WIXLIB

Beamwidth Half-power beamwidth (HPBW) is the anglebetween two vectors from the pattern’s origin tothe points of the major lobe where the radiationintensity is half its maximum Often used to describe the antenna resolution properties» Important in radar technology, radioastronomy, etc. First-null beamwidth (FNBW) is the anglebetween two vectors, originating at the pattern’sorigin and tangent to the main beam at its base.» Often FNBW 2*HPBWR Struzak49

Antenna Mask (Example 1)Typical relativedirectivity- maskof receivingantenna (Yagiant., TV dcmwaves)-5-10-15180120600-60-120-20-180Relative gain, dB0[CCIR doc. 11/645, 17-Oct 1989)Azimith angle, degreesR Struzak50

Antenna Mask (Example 2)00dBRR/1998 APS30 Fig.9Relative gain 00Phi/Phi0Reference pattern for co-polar and cross-polar components forsatellite transmitting antennas in Regions 1 and 3 (Broadcasting 12 GHz)R Struzak51

VoltsEquivalent half-power beamwidth representationsof an antenna’s radiation pattern.R Struzak52

Anisotropic sources: gainHypothetic directionalantennaHypotheticisotropicantennaEvery real antenna radiates moreenergy in some directions than inothers (i.e. has directional properties) Idealized example of directionalantenna: the radiated energy isconcentrated in the yellow region(cone). Directive antenna gain: the power fluxdensity is increased by (roughly) theinverse ratio of the yellow area and thetotal surface of the isotropic sphere– Gain in the field intensity may also beconsidered - it is equal to the squareroot of the power gain.R Struzak53

Plane angle: radian Angle in radians,lω ω*rω lω / r;lωrω– lω is the length of the arcsegment supported by theangle ω in a circle of radius r.– There are 2π rad in a fullcircle– 1 rad (360 / 2π) degR Struzak54

Solid angle: steradian Solid angle in steradians (sr),Ω (SΩ)/r2; SΩ Ωr2SΩ is the spherical surface area supportedby the solid angle Ω in a sphere of radius r The steradian is the area cut out by the solidangle, divided by the sphere’s radiussquared - ‘squared radian’. If the area is S, and the radius is d, then theangle is S/d2 steradians. The total solidangle (a full sphere) is thus 4π steradians. As one radian is 180/π 57.3 degrees, thetotal solid angle is 4π x (57.3)2 41253square degrees, one steradian is 3282.806square degrees, and one square degree isabout 305 x 10-6 steradiansR Struzak55

Example: gain of 1 deg2 antennaP/αd2 α sr IsotropicP/(4πd2) G 4π/α If α 1 deg2, thenA hypothetical source radiates Pwatts uniformly within the solid angleof α steradians in a given directionand zero outsideThe total surface of the sphere is4πd2 and the average irradiance isthe power divided by the surface:[P/(4πd2)] w/m2α steradians corresponds tospherical surface of αd2 andirradiance within that angle is[P/αd2] w/m2The antenna gain equals the ratio ofthese two, or 4π/αFor α 1 deg2 ( 305*10-6 sr); thegain 4π/305*10-6 46 dB. ,G 4π/305*10-6 46 dBR Struzak56

Effect of sidelobesLet the main beamwidth of an antenna be Ω square degrees, with uniform irradiance ofW watts per square meter. Let the sidelobe irradiance (outside the main beam) beuniform and k times weaker, i.e. (W/k) watts per square meter, k 1. Then:WW/kG .The gain decreases with the sidelobelevel and beamwidth.If the main lobe is 1 square degree andthe sidelobes are attenuated by 20 dB,then k 100 and G 100 (or 20dB) ,much less than in the previous example(46dB).In the limit, when k 1, the gain tends to1 (isotropic antenna).W1 W0 1 k 1 Ω k k 41253 R Struzak57

PM W Ωd 2- power radiated within the main lobe W PS ( 41253 Ω ) d 2 - power radiated by sidelobes k 41253 Ω 2 - total powerPT PM PS Wd Ω kk PT 1 kΩ Ω W0 W 2 41253d41253kk 1WG W0 1 k 1 Ω k k 41253 - average irradiation- antenna gain.R Struzak58

Antenna gain measurementReferenceantennaMeasuringequipmentPo Powerdelivered tothe referenceantennaS0 Powerreceived(the same inboth steps)ActualantennaP Powerdelivered tothe actualantennaStep 1: referenceMeasuringequipmentS Powerreceived(the same inboth steps)Step 2: substitutionAntenna Gain (P/Po) S S0R Struzak59

