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Critical Frequency
In telecommunication, the term critical frequency has the following meanings: * In radio propagation by way of the ionosphere, the limiting frequency at or below which a wave component is reflected by, and above which it penetrates through, an ionospheric layer. * At near vertical incidence, the limiting frequency at or below which incidence, the wave component is reflected by, and above which it penetrates through, an ionospheric layer. Critical Frequency changes with time of day, atmospheric conditions and angle of fire of the radio waves by antenna. The existence of the critical frequency is the result of electron limitation, ''i.e.,'' the inadequacy of the existing number of free electrons to support reflection at higher frequencies. In signal processing the ''critical frequency'' it is also another name for the Nyquist frequency. Critical frequency is the highest magnitude of frequency above which the waves penetrate the ionosphere and below which the waves are reflected b ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Telecommunication
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that feasible with the human voice, but with a similar scale of expediency; thus, slow systems (such as postal mail) are excluded from the field. The transmission media in telecommunication have evolved through numerous stages of technology, from beacons and other visual signals (such as smoke signals, semaphore telegraphs, signal flags, and optical heliographs), to electrical cable and electromagnetic radiation, including light. Such transmission paths are often divided into communication channels, which afford the advantages of multiplexing multiple concurrent communication sessions. ''Telecommunication'' is often used in its plural form. Other examples of pre-modern long-distance communication included audio messages, such as coded d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Permittivity Of Free Space
Vacuum permittivity, commonly denoted (pronounced "epsilon nought" or "epsilon zero"), is the value of the absolute dielectric permittivity of classical vacuum. It may also be referred to as the permittivity of free space, the electric constant, or the distributed capacitance of the vacuum. It is an ideal (baseline) physical constant. Its CODATA value is: : (farads per meter), with a relative uncertainty of It is a measure of how dense of an electric field is "permitted" to form in response to electric charges, and relates the units for electric charge to mechanical quantities such as length and force. For example, the force between two separated electric charges with spherical symmetry (in the vacuum of classical electromagnetism) is given by Coulomb's law: :F_\text = \frac \frac Here, ''q''1 and ''q''2 are the charges, ''r'' is the distance between their centres, and the value of the constant fraction 1/4 \pi \varepsilon_0 (known as the Coulomb constant, ''k''e) is ap ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radio Propagation
Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another in vacuum, or into various parts of the atmosphere. As a form of electromagnetic radiation, like light waves, radio waves are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization, and scattering. Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for amateur radio communications, international shortwave broadcasters, to designing reliable mobile telephone systems, to radio navigation, to operation of radar systems. Several different types of propagation are used in practical radio transmission systems. '' Line-of-sight propagation'' means radio waves which travel in a straight line from the transmitting antenna to the receiving antenna. Line of sight transmission is used for medium-distance radio transmission, such as cell phones, cordless phones ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Low Frequency
Low frequency (LF) is the ITU designation for radio frequencies (RF) in the range of 30–300 kHz. Since its wavelengths range from 10–1 km, respectively, it is also known as the kilometre band or kilometre wave. LF radio waves exhibit low signal attenuation, making them suitable for long-distance communications. In Europe and areas of Northern Africa and Asia, part of the LF spectrum is used for AM broadcasting as the " longwave" band. In the western hemisphere, its main use is for aircraft beacon, navigation (LORAN), information, and weather systems. A number of time signal broadcasts also use this band. Propagation Because of their long wavelength, low frequency radio waves can diffract over obstacles like mountain ranges and travel beyond the horizon, following the contour of the Earth. This mode of propagation, called '' ground wave'', is the main mode in the LF band. Ground waves must be vertically polarized (the electric field is vertical while the m ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
High Frequency Internet Protocol
High Frequency Internet Protocol (HFIP or HF-IP) is usually associated with Automatic Link Establishment and HF radio data communications. HFIP provides protocol layers enabling internet file transfer, chat, web and email. HFIP commonly uses ionospheric propagation of radio waves to form a wide area network that can span thousands of kilometers. HF transceivers in HFIP service typically run 20 to 150 watts for portable or mobile units, up to approximately 2000 watts transmitter output for high power base stations with HFIP servers. STANAG 5066 is a common HFIP standard. An amateur radio HFIP network called HFLINK uses Automatic Link Establishment for initiating data communications, with ARQ 8FSK frequency-shift keying and PSK phase-shift keying Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
High Frequency Active Auroral Research Program
The High-frequency Active Auroral Research Program (HAARP) was initiated as an ionospheric research program jointly funded by the U.S. Air Force, the U.S. Navy, the University of Alaska Fairbanks, and the Defense Advanced Research Projects Agency (DARPA). It was designed and built by BAE Advanced Technologies. Its original purpose was to analyze the ionosphere and investigate the potential for developing ionospheric enhancement technology for radio communications and surveillance. Since 2015 it has been operated by the University of Alaska Fairbanks. The most prominent instrument at HAARP is the Ionospheric Research Instrument (IRI), a high-power radio frequency transmitter facility operating in the high frequency (HF) band. The IRI is used to temporarily excite a limited area of the ionosphere. Other instruments, such as a VHF and a UHF radar, a fluxgate magnetometer, a digisonde (an ionospheric sounding device), and an induction magnetometer, are used to study the phys ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
High Frequency
High frequency (HF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten decameters (ten to one hundred meters). Frequencies immediately below HF are denoted medium frequency (MF), while the next band of higher frequencies is known as the very high frequency (VHF) band. The HF band is a major part of the shortwave band of frequencies, so communication at these frequencies is often called shortwave radio. Because radio waves in this band can be reflected back to Earth by the ionosphere layer in the atmosphere – a method known as "skip" or " skywave" propagation – these frequencies are suitable for long-distance communication across intercontinental distances and for mountainous terrains which prevent line-of-sight communications. The band is used by international shortwave broadcasting station ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sellmeier Equation
The Sellmeier equation is an empirical relationship between refractive index and wavelength for a particular transparent medium. The equation is used to determine the dispersion of light in the medium. It was first proposed in 1872 by Wolfgang Sellmeier and was a development of the work of Augustin Cauchy on Cauchy's equation for modelling dispersion. The equation In its original and the most general form, the Sellmeier equation is given as : n^2(\lambda) = 1 + \sum_i \frac , where ''n'' is the refractive index, ''λ'' is the wavelength, and ''B''i and ''C''i are experimentally determined ''Sellmeier coefficients''. These coefficients are usually quoted for λ in micrometres. Note that this λ is the vacuum wavelength, not that in the material itself, which is λ/n. A different form of the equation is sometimes used for certain types of materials, e.g. crystals. Each term of the sum representing an absorption resonance of strength ''B''i at a wavelength . For example, the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, troughs, or zero crossings, and is a characteristic of both traveling waves and standing waves, as well as other spatial wave patterns. The inverse of the wavelength is called the spatial frequency. Wavelength is commonly designated by the Greek letter '' lambda'' (λ). The term ''wavelength'' is also sometimes applied to modulated waves, and to the sinusoidal envelopes of modulated waves or waves formed by interference of several sinusoids. Assuming a sinusoidal wave moving at a fixed wave speed, wavelength is inversely proportional to frequency of the wave: waves with higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. Wavelength depends on the medium (for example, vacuum, air, or water) tha ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Refractive Index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or refracted, when entering a material. This is described by Snell's law of refraction, , where ''θ''1 and ''θ''2 are the angle of incidence and angle of refraction, respectively, of a ray crossing the interface between two media with refractive indices ''n''1 and ''n''2. The refractive indices also determine the amount of light that is reflected when reaching the interface, as well as the critical angle for total internal reflection, their intensity ( Fresnel's equations) and Brewster's angle. The refractive index can be seen as the factor by which the speed and the wavelength of the radiation are reduced with respect to their vacuum values: the speed of light in a medium is , and similarly the wavelength in that medium is , where ''Π... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Electron Mass
The electron mass (symbol: ''m''e) is the mass of a stationary electron, also known as the invariant mass of the electron. It is one of the fundamental constants of physics. It has a value of about or about , which has an energy-equivalent of about or about Terminology The term "rest mass" is sometimes used because in special relativity the mass of an object can be said to increase in a frame of reference that is moving relative to that object (or if the object is moving in a given frame of reference). Most practical measurements are carried out on moving electrons. If the electron is moving at a relativistic velocity, any measurement must use the correct expression for mass. Such correction becomes substantial for electrons accelerated by voltages of over . For example, the relativistic expression for the total energy, ''E'', of an electron moving at speed v is :E = \gamma m_\text c^2 , where the Lorentz factor is \gamma = 1/\sqrt . In this expression ''m''e is the "r ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radio Propagation
Radio propagation is the behavior of radio waves as they travel, or are propagated, from one point to another in vacuum, or into various parts of the atmosphere. As a form of electromagnetic radiation, like light waves, radio waves are affected by the phenomena of reflection, refraction, diffraction, absorption, polarization, and scattering. Understanding the effects of varying conditions on radio propagation has many practical applications, from choosing frequencies for amateur radio communications, international shortwave broadcasters, to designing reliable mobile telephone systems, to radio navigation, to operation of radar systems. Several different types of propagation are used in practical radio transmission systems. '' Line-of-sight propagation'' means radio waves which travel in a straight line from the transmitting antenna to the receiving antenna. Line of sight transmission is used for medium-distance radio transmission, such as cell phones, cordless phones ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
