In mechanical systems, resonance is a phenomenon that occurs when the frequency at which a force is periodically applied is equal or nearly equal to one of the natural frequencies of the system on which it acts. This causes the system to oscillate with larger amplitude than when the force is applied at other frequencies. Frequencies at which the response amplitude is a relative maximum are known as resonant frequencies or resonance frequencies of the system. Near resonant frequencies, small periodic forces have the ability to produce large amplitude oscillations, due to the storage of vibrational energy. In other systems, such as electrical or optical, phenomena occur which are described as resonance but depend on interaction between different aspects of the system, not on an external driver. For example, electrical resonance occurs in a circuit with capacitors and inductors because the collapsing magnetic field of the inductor generates an electric current in its windings that charges the capacitor, the discharging capacitor provides an electric current that builds the magnetic field in the inductor.
Once the circuit is charged, the oscillation is self-sustaining, there is no external periodic driving action. This is analogous to a mechanical pendulum, where mechanical energy is converted back and forth between kinetic and potential, both systems are forms of simple harmonic oscillators. In optical cavities, light confined in the cavity reflects forth multiple times; this produces standing waves, only certain patterns and frequencies of radiation are sustained, due to the effects of interference, while the others are suppressed by destructive interference. Once the light enters the cavity, the oscillation is self-sustaining, there is no external periodic driving action; some behavior is mistaken for resonance but instead is a form of self-oscillation, such as aeroelastic flutter, speed wobble, or Hunting oscillation. In these cases, the external energy source does not oscillate, but the components of the system interact with each other in a periodic fashion. Resonance occurs when a system is able to store and transfer energy between two or more different storage modes.
However, there are some losses from cycle to cycle, called damping. When damping is small, the resonant frequency is equal to the natural frequency of the system, a frequency of unforced vibrations; some systems have multiple, resonant frequencies. Resonance phenomena occur with all types of vibrations or waves: there is mechanical resonance, acoustic resonance, electromagnetic resonance, nuclear magnetic resonance, electron spin resonance and resonance of quantum wave functions. Resonant systems can be used to generate vibrations of a specific frequency, or pick out specific frequencies from a complex vibration containing many frequencies; the term resonance originates from the field of acoustics observed in musical instruments, e.g. when strings started to vibrate and to produce sound without direct excitation by the player. A familiar example is a playground swing. Pushing a person in a swing in time with the natural interval of the swing makes the swing go higher and higher, while attempts to push the swing at a faster or slower tempo produce smaller arcs.
This is because the energy the swing absorbs is maximized when the pushes match the swing's natural oscillations. Resonance occurs in nature, is exploited in many manmade devices, it is the mechanism by which all sinusoidal waves and vibrations are generated. Many sounds we hear, such as when hard objects of metal, glass, or wood are struck, are caused by brief resonant vibrations in the object. Light and other short wavelength electromagnetic radiation is produced by resonance on an atomic scale, such as electrons in atoms. Other examples of resonance: Timekeeping mechanisms of modern clocks and watches, e.g. the balance wheel in a mechanical watch and the quartz crystal in a quartz watch Tidal resonance of the Bay of Fundy Acoustic resonances of musical instruments and the human vocal tract Shattering of a crystal wineglass when exposed to a musical tone of the right pitch Friction idiophones, such as making a glass object vibrate by rubbing around its rim with a fingertip Electrical resonance of tuned circuits in radios and TVs that allow radio frequencies to be selectively received Creation of coherent light by optical resonance in a laser cavity Orbital resonance as exemplified by some moons of the solar system's gas giants Material resonances in atomic scale are the basis of several spectroscopic techniques that are used in condensed matter physics Electron spin resonance Mössbauer effect Nuclear magnetic resonance The visible, rhythmic twisting that resulted in the 1940 collapse of "Galloping Gertie", the original Tacoma Narrows Bridge, is mistakenly characterized as an example of resonance phenomenon in certain textbooks.
