11.8.5 Why Can Radio Waves Bend Around Hills and Buildings?
- Don't Radio Waves Travel in Straight Lines?
- What Is Diffraction?
- Why Does Diffraction Occur?
- Why Do Lower Frequencies Diffract More Easily?
- Why Don't Hills Completely Block Radio Signals?
- What Is a Knife-Edge Obstacle?
- What Are Fresnel Zones?
- Why Is Fresnel Zone Clearance Important?
- Does Diffraction Always Improve Communication?
- How Do Engineers Reduce Diffraction Losses?
- Where Is Diffraction Important?
- Why Is Understanding Diffraction Important?
- What Should You Remember?
Short Answer
Radio waves can often reach locations that appear to be completely blocked because they undergo diffraction. Diffraction causes electromagnetic waves to bend around the edges of obstacles and spread into regions that would otherwise lie in radio shadow. The amount of bending depends primarily on the wavelength of the signal and the size of the obstruction. Lower-frequency signals, with their longer wavelengths, diffract much more readily than higher-frequency signals. Understanding diffraction is essential for designing reliable communication systems because it explains why hills, buildings, and other obstacles do not always completely block radio transmission.
Don't Radio Waves Travel in Straight Lines?
In free space, radio waves propagate in straight lines.
If an opaque object is placed directly between a transmitter and a receiver, it might therefore seem reasonable to expect communication to cease immediately. Experience shows otherwise. AM radio stations are often received behind hills. Mobile phones frequently continue working inside cities where buildings block the direct path. Emergency communication systems operate successfully in mountainous terrain.
These observations demonstrate that radio waves are capable of reaching locations that have no direct line of sight to the transmitter.
The principal reason is diffraction.
What Is Diffraction?
Diffraction is the tendency of waves to bend around obstacles and spread beyond sharp edges.
It is not unique to radio waves. Water waves spread around the end of a breakwater. Sound can often be heard around the corner of a building. Light also diffracts, although its extremely short wavelength usually makes the effect difficult to observe without specialised equipment. Radio waves behave in exactly the same way.
Whenever a radio wave encounters an obstacle, part of the energy bends around its edges and continues propagating beyond the obstruction.
Why Does Diffraction Occur?
Diffraction arises because electromagnetic waves are not narrow rays but continuously distributed wavefronts.
According to Huygens' Principle, every point on a wavefront may be regarded as a source of tiny secondary wavelets. When part of the original wavefront is blocked by an obstacle, these secondary wavelets continue spreading beyond the obstacle and combine to form a new wavefront. The result is that energy appears within the geometric shadow of the obstruction.
Rather than stopping abruptly at the edge of a hill or building, the wave gradually bends into the shadowed region.
Why Do Lower Frequencies Diffract More Easily?
One of the most important factors affecting diffraction is wavelength. Long wavelengths bend around obstacles much more effectively than short wavelengths. This explains why:
- long-wave and medium-wave broadcasts often remain audible behind hills;
- HF communication frequently reaches valleys that have no direct line of sight;
- VHF communication experiences moderate diffraction; and
- microwave signals require much clearer paths.
A useful way to think about this is to compare the wavelength with the size of the obstacle.
If the wavelength is large relative to the obstruction, the wave bends readily around it. If the wavelength is much smaller than the obstacle, diffraction becomes much weaker.
Why Don't Hills Completely Block Radio Signals?
Consider a transmitter located behind a hill.
Part of the transmitted wave strikes the crest of the hill. Instead of stopping completely, some of the energy bends around the summit and continues into the region beyond.
The receiver therefore still receives a signal, although it is usually weaker than if a clear line of sight existed. This phenomenon is particularly important for communication in mountainous regions.
Without diffraction, many rural radio services would require far more relay stations than they do today.
What Is a Knife-Edge Obstacle?
Engineers often simplify diffraction analysis by representing an obstacle as a single sharp edge.
