Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
SPRAWDZIC #4--6!!!! Antenna Basics 15 Jan 2003 Property of R. Struzak 1 Outline • • • • • • Reciprocity Theorem Point Radiator Concept Irradiance, PFD Directivity, Gain, Radiation Efficiency EIRP Power Transfer 15 Jan 2003 Property of R. Struzak 2 EM Field = EM Forces • EM Field is a spatial distribution of forces which may be exerted on an electric charge – Force = a vector characterized by its intensity, direction, & orientation • Classical physics – Coulomb (1736-1806), Galvani (1737-1798) Volta (1745-1827), Ampere (1775-1836), Faraday (17911867), Maxwell (1831-1879), Hertz (1857-1894), Marconi (1874-1937), Popov (1874-1937) 15 Jan 2003 Property of R. Struzak 3 EM Field • EM forces fill-in the whole space without limits. • They interact with the matter. – Magnetic forces and electric forces act differently, e.g. the magnetic field interact with electric charges only when the charges move. – For many years Electric and Magnetic forces were considered as being different phenomena and different branches of physics. Only in 19 century ……. realized that they both are different faces of the same EM phenomenon 15 Jan 2003 Property of R. Struzak 4 • Abdus Salam, (XX-XX), 1979 Nobel Prize Laureate, indicated further that electromagnetism and weak interaction known from quantum physics are various facets of the same phenomenon. • Richard Feynmann (1918-1988), 1965 Nobel Prize Laureate ( XXX quantum electrodynamics) 15 Jan 2003 Property of R. Struzak 5 EM forces are stronger than gravity forces, but how strong they are? • Imagine 2 persons at 1 m distance. By some magic, we decrease the number of protons by 1% in each, so that each has more electrons than protons, and is no more electrically neutral: they repulse each other. How strong would be the repulsive force? • Could it be enough to move a sheet of paper? Or this table? Or, perhaps, this building? 15 Jan 2003 Property of R. Struzak 6 • Feynman calculated that the repulsive force would be strong enough to lift the whole Earth! • EM forces generated in far galaxies can move electrons on the Earth: Panzias & Wilson, Nobel Prize Laureates 1978, showed that the EM residual noise was generated during the Big Bang 15 Jan 2003 Property of R. Struzak 7 15 Jan 2003 Property of R. Struzak 8 Maxwell Equations • Concept of unlimited EM field interacting with the matter – Mathematics: 2 coupled vectors E and H (6 numbers) varying with time and space • Summary: The magnetic and electric components of the time-space-variable electromagnetic field and the time-variable electric current are mutually coupled. 15 Jan 2003 Property of R. Struzak 9 EM Field of Linear Antennas • Summation of all vector components E (or H) produced by each antenna element O E E1 E2 E3 ... H H1 H 2 H 3 ... • In the far-field region, the vector components are parallel to each other • Method of moments 15 Jan 2003 Property of R. Struzak 10 EM Field of Current Element Er z E Er E E H H r H H E OP I, dz r E y 2 E Er E E H H r H H 2 2 2 2 2 x I: monochromatic AC [ampere]; dz: short element [meter] 15 Jan 2003 Property of R. Struzak 11 EM Field of Current Element 2 E jA FF jQ C (sin )e j r Er 2 A Q C (cos )e j r E 0 jA H FF Q (sin )e j r 120 H r H 0 2 A 30 2 Idz 1 FF r 1 Q ( r ) 2 1 C ( r )3 Idz: “moment of linear current element” Johnson & Jasik: Antenna Engineering Handbook; T. Dvorak: Basics of Radiation Measurements, EMC Zurich 1991; J. Dunlop, D. Smith Telecommunications Engineering1995, p. 216 15 Jan 2003 Property of R. Struzak 12 EM Field of Current Element 3 • The components of the EM field – are proportional to the current moment Idz – are azimuth-independent (axial symmetry) – decrease with distance as (r)-1, (r)-2, or (r)-3; if r = 1, [r = /(2)], C = FF = Q • E maximal in the equatorial plane • Er maximal in the direction of current dz • H maximal in the equatorial plane 15 Jan 2003 Property of