The previous post used to introduce Fresnel Zone and How to calculate Fresnel Zone in Microwave System . This post continue discussing influence of ground reflection on receive level of microwave.
To make explanation more simple and easy to understand, the convex of the earth is ignored, that is, distance between stations is considered to be short and the ground is with little fluctuation.
Actual microwave propagation may be blocked by buildings, trees and mountains. If obstacles are high enough to enter the first Fresnel zone, additional loss is caused and the receiving level decreases, and then the transmission quality is affected. To avoid the phenomenon, the clearance is introduced.
The vertical distance from the obstacle to line section AB is called clearance of the obstacle in the path, and line section hc that is vertical to the ground is used to indicate the clearance. If the first Fresnel diameter of this point is F1, hc/F1 is the relative clearance of this point.
In actual microwave project, knife-edge obstacles always block the transmission path. The knife-edge obstacles cannot block all the Fresnel zones, at the receiving point, there is only partial Fresnel zone energy is diffracted and make the receiving point have somewhat level. The level must be lower than that in the free space. Loss caused by the knife-edge obstacles is called additional loss. When the peak of the obstacle just falls down on the link between the transmitting and receiving points, that is, H C=0, the additional loss is 6 dB. When the peak of the obstacle surpasses the link between the two points, the additional loss may rapidly increase. When the peak of the obstacle is below the link between the two points, additional loss may vary slightly around 0 dB, at this time, the transmission loss (or receiving level) on the path is close to that of the free space.
Flat terrain indicates that the earth curvature is not considered and the terrain between two points is considered to be flat. In actual microwave communication project lines, the receiving and transmitting antennas are leveled to make the receive end receive stronger direct waves. But based on the Huygens theory, some electric waves are always sent to the ground, therefore, at the receiving point, besides the direct waves, there are reflected waves reflected by the ground and meeting the reflection conditions (angle of arrival equals to angle of reflection). We can use the following geometrical relationship to deduce the expression of virtual value of the composite field strength.
If: transient value of field strength of direct wave is expressed by:
e1=21/2 E0cosωt
Transient value of field strength of reflected wave is expressed by:
e2=21/2 E0Φcos (ωt-ψ-2π (r2-r1) /2)
In the two formulas:
Bu deduction, the effective value of composite field strength is:
E=(E02+E02Φ2+2 E02ΦCOS (ψ+2π (r2-r1) /λ) ) 1/2
=E0 (1+Φ2–2ΦCOS (ψ+2Ï€ (r2-r1) /λ) ) 1/2
The ratio of composite field strength E to the field strength of free space is called fading factor V when the ground influence is considered. The V is expressed by:
V=E/E0
= (1+Φ2–2ΦCOS (ψ+2Ï€ (r2-r1) /λ) ) 1/2
Expressed in dB:
V dB=20logV
When the ground influence is considered, the actual receiving level is:
PR (dBm) =PR0 (dBm) +V dB
To make explanation more simple and easy to understand, the convex of the earth is ignored, that is, distance between stations is considered to be short and the ground is with little fluctuation.
1. Clearance
The vertical distance from the obstacle to line section AB is called clearance of the obstacle in the path, and line section hc that is vertical to the ground is used to indicate the clearance. If the first Fresnel diameter of this point is F1, hc/F1 is the relative clearance of this point.
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| definition-clearance-schematic |
2. Loss caused by Knife-edge Obstacles on the Path
In actual microwave project, knife-edge obstacles always block the transmission path. The knife-edge obstacles cannot block all the Fresnel zones, at the receiving point, there is only partial Fresnel zone energy is diffracted and make the receiving point have somewhat level. The level must be lower than that in the free space. Loss caused by the knife-edge obstacles is called additional loss. When the peak of the obstacle just falls down on the link between the transmitting and receiving points, that is, H C=0, the additional loss is 6 dB. When the peak of the obstacle surpasses the link between the two points, the additional loss may rapidly increase. When the peak of the obstacle is below the link between the two points, additional loss may vary slightly around 0 dB, at this time, the transmission loss (or receiving level) on the path is close to that of the free space.
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| Loss caused by knife-edge obstacle |
3. Reflection of Flat Terrain to Electric Waves
Flat terrain indicates that the earth curvature is not considered and the terrain between two points is considered to be flat. In actual microwave communication project lines, the receiving and transmitting antennas are leveled to make the receive end receive stronger direct waves. But based on the Huygens theory, some electric waves are always sent to the ground, therefore, at the receiving point, besides the direct waves, there are reflected waves reflected by the ground and meeting the reflection conditions (angle of arrival equals to angle of reflection). We can use the following geometrical relationship to deduce the expression of virtual value of the composite field strength.
If: transient value of field strength of direct wave is expressed by:
e1=21/2 E0cosωt
Transient value of field strength of reflected wave is expressed by:
e2=21/2 E0Φcos (ωt-ψ-2π (r2-r1) /2)
In the two formulas:
- e1, e2 are the transient values of field strength of direct wave and reflected wave respectively.
- E0 is the effective wave of the field strength of waves propagates in free space.
- Φ is the modulus of reflectance.
- ψ is the phase angle of reflectance (when the angle of arrival formed by the incoming wave and the ground are small, ψ is close to 180°)
- r2-r1 is the progressive error of the field strength of reflected wave and the direct wave.
Bu deduction, the effective value of composite field strength is:
E=(E02+E02Φ2+2 E02ΦCOS (ψ+2π (r2-r1) /λ) ) 1/2
=E0 (1+Φ2–2ΦCOS (ψ+2Ï€ (r2-r1) /λ) ) 1/2
The ratio of composite field strength E to the field strength of free space is called fading factor V when the ground influence is considered. The V is expressed by:
V=E/E0
= (1+Φ2–2ΦCOS (ψ+2Ï€ (r2-r1) /λ) ) 1/2
Expressed in dB:
V dB=20logV
When the ground influence is considered, the actual receiving level is:
PR (dBm) =PR0 (dBm) +V dB
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| Influence of terrain on electric waves |
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