This model is known as the knife-edge diffraction model. Although real hills are rarely knife-edged, the model provides a remarkably useful approximation for many propagation problems. The amount of diffraction depends upon how deeply the obstacle intrudes into the direct propagation path.
A small obstruction causes relatively little additional loss. A large obstruction produces much greater attenuation.
Modern radio-planning software routinely performs knife-edge diffraction calculations when predicting communication coverage.
What Are Fresnel Zones?
Many people assume that successful communication requires only a clear straight line between the two antennas.
In reality, the radio wave occupies a three-dimensional region surrounding the direct path. This region is divided into a series of ellipsoidal volumes known as Fresnel zones. The first Fresnel zone is particularly important because most of the transmitted energy passes through it. If large obstacles intrude into this region, diffraction increases and additional signal loss occurs, even though the direct line of sight itself remains unobstructed.
For this reason, microwave engineers aim not merely to establish line of sight but also to maintain adequate clearance of the first Fresnel zone.
Why Is Fresnel Zone Clearance Important?
Imagine two microwave towers with a clear visual path between them.
At first glance the link appears ideal. However, if a hill, trees, or buildings extend into the first Fresnel zone, diffraction losses may become significant. The signal may therefore be much weaker than expected. A common design objective is to keep at least about 60 percent of the first Fresnel zone free from obstructions under normal operating conditions. This usually provides reliable communication while avoiding unnecessary tower height.
Fresnel-zone clearance is therefore one of the most important considerations when designing terrestrial microwave links.
Does Diffraction Always Improve Communication?
Although diffraction often enables communication where no direct path exists, it also introduces additional attenuation.
The diffracted signal is always weaker than the unobstructed wave. Furthermore, diffraction frequently combines with reflected and scattered signals, contributing to multipath propagation and fading. Engineers therefore regard diffraction as both beneficial and unavoidable.
It allows communication into shadowed regions while simultaneously introducing additional propagation losses that must be included in the link budget.
How Do Engineers Reduce Diffraction Losses?
Several approaches can reduce diffraction-related attenuation.
These include:
- increasing antenna height;
- relocating antennas;
- selecting a different propagation path;
- using lower operating frequencies;
- providing additional fade margin;
- installing repeater stations; or
- using satellite communication where terrestrial paths are unsuitable.
The most appropriate solution depends upon the communication system and the surrounding terrain.
Where Is Diffraction Important?
Diffraction influences a wide variety of communication systems, including:
- HF communication;
- VHF and UHF land-mobile radio;
- emergency service radio networks;
- television broadcasting;
- microwave relay systems;
- cellular networks;
- rural broadband;
- radar; and
- satellite links near the horizon.
In mountainous regions, diffraction often determines whether reliable communication is possible without constructing additional infrastructure.
Why Is Understanding Diffraction Important?
Diffraction demonstrates that radio propagation is fundamentally different from simple geometric optics.
Signals do not merely travel in straight lines until blocked by an obstacle. Instead, they interact continuously with the environment. Understanding diffraction enables engineers to predict coverage, estimate additional propagation losses, determine suitable antenna heights, and design reliable communication systems under realistic operating conditions.
Together with reflection, refraction, and scattering, diffraction forms one of the principal mechanisms governing radio-wave propagation.
What Should You Remember?
- Diffraction allows radio waves to bend around obstacles and reach regions that have no direct line of sight.
- Diffraction occurs because electromagnetic waves spread beyond the edges of obstacles rather than behaving as perfectly narrow rays.
- Longer wavelengths diffract much more readily than shorter wavelengths.
- Knife-edge diffraction provides a useful engineering model for estimating propagation over hills and ridges.
- The first Fresnel zone contains most of the transmitted energy and should be kept largely free of obstructions.
- Microwave engineers typically aim to maintain approximately 60 percent first-Fresnel-zone clearance for reliable communication.
- Diffraction often makes communication possible beyond obstacles but always introduces additional attenuation that must be considered during system design.