1、Antennas and PropagationChapter 5IntroductionnAn antenna is an electrical conductor or system of conductorsnTransmission-radiates electromagnetic energy into spacenReception-collects electromagnetic energy from spacenIn two-way communication,the same antenna can be used for transmission and receptio
2、nRadiation PatternsnRadiation patternnGraphical representation of radiation properties of an antennanDepicted as two-dimensional cross sectionnBeam width(or half-power beam width)nMeasure of directivity of antennanReception patternnReceiving antennas equivalent to radiation patternTypes of Antennasn
3、Isotropic antenna(idealized)nRadiates power equally in all directionsnDipole antennasnHalf-wave dipole antenna(or Hertz antenna)nQuarter-wave vertical antenna(or Marconi antenna)nParabolic Reflective Antenna Antenna GainnAntenna gainnPower output,in a particular direction,compared to that produced i
4、n any direction by a perfect omnidirectional antenna(isotropic antenna)nEffective areanRelated to physical size and shape of antennaAntenna GainnRelationship between antenna gain and effective areanG=antenna gainnAe=effective areanf=carrier frequencync=speed of light(3 108 m/s)n=carrier wavelength22
5、244cAfAGeePropagation ModesnGround-wave propagationnSky-wave propagationnLine-of-sight propagationGround Wave PropagationGround Wave PropagationnFollows contour of the earthnCan Propagate considerable distancesnFrequencies up to 2 MHznExamplenAM radioSky Wave PropagationSky Wave PropagationnSignal r
6、eflected from ionized layer of atmosphere back down to earthnSignal can travel a number of hops,back and forth between ionosphere and earths surfacenReflection effect caused by refractionnExamplesnAmateur radionCB radioLine-of-Sight PropagationLine-of-Sight PropagationnTransmitting and receiving ant
7、ennas must be within line of sightnSatellite communication signal above 30 MHz not reflected by ionospherenGround communication antennas within effective line of site due to refractionnRefraction bending of microwaves by the atmospherenVelocity of electromagnetic wave is a function of the density of
8、 the mediumnWhen wave changes medium,speed changesnWave bends at the boundary between mediumsLine-of-Sight EquationsnOptical line of sightnEffective,or radio,line of sightnd=distance between antenna and horizon(km)nh=antenna height(m)nK=adjustment factor to account for refraction,rule of thumb K=4/3
9、hd57.3hd57.3Line-of-Sight EquationsnMaximum distance between two antennas for LOS propagation:nh1=height of antenna onenh2=height of antenna two2157.3hhLOS Wireless Transmission ImpairmentsnAttenuation and attenuation distortionnFree space lossnNoisenAtmospheric absorptionnMultipathnRefractionnTherm
10、al noiseAttenuationnStrength of signal falls off with distance over transmission mediumnAttenuation factors for unguided media:nReceived signal must have sufficient strength so that circuitry in the receiver can interpret the signalnSignal must maintain a level sufficiently higher than noise to be r
11、eceived without errornAttenuation is greater at higher frequencies,causing distortionFree Space LossnFree space loss,ideal isotropic antenna nPt=signal power at transmitting antennanPr=signal power at receiving antennan=carrier wavelengthnd=propagation distance between antennasnc=speed of light(3 10
12、 8 m/s)where d and are in the same units(e.g.,meters)222244cfddPPrtFree Space LossnFree space loss equation can be recast:dPPLrtdB4log20log10 dB 98.21log20log20d dB 56.147log20log204log20dfcfdFree Space LossnFree space loss accounting for gain of other antennasnGt=gain of transmitting antennanGr=gai
13、n of receiving antennanAt=effective area of transmitting antennanAr=effective area of receiving antenna trtrtrrtAAfcdAAdGGdPP2222224Free Space LossnFree space loss accounting for gain of other antennas can be recast as rtdBAAdLlog10log20log20 dB54.169log10log20log20rtAAdfCategories of NoisenThermal
14、NoisenIntermodulation noisenCrosstalknImpulse Noise Thermal NoisenThermal noise due to agitation of electronsnPresent in all electronic devices and transmission medianCannot be eliminatednFunction of temperaturenParticularly significant for satellite communicationThermal NoisenAmount of thermal nois
15、e to be found in a bandwidth of 1Hz in any device or conductor is:nN0=noise power density in watts per 1 Hz of bandwidthnk=Boltzmanns constant=1.3803 10-23 J/KnT=temperature,in kelvins(absolute temperature)W/Hz k0TN Thermal NoisenNoise is assumed to be independent of frequencynThermal noise present
16、in a bandwidth of B Hertz(in watts):or,in decibel-wattsTBNkBTNlog10 log 10k log10BTlog10 log 10dBW 6.228Noise TerminologynIntermodulation noise occurs if signals with different frequencies share the same mediumnInterference caused by a signal produced at a frequency that is the sum or difference of
17、original frequenciesnCrosstalk unwanted coupling between signal pathsnImpulse noise irregular pulses or noise spikesnShort duration and of relatively high amplitudenCaused by external electromagnetic disturbances,or faults and flaws in the communications systemExpression Eb/N0nRatio of signal energy
18、 per bit to noise power density per HertznThe bit error rate for digital data is a function of Eb/N0nGiven a value for Eb/N0 to achieve a desired error rate,parameters of this formula can be selectednAs bit rate R increases,transmitted signal power must increase to maintain required Eb/N0TRSNRSNEbk/
19、00Other ImpairmentsnAtmospheric absorption water vapor and oxygen contribute to attenuationnMultipath obstacles reflect signals so that multiple copies with varying delays are receivednRefraction bending of radio waves as they propagate through the atmosphereMultipath PropagationMultipath Propagatio
20、nnReflection-occurs when signal encounters a surface that is large relative to the wavelength of the signalnDiffraction-occurs at the edge of an impenetrable body that is large compared to wavelength of radio wavenScattering occurs when incoming signal hits an object whose size in the order of the w
21、avelength of the signal or lessThe Effects of Multipath PropagationnMultiple copies of a signal may arrive at different phasesnIf phases add destructively,the signal level relative to noise declines,making detection more difficultnIntersymbol interference(ISI)nOne or more delayed copies of a pulse m
22、ay arrive at the same time as the primary pulse for a subsequent bitTypes of FadingnFast fadingnSlow fadingnFlat fadingnSelective fadingnRayleigh fadingnRician fadingError Compensation MechanismsnForward error correctionnAdaptive equalizationnDiversity techniquesForward Error CorrectionnTransmitter
23、adds error-correcting code to data blocknCode is a function of the data bitsnReceiver calculates error-correcting code from incoming data bitsnIf calculated code matches incoming code,no error occurrednIf error-correcting codes dont match,receiver attempts to determine bits in error and correctAdapt
24、ive EqualizationnCan be applied to transmissions that carry analog or digital informationnAnalog voice or videonDigital data,digitized voice or videonUsed to combat intersymbol interferencenInvolves gathering dispersed symbol energy back into its original time intervalnTechniquesnLumped analog circu
25、itsnSophisticated digital signal processing algorithmsDiversity TechniquesnDiversity is based on the fact that individual channels experience independent fading eventsnSpace diversity techniques involving physical transmission pathnFrequency diversity techniques where the signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriersnTime diversity techniques aimed at spreading the data out over time
