In physics, surface waves are mechanical waves that propagate along the interface between different media. A common example is a gravitational wave along the liquid surface, like a sea wave. Gravitational waves may also occur in the liquid, at the interface between two liquids of different densities. Elastic surface waves may move along the surface of solid objects, such as Rayleigh or Love waves. Electromagnetic waves can also spread as "surface waves" where they can be guided along the refractive index gradient or along the interface between two media having different dielectric constants. In radio transmission, the ground wave is a guided wave spreading close to the Earth's surface.
Video Surface wave
Mechanical waves
In seismology, several types of surface waves are encountered. Surface waves, in this mechanical sense, are commonly known as "Love Waves" (wave L) or Rayleigh waves. . The seismic waves are waves that travel the Earth, often as a result of earthquakes or explosions. The wave of love has a transverse motion (movement perpendicular to the direction of travel, like light waves), whereas Rayleigh Waves have longitudinal (movement parallel to the direction of travel, like sound waves) and transverse movement. Seismic waves are studied by seismolog and measured by seismograph or seismometer. Surface waves reach a wide frequency range, and the most damaging wave periods are usually 10 seconds or longer. Surface waves can encircle the earth many times from the greatest earthquakes. Surface waves are caused when P wave and S wave appear to the surface.
Examples are waves on the surface of water and air (sea surface waves). Another example is the internal wave, which can be transmitted along the two water mass interfaces with different densities.
In theory of hearing physiology, the traveling wave (TW) of Von Bekesy, is produced from the acoustic surface wave of the basilar membrane into the cochlear ducts. His theory pretends to explain every feature of the hearing sensation due to this passive mechanical phenomenon. Jozef Zwislocki, and then David Kemp, suggests that it is unrealistic and active feedback is needed.
Maps Surface wave
Electromagnetic waves
Waves of land refers to the propagation of radio waves parallel to and adjacent to the Earth's surface, following the curvature of the Earth. This radiation ground wave is known as the surface wave of Norton . Another type of surface wave is the non-radiation Zenneck surface wave Zenneck-Sommerfeld surface wave , the trapped surface wave and the wave glide . See also Dyakonov surface wave (DSW) propagating at the interface of transparent material with different symmetry.
Radio propagation
Lower frequency radio waves, below 3 MHz, travel efficiently as ground waves. In the ITU nomenclature, this includes (in sequence): medium frequency (MF), low frequency (LF), very low frequency (VLF), ultra low frequency (ULF), super low frequency (SLF), very low frequency (ELF) waves.
Soil propagation works because low frequency waves are diffracted around obstacles because of their long wavelengths, allowing them to follow the curvature of the Earth. The earth has one refractive index and the atmosphere has one again, so it is the interface that supports guided wave transmission. Soil waves propagate in vertical polarization, with their horizontal magnetic field and a vertical (approximate) electric field. With VLF waves, the ionosphere and the earth's surface act as waveguides.
Surface conductivity affects ground wave propagation, with more conductive surfaces such as seawater providing better propagation. Increasing surface conductivity results in less dissipation. The index of refraction depends on the spatial and temporal changes. Since the soil is not a perfect electrical conductor, the ground waves are attenuated as they follow the surface of the earth. The initial wave edge is vertical, but the ground, acting as a lossy dielectric, causes a forward tilt wave as it travels. This directs some energy to the earth where it is lost, so the signal decreases exponentially.
Most long-distance LF radio communications (between 30 kHz and 300 kHz) are ground wave propagation results. Transmission of medium wave radios (frequencies between 300 kHz and 3000 kHz), including AM broadcast bands, travels both as ground waves and, for longer distances at night, as waves. Losing ground becomes lower at lower frequencies, greatly increasing the coverage of AM stations using the lower end of the band. The frequencies of VLF and LF are mostly used for military communications, especially with boats and submarines. The lower the frequency, the better the waves penetrate the sea water. ELF waves (below 3kHz) have even been used to communicate with submerged submarines.
Land waves have been used on over-the-horizon radar, which operate mainly at frequencies between 2-20 MHz above the ocean, which have high enough conductivity to transport them to and from a reasonable distance (up to 100 km or more, over-horizon also uses skywave propagation at much greater distances). In the development of radio, ground waves are used extensively. Initial commercial and professional radio services rely solely on long wave, low frequency, and groundwave propagation. To prevent disruption to these services, amateur and experimental transmitters are limited to high frequencies (HF), feeling useless because their ground wave range is limited. Following the discovery of other propagation modes that may occur in the medium wave and shortwave frequencies, the advantages of HF for commercial and military purposes become apparent. The amateur experiment is then limited only to official frequencies within range.
Medium and shortwave reflects the ionosphere at night, known as the skywave. During daylight, the D layer is lower than the ionosphere form and absorbs the lower frequency energy. This prevents skywave propagation from being very effective at mid-day wave frequencies. At night, when the D layer disappears, the medium wave transmission goes better with the skywave. Earthwaves do not include ionosphere and tropospheric waves.
The spread of sound waves through the ground utilizes the ability of the Earth to more efficiently transmit low frequencies known as ground wave audio (AGR).
The micro-field theory
In the theory of microwave field, dielectric and conductor interfaces support "surface wave transmission". Surface waves have been studied as part of a transmission line and some can be considered as single wire transmission lines.
Characteristics and utilization of electric surface wave phenomenon include:
- The field component of the waveform decreases with the distance from the interface.
- Electromagnetic energy is not changed from the surface wave field to other forms of energy (except in surface waves leaking or crushing) so the waves do not transmit normal power to the interface, which is evanescent throughout the dimension.
- In optical fiber transmission, evanescent waves are surface waves.
- In coaxial cables in addition to TEM mode, there is also a transverse-magnetic (TM) mode that spreads as surface waves in the area around the central conductor. To induce general impedance, this mode is effectively suppressed but in high impedance blocking and on a single central conductor with no external shield, low attenuation and broadband propagation are strongly supported. The operation of the transmission line in this mode is called E-Line.
Plasma surface polariton
Plasma surface polariton (SPP) is an electromagnetic surface wave that runs along the interface between two media with different dielectric constants. It exists under the condition that the permittivity of one of the materials forms a negative interface, while the other is positive, as is the case for the interface between air and medium performing lossy below the plasma frequency. The wave propagates parallel to the interface and decays exponentially vertically to it, a property called evanescence. Since waves are in the boundary of lossy and second-medium conductors, these oscillations can become sensitive to changes in boundaries, such as the adsorption of molecules by the conduction surface.
Sommerfeld-Zenneck surface wave
Source of the article : Wikipedia
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