Radio waves are defined asfree space(including air and vacuum)radio frequencyBandelectromagnetic wave。The shorter the wavelength and the higher the frequency of the radio wave, the more information will be transmitted in the same time.
electromagnetic waveOne of.The electromagnetic wave with a frequency below 30000000KHz (300GHz) or a wavelength greater than 1mm is called radio wave because it is generated by the alternating current of the oscillation circuit and can be transmitted and absorbed by the antenna.[1][4]
There are many kinds of electromagnetic waves, which are in the order of frequency from low to high: radio wavesinfrared、visible light、ultraviolet rays、X-rayandGamma ray。The radio waves are distributed in the frequency range below 300GHz.
Radio waves in different bands have different propagation characteristics.
The lower the frequency, the propagationlossThe smaller the coverage distance, the farther the coverage distance,diffractionThe stronger the ability.However, the frequency resources in the low frequency band are tight, and the system capacity is limited, so the radio waves in the low frequency band are mainly used in broadcasting, television, paging and other systems.
High frequency resources are abundant and the system capacity is large.However, the higher the frequency, the greater the propagation loss, the closer the coverage distance, and the weaker the diffraction ability.In addition, the higher the frequency, the greater the technical difficulty, and the cost of the system will increase accordingly.
The coverage effect and capacity shall be comprehensively considered when the mobile communication system selects the frequency band.Compared with other bands, UHF band has a good compromise between coverage effect and capacity, so it is widely used in mobile communication field of mobile phones and other terminals.Of course, with the increasing demand for mobile communication and the increasing capacity required, mobile communication systems will inevitably develop to high frequency band.
The speed of radio waves varies only with the electrical and magnetic properties of the propagating medium.The speed of radio wave propagation in vacuum is equal to the speed of light propagation in vacuum, because both radio wave and light belong to electromagnetic wave.The speed of radio wave propagation in other media is V ε=C/sqrt (ε).Where ε is the dielectric constant of the propagation medium.The dielectric constant of air is very close to that of vacuum, slightly greater than 1, so the propagation speed of radio waves in air is slightly less than the speed of light, which is generally considered to be equal to the speed of light.
Radio wave propagation
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The propagation mode of radio waves
For free space, because there is no barrier in the free space, the radio wave propagation is only direct, and there is no other phenomenon.
For the actual propagation environment in daily life, due to the existence of various objects on the ground, the propagation of radio waves includes direct reflection, reflection, diffraction (diffraction), etc. In addition, for indoor or train users, there are also some signals from the penetration of radio waves through buildings.All these have resulted in the diversity and complexity of radio wave propagation, which has increased the difficulty of radio wave propagation research.
Direct projection
Direct radiation within sight distance can be regarded as radio wave propagation in free space.The propagation loss formula of direct wave is the same as the path loss formula in free space: PL=32.44+20lgf+20lgd.Where, PL is the road loss in free space, in dB.F is the carrier frequency, in MHz.D is the distance between the transmitting source and the receiving point, in km.
Reflection, refraction and penetration
stayElectromagnetic wave propagationWhen encountering an obstacle in the process, when the size of the obstacle is far larger than the wavelength of the electromagnetic wave, the electromagnetic wave will be reflected and refracted at the junction of different media.In addition, the medium properties of obstacles will also affect reflection.For good conductors, reflection will not bring attenuation;For insulator, it only reflects part of the incident energy, and the rest is refracted into a new medium to continue propagation;For non ideal medium, electromagnetic wave penetrates through the medium, that is, when penetrating, the medium will absorb the energy of electromagnetic wave and produce penetration fading.The size of penetration loss is related not only to the frequency of electromagnetic wave, but also to the material and size of the penetrating object.
