# التطور في مجال الاتصالات



## المتوكلة على الله (25 أبريل 2007)

بسم الله الرحمن الرحيم
السلام عليكم ورحمة الله وبركاته
أحببت ان أتناول موسوعة شاملة عن التطور في مجال الاتصالات
وسأبدأ بالحديث عن
[Mobile phone
Several mobile phones
A mobile or cellular telephone is a long-range, portable electronic device for personal telecommunications over long distances. In addition to the standard voice function of a telephone, current mobile phones can support many additional services such as SMS for text messaging, email, packet switching for access to the Internet, and MMS for sending and receiving photos and video. Most current mobile phones connect to a cellular network of base stations (cell sites), which is in turn interconnected to the public switched telephone network (PSTN) (the exception are satellite phones).
Mobile phones are distinct from cordless telephones, which generally operate only within a limited range of a specific base station. Technically, the term _mobile phone_ includes such devices as satellite phones and pre-cellular mobile phones such as those operating via MTS which do not have a cellular network, whereas the related term _cell(ular) phone_ does not. In practice, the two terms are used nearly interchangeably.
History
Mobile phones from various years, ranging from a large late 1980s-era phone to tiny 2000s phones
The Mobile phone is one of the most used pieces of equipment today. The concept of using hexagonal cells for mobile phone base stations was invented in 1947 by Bell Labs engineers at AT&T and was further developed by Bell Labs during the 1960s. Radiophones have a long and varied history that stretches back to the Second World War when the military started to use radio telephony links and civil services in the 1950s, with hand-held cellular radio devices being available since 1983. Due to their low establishment costs and rapid deployment, mobile phone networks have since spread rapidly throughout the world, outstripping the growth of fixed telephony
In 1945, the 0G generation of mobile telephones were introduced. 0G mobile telephones, such as Mobile Telephone Service, were not officially categorized as mobile phones, since they did not support the automatic change of channel frequency in the middle of a call, when the user moved from one cell (base station coverage area) to another cell, a feature called "handover".
Mock-up of the "portable phone of the future", from a mid-1960s Bell System advertisement, shows a device not too different from today's mobile telephones.
In 1970 Amos Joel of Bell Labs invented the "call handoff" feature, which allowed a mobile-phone user to travel through several cells during the same conversation. Martin Cooper of Motorola is widely considered to be the inventor of the first practical mobile phone for handheld use in a non-vehicle setting. Using a modern, if somewhat heavy portable handset, Cooper made the first call on a handheld mobile phone on April 3, 1973. At the time he made his call, Cooper was working as Motorola's General Manager of its Communications Division.
Fully automatic cellular networks were first introduced in the early to mid-1980s (the 1G generation). The first fully automatic mobile phone system was the 1981 Nordic Mobile Telephone (NMT) system. Until the late 1980s, most mobile phones were too large to be carried in a jacket pocket, so they were usually permanently installed in vehicles as car phones. With the advance of miniaturization and smaller digital components, mobile phones got smaller and lighter.
Manufacturers
Nokia Corporation is currently the world's largest manufacturer of mobile telephones, with a global market share of approximately 36% in Q4 of 2006. Other mobile phone manufacturers include Apple Inc., Audiovox (now UT Starcom), Benefon, BenQ-Siemens, High Tech Computer Corporation, Fujitsu, Kyocera, 3G, LG, Motorola, NEC, HTC, Panasonic (Matsushita Electric), Pantech Curitel, Philips, Research in Motion, Sagem, Samsung, Sanyo, Sharp, Siemens, Sierra Wireless, SK Teletech, Sony Ericsson, T&A Alcatel, and Toshiba. There are also specialist communication systems related to, but distinct from mobile phones, such as for exampleProfessional Mobile Radio.
Applications
Mobile news services are expanding with many organizations providing "on-demand" news services by SMS. Some also provide "instant" news pushed out by SMS. Mobile telephony also facilitates activism and public journalism being explored by Reuters and Yahoo and small independent news companies such as Jasmine News in Sri Lanka.​ 
Health impacts
Since the introduction of mobile phones, concerns have been raised about the potential health impacts from cellular phone use.[13] Studies from the National Cancer Institute and researchers at the Danish Institute of Cancer Epidemiology in Copenhagen do not show any link between cellular phone use and cancer. The Danish study only covered analog mobile phone usage up through 1995, and subjects who started mobile phone usage after 1995 were counted as non-users in the study. However, a study by the International Agency for Research on Cancer of 4,500 users found a statistically significant link between tumor frequency and mobile phone use. ​


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## المتوكلة على الله (25 أبريل 2007)

*تابع للحديث عن الموبايل*

Technology

Mobile phone tower
Mobile phones and the network they operate under vary significantly from provider to provider, and nation to nation. However, all of them communicate through electromagnetic microwaves with a cell site base station, the antennas of which are usually mounted on a tower, pole, or building.
The phones have a low-power transceiver that transmits voice and data to the nearest cell sites, usually not more than 5 to 8 miles (approximately 8 to 13 kilometers) away. When the mobile phone or data device is turned on, it registers with the mobile telephone exchange, or switch, with its unique identifiers, and will then be alerted by the mobile switch when there is an incoming telephone call. The handset constantly listens for the strongest signal being received from the surrounding base stations. As the user moves around the network, the mobile device will "handoff" to various cell sites during calls, or while waiting (idle) between calls it will reselect cell sites.
Cell sites have relatively low-power (often only one or two watts) radio transmitters which broadcast their presence and relay communications between the mobile handsets and the switch. The switch in turn connects the call to another subscriber of the same wireless service provider or to the public telephone network, which includes the networks of other wireless carriers. Many of these sites are camouflaged to blend with existing environments, particularly in high-scenery areas.
The dialogue between the handset and the cell site is a stream of digital data that includes digitized audio (except for the first generation analog networks). The technology that achieves this depends on the system which the mobile phone operator has adopted. Some technologies include AMPS for analog, and D-AMPS, CDMA2000, GSM, GPRS, EV-DO, and UMTS for digital communications. Each network operator has a unique radio frequency band.
[FONT='Cambria','serif']Satellite phones[/FONT]
Some mobile telephones, especially those used in remote locations, where constructing a cell network would be too unprofitable or difficult, instead communicate directly with an orbiting satellite. Such devices tend to be bulkier than cell-based mobile phones, as they require a large antenna or dish for communicating with the satellite, but do not require ground based transmitters, making them useful for communicating from remote areas and disaster zones.
[FONT='Cambria','serif']Semi-Cordless Phone[/FONT]
There are phones that work as a cordless phone when near their corresponding base station (and sometimes other base stations) and work as a wireless phone when in other locations but for a variety of reasons did not become popular.
[FONT='Cambria','serif']IP (Internet Protocol) Telephony[/FONT]

A WiFi-based VoIP phone
Also known as Internet telephony, IP Telephony is a service based on Voice over IP (VoIP), a disruptive technology that is rapidly gaining ground against traditional telephone network technologies. In Japan and South Korea up to 10% of subscribers, as of January 2005, have switched to this digital telephone service. A January 2005 Newsweek article suggested that Internet telephony may be "the next big thing." 
As of 2006 many VoIP companies offer service to consumers and businesses.
IP telephony uses a broadband Internet connection to transmit conversations as data packets. In addition to replacing POTS, IP telephony is also competing with mobile phone networks by offering free or lower cost connections via WiFi hotspots. As mentioned above VoIP is also used on private wireless networks which may or may not have a connection to the outside telephone network.​


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## المتوكلة على الله (25 أبريل 2007)

وفي خضم الحديث عن التطور في الاتصالات
فلا بد من الحديث عن الألياف البصرية
An *optical fiber* (or *fibre*) is a glass or plastic fiber designed to guide light along its length by confining as much light as possible in a propagating form. In fibers with large core diameter, the confinement is based on total internal reflection. In smaller diameter core fibers, (widely used for most communication links longer than 200m) the confinement relies on establishing a waveguide. *Fiber optics* is the overlap of applied science and engineering concerned with such optical fibers. Optical fibers are widely used in fiber-optic communication, which permits digital data transmission over longer distances and at higher data rates than other forms of wired and wireless communications. They are also used to form sensors, and in a variety of other applications.
The term optical fiber covers a range of different designs including graded-index optical fibers, step-index optical fibers, birefringent polarization-maintaining fibers and more recently photonic crystal fibers, with the design and the wavelength of the light propagating in the fiber dictating whether or not it will be multi-mode optical fiber or single-mode optical fiber. Because of the mechanical properties of the more common glass optical fibers, special methods of splicing fibers and of connecting them to other equipment are needed. Manufacture of optical fibers is based on partially melting a chemically doped preform and pulling the flowing material on a draw tower. Fibers are built into different kinds of cables depending on how they will be used.
The light-guiding principles behind optical fibers was first demonstrated in Victorian times, but modern optical fibers were only developed beginning in the 1950s. Optical fibers became practical for use in communications in the late 1970s, once the attenuation was reduced sufficiently; since then, several technical advances have been made to improve the attenuation and dispersion properties of optical fibers (i.e., allowing signals to travel further and carry more information), and lower the cost of fiber communications systems.