The catastrophic vibrations that destroyed the bridge were not due to simple mechanical resonance, but to a more complicated interaction between the bridge and the winds passing through it—a phenomenon known as aeroelastic flutter, a kind of "self-sustaining vibration" as referred to in the nonlinear theory of vibrations. Robert H. Scanlan, father of bridge aerodynamics, has written an article about this misunderstanding; the rocket engines for the International Space Station are controlled by an autopilot. Ordinarily, uploaded parameters for controlling the engine control system for the Zvezda modu
Lightning is a violent and sudden electrostatic discharge where two electrically charged regions in the atmosphere temporarily equalize themselves during a thunderstorm. Lightning creates a wide range of electromagnetic radiations from the hot plasma created by the electron flow, including visible light in the form of black-body radiation. Thunder is the sound formed by the shock wave formed as gaseous molecules experience a rapid pressure increase; the three main kinds of lightning are: created either inside one thundercloud, or between two clouds, or between a cloud and the ground. The 15 recognized observational variants include "heat lightning", seen but not heard, dry lightning, which causes many forest fires, ball lightning, observed scientifically. Humans have deified lightning for millennia, lightning inspired expressions like "Bolt from the blue", "Lightning never strikes twice", "blitzkrieg" are common. In some languages, "Love at first sight" translates as "lightning strike"; the details of the charging process are still being studied by scientists, but there is general agreement on some of the basic concepts of thunderstorm electrification.
The main charging area in a thunderstorm occurs in the central part of the storm where air is moving upward and temperatures range from −15 to −25 °C, see figure to the right. At that place, the combination of temperature and rapid upward air movement produces a mixture of super-cooled cloud droplets, small ice crystals, graupel; the updraft carries the super-cooled cloud droplets and small ice crystals upward. At the same time, the graupel, larger and denser, tends to fall or be suspended in the rising air; the differences in the movement of the precipitation cause collisions to occur. When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged. See figure to the left; the updraft carries. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm; the result is that the upper part of the thunderstorm cloud becomes positively charged while the middle to lower part of the thunderstorm cloud becomes negatively charged.
The upward motions within the storm and winds at higher levels in the atmosphere tend to cause the small ice crystals in the upper part of the thunderstorm cloud to spread out horizontally some distance from thunderstorm cloud base. This part of the thunderstorm cloud is called the anvil. While this is the main charging process for the thunderstorm cloud, some of these charges can be redistributed by air movements within the storm. In addition, there is a small but important positive charge buildup near the bottom of the thunderstorm cloud due to the precipitation and warmer temperatures. A typical cloud-to-ground lightning flash culminates in the formation of an electrically conducting plasma channel through the air in excess of 5 km tall, from within the cloud to the ground's surface; the actual discharge is the final stage of a complex process. At its peak, a typical thunderstorm produces three or more strikes to the Earth per minute. Lightning occurs when warm air is mixed with colder air masses, resulting in atmospheric disturbances necessary for polarizing the atmosphere.
However, it can occur during dust storms, forest fires, volcanic eruptions, in the cold of winter, where the lightning is known as thundersnow. Hurricanes generate some lightning in the rainbands as much as 160 km from the center; the science of lightning is called fulminology, the fear of lightning is called astraphobia. Lightning is not distributed evenly around the planet. On Earth, the lightning frequency is 44 times per second, or nearly 1.4 billion flashes per year and the average duration is 0.2 seconds made up from a number of much shorter flashes of around 60 to 70 microseconds. Many factors affect the frequency, distribution and physical properties of a typical lightning flash in a particular region of the world; these factors include ground elevation, prevailing wind currents, relative humidity, proximity to warm and cold bodies of water, etc. To a certain degree, the ratio between IC, CC and CG lightning may vary by season in middle latitudes; because human beings are terrestrial and most of their possessions are on the Earth where lightning can damage or destroy them, CG lightning is the most studied and best understood of the three types though IC and CC are more common types of lightning.
