Assignment Comm3

7/31/2019 Assignment Comm3

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frequencies, a receiving antenna may include not only the actual electrical antenna but an integratedpreamplifier and/or mixer.

PARAMETER OF ANTENNA

Antennas are characterized by a number of performance measures which a user would be concerned within selecting or designing an antenna for a particular application. Chief among these relate to thedirectional characteristics (as depicted in the antenna's radiation pattern) and the resulting gain. Even inomnidirectional (or weakly directional) antennas, the gain can often be increased by concentrating moreof its power in the horizontal directions, sacrificing power radiated toward the sky and ground. The

antenna's power gain (or simply "gain") also takes into account the antenna's efficiency, and is often theprimary figure of merit.

Resonant antennas are expected to be used around a particular resonant frequency; an antenna musttherefore be built or ordered to match the frequency range of the intended application. A particularantenna design will present a particular feedpoint impedance. While this may affect the choice of anantenna, an antenna's impedance can also be adapted to the desired impedance level of a system usingan matching network while maintaining the other characteristics (except for a possible loss of efficiency).

Although these parameters can be measured in principle, such measurements are difficult and requirevery specialized equipment. Beyond tuning a transmitting antenna using an SWR meter, the typical userwill depend on theoretical predictions based on the antenna design and/or on claims of a vendor.

An antenna transmits and receives radio waves with a particular polarization which can be reoriented by

tilting the axis of the antenna in many (but not all) cases. The physical size of an antenna is often apractical issue, particularly at lower frequencies (longer wavelengths). Highly directional antennas needto be significantly larger than the wavelength. Resonant antennas use a conductor, or a pair ofconductors, each of which is about one quarter of the wavelength in length. Antennas that are required tobe very small compared to the wavelength sacrifice efficiency and cannot be very directional. Fortunatelyat higher frequencies (UHF, microwaves) trading off performance to obtain a smaller physical size isusually not required.
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Resonant antennas

While there are broadband designs for antennas, the vast majority of antennas are based on the half-wave dipole which has a particular resonant frequency. At its resonant frequency, the wavelength(figured by dividing the speed of light by the resonant frequency) is slightly over twice the length of thehalf-wave dipole (thus the name). The quarter-wave vertical antenna consists of one arm of a half-wavedipole, with the other arm replaced by a connection to ground or an equivalent groundplane (or counterpoise). A Yagi-Uda array consists of a number of resonant dipole elements, only one ofwhich is directly connected to the transmission line. The quarter-wave elements of a dipole or verticalantenna imitate a series-resonant electrical element, since if they are driven at the resonant frequencya standing wave is created with the peak current at the feed-point and the peak voltage at the far end.

A common misconception is that the ability of a resonant antenna to transmit (or receive) fails atfrequencies far from the resonant frequency. The reason a dipole antenna needs to be used at the resonantfrequency has to do with the impedance match between the antenna and the transmitter or receiver (andits transmission line). For instance, a dipole using a fairly thin conductor will have a purely resistive

feedpoint impedance of about 63 ohms at its design frequency. Feeding that antenna with a current of 1ampere will require 63 volts of RF, and the antenna will radiate 63 watts (ignoring losses) of radiofrequency power. If that antenna is driven with 1 ampere at a frequency 20% higher, it will still radiate asefficiently but in order to do that about 200 volts would be required due to the change in the antenna'simpedance which is now largely reactive (voltage out of phase with the current). A typical transmitterwould not find that impedance acceptable and would deliver much less than 63 watts to it; thetransmission line would be operating at a high (poor) standing wave ratio. But using an appropriatematching network, that large reactive impedance could be converted to a resistive impedance satisfyingthe transmitter and accepting the available power of the transmitter.

This principle is used to construct vertical antennas substantially shorter than the 1/4 wavelength atwhich the antenna is resonant. By adding an inductance in series with the vertical antenna (a so-called loading coil) the capacitative reactance of this antenna can be cancelled leaving a pure resistance

which can then be matched to the transmission line. Sometimes the resulting resonant frequency of such asystem (antenna plus matching network) is described using the construct of "electrical length" and the useof a shorter antenna at a lower frequency than its resonant frequency is termed "electrical lengthening".For example, at 30 MHz (wavelength = 10 meters) a true resonant monopole would be almost 2.5 meters(1/4 wavelength) long, and using an antenna only 1.5 meters tall would require the addition of a loadingcoil. Then it may be said that the coil has "lengthened" the antenna to achieve an "electrical length" of 2.5meters, that is, 1/4 wavelength at 30 MHz where the combined system now resonates. However, theresulting resistive impedance achieved will be quite a bit lower than the impedance of a resonantmonopole, likely requiring further impedance matching.

Current and voltage distribution

The antenna conductors have the lowest feed-point impedance at the resonant frequency where they arejust under 1/4 wavelength long; two such conductors in line fed differentially thus realize the familiar"half-wave dipole". When fed with an RF current at the resonant frequency, the quarter wave elementcontains a standing wave with the voltage and current largely in phase quadrature, as would be obtainedusing a quarter wave stub of transmission line. The current reaches a minimum at the end of the elementand is maximum at the feed-point. The voltage, on the other hand, is the greatest at the end of theconductor and reaches a minimum (but not zero) at the feedpoint. Making the conductor shorter orlonger than 1/4 wavelength means that the voltage pattern reaches its minimum somewhere beyond thefeed-point, so that the feed-point has a higher voltage and thus sees a higher impedance, as we have
http://en.wikipedia.org/wiki/Log_periodic_antennahttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/Resonant_frequencyhttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Groundhttp://en.wikipedia.org/wiki/Ground_planehttp://en.wikipedia.org/wiki/Ground_planehttp://en.wikipedia.org/wiki/Counterpoisehttp://en.wikipedia.org/wiki/Yagi-Udahttp://en.wikipedia.org/wiki/Standing_wavehttp://en.wikipedia.org/wiki/Impedance_matchhttp://en.wikipedia.org/wiki/Standing_wave_ratiohttp://en.wikipedia.org/wiki/Loading_coil#Radio_antennaehttp://en.wikipedia.org/wiki/Electrical_lengtheninghttp://en.wikipedia.org/wiki/Standing_wavehttp://en.wikipedia.org/wiki/Standing_wavehttp://en.wikipedia.org/wiki/Electrical_lengtheninghttp://en.wikipedia.org/wiki/Loading_coil#Radio_antennaehttp://en.wikipedia.org/wiki/Standing_wave_ratiohttp://en.wikipedia.org/wiki/Impedance_matchhttp://en.wikipedia.org/wiki/Standing_wavehttp://en.wikipedia.org/wiki/Yagi-Udahttp://en.wikipedia.org/wiki/Counterpoisehttp://en.wikipedia.org/wiki/Ground_planehttp://en.wikipedia.org/wiki/Ground_planehttp://en.wikipedia.org/wiki/Groundhttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Resonant_frequencyhttp://en.wikipedia.org/wiki/Dipole_antennahttp://en.wikipedia.org/wiki/Log_periodic_antenna


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noted. Since that voltage pattern is almost in phase quadrature with the current, the impedance seen atthe feed-point is not only much higher but mainly reactive.