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## م.رائد الجمّال (25 أبريل 2007)

مشكوره و جزاك الله كل خير


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## المهندس الاسلامي (25 أبريل 2007)

مشكووووووووووووووووووووووور


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## المتوكلة على الله (26 أبريل 2007)

مشكور جزيلا اخي الفاضل م. رائد الجمال
على مرورك الكريم
وبوركت اخي الفاضل المهندس الاسلامي على مشاركتك الكريمة


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## المتوكلة على الله (26 أبريل 2007)

Principle of operation
An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis, by the process of total internal reflection. The fiber consists of a _core_ surrounded by a cladding layer. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in _step-index fiber_, or gradual, in _graded-index fiber_.

*[Multimode fiber*



 


The propagation of light through a multi-mode optical fiber.


Fiber with large (greater than 10 μm) core diameter may be analyzed by geometric optics. Such fiber is called _multimode fiber_, from the electromagnetic analysis (see below). In a step-index multimode fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a line normal to the boundary), greater than the critical angle for this boundary, are completely reflected. The critical angle (minimum angle for total internal reflection) is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the core into the cladding, and do not convey light and hence information along the fiber. The critical angle determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber. However, this high numerical aperture increases the amount of dispersion as rays at different angles have different path lengths and therefore take different times to traverse the fiber. A low numerical aperture may therefore be desirable.
In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis.

*[ Singlemode fiber*



 


A typical single-mode optical fiber, showing diameters of the component layers.


Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics. Instead, it must be analyzed as an electromagnetic structure, by solution of Maxwell's equations as reduced to the electromagnetic wave equation. The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called single-mode or _mono-mode_ fiber. The behavior of larger-core multimode fiber can also be modeled using the wave equation, which shows that such fiber supports more than one mode of propagation (hence the name). The results of such modeling of multi-mode fiber approximately agree with the predictions of geometric optics, if the fiber core is large enough to support more than a few modes.
The waveguide analysis shows that the light energy in the fiber is not completely confined in the core. Instead, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave.
The most common type of single-mode fiber has a core diameter of 8 to 10 μm and is designed for use in the near infrared. It is notable that the mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths. Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 microns and as large as hundreds of microns.


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## المتوكلة على الله (26 أبريل 2007)

Special-purpose fiber
Some special-purpose optical fiber is constructed with a non-cylindrical core and/or cladding layer, usually with an elliptical or rectangular cross-section. These include polarization-maintaining fiber and fiber designed to suppress whispering gallery mode propagation.

* Materials*

Glass optical fibers are almost always made from silica, but some other materials, such as fluorozirconate, fluoroaluminate, and chalcogenide glasses, are used for longer-wavelength infrared applications. Like other glasses, these glasses have a refractive index of about 1.5. Typically the difference between core and cladding is less than one percent.
Plastic optical fiber (POF) is commonly step-index multimode fiber, with core diameter of 1 mm or larger. POF typically has much higher attenuation than glass fiber (that is, the amplitude of the signal in it decreases faster), 1 dB/m or higher, and this high attenuation limits the range of POF-based systems.

*Fiber fuse*

At high optical intensities, above 2 megawatts per square centimetre, when a fiber is subjected to a shock or is otherwise suddenly damaged, a _fiber fuse_ can occur. The reflection from the damage vaporizes the fiber immediately before the break, and this new defect remains reflective so that the damage propagates back toward the transmitter at 1–3 meters per second The open fiber control system, which ensures laser eye safety in the event of a broken fiber, can also effectively halt propagation of the fiber fuse . In situations, such as undersea cables, where high power levels might be used without the need for open fiber control, a "fiber fuse" protection device at the transmitter can break the circuit to prevent any damage.


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## المتوكلة على الله (26 أبريل 2007)

*Termination and splicing*

Termination and splicing


 


ST fiber connector on multimode fiber


Optical fibers are connected to terminal equipment by optical fiber connectors. These connectors are usually of a standard type such as _FC_, _SC_, _ST_, _LC_, or _MTRJ_.
Optical fibers may be connected to each other by connectors or by _splicing_, that is, joining two fibers together to form a continuous optical waveguide. The generally accepted splicing method is arc fusion splicing, which melts the fiber ends together with an electric arc. For quicker fastening jobs, a "mechanical splice" is used.
Fusion splicing is done with a specialized instrument that typically operates as follows: The two cable ends are fastened inside a splice enclosure that will protect the splices, and the fiber ends are stripped of their protective polymer coating (as well as the more sturdy outer jacket, if present). The ends are _cleaved_ (cut) with a precision cleaver to make them perpendicular, and are placed into special holders in the splicer. The splice is usually inspected via a magnified viewing screen to check the cleaves before and after the splice. The splicer uses small motors to align the end faces together, and emits a small spark between electrodes at the gap to burn off dust and moisture. Then the splicer generates a larger spark that raises the temperature above the melting point of the glass, fusing the ends together permanently. The location and energy of the spark is carefully controlled so that the molten core and cladding don't mix, and this minimizes optical loss. A splice loss estimate is measured by the splicer, by directing light through the cladding on one side and measuring the light leaking from the cladding on the other side. A splice loss under 0.1 dB is typical. The complexity of this process is the major thing that makes fiber splicing more difficult than splicing copper wire.
Mechanical fiber splices are designed to be quicker and easier to install, but there is still the need for stripping, careful cleaning and precision cleaving. The fiber ends are aligned and held together by a precision-made sleeve, often using a clear gel (index matching gel) that enhances the transmission of light across the joint. Such joints typically have higher optical loss, and are less robust than fusion splices, especially if the gel is used. All splicing techniques involve the use of an enclosure into which the splice is placed for protection afterward.
Fibers are terminated in connectors so that the fiber end is held at the end face precisely and securely. A fiber optic connector is basically a rigid cylindrical barrel surrounded by a sleeve that holds the barrel in its mating socket. It can be push and click, turn and latch, or threaded. A typical connector is installed by preparing the fiber end and inserting it into the rear of the connector body. Quick set glue is usually used so the fiber is held securely, and a strain relief is secured to the rear. Once the glue has set, the end is polished to a mirror finish. Various types of polish profile are used, depending on the type of fiber and the application. For singlemode fiber, the fiber ends are typically polished with a slight curvature, such that when the connectors are mated the fibers touch only at their cores. This is known as a "physical contact" (PC) polish. The curved surface may be polished at an angle, to make an angled physical contact (APC) connection. Such connections have higher loss than PC connections, but greatly reduced backreflection, because light that reflects from the angled surface leaks out of the fiber core; the resulting loss in signal strength is known as gap loss.
Various methods to align two fiber ends to each other or one fiber to an optical device (VCSEL, LED, waveguide etc.) have been reported. They all follow either an active fiber alignment approach or a passive fiber alignment approach.​


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## م.رائد الجمّال (26 أبريل 2007)

جزاك الله كل خير


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## المتوكلة على الله (26 أبريل 2007)

*antenna*

جزاك الله خيرا اخي م. رائد جمال على مشاركتك الكريمة في هذا الموضوع مرة أخرى
والآن أود الحديث عن الantenna
لأهميتها في مجال الاتصالات
An *antenna* or *aerial* is a transducer designed to transmit or receive radio waves which are a class of electromagnetic waves. In other words, antennas convert radio frequency electrical currents into electromagnetic waves and vice versa. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless lan, radar, and space exploration. Antennas usually work in air or outer space, but can also be operated under water or even through soil and rock at certain frequencies for short distances.
Physically, an antenna is an arrangement of conductors that generate a radiating electromagnetic field in response to an applied alternating voltage and the associated alternating electric current, or can be placed in an electromagnetic field so that the field will induce an alternating current in the antenna and a voltage between its terminals. Some antenna devices (parabola, horn antenna) just adapt the free space to another type of antenna.​


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## المتوكلة على الله (26 أبريل 2007)

*antenna parameters*

Antenna parameters
There are several critical parameters that affect an antenna's performance and can be adjusted during the design process. These are resonant frequency, impedance, gain, aperture or radiation pattern, polarization, efficiency and bandwidth. Transmit antennas may also have a maximum power rating, and receive antennas differ in their noise rejection properties.​*[Resonant frequency*