Lightning's relative unpredictability limits a complete explanation of how or why it occurs after hundreds of years of scientific investigation. About 70 % of lightning occurs over land in the tropics; this occurs from both the mixture of warmer and colder air masses, as well as differences in moisture concentrations, it happens at the boundaries between them. The flow of warm ocean currents past drier land masses, such as the Gulf Stream explains the elevated frequency of lightning in the Southeast United States; because the influence of small or absent land masses in the vast stretches of the world's oceans limits the differences between these variants in the atmosphere, lightning is notably less frequent there than over larger landforms. The North and South Poles are limited in their coverage of thunderstorms and theref
Ultra low frequency
Ultra low frequency is the ITU designation for the frequency range of electromagnetic waves between 300 hertz and 3 kilohertz. In magnetosphere science and seismology, alternative definitions are given, including ranges from 1 mHz to 100 Hz, 1 mHz to 1 Hz, 10 mHz to 10 Hz. Frequencies above 3 Hz in atmospheric science are assigned to the ELF range. Many types of waves in the ULF frequency band can be observed in the magnetosphere and on the ground; these waves represent important physical processes in the near-Earth plasma environment. The speed of the ULF waves is associated with the Alfvén velocity that depends on the ambient magnetic field and plasma mass density; this band is used for communications in mines. Some monitoring stations have reported that earthquakes are sometimes preceded by a spike in ULF activity. A remarkable example of this occurred before the 1989 Loma Prieta earthquake in California, although a subsequent study indicates that this was little more than a sensor malfunction.
On December 9, 2010, geoscientists announced that the DEMETER satellite observed a dramatic increase in ULF radio waves over Haiti in the month before the magnitude 7.0 Mw 2010 earthquake. Researchers are attempting to learn more about this correlation to find out whether this method can be used as part of an early warning system for earthquakes. ULF has been used by the military for secure communications through the ground. NATO AGARD publications from the 1960s detailed many such systems, although it is possible that the published papers left a lot unsaid about what was developed secretly for defense purposes. Communications through the ground using conduction fields is known as "Earth-Mode" communications and was first used in World War I. Radio amateurs and electronics hobbyists have used this mode for limited range communications using audio power amplifiers connected to spaced electrode pairs hammered into the soil. At the receiving end, the signal is detected as a weak electric current between a further pair of electrodes.
Using weak signal reception methods with PC-based DSP filtering with narrow bandwidths, it is possible to receive signals at a range of a few kilometers with a transmitting power of 10-100 W and electrode spacing of around 10–50 m. Earth's field NMR Through the earth mine communications Voice frequency Tomislav Stimac, "Definition of frequency bands". IK1QFK Home Page. NASA live streaming ELF -> VLF Receiver Amateur Radio Below 10 kHz "G3XBM's page on Earth Mode Communication" Review of Earth Mode Communications "1966 abstract about Earth Mode Comms by Ames and Orange" Radio communications within the Earth's crust "Abstract of article by Burrows written in 1963"
Shortwave radio is radio transmission using shortwave radio frequencies. There is no official definition of the band, but the range always includes all of the high frequency band, extends from 1.7–30 MHz. Radio waves in the shortwave band can be reflected or refracted from a layer of electrically charged atoms in the atmosphere called the ionosphere. Therefore, short waves directed at an angle into the sky can be reflected back to Earth at great distances, beyond the horizon; this is called skywave or "skip" propagation. Thus shortwave radio can be used for long distance communication, in contrast to radio waves of higher frequency which travel in straight lines and are limited by the visual horizon, about 64 km. Shortwave radio is used for broadcasting of voice and music to shortwave listeners over large areas, it is used for military over-the-horizon radar, diplomatic communication, two-way international communication by amateur radio enthusiasts for hobby and emergency purposes, as well as for long distance aviation and marine communications.
The widest popular definition of the shortwave frequency interval is the ITU Region 1 definition, is the span 1.6–30 MHz, just above the medium wave band, which ends at 1.6 MHz. There are other definitions of the shortwave frequency interval: 1.71 to 30 MHz in ITU Region 2 1.8 to 30 MHz 2.3 to 30 MHz 2.3 to 26.1 MHz In Germany and Austria the ITU Region 1 shortwave radio frequency interval can be subdivided in: de:Grenzwelle: 1.605–3.8 MHz In Germany these shortwave radio frequency intervals have been seen used: the above other definitions The name "shortwave" originated during the early days of radio in the early 20th century, when the radio spectrum was considered divided into long wave, medium wave and short wave bands based on the wavelength of the radio waves. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used for radio communications; the broadcast medium wave band now extends above the 200 m/1,500 kHz limit, the amateur radio 1.8 MHz – 2.0 MHz band is the lowest-frequency band considered to be'shortwave'.
