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  1. [21]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


    تاريخ التسجيل: Apr 2007
    المشاركات: 3,104
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    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 streamcontainer 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.

    0 Not allowed!




    على الله توكلوا .........ولا تتواكلوا
    يا سادتي..
    لا ترفعوا تلك الأيادي للسماء..
    لا ترفعوها إنها لن تستجيب..
    هل يستجيب الله صوت العاجزين؟!
    من قد أضاعوا الدين واحترفوا البكاء!!
    من حرروا الأرض السليبة بالقعود وبالدعاء!!
    من واجهوا كيد الأعادي بالتناحر والجفاء !!
    فلنأخذ بالأسباب ولنتوكل على الله
    وبإذن الله لن نرد خائبين

  2. [22]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


    تاريخ التسجيل: Apr 2007
    المشاركات: 3,104
    Thumbs Up
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    Given: 0

    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 spectrumELFSLFULFVLFLFMFHFVHFUHFSHFEHF3 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.


    0 Not allowed!




    على الله توكلوا .........ولا تتواكلوا
    يا سادتي..
    لا ترفعوا تلك الأيادي للسماء..
    لا ترفعوها إنها لن تستجيب..
    هل يستجيب الله صوت العاجزين؟!
    من قد أضاعوا الدين واحترفوا البكاء!!
    من حرروا الأرض السليبة بالقعود وبالدعاء!!
    من واجهوا كيد الأعادي بالتناحر والجفاء !!
    فلنأخذ بالأسباب ولنتوكل على الله
    وبإذن الله لن نرد خائبين

  3. [23]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


    تاريخ التسجيل: Apr 2007
    المشاركات: 3,104
    Thumbs Up
    Received: 18
    Given: 0

    telephone

    Telephone







    A French rotary telephone



    A basic modern telephone



    Touch Tone® telephone



    Copy of the original phone of Graham Bell at the Musée des Arts et Métiers in Paris



    1896 Telephone (Sweden)



    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.

    0 Not allowed!




    على الله توكلوا .........ولا تتواكلوا
    يا سادتي..
    لا ترفعوا تلك الأيادي للسماء..
    لا ترفعوها إنها لن تستجيب..
    هل يستجيب الله صوت العاجزين؟!
    من قد أضاعوا الدين واحترفوا البكاء!!
    من حرروا الأرض السليبة بالقعود وبالدعاء!!
    من واجهوا كيد الأعادي بالتناحر والجفاء !!
    فلنأخذ بالأسباب ولنتوكل على الله
    وبإذن الله لن نرد خائبين

  4. [24]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


    تاريخ التسجيل: Apr 2007
    المشاركات: 3,104
    Thumbs Up
    Received: 18
    Given: 0

    Early commercial instruments

    Early commercial instruments
    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.

    0 Not allowed!




    على الله توكلوا .........ولا تتواكلوا
    يا سادتي..
    لا ترفعوا تلك الأيادي للسماء..
    لا ترفعوها إنها لن تستجيب..
    هل يستجيب الله صوت العاجزين؟!
    من قد أضاعوا الدين واحترفوا البكاء!!
    من حرروا الأرض السليبة بالقعود وبالدعاء!!
    من واجهوا كيد الأعادي بالتناحر والجفاء !!
    فلنأخذ بالأسباب ولنتوكل على الله
    وبإذن الله لن نرد خائبين

  5. [25]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


    تاريخ التسجيل: Apr 2007
    المشاركات: 3,104
    Thumbs Up
    Received: 18
    Given: 0

    digital telephony


    Telephones (and Teléfonos) on sale at a Best Buy store.


    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


    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.

    0 Not allowed!




    على الله توكلوا .........ولا تتواكلوا
    يا سادتي..
    لا ترفعوا تلك الأيادي للسماء..
    لا ترفعوها إنها لن تستجيب..
    هل يستجيب الله صوت العاجزين؟!
    من قد أضاعوا الدين واحترفوا البكاء!!
    من حرروا الأرض السليبة بالقعود وبالدعاء!!
    من واجهوا كيد الأعادي بالتناحر والجفاء !!
    فلنأخذ بالأسباب ولنتوكل على الله
    وبإذن الله لن نرد خائبين

  6. [26]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


    تاريخ التسجيل: Apr 2007
    المشاركات: 3,104
    Thumbs Up
    Received: 18
    Given: 0

    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 radiowaves 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


    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 sugarmolecules 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.

    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 distancetelephone 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 LANprotocols, such as Bluetooth and the IEEE802.11 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 IEEE802.16 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.
    • 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.

    0 Not allowed!




    على الله توكلوا .........ولا تتواكلوا
    يا سادتي..
    لا ترفعوا تلك الأيادي للسماء..
    لا ترفعوها إنها لن تستجيب..
    هل يستجيب الله صوت العاجزين؟!
    من قد أضاعوا الدين واحترفوا البكاء!!
    من حرروا الأرض السليبة بالقعود وبالدعاء!!
    من واجهوا كيد الأعادي بالتناحر والجفاء !!
    فلنأخذ بالأسباب ولنتوكل على الله
    وبإذن الله لن نرد خائبين

  7. [27]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


    تاريخ التسجيل: Apr 2007
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    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 nonionizingwave 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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    على الله توكلوا .........ولا تتواكلوا
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  8. [28]
    bebo13
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    بسم الله ما شاء الله على المعلومات القيمة وجعلها الله في ميزان حسناتك ان شاء الله

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  9. [29]
    المتوكلة على الله
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    مشكورين على الردود
    حياكم الله
    وبارك الله فيكم

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    على الله توكلوا .........ولا تتواكلوا
    يا سادتي..
    لا ترفعوا تلك الأيادي للسماء..
    لا ترفعوها إنها لن تستجيب..
    هل يستجيب الله صوت العاجزين؟!
    من قد أضاعوا الدين واحترفوا البكاء!!
    من حرروا الأرض السليبة بالقعود وبالدعاء!!
    من واجهوا كيد الأعادي بالتناحر والجفاء !!
    فلنأخذ بالأسباب ولنتوكل على الله
    وبإذن الله لن نرد خائبين

  10. [30]
    hammhamm44
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    د.م. فكرى نور

  
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