The "_resonant frequency_" and "_electrical resonance_" is related to the electrical length of the antenna. The electrical length is usually the physical length of the wire divided by its velocity factor (the ratio of the speed of wave propagation in the wire to _c0_, the speed of light in a vacuum). Typically an antenna is tuned for a specific frequency, and is effective for a range of frequencies usually centered on that resonant frequency. However, the other properties of the antenna (especially radiation pattern and impedance) change with frequency, so the antenna's resonant frequency may merely be close to the center frequency of these other more important properties.
Antennas can be made resonant on harmonic frequencies with lengths that are fractions of the target wavelength. Some antenna designs have multiple resonant frequencies, and some are relatively effective over a very broad range of frequencies. The most commonly known type of wide band aerial is the logarithmic or log periodic, but its gain is usually much lower than that of a specific or narrower band aerial.​*[ Gain*

"Gain" as a parameter measures the directionality of a given antenna. An antenna with a low gain emits radiation in all directions equally, whereas a high-gain antenna will preferentially radiate in particular directions. Specifically, the *Gain*, *Directive gain* or *Power gain* of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in a given direction at an arbitrary distance divided by the intensity radiated at the same distance by an hypothetical isotropic antenna:






We write "hypothetical" because a perfect isotropic antenna cannot exist in reality (the electric and magnetic field would not satisfy Maxwell equations for electromagnetic fields). Gain is a dimensionless number (without units).
The gain of an antenna is a passive phenomenon - power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. If an antenna has a greater than one gain in some directions, it must have a less than one gain in other directions since energy is conserved by the antenna. An antenna designer must take into account the application for the antenna when determining the gain. High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully in a particular direction. Low-gain antennas have shorter range, but the orientation of the antenna is inconsequential. For example, a dish antenna on a spacecraft is a high-gain device (must be pointed at the planet to be effective), while a typical WiFi antenna in a laptop computer is low-gain (as long as the base station is within range, the antenna can be in an any orientation in space).
As an example, consider an antenna that radiates an electromagnetic wave whose electrical field has an amplitude



at a distance



. This amplitude is given by:





where:​




is the current fed to the antenna and ​




is a constant characteristic of each antenna. ​
For a large distance



. The radiated wave can be considered locally as a plane wave. The intensity of an electromagnetic plane wave is:





where



is a universal constant called vacuum impedance. and





If the resistive part of the series impedance of the antenna is



, the power fed to the antenna is



. The intensity of an isotropic antenna is the power so fed divided by the surface of the sphere of radius



:





The directive gain is:






For the commonly utilized half-wave dipole, the particular formulation works out to the following, including its decibel equivalency, expressed as *dBi* (decibels referenced to *i*sotropic radiator):






_(In most cases *73.1296*, or even *73.13*, is adequate)_ ​​




_(Likewise, *1.64* and *2.15 dBi* are usually the cited values)_ 

Sometimes, the half-wave dipole is taken as a reference instead of the isotropic radiator. The gain is then given in *dBd* (decibels over *d*ipole):
*0 dBd = 2.15 dBi* ​


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## المتوكلة على الله (26 أبريل 2007)

Bandwidth
The "_bandwidth_" of an antenna is the range of frequencies over which it is effective, usually centered around the resonant frequency. The bandwidth of an antenna may be increased by several techniques, including using thicker wires, replacing wires with _cages_ to simulate a thicker wire, tapering antenna components (like in a feed horn), and combining multiple antennas into a single assembly and allowing the natural impedance to select the correct antenna. Small antennas are usually preferred for convenience, but there is a fundamental limit relating bandwidth, size and efficiency.

*[Impedance*

As an electro-magnetic wave travels through the different parts of the antenna system (radio, feed line, antenna, free space) it may encounter differences in impedance (E/H, V/I, etc). At each interface, depending on the impedance match, some fraction of the wave's energy will reflect back to the source, forming a standing wave in the feed line. The ratio of maximum power to minimum power in the wave can be measured and is called the standing wave ratio (*SWR*). A SWR of 1:1 is ideal. A SWR of 1.5:1 is considered to be marginally acceptable in low power applications where power loss is more critical, although an SWR as high as 6:1 may still be usable with the right equipment. Minimizing impedance differences at each interface (impedance matching) will reduce SWR and maximize power transfer through each part of the antenna system.
Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line, using the feed line as an impedance transformer. More commonly, the impedance is adjusted at the load (see below) with an antenna tuner, a balun, a matching transformer, matching networks composed of inductors and capacitors, or matching sections such as the gamma match.


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## المتوكلة على الله (26 أبريل 2007)

Radiation pattern
The radiation pattern is a graphical depiction of the relative field strength transmitted from or received by the antenna. As antennas radiate in space often several curves are necessary to describe the antenna. If the radiation of the antenna is symmetrical about an axis (as is the case in dipole, helical and some parabolic antennas) a unique graph is sufficient.
Each antenna supplier/user has different standards as well as plotting formats. Each format has its own advantages and disadvangages. Radiation pattern of an antenna can be defined as the locus of all points where the emitted power per unit surface is the same. As the radiated power per unit surface is proportional to the squared electrical field of the electromagnetic wave. The radiation pattern is the locus of points with the same electrical field. In this representation, the reference is, usually, the best angle of emission. It is also possible to depict the directive gain of the antenna as a function of the direction. Often the gain is given in decibels.
The graphs can be drawn using cartesian (rectangular) coordinates or a polar plot. The shape of curves can be very different in cartesian or polar coordinates and with the choice of the limits of the logarithmic scale. The four drawings below are the radiation patterns of a same half-wave antenna.


Radiation pattern of a half-wave dipole antenna. Linear scale.




Gain of a half-wave dipole. The scale is in dBi.




Gain of a half-wave dipole. Cartesian representation.




3D Radiation pattern of a half-wave dipole antenna.


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## المتوكلة على الله (26 أبريل 2007)

Polarization
The "_polarization_" of an antenna is the orientation of the electric field (E-plane) of the radio wave with respect to the Earth's surface and is determined by the physical structure of the antenna and by its orientation. It has nothing in common with antenna directionality terms: "horizontal", "vertical" and "circular". Thus, a simple straight wire antenna will have one polarization when mounted vertically, and a different polarization when mounted horizontally. "Electromagnetic wave polarization filters" are structures which can be employed to act directly on the electromagnetic wave to filter out wave energy of an undesired polarization and to pass wave energy of a desired polarization.
Reflections generally affect polarization. For radio waves the most important reflector is the ionosphere - signals which reflect from it will have their polarization changed unpredictably. For signals which are reflected by the ionosphere, polarization cannot be relied upon. For line-of-sight communications for which polarization can be relied upon, it can make a large difference in signal quality to have the transmitter and receiver using the same polarization; many tens of dB difference are commonly seen and this is more than enough to make the difference between reasonable communication and a broken link.
Polarization is largely predictable from antenna construction, but especially in directional antennas, the polarization of side lobes can be quite different from that of the main propagation lobe. For radio antennas, polarization corresponds to the orientation of the radiating element in an antenna. A vertical omnidirectional WiFi antenna will have vertical polarization (the most common type). An exception is a class of elongated waveguide antennas in which vertically placed antennas are horizontally polarized. Many commercial antennas are marked as to the polarization of their emitted signals.
Polarization is the sum of the E-plane orientations over time projected onto an imaginary plane perpendicular to the direction of motion of the radio wave. In the most general case, polarization is elliptical (the projection is oblong), meaning that the antenna varies over time in the polarization of the radio waves it is emitting. Two special cases are linear polarization (the ellipse collapses into a line) and circular polarization (in which the ellipse varies maximally). In linear polarization the antenna compels the electric field of the emitted radio wave to a particular orientation. Depending on the orientation of the antenna mounting, the usual linear cases are horizontal and vertical polarization. In circular polarization, the antenna continuously varies the electric field of the radio wave through all possible values of its orientation with regard to the Earth's surface. Circular polarizations, like elliptical ones, are classified as right-hand polarized or left-hand polarized using a "thumb in the direction of the propagation" rule. Optical researchers use the same rule of thumb, but pointing it in the direction of the emitter, not in the direction of propagation, and so are opposite to radio engineers' use.
In practice, regardless of confusing terminology, it is important that linearly polarized antennas be matched, lest the received signal strength be greatly reduced. So horizontal should be used with horizontal and vertical with vertical. Intermediate matchings will lose some signal strength, but not as much as a complete mismatch. Transmitters mounted on vehicles with large motional freedom commonly use circularly polarized antennas so that there will never be a complete mismatch with signals from other sources. In the case of radar, this is often reflections from rain drops.