Generally, the indoor radio wave signal is the superposition of the penetration component and the diffraction component, and the diffraction component accounts for the majority.Therefore, in general, the indoor and outdoor level difference of high-frequency signal (e.g. 1800MHz) is larger than that of low-frequency signal (800MHz).In addition, after low-frequency signals enter the room, due to the poor penetration ability, the field strength distribution is more uniform after various indoor reflections;However, after high-frequency signals enter the room, some of them penetrate and go out, so the indoor signal distribution is not uniform, which makes users feel that the signal fluctuates greatly.
Diffraction
stayElectromagnetic wave propagationWhen encountering an obstacle in the process, when the size of the obstacle is close to the wavelength of the electromagnetic wave, the electromagnetic wave can be diffracted from the edge of the object.Diffraction can help cover shadow areas.
scattering
When encountering obstacles in the process of electromagnetic wave propagation, the size of the obstacles is smaller than the wavelength of the electromagnetic wave, and the number of such obstacles in the unit volume is very large, scattering will occur.Scattering occurs on the surface of rough objects, small objects or other irregular objects, such as leaves, street signs and lamp posts.
The general form of radio wave line of sight propagation is mainly the superposition of direct wave and ground reflection wave, which may strengthen the signal or weaken the signal.
Since the earth is spherical, there is a limit distance Rmax for line of sight propagation affected by the radius of curvature of the earth, which is affected by the height of the transmitting antenna, the height of the receiving antenna and the radius of the earth.
NLOS propagation
The general forms of non line of sight propagation of radio waves include diffracted waves, tropospheric reflected waves and ionospheric reflected waves.
① Diffracted wave
Diffraction wave is the main source of signals inside buildings or in shaded areas.The intensity of the diffracted wave is greatly affected by the propagation environment, and the higher the frequency, the weaker the diffracted signal.
Tropospheric reflected waves originate in the troposphere.The troposphere is a heterogeneous medium, which changes with time due to weather conditions.Its reflection coefficient decreases with height.This slowly changing reflection coefficient bends the radio wave.Tropospheric reflection mode is applied to wireless communication with wavelength less than 10 meters (i.e. frequency greater than 30 MHz).Tropospheric reflection wave has great randomness.
③ Ionospheric reflection
When the radio wave wavelength is greater than 1 m (that is, the frequency is less than 300 MHz), the ionosphere is a reflector.The radio waves reflected from the ionosphere may have one or more jumps, so this kind of propagation is used for long-distance communication. Like the troposphere, the ionosphere is also characterized by continuous fluctuations.
radio waves
Characteristics of radio waves in land mobile communication environment
Complexity of communication environment
Because the antenna height of mobile terminal is relatively low, the propagation path is always affected by terrain and human environment, which makes a large number of received signals scattered, reflected or superimposed.
The complexity of communication environment is reflected in the diversity of terrain, man-made buildings and man-made interference.For example, in the terrain with trees around, the leaves will cause a lot of scattering of radio waves.For the urban environment, the wave guide effect caused by tall buildings on both sides of the street makes the signal along the propagation direction on the street strengthen and the signal perpendicular to the propagation direction weaken. The difference between the two can reach about 10dB.In addition, the ignition of motor vehicles, power lines, industry and others will interfere with the received signal.
Random mobility of mobile terminals
The mobile terminal is always moving. Even if the mobile terminal is not moving, the surrounding environment is changing all the time, such as the movement of people and vehicles, and the wind blowing leaves, which makes the propagation path between the base station and the mobile terminal constantly change.And the change of mobile terminal's moving direction and speed relative to the base station will lead to the change of signal level, which can only be described by the probability distribution of random process.
Openness of communication
The openness of radio wave propagation space leads to serious spatial interference.Common interference includes co frequency interference, adjacent frequency interference, intermodulation interference, etc.With the increase of frequency reuse coefficient, co adjacent frequency interference will become the main interference.
