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  1. [11]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


    تاريخ التسجيل: Apr 2007
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    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 receiveradio 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.

    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. [12]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


    تاريخ التسجيل: Apr 2007
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    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 isotropic 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 dipole):
    0 dBd = 2.15 dBi

    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. [13]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


    تاريخ التسجيل: Apr 2007
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    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.

    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. [14]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


    تاريخ التسجيل: Apr 2007
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    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.






    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. [15]
    المتوكلة على الله
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    عضو متميز


    تاريخ التسجيل: Apr 2007
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    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.

    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. [16]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


    تاريخ التسجيل: Apr 2007
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    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.

    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. [17]
    hammhamm44
    hammhamm44 غير متواجد حالياً
    عضو متميز
    الصورة الرمزية hammhamm44


    تاريخ التسجيل: Apr 2004
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    thanks for a good informations

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    د.م. فكرى نور

  8. [18]
    المتوكلة على الله
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    عضو متميز


    تاريخ التسجيل: Apr 2007
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    thanks alot for your reply

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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

  9. [19]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


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

    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'sFourteen 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 (RAdio Detection And Ranging).
    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..

    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

  10. [20]
    المتوكلة على الله
    المتوكلة على الله غير متواجد حالياً
    عضو متميز


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

    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.

    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

  
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