? Warning radar, guidance radar, artillery aiming radar, airborne fire control radar, altimeter radar, blind descent radar, terrain avoidance radar, terrain tracking radar, imaging radar, weather radar, etc.
? Conical scanning radar, monopulse radar, passive phased array radar, active phased array radar, pulse compression radar, frequency agile radar, MTI radar, MTD radar, PD radar, synthetic aperture radar, noise radar, impact radar, bistatic/multistatic radar, sky/ground wave over-the-horizon radar, etc.
? Meter wave radar, decimeter wave radar, centimeter wave radar, millimeter wave radar, laser/infrared radar, etc.
? Two-coordinate radar, three-coordinate radar, speed measuring radar, height measuring radar, guidance radar, etc.
? Everyone knows that bats hide in dark places during the day and fly out in search of food at night. However, why can it fly in the dark without hitting obstacles, such as trees or houses? For thousands of years, people have said that this is because its eyes are afraid of light, but it can see in the dark. In fact, this speculation is not quite correct. Let's uncover the secret of bats. Bats' mouths can make sounds that we can't hear. This kind of sound oscillates between 25,000 and 70,000 times per second (the number of vibrations that the human ear can feel per second is about16 to 20,000 times), which is beyond the range that the human ear can feel, so it is called ultrasonic. Bats have special hearing organs. It can feel this ultrasonic wave. When it flies in the dark, its mouth often emits ultrasonic waves. When the sound wave meets an obstacle in a certain direction, it immediately reflects back from that direction, and part of it is reflected to the bat's ear, so it knows that there is an obstacle in that direction and avoids it in time. With experience, it can also know that the echo is urgent, the obstacle is near, the echo is slow and the obstacle is far away. In other words, it judges the distance of obstacles according to the speed of echo and the direction of obstacles according to the direction of echo.
? Ultrasonic wave belongs to mechanical wave, and its inherent reflection characteristics can be used to measure distance. Similarly, sound waves, water waves and even electromagnetic waves (including radio waves, infrared rays, visible rays, ultraviolet rays, roentgen rays and gamma rays), which are both mechanical waves, have reflection characteristics in certain circumstances, and radar uses electromagnetic waves.
? The speed of electromagnetic waves is many times larger than that of sound waves, and the speed is 300,000 km/s. I want to know how fast this speed is. For example, electromagnetic waves can circle the earth seven and a half times a second. Another interesting example is that if you listen to singing in a recording studio, before you hear the singer's voice, the voice has spread to a considerable distance through the radio. A listener who is 0/000 kilometers away from you/kloc-can even hear beautiful songs in front of you on the radio.
? The so-called directional emission is to emit pulsed electromagnetic energy with a narrow beam at a specific moment. It has an important advantage that it can save energy. The radar station can "see" far with small power, and the effect is good. For example, a person reading in a big room at night does not need to light up the whole big room with a lamp. He only needs a desk lamp to shine light on the book, and even a few light bulbs are enough to avoid hurting his eyes. It is true that the rest of the room is dark at this time, but this does not hinder reading. On the contrary, it will make us see the book more clearly.
? The same is true of radar work. Because it concentrates energy in a small space, it is the best way to "illuminate" the target.
? How narrow should the radar beam be? The narrower the beam, the greater the energy concentrated in it, and the more correct the direction of finding the target. Suppose an enemy plane appears in the area illuminated by radar. How long does it take to scan the space around the radar station with a narrow needle beam? At the same time, can we keep the enemy from escaping? The time spent must be a lot. During this period, any aircraft can escape from the radar search area. In other words, the "needle" bundle is not suitable. There is a reasonable limit for beam narrowing, which is to meet all kinds of requirements for radar to the same extent. For radar stations with different tasks, the methods to solve this problem are absolutely different. The beam of air defense sentry radar station should be wide, and the radar station aiming at artillery should be narrow. In many cases, in order to meet the needs of special tasks of radar stations, beams often have special shapes.
? From this point of view, the basic principle of radar is the directional emission of electromagnetic waves, the reflection of electromagnetic waves on conductors, and the first correct understanding of the propagation speed of electric waves. Obviously, a deep understanding and mastery of radar technology is an indispensable foundation.