Communication classification
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There are three propagation modes of radio waves from the transmitting place to the receiving place: sky wave, ground wave and space straight wave. The characteristics of each wave are as follows:
Ground wave: radio waves propagating along the earth's surface, called ground waves.In the process of propagation, because the radio wave is absorbed by the ground, its propagation distance is not far.The higher the frequency, the greater the ground absorption. Therefore, when short wave and ultrashort wave propagate along the ground, the distance is relatively short, generally not more than 100 km, while medium wave propagation distance is relatively long.The advantages are less affected by climate, stable signal and high communication reliability.[1]
Sky wave: The radio wave that is reflected and propagated by the ionosphere in the atmosphere is called sky wave, also called ionospheric reflection wave.The transmitted radio waves are reflected by the ionosphere more than 70-80 kilometers from the ground to the receiving place, and their propagation distance is relatively long, generally more than 1000 kilometers.The disadvantage is that it is greatly affected by the ionospheric climate and the transmission signal is very unstable.Shortwave band is the best frequency band for skywave propagation, and fishing vessels are equipped withShort wave single sideband radio stationIt is a device for long-distance communication by means of skywave propagation.[1]
Space linear wave: the radio wave that propagates in a straight line from the transmitting place to the receiving place in space, called space linear wave, also called linear wave or line of sight wave.The transmission distance is within the sight distance, only tens of kilometers.The interphones and radars equipped on fishing vessels are all devices that communicate by means of space wave propagation.[1]
transverse wave
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Radio waves are shear waves.That is, the direction of the electric field and magnetic field is perpendicular to the propagation direction of the wave. When the radio wave propagates in space, it must be affected by the atmosphere, especially the ionosphere, which causes the radio wave to refract and attenuate. The larger the wavelength, the greater the refraction and attenuation.
According to different propagation characteristics of radio wave wavelength, different communication services use different wave bands. For example, long wave is used for navigation and fixed services;Medium and short wave for mobile services;Microwave is used for radio astronomy and space communication.[2]
Radio wave characteristics
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The fading characteristics of radio waves
The fading of radio waves in the process of propagation is a very important characteristic of radio waves, which can be described from three scales: large, medium and small.
Large scale is used to describe the median signal (regional mean).It has the propagation characteristic of power law, that is, the median signal power is inversely proportional to a certain power of distance length increase.
Mesoscale is used to describe slow fading.It is the average power change overlapping the median level of large-scale propagation characteristics.When expressed in decibels, this change tends to normal distribution.
Small scale is used to describe fast fading.It usually obeys Rayleigh probability density function, also known as Rayleigh fading.
Doppler shift
According to the Doppler effect, due to the relative movement between the radio wave transmitting end and the receiving end, there will be a difference between the signal frequency received by the receiving end and the signal frequency sent by the transmitting end. The difference is the Doppler frequency shift.
The Doppler frequency shift conforms to the following formula:
Is the frequency after synthesis
Is the operating frequency
Is the maximum Doppler frequency
V is the maximum speed of the mobile terminal
Is the wavelength
The included angle between the propagation direction of multi-path signal synthesis and the travel direction of mobile terminal
Time dispersion and equalization
Time dispersion originates from reflection, and its reflected signal comes from objects thousands of meters away from the receiving antenna.For example, if the sequence of "1" and "0" is continuously transmitted by the base station, if the time when the remote reflection signal arrives at the mobile terminal just lags behind the direct signal by one bit, then the receiving terminal will detect "0" from the direct signal and "1" from the reflection signal, thus leading to inter symbol interference, which is called time dispersion.Adaptive equalization technology can reduce the influence of time dispersion.
Radio wave history
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maxwellThe earliest time he submitted it to Britainroyal society In his paper "Dynamic Theory of Electromagnetic Field"electromagnetic waveThe theoretical basis of communication.His work was completed between 1861 and 1865.
Christmas Eve in 1906,Fan Xinda(Reginald Fessenden) in the United StatesMassachusettsuseHeterodyne methodRealized the first radio broadcast in history.Fan Xinda broadcast that he played "Christmas Eve" and read aloud on the violin《Bible》Clip.The Marconi Research Center in Chermsford, England, launched the world's first regular radio entertainment program in 1922.