* Efficiency*

"_Efficiency_" is the ratio of power actually radiated to the power put into the antenna terminals. A dummy load may have a SWR of 1:1 but an efficiency of 0, as it absorbs all power and radiates heat but not RF energy, showing that SWR alone is not an effective measure of an antenna's efficiency. Radiation in an antenna is caused by radiation resistance which can only be measured as part of total resistance including loss resistance. Loss resistance usually results in heat generation rather than radiation, and reduces efficiency. Mathematically, efficency is calculated as radiation resistance divided by total resistance.

*Overview of antenna parameters*

Except for polarization, the SWR is the most easily measured of the parameters above. Impedance can be measured with specialized equipment, as it relates to the complex SWR. Measuring radiation pattern requires a sophisticated setup including significant clear space (enough to put the sensor into the antenna's far field, or an anechoic chamber designed for antenna measurements), careful study of experiment geometry, and specialized measurement equipment that rotates the antenna during the measurements.
Bandwidth depends on the overall effectiveness of the antenna, so all of these parameters must be understood to fully characterize the bandwidth capabilities of an antenna. However, in practice, bandwidth is typically determined by looking only at SWR, i.e., by finding the frequency range over which the SWR is less than a given value. Bandwidth over which an antenna exhibits a particular radiation pattern is also important, for in practical use the performance of an antenna at the extremes of an assigned frequency band is important.

*[Transmission and reception*

All of these parameters are expressed in terms of a transmission antenna, but are identically applicable to a receiving antenna, due to reciprocity. Impedance, however, is not applied in an obvious way; for impedance, the impedance at the load (where the power is consumed) is most critical. For a transmitting antenna, this is the antenna itself. For a receiving antenna, this is at the (radio) receiver rather than at the antenna. Tuning is done by adjusting the length of an electrically long linear antenna to alter the electrical resonance of the antenna.
Antenna tuning is done by adjusting an inductance or capacitance combined with the active antenna (but distinct and separate from the active antenna). The inductance or capacitance provides the reactance which combines with the inherent reactance of the active antenna to establish a resonance in a circuit including the active antenna. The established resonance being at a frequency other than the natural electrical resonant frequency of the active antenna. Adjustment of the inductance or capacitance changes this resonance.
Antennas used for transmission have a maximum power rating, beyond which heating, arcing or sparking may occur in the components, which may cause them to be damaged or destroyed. Raising this maximum power rating usually requires larger and heavier components, which may require larger and heavier supporting structures. This is a concern only for transmitting antennas, as the power received by an antenna rarely exceeds the microwatt range.
Antennas designed specifically for reception might be optimized for noise rejection capabilities. An "_antenna shield_" is a conductive or low reluctance structure (such as a wire, plate or grid) which is adapted to be placed in the vicinity of an antenna to reduce, as by dissipation through a resistance or by conduction to ground, undesired electromagnetic radiation, or electric or magnetic fields, which are directed toward the active antenna from an external source or which emanate from the active antenna. Other methods to optimized for noise rejection can be done by selecting a narrow bandwidth so that noise from other frequencies is rejected, or selecting a specific radiation pattern to reject noise from a specific direction, or by selecting a polarization different from the noise polarization, or by selecting an antenna that favors either the electric or magnetic field.
For instance, an antenna to be used for reception of low frequencies (below about ten megahertz) will be subject to both man-made noise from motors and other machinery, and from natural sources such as lightning. Successfully rejecting these forms of noise is an important antenna feature. A small coil of wire with many turns is more able to reject such noise than a vertical antenna. However, the vertical will radiate much more effectively on transmit, where extraneous signals are not a concern.


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## المتوكلة على الله (26 أبريل 2007)

Basic antenna models
There are many variations of antennas that have various configurations. These configurations contain space or medium which tends to confine the energy within specified boundaries along a predetermined path (known as "_restricted space_"), such as wave guides, hollow resonators, and conductive wires. Below are a few common models. More can be found in Category:Radio frequency antenna types.


 


A multiband rotary directional antenna for amateur radio use




 


Rooftop television antenna. It is actually three Yagi antennas in one. The longest elements are for the low band (channels 2-6) the medium-length elements are for the high band (channels 7-13) and the shortest elements are for the UHF band (channels 14-69)



The isotropic radiator is a purely theoretical antenna that radiates equally in all directions. It is considered to be a point in space with no dimensions and no mass. This antenna cannot physically exist, but is useful as a theoretical model for comparison with all other antennas. Most antennas' gains are measured with reference to an isotropic radiator, and are rated in dBi (decibels with respect to an isotropic radiator). 
The dipole antenna is simply two wires pointed in opposite directions arranged either horizontally or vertically, with one end of each wire connected to the radio and the other end hanging free in space. Since this is the simplest practical antenna, it is also used as reference model for other antennas; gain with respect to a dipole is labeled as dBd. Generally, the dipole is considered to be omnidirectional in the plane perpendicular to the axis of the antenna, but it has deep nulls in the directions of the axis. Variations of the dipole include the folded dipole, the half wave antenna, the groundplane antenna, the whip, and the J-pole. 
The Yagi-Uda antenna is a directional variation of the dipole with parasitic elements added with functionality similar to adding a reflector and lenses (directors) to focus a filament lightbulb. 
Loop antennas have a continuous conducting path leading from one conductor of a two-wire transmission line to the other conductor. "Symmetric" loop antennas have a plane of symmetry running along the feed and through the loop. "Planar" loop antennas lie in a single plane which also contains the conductors of the feed. "Three-dimensional" loop antennas have wire which runs in all of the x,y, and z directions. By definition they are not planar. They may, however, be symmetric about planes which contain the feed. 
The (large) loop antenna is similar to a dipole, except that the ends of the dipole are connected to form a circle, triangle (delta loop antenna) or square. Typically a loop is a multiple of a half or full wavelength in circumference. A circular loop gets higher gain (about 10%) than the other forms of large loop antenna, as gain of this antenna is directly proportional to the area enclosed by the loop, but circles can be hard to support in a flexible wire, making squares and triangles much more popular. Large loop antennas are more immune to localized noise partly due to lack of a need for a groundplane. The large loop has its strongest signal in the plane of the loop, and nulls in the axis perpendicular to the plane of the loop. 
The small loop antenna, also called the magnetic loop antenna is a loop of wire (in other words, both ends of the wire connect to the radio) less than a wavelength in circumference. Typically, the circumference is less than 1/10 for a receiving loop, and less than 1/4 for a transmitting loop. Unlike nearly all other antennas in this list, this antenna detects the magnetic component of the electromagnetic wave. As such, it is less sensitive to near field electric noise when properly shielded. The received voltage can be greatly increased by bringing the loop into resonance with a tuning capacitor. The small loop has a maximum output when the magnetic field is normal to the plane of the loop, and since this field is transverse to the direction of the wave, has a maximum in the plane of the loop. This is the same mechanism as the large loop. 
The electrically short antenna is an open-end wire far less than 1/4 wavelength in length - in other words only one end of the antenna is connected to the radio, and the other end is hanging free in space. Unlike nearly all other antennas in this list, this antenna detects only the electric field of the wave instead of the electromagnetic field - think of the free end of the wire as measuring the voltage of that point in space, as opposed to measuring both the voltage and the magnetic field. Its receiving aperture cannot be changed by adding lumped components, but more efficient power transfer can be achieved by impedance matching with such circuits. Electrically short antennas are typically used where operating wavelength is large and space is limited, e.g. for mobile transceivers operating at long wavelengths. 
The fractal antenna is a class where the structure is self similar, and includes *log periodic* antennas and fractal element antennas, which are used for smaller and wideband or multiband applications. 
The parabolic antenna is a special antenna where a reflector dish is used to focus the signal from a directional antenna feeder. Antennas of this type are commonly found as Satellite television antennas, Wi-fi / WLAN, radio astronomy, radio-links, mobile phone backhaul and military tactical radio link -antennas. They are characterized by high directionality and gain but can only be used at UHF to microwave and higher frequencies due to dimensions getting too large at lower frequencies. 
The microstrip antenna consists of a patch of metalization on a ground plane. These are low profile, light weight antennas, most suitable for aerospace and mobile applications. Because of their low power handling capability, these antennas can be used in low-power transmitting and receiving applications. Microstrip antennas are the most commonly used antennas in mobile communications, satellite links, W-LAN and so on because circuit functions can be directly integrated to the microstrip antenna to form compact transceivers and spatial power combiners. 
The quad antenna is an array of square loops that vary in size. The quad is related to the loop in exactly the same way the yagi is related to the dipole. Typically, the quad needs fewer elements to get the same gain as a yagi. Variations of the quad include the delta loop antenna which uses a triangle instead of a square, requiring fewer supports for large wavelength antennas. 
The random wire antenna is simply a very long (greater than one wavelength) wire with one end connected to the radio and the other in free space, arranged in any way most convenient for the space available. Folding will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.) Typically, a random wire antenna will also require an antenna tuner, as it might have a random impedance that varies nonlinearly with frequency. 
The Beverage antenna is a form of directional long-wire antenna which uses a resistive termination at one end and feed from the other. 
The endfire helical antenna is a directional antenna suited for receiving signals that are either circular polarized or randomly polarized. These are usually used with satellites, and are frequently used for the driven element on a dish. 
The broadside helical antenna is a variation of the dipole, which has been coiled up to decrease its physical size. A typical broadside helical will have lower gain than the equivalent full length dipole, but will be flexible and smaller. The stock antenna for most hand held radios ("rubber duck") is a broadside helical. 
The Phased array antenna is a group of independently fed active elements in which the relative phases of the respective signals feeding the elements are varied in such a way that the effective radiation pattern of the array is reinforced in a desired direction and suppressed in undesired directions. In plain language, this is a directional antenna that can be aimed without moving any parts. 
Synthetic aperture radar uses a series of observations separated in time and space to simulate a very large antenna. Interferometry allows the monitor to combine signals from several radio receivers or a single moving receiver. 
A _trailing wire antenna_ is used by submarines when submerged. These antennas are designed to pick up transmissions in the low frequency (LF) and very low frequency (VLF) ranges. Trailing wire antennas are also used in some aircraft, in the HF, LF and VLF ranges. 
An evolved antenna refers to an antenna fully or substantially designed using a computer algorithm based on Darwinian evolution. 
A _dielectric resonator_ is a variation on the conventional antenna in which an insulator with a large dielectric constant is used to modify the electromagnetic field. It is claimed that the dielectric contains the antenna's near field and therefore prevents it from interfering with other nearby antennas or circuits, making it suitable for miniature equipment such as mobile phones. 
A _feed horn_ is an antenna system that handles the incoming waveform from the dish to the focal point. It usually comprises a series of rings with decreasing radius in order to drive the signal to the polarizer.