It can be seen that if such an element is resonant at f0 to produce such a standing wave pattern, thenfeeding that element with 3f0 (whose wavelength is 1/3 that of f0) will lead to a standing wave pattern inwhich the voltage is likewise a minimum at the feed-point (and the current at a maximum there). Thus,an antenna element is also resonant when its length is 3/4 of a wavelength (3/2 wavelength for acomplete dipole). This is true for all odd multiples of 1/4 wavelength, where the feed-point impedance ispurely resistive, though larger than the resistive impedance of the 1/4 wave element. Although such anantenna is resonant and works perfectly well at the higher frequency, the antenna radiation pattern is alsoaltered compared to the half-wave dipole.

The use of a monopole or dipole at odd multiples of the fundamental resonant frequency, however,does not extend to even multiples (thus a 1/2 wavelength monopole or 1 wavelength dipole). Now thevoltage standing wave is at its peak at the feed-point, while that of the current (which must be zero at theend of the conductor) is at a minimum (but not exactly zero). The antenna is anti-resonant at thisfrequency. Although the reactance at the feedpoint can be cancelled using such an element length, thefeed-point impedance is very high, and is highly dependent on the diameter of the conductor (whichmakes only a small difference at the actual resonant frequency). Such an antenna does not match the

much lower characteristic impedance of available transmission lines, and is generally not used. Howeversome equipment where transmission lines are not involved which desire high driving point impedancemay take advantage of this anti-resonance.

Bandwidth

Although a resonant antenna has a purely resistive feed-point impedance at a particular frequency, many(if not most) applications require using an antenna over a range of frequencies. An antenna'sbandwidth specifies the range of frequencies over which its performance does not suffer due to a poorimpedance match. Also in the case of a Yagi-Uda array, the use of the antenna very far away from itsdesign frequency reduces the antenna's directivity, thus reducing the usable bandwidth regardless ofimpedance matching.

Except for the latter concern, the resonant frequency of a resonant antenna can always be altered byadjusting a suitable matching network. To do this efficiently one would require remotely adjusting amatching network at the site of the antenna, since simply adjusting a matching network at the transmitter(or receiver) would leave the transmission line with a poor standing wave ratio.

Instead, it is often desired to have an antenna whose impedance does not vary so greatly over a certainbandwidth. It turns out that the amount of reactance seen at the terminals of a resonant antenna when thefrequency is shifted, say, by 5%, depends very much on the diameter of the conductor used. A long thinwire used as a half-wave dipole (or quarter wave monopole) will have a reactance significantly greaterthan the resistive impedance it has at resonance, leading to a poor match and generally unacceptableperformance. Making the element using a tube of a diameter perhaps 1/50 of its length, however, resultsin a reactance at this altered frequency which is not so great, and a much less serious mismatch whichwill only modestly damage the antenna's net performance. Thus rather thick tubes are typically used forthe solid elements of such antennas, including Yagi-Uda arrays.

Rather than just using a thick tube, there are similar techniques used to the same effect such as replacingthin wire elements with cages to simulate a thicker element. This widens the bandwidth of the resonance.On the other hand, amateur radio antennas need to operate over several bands which are widelyseparated from each other. This can often be accomplished simply by connecting resonant elements forthe different bands in parallel. Most of the transmitter's power will flow into the resonant element whilethe others present high (reactive) impedance and draw little current from the same voltage. A popularsolution uses so-called traps consisting of parallel resonant circuits which are strategically placed in
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breaks along each antenna element. When used at one particular frequency band the trap presents a veryhigh impedance (parallel resonance) effectively truncating the element at that length, making it a properresonant antenna. At a lower frequency the trap allows the full length of the element to be employed,albeit with a shifted resonant frequency due to the inclusion of the trap's net reactance at that lowerfrequency.

The bandwidth characteristics of a resonant antenna element can be characterized according to its Q, justas one uses to characterize the sharpness of an L-C resonant circuit. However it is often assumed thatthere is an advantage in an antenna having a high Q. After all, Q is short for "quality factor" and a low Qtypically signifies excessive loss (due to unwanted resistance) in a resonant L-C circuit. However thisunderstanding does not apply to resonant antennas where the resistance involved is the radiationresistance, a desired quantity which removes energy from the resonant element in order to radiate it (thepurpose of an antenna, after all!). The Q is a measure of the ratio of reactance to resistance, so with afixed radiation resistance (an element's radiation resistance is almost independent of its diameter) agreater reactance off-resonance corresponds to the poorer bandwidth of a very thin conductor. The Q ofsuch a narrowband antenna can be as high as 15. On the other hand a thick element presents lessreactance at an off-resonant frequency, and consequently a Q as low as 5. These two antennas willperform equivalently at the resonant frequency, but the second antenna will perform over a bandwidth 3times as wide as the "hi-Q" antenna consisting of a thin conductor.

Gain

Gain is a parameter which measures the degree of directivity of the antenna's radiation pattern. A high-gain antenna will preferentially radiate in a particular direction. Specifically, the antenna gain, or powergain of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antennain the direction of its maximum output, at an arbitrary distance, divided by the intensity radiated at thesame distance by a hypothetical isotropic antenna.

The gain of an antenna is a passive phenomenon - power is not added by the antenna, but simplyredistributed to provide more radiated power in a certain direction than would be transmitted by anisotropic 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 theorientation of the antenna is relatively inconsequential. For example, a dish antenna on a spacecraft is ahigh-gain device that must be pointed at the planet to be effective, whereas a typical Wi-Fi antenna in alaptop computer is low-gain, and as long as the base station is within range, the antenna can be in anyorientation in space. It makes sense to improve horizontal range at the expense of reception above orbelow the antenna. Thus most antennas labeled "omnidirectional" really have some gain.

In practice, the half-wave dipole is taken as a reference instead of the isotropic radiator. The gain is thengiven in dBd (decibels over dipole):

NOTE: 0 dBd = 2.15 dBi. It is vital in expressing gain values that the reference point be included..

Effective area or aperture

The effective area or effective aperture of a receiving antenna expresses the portion of the power of apassing electromagnetic wave which it delivers to its terminals, expressed in terms of an equivalent area.For instance, if a radio wave passing a given location has a flux of 1 pW / m 2 (1012 watts per squaremeter) and an antenna has an effective area of 12 m2, then the antenna would deliver 12 pW of RF powerto the receiver (30 microvolts rms at 75 ohms). Since the receiving antenna is not equally sensitive tosignals received from all directions, the effective area is a function of the direction to the source.
http://en.wikipedia.org/wiki/Q_factorhttp://en.wikipedia.org/wiki/LC_circuithttp://en.wikipedia.org/wiki/Q_factorhttp://en.wikipedia.org/wiki/LC_circuithttp://en.wikipedia.org/wiki/Radiation_resistancehttp://en.wikipedia.org/wiki/Radiation_resistancehttp://en.wikipedia.org/wiki/Radiation_resistancehttp://en.wikipedia.org/wiki/Antenna_gainhttp://en.wikipedia.org/wiki/Intensity_(physics)http://en.wikipedia.org/wiki/Isotropic_antennahttp://en.wikipedia.org/wiki/Wi-Fihttp://en.wikipedia.org/wiki/Antenna_effective_areahttp://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Root_mean_squarehttp://en.wikipedia.org/wiki/Root_mean_squarehttp://en.wikipedia.org/wiki/Radio_frequencyhttp://en.wikipedia.org/wiki/Antenna_effective_areahttp://en.wikipedia.org/wiki/Wi-Fihttp://en.wikipedia.org/wiki/Isotropic_antennahttp://en.wikipedia.org/wiki/Intensity_(physics)http://en.wikipedia.org/wiki/Antenna_gainhttp://en.wikipedia.org/wiki/Radiation_resistancehttp://en.wikipedia.org/wiki/Radiation_resistancehttp://en.wikipedia.org/wiki/Radiation_resistancehttp://en.wikipedia.org/wiki/LC_circuithttp://en.wikipedia.org/wiki/Q_factorhttp://en.wikipedia.org/wiki/LC_circuithttp://en.wikipedia.org/wiki/Q_factor


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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 wavecan be measured and is called the standing wave ratio (SWR). A SWR of 1:1 is ideal. A SWR of 1.5:1 isconsidered 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 impedancedifferences at each interface (impedance matching) will reduce SWR and maximize power transferthrough each part of the antenna system.

Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength inuse. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedanceof the feed line, using the feed line as an impedance transformer. More commonly, the impedance isadjusted at the load (see below) with an antenna tuner, a balun, a matching transformer, matchingnetworks composed of inductors and capacitors, or matching sections such as the gamma match.

Efficiency

Efficiency of a transmitting antenna is the ratio of power actually radiated (in all directions) to the powerabsorbed by the antenna terminals. The power supplied to the antenna terminals which is not radiated isconverted into heat. This is usually through loss resistance in the antenna's conductors, but can also bedue to dielectric or magnetic core losses in antennas (or antenna systems) using such components. Suchloss effectively robs power from the transmitter, requiring a stronger transmitter in order to transmit asignal of a given strength.

For instance, if a transmitter delivers 100 W into an antenna having an efficiency of 80%, then the antennawill radiate 80 W as radio waves and produce 20 W of heat. In order to radiate 100 W of power, onewould need to use a transmitter capable of supplying 125 W to the antenna. Note that antenna efficiencyis a separate issue from impedance matching, which may also reduce the amount of power radiated usinga given transmitter. If an SWR meter reads 150 W of incident power and 50 W of reflected power whichmeans that 100 W have actually been absorbed by the antenna (ignoring transmission line losses). How

much of that power has actually been radiated cannot be directly determined through electricalmeasurements at (or before) the antenna terminals, but would require (for instance) careful measurementof field strength. Fortunately the loss resistance of antenna conductors such as aluminum rods can becalculated and the efficiency of an antenna using such materials predicted.

However loss resistance will generally affect the feedpoint impedance, adding to its resistive (real)component. That resistance will consist of the sum of the radiation resistance Rr and the loss resistanceRloss. If an rms current I is delivered to the terminals of an antenna, then a power of I2Rr will be radiatedand a power of I2Rloss will be lost as heat. Therefore the efficiency of an antenna is equal to Rr / (Rr + Rloss).Of course only the total resistance Rr + Rloss can be directly measured.

Amount of atmospheric noise atvarious elevation angles versus

frequency according CCIR 322
http://en.wikipedia.org/wiki/Standing_wave_ratiohttp://en.wikipedia.org/wiki/Impedance_matchinghttp://en.wikipedia.org/wiki/Complex_numberhttp://en.wikipedia.org/wiki/Electrical_length_(antenna)http://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Antenna_tunerhttp://en.wikipedia.org/wiki/Balunhttp://en.wikipedia.org/wiki/Inductorhttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Electrical_efficiencyhttp://en.wikipedia.org/wiki/Copper_losshttp://en.wikipedia.org/wiki/Impedance_matchinghttp://en.wikipedia.org/wiki/Standing_wave_ratiohttp://en.wikipedia.org/wiki/Field_strengthhttp://en.wikipedia.org/wiki/Copper_losshttp://en.wikipedia.org/wiki/Radiation_resistancehttp://en.wikipedia.org/wiki/Root_mean_squarehttp://en.wikipedia.org/wiki/Root_mean_squarehttp://en.wikipedia.org/wiki/Radiation_resistancehttp://en.wikipedia.org/wiki/Copper_losshttp://en.wikipedia.org/wiki/Field_strengthhttp://en.wikipedia.org/wiki/Standing_wave_ratiohttp://en.wikipedia.org/wiki/Impedance_matchinghttp://en.wikipedia.org/wiki/Copper_losshttp://en.wikipedia.org/wiki/Electrical_efficiencyhttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Inductorhttp://en.wikipedia.org/wiki/Balunhttp://en.wikipedia.org/wiki/Antenna_tunerhttp://en.wikipedia.org/wiki/Transformerhttp://en.wikipedia.org/wiki/Electrical_length_(antenna)http://en.wikipedia.org/wiki/Complex_numberhttp://en.wikipedia.org/wiki/Impedance_matchinghttp://en.wikipedia.org/wiki/Standing_wave_ratiohttp://en.wikipedia.org/wiki/File:Atmosphericnoise.PNG


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According to reciprocity, the efficiency of an antenna, when used as a receiving antenna, is identical tothe efficiency as defined above. The power that an antenna will deliver to a receiver (with aproper impedance match) is reduced by the same amount. However often in a receiving application,

inefficiency of an antenna may be of lesser consequence or even of no consequence, notably at lowerfrequencies or when used to receive signals in "crowded" bands. That is true in cases where the receivedsignal competes not against receiver noise, but against atmospheric noise or interference received by theantenna itself. The loss within the antenna will affect the intended signal and the noise/interferenceidentically, leading to no reduction in signal to noise ratio (SNR). According to the graph shownillustrating the frequency dependence of atmospheric and man-made noise, one can see that using areceiving antenna with an efficiency of only 10% at frequencies below 10 MHz will still supply a signal tothe receiver which includes noise well above the thermal limit. A decent RF amplifier in the receiver willnot significantly add to this noise level or reduce the resulting SNR.

This is fortunate, since antennas at lower frequencies which are not rather large (a good fraction of awavelength in size) are inevitably inefficient (due to the small radiation resistance Rr of small antennas).Most AM broadcast radios (except for car radios) take advantage of this principle by including a

small loop antenna for reception which has an extremely poor efficiency. Using such an inefficientantenna at this low frequency (5301650 kHz) thus has little effect on the receiver's net performance, butsimply requires greater amplification by the receiver's electronics. Contrast this tiny component to themassive and very tall towers used at AM broadcast stations for transmitting at the very same frequency,where every percentage point of reduced antenna efficiency entails a substantial cost.

The definition of antenna gain or power gain already includes the effect of the antenna's efficiency.Therefore if one is trying to radiate a signal toward a receiver using a transmitter of a given power, oneneed only compare the gain of various antennas rather than considering the efficiency as well. This islikewise true for a receiving antenna at very high (especially microwave) frequencies, where the point isto receive a signal which is strong compared to the receiver's noise temperature. However in the case of adirectional antenna used for receiving signals with the intention of rejecting interference from differentdirections, one is no longer concerned with the antenna efficiency, as discussed above. In this case, ratherthan quoting the antenna gain, one would be more concerned with the directive gain whichdoes not include the effect of antenna (in) efficiency. The directive gain of an antenna can be computedfrom the published gain divided by the antenna's efficiency.