Antenna Gains Gi, Gd Unless otherwise specified, the gain refersto the direction of maximum radiation. Gain is a dimension-less factor related topower and usually expressed in decibels Gi “Isotropic Power Gain” – theoreticalconcept, the reference antenna is isotropic Gd - the reference antenna is a half-wavedipoleR Struzak60

Typical Gain and BeamwidthType of antennaGi [dB]BeamW.Isotropic03600x3600Half-wave Dipole23600x1200Helix (10 turn)14350x350Small dish16300x300Large dish4510x10R Struzak61

Gain, Directivity, RadiationEfficiency The radiation intensity, directivityand gain are measures of theability of an antenna toconcentrate power in a particulardirection.Directivity relates to the powerradiated by antenna (P0 )Gain relates to the powerdelivered to antenna (PT)G (ϑ , ϕ ) ηD(ϑ , ϕ )PTη P0 η: radiation efficiency(0.5 - 0.75)R Struzak62

Antenna gain and effective area Effective area: Measure of the effectiveabsorption area presented by an antenna to anincident plane wave. Depends on the antenna gain and wavelengthλ2Ae ηG (θ , ϕ ) [m ]4π2Aperture efficiency: ηa Ae / AA: physical area of antenna’s aperture, square metersR Struzak63

Power Transfer in Free Space PR PFD Ae GT PT 2π4r λ GR 4π 2 λ PT GT GR 4πr 2R Struzak λ: wavelength [m]PR: power available at thereceiving antennaPT: power delivered to thetransmitting antennaGR: gain of the transmittingantenna in the direction ofthe receiving antennaGT: gain of the receivingantenna in the direction ofthe transmitting antennaMatched polarizations64

e.i.r.p. Equivalent Isotropically RadiatedPower (in a given direction):e.i.r. p. PGi The product of the power supplied to theantenna and the antenna gain (relativeto an isotropic antenna) in a givendirectionR Struzak65

Linear Polarization In a linearly polarizedplane wave the directionof the E (or H) vector isconstant .R Struzak66

Polarization ellipseExEyMψN The superposition of twocoherent plane-wavecomponents results in anelliptically polarized wave The polarization ellipse isdefined by its axial ratioN/M (ellipticity), tilt angleψ and sense of rotation Polarization (Java mlR Struzak67

Elliptical PolarizationLHCEx cos (wt)Ey cos (wt)Ex cos (wt)Ey cos (wt pi/4)Ex cos (wt)Ey -sin (wt)RHCEx cos (wt)Ey -cos (wt pi/4)R StruzakEx cos (wt)Ey cos (wt 3pi/4)Ex cos (wt)Ey sin (wt)68

Polarization statesLHCUPPER HEMISPHERE:ELLIPTIC POLARIZATIONLEFT HANDED SENSELATTITUDE:REPRESENTSAXIAL RATIOEQUATOR:LINEAR POLARIZATIONLOWER HEMISPHERE:ELLIPTIC POLARIZATIONRIGHT HANDED SENSEPOLES REPRESENTCIRCULAR POLARIZATIONS(Poincaré sphere)450 LINEARRHCR StruzakLONGITUDE:REPRESENTSTILT ANGLE69

Comments on Polarization At any moment in a chosen reference point inspace, there is actually a single electric vector E(and associated magnetic vector H). This is the result of superposition (addition) ofthe instantaneous fields E (and H) produced byall radiation sources active at the moment. The separation of fields by their wavelength,polarization, or direction is the result of ‘filtration’.R Struzak70

Antenna Polarization The polarization of an antenna in a specificdirection is defined to be the polarization of thewave produced by the antenna at a greatdistance at this directionR Struzak71

Polarization Efficiency The power received by an antennafrom a particular direction is maximal if thepolarization of the incident wave and thepolarization of the antenna in the wave arrivaldirection have:– the same axial ratio– the same sense of polarization– the same spatial orientation.R Struzak72

Polarization filters/ reflectorsWall of thin parallel wires (conductors) E1 0 E1 0 E2 0 E2 E2 Vector E wiresVector E wiresReflectingTransparentWire distance 0.1λ At the surface of ideal conductor the tangentialelectrical field component 0R Struzak73

Outline IntroductionReview of basic antenna typesRadiation pattern, gain, polarizationEquivalent circuit & radiation efficiencySmart antennasSome theorySummaryR Struzak74

