The first Fresnel zone is the region where the microwave transmission energy is the most intense. The obstruction in the Fresnel zone should be as little as possible. Due to obstruction of the first Fresnel Zone, cause link failures. When planning a new link try to maintain good clearance between two stations, avoiding obstruct the first Fresnel zone.
As shown in figure, the fi
rst Fresnel zone is obstructed. When the height of the obstacle enters the first Fresnel zone, additional loss might be added. As a result, the received level is decreased and the transmission quality id affected.
Factors Affecting in Electric Wave Propagation
When the electric wave is propagated in the atmosphere, the reflected wave from the ground surface is the major factor that affects the received level. Smooth ground or water surface can reflect a part of the signal energy transmitted by the antenna to the receiving antenna and cause interference to the main wave (direct wave). The vector sum of the reflected wave and main wave increases or decreases 
the composite wave. As a result, the transmission becomes unstable. Therefore, during the microwave link design, avoid reflected waves as much as possible. If reflection is inevitable, make use of the terrain ups and downs to block the reflected waves.
Different reflection conditions of different terrains have different effects on electric wave propagation. Terrains are classified into the following four types:
- Mountains (or cities with dense buildings)
- Hills
- Plain
- Large-area water surface
The reflection coefficient of mountains is the smallest, and thus the mountain terrain is most suitable for microwave transmission. The hill terrain is less suitable. When designing the circuits, avoid smooth plane such as water surface.
Generally there are two methods, which can be used to reduce the influence of ground reflection.
- Avoid smooth plane such as water surface during circuit design. This reduces the influence of ground reflection.
- Make use of the terrain ups and downs to block the reflected waves if reflection is inevitable.
Atmosphere
Microwave is propagated in atmosphere, so atmosphere will have a big impact on microwave propagation. Troposphere is the low altitude atmosphere within 10 km from the ground. Microwave antennas will not be higher than troposphere, so the electric wave propagation in aerosphere can be narrowed down to that in troposphere.
The main effects of troposphere on electric wave propagation are listed below
- Absorption caused by gas resonance. This type of absorption can affect the microwave at 12 GHz or higher.
- Absorption and scattering caused by rain, fog and snow. This type of absorption can affect the microwave at 10 GHz or higher. Refraction, absorption, reflection and scattering caused by in homogeneity of atmosphere.
- Refraction is the most significant impact to microwave propagation.
Fading
Fading means random variation of the received level. By the mechanism of fading, fading may fall into;
- Duct type fading - Due to the effects of the meteorological conditions such as ground cooling at night, burnt warm by the sun in the morning, smooth sea surface and anticyclone, a non-uniform structure is formed in atmosphere.
- K-type fading
- Scintillation fading - When the dielectric constant of local atmosphere is different from the ambient due to the particle clusters formed under different pressure, temperature, and humidity conditions, scattering occurs to the electric wave.
- Rain fading, absorption fading
- Free space propagation fading.
Anti-fading Technologies for Digital Microwave System
In digital microwave propagation, various fading will occur due to factors such as atmosphere, ground, and climate. This will cause receive power decrease or waveform distortion and thus further result in circuit performance degradation. Therefore, anti-fading measures are used to improve the performance of the transmission circuit system.
Fading factors can cause waveform distortion or power decrease. The anti-fading technologies can be classified according to equipment -level countermeasures and system-level countermeasures. Each anti-fading technology can only solve limited problems.
The equipment-level countermeasures include adaptive equalization, automatic transmit power control (ATPC) and forward error correction (FEC). Adaptive equalization can reduce the influence of waveform distortion, whereas ATPC and FEC can reduce the influence of power decrease.
The system-level countermeasures include the diversity receiving technology, which can reduce the influence of both power decrease and waveform distortion.