Polarization

The polarization of an antenna is the orientation of the electric field (E-plane) of the radio wave withrespect to the Earth's surface and is determined by the physical structure of the antenna and by itsorientation. 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, anda different polarization when mounted horizontally. "Electromagnetic wave polarization filters" arestructures 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 arereflected by the ionosphere, polarization cannot be relied upon. For line-of-sight communications forwhich polarization can be relied upon, it can make a large difference in signal quality to have thetransmitter and receiver using the same polarization; many tens of dB difference is commonly seen andthis is more than enough to make the difference between reasonable communication and a broken link.
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Polarization is largely predictable from antenna construction but, especially in directional antennas, thepolarization of side lobes can be quite different from that of the main propagation lobe. For radioantennas, polarization corresponds to the orientation of the radiating element in an antenna. Avertical omnidirectional Wi-Fi antenna will have vertical polarization (the most common type). Anexception is a class of elongated waveguide antennas in which vertically placed antennas is horizontallypolarized. 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 planeperpendicular to the direction of motion of the radio wave. In the most general case, polarization iselliptical, meaning that the polarization of the radio waves varies over time. Two special cases are linearpolarization (the ellipse collapses into a line) and circular polarization (in which the two axes of theellipse are equal). In linear polarization the antenna compels the electric field of the emitted radio wave toa particular orientation. Depending on the orientation of the antenna mounting, the usual linear cases arehorizontal and vertical polarization. In circular polarization, the antenna continuously varies the electricfield 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 polarizedusing 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 toradio engineers' use.

In practice, regardless of confusing terminology, it is important that linearly polarized antennas bematched, lest the received signal strength be greatly reduced. So horizontal should be used withhorizontal and vertical with vertical. Intermediate matching will lose some signal strength, but not asmuch as a complete mismatch. Transmitters mounted on vehicles with large motional freedomcommonly use circularly polarized antennasso that there will never be a complete mismatch with signalsfrom other sources.

Impedance matching

Maximum power transfer requires matching the impedance of an antenna system (as seen looking intothe transmission line) to the complex conjugate of the impedance of the receiver or transmitter. In the case

of a transmitter, however, the desired matching impedance might not correspond to the dynamic outputimpedance of the transmitter as analyzed as a source impedance but rather the design value (typically 50ohms) required for efficient and safe operation of the transmitting circuitry. The intended impedance isnormally resistive but a transmitter (and some receivers) may have additional adjustments to cancel acertain amount of reactance in order to "tweak" the match. When a transmission line is used in betweenthe antenna and the transmitter (or receiver) one generally would like an antenna system whoseimpedance is resistive and near the characteristic impedance of that transmission line in order tominimize the standing wave ratio (SWR) and the increase in transmission line losses it entails, in additionto supplying a good match at the transmitter or receiver itself.

Antenna tuning generally refers to cancellation of any reactance seen at the antenna terminals, leavingonly a resistive impedance which might or might not be exactly the desired impedance (that of thetransmission line). Although an antenna may be designed to have a purely resistive feedpoint impedance

(such as a dipole 97% of a half wavelength long) this might not be exactly true at the frequency that it iseventually used at. In some cases the physical length of the antenna can be "trimmed" to obtain a pureresistance. On the other hand, the addition of a series inductance or parallel capacitance can be used tocancel a residual capacitative or inductive reactance, respectively.

In some cases this is done in a more extreme manner, not simply to cancel a small amount of residualreactance, but to resonate an antenna whose resonance frequency is quite different than the intendedfrequency of operation. For instance, a "whip antenna" can be made significantly shorter than 1/4wavelength long, for practical reasons, and then resonated using a so-called loading coil. This physically
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large inductor at the base of the antenna has an inductive reactance which is the opposite of thecapacitative reactance that such a vertical antenna has at the desired operating frequency. The result is apure resistance seen at feedpoint of the loading coil; unfortunately that resistance is somewhat lower thanwould be desired to match commercial coax[citation needed].

So an additional problem beyond canceling the unwanted reactance is of matching the remainingresistive impedance to the characteristic impedance of the transmission line. In principle this can alwaysbe done with a transformer, however the turns ratio of a transformer is not adjustable. A generalmatching network with at least two adjustments can be made to correct both components of impedance.Matching networks using discrete inductors and capacitors will have losses associated with thosecomponents, and will have power restrictions when used for transmitting. Avoiding these difficulties,commercial antennas are generally designed with fixed matching elements and/or feeding strategies toget an approximate match to standard coax, such as 50 or 75 Ohms. Antennas based on the dipole (ratherthan vertical antennas) should include a balun in between the transmission line and antenna element,which may be integrated into any such matching network.

Another extreme case of impedance matching occurs when using a small loop antenna (usually, but notalways, for receiving) at a relatively low frequency where it appears almost as a pure inductor.Resonating such an inductor with a capacitor at the frequency of operation not only cancels the reactance

but greatly magnifies the very small radiation resistance of such a loop. This is implemented in most AMbroadcast receivers, with a small ferrite loop antenna resonated by a capacitor which is varied along withthe receiver tuning in order to maintain resonance over the AM broadcast band
http://en.wikipedia.org/wiki/Coaxial_cablehttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Characteristic_impedancehttp://en.wikipedia.org/wiki/Balunhttp://en.wikipedia.org/wiki/Loop_antennahttp://en.wikipedia.org/wiki/Radiation_resistancehttp://en.wikipedia.org/wiki/Radiation_resistancehttp://en.wikipedia.org/wiki/Loop_antennahttp://en.wikipedia.org/wiki/Balunhttp://en.wikipedia.org/wiki/Characteristic_impedancehttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Coaxial_cable


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SKYWAVE PROPAGATION

Propagation beyond the line of sight is possible through sky waves. Sky waves are radio waves that

propagate into the atmosphere and then are returned to earth at some distance from the transmitter. Wewill consider two cases:

ionospheric refractiontropospheric scatter

IONOSPHERIC REFRACTION

This propagation mode occurs when radio waves travel into the ionosphere, a region of charged particles50 300 miles above the earths surface. The ionosphere is created when the sun ionizes the upper regionsof the earths atmosphere. These charged regions are electrically active. The ionosphere bends andattenuates radio waves at frequencies below 30 MHz. Above 200 MHz the ionosphere becomescompletely transparent. The ionosphere is responsible for most propagation phenomena observed at HF,

MF, LF and VLF. The ionosphere consists of 4 highly ionized regions

The D layer at a height of 38 55 miThe E layer at a height of 62 75 miThe F1 layer at a height of 125 150 mi (winter) and 160 180 mi (summer)The F2 layer at a height of 150 180 mi (winter) and 240 260 mi (summer)

The density of ionization is greatest in the F layers and least in the D layer Though created by solarradiation, the ionosphere does not completely disappear shortly after sunset. The D and E layersdisappear almost immediately, but the F1 and F2 layers do not disappear; rather they merge into a singleF layer located at a distance of 150 250 mi above the earth. Recombination or charged particles is quiteslow at that altitude, so the F layer lasts until dawn.

The diagram below shows the geometry of ionospheric refraction. The maximum frequency that can bereturned by the ionosphere when the radio waves are vertically incident on the ionosphere (transmittedstraight up) is called the critical frequency.