? Usually, radio broadcasts use medium wave or short wave. Radar generally works in ultrashort wave or microwave. Radar working in ultrashort wave band is called ultrashort wave radar or meter wave radar; Radar working in microwave band is generally called microwave radar. Microwave radar is sometimes subdivided into decimeter wave radar, centimeter wave radar and millimeter wave radar.
? So, why can't radar work on medium wave or short wave like radio? This is determined by the working principle of radar. Clairvoyance can only "see" the target by the reflection of electromagnetic waves on the target. There is a law of wave reflection: the bigger the target, the stronger the reflection. Therefore, the shorter the wavelength of electromagnetic wave used by radar, the stronger the reflection on other targets such as aircraft or missiles. Therefore, radar must work in ultrashort wave or microwave band to effectively play its role in detecting targets. At the same time, radar antenna is an important part of radar. If the radar works in the medium wave band, in order to realize directional emission, dozens or even hundreds of metal rods that can excite electromagnetic waves must be arranged into an array antenna, which will be a very large antenna, which is neither economical nor feasible in practical application. Therefore, the working wavelength of radar should not be too long.
? If the radar wants to detect the target, it must have electromagnetic waves. The tool that can excite electromagnetic waves in space in radar is vibrator, which is actually a metal bar.
? The process of electrons bouncing back and forth in a metal bar is called electric oscillation. If there is no resistance in the process of bouncing, this kind of bouncing will continue. The electron flow oscillates from the left end of the metal bar to the right end, and then from the right end back to the left end, which is called oscillation period, and the number of cycles per second is called oscillation frequency.
? The electron flow on the metal rod is extremely fast, close to the speed of light, and it is constant. Therefore, the longer the metal is, the longer the electron current oscillates back and forth, and the lower the oscillation frequency. In an oscillation period, the distance that the electron flows through is the wavelength. Obviously, during this time, the distance traveled by the electron flow is exactly twice the length of the metal bar. In other words, the length of a metal rod is exactly half the wavelength. Therefore, this kind of metal rod is often called a half-wave oscillator.
? The high-frequency electric oscillation of the electron flow on the half-wave oscillator will excite electromagnetic waves with the same frequency in space, leave the oscillator at the speed of light and fly in all directions; Half-wave oscillator is a device for radar to emit electromagnetic waves into space. It is equivalent to a wooden block or a flashlight bulb that is stirred in water, and plays the role of stirring water waves in water or emitting light waves into space. Because a half-wave oscillator can emit electromagnetic waves into space, it is sometimes called a radiator.
? The back-and-forth oscillation of electron flow in a half-wave oscillator will encounter resistance. If it is not provided with energy to overcome all kinds of resistance, this oscillation will soon stop. So there must be a machine in the radar that can drive the oscillation of the electron flow on the half-wave oscillator and force it according to our needs. This machine is called radar transmitter. It is the energy source of half-wave oscillator, which is equivalent to the battery in flashlight.
? The radar transmitter provides high-frequency electric oscillation energy to the half-wave oscillator, which excites electromagnetic waves in space. Once the radar transmitter is turned off, the half-wave oscillator will stop emitting electromagnetic waves into space. So by controlling the switch of the transmitter, you can control the emission of electromagnetic waves into space.
? With a transmitter and a half-wave oscillator, you can send radio waves into space. However, the radio waves emitted in this way cannot be used to search and detect targets. Because it emits radio waves in all directions in space. These radio waves hit the target from all directions and reflect back together, so there is no way to know which target is in which direction.
? How to make the radar emit radio waves in only one direction? We know that if the cover and reflective bowl around the flashlight bulb are removed, the light emitted by the bare small bulb will have no direction. With the reflective bowl and cover, the light will only be emitted in one direction, and the reflective bowl plays a role in gathering light waves.
? The method of radar directional emission of radio waves is the same as that of flashlight focusing. That is, instead of letting the half-wave oscillator directly emit radio waves into space, let it first emit radio waves to a reflector like a cauldron, and the radio waves reflected by the reflector will only be emitted in one direction. This kind of reflector like a cauldron is called a parabolic reflector.