Early long distance radio telegraphy used long waves, below 300 kilohertz. The drawbacks to this system included a limited spectrum available for long distance communication, the expensive transmitters and gigantic antennas that were required, it was difficult to beam the radio wave directionally with long wave, resulting in a major loss of power over long distances. Prior to the 1920s, the shortwave frequencies above 1.5 MHz were regarded as useless for long distance communication and were designated in many countries for amateur use. Guglielmo Marconi, pioneer of radio, commissioned his assistant Charles Samuel Franklin to carry out a large scale study into the transmission characteristics of short wavelength waves and to determine their suitability for long distance transmissions. Franklin rigged up a large antenna at Poldhu Wireless Station, running on 25 kW of power. In June and July 1923, wireless transmissions were completed during nights on 97 meters from Poldhu to Marconi's yacht Elettra in the Cape Verde Islands.
In September 1924, Marconi transmitted daytime and nighttime on 32 meters from Poldhu to his yacht in Beirut. Franklin went on to refine the directional transmission, by inventing the curtain array aerial system. In July 1924, Marconi entered into contracts with the British General Post Office to install high speed shortwave telegraphy circuits from London to Australia, South Africa and Canada as the main element of the Imperial Wireless Chain; the UK-to-Canada shortwave "Beam Wireless Service" went into commercial operation on 25 October 1926. Beam Wireless Services from the UK to Australia, South Africa and India went into service in 1927. Shortwave communications began to grow in the 1920s, similar to the internet in the late 20th century. By 1928, more than half of long distance communications had moved from transoceanic cables and longwave wireless services to shortwave and the overall volume of transoceanic shortwave communications had vastly increased. Shortwave stations had cost and efficiency advantages over massive longwave wireless installations, however some commercial longwave communications stations remained in use until the 1960s.
Long distance radio circuits reduced the load on the existing transoceanic telegraph cables and hence the need for new cables, although the cables maintained their advantages of high security and a much more reliable and better quality signal than shortwave. The cable companies began to lose large sums of money in 1927, a serious financial crisis threatened the viability of cable companies that were vital to strategic British interests; the British government convened the Imperial Wireless and Cable Conference in 1928 "to examine the situation that had arisen as a result of the competition of Beam Wireless with the Cable Services". It recommended and received Government approval for all overseas cable and wireless resources of the Empire to be merged into one system controlled by a newly formed company in 1929, Imperial and International Communications Ltd; the name of the company was changed to Cable and Wireless Ltd. in 1934. Long-distance cables had a
In physics, electromagnetic radiation refers to the waves of the electromagnetic field, propagating through space, carrying electromagnetic radiant energy. It includes radio waves, infrared, ultraviolet, X-rays, gamma rays. Classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields that propagate at the speed of light, which, in a vacuum, is denoted c. In homogeneous, isotropic media, the oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave; the wavefront of electromagnetic waves emitted from a point source is a sphere. The position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic waves of different frequency are called by different names since they have different sources and effects on matter. In order of increasing frequency and decreasing wavelength these are: radio waves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
Electromagnetic waves are emitted by electrically charged particles undergoing acceleration, these waves can subsequently interact with other charged particles, exerting force on them. EM waves carry energy and angular momentum away from their source particle and can impart those quantities to matter with which they interact. Electromagnetic radiation is associated with those EM waves that are free to propagate themselves without the continuing influence of the moving charges that produced them, because they have achieved sufficient distance from those charges. Thus, EMR is sometimes referred to as the far field. In this language, the near field refers to EM fields near the charges and current that directly produced them electromagnetic induction and electrostatic induction phenomena. In quantum mechanics, an alternate way of viewing EMR is that it consists of photons, uncharged elementary particles with zero rest mass which are the quanta of the electromagnetic force, responsible for all electromagnetic interactions.