The critical frequency varies from place to place, and it is possible to view this variation by looking ata real-time critical frequency map

The critical frequency varies from 1 to 15 MHzunder normal conditions. Most communicationsis done using radio waves transmitted at thehorizon, to get the maximum possible distanceper hop. The highest frequency that can bereturned when the takeoff angle is zero degrees iscalled the MUF, maximum usable frequency. TheMUF and critical frequency are related by thefollowing formula:
http://www.ngdc.noaa.gov/stp/IONO/ionointro.htmlhttp://www.ycars.org/EFRA/Glossary/Radiowave%20frequency%20ranges%20%5bOulu%5d.htmhttp://www.ycars.org/EFRA/Glossary/Radiowave%20frequency%20ranges%20%5bOulu%5d.htmhttp://www.radio-electronics.com/info/propagation/HF/Critical.htmlhttp://www.spacew.com/www/fof2.html%C2%A0http://www.spacew.com/www/fof2.html%C2%A0http://www.radio-electronics.com/info/propagation/HF/Critical.htmlhttp://www.ycars.org/EFRA/Glossary/Radiowave%20frequency%20ranges%20%5bOulu%5d.htmhttp://www.ycars.org/EFRA/Glossary/Radiowave%20frequency%20ranges%20%5bOulu%5d.htmhttp://www.ngdc.noaa.gov/stp/IONO/ionointro.html


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The MUF can range from 3 to 50 MHz.

The ionosphere also attenuates radio waves. The amount of attenuation is roughly inversely proportionalto the square of the frequency of the wave. Thus attenuation is a severe problem at lower frequencies,making daytime global communications via sky wave impossible at frequencies much below 5 MHz.

The properties of the ionosphere are variable. There are 3 periodic cycles of variation:diurnal (daily) cycleseasonal cyclesunspot cycle

The daily cycle is driven by the intensity of the solar radiation ionizing the upper atmosphere. The D andE layers form immediately after sunrise, and the F layer splits into two layers, the F1 and F2. The densityof the layers increases until noon and then decreases slowly throughout the afternoon. After sunset, the Dand E layers disappear and the F1 and F2 merge to form the F layer. Take another look at the real-timeMUF map and notice the difference between the MUF numbers in the day and night regions. If you aren'tsure which region is the daytime region, it has a small yellow sun icon in its center. The thick gray linesindicate the location of the terminator - the division between day and night.

Seasonal variation is linked to the tilt of the earths axis and the distance between the earth and sun. Theeffects are complex, but the result is that ionospheric propagation improves dramatically during the forthe northern hemisphere during their winter, while seasonal variation in the southern hemisphere ismuch smaller.
http://www.spacew.com/www/realtime.htmlhttp://www.spacew.com/www/realtime.htmlhttp://www.spacew.com/www/realtime.htmlhttp://www.spacew.com/www/realtime.html


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The 11 year sunspot cycle exerts a tremendous effect on the atmosphere. Near the peak of the cycle (thelast peak occurred in December 2001) the suns surface is very active, emitting copious amounts of UVradiation and charged particles, which increase the density of the ionosphere. This leads to a generalincrease in MUFs and attenuation at lower frequencies. When the sun becomes extremely active, or amajor solar flare occurs, the ionosphere can become so dense that global ionospheric communications are

disrupted.

The maximum distance that can be covered by a single hop using ionospheric propagation is about 2500miles. Greater distances can be covered using multi-hop propagation, in which radio waves are reflectedby the ground back up to the ionosphere.

The ionosphere is not uniform and different regions refract RF differently. Multipath propagation is theresult. This leads to rapid variations in the received signal amplitude known as fading.

One of the consequences of ionospheric propagation is that reception of signals on the AM broadcastband varies greatly from day to night.

SPORADIC-E PROPAGATION

For reasons that are not clearly understood, clouds of densely ionized gases appear in the E -layer of theionosphere. The clouds are generally relatively small and can happen at any time of day. These clouds areformed throughout the year, but are most common in the summer months. Because these clouds are sodensely ionized, they can support ionospheric propagation at frequencies well above the normal MUF.Sporadic E propagation has been observed at frequencies as high as 144 MHz, and is relatively commonat 50 MHz.

The E-layer is lower than the F-layer and as a result, the distance covered by a sporadic-E hop isapproximately 1000-1300 miles, depending on the cloud's height. The sporadic-E clouds drift through theE-layer, adding to the unpredictability of sporadic-E propagation.

Sporadic-E propagation is not generally useful because of its unpredictability. Its main impact is negative,causing VHF-TV and FM broadcasters in different markets to interfere with each other.

TROPOSPHERIC SCATTER
http://science.nasa.gov/ssl/pad/solar/sunspots.htmhttp://hesperia.gsfc.nasa.gov/sftheory/flare.htmhttp://www.free-definition.com/Radio-propagation.htmlhttp://www.free-definition.com/Radio-propagation.htmlhttp://hesperia.gsfc.nasa.gov/sftheory/flare.htmhttp://science.nasa.gov/ssl/pad/solar/sunspots.htm


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Regional over the horizon communications are possible through a sky wave technique called

tropospheric scatter (troposcatter or just tropo). As shown in the diagram below, the troposphere, which

is the layer of the atmosphere closest to the ground, has pockets or cells of air within it that have a

different water vapor content and therefore a different refractive index for radio waves. As a result, radio

waves are scattered by the cells over the horizon. This scatter occurs at frequencies of 0.3 10 GHz.

Operation above 10 GHz is not possible because water vapor in the air strongly absorbs the signals Thisscattering process is not efficient and very little of the transmitted signal is scattered in the direction of the

receiver. High power transmitters and sensitive receivers are required.

The troposphere contains almost all of the earths weather patterns, which makes the tropospheresproperties quite variable. This makes troposcatter communications subject to weather induced fading andcommunications blackouts. To improve the reliability of troposcatter links, a technique called diversityoperation is used. There are three types of diversity:

Frequency Diversity two frequencies simultaneously transmit the same signal

Polarization Diversity radio waves of both polarizations are transmitted simultaneously

Space Diversity pairs of widely separated antennas are used for transmitting and receiving

Diversity operation greatly increases the reliability of troposcatter links, but it comes at a significant cost,because at least double the amount of equipment is needed at each installation.


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FIBER OPTICS

In recent years it has become apparent that fiber-optics are steadily replacing copper wire as anappropriate means of communication signal transmission. They span the long distances between localphone systems as well as providing the backbone for many network systems. Other system users includecable television services, university campuses, office buildings, industrial plants, and electric utilitycompanies.

A fiber-optic system is similar to the copper wire system that fiber-optics is replacing. The difference isthat fiber-optics use light pulses to transmit information down fiber lines instead of using electronicpulses to transmit information down copper lines. Looking at the components in a fiber-optic chain will

give a better understanding of how the system works in conjunction with wire based systems.

At one end of the system is a transmitter. This is the place of origin for information coming on to fiber-optic lines. The transmitter accepts coded electronic pulse information coming from copper wire. It thenprocesses and translates that information into equivalently coded light pulses. A light-emitting diode(LED) or an injection-laser diode (ILD) can be used for generating the light pulses. Using a lens, the lightpulses are funneled into the fiber-optic medium where they travel down the cable. The light (nearinfrared) is most often 850nm for shorter distances and 1,300nm for longer distances on Multi-mode fiberand 1300nm for single-mode fiber and 1,500nm is used for longer distances.

Think of a fiber cable in terms of very long cardboard roll (from the inside roll of paper towel) that iscoated with a mirror on the inside.

If you shine a flashlight in one end you can see light come out at the far end - even if it's been bent arounda corner.