Transmitting antenna equivalent circuitAntennaTransmitterTransm. lineRadio waveThe transmitter with the transmission line is represented by an(Thevenin) equivalent generatorjXARGVGGeneratorjXGRrThe antenna is represented by its input impedance(which is frequency-dependent and is influenced by objectsnearby) as seem from the generatorjXA represents energy stored in electric (Ee) and magnetic (Em)near-field components; if Ee Em then XA 0 (antennaresonance)Rr represents energy radiated into space (far-field components)Rl represents energy lost, i.e. transformed into heat in theantenna structureRlNote: Transmission-line model offers better approximationR Struzak75

Power transfer: Tx antennaTransmitter is represented by an eqivalentgenerator with VG , RG , X G const.Let RA RR RL ; RA , X A var.jXARGVGGeneratorjXGRRThe power absorbed by antenna P I 2 RA I2 P VG2RL VG 22( RG RA ) ( XG X A ) RA( RG RA ) ( XG X A )222RARG VG2 P 22 RG RA X G X A 1 RRRG G GR Struzak76

P VG2( RG RA )RA2Let X G X A 0. Then P VG2 X G2 2 X G X A X A2 RA ( 2 X G 2 X A ) P VG2 22 2 X A RRXX ( G( G A ) A) P 0, when X A X G X AMaximum : P P 0 RA X ARA RG ,X A XGRA( RG RA )2 2 RRRRR2()() P AAGA VG2 G2 2 RA ( RG RA ) 2 22 R 2 RG RA RA 2 RG RA 2 RA VG2 G 2 2 R RA ) ( G P 0, when RG RA RAVG2P 4 RGR Struzak77

Impedance matchingRA Rr Rl RgX A XgPA Pg Vg24 RAVg24 Rg( PA )Pr PARr( Rr Rl )Pl PARl( Rr Rl )R Struzak78

Power vs. field strengthPr E2 E Pr Z 0Z0E Eθ Eϕ2H 2EZ0Z 0 377 ohmsfor plane wavein free spaceR Struzak79

Receiving antenna equivalent circuitAntennaRadio waveTransm.lineReceiverThe antenna with the transmission line is representedby an (Thevenin) equivalent generatorRrRlAntennajXAjXLRLThe receiver is represented by its input impedance asseen from the antenna terminals (i.e. transformed bythe transmission line)VA is the (induced by the incident wave) voltage at theantenna terminals determined when the antenna isopen circuitedNote1: The antenna impedance is the same when the antenna is used toradiate and when it is used to receive energyVAThevenin equivalentNote 2: Transmission-line model offers better approximationR Struzak80

Power transferPA / PAmax10.500.11RA / RG; (XA XG 0)10 The maximumpower is deliveredto (or from) theantenna when theantennaimpedance and theimpedance of theequivalentgenerator (or load)are matchedR Struzak81

When the impedances are matched– Half of the source power is delivered to the load andhalf is dissipated within the (equivalent) generator asheat– In the case of receiving antenna, a part (Pl) of thepower captured is lost as heat in the antennaelements, the other part being reradiated (scattered)back into space Even when the antenna losses tend to zero, still only half ofthe power captured is delivered to the load (in the case ofconjugate matching), the other half being scattered back intospaceR Struzak82

The antenna impedance must be matchedto the transmitter output impedance (or tothe receiver input impedance) and totransmission line between them to assureeffective power transfer Inexpensive impedance-matching devicesare usually narrow-bandR Struzak83

Radiation efficiency The radiation efficiency e indicates howefficiently the antenna uses the RF power It is the ratio of the power radiated by theantenna and the total power delivered tothe antenna terminals (in transmittingmode). In terms of equivalent circuitparameters:Rre Rr RlR Struzak84

Outline IntroductionReview of basic antenna typesRadiation pattern, gain, polarizationEquivalent circuit & radiation efficiencySmart antennasSome theorySummaryR Struzak85

Antenna arrays Consist of multiple (usually identical) antennas(elements) ‘collaborating’ to synthesize radiationcharacteristics not available with a single antenna. Theyare able– to match the radiation pattern to the desired coverage area– to change the radiation pattern electronically (electronicscanning) through the control of the phase and the amplitude ofthe signal fed to each element– to adapt to changing signal conditions– to increase transmission capacity by better use of the radioresources and reducing interference Complex & costly– Intensive research related to military, space, etc. activities» Smart antennas, signal-processing antennas, tracking antennas,phased arrays, etc.Source: adapted from N GregorievaR Struzak86

