What is a microwave station?

Microwave station is the terminal station or relay station in terrestrial microwave relay system. The availability and transmission quality of microwave relay circuit are closely related to the design of microwave station. The problems that should be paid attention to when designing different types of microwave stations are also different. Microwave stations can be divided into terminal stations and relay stations, and relay stations can be subdivided into substations and relay stations.

terminal

When there is a feeder, the station or feeder terminal at both ends of the link. Demodulate signals into baseband signals and send them to baseband terminal equipment (and vice versa). Generally, each link of a terminal station has only one communication direction, and there may be links in several directions in a big city. The terminal station plays the main role in each direction link.

Transfer station

A substation (or main station) has a relay station, which has a landing call and a transfer call. Besides some characteristics of the terminal station, it also has more than two microwave link communication directions. Substation can play an auxiliary control role on the link.

relay station

A relay station is a station that only plays a simple relay role. It sometimes plays the role of rearranging digital code streams in digital links, and is called a regenerative relay station, but it has no uplink and downlink functions. In the microwave link for transmitting radio and television, relay stations can separate video signals for local TV stations to relay. In a link, relay stations usually have only two communication directions. In most cases, the number of relay stations is larger than other types of stations. Therefore, reducing the maintenance of relay stations is an important issue.

Requirements of each subsystem of microwave station

The requirements of each subsystem in the microwave station are respectively considered according to the functions of each subsystem:

① Main communication system: When there are more than two channels in the transceiver of the microwave station, it is best to arrange them in a row, and only in this way can the waveguide shunt system of the feeder be conveniently arranged. For example, when a link has 8 channels and 4 channels * * * share a feeder (in which odd and even channels are transmitted with different polarizations, so 8 channels need two feeders), it is necessary to use a branch system to send signals of 4 different channels (or 4 frequencies) to each wireless receiver (the transmitter transmits reversely, with the same structure). This shunt system is the simplest and most widely used. Because every time the radio wave passes through the stopband reflection of circulator and bandpass filter, it will lead to the increase of signal loss and group delay unevenness. In order to make the quality of each channel basically equal, the number of circulators and filters passed by each channel can be the same only if the frame arrangement of adjacent stations adopts the opposite arrangement order. If the front stations are arranged according to channels 1, 3, 5 and 7 from the inlet end of the feeder, the rear stations are arranged according to channels 7, 5, 3 and 1. The same is true for channels 2, 4, 6 and 8.

When the terminal equipment is far away from microwave equipment, baseband signals need to be transmitted through long cables. In addition to maintaining the necessary return loss (or matching situation), the protective grounding of the system is very important. Because the frequency spectrum of baseband signal mostly coincides with the broadcast segment, when the transmission cable is poorly sealed or grounded, broadcast crosstalk will occur.

② Power supply system In order to obtain high link availability, most microwave station equipment adopts DC power supply. According to the electricity consumption, the primary power supply can be AC mains, solar cells and wind power, and an appropriate amount of oil generators can be equipped as backup according to the reliability of various equipment. General basic power supply needs to be unattended. In order to avoid electrical corrosion of different metals, DC power supply systems are mostly grounded at the positive electrode. If the AC power supply system of microwave station is commercial power, the load from substation to computer room mostly adopts three-phase five-wire system. That is, the neutral wire of the three-phase power supply is separated from the ground wire, so as to avoid interference caused by excessive 50Hz ripple caused by three-phase unbalanced current flowing through the ground wire, and the line is not allowed to be grounded in the computer room.

(3) Grounding In microwave stations, it is difficult to separate lightning protection grounding from protection grounding or DC power supply grounding, so common grounding is often used. And in order to avoid stray current interference in the computer room, a point grounding is also adopted, as shown in the figure. Therefore, the lead from the DC power supply to the rack is grounded at the grounding point and the power input of the rack.

④ The antenna or tower of lightning protection microwave station is vulnerable to lightning strike. Because there is no wired connection between sites, it is usually easier to handle. Only after taking lightning protection measures (such as lightning rod) can the whole station ground potential balance method be used to eliminate the harm caused by ground potential difference during lightning stroke. As shown in figure 1, the grounding grid is the standard to measure the grounding potential balance. When lightning current flows into the earth, the potential of each point in the grounding grid is equal, so there will be no step voltage or potential difference between devices, and the requirements for grounding resistance of the grounding grid are not very strict.

When the microwave station has power lines or telephone lines introduced from far away, these wires may be directly struck by lightning; It is also possible to introduce electromagnetic induction to cause surge when the earth is struck by lightning, resulting in lightning damage. In addition to lightning protection measures, lightning protection measures should also be taken when connecting these wires with the equipment in the station. Such as a varistor or a lightning arrester.

⑤ The antenna of the antenna is used to receive the diffused waves in the air or convert the guided waves in the antenna into diffused waves in the air. The antenna is the transmission medium between the antenna and the wireless equipment. Its number varies with link capacity or whether there is spatial diversity reception. A small-capacity circuit can receive and transmit using a duplexer and a pair of antennas. When the capacity is large, orthogonal polarization technology can be used to isolate the receiving and transmitting signals of an antenna, and two feeders can be used to transmit them separately. No matter how big the capacity is, you can use two antennas, each of which is polarized and uses four feeders. However, attention should be paid to the partial spectrum overlap between adjacent channels when combining. Decoupling between channels can be enhanced by * * * transmitting and * * receiving pole decoupling, and pole decoupling can be easily adjusted to the optimal value on a pair of antennas. Therefore, when two antennas are required to be in the same direction, it is best to share one antenna for all transmitting channels, separate the single and double channels by polarization, and treat the receiving antennas in the same way.

⑥ Iron Tower: There are two types of iron towers: self-supporting and cable-pulling. When the building area is small, self-supporting towers are often used. When there is sufficient construction site, low-cost cable towers can be used. And the spatial diversity antennas can be easily arranged on the same vertical plane. In order to avoid the cable tower from being easily twisted, it can be overcome by lengthening the cable arm.

Cement towers can also be used in coastal areas or areas with suitable conditions. Although the cost is slightly higher, its maintenance cost is lower than that of metal towers in strong typhoon areas or areas with severe sea breeze corrosion, and its service life is longer.