Light pulses move easily down the fiber-optic line because of a principle known as total internalreflection. "This principle of total internal reflection states that when the angle of incidence exceeds acritical value, light cannot get out of the glass; instead, the light bounces back in. When this principle isapplied to the construction of the fiber-optic strand, it is possible to transmit information down fiber linesin the form of light pulses. The core must a very clear and pure material for the light or in most cases nearinfrared light (850nm, 1300nm and 1500nm). The core can be Plastic (used for very short distances) butmost are made from glass. Glass optical fibers are almost always made from pure silica, but some othermaterials, such as fluorozirconate, fluoroaluminate, andchalcogenide glasses, are used for longer-wavelength infrared applications.
http://en.wikipedia.org/wiki/Silicahttp://en.wikipedia.org/w/index.php?title=Fluorozirconate_glass&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Fluoroaluminate_glass&action=edit&redlink=1http://en.wikipedia.org/wiki/Chalcogenide_glasshttp://en.wikipedia.org/wiki/Chalcogenide_glasshttp://en.wikipedia.org/w/index.php?title=Fluoroaluminate_glass&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Fluorozirconate_glass&action=edit&redlink=1http://en.wikipedia.org/wiki/Silica


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There are three types of fiber optic cable commonly used: single mode, multimode and plastic opticalfiber (POF).

Transparent GlassTransparent glass or plastic fibers which allow light to be guided from one end to the other with minimalloss.

Fiber optic cable functions as a "light guide," guiding the light introduced at one end of the cable throughto the other end. The light source can either be a light-emitting diode (LED)) or a laser.

The light source is pulsed on and off, and a light-sensitive receiver on the other end of the cable convertsthe pulses back into the digital ones and zeros of the original signal.

Even laser light shining through a fiber optic cable is subject to loss of strength, primarily throughdispersion and scattering of the light, within the cable itself. The faster the laser fluctuates, the greater therisk of dispersion. Light strengtheners, called repeaters, may be necessary to refresh the signal in certainapplications.

While fiber optic cable itself has become cheaper over time - a equivalent length of copper cable cost less

per foot but not in capacity. Fiber optic cable connectors and the equipment needed to install them arestill more expensive than their copper counterparts.

Single Mode cable

Single Mode cable is a single stand (most applications use 2 fibers) of glass fiber with a diameter of 8.3 to10 microns that has one mode of transmission. Single Mode Fiber with a relatively narrow diameter,through which only one mode will propagate typically 1310 or 1550nm. Carries higher bandwidth than


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multimode fiber, but requires a light source with a narrow spectral width. Synonyms mono-mode opticalfiber, single-mode fiber, single-mode optical waveguide, uni-mode fiber.

Single Modem fiber is used in many applications where data is sent at multi-frequency (WDM Wave-Division-Multiplexing) so only one cable is needed - (single-mode on one single fiber)

Single-mode fiber gives you a higher transmission rate and up to 50 times more distance than multimode,but it also costs more. Single-mode fiber has a much smaller core than multimode. The small core andsingle light-wave virtually eliminate any distortion that could result from overlapping light pulses,providing the least signal attenuation and the highest transmission speeds of any fiber cable type.

Single-mode optical fiber is an optical fiber in which only the lowest order bound mode can propagate atthe wavelength of interest typically 1300 to 1320nm.

Multi-Mode cable

Multi-Mode cable has a little bit biggerdiameter, with a common diameters inthe 50-to-100 micron range for the lightcarry component (in the US the mostcommon size is 62.5um). Mostapplications in which Multi-mode fiberis used, 2 fibers are used (WDM is notnormally used on multi-mode fiber).POF is a newer plastic-based cable

which promises performance similar toglass cable on very short runs, but at alower cost.

Multimode fiber gives you highbandwidth at high speeds (10 to 100MBS - Gigabit to 275m to 2km) over medium distances. Light wavesare dispersed into numerous paths, or modes, as they travel through the cable's core typically 850 or1300nm. Typical multimode fiber core diameters are 50, 62.5, and 100 micrometers. However, in longcable runs (greater than 3000 feet [914.4 meters), multiple paths of light can cause signal distortion at the


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receiving end, resulting in an unclear and incomplete data transmission so designers now call for singlemode fiber in new applications using Gigabit and beyond.

Fiber

Some 10 billion digital bits can be transmitted per second along an optical fiber link in a commercialnetwork, enough to carry tens of thousands of telephone calls. Hair-thin fibers consist of two concentriclayers of high-purity silica glass the core and the cladding, which are enclosed by a protective sheath.Light rays modulated into digital pulses with a laser or a light-emitting diode move along the corewithout penetrating the cladding.

The light stays confined to the core because the cladding has a lower refractive index a measure of itsability to bend light. Refinements in optical fibers, along with the development of new lasers and diodes,may one day allow commercial fiber-optic networks to carry trillions of bits of data per second.

Total internal refection confines light within optical fibers (similar to looking down a mirror made in theshape of a long paper towel tube). Because the cladding has a lower refractive index, light rays reflect

back into the core if they encounter the cladding at a shallow angle (red lines). A ray that exceeds acertain "critical" angle escapes from the fiber (yellow line).

STEP-INDEX MULTIMODE FIBER

STEP-INDEX MULTIMODE FIBER has a large core, up to 100 microns in diameter. As a result, some of

the light rays that make up the digital pulse may travel a direct route, whereas others zigzag as theybounce off the cladding. These alternative pathways cause the different groupings of light rays, referredto as modes, to arrive separately at a receiving point. The pulse, an aggregate of different modes, beginsto spread out, losing its well-defined shape. The need to leave spacing between pulses to preventoverlapping limits bandwidth that is, the amount of information that can be sent. Consequently, this typeof fiber is best suited for transmission over short distances, in an endoscope, for instance.

GRADED-INDEX MULTIMODE FIBER


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GRADED-INDEX MULTIMODE FIBER contains a core in which the refractive index diminishesgradually from the center axis out toward the cladding. The higher refractive index at the center makesthe light rays moving down the axis advance more slowly than those near the cladding. Also, rather thanzigzagging off the cladding, light in the core curves helically because of the graded index, reducing itstravel distance. The shortened path and the higher speed allow light at the periphery to arrive at areceiver at about the same time as the slow but straight rays in the core axis. The result: a digital pulse

suffers less dispersion.

SINGLE-MODE FIBER

SINGLE-MODE FIBER has a narrow core (eight microns or less), and the index of refraction between thecore and the cladding changes less than it does for multimode fibers. Light thus travels parallel to theaxis, creating little pulse dispersion. Telephone and cable television networks install millions ofkilometers of this fiber every year.


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WAVEGUIDES

Waveguides are used to transfer electromagnetic power efficiently from one point in space to another.Some common guiding structures are shown in the figure below. These include the typical coaxial cable,the two-wire and mictrostrip transmission lines, hollow conducting waveguides, and optical fibers. In

practice, the choice of structure is dictated by: (a) the desired operating frequency band, (b) the amount ofpower to be transferred, and (c) the amount of transmission losses that can be tolerated.

Coaxial cables are widely used to connect RF components. Their operation is practical for frequencies

below 3 GHz. Above that the losses are too excessive. For example, the attenuation might be 3 dB per 100m at 100 MHz, but 10 dB/100 m at 1 GHz, and 50 dB/100 m at 10 GHz. Their power rating is typically ofthe order of one kilowatt at 100 MHz, but only 200 W at 2 GHz, being limited primarily because of theheating of the coaxial conductors and of the dielectric between the conductors (dielectric voltagebreakdown is usually a secondary factor.) However, special short-length coaxial cables do exist thatoperate in the 40 GHz range. Another issue is the single-mode operation of the line. At higherfrequencies, in order to prevent higher modes from being launched, the diameters of the coaxialconductors must be reduced, diminishing the amount of power that can be transmitted.