Satellite antennas (TV) Not an array!R Struzak87

Owens Valley RadioObservatory ArrayThe Earth’satmosphere istransparent inthe narrowvisible-lightwindow(4000-7000angstroms) andthe radio bandbetween 1 mmand 10 m.[Sky & TelescopeFeb 1997 p.26]R Struzak88

New Mexico Very Large Array[Sky & TelescopeFeb 1997 p. 30]27 antennas along 3 railroad tracks provide baselines up to 35 km.Radio images are formed by correlating the signals garnered byeach antenna.R Struzak89

2 GHz adaptive antenna array A set of 482 GHzantennas– Source:ArraycommR Struzak90

Phased Arrays Array of N antennas in a linear or twodimensional configuration beam-forming& control device The amplitude and phase excitation of eachindividual antenna controlled electronically(“software-defined”)– Diode phase shifters– Ferrite phase shifters Inertia-less beam-forming and scanning (µsec)with fixed physical structureR Struzak91

Simulation 2 omnidirectional antennas (equalamplitudes)– Variables distance increment phase increment N omnidirectional antennas– Group factor (N omnidirectional antennasuniformly distributed along a straight line,equal amplitudes, equal phase increment)R Struzak92

2 omnidirectional 1-0.5-0.5-0.5-1-1-1D 0.5λ, θ 900D 0.5λ, θ 1800D 0.5λ, θ 00 000-1Java applet 2 antennas:Simple demo: 2-2.htmlDetailed analysis: -2.htmlR Struzak93

N omnidirectional antennas62.510952871Relative gain1.5Relative gainRelative gain4320.565432110-1800-90090Azimuth angle, degreesN 2, θ 900180-180-90090Azimuth angle, degreesN 5, θ 18001800-180-90090180Azimuth angle, degreesN 9, θ 450 Array gain (line, uniform, identical power)R Struzak94

Switched beam antennas– Based on switching function betweenseparate directive antennas orpredefined beams of an array Space Division Multiple Access(SDMA) allocating an angledirection sector to each user– In a TDMA system, two users will beallocated to the same time slot andthe same carrier frequency– They will be differentiated by differentdirection anglesR Struzak95

Dynamically phased array(PA):– A generalization of theswitched lobe concept– The radiation patterncontinuously track thedesignated signal (user)– Include a direction of arrival(DoA) tracking algorithmR Struzak96

Beam SteeringBeam directionθEqui-phasewave frontd3 [(2π/λ)d sinθ]2 0RadiatingelementsPhaseshifters Beamsteeringusingphaseshifters ateachradiatingelementPowerdistributionR Struzak97

4-Bit Phase-Shifter (Example)Bit #3Bit #4Input00or22.5000or450Bit #1Bit #200or90000or1800OutputSteering/ Beam-forming CircuitryAlternative solution: Transmission line with controlled delayR Struzak98

Switched-Line Phase BitDelay line #1aInputDiode switchOutputDelay line #1bPhase bit delay differenceR Struzak99

Antenna Arrays: Benefits Possibilities to control electronically–––––Direction of maximum radiationDirections (positions) of nullsBeam-widthDirectivityLevels of sidelobesusing standard antennas (or antenna collections)independently of their radiation patterns Antenna elements can be distributed alongstraight lines, arcs, squares, circles, etc.R Struzak100

Adaptive (“Intelligent”)Antennas Array of N antennas in a linear,circular, or planar configuration Used for selection signals fromdesired sources and suppressincident signals from undesiredsources The antenna pattern track thesources It is then adjusted to null out theinterferers and to maximize thesignal to interference ratio (SIR) Able to receive and combineconstructively multipath signalsR Struzak101

The amplitude/ phaseexcitation of eachantenna controlledelectronically(“software-defined”) The weight-determiningalgorithm uses a-prioriand/ or measuredinformation to adaptantenna to changingenvironment The weight andsumming circuits canoperate at the RF and/or at an orithmR Struzak102

Antenna sitting Radio horizon Effects of obstacles & structures nearby Safety– operating procedures– Grounding lightning strikes static charges– Surge protection lightning searches for a second path to groundR Struzak103

Outline IntroductionReview of basic antenna typesRadiation pattern, gain, polarizationEquivalent circuit & radiation efficiencySmart antennasSome theorySummaryR Struzak104

Maxwell’s Equations EM field interacting wi

archive/wavesA.html -Elliptical polarization: change of the rotation sense! R Struzak 22 Planar reflectors Uda-Yagi, Log-periodic antennas d 2d Intended reflector antenna allows maintaining radio link in non-LOS conditions (avoiding propagation obstacles) Unintended reflector antennas