Wireless transmission distance of microwave signal
Time:2022-11-09
Views:1568
1. Which way can FM signal and AM signal be transmitted farther?
In other words, under the same conditions of power, frequency and environment, the transmission distance of FM signal is much larger than that of AM signal.
Taking TV signal transmission as an example, when the FM signal is a few decibels higher than the noise level, the demodulation threshold is often exceeded. If it is more than ten decibels higher, good images and sound can be obtained. At this time, it is not wrong to find the trace of the AM signal on the TV screen. In order to obtain a better TV image, the signal level must be more than thirty decibels higher, and the distance of transmission can be imagined, right?
However, in recent years, due to the large use of wireless transmission equipment, the number of interference signals in the air has increased year by year. FM signals are easy to be interfered because they occupy a wide frequency band. In the crowded frequency band, the advantage of strong anti-interference ability of FM mode has disappeared. It is felt that the amplitude modulation mode is more distant when compared with various equipment on the site.
2. Which method is more effective to increase transmission distance, transmission power and antenna gain?
Both methods are effective, and which one is more effective depends on the specific situation. Here are some common cases.
A - Small, portable and vehicle mounted, it is inconvenient to use high gain transmission antenna. To increase the transmission distance, only the transmission power can be increased. In addition, the first two modes should be the FM mode, because the transmitter power supply efficiency of FM mode is high, the circuit structure is simple, and it is easy to do smaller. On board transmitters do not need to consider modulation methods, because weight and power efficiency are not a problem, as long as the transmission antenna is not too large.
B - Long distance transmission with fixed receiving and transmitting points, suitable for using high gain antennas. Generally, the transmission distance is more than 10km, and the influence of the earth‘s surface has to be considered. When the transmission distance is more than 50km or 60km, the height of the transceiver antenna needs to be dozens of meters. It is often necessary to build an iron tower or use tall buildings, or even use mountains. It is not difficult to install a relatively bulky high gain antenna, and naturally it does not need a lot of power. Of course, it is not impossible to install a low gain antenna with a high-power transmitter at a high place, but it will cost more to pollute the environment and harm others rather than benefit themselves.
C - When transmitting in a complex terrain environment, the distribution of signal field strength is also very complex. At this time, it is meaningless to use a high gain antenna, and the transmission distance can only be increased by increasing the transmission power.
3. Why is the effect of high gain antenna bad in complex environment?
In fact, it is not absolutely bad to use high gain antennas for transmission in complex environments. The reason why the high gain antenna has high gain is that it can transmit energy from a larger space to the feed point as much as possible in an ideal environment. In a complex environment, the relative relationship of space energy will also become irregular. The energy transmitted to the feed point may be large, small or negative after being added.
It is easy to verify this at the project site.
4. How to improve the transmission effect of analog TV signals when wireless transmission must be carried out in buildings?
We all have such experience that when transmitting in buildings, the image quality may change a lot if the antenna orientation is slightly changed; If the detection instrument is connected, it can be found that the signal strength can change thousands of times by slightly adjusting the orientation of the antenna. Therefore, it is better to use diversity antenna for wireless transmission in buildings.
The so-called diversity antenna is to divide the signal into multiple channels at the transmitting end and transmit it with multiple antennas; At the receiving end, the signals received by multiple antennas are combined into one channel and sent to the receiver. In this way, the signal loss caused by the division and integration of multiple signals is limited, but it is far from thousands of times. Therefore, this method is cost-effective.
There should be at least two diversity antennas. The higher the frequency, the more obvious the improvement effect. This method is often used in our common wireless network equipment. Sometimes we see that the wireless network equipment has only one antenna. In fact, when we open the shell, we can see that there is often one inside.
5. What are the advantages of wireless transmission of analog TV signals, FM, AM, higher frequency and lower frequency in complex environments?
It is known that the same distance is transmitted wirelessly and the corresponding frequency is used. The power required for FM TV signal is much smaller than that for AM TV signal. Therefore, FM is certainly better for transmission in open areas. In complex environments, FM has some advantages because it requires much less strength of the received signal. However, the frequency bandwidth occupied by FM mode is up to 27MHz, while that of AM mode is only 8MHz. From transmission to reception, it can also be regarded as a transmission line. In open areas, as long as the equipment of the transmitter and receiver are well matched, it can be considered that the frequency characteristics of this line are relatively flat. However, in complex environments, due to the impact of multi-channel transmission, the frequency characteristics of the equivalent line are extremely poor, It is much more difficult to transmit 27MHz bandwidth than 8MHz bandwidth. Therefore, in practice, it can be clearly felt that it is easier to obtain stable images by AM transmission in buildings.
In recent years, we have done a lot of experiments in the low carrier frequency FM TV transmission. Normally, the requirements for the transmission environment should be relatively low if the frequency is low. In fact, it is difficult to transmit FM TV signals wirelessly at more than 100 to more than 200 MHz. We analyze that the reason may be that the relative bandwidth is too large. Similarly, when the operating frequency is high, the relative bandwidth is small. Sometimes, transmission in complex environments adds this advantage.
Therefore, there are many kinds of complex environments. It is necessary to have a more comprehensive understanding of the transmission site environment, make specific analysis in a targeted way, and maybe (just maybe) draw a conclusion that is closer to reality.
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