R. Struzak 13 EM Field: Elementary Current Loop H 120BFF jQ C (sin )e jr H r 2 BQ C (cos )e jr E BFF Q (sin )e jr 3dm B 4 H Er E 0 2 dm I LoopArea dm: “magnetic dipole moment” 15 Jan 2003 Property of R. Struzak 14 Field Components Intensity 1000 C C, Q: Induction fields Relative fieldstrength 100 Q 10 FF 1 FF: Radiation field 0.1 FF Q 0.01 C 0.001 0.1 1 10 Relative distance, Br 15 Jan 2003 Property of R. Struzak 15 Field Impedance 100 Short dipole Z / 377 10 1 0.1 Small loop 0.01 0.01 0.1 1 10 Distance / (lambda/ 2Pi) 15 Jan 2003 Property of R. Struzak 100 Field impedance Z = E/H depends on the antenna type and on distance 16 Far-Field, Near-Field • Near-field region: – – • Angular distribution of energy depends on distance from the antenna; Reactive field components dominate (L, C) Far-field region: – – – 15 Jan 2003 Angular distribution of energy is independent on distance; Radiating field component dominates (R) The resultant EM field can locally be treated as uniform (TEM) Property of R. Struzak 17 Source Characteristics 1 • The radiated (far) field in all direction from a single monochromatic source in free space is completely specified by 4 quantities: 1. Amplitude of the E component of the electric field as functions of r, , and 2. Amplitude of the E component of the electric field as functions of r, , and 15 Jan 2003 Property of R. Struzak 18 Source Characteristics 2 3. Phase lag of E behind E as a function of , r, and 4. Phase lag of a field component behind its value at a reference point as a function of r, , and • Phase characteristics are often disregarded but they are important when the fields from 2 or more sources are to be added. 15 Jan 2003 Property of R. Struzak 19 Reciprocity Theorem • The proprieties of a receiving antenna are identical with the proprieties of the same antenna when used for transmitting – Note: This theorem is valid only for linear passive antennas (i.e. antennas that do not contain nonlinear elements and/or amplifiers) 15 Jan 2003 Property of R. Struzak 20 Antenna Functions • To transform the power of time-dependent electrical current into the power of the time-and-spacedependent electro-magnetic (EM) wave (transmitting antenna) • To transform the power of the time-and-spacedependent EM wave into the power of the timedependent electrical current (receiving antenna) 15 Jan 2003 Property of R. Struzak 21 Intended & Unintended Antennas • Intended antennas – Radiocommunication antennas – Measuring antennas, EM sensors, probes – EM applicators (Industrial, Medical) • Unintended antennas – Radiating (any conductor/ installation carrying electrical current: e.g. electrical installation of vehicles) – Receiving/ Re-radiating (any conducting structure/ installation irradiated by EM waves) – Stationary (e.g. antenna masts or power line wires) – Time-varying (e.g. windmill or helicopter propellers) – Transient (e.g. aeroplanes, missiles) 15 Jan 2003 Property of R. Struzak 22 Basic Antenna Characteristics • In terms of field theory (Electromagnetics) – Gain – Radiation pattern (Half-power beam width, unintended lobes) – Polarization (Cross-polarization) • In terms of circuit theory – Radiation resistance (Impedance) – VSWR 15 Jan 2003 Property of R. Struzak 23 Point Source • For many purposes, it is sufficient to know the direction (angle) variation of the power radiated by antenna at large distances. • For that purpose, any practical antenna, regardless of its size and complexity, can be represented as a point-source. • The actual field near the antenna is then disregarded. 