Radio wave transmitter
hertz(Heinrich Rudolf Hertz) was the first person to test and verifymaxwellThe theory of.He proved that radio radiation has all the characteristics of waves and found thatelectromagnetic fieldThe equation can be expressed by partial differential equation, usually calledwave equation 。[3]
Using radio waves to determine ship position in navigation
In 1893,Nikola Tesla (Nikola Tesla) in the United StatesMissouriSt. Louis first publicly displayedradio communication。He described and demonstrated the basic principles of radio communication in his reports for "Philadelphia Franklin College" and "National Electric Light Association".The instruments he made includeElectronic tubeAll the basic elements of a pre invention radio system.
Guglielmo Marconi has what is generally considered to be the world's first patent for radio technology, British Patent No. 12039, "Improvement of electric pulse and signal transmission technology and required equipment".
In 1897,Nikola Tesla Obtained a patent for radio technology in the United States.In 1898, Marconi opened the world's first radio factory in Hall Street, Chermsford, England, employing about 50 people.However, the United States Patent Office revoked its patent in 1904 and granted Marconi a patent for radio invention instead.This move may be affected byMarconi Economic backers in the United States, includingEdisonThe result of Andrew Carnegie's influence.In 1909, Marconi and Karl Ferdinand Braun were awarded the title of "Contribution to the Invention of Wireless Telegraph"The nobel prize in physics。
In 1943, shortly after Tesla's death,United States Supreme CourtRe recognize Tesla's patent as valid.This decision acknowledged that his invention had been completed before Marconi's patent.Some people believe that the decision was made for obvious economic reasons.In this way, the US government in World War II could avoid payingMarconiRoyalties.
Radio wave application
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Radio was first used in navigation, using Morse telegrams to transmit information between ships and land.Radio has a variety of applications, including wireless data networks, various mobile communications and radio broadcasting.
The following are some of the main applications of radio technology:
navigation
The earliest form of broadcasting was maritime radio telegraphy.It uses switches to control whether continuous wave is transmitted or not, so the signal generated in the receiver, namelyMorse code。
Propagation characteristics of radio waves
*AM broadcasting can transmit radio signals.AM broadcasting adopts amplitude modulation technology, that is, the greater the received intensity, the greater the energy transmitted by the radio station.Such signals are vulnerable to interference such as lightning or other interference sources.
*FM radio can transmit radio signals with higher fidelity than AM radio.yesfrequency modulation In terms of, the larger the bandwidth, the higher the frequency of the transmitted signal.FM radio works at very high altitudeFrequency band(Very High Frequency, VHF). The higher the frequency band, the greater the frequency bandwidth it has, so it can accommodate more radio stations.At the same time, the shorter the wavelength, the closer the propagation of radio waves is to the linear propagation of light waves.
*The sideband of FM radio can be used to transmit digital signals such as radio station logo, program name introduction, website, stock market information, etc.In some countries, when being moved to a new area, FM radio can automatically find the original channel according to the sideband information.
*Radio stations used in navigation and aviation shall apply VHF amplitude modulation technology.This allows light antennas to be used on aircraft and ships.
*Government, fire protection, police and commercial radio stations usually apply narrowband FM technology in the dedicated frequency band.These applications typically use a bandwidth of 5KHz.Compared with the bandwidth of FM radio or TV, fidelity has to be sacrificed.
*Civil or military HF services use short waves for communication between ships, aircraft or isolated locations.In most cases, the single sideband technology is used, which can save half of the frequency band compared with the AM technology, and use the transmission power more effectively.
*Terrestrial Trunked Radio (TETRA) is a digital trunked telephone system designed for special departments such as the military, police, emergency services, etc.
