How far is the wireless device?
When it comes to this topic, it is necessary to think about it. For example, before 1949, there was no communication satellite in the sky, and the Shanghai Underground Party could work with Yan'an with a few watts of power. Of course, this used shortwaves reflected by the ionosphere. Even today, a large number of amateur radio enthusiasts can still use thousands of kilowatts of power for thousands of kilometers of communication. Even with the ultra-short-wave communication in the commonly used 430MHz band, a large number of amateur radio enthusiasts can use a few watts of power to transmit and transmit images across the continent using amateur communication satellites that use a perigee to reach more than 1,000 kilometers.
How far is the actual wireless device, such as our commonly used 430Mhz band, this band is also called the line-of-sight band. For example, it can be used with the eyes to see. Some people say that I saw the moon, and it is completely acceptable. Not only can it be passed, a large number of radio enthusiasts around the world use the 430M frequency band for lunar reflection communication, which can cover half of the earth. A better example is the availability of amateur stations on the International Space Station, and the power of communication is not great.
figure 1
The reality is just now, how far can a milliwatt of power go? This is very easy to try, find an RF signal source connected to the antenna to transmit FM signals with a frequency offset of 75khz and 5khz respectively, and use a better amateur radio intercom to receive in wideband mode and narrowband mode respectively. It is not enough to find that the broadband is less than 100 meters, and the narrow band can be many times far. So bandwidth determines the distance because the density of energy is not concentrated. Try 20mw again and find that narrow-band fm communication is easy to pass 20km when the antenna is good, and the broadband is very close. This makes it possible to understand why radio stations and television stations have such high-power towers. Therefore, when there is a communication distance requirement, the user communication mode should be selected first.
How much power is used
Find a spectrum analyzer and connect the antenna. Then find a walkie-talkie and leave the instrument for 5 meters. Press the launch button to see the spectrum of the launch. By shaking the walkie-talkie with a large range of hands, you can find that the spectrum changes greatly, and there can be more than 10db changes, so if you use mobile, you must have about 10 times the power headroom.
How to expand communication distance
There is only one way to go, the other is 瞎掰, this is the networking. Whether it is a large-area network or a complex community-based network, the reliability of long-distance communication can be greatly improved.
Assume that a wireless communication IC using the ism band has been selected, such as nrf905
This means that you have decided on the scope of use and are small-scale applications. Any measures such as increasing the transmit power will not change qualitatively. Because this chip is not used for long distances.
The main focus of long-distance communication is the receiver. In the 430M band, for example, there must be at least two intermediate frequencies with good filters. For narrow and large-scale off-the-shelf crystal and ceramic filters, the selection of wideband filters is not so easy. It can be said that the high indicators are very complicated and costly. The filter basically determines the quality of your receiver.
Antenna is a problem that must be taken seriously
Even with an IC such as nrf905, the change of the communication antenna can greatly improve the communication distance. The most difficult to use is to find a small antenna directly connected to the module. Needless to say, it is assumed that the RF signal reflected by the 1W transmit power is likely to damage the host device. I have a painful experience: once I debugged a 150W power PA (430MHz) to play satellite communication, because the antenna is not particularly matched, the transmitted signal directly destroys the SSD of the computer 5 meters away, but there is a warranty. Therefore, in the absence of the necessary measurement methods, try to avoid using more than +20dbm power.
What kind of module should I choose?
If you want to use a ready-made network, such as GPRS. People who spend a lot of money to build a network will be very useful. If you want to go far, you must use a dedicated narrowband device. Since the general unit group wireless network is not allowed in China, the networking is not mentioned. If you choose a chip such as nrf905, you have actually given up the distance, but even this way you can completely improve the communication distance by some means.
Properly increase the transmit power to improve the reliability of communication
In the receiving end, the front end introduces good filtering, and the controllable LNA, is not doing the wireless line, the kung fu is generally reflected here. When you see a module that only knows how to increase the transmit power without receiving it, you can know the designer's level. In particular, some engineers who have been exposed to RF for more than 10 years are still doing so.
Increase the gain of the antenna and select the antenna system with small reactance
Shield the machine as much as possible, and actually shield the magnetic shield and the field shield. The field shield is used here. If it is used in some industrial environments with bad environment, magnetic shield should be added.
Try not to use the switching power supply on the system. In fact, it can be used. It is mainly used in the test to eliminate the bad communication.
Find an instrument to test how much power the purchased module has. Some merchants have a power consumption of 33 mA at 33 db when 3.3 V is written in the data sheet. I don't know where the energy comes from.
The module uses battery power first when testing. Some of the cheaper power modules are greatly reduced in performance when they are connected to non-battery power supplies.
How beginners choose instruments
For the frequency band such as 433, because it is allocated to the amateur radio frequency band internationally, an inexpensive antenna analyzer can be purchased, and the standing wave and the reactance can be measured. Then come to a second-hand spectrum analyzer (about 3,000 yuan) to start, and then slowly add later.
How to determine the status of communication
Look for two amateur walkie-talkies with micro-power files, such as 7R, set the power to less than 50 milliwatts. If you can't talk in the set occasions, don't try hard. You can't pass the 905 and 3W PA. Other ways, don't waste time.
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