15 Jan 2003 Property of R. Struzak 24 Point Source 2 • The EM field at large distances from an antenna can be treated as originated at a point source - fictitious volume-less emitter. • The EM field in a homogenous unlimited medium at large distances from an antenna can be approximated by an uniform plane TEM wave 15 Jan 2003 Property of R. Struzak 25 Power Flow • The time rate of EM energy flow per unit area in free space is the Poynting vector. • It is the cross-product (vector product, right-hand screw direction) of the electric field vector (E) and the magnetic field vector (H) P = E x H. • For the elementary dipole E H and only ExH carry energy into space with the speed of light 15 Jan 2003 Property of R. Struzak 26 Power Flow 2 • The Poynting vector gives the irradiance and direction of propagation of the electromagnetic wave in free space. • Irradiance = radiant power incident per unit area upon a surface. It is usually expressed in watts per square meter, but may also be expressed in joules per square meter. • Synonyms: Power Density, Power Flow Density (PFD). 15 Jan 2003 Property of R. Struzak 27 Power Flow 3 • In free space, the radiated energy streams from the point source in radial lines, i.e. the Poynting vector has only the radial component in spherical coordinates. • A source that radiates uniformly in all directions is an isotropic source (radiator, antenna). For such a source the radial component of the Poynting vector is independent of and . 15 Jan 2003 Property of R. Struzak 28 Spherical coordinates for a point source of radiation in free space Observation point (r,,) Polar axis Z Poynting vector E Point source at origin r X 15 Jan 2003 E Y Equatorial plane Property of R. Struzak 29 Power Flow From Point Source Z r sin Polar axis Element of area ds = r2 sin d d r d r X Equatorial plane 15 Jan 2003 Property of R. Struzak Y r sin d 30 Power Flow - General Case The total complex power flow through any closed surface : 1 W ' ( E H *)ds 2 E and H * are complex ve ctors of electric and magnetic fields, H * is the complex conjugate of H . ds r 2 sin d d is the infinitisi mal elemet of spherical surface The total real power thro ugh the surface : 1 W Re W ' Re ( E H *)ds Pds Pr ds 2 1 P Re E H * is the Poynting vector. 2 15 Jan 2003 Property of R. Struzak 31 Power Flow - Isotropic Source For an isotropic source in loss-less medium, Pr is independent of and so that r W Pr ds Pr ds The integral is equal to the area of the sphere (4 r 2 ) and W Pr 4 r 2 . W 4 r 2 Pr PFD (Poynting vector), [Wm -2 ] Pr W power radiated, [W] r distance [m] 15 Jan 2003 Notes • • • • PFD does not depend on frequency/ wavelength Distance increases x 2 → PFD decreases x 4 Distance increases x 2 → E decreases x 2 Isotropic radiator cannot be physically realized Property of R. Struzak 32 Anisotropic sources • Every antenna has directional properties (radiates more energy in some directions than in others). Isotropic sphere • Idealized example of directional antenna: the radiated energy is concentrated in the yellow region (cone). • The power flux density gains: it is increased by (roughly) the inverse ratio of the yellow area and the total surface of the isotropic sphere. 15 Jan 2003 Property of R. Struzak 33 Antenna Gain • The ratio of the power required at the input of a loss-free reference antenna to the power supplied to the input of the given antenna to produce, in a given direction, the same field strength at the same distance. 15 Jan 2003 Property of R. Struzak 34 Antenna Gain 2 Step 2 Step 1 Actual antenna Measuring equipment Reference antenna Measuring equipment P = Power delivered to the actual antenna S = Power received Po = Power delivered to the reference antenna S0 = Power received Antenna Gain = (P/Po) S=S0 15 Jan 2003 Property of R. Struzak 35 Antenna Gains Gi, Gd, Gr • Gi “Isotropic Power Gain” - the reference antenna is isotropic • Gd - the reference antenna is a half-wave dipole isolated in space • Gr - the reference antenna is linear much shorter than one quarter of the wavelength, normal to the surface of a perfectly conducting plane 15 Jan 2003 Property of R. Struzak 36 Antenna Gain: Comments • Unless otherwise specified, the gain refers to the direction of maximum radiation. • Gain in the field intensity may also be considered - it is equal to the square root of the power gain. • Gain is a dimension-less factor, usually expressed in decibels 15 Jan 2003 Property of R. Struzak 37 Radiant Intensity z Transmitting antenna = Radiated power per unit solid angle (steradian), (,), in watts per steradian Observation • A measure of the ability of Point an antenna to concentrate radiated power in a r particular direction y • Does not depend on distance x Assumption: Distance (r) is very large 15 Jan 2003 Property of R. Struzak 38 Directivity ( , ) ( , ) D( , ) avg P0 4 Average radiation intensity P avg 0 4 Total power radiated P0 2 0 15 Jan 2003 0 ( , ) sin d d Property of R. Struzak • D Has no units • P0 = power radiated 39 Gain, Directivity, Radiation Efficiency • The radiation intensity, directivity and gain are measures of the ability of an antenna to concentrate power in a particular direction. • Directivity relates to the power radiated by antenna (P0 ) • Gain relates to the power delivered to antenna (PT) 15 Jan 2003 G ( , ) D ( , ) PT P0 • : radiation efficiency (0.5 - 0.75) Property of R. Struzak 40 Antenna Gain & PFD ( , ) ( , ) S ( , ) (r )( r ) r2 P0 G ( , ) 4r 2 G ( , ) S 0 S0 = PFD produced by a loss-less isotropic radiator 15 Jan 2003 Property of R. Struzak 41 Directivity Pattern – The variation of the field intensity of an antenna as an angular function with respect to the axis. – Usually represented graphically for the far-field conditions. – May be considered for a specified polarization and/or plane (horizontal, vertical). – Depends on the polarization and the reference plane for which it is defined/measured – Synonym: Radiation pattern. 15 Jan 2003 Property of R. Struzak 42 Antenna patterns Pmax() 1 E2 Pr E Pr Z 0 Z0 E E2 E2 P() Power pattern Z 0 377 ohms P()/Pmax() for plane wave Relative (normalized) power pattern • Usually represented in 2 reference planes =const. and =const. • E & PDF relate to the equivalent uniform plane wave • Note: Peak value = 2 x Effective value for sinusoidal quantities 15 Jan 2003 Property of R. Struzak 43 Elements of Radiation Pattern Main lobe Emax Sidelobes Emax /2 Nulls -180 0 Beamwidth 15 Jan 2003 180 • • • • Gain Beam width Nulls (positions) Side-lobe levels (envelope) • Front-to-back ratio Property of R. Struzak 44 Beam width • Beamwidth of an antenna pattern: the angle between the half-power points of the main lobe. • Defined separately for the horizontal plane and for the vertical plane. • Usually expressed in degrees. 15 Jan 2003 Property of R. Struzak 46 Antenna Mask (Example 1) Typical relative directivity- mask of receiving antenna (Yagi ant., TV dcm waves) Relative gain, dB 0 -5 -10 -15 180 120 60 0 -60 -120 -180 -20 [CCIR doc. 11/645, 17-Oct 1989) Azimith angle, degrees 15 Jan 2003 Property of R. Struzak 47 Antenna Mask (Example 2) 0 0dB RR/1998 APS30 Fig.9 Relative gain (dB) -10 COPOLAR -3dB -20 Phi -30 -40 CROSSPOLAR -50 0.1 10 1 100 Phi/Phi0 Reference pattern for co-polar and cross-polar components for satellite transmitting antennas in Regions 1 and 3 (Broadcasting ~12 GHz) 15 Jan 2003 Property of R. Struzak 48 Typical Gain and Beamwidth Type of antenna Gi [dB] BeamW. Isotropic 0 3600x3600 Half-wave Dipole 2 3600x1200 Helix (10 turn) 14 350x350 Small dish 16 300x300 Large dish 45 10x10 15 Jan 2003 Property of R. Struzak 49 Gain and Beamwidth • Gain and beam-width of highly directive antennas are inter-related: G ~ 30000 / (1*2) • 1 and 2 are the half-power beamwidths in the two orthogonal principal planes of antenna radiation pattern in degrees. 15 Jan 2003 Property of R. Struzak 50 Increasing Gain Using multiple antenna Using lenses 15 Jan 2003 Using reflector Property of R. Struzak 51 Parabolic Antenna L A” A A’ B’ B” B F C” C C’ L’ 15 Jan 2003 Wave front • For the planar wave front, the times/distances FA’A = FB’B = CC’C =… • Extend AA’ by A’A” … • Require A’A” = A’F … • Locus of points equidistant from F and LL’ is parabