11.8.8 Why Do Rain, Fog, and the Atmosphere Affect Microwave Communication?
- Why Doesn't the Atmosphere Affect All Radio Waves Equally?
- What Is Atmospheric Attenuation?
- Which Atmospheric Gases Cause the Greatest Losses?
- Why Does Rain Cause Signal Loss?
- Why Are Higher Frequencies More Affected?
- Do Clouds and Fog Matter?
- Does Snow Cause Attenuation?
- What Is Atmospheric Scintillation?
- How Do Engineers Overcome Atmospheric Attenuation?
- Why Doesn't Everyone Use Lower Frequencies?
- Where Are These Effects Important?
- Why Is Understanding Atmospheric Attenuation Important?
- What Should You Remember?
Short Answer
As radio frequency increases into the microwave and millimetre-wave bands, electromagnetic waves interact much more strongly with atmospheric gases, rain, clouds, and fog. Water droplets and oxygen molecules absorb and scatter part of the transmitted energy, reducing the strength of the received signal. This additional attenuation, known as atmospheric attenuation, becomes one of the principal limitations of high-frequency terrestrial microwave and satellite communication systems. Understanding these effects allows engineers to choose appropriate frequencies, provide sufficient fade margin, and design reliable communication networks.
Why Doesn't the Atmosphere Affect All Radio Waves Equally?
Most people think of the atmosphere as being transparent to radio waves.
To a large extent this is true at lower frequencies. Signals in the LF, MF, and HF bands experience relatively little attenuation while travelling through the atmosphere. As frequency increases, however, radio waves begin interacting much more strongly with atmospheric gases and suspended water droplets. The atmosphere therefore becomes an increasingly important part of the communication channel.
By the time frequencies reach tens of gigahertz, atmospheric effects often dominate link performance.
What Is Atmospheric Attenuation?
Atmospheric attenuation is the reduction in signal strength caused by the absorption and scattering of electromagnetic energy by the atmosphere.
Unlike free-space path loss, which results simply from geometric spreading, atmospheric attenuation represents a genuine loss of energy. Some of the transmitted energy is converted into heat within atmospheric gases. Some is scattered in directions away from the receiving antenna.
Both mechanisms reduce the signal available at the receiver.
Which Atmospheric Gases Cause the Greatest Losses?
The atmosphere contains many gases, but two are particularly important for radio propagation:
- oxygen; and
- water vapour.
These molecules absorb microwave energy most strongly at particular frequencies. For example, oxygen produces a significant absorption peak near 60 GHz, while water vapour has a major absorption peak near 22 GHz and additional peaks at higher frequencies. Communication engineers normally avoid these absorption bands unless the additional attenuation is actually desirable.
For example, the strong oxygen absorption around 60 GHz greatly limits communication range, allowing dense frequency reuse in short-range wireless networks with minimal interference.
Why Does Rain Cause Signal Loss?
Rain is one of the most significant sources of attenuation at microwave frequencies. Raindrops interact with electromagnetic waves in two principal ways. First, part of the energy is absorbed within the water. Second, part is scattered away from the receiver.
The combined effect is known as rain attenuation or rain fade. The amount of attenuation depends upon:
- operating frequency;
- rainfall intensity;
- path length through the rain;
- elevation angle; and
- drop-size distribution.
Heavy tropical rainfall may introduce many decibels of additional attenuation, particularly above about 10 GHz.
Why Are Higher Frequencies More Affected?
Rain attenuation increases rapidly as wavelength becomes comparable with the size of the raindrops.
At lower frequencies the wavelength is much larger than the drops, so very little interaction occurs. As frequency increases, the wavelength shortens and the droplets become increasingly effective at absorbing and scattering energy. This explains why:
- L-band satellite systems experience very little rain fade;
- C-band systems are only modestly affected;
- Ku-band systems experience noticeable attenuation during heavy rain; and
- Ka-band systems require careful engineering to maintain reliable service during severe weather.
These trade-offs strongly influence the design of modern satellite communication systems.
