The basic discipline to study the propagation phenomena and laws of electromagnetic waves in the earth environment and the sun earth environment, as well as the application problems
Propagation of electromagnetic wave is a basic discipline to study the propagation phenomena and laws of electromagnetic waves in the earth environment and the sun earth environment, as well as application problems.The earth environment includes the natural environment and artificial environment above and below the surface of the earth.This subject has a strong practicality, developed with the needs of human activities in information, environment and space, and has a basic nature.Many of the problems it involves are closely related to the earth's atmospheric physics and solar terrestrial physics.
In 1864, James C. Maxwell first proposed the theory of electromagnetic field. More than 20 years later, Heinrich R. Hertz's electromagnetic wave experiment was successful, which inspired people to actively explore ways to achieve wireless communication by using electromagnetic waves.Some famous scientists and mathematicians have explored the theory of ground wave propagation. For example, A. Sommerfeld established the basic theory of radio wave propagation along flat ground, B. VanderPol and W. Watson established the basic theory of radio wave propagation around conductive spherical ground.Since then, many scientists have made significant progress in the theory of propagation around the earth.Some inventors and engineers invented electronic tubes, developed radio receiving and transmitting equipment, and carried out research and experiments on ground wave propagation. They found that the ground wave field strength decayed rapidly with the increase of distance, and the higher the pause rate, the faster the attenuation, and the ground wave communication can only be relatively close.
In the first decade of the 20th century, G. Marconi conducted a successful transatlantic radio transmission and communication experiment.Some scientists have realized that there may be a layered structure composed of free electrons above the earth's atmosphere to return radio waves to the earth.In the 1920s, some scientists observed the free electron layer in the atmosphere with different methods, and measured its stratification, named it the ionosphere, which created the discipline field of ionospheric physics and the propagation of electromagnetic waves in the ionosphere, and provided a scientific basis for establishing long-distance short wave radio communication and broadcasting.
Radar remote sensing technology
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During the Second World War, both belligerents made great efforts in the research and development of radar and microwave technology, which greatly expanded the application of radio wave spectrum and raised new propagation problems.For example, if the radio wave used for radar positioning passes through the atmosphere, it will have a slight bend in range and a slight change in speed. These effects must be considered under the requirements of long distance and high accuracy.In addition to positioning, it is also required to have the function of reconnaissance target characteristics and anti-jamming capability.In order to reconnaissance target characteristics, signals with multiple frequencies or wide spectrum (including spectrum with special distribution shape) are often used, or frequency sweeping and beam scanning technologies are used to increase the amount of information, and imaging technology can be combined to make it have imaging function, such as synthetic aperture radar.Radar can also be used to detect clouds and even the surface of the moon.In addition, low frequency monopulse radar has been developed to detect underground and underwater targets and detect stealth aircraft.The development of remote sensing technology has been rapidly promoted due to the reconnaissance requirements for a series of environmental and ecological problems, such as agricultural growth, distribution of mineral resources, pollution on the sea surface, disastrous climate, etc.New radar technology and remote sensing technology have opened up a new direction problem of electromagnetic wave propagation, namely the so-called inverse problem or inversion problem.
Microwave and ultrashort wave communication
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The development of microwave and ultrashort wave technology provides various ways of communication:
① It is relatively simple and practical to use short wave ionospheric propagation for communication. However, because the ionosphere is a dispersive medium with narrow frequency band, it is not suitable for transmitting television signals and fast digital signals. When the sun flares, the ionosphere will appear sudden disturbance, which will interrupt short wave communication.Then there will be ionospheric storms, which will damage the ionization layer structure to some extent and seriously affect short wave communication.
② Because of the high microwave frequency, the troposphere is basically non dispersive, and the influence of the ionosphere on it can generally be ignored, so the frequency band can be very wide, which can transmit multiple TV signals and digital signals.Generally, microwave can only spread within the line of sight distance. In order to increase the communication distance, the segmented relay method can be used.
③ For ultrashort wave, in addition to communication within the line of sight distance, forward scattering communication can be carried out by using the random nonuniformity of refractive index generated by turbulence movement in the troposphere. The communication distance is generally between 200 and 300 km, and its bandwidth is larger than that of short wave ionospheric communication. Although there is fading, communication is often reliable, but its disadvantage is that the communication distance is small,The power used is large.
Long wave and ultra long wave communication
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The discovery of the ionosphere has made people realize that a concentric spherical waveguide is formed between the bottom of the ionosphere and the surface of the earth, and long wave and ultra long wave signals can be transmitted from one place to another.Both theory and practice have proved that the attenuation is small and the phase is quite stable except for the sunrise and sunset time. It is suitable for broadcasting standard time signals and hyperbolic navigation, and can also send commands and information from the ground using strong long wave or ultra long wave launchers to submarines that are not very deep off the sea.