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## hammhamm44 (27 أبريل 2007)

thanks for a good informations


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## المتوكلة على الله (27 أبريل 2007)

thanks alot for your reply


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## المتوكلة على الله (29 أبريل 2007)

*The radio*

*Radio* is the wireless transmission of signals, by modulation of electromagnetic waves with frequencies below those of visible light.
Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space. It does not require a medium of transport. Information is carried by systematically changing (modulating) some property of the radiated waves, such as their amplitude or their frequency. When radio waves pass an electrical conductor, the oscillating fields induce an alternating current in the conductor. This can be detected and transformed into sound or other signals that carry information.
The word 'radio' is used to describe this phenomenon, and television and radio transmissions are classed as radio frequency emissions
Uses of radio
Early uses were maritime, for sending telegraphic messages using Morse code between ships and land. The earliest users included the Japanese Navy scouting the Russian fleet during the Battle of Tsushima in 1905. One of the most memorable uses of marine telegraphy was during the sinking of the RMS _Titanic_ in 1912, including communications between operators on the sinking ship and nearby vessels, and communications to shore stations listing the survivors.
Radio was used to pass on orders and communications between armies and navies on both sides in World War I; Germany used radio communications for diplomatic messages once its submarine cables were cut by the British. The United States passed on President Woodrow Wilson's Fourteen Points to Germany via radio during the war. Broadcasting began from San Jose in 1909[4], and became feasible in the 1920s, with the widespread introduction of radio receivers, particularly in Europe and the United States. Besides broadcasting, point-to-point broadcasting, including telephone messages and relays of radio programs, became widespread in the 1920s and 1930s. Another use of radio in the pre-war years was the development of detecting and locating aircraft and ships by the use of radar (_RA_dio _D_etection _A_nd _R_anging).
Today, radio takes many forms, including wireless networks, mobile communications of all types, as well as radio broadcasting. Before the advent of television, commercial radio broadcasts included not only news and music, but dramas, comedies, variety shows, and many other forms of entertainment. Radio was unique among dramatic presentation that it used only sound. For more, see radio programming..​


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## المتوكلة على الله (29 أبريل 2007)

*Audio*






A Fisher 500 AM/FM hi-fi receiver from 1959.


AM broadcast radio sends music and voice in the Medium Frequency (MF—0.300 MHz to 3 MHz) radio spectrum. AM radio uses amplitude modulation, in which the amplitude of the transmitted signal is made proportional to the sound amplitude captured (transduced) by the microphone while the transmitted frequency remains unchanged. Transmissions are affected by static and interference because lightning and other sources of radio that are transmitting at the same frequency add their amplitudes to the original transmitted amplitude. The most wattage an AM radio station is allowed to use is 50,000 watts and the only stations that can blast out signals this high were grandfathered in; these include WJR and CKLW.
FM broadcast radio sends music and voice with higher fidelity than AM radio. In frequency modulation, amplitude variation at the microphone cause the transmitter frequency to fluctuate. Because the audio signal modulates the frequency and not the amplitude, an FM signal is not subject to static and interference in the same way as AM signals. FM is transmitted in the Very High Frequency (VHF—30 MHz to 300 MHz) radio spectrum. VHF radio waves act more like light, traveling in straight lines, hence the reception range is generally limited to about 50-100 miles. During unusual upper atmospheric conditions, FM signals are occasionally reflected back towards the Earth by the ionosphere, resulting in Long distance FM reception. FM receivers are subject to the capture effect, which causes the radio to only receive the strongest signal when multiple signals appear on the same frequency. FM receivers are relatively immune to lightning and spark interference.
FM Subcarrier services are secondary signals transmitted "piggyback" along with the main program. Special receivers are required to utilize these services. Analog channels may contain alternative programming, such as reading services for the blind, background music or stereo sound signals. In some extremely crowded metropolitan areas, the subchannel program might be an alternate foreign language radio program for various ethnic groups. Subcarriers can also transmit digital data, such as station identification, the current song's name, web addresses, or stock quotes. In some countries, FM radios automatically retune themselves to the same channel in a different district by using sub-bands.
Aviation voice radios use VHF AM. AM is used so that multiple stations on the same channel can be received. (Use of FM would result in stronger stations blocking out reception of weaker stations due to FM's capture effect). Aircraft fly high enough that their transmitters can be received hundreds of miles (kilometres) away, even though they are using VHF.
Marine voice radios can use AM in the shortwave High Frequency (HF—3 MHz to 30 MHz) radio spectrum for very long ranges or narrowband FM in the VHF spectrum for much shorter ranges. Government, police, fire and commercial voice services use narrowband FM on special frequencies. Fidelity is sacrificed to use a smaller range of radio frequencies, usually five kHz of deviation, rather than the 75 kHz used by FM broadcasts and 25 kHz used by TV sound.
Civil and military HF (high frequency) voice services use shortwave radio to contact ships at sea, aircraft and isolated settlements. Most use single sideband voice (SSB), which uses less bandwidth than AM. On an AM radio SSB sounds like ducks quacking. Viewed as a graph of frequency versus power, an AM signal shows power where the frequencies of the voice add and subtract with the main radio frequency. SSB cuts the bandwidth in half by suppressing the carrier and (usually) lower sideband. This also makes the transmitter about three times more powerful, because it doesn't need to transmit the unused carrier and sideband.
TETRA, Terrestrial Trunked Radio is a digital cell phone system for military, police and ambulances. Commercial services such as XM, WorldSpace and Sirius offer encrypted digital Satellite radio.​


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## المتوكلة على الله (29 أبريل 2007)

*Radio*

*Telephony*
Mobile phones transmit to a local cell site (transmitter/receiver) that ultimately connects to the public switched telephone network (PSTN) through an optic fiber or microwave radio and other network elements. When the mobile phone nears the edge of the cell site's radio coverage area, the central computer switches the phone to a new cell. Cell phones originally used FM, but now most use various digital modulation schemes. Satellite phones use satellites rather than cell towers to communicate. They come in two types: INMARSAT and Iridium. Both types provide world-wide coverage. INMARSAT uses geosynchronous satellites, with aimed high-gain antennas on the vehicles. Iridium uses 66 Low Earth Orbit satellites as the cells.​* Video*