Do Clouds and Fog Matter?
Clouds and fog also contain suspended water droplets.
Although the droplets are much smaller than raindrops, they can still introduce measurable attenuation at higher microwave frequencies. For most terrestrial communication systems the effect is relatively small. For satellite links operating above approximately 20 GHz, however, cloud attenuation may become significant, particularly when combined with rainfall.
Engineers therefore include cloud attenuation in detailed propagation calculations for high-frequency satellite systems.
Does Snow Cause Attenuation?
Snow generally produces less attenuation than rain because frozen water absorbs less microwave energy than liquid water.
Nevertheless, wet snow, sleet, and hail can produce appreciable attenuation, particularly at the higher microwave frequencies. Accumulated snow or ice on antenna surfaces may also reduce antenna performance by changing the antenna's electrical characteristics.
For communication systems operating in cold climates, these effects must also be considered during system design.
What Is Atmospheric Scintillation?
The atmosphere is rarely perfectly uniform.
Small variations in temperature, humidity, and pressure create tiny fluctuations in the refractive index. As radio waves pass through these irregularities, the received signal may fluctuate slightly. These rapid variations are known as atmospheric scintillation.
Scintillation is usually much less severe than rain attenuation but may become noticeable on long microwave and satellite links, particularly at high frequencies and low elevation angles.
How Do Engineers Overcome Atmospheric Attenuation?
A variety of engineering techniques are used to maintain reliable communication.
These include:
- increasing antenna gain;
- increasing transmitter power;
- providing additional fade margin;
- adaptive coding and modulation;
- uplink power control;
- site diversity;
- frequency diversity; and
- selecting lower operating frequencies where appropriate.
Modern satellite systems continuously monitor link quality and automatically adjust transmission parameters to compensate for changing weather conditions.
These adaptive techniques greatly improve service availability.
Why Doesn't Everyone Use Lower Frequencies?
Lower frequencies clearly offer better propagation through the atmosphere.
However, they also provide much less available bandwidth. Modern broadband communication requires very wide frequency allocations to support high data rates. Higher-frequency bands therefore remain attractive despite their greater atmospheric attenuation. The engineer's task is to balance capacity against propagation reliability.
This is one reason why satellite television commonly uses the Ku-band, while very-high-capacity broadband systems increasingly employ the Ka-band despite its greater susceptibility to rain fade.
Where Are These Effects Important?
Atmospheric attenuation influences many communication systems, including:
- terrestrial microwave links;
- satellite communication;
- satellite television;
- satellite broadband;
- weather radar;
- automotive radar;
- millimetre-wave 5G and emerging 6G systems;
- Earth observation satellites; and
- deep-space communication.
In tropical regions, where heavy rainfall is common, rain attenuation often becomes the dominant factor limiting communication reliability.
Why Is Understanding Atmospheric Attenuation Important?
As communication systems continue moving towards higher frequencies, atmospheric effects become increasingly significant.
Future wireless technologies will rely heavily upon millimetre-wave and even sub-terahertz frequencies to provide the enormous bandwidth demanded by emerging applications. These systems will offer exceptional capacity but will also require increasingly sophisticated propagation modelling and adaptive transmission techniques.
Understanding atmospheric attenuation therefore remains one of the fundamental skills of the communications engineer.
What Should You Remember?
- Atmospheric attenuation is caused by the absorption and scattering of radio waves by atmospheric gases and water.
- Oxygen and water vapour produce the most important gaseous absorption bands.
- Rain attenuation becomes increasingly significant above approximately 10 GHz.
- Cloud, fog, snow, and atmospheric scintillation may also reduce signal strength at microwave frequencies.
- Ku-band and especially Ka-band satellite systems are more susceptible to weather-related attenuation than lower-frequency systems.
- Engineers compensate using fade margin, adaptive coding and modulation, uplink power control, antenna gain, and diversity techniques.
- As communication systems move to ever higher frequencies, atmospheric attenuation becomes an increasingly important design consideration for terrestrial microwave and satellite communication systems.