fresnel zones. > >The Fresnel zone is relevant to UAV operations for command and control (C2) and for data links (e.g., video, telemetry). For reliable communications, at least 60% of the first Fresnel zone should be free of obstructions. Higher frequencies (e.g., 2.4 GHz) have smaller Fresnel zones than lower frequencies (e.g., 900 MHz), making them more vulnerable to obstructions. ## Radius The radius of the first Fresnel zone at any point between the transmitter and receiver is calculated as: $r = \sqrt{\frac{n \lambda d_1 d_2}{d_1 + d_2}}$ where: - $r$ is the radius of the $n^{th}$ Fresnel zone (usually just the first zone is most critical). - $\lambda$ is the wavelength of the signal, which is $\lambda = c / f$ where $c$ is the speed of light ($\approx 3 \cdot 10^8$ m/s) and $f$ is the frequency (for example, $2.4 \cdot 10^9$ Hz). - $d1$ and $d2$ are the distances from the point to the transmitter and receiver. - $n$ is the Fresnel zone number ($1$ for the first). For example, assuming a radio frequency of 2.4 GHz and a transmitter and receiver located 1 km apart, the radius of the Fresnel zone at mid-point will be: $r = \sqrt{\frac{1 \cdot (3 \cdot 10^8 / 2.4 \cdot 10^9) \cdot 500 \cdot 500}{500 + 500}} = \sqrt{\frac{0.125 \times 250,000}{1,000}} = \sqrt{31.25} \approx 5.59 ~ m$ >[!related] >- **North** (upstream): [[Line of sight]] >- **West** (similar): — >- **East** (different): — >- **South** (downstream): —