Sliding Propagation
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With the development of space activities, more and more research topics on electromagnetic wave propagation have been proposed.One of the problems is how to overcome the earth's obstruction to maintain regular radio communication between the earth station and orbiters, especially manned spacecraft or space shuttles.The only direct way is to use the ionosphere to refract short wave radio waves.When the propagation distance between two points on the ground is greater than the maximum distance (about 4000km) of one hop in the ionosphere F2, it is generally believed that the propagation mode will be two or more hops, but practice has proved that the actual maximum available frequency (MUF) of the remote circuit is always higher than the MUF value predicted by the multiple hop propagation mode,Frequency prediction professionals all use an empirical method called control point method. The method is to take a point 2000km away from both ends of the line between the sending and receiving points as the control point. According to the ionosphere conditions above the two control points, predict the MUF value of 4000km one hop with the two control points as the center. The lower of the two values is the MUF of the whole circuit,No matter how much the whole circuit length exceeds 4000km.This shows two points. One is that for the whole circuit, the ionization condition above the control points at both ends of the transmitter and receiver is critical;Second, the control point method is used to ensure that there must be rays propagating along the horizontal direction in the sky where the distance between the departure and reception points is less than 2000km.In the early 1950s, Chinese scholars put forward the concept of gliding propagation.In 1961, the Soviet Union successfully launched a manned spacecraft. The spacecraft carried a transmitter with a frequency of about 20MHz for radio communication between the ground station and astronauts, which can also be explained by gliding propagation.It is assumed that the refractive index n of the ionosphere is only a function of the vector radius distance r calculated from the center of the earth, and the minimum value (nr) min of nr forms a sphere.Let a be the radius of the earth, set at a certain point on the ground as the transmitting point, and consider a ray with an elevation angle α 0 with the ground in the transmitting beam, such as
The radiation will be "reflected" by the ionosphere, such as
Here α 0 takes its critical value α 0 m, and the ray will be tangent to the (nr) min plane, and tend to continue to propagate along the arc where the plane (nr) min intersects the projection plane of the ray.However, the ray can not spread directly over the whole circle, because the ray path is very sensitive to the elevation angle α 0. As long as α 0 is slightly reduced, the ray will bend to the ground. The less the reduction, the farther the gliding distance will be.On the contrary, as long as α 0 is slightly increased, the ray will bend into space. The less the increase, the farther the gliding distance will be.In the actual transmitted beam,α 0 has rays slightly smaller and larger than the critical elevation angle α 0 m. Therefore, gliding propagation is a unified concept, which is applicable to long-distance short wave ionospheric propagation between two points on the ground and between a point on the ground and satellites or spacecraft. The difference is that for the propagation between two points on the ground, the frequency selection must be limited by the requirements of the control point method at both sending and receiving ends,The transmission between a point on the ground and a satellite spacecraft is only subject to the requirements of the control point method at one end of the launch point on the ground.The gliding propagation can reach all the distant points on the ground beyond the one hop range of F2 layer, and can also reach all the spaces below and above the (nr) min height.actually.In general, n is not only a function of r, but also a slowly varying function of the spherical angle θ and azimuth φ. We can approximately relate it to the gliding propagation: the band on the (nr) min plane is divided into several segments of (nr) min1, (nr) min2, etc. The (nr) min plane of each segment can be regarded as a segment on its corresponding spherical surface. From the total transmitted beam, there must be a very narrow beam in the first segment (nr)In the second segment (nr) min, a very narrow beam must glide and spread further, and so on.Unless there are ionospheric storms or sudden disturbances and other special circumstances, gliding propagation is always established.
Geosynchronous satellite communication
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The development of space technology has provided an advanced means of communication for realizing long-distance, broadband, multi-channel, high stability and reliability on the earth.Three geostationary satellites are evenly distributed over the equator, which can cover all areas on the earth surface except a small part around the two poles.
ionospheric sounding
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When the ionospheric short wave communication is limited to different points on the ground, only the part of the ionosphere below the maximum electron density of F2 layer needs to be understood, and the detectors used by general ionospheric observatories can only detect this part;But when it comes to radio communication with space, it is often necessary to detect the entire ionosphere.In addition to sounding rockets and spaceborne instruments, there are two means of detection: one is to use the same principle as the ground vertical detector to develop a spaceborne top vertical detector to detect the top of the ionosphere from the satellite down;The other is to use high-power ultrashort wave radar to measure the scattering echo generated by the random non-uniform structure of electronic density, and obtain the relevant parameters of the entire ionosphere through theoretical calculation.
Laser propagation in atmosphere
The laser came out in the early 1960s. Since then, coherent light sources with various frequencies and energies can be generated artificially, and many optical components, lidars and laser communication equipment corresponding to microwave have been developed.Therefore, the propagation of laser beams, especially those with high energy density, in the atmosphere and other media has attracted much attention.If the energy density of the laser beam is large enough, when it passes through the atmosphere and other objects, it will produce nonlinear effects, heating and expansion effects, and even ionization and other chemical effects.
Nuclear electromagnetic pulse
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In nuclear war, the explosion of an atomic bomb can release nuclear electromagnetic pulse with large energy and wide frequency band, and make the irradiated electronic system completely lose its function.Therefore, it is an important subject for national defense to find and destroy such atomic bombs as early as possible, to spread the spectrum and frequency bands of nuclear electromagnetic pulse, and to damage the electronic components and equipment, especially to protect them.