*Amateur radioyesRadio enthusiastParticipating radio communication.Amateur radio stationYou can use the entirespectrumThere are many open frequency bands.Enthusiasts use different forms of coding methods and techniques.Some later commercially available technologies, such as FM, SSB AM, digital packet radio and satellite signal repeaters, were first applied by amateurs.
conversation
*Cellular telephoneOr mobile phone is the most widely used wireless communication mode.The cellular telephone coverage area is usually divided into multiple cells.Each cell consists of onebase stationTransmitter coverage.In theory, the cell is shaped like a honeycomb hexagon, which is also the source of the cell phone name.Currently, the widely used mobile phone system standards include GSM, CDMA and TDMA.A few operators have begun to provide the next generation of 3G mobile communication services, and their leading standards are UMTS and CDMA2000.
*Generally, analog TV signals are transmitted in the same signal by amplitude modulation, frequency modulation and synthesis of images.
*Digital TV adoptsMPEG-2Image compression technology, thus only half of the bandwidth of analog TV signal is needed.
Emergency services
The way of radio wave propagation
*Emergency position indicating radio beacons (EPIRBs), emergency positioning transmitters or personal positioning beacons are small ones used to locate people or measurements through satellites in emergencyRadio transmitter。Its function is to provide the accurate location of the target for the rescue personnel so as to provide timely rescue.
*Active and passive radio devices can identify and indicate the identity of objects.
*Allsatellite navigation system Both use satellites equipped with accurate clocks.Navigation satellites broadcast their position and timing information.The receiver receives signals from multiple navigation satellites at the same time.Receiver passes measurementair wavesIts distance to each satellite can be obtained from its propagation time of, and then its precise position can be calculated.
*Loran system also uses the propagation time of radio waves for positioning, but its transmitting stations are all located on land.
*VOR system is usually used for flight positioning.It uses two transmitters. One directional transmitter always emits and rotates at a fixed rate like the spotlights of a lighthouse.When the directional transmitter is facing north, another omnidirectional transmitter will send pulses.The aircraft can receive signals from two VOR stations, so as to determine its position by calculating the intersection of two beams.
*Radio orientation YesRadio navigationThe earliest form of.Radio orientation uses mobileLoop antennaTo find the direction of the radio station.
*The radar calculates the distance of the target by measuring the delay of the reflected radio wave.The surface type of the target is sensed by the polarization and frequency of the reflected wave.
Multi channel transmission effect of radio waves
*Use of navigation radarultrashort waveScan the target area.Generally, the scanning frequency is two to four times per minute, and the terrain is determined by reflected waves.This technology is usually applied to merchant ships and long-distance commercial aircraft.
*Multi purpose radar usually uses the frequency band of navigation radar.However, the pulses it emits are modulated and polarized to determine the surface type of the reflector.The excellent multi-purpose radar can distinguish rainstorm, land, vehicles, etc.
*The search radar uses shortwave pulses to scan the target area, usually 2-4 times per minute.Some search radar applicationsDoppler effectA radar that can distinguish moving objects from the background to seek is similar to a search radar, but it can scan small areas quickly and repeatedly, usually several times per second.
*weather radarSimilar to search radar, but usingCircular polarizationWavelength at which water droplets are easily reflected.Some weather radars also use the Doppler effect to measure wind speed.
heating
*Microwave ovens use high-power microwaves to heat food.(Note: A common misconception is that the frequency used by microwave ovens is the resonance frequency of water molecules. In fact, the frequency used is about one tenth of the resonance frequency of water molecules.)
power
*Radio waves can produce weak electrostatic force and magnetic force.In microgravity, this can be used to fix the position of objects.
*Aerospace power: it is proposed to use high strengthmicrowave radiationThe pressure generated is used as the power of the interstellar probe.
astronomy
*By radioAstronomical telescopeThe radio wave signals received from cosmic objects can be used to study the physical and chemical properties of celestial bodies.This subject is calledRadio astronomy。