Two-wire lines are not used at microwave frequencies because they are not shielded and can radiate. Onetypical use is for connecting indoor antennas to TV sets. Microstrip lines are used widely in microwaveintegrated circuits.

Impedance matching

In circuit theory, the impedance is a generalization of electrical resistivity in the case of alternating

current, and is measured in ohms (). A waveguide in circuit theory is described by a transmission

line having a length and self impedance. In other words the impedance is the resistance of the circuit

component (in this case a waveguide) to the propagation of the wave. This description of the waveguide

was originally intended for alternating current, but is also suitable for electromagnetic and sound waves,

once the wave and material properties (such as pressure, density, dielectric constant) are properly

converted into electrical terms (current and impedance for example).

Impedance matching is important when components of an electric circuit are connected (waveguide to

antenna for example): The impedance ratio determines how much of the wave is transmitted forward and

how much is reflected. In connecting a waveguide to an antenna a complete transmission is usually

required, so that their impedances are matched.
http://en.wikipedia.org/wiki/Circuit_theoryhttp://en.wikipedia.org/wiki/Electrical_impedancehttp://en.wikipedia.org/wiki/Electrical_resistivityhttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Transmission_linehttp://en.wikipedia.org/wiki/Transmission_linehttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Dielectric_constanthttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Dielectric_constanthttp://en.wikipedia.org/wiki/Densityhttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Transmission_linehttp://en.wikipedia.org/wiki/Transmission_linehttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Electrical_resistivityhttp://en.wikipedia.org/wiki/Electrical_impedancehttp://en.wikipedia.org/wiki/Circuit_theory


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The reflection coefficient can be calculated using:

,

where is the reflection coefficient (0 denotes full transmission, 1 full reflection, and 0.5 is a reflection of

half the incoming voltage), Z1 and Z2 are the impedance of the first component (from which the wave

enters) and the second component, respectively.

An impedance mismatch creates a reflected wave, which added to the incoming waves creates a standing

wave. An impedance mismatch can be also quantified with the standing wave ratio (SWR or VSWR for

voltage), which is connected to the impedance ratio and reflection coefficient by:

,

where are the minimum and maximum values of the voltage absolute value, and the

VSWR is the voltage standing wave ratio, which value of 1 denotes full transmission, without reflection

and thus no standing wave, while very large values mean high reflection and standing wave pattern.
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SATELLITE COMMUNICATION

In satellite communication, signal transferring between the sender and receiver is done with the help ofsatellite. In this process, the signal which is basically a beam of modulated microwaves is sent towardsthe satellite. Then the satellite amplifies the signal and sent it back to the receivers antenna present on the

earths surface. So, all the signal transferring is happening in space. Thus this type of communication isknown as space communication.

Two satellites which are commonly used in satellite communication are Active and passive satellites.

Passive satellites: It is just a plastic balloon having a metal coated over it. This sphere reflects thecoming microwave signals coming from one part of the earth to other part. This is also known as passivesphere. Our earth also has a passive satellite i.e. moon.

Active satellites: It basically does the work of amplifying the microwave signals coming. Inactive satellites an antenna system, transmitter, power supply and a receiver is used. Thesesatellites arealso called as transponders. The transmitters fitted on the earth generate themicrowaves. These rays are

received by the transponders attached to the satellite. Then after amplifying, these signals are transmittedback to earth. This sending can be done at the same time or after some delay. These amplified signals arestored in the memory of the satellites, when earth properly faces the satellite. Then the satellite startssending the signals to earth. Some active satellites also have programming and recording features. Thenthese recording can be easily played and watched. The first active satellite was launched by Russia in1957. The signals coming from the satellite when reach the earth, are of very low intensity. Theiramplification is done by the receivers themselves. After amplification these become available for furtheruse.

Development of satellite communication

The idea of communicating through a satellite first appeared in the short story titled The Brick Moon,written by the American clergyman and author Edward Everett Hale and published inThe AtlanticMonthlyin 186970. The story describes the construction and launch into Earth orbit of a satellite 200 feet(60 metres) in diameter and made of bricks. The brick moon aided mariners in navigation, as peoplesent Morse code signals back to Earth by jumping up and down on the satellites surface.

The first practical concept of satellite communication was proposed by 27-year-old Royal AirForce officer Arthur C. Clarke in a paper titled Extra-Terrestrial Relays: Can Rocket Stations Give World-wide Radio Coverage? published in the October 1945 issue of Wireless World. Clarke, who would laterbecome an accomplished science fiction writer, proposed that a satellite at an altitude of 35,786 km(22,236 miles) above Earths surface would be moving at the same speed as Earths rotation. At thisaltitude the satellite would remain in a fixed position relative to a point on Earth. Thi sorbit, now called ageostationary orbit, is ideal for satellite communications, since an antenna on the ground can bepointed to a satellite 24 hours a day without having to track its position. Clarke calculated in his paperthat three satellites spaced equidistantly in geostationary orbit would be able to provide radio coveragethat would be almost worldwide with the sole exception of some of thepolar regions.

The first artificial satellite, Sputnik 1, was launched successfully by the Soviet Union on Oct. 4, 1957.Sputnik 1 was only 58 cm (23 inches) in diameter with four antennas sending low-frequency radio signalsat regular intervals. It orbited Earth in a elliptical orbit, taking 96.2 minutes to complete one revolution. It
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transmitted signals for only 22 days until its battery ran out and was in orbit for only three months, butits launch sparked the beginning of the space race between the United States and the Soviet Union.

The first satellite to relay voice signals was launched by the U.S. governments Project SCORE (SignalCommunication by Orbiting Relay Equipment) from Cape Canaveral, Florida, on Dec. 19, 1958. Itbroadcast a taped message conveying peace on earth and goodwill toward men everywhere from U.S.

Pres. Dwight D. Eisenhower.

American engineers John Pierce of American Telegraph and Telephone Companys (AT&Ts) BellLaboratories and Harold Rosen of Hughes Aircraft Company developed key technologies in the 1950sand 60s that made commercial communication satellites possible. Pierce outlined the principles ofsatellite communications in an article titled Orbital Radio Relays published in the April 1955 issue of JetPropulsion. In it he calculated the precise power requirements to transmit signals to satellites in variousEarth orbits. Pierces main contribution to satellite technology was the development of the traveling wavetube amplifier, which enabled a satellite to receive, amplify, and transmit radio signals. Rosen developedspin-stabilization technology that provided stability to satellites orbiting in space.

When the U.S. National Aeronautics and Space Administration (NASA) was established in 1958, it

embarked on a program to develop satellite technology. NASAs first project was the Echo 1 satellite thatwas developed in coordination with AT&T s Bell Labs. Pierce led a team at Bell Labs that developedthe Echo 1 satellite, which was launched on Aug. 12, 1960. Echo 1 was a 30.5-metre (100-foot) aluminum-coatedballoon that contained no instruments but was able to reflect signals from the ground. Since Echo 1only reflected signals, it was considered a passive satellite. Echo 2, managed by NASAs Goddard SpaceFlight Center in Beltsville, Md., was launched on Jan. 25, 1964. After Echo 2, NASA abandoned passivecommunications systems in favour of active satellites. The Echo 1 and Echo 2 satellites were credited withimproving the satellite tracking and ground station technology that was to prove indispensable later inthe development of active satellite systems.