? The size of the reflector is closely related to the wavelength of radio waves. The shorter the wavelength, the smaller the reflector can be; The longer the wavelength, the larger the reflector. Otherwise, it will have a bad influence on the concentration of radio waves. Of course, at the same wavelength, the larger the reflector, the better the concentration of electric waves.
? Half-wave oscillator (radiator) and pot reflector are combined into one, which is called radar antenna. This kind of radar antenna is specially called parabolic antenna.
? For microwave radar with the wavelength of 10cm, the length of the half-wave oscillator is 5cm, and the diameter of the parabolic reflector must reach about 9m, so that the emitted radio waves can have sufficient directivity. For a meter-wave radar with a wavelength of 3m, the length of its half-wave oscillator is1.5m.. If calculated in proportion, there must be at least one cauldron with a diameter of 270m to make the emitted radio waves have sufficient directivity. This is obviously not practical. Therefore, for meter-wave radar, it is necessary to find another way to realize the directional emission of electric waves.
? Through practice, it is found that directional emission can also be realized by arranging dozens or even hundreds of half-wave oscillators according to certain rules. Moreover, the more the number of half-wave oscillators, the better the directivity.
? Under the same directional emission performance, the shorter the working wavelength of the radar, the smaller the size of the radar antenna. But we can't go to the other extreme, saying that the shorter the working wavelength of radar, the better. If the wavelength is long, it will be difficult; Too short also has disadvantages. Radio waves with too short wavelength will suffer great losses when propagating in the atmosphere. Therefore, it will not spread far. Therefore, the working wavelength of radar can not be too long or too short, and generally works in ultrashort wave or microwave band.
? Before the radar antenna has time to turn from one direction to another, the radio waves emitted from the target have returned to the radar antenna. In order to know the information of the target (its position, height, distance, etc. Judging from these reflected radio waves, we need something like bat ears. On radar, this part is called radar receiver, which is a particularly sensitive "ear". In order to make the detection distance of radar as far as possible, the power of radar transmitter is very large. However, the power of radio waves reflected from distant targets is extremely small. The radio wave of 1000Kw is emitted. When it reaches the fighter plane 500km away, the power of the radio wave reflected from it is only a tiny part of the emission power. When the reflected radio waves return to the radar antenna and enter the radar receiver, the reflected radio waves are even smaller. Its power is less than 1 picowatt.
? The radio wave signal reflected from distant targets is very weak, and it usually needs to be amplified by millions of times to be observed on the radar display screen. This task of magnifying millions of times will be completed by the radar receiver.
? The radar receiver is exactly the same as the ordinary superheterodyne radio in principle. The only difference is that it does not receive medium-wave or short-wave radio broadcast signals, but receives ultra-short-wave or microwave radar signals reflected by the target.
? Because the working frequency of radar is too high, it is not easy to directly amplify such a high-frequency signal by millions of times. Therefore, after the signal enters the receiver, it must first change its frequency, that is, from the higher ultrashort wave or microwave band to the lower intermediate frequency, which is called frequency conversion. The down-converted signal is amplified again and again by a multistage amplifier composed of transistors or electron tubes, so it is easier to be amplified by millions of times. This kind of receiver after frequency conversion and amplification is called superheterodyne receiver.
? The terminal equipment for real-time automatic display of radar information is man-machine interface. Radar display usually displays the information contained in radar echo in the form of radar image, which is convenient for operators to understand and operate. The traditional radar image is the original radar video directly output by the receiver or the radar video image after signal processing. It's called showing. The radar data processed by computer or the radar image displayed by comprehensive video is called secondary display. The display can have two display modes at the same time. Radar images can be inserted with various marker signals, such as distance markers, angle markers, gates, etc. You can even insert or project the superimposed map background as an auxiliary observation means. In order to record the target signal or select data, digital data, marks or symbols can be inserted into the radar image. The radar display can also comprehensively display information from other radar stations or information sources, and add other status and commands. , as a command and control display. The display console connected to the computer often uses keyboard, light pen, trackball and even voice input devices as input devices for man-machine dialogue.