edVirtus Reference

What Is Diffraction Loss?

Why Do Radio Signals Become Weaker When They Pass Around Obstacles?

Diffraction loss is the reduction in radio signal strength that occurs when electromagnetic waves encounter an obstacle and bend around its edges rather than travelling along a clear line-of-sight path. Although diffraction allows radio signals to reach locations that would otherwise be completely shadowed, the bending process is accompanied by a reduction in signal power. This reduction is known as diffraction loss.

The basic principle is straightforward. When a radio wave encounters an obstacle such as a hill, ridge, building, or mountain, part of the wavefront bends around the edge of the obstruction and continues to propagate into the region behind it. However, only a portion of the original energy follows this diffracted path, so the received signal is weaker than it would be if an unobstructed line-of-sight path existed. The amount of diffraction loss depends on the size and shape of the obstacle, the operating frequency, and the degree to which the line of sight is obstructed.

A useful analogy is hearing someone speaking from behind a doorway. Although the speaker cannot be seen directly, some of the sound bends around the edge of the doorway and reaches the listener. The voice is still audible, but it is weaker than if there were a direct line of sight. Radio waves behave in a similar manner when they diffract around obstacles.

Diffraction is particularly important in terrestrial radio systems operating in hilly or urban environments, where direct line-of-sight propagation is often blocked. Lower-frequency radio waves, with their longer wavelengths, generally diffract more effectively around obstacles than higher-frequency signals. Consequently, VHF and UHF signals often provide better coverage behind hills than microwave or millimetre-wave signals, which require a much clearer line of sight.

Engineers frequently estimate diffraction loss using the knife-edge diffraction model, which assumes that the obstacle has a sharp edge. Although real terrain is usually more complex, this model provides a useful approximation for predicting signal attenuation and planning radio links. More sophisticated propagation models account for multiple obstacles and irregular terrain profiles.

It is important to distinguish diffraction loss from clutter loss. Diffraction loss arises primarily because radio waves bend around large obstacles that obstruct the propagation path, whereas clutter loss results from the absorption, reflection, and scattering caused by numerous smaller objects such as buildings, vegetation, and vehicles. Diffraction loss also differs from free-space path loss, which occurs even in completely unobstructed propagation.

Today, diffraction loss is a fundamental consideration in the planning of terrestrial communication systems, including broadcast networks, land mobile radio, cellular systems, microwave links, and public safety communications. By understanding and predicting diffraction loss, engineers can estimate coverage behind obstacles, optimise antenna locations, and design communication systems that remain reliable even in challenging terrain.