The frequency of FM radio is 88-108MHz, and the coverage ranges from several hundred kilometers to several thousand kilometers. The frequency of wireless communication cellular network is 400MHz-3GHz. The coverage ranges from several kilometers to tens of kilometers. The communication frequency of WIFI network is 2.4-2.5GHz and 5.2-5.8GHz, and the coverage ranges from a few meters to a few hundred meters. While the communication frequency of the satellite is 5GHz-40GHz and the communication distance is 36,000 kilometers.
We know that the lower the frequency of the electromagnetic wave, the farther it is free to travel in the air. So why can transmit such a long distance (36,000 kilometers, but in the communication frequency as high as 5GHz-40GHz? This section will show you the reason about the satellite signal from 5 parts.
1. Atmospheric Ionosphere
The ionosphere is an ionized region of the Earth’s atmosphere. The ionosphere extends from 50 kilometers to about 1000 kilometers. There are quite a lot of free electrons and ions, which can change the propagation speed of radio waves, refraction, reflection and scattering, and polarization. The rotation of the face is absorbed to varying degrees. Radio waves include microwave, short wave, medium wave, and long wave. Among them, the microwave is not affected by the ionosphere and can directly penetrate the ionosphere. Therefore, satellite communication signals must use microwave frequency band to achieve communication.
2. Free Space Attenuation of Electromagnetic Waves
Electromagnetic waves are depleted when they penetrate any medium, so as the same as travelling through the atmosphere. The transmission loss (L) of electromagnetic waves in the free space of the atmosphere is proportional to the frequency (F) and transmission distance (D). The formula is as follows:
3. Wireless Communication Receiving Power Link
A complete wireless communication system includes transmitter, transmitting antenna, propagation medium, receiving antenna, and receiver. How many useful signals the receiver will eventually receive depends on the front four cells. The formula of the wireless communication receiving power link is as follows:
So as long as the antenna gain (Gt and Gr) is high, it is possible to compensate for the free space loss.
4. High Gain Antenna
If the antenna can be transmitted in only one direction, the antenna gain will be greatly improved. This is the origin of directional high gain antennas. A classic directional high gain antenna is the Cassegrain antenna, also known as a parabolic antenna. The higher the frequency of the wireless link, the higher the efficiency, regardless of weather factors (such as rain, cloudy clouds).
5. High Frequency, Brand Bandwidth, High Speed
Satellites need to work on batteries in space, and battery energy is limited. Therefore, shortening the communication time is a good way to save battery energy. In order to shorten the communication time under the condition that the amount of information to be transmitted is constant, the transmission rate must be increased. The way to increase the information transmission rate is to increase the communication bandwidth based on the Shannon formula. Brand bandwidth can carry more information.
There are several reasons why satellites use operating frequencies above 5 GHz:
(1) Can smoothly pass through the ionosphere without being reflected or absorbed.
(2) High-frequency transmission is supported by high-gain antennas without much power loss.
(3) High frequency means large bandwidth. According to Shannon formula, the wider the bandwidth, the higher the information transmission rate, which is helpful for saving energy and improving the service life of the satellite.
(4) There are relatively complete spectrum resources above 5G, which are not easy to be interfered.