Pierces team at Bell Labs also developed Telstar 1, the first active communications satellite capable oftwo-way communications. Telstar 1 was launched into low Earth orbit on July 10, 1962, by a Delta rocket.NASA provided the launch services and some tracking and telemetry support. Telstar 1 was the first

satellite to transmit live television images between Europe and North America. Telstar 1 alsotransmittedthe first phone call via satellitea brief call from AT&T chairman Frederick Kappel transmitted from theground station in Andover, Maine, to U.S. Pres. Lyndon Johnson in Washington, D.C.

Rosensteam at Hughes Aircraft attempted to place the first satellite in geostationary orbit, Syncom 1, onFeb. 14, 1963. However, Syncom 1 was lost shortly after launch. Syncom 1 was followed by the successfullaunch of Syncom 2, the first satellite in a geosynchronous orbit (an orbit that has a period of 24 hours butis inclined to the Equator), on July 26, 1963, and Syncom 3, the first satellite in geostationary orbit, onAug. 19, 1964. Syncom 3 broadcast the 1964 Olympic Games from Tokyo, Japan, to the United States, thefirst major sporting event broadcast via satellite.

The successful development of satellite technology paved the way for a global communications

satellite industry. The United States spearheaded the development of the satellite communicationsindustry with the passing of the Communications Satellite Act in 1962. The act authorized the formationof the Communications Satellite Corporation(Comsat), a private company that would represent theUnited States in an international satellite communications consortium called Intelsat.

Intelsat was formed on Aug. 20, 1964, with 11 signatories to the Intelsat Interim Agreement. The original11 signatories were Austria, Canada, Japan, the Netherlands, Norway, Spain, Switzerland, the UnitedKingdom, the United States, the Vatican, and West Germany.
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On April 6, 1965, the first Intelsat satellite, Early Bird (also called Intelsat 1), was launched; it wasdesigned and built byRosensteam at Hughes Aircraft Company. Early Bird was the first operationalcommercial satellite providing regular telecommunications and broadcasting services between NorthAmerica and Europe. Early Bird was followed by Intelsat 2B and 2D, launched in 1967 and covering

the Pacific Ocean region, and Intelsat 3 F-3, launched in 1969 and covering the Indian Ocean region.Intelsats satellites in geostationary orbit provided nearly global coverage, as Arthur C. Clarke hadenvisioned 24 years earlier. Nineteen days after Intelsat 3 F-3 was placed over the Indian Ocean, thelanding of the first human on the Moon on July 20, 1969, was broadcast live through the global networkof Intelsat satellites to over 600 million television viewers.

The Soviet Union continued its development of satellite technology with the Molniya series of satellites,which were launched in a highly elliptical orbit to enable them to reach the far northern regions of thecountry. The first satellite in this series, Molniya 1, was launched on April 23, 1965. By 1967 six Molniyasatellites provided coverage throughout the Soviet Union. During the 50th anniversary of the SovietUnion on Oct. 1, 1967, the annual parade in Red Square was broadcast nationwide via the Molniyasatellite network. In 1971 the Intersputnik International Organization of Space Communications was

formed by several communist countries, led by the Soviet Union.

The potential application of satellites for development and their ability to reach remote regions led othercountries to build and operate their own national satellite systems. Canada was the first country after theSoviet Union and the United States to launch its own communications satellite, Anik 1, on Nov. 9, 1972.This was followed by the launch of Indonesias Palapa 1 satellite on July 8, 1976. Many other countriesfollowed suit and launched their own satellites.

Satellite applications

Telephone

The first and historically most important application for communication satellites was inintercontinental long distance telephony. The fixed Public Switched Telephone Networkrelays telephonecalls from land line telephones to an earth station, where they are then transmitted to a geostationarysatellite. The downlink follows an analogous path. Improvements in submarine communications cables,through the use of fiber-optics, caused some decline in the use of satellites for fixed telephony in the late20th century, but they still serve remote islands such as Ascension Island, Saint Helena, Diego Garcia,and Easter Island, where no submarine cables are in service. There are also regions of some continentsand countries where landline telecommunications are rare to nonexistent, for example large regionsof South America, Africa, Canada, China, Russia, andAustralia. Satellite communications also provideconnection to the edges of Antarctica and Greenland.

Satellite phones connect directly to a constellation of either geostationary or low-earth-orbit satellites.Calls are then forwarded to a satellite teleport connected to the Public Switched Telephone Network

Satellite television

As television became the main market, its demand for simultaneous delivery of relatively few signals oflarge bandwidth to many receivers being a more precise match for the capabilities of geosynchronous
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comsats. Two satellite types are used for North American television and radio: Direct broadcastsatellite (DBS), and Fixed Service Satellite (FSS)

The definitions of FSS and DBS satellites outside of North America, especially in Europe, are a bit moreambiguous. Most satellites used for direct-to-home television in Europe have the same high power outputas DBS-class satellites in North America, but use the same linear polarization as FSS-class satellites.

Examples of these are the Astra, Eutelsat, and Hotbird spacecraft in orbit over the European continent.Because of this, the terms FSS and DBS are more so used throughout the North American continent, andare uncommon in Europe.

Fixed Service Satellite

Fixed Service Satellites use the C band, and the lower portions of the Ku bands. They are normally usedfor broadcast feeds to and from television networks and local affiliate stations (such as program feeds fornetwork and syndicated programming, live shots, and backhauls), as well as being used for distancelearning by schools and universities, business television (BTV),Videoconferencing, and generalcommercial telecommunications. FSS satellites are also used to distribute national cable channels to cabletelevision headends.

Free-to-air satellite TV channels are also usually distributed on FSS satellites in the Ku band. The IntelsatAmericas 5, Galaxy 10R and AMC 3 satellites over North America provide a quite large amount of FTAchannels on their Ku band transponders.

The American Dish Network DBS service has also recently utilized FSS technology as well for theirprogramming packages requiring their SuperDish antenna, due to Dish Network needing more capacityto carry local television stations per the FCC's "must-carry" regulations, and for more bandwidth tocarry HDTV channels.

Direct broadcast satellite

A direct broadcast satellite is a communications satellite that transmits to small DBS satellite dishes(usually 18 to 24 inches or 45 to 60 cm in diameter). Direct broadcast satellites generally operate in theupper portion of the microwave Ku band. DBS technology is used for DTH-oriented (Direct-To-Home)satellite TV services, such as DirecTV and DISH Network in the United States, Bell TV and Shaw Direct inCanada, Freesat and Sky Digital in the UK, the Republic of Ireland, and New Zealand and DSTV in SouthAfrica .

Operating at lower frequency and lower power than DBS, FSS satellites require a much larger dish forreception (3 to 8 feet (1 to 2.5m) in diameter for K u band, and 12 feet (3.6m) or larger for C band). Theyuse linear polarization for each of the transponders' RF input and output (as opposed to circularpolarization used by DBS satellites), but this is a minor technical difference that users do not notice. FSSsatellite technology was also originally used for DTH satellite TV from the late 1970s to the early 1990s in

the United States in the form of TVRO (TeleVision Receive Only) receivers and dishes. It was also used inits Ku band form for the now-defunct Primestar satellite TV service.

Satellites for communication have now been launched that have transponders in the Ka band, such asDirecTV's SPACEWAY-1 satellite, and Anik F2. NASA as well has launched experimental satellites usingthe Ka band recently.
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