Quantum electrodynamics is the theory of. Quantum effects provide additional sources of EMR, such as the transition of electrons to lower energy levels in an atom and black-body radiation; the energy of an individual photon is greater for photons of higher frequency. This relationship is given by Planck's equation E = hν, where E is the energy per photon, ν is the frequency of the photon, h is Planck's constant. A single gamma ray photon, for example, might carry ~100,000 times the energy of a single photon of visible light; the effects of EMR upon chemical compounds and biological organisms depend both upon the radiation's power and its frequency. EMR of visible or lower frequencies is called non-ionizing radiation, because its photons do not individually have enough energy to ionize atoms or molecules or break chemical bonds; the effects of these radiations on chemical systems and living tissue are caused by heating effects from the combined energy transfer of many photons. In contrast, high frequency ultraviolet, X-rays and gamma rays are called ionizing radiation, since individual photons of such high frequency have enough energy to ionize molecules or break chemical bonds.
These radiations have the ability to cause chemical reactions and damage living cells beyond that resulting from simple heating, can be a health hazard. James Clerk Maxwell derived a wave form of the electric and magnetic equations, thus uncovering the wave-like nature of electric and magnetic fields and their symmetry; because the speed of EM waves predicted by the wave equation coincided with the measured speed of light, Maxwell concluded that light itself is an EM wave. Maxwell's equations were confirmed by Heinrich Hertz through experiments with radio waves. According to Maxwell's equations, a spatially varying electric field is always associated with a magnetic field that changes over time. A spatially varying magnetic field is associated with specific changes over time in the electric field. In an electromagnetic wave, the changes in the electric field are always accompanied by a wave in the magnetic field in one direction, vice versa; this relationship between the two occurs without either type of field causing the other.
In fact, magnetic fields can be viewed as electric fields in another frame of reference, electric fields can be viewed as magnetic fields in another frame of reference, but they have equal significance as physics is the same in all frames of reference, so the close relationship between space and time changes here is more than an analogy. Together, these fields form a propagating electromagnetic wave, which moves out into space and need never again interact with the source; the distant EM field formed in this way by the acceleration of a charge carries energy with it that "radiates" away through space, hence the term. Maxwell's equations established that some charges and currents produce a local type of electromagnetic field near them that does not have the behaviour of EMR. Currents directly produce a magnetic field, but it is of a magnetic dipole type that dies out with distance from the current. In a similar manner, moving charges pushed apart in a conductor by a changing electrical potential produce an electric dipole type electric
In telecommunications, an atmospheric duct is a horizontal layer in the lower atmosphere in which the vertical refractive index gradients are such that radio signals are guided or ducted, tend to follow the curvature of the Earth, experience less attenuation in the ducts than they would if the ducts were not present. The duct acts as an atmospheric dielectric waveguide and limits the spread of the wavefront to only the horizontal dimension. Atmospheric ducting is a mode of propagation of electromagnetic radiation in the lower layers of Earth’s atmosphere, where the waves are bent by atmospheric refraction. In over-the-horizon radar, ducting causes part of the radiated and target-reflection energy of a radar system to be guided over distances far greater than the normal radar range, it causes long distance propagation of radio signals in bands that would be limited to line of sight. Radio "ground waves" propagate along the surface as creeping waves; that is, they are only diffracted around the curvature of the earth.
This is one reason. The best known exception is; the reduced refractive index due to lower densities at the higher altitudes in the Earth's atmosphere bends the signals back toward the Earth. Signals in a higher refractive index layer, i.e. duct, tend to remain in that layer because of the reflection and refraction encountered at the boundary with a lower refractive index material. In some weather conditions, such as inversion layers, density changes so that waves are guided around the curvature of the earth at constant altitude. Phenomena of atmospheric optics related to atmospheric ducting include the green flash, Fata Morgana, superior mirage, mock mirage of astronomical objects and the Novaya Zemlya effect. Sky wave Thermal fade Temperature inversion Tropospheric ducting Earth-Ionosphere waveguide
Medium frequency is the ITU designation for radio frequencies in the range of 300 kilohertz to 3 megahertz. Part of this band is the medium wave AM broadcast band; the MF band is known as the hectometer band as the wavelengths range from ten to one hectometer. Frequencies below MF are denoted low frequency, while the first band of higher frequencies is known as high frequency. MF is used for AM radio broadcasting, navigational radio beacons, maritime ship-to-shore communication, transoceanic air traffic control. Radio waves at MF wavelengths propagate via reflection from the ionosphere. Ground waves follow the contour of the Earth. At these wavelengths they can bend over hills, travel beyond the visual horizon, although they may be blocked by mountain ranges. Typical MF radio stations can cover a radius of several hundred miles from the transmitter, with longer distances over water and damp earth. MF broadcasting stations use ground waves to cover their listening areas. MF waves can travel longer distances via skywave propagation, in which radio waves radiated at an angle into the sky are reflected back to Earth by layers of charged particles in the ionosphere, the E and F layers.
However at certain times the D layer can be electronically noisy and absorb MF radio waves, interfering with skywave propagation. This happens when the ionosphere is ionised, such as during the day, in summer and at times of high solar activity, At night in winter months and at times of low solar activity, the ionospheric D layer can disappear; when this happens, MF radio waves can be received hundreds or thousands of miles away as the signal will be refracted by the remaining F layer. This can be useful for long-distance communication, but can interfere with local stations. Due to the limited number of available channels in the MW broadcast band, the same frequencies are re-allocated to different broadcasting stations several hundred miles apart. On nights of good skywave propagation, the signals of distant stations may reflect off the ionosphere and interfere with the signals of local stations on the same frequency; the North American Regional Broadcasting Agreement sets aside certain channels for nighttime use over extended service areas via skywave by a few specially licensed AM broadcasting stations.
These channels are called clear channels, the stations, called clear-channel stations, are required to broadcast at higher powers of 10 to 50 kW. A major use of these frequencies is AM broadcasting. Although these are medium frequencies, 120 meters is treated as one of the shortwave bands. There are a number of coast guard and other ship-to-shore frequencies in use between 1600 and 2850 kHz; these include, as examples, the French MRCC on 1696 kHz and 2677 kHz, Stornoway Coastguard on 1743 kHz, the US Coastguard on 2670 kHz and Madeira on 2843 kHz. RN Northwood in England broadcasts Weather Fax data on 2618.5 kHz. Non-directional navigational radio beacons for maritime and aircraft navigation occupy a band from 190 to 435 kHz, which overlaps from the LF into the bottom part of the MF band. 2182 kHz is the international distress frequency for SSB maritime voice communication. It is analogous to Channel 16 on the marine VHF band. 500 kHz was for many years the maritime distress and emergency frequency, there are more NDBs between 510 and 530 kHz.
Navtex, part of the current Global Maritime Distress Safety System occupies 518 kHz and 490 kHz for important digital text broadcasts. Lastly, there are aeronautical and other mobile SSB bands from 2850 kHz to 3500 kHz, crossing the boundary from the MF band into the HF radio band. An amateur radio band known as 160 meters or ` top-band' is between 2000 kHz. Amateur operators transmit digital signals and SSB voice signals on this band. Following World Radiocommunication Conference 2012, the amateur service received a new allocation between 472 and 479 kHz for narrow band modes and secondary service, after extensive propagation and compatibility studies made by the ARRL 600 meters Experiment Group and their partners around the world. In recent years, some limited amateur radio operation has been allowed in the region of 500 kHz in the US, UK, Germany and Sweden. Many home-portable or cordless telephones those that were designed in the 1980s, transmit low power FM audio signals between the table-top base unit and the handset on frequencies in the range 1600 to 1800 kHz.
Transmitting antennas used on this band include monopole mast radiators, top-loaded wire monopole antennas such as the inverted-L and T antennas, wire dipole antennas. Ground wave propagation, the most used type at these frequencies, requires vertically polarized antennas like monopoles; the most common transmitting antenna, the quarter wave monopole, is physically large at these frequencies, 25 to 250 metres requiring a tall radio mast. The metal mast itself is used as the antenna, is mounted on a large porcelain insulator to isolate it from the ground; the monopole antenna if electrically short requires a good, l