Microwave is a frequency band of electromagnetic waves with a wavelength between 1 mm and 1 m. Let's start with the history of electromagnetic waves, discuss the theory and main frequency bands of electromagnetic waves, then talk about various applications of microwaves, and pick a few Taiwan's related applications to explain, and finally introduce the principle of microwave oven and microwave heating.
The theory of electromagnetic wavesThe two most important people in the history of electromagnetic wave development are Faraday and Maxwell. Both of them are the top 10 physicists. Their main contribution is what we want to talk about. Faraday was born in 1791. He discovered "Faraday's Law" through experiments in 1831: a magnetic field that changes with time produces an electric field. For example, when a magnet is passed through a coil, voltage and current are induced on the coil. Faraday's law is important because only one method can be used to generate an electric field, which is a charge, and Faraday has discovered another way to generate an electric field.
In the same year that Faraday's law was discovered, Maxwell was born. In 1873, Maxwell proposed an important theory: an electric field that changes with time produces a magnetic field. This is another epoch-making milestone, because at the time it was only known that current could generate a magnetic field. Because of the reasoning, Maxwell's theory was not accepted until his death in 1879. Until 1887, Hertz used the LC oscillator to generate electromagnetic waves, and Maxwell's theory was finally confirmed!
At that time, everyone only knew that light was a wave, and the phenomenon of light fluctuations could be detected by an interferometer, but it was not known what light was. Maxwell said that light waves are electromagnetic waves, composed of electric and magnetic fields, but they are too innovative to end in regret. The great thing about Faraday and Maxwell is that they found a new way to generate electric and magnetic fields.
Here we use mass and spring to compare the oscillation of electromagnetic waves. A spring mass is attached to the spring, and the spring is removed from the equilibrium position, and a bit energy can be generated. After the loose hand, the position can gradually become kinetic energy. When the maximum energy of the kinetic energy is the smallest, the kinetic energy begins to be converted into potential energy, and finally all changes. In place. The second half begins the same cycle, so the oscillation of the spring and mass is the mutual conversion between kinetic energy and potential energy.
The same principle can be applied to a circuit having a capacitor and an inductor. After the capacitor is charged, the circuit is turned on, and the voltage is generated by the positive and negative charges on the capacitor, so there is a current, and a current flows through the inductor to generate a magnetic field. At this time, the electric field energy in the capacitor becomes smaller as the electric charge decreases. When the electric charge flows, the electric field does not, and the energy becomes the magnetic field energy. When the magnetic field is maximum, the current is also the largest, but because the current is always flowing, it cannot be reduced to zero at once, so that the electric charge flows to the capacitor, and then the magnetic field energy gradually changes back to the electric field energy, and finally all becomes the electric field energy. The second half of the cycle begins the same cycle again. This is a phenomenon of electromagnetic oscillation, which is used by Hertz to generate electromagnetic waves.
Summarizing these two forms of oscillation, we can see the general rule of an oscillation phenomenon:
Energy form one, energy form twoIt can be seen from this general rule that the oscillation requires two mechanisms for storing energy. For example, the energy storage mechanism of the mechanical oscillation of the mass and the spring is kinetic energy and potential energy, and the energy storage mechanism of the LC oscillator and the electromagnetic oscillation is an electric field and a magnetic field. In addition, there is a need for mechanisms for energy exchange, such as mass and spring energy exchange mechanisms are spring resilience, LC oscillator energy exchange system is current and charge.
Although electromagnetic waves store energy by electric and magnetic fields, the mechanism of energy exchange is completely different, and is exchanged by the time variation of electric and magnetic fields. Since the electromagnetic wave does not need to generate a magnetic field by a current and does not need to generate an electric field by the electric charge, it can exist in a space without a medium, such as an outer space.
Hertz uses the LC oscillator to generate electromagnetic waves. In the process of oscillation, a part of the energy is coupled by the inductor and transmitted to the electric dipole antenna through the transmission line. On the antenna, the current generates a magnetic field and accumulates electric charge, thus generating an electric field. . The electric field and the magnetic field are generated at the antenna, and the two are substantially perpendicular to each other. Then, according to the theory of Maxwell and Faraday, the electromagnetic waves that are completely perpendicular to the electric field and the magnetic field are formed and propagated at the speed of light.
Main band of electromagnetic wavesThe frequency of the electromagnetic wave is from a few Hertz (1 Hz equal to the frequency of one oscillation per second, expressed in Hz), up to 1024 Hz or more, and the range can be said to be very wide. The entire spectrum area can be roughly divided into long waves, radio waves (including microwaves in radio waves), infrared rays, visible light, ultraviolet rays, and then X-rays, gamma rays, and the like.
One interesting phenomenon is the relationship between the absorption coefficient of water and the frequency of water. There is a lot of water vapor in the atmosphere. In the narrow visible light frequency band, the absorption coefficient of water is like a canyon, suddenly falling by 10 to 10 billion times, so that the atmosphere seems to have a window that allows sunlight to penetrate water vapor to the ground. . If there is no such a magical Grand Canyon, the earth will be black now, there will be no plants and no photosynthesis, no energy, no food, no burning gasoline, of course, we do not exist! Therefore, the absorption coefficient of water on electromagnetic waves seems to be a natural wonder, which is not found by other substances, and is almost designed by supernatural forces.
Let's talk about the low frequency band. 60 Hz is a low frequency that vibrates 60 times per second. It has a lot to do with us. This is the frequency band used by household electric power. The high voltage line transmits energy at 60 Hz. Everyone hates the high voltage line, but we have to rely on it to transmit energy. How does the high voltage line transmit energy?
It transmits energy through the electric and magnetic fields of electromagnetic waves. The current of the high-voltage line generates a magnetic field, and the voltage difference generates an electric field, and the electric field and the magnetic field are exactly perpendicular to each other, so that energy can be transmitted. In this way, the high-voltage line is surrounded by a large torrent of electromagnetic fields, but the inside of the high-voltage line cannot transmit energy. The reason why the high-voltage line is so high is not only because it is dangerous to hit the line, but it is dangerous to enter the torrent of the electromagnetic field.
In addition to the high voltage line, the same principle is used as long as it is a signal that transmits signals or transmits energy. The batteries of steam and locomotives are direct current, but there are also voltages and currents in the direct current wires. The electric and magnetic fields are also perpendicular to each other, and the energy can be transmitted.
Microwave applicationAfter talking about the light waves and low-frequency waves on both sides of the microwave, I began to enter another topic: the application of microwaves. We first introduce several frequency bands related to communication and radar from the spectrum of electromagnetic waves.
Optical fiber communication uses light waves, in addition to radio waves. There are medium waves in the radio wave band, which are used by early radios, as well as short-wave, AM, FM, and VHF TV channels. The most important one is microwave, which is the most important frequency band for communication and radar. International organizations divide the radio frequency band into many channels, and even specify the channels used by military equipment, otherwise they will interfere with each other, so military equipment, civilian equipment, satellite, television, etc., each have a well-defined channel. Space communication has channels up and down, which are different from the channels used for terrestrial communications.
Next, talk about daily communication. The TV show needs to be sent to a remote place for transmission. It needs to be transferred on the ground. After receiving the signal, a transfer station will transmit it to another transfer station and finally to the TV station at the receiving place. It can also be sent via satellite. Go further afield. Transoceanic telephones and telecommunications are also sent out by satellites, all using microwaves.
When it comes to the defense system, this is of course absolutely important. Take the United States as an example. The global US military is under the command of 24 hours. There are warships, planes, and tanks scattered in different corners of the earth. They are connected together by satellite communication. In addition, each warship has a variety of radar and communication equipment, hundreds of microwave transmitters, dozens of new fighters, and a variety of functions, including communication, detection , navigation, interference, fire control, etc.
For example, in-flight missiles, in order to hit the target, require radar navigation. The combat aircraft must emit signals to interfere with the enemy's radar, so that the enemy's radar can't grasp its position. Smarter can even send a deception signal to the enemy. The square radar mistaken its position and wasted a missile. The communication between the aircraft and the headquarters is also dependent on microwaves, and other like the chariots, etc., are similar situations.
So the impact of microwaves on us is very large. Warships and fighters protect us. It is an indirect influence. Ground communication is a direct influence. Now almost everyone is calling a mobile phone, that is, microwave is helping us.
Domestic related microwave researchLet's talk about a few practical examples related to China. The author specializes in high-power microwaves at Tsinghua University, and there are very few teams conducting research in this area in China, so I use the example of Tsinghua University. Starting from the generation of microwaves, Tsinghua University's "High Frequency Electromagnetic Laboratory" cooperated with the Chinese Academy of Sciences to develop microwave transmitters. After years of efforts, a series of microwave emitters were produced. Some of the technologies we develop must be built from scratch.
For example, to generate microwaves, first, an electron gun is used to generate a stream of electrons, and then the energy of the stream is converted into energy of an electromagnetic wave. The electron gun is the core component of the microwave transmitter. After the computer simulation design, the engineering design is required, followed by precision machining, manufacturing various parts, and then welding. The manufacturing process of other various components is the same. Finally, all the components are welded into one emitter, and the contact surface is more than 100. The welding is performed again and again at different temperatures. Once an error occurs, the previous work is abandoned. After the finished product is manufactured, it is tested with a high-voltage power supply. If it is not qualified, it will be abandoned.
The process requires repeated soldering because the emitter must maintain a high vacuum to prevent electrons from colliding with the gas, just like a vacuum tube, so this high power microwave emitter is often referred to simply as a "microwave tube." Since the manufacture of microwave tubes is so difficult, the higher the frequency, the more difficult it is. In advanced countries, the microwave tubes of the millimeter wave band are listed as output tube products. Our research focus is on the millimeter band, so this work is very important to our defense.
All of the above mentioned are mature technologies. Microwaves began to develop during the Second World War. Now they have entered the industry and are a very successful study, but they also need constant innovation. Tsinghua University is mainly researching on the research of "magnetic rotary wave amplifier" and "single-anode magnetron gun". The magnetic rotary traveling wave amplifier developed by the company can amplify a signal by 10 million times, regardless of In terms of power, efficiency, gain or bandwidth, it has surpassed the traditional limit and brought new opportunities in application.
Advanced countries like the United States are ready to apply magnetic rotary traveling wave amplifiers to space technology. There are many space detection facilities in the United States, which have a variety of radars to detect space objects, such as enemies and their own missiles, satellites in the sky, and even space debris in the sky. The space debris is very fast. When the spacecraft is accidentally hit, it will be like a shell, but the density of the debris is not high, and the chance of being hit is not big. Fragments are large and small. To see space debris of 1 cm in size, it is necessary to use a new device such as a magnetic whir-wave amplifier.
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