Fresnel Zones

In radio communications, line of sight (LoS) is not enough. The radio waves spread out in an elliptical region called the Fresnel Zone. Any obstacles within this zone can cause diffraction and signal loss, even if they don't block the visual line of sight.

The First Fresnel Zone (F1)

The most critical region is the First Fresnel Zone (F1). This contains the primary energy of the wave. The radius of the nth Fresnel zone at a point $d_1$ from the transmitter and $d_2$ from the receiver is given by:

Where:

• $n$ is the zone number (1, 2, 3...)
• $\lambda$ is the wavelength ($\lambda = c/f$)
• $d_1, d_2$ are distances to Tx and Rx

The 60% Rule

For a reliable RF link, it is widely accepted that you must keep 60% of the First Fresnel Zone clear of obstacles.

• If clearance > 60% F1: Propagation is roughly equivalent to Free Space.
• If clearance < 60% F1: Diffraction loss begins to occur significantly.

Interactive Visualizer

Drag the obstacle below to see how clearance affects the link status.