Television sends the picture as AM and the sound as FM, with the sound carrier a fixed frequency (4.5 MHz in the NTSC system) away from the video carrier. Analog television also uses a vestigial sideband on the video carrier to reduce the bandwidth required.
Digital television uses quadrature amplitude modulation. A Reed-Solomon error correction code adds redundant correction codes and allows reliable reception during moderate data loss. Although many current and future codecs can be sent in the MPEG-2 transport stream container format, as of 2006 most systems use a standard-definition format almost identical to DVD: MPEG-2 video in Anamorphic widescreen and MPEG layer 2 (_MP2_) audio. High-definition television is possible simply by using a higher-resolution picture, but H.264/AVC is being considered as a replacement video codec in some regions for its improved compression. With the compression and improved modulation involved, a single "channel" can contain a high-definition program and several standard-definition programs.​* Navigation*

All satellite navigation systems use satellites with precision clocks. The satellite transmits its position, and the time of the transmission. The receiver listens to four satellites, and can figure its position as being on a line that is tangent to a spherical shell around each satellite, determined by the time-of-flight of the radio signals from the satellite. A computer in the receiver does the math.
Radio direction-finding is the oldest form of radio navigation. Before 1960 navigators used movable loop antennas to locate commercial AM stations near cities. In some cases they used marine radiolocation beacons, which share a range of frequencies just above AM radio with amateur radio operators. Loran systems also used time-of-flight radio signals, but from radio stations on the ground. VOR (Very High Frequency Omnidirectional Range), systems (used by aircraft), have an antenna array that transmits two signals simultaneously. A directional signal rotates like a lighthouse at a fixed rate. When the directional signal is facing north, an omnidirectional signal pulses. By measuring the difference in phase of these two signals, an aircraft can determine its bearing or radial from the station, thus establishing a line of position. An aircraft can get readings from two VOR and locate its position at the intersection of the two radials, known as a "fix." When the VOR station is collocated with DME (Distance Measuring Equipment), the aircraft can determine its bearing and range from the station, thus providing a fix from only one ground station. Such stations are called VOR/DMEs. The military operates a similar system of navaids, called TACANs, which are often built into VOR stations. Such stations are called VORTACs. Because TACANs include distance measuring equipment, VOR/DME and VORTAC stations are identical in navigation potential to civil aircraft.​*Radar*

Radar (Radio Detection And Ranging) detects things at a distance by bouncing radio waves off them. The delay caused by the echo measures the distance. The direction of the beam determines the direction of the reflection. The polarization and frequency of the return can sense the type of surface. Navigational radars scan a wide area two to four times per minute. They use very short waves that reflect from earth and stone. They are common on commercial ships and long-distance commercial aircraft
General purpose radars generally use navigational radar frequencies, but modulate and polarize the pulse so the receiver can determine the type of surface of the reflector. The best general-purpose radars distinguish the rain of heavy storms, as well as land and vehicles. Some can superimpose sonar data and map data from GPS position.
Search radars scan a wide area with pulses of short radio waves. They usually scan the area two to four times a minute. Sometimes search radars use the doppler effect to separate moving vehicles from clutter. Targeting radars use the same principle as search radar but scan a much smaller area far more often, usually several times a second or more. Weather radars resemble search radars, but use radio waves with circular polarization and a wavelength to reflect from water droplets. Some weather radar use the doppler to measure wind speeds.​*Emergency services*

Emergency Position-Indicating Radio Beacons (EPIRBs), Emergency Locating Transmitters (ELTs) or Personal Locator Beacons (PLBs) are small radio transmitters that satellites can use to locate a person or vehicle needing rescue. Their purpose is to help rescue people in the first day, when survival is most likely. There are several types, with widely-varying performance.​


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## المتوكلة على الله (29 أبريل 2007)

*The electromagnetic spectrum*

*The electromagnetic spectrum*
Radio waves are a form of electromagnetic radiation, created whenever a charged object (in normal radio transmission, an electron) accelerates with a frequency that lies in the radio frequency (RF) portion of the electromagnetic spectrum. In radio, this acceleration is caused by an alternating current in an antenna. Radio frequencies occupy the range from a few tens of hertz to three hundred gigahertz, although commercially important uses of radio use only a small part of this spectrum.[1]
*Radio spectrum*ELFSLFULFVLFLFMFHFVHFUHFSHFEHF3 Hz30 Hz300 Hz3 kHz30 kHz300 kHz3 MHz30 MHz300 MHz3 GHz30 GHz30 Hz300 Hz3 kHz30 kHz300 kHz3 MHz30 MHz300 MHz3 GHz30 GHz300 GHz​​Other types of electromagnetic radiation, with frequencies above the RF range, are microwave, infrared, visible light, ultraviolet, X-rays and gamma rays. Since the energy of an individual photon of radio frequency is too low to remove an electron from an atom, radio waves are classified as non-ionizing radiation.


 


Electromagnetic spectrum and diagram of radio transmission of an audio signal. *NB* The colours used in this diagram of the electromagnetic spectrum are for decoration only. They do not correspond to the wavelengths and frequencies indicated on the scale.
​


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## المتوكلة على الله (30 أبريل 2007)

*telephone*

​*Telephone*




http://en.wikipedia.org/wiki/Telephone#searchInput 



 http://en.wikipedia.org/wiki/Image:Telephone-modele-W48.jpg
A French rotary telephone




 http://en.wikipedia.org/wiki/Image:Tphone2.jpg
A basic modern telephone




 http://en.wikipedia.org/wiki/Image:ATTtelephone-large.jpg
Touch Tone® telephone




 http://en.wikipedia.org/wiki/Image:CNAM-IMG_0564.jpg
Copy of the original phone of Graham Bell at the _Musée des Arts et Métiers_ in Paris




 http://en.wikipedia.org/wiki/Image:1896_telephone.jpg
1896 Telephone (Sweden)




 http://en.wikipedia.org/wiki/Image:Phone_handset.jpeg
A telephone handset



The *telephone* is a telecommunications device which is used to transmit and receive sound (most commonly voice and speech) across distance. Most telephones operate through transmission of electric signals over a complex telephone network which allows almost any phone user to communicate with almost any other.


​//​*Basic principle*

The telephone handles two kinds of information: signals and voice, at different times on the same twisted pair of wires. The signaling equipment consists of a bell to alert the user of incoming calls, and a dial to enter the phone number for outgoing calls. A calling party wishing to speak to another telephone will pick up the handset, thus operating the switch hook, which puts the telephone into active state or off hook with a resistance short across the wires, causing current to flow. The telephone exchange detects the DC current, attaches a digit receiver, and sends dial tone to indicate readiness. The user pushes the number buttons, which are connected to a tone generator inside the dial, which generates DTMF tones. The exchange connects the line to the desired line and alerts that line.
When a phone is inactive, that is on hook, its bell, beeper, flasher or other alerting device is connected across the line through a capacitor. The inactive phone does not short the line, thus the exchange knows it is on hook and only the bell is electrically connected. When someone calls this phone, the telephone exchange applies a high voltage pulsating signal, which causes the sound mechanism to ring, beep or otherwise alert the called party. When that user picks up the handset, the switchhook disconnects the bell, connects the voice parts of the telephone, and puts a resistance short on the line, confirming that the phone has been answered and is active. Both lines being off hook, the signaling job is complete. The parties are connected together, and may converse using the voice parts of their telephones.
The voice parts of the telephone are in the handset, and consist of a transmitter (often called microphone) and a receiver. The transmitter, powered from the line, puts out an electric current which varies in response to the acoustic pressure waves produced by the voice. The resulting variations in electric current are transmitted along the telephone line to the other phone, where they are fed into the coil of the receiver, which is a miniature loudspeaker. The varying electric current in the coil causes it to move back and forth, reproducing the acoustic pressure waves of the transmitter. Thus, it speaks.
When a party "hangs up", that is puts the handset on the cradle, DC current ceases to flow in that line, thus signaling to the exchange switch to disconnect the telephone call.​


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## المتوكلة على الله (30 أبريل 2007)

*Early commercial instruments*

*Early commercial instrument*s
As hinted in the above timeline, early telephones were technically diverse. Some used a liquid transmitter, which was dangerous and inconvenient and soon went out of use. Some were dynamic, i.e. their diaphragm wriggled a coil of wire in the field of a permanent magnet or vice versa. This kind survived in small numbers through the 20th century in military and maritime applications where its ability to create its own electrical power was crucial. Most, however, used the Edison/Berliner carbon transmitter, which was much louder than the other kinds, even though it required an induction coil, actually acting as an impedance matching transformer to make it compatible to the impedance of the line. The Edison patents kept the Bell monopoly viable into the 20th century, by which time the network was more important than the instrument anyway.
Early telephones were locally powered; ie they used a dynamic transmitter or else powered their transmitter with a local battery. One of the jobs of outside plant personnel was to visit each telephone periodically to inspect the battery. During the 20th century "common battery" operation came to dominate, powered by "talk battery" from the telephone exchange over the same wires that carried the voice signals. Late in the century, wireless handsets brought a revival of local battery power.
Early telephones had one wire for both transmitting and receiving of audio, with ground return as used in telegraphs. The earliest dynamic telephones also had only opening for sound, and the user alternately listened and spoke (rather, shouted) into the same hole. Sometimes the instruments were operated in pairs at each end, making conversation more convenient if more expensive.
At first, the benefits of an exchange were not exploited. Telephones instead were leased in pairs to the subscriber, for example one for his home and one for his shop, who must arrange with telegraph contractors to construct a line between them. Users who wanted the ability to speak to three or four different shops, suppliers etc would obtain and set up three or four pairs of telephones. Western Union, already using telegraph exchanges, quickly extended the principle to its telephones in New York and San Francisco, and Bell was not slow in appreciating the potential.
Signalling began in an appropriately primitive manner. The user alerted the other end, or the exchange operator, by whistling into the transmitter. Exchange operation soon resulted in telephones being equipped with a bell, first operated over a second wire and later with the same wire using a condenser. Telephones connected to the earliest Strowger automatic exchanges had seven wires, one for the knife switch, one for each telegraph key, one for the bell, one for the push button and two for speaking.
Rural and other telephones that were not on a common battery exchange had a "magneto" or hand cranked generator to produce a high voltage alternating signal to ring the bells of other telephones on the line and to alert the operator.
In the 1890s a new smaller style of telephone was introduced, packaged in three parts. The transmitter stood on a stand, known as a "candlestick" for its shape. When not in use, the receiver hung on a hook with a switch in it, known as a "switchhook." Previous telephones had required operating a separate switch to connect either the voice parts of the telephone or the bell. With the new kind, they less often forgot and left the phone "off the hook". The bell, induction coil, battery and magneto were in a separate "bell box" if it was a magneto exchange. For a common battery exchange, the bell box could be installed under a desk or otherwise out of the way, since it didn't need a battery or magneto.
Cradle designs were also used at this time, having a handle with the receiver and transmitter attached, separate from the cradle base that housed the magneto crank and other parts. They were larger than the "candlestick" and more popular.
Disadvantages of single wire operation such as crosstalk and hum from nearby AC power wires had already led to the use of twisted pairs and, for long distance telephones, four-wire circuits. Users at the beginning of the 20th century did not place long distance calls from their own telephones but made an appointment to use a special sound proofed long distance telephone booth furnished with the latest high technology equipment where, for a workingman's week's pay, they could sit comfortably for three minutes and shout across hundreds of miles without waking the neighbors.
What turned out to be the most popular and long lasting physical style of telephone was introduced in the early 20th century, including Bell's Model 102. A carbon granule transmitter and electromagnetic receiver were united in a single molded plastic handle, which when not in use sat in a cradle in the base unit. The circuit diagram of the Model 102 shows the direct connection of the receiver to the line, while the transmitter was induction coupled, with energy supplied by a local battery. The coupling transformer, battery, and ringer were in a separate enclosure. The dial switch in the base interrupted the line current by repeatedly but very briefly disconnecting the line 1-10 times for each digit, and the hook switch (in the center of the circuit diagram) permanently disconnected the line and the transmitter battery while the handset was on the cradle.
After the 1930s the base also enclosed the bell and induction coil, obviating the old separate bell box. Power was supplied to each subscriber line by central office batteries instead of a local battery, which required periodic service. For the next half century, the network behind the telephone became progressively larger and much more efficient, but after the dial was added the instrument itself changed little until Touch Tone replaced the dial in the 1960's.​


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## المتوكلة على الله (30 أبريل 2007)

*digital telephony*



http://en.wikipedia.org/wiki/Image:Telé.jpg
Telephones (and Teléfonos) on sale at a Best Buy store.

​* Digital Telephony*

_http://en.wikipedia.org/wiki/Digital_Telephony_ 
The Public Switched Telephone Network (PSTN) has gradually evolved towards digital telephony which has improved the capacity and quality of the network. End-to-end analog telephone networks were first modified in the early 1960s by upgrading long-haul transmission networks with T1 carrier systems. Later technologies such as SONET and fiber optic transmission methods further advanced digital transmission. Although analog carrier systems existed, digital transmission made it possible to significantly increase the number of channels multiplexed on a single transmission medium. While today the end instrument remains analog, the analog signals reaching the aggregation point (Serving Area Interface (SAI) or the central office (CO) ) are typically converted to digital signals. Digital loop carriers (DLC) are often used, placing the digital network ever closer to the customer premises, relegating the analog local loop to legacy status​*Wireless phone systems*

While the term "wireless" in this context means radio and can refer to any telephone that uses radio waves (such telephones have existed since 1915: _see_ "Hello, Hawaii, How Are You?"), it is primarily used for cellular mobile phones. In the United States wireless companies tend to use the term wireless to refer to a wide range of services while the cell phone itself is called a mobile phone, mobile, PCS phone, cell phone or simply cell with the trend now moving towards mobile.
The changes in terminology is partially due to providers using different terms in marketing to differentiate newer digital services from older analog systems and services of one company from another.​*Cordless telephone*



http://en.wikipedia.org/wiki/Image:CordlessUniden-large.jpg
Cordless handset


Cordless telephones, invented by Teri Pall in 1965, consist of a base unit that connects to the land-line system and also communicates with remote handsets by low power radio. This permits use of the handset from any location within range of the base. Because of the power required to transmit to the handset, the base station is powered with an electronic power supply. Thus, cordless phones typically do not function during power outages. Initially, cordless phones used the 1.7 MHz frequency range to communicate between base and handset. Because of quality and range problems, these units were soon superseded by systems that used frequency modulation (FM) at higher frequency ranges (49 MHz, 900 MHz, 2.4 GHz, and 5.8 GHz). The 2.4 GHz cordless phones can interfere with certain wireless LAN protocols (802.11b/g) due to the usage of the same frequencies. On the 2.4 GHz band, several "channels" are utilized in an attempt to guard against degradation in the quality of the voice signal due to crowding. The range of modern cordless phones is normally on the order of a few hundred meters.​* Mobile phones*


Most modern mobile phone systems are cell-structured. Radio is used to communicate between a handset and nearby cell sites.
When a handset gets too far from a cell site, a computer system commands the handset and a closer cell site to take up the communications on a different channel without interrupting the call.
Radio frequencies are a limited, shared resource. The higher frequencies used by cell phones have advantages over short distances. Connection distance is somewhat predictable and can be controlled by adjusting the power level. By only using enough power to connect to the "nearest" cell site phones using one cell site will cause almost no interference with phones using the same frequencies on another cell site. The higher frequencies also work well with various forms of multiplexing which allows more than one phone to connect to the same tower with the same set of frequencies.​* Satellite phones*


Some mobile telephones, especially those used in remote locations, where constructing a cell network would be too unprofitable or difficult, instead communicate directly with an orbiting satellite. Such devices tend to be bulkier than cell-based mobile phones, as they require a large antenna or dish for communicating with the satellite, but do not require ground based transmitters, making them useful for communicating from remote areas and disaster zones.​* Semi-Cordless Phone*

There are phones that work as a cordless phone when near their corresponding base station (and sometimes other base stations) and work as a wireless phone when in other locations but for a variety of reasons did not become popular.​


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## المتوكلة على الله (30 أبريل 2007)

*microwave*

*Microwave*

*Microwaves* are electromagnetic waves with wavelengths longer than those of terahertz (THz) frequencies, but relatively short for radio waves. Microwaves have wavelengths approximately in the range of 30 cm (frequency = 1 GHz) to 1 mm (300 GHz). This range of wavelengths has led many to question the naming convention used for microwaves as the name suggests a micrometer wavelength. However, the boundaries between far infrared light, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly arbitrary and are used variously between different fields of study. The term microwave generally refers to "alternating current signals with frequencies between 300 MHz (3×108 Hz) and 300 GHz (3×1011 Hz)."[1]​//​* Discovery*

The existence of electromagnetic waves, of which microwaves are part of the frequency spectrum, was predicted by James Clerk Maxwell in 1864 from his Maxwell's equations. In 1888, Heinrich Hertz was the first to demonstrate the existence of electromagnetic waves by building an apparatus that produced and detected microwaves in the UHF region. The design necessarily used horse-and-buggy materials, including a horse trough, a wrought iron point spark, Leyden jars, and a length of zinc gutter whose parabolic cross-section worked as a reflection antenna.​* Frequency range*



 http://en.wikipedia.org/wiki/Image:Atmospheric_microwave_transmittance_at_mauna_kea(simulated).gif
Plot of the zenith atmospheric transmission on the summit of Mauna Kea throughout the entire gigahertz range of the electromagnetic spectrum at a precipitable water vapor level of 0.001 mm. (simulated)


The microwave range includes ultra-high frequency (UHF) (0.3–3 GHz), super high frequency (SHF) (3–30 GHz), and extremely high frequency (EHF) (30–300 GHz) signals.
Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that it is effectively opaque, until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges.​* Devices*

Vacuum tube based devices operate on the ballistic motion of electrons in a vacuum under the influence of controlling electric or magnetic fields, and include the magnetron, klystron, traveling wave tube (TWT), and gyrotron. These devices work in the density modulated mode, rather than the current modulated mode. This means that they work on the basis of clumps of electrons flying ballistically through them, rather than using a continuous stream.​* Uses*


A microwave oven works by passing microwave radiation, usually at a frequency of 2450 MHz (a wavelength of 12.24 cm), through the food. Water, fat, and sugar molecules in the food absorb energy from the microwave beam in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a positive charge at one end and a negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field induced by the microwave beam. This molecular movement creates heat as the rotating molecules hit other molecules and put them into motion. Microwave heating is most efficient on liquid water, and much less so on fats and sugars (which have less molecular dipole moment), and frozen water (where the molecules are not free to rotate). Microwave heating is sometimes incorrectly explained as a rotational resonance of water molecules: such resonance only occurs at much higher frequencies, in the tens of gigahertz. Moreover, large industrial/commercial microwave ovens operating in the 900 MHz range also heat water and food perfectly well. ​
A common misconception is that microwave ovens cook food from the "inside out". In reality, microwaves are absorbed in the outer layers of food in a manner somewhat similar to heat from other methods. The misconception arises because microwaves penetrate dry nonconductive substances at the surfaces of many common foods, and thus often deposit initial heat more deeply than other methods. Depending on water ******* the depth of initial heat deposition may be several centimeters or more with microwave ovens, in contrast to grilling ("broiling" in American English), which relies on infrared radiation, or convection heating, which deposit heat shallowly at the food surface. Depth of penetration of microwaves is dependent on food composition and the frequency, with lower microwave frequencies being more penetrating. ​



 http://en.wikipedia.org/wiki/Image:Milfordsun300.jpg
AT&T Long Lines Microwave Relay Tower, Utah. The lower horn antennas are for TD Radio, 3.7-4.2 GHz, and can simultaneously carry signals in the 6 and 11 GHz bands.
​
Microwave radio is used in broadcasting and telecommunication transmissions because, due to their short wavelength, highly directive antennas are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies). There is also more bandwidth in the microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300 MHz is less than 300 MHz while many GHz can be used above 300 MHz. Typically, microwaves are used in television news to transmit a signal from a remote location to a television station from a specially equipped van. ​

Before the advent of fiber optic transmission, most long distance telephone calls were carried via microwave point-to-point links through sites like the AT&T Long Lines facility shown in the photograph. Starting in the early 1950's, frequency division multiplex was used to send up to 5,400 telephone channels on each microwave radio channel, with as many as ten radio channels combined into one antenna for the _hop_ to the next site, up to 70 km away. ​

Radar also uses microwave radiation to detect the range, speed, and other characteristics of remote objects. ​

Wireless LAN protocols, such as Bluetooth and the IEEE 802.11 specifications, also use microwaves in the 2.4 GHz ISM band, although 802.11a uses ISM band and UNII frequencies in the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services can be found in many countries (but not the USA) in the 3.5–4.0 GHz range. ​

Metropolitan Area Networks: MAN protocols, such as WiMAX (Worldwide Interoperability for Microwave Access) based in the IEEE 802.16 specification. The IEEE 802.16 specification was designed to operate between 2 to 11 GHz. The commercial implementations are in the 2.5 GHz, 3.5 GHz and 5.8 GHz ranges. ​

Wide Area Mobile Broadband Wireless Access: MBWA protocols based on standards specifications such as ATIS/ANSI HC-SDMA (e.g. iBurst) are designed to operate between 1.6 and 2.3 GHz to give mobility and in-building penetration characteristics similar to mobile phones but with vastly greater spectral efficiency. ​

Cable TV and Internet access on coax cable as well as broadcast television use some of the lower microwave frequencies. Some mobile phone networks, like GSM, also use the lower microwave frequencies. ​

Many semiconductor processing techniques use microwaves to generate plasma for such purposes as reactive ion etching and plasma-enhanced chemical vapor deposition (PECVD). ​

Microwaves can be used to transmit power over long distances, and post-World War II research was done to examine possibilities. NASA worked in the 1970s and early 1980s to research the possibilities of using Solar power satellite (SPS) systems with large solar arrays that would beam power down to the Earth's surface via microwaves. ​

A maser is a device similar to a laser, except that it works at microwave frequencies. ​

Most radio astronomy uses microwaves. ​


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## المتوكلة على الله (30 أبريل 2007)

*Microwave frequency bands*

Microwave frequency bands
The microwave spectrum is usually defined as electromagnetic energy ranging from approximately 1 GHz to 1000 GHz in frequency, but older usage includes lower frequencies. Most common applications are within the 1 to 40 GHz range. Microwave Frequency Bands as defined by the Radio Society of Great Britain in the table below:
Microwave frequency bandsDesignationFrequency rangeL band1 to 2 GHzS band2 to 4 GHzC band4 to 8 GHzX band8 to 12 GHzKu band12 to 18 GHzK band18 to 26.5 GHzKa band26.5 to 40 GHzQ band30 to 50 GHzU band40 to 60 GHzV band50 to 75 GHzE band60 to 90 GHzW band75 to 110 GHzF band90 to 140 GHzD band110 to 170 GHz
The above table reflects Radio Society of Great Britain (RSGB) usage. The term P band is sometimes used for Ku Band. For other definitions see Letter Designations of Microwave Bands​*Health effects*

First, it is imperative to understand that the word "radiation" applies to different things. One kind is ionizing radiation or particle radiation, which can (for example) damage organic molecules by impacting them and imparting energy. Another is nonionizing wave radiation, like microwaves, which cannot. It is common for the terms to be crossed, so that people get the impression a microwave oven might make food "radioactive", which is literally impossible no matter how much microwave energy is used. Similarly, microwave radiation could not cause cancer in the same way that uranium could. The word "radiation" refers to the fact that energy can radiate, and not to the different nature and effects of different kinds of energy.
The health effects of microwaves are highly controversial. A great number of studies have been undertaken in the last two decades, some concluding that microwaves pose a hazard to health, and others concluding they are safe. It is understood that microwave radiation of a level that causes even minute heating of living tissue is hazardous, and most countries have standards limiting exposure, such as the Federal Communications Commission RF safety regulations. Still at issue is whether lower levels of microwave energy have bioeffects.
Synthetic reviews of literature indicate the predominance of their safety of utilisation. The motivations of each "side" in this debate are held in suspect by the other.
For example, in a paper published in January 2007, Panagopoulos et al. showed that exposing flies to a cellular phone in similar conditions to those to which a mobile phone user is exposed resulted in cell death. 
Microwave radiation is a form of non-ionizing radiation and therefore has little to no effect at the molecular level. It is, for instance, unable to cause cancer via DNA damage, something that more energetic X-rays and gamma rays can do. Heating is often observed, but the harm this may cause is unknown at low levels, the overall effect being similar to using an electric blanket to provide the same level of heat. Other possible hazards are, as mentioned, still being studied. Any level that causes heating also has the potential to cause localized areas of intense heating due to standing waves.​


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## bebo13 (28 يوليو 2007)

بسم الله ما شاء الله على المعلومات القيمة وجعلها الله في ميزان حسناتك ان شاء الله


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## المتوكلة على الله (1 أكتوبر 2007)

مشكورين على الردود
حياكم الله
وبارك الله فيكم


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## hammhamm44 (1 أكتوبر 2007)

رمضان كريم 
بصراحة استاذ فى العرض الجميل والواضح


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## فوزي فالح (1 أكتوبر 2007)

المتوكلة علي الله 
جزاك الله خير 
بصراحة لم اجد جزئية في الاتصالات لم تتطرق اليها 
مجهود يكتب في ميزان حسناتك
تحياتي


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## المتوكلة على الله (2 أكتوبر 2007)

شكرا لكم على الردود
بارك الله فيكم


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## محمود حمدي السعدني (3 أكتوبر 2007)

جزاك الله خير


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## المتوكلة على الله (4 أكتوبر 2007)

محمود حمدي السعدني قال:


> جزاك الله خير


جزاكم الله خيرا مثله
ومشكورين على الرد


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## بوعامرالسالك (9 فبراير 2008)

mersiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii


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## المتوكلة على الله (9 يوليو 2008)

العفو منكم جميعا
وجزاكم الله كل الخير


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