When designing a microwave link, following four aspects should be covered;
- Requirement on the point-to-point line-of-sight communication
- Objective of designing a microwave transmission line
- Transmission clearance
- Meanings of K value in the microwave transmission planning.
Because the microwave is a short wave and has weak ability of diffraction, the normal communication can be realized only in the line-of-sight transmission without obstacles.
The transmit power is very small as compared with radar. The only way to implement long-haul communication is to use the antenna of a greater diameter or increase the transmit power of the antenna.
In common geographical conditions, it is recommended that there should no be obstacles within the first Fresnel zone if K-factor is equal to 4/3.
Steps to Design a Microwave Transmission Link
- Determine the route according to the engineering map
- Select the site of the microwave station
- Draw the cross-sectional chart of the terrain
- Calculate the parameters for site construction
As the first step, the area should be selected, which is curly as much as possible, such as the hilly area. Also the water surface and the flat and wide area should be avoided, because that is not suitable for the transmission of the electric wave. In this way, the strong reflection signal and the accordingly caused deep fading can be avoided. The line should avoid crossing through or penetrating into the mountainous area. The line should go along the railway, road and other areas with convenient transportation
As the second step, the site of the microwave station should be selected. The distance between two sites should not be too long. The distance between two relay stations should be equal, and each relay section should have proper clearance. Avoid the interference from other radio services, such as the satellite communication system, radar site, TV station, and broadcast station.
As the third step, cross-sectional chart of the terrain should be drawn. Calculate the antenna height and transmission situation of each site. For the line that has strong reflection, adjust the mounting height of the antenna to block the reflected wave, or have the reflection point fall on the earth surface with small reflection factor. Consider the path clearance. The clearance in the plain area should not be too big, and that in the mountainous area should not be too small.
As the fourth step, the parameters for site construction should be calculated. Calculate the terrain parameters when the route and the site are already determined. Calculate the azimuth and the elevation angles of the antenna, distance between sites, free space transmission loss & received level, rain fading index, line interruption probability, and allocated values and margin of the line index. When the margin of the line index is eligible, plan the equipment and frequencies, make the approximate budget, and deliver the construction drawings.
