Depending on the spectrum of electromagnetic waves, it can be divided into radio waves, infrared, visible light, ultraviolet rays, X-rays, gamma rays, and cosmic rays.
The most commonly used radio waves, which frequencies from 3kHz to 3000GHz.
Our secondary school physics has learned [Light Speed %= Frequency %& Times; Wavelength] This important formula is 3 %& Times; 10 ^ 8 m / sec, the frequency and wavelength of radio waves are inversely proportional.
According to the low to high sort, 3kHz to 30kHz is very low frequencies, and VLF is a short-term version of Very Low frequence, and then as shown in the figure, the right is pushed to the right, the frequency is getting higher and shorter, and the wavelength is getting shorter.
There is a very interesting, which is usually divided by frequency, and China usually naming according to the wavelength. For example, the foreign will speak VLF, and China will say that this is very long, in fact, the meaning is consistent, all refers to this spectrum.
In order to effectively radiation energy, the length of the antenna is proportional to the wavelength, usually 1/4 of the wavelength, and the shortestity cannot be shorter than 1/10, and then the signal can be effectively radiated.
For very wave, the wavelength is 10 km to 100 kilometers, and the length of the appropriate antenna has a minimum of more than 1 km, and a thousands of thousands of watts of power supply equipment will be used. This huge integrated complex wireless communication system has a strong characteristic of diffraction and penetration, and the signal is relatively stable, and the transmission distance is far, especially to communicate with a submarine under the sea level. Other wavelengths of short signals are reflected in the sea level, only this wavelength is very long to be drilled into the seawater to communicate with the submarine, so long wave communications has become an important means of military to the potential.
You may ask: Is there a few kilometers of antennas? Why didn't I have seen it? Usually this antenna is between two hills, using two hills as antenna bracket, and forms an end of the sub-communication.
Why have you not seen it? Because this antenna belongs to military secrets, it will be set as a military penalty area in the square, you certainly won't see it. Even the soldiers working inside are strictly forbidden to take pictures, do not say to the Internet.
Why is it confidential? This is because the inline can calculate the frequency bands and even activity ranges used by the submarine and nuclear submarines by antenna length, an array shape, a transmit angle.
The radio wave is used as a very significant feature of communication, the longer the wavelength, the longer the antenna, the greater the power, the better the diffractionability of the radio wave, and of course the equipment will become increasingly large. The shorter the wavelength, the shorter the antenna, the more radio waves approach the linear transmission, the worse the diffractionability of the diffusion, but the equipment can do very small and cheap. The medium wave belongs to the two unreaches, the equipment is neither a very small, the diffusion of the signal is not too strong, and it is very strong for military communications, so this will give the civilian use, we It is a medium wave used as civil broadcast.
Short waves are a very old military communication means, which has a significant feature that can achieve ultra-long distance communication through the reflection of ionization layers.
Located in the atmospheric layer above the surface of the Earth's surface, it is inexpensive under solar irradiation to form an ionizing layer that reflects radio waves.
If we calculate the height of the ionization layer and control the emission angle of the short-wave signal, you can reach a far distance after a few hops.
Of course, after multiple reflections of the ionization layer and the ground, the quality of its signal will have a lot loss. However, due to its very small equipment, the backpacking short-wave radio can reach thousands of kilometers, so it has become an important military communication method.
Microwaves are also divided into many bands, in summary, the frequency is getting higher and higher, and the wavelength is getting shorter, and its linearity is getting stronger. Drawing and penetratingness are getting worse.
Therefore, microwaves are often used in satellite communication. After all, there is no obstruction between the earth and the satellite, which is very conforming to the requirements of linear communication.
The wavelength of the microwave communication is only a few centimeters or even shorter, and the length of the antenna is shorter, such as the satellite antenna in the figure.
"Cot" is not an antenna, it is just a parabolic reflecting surface that is the aim to reflect the straight line into the parabolic focus, so that the antennas at the focus get more energy to facilitate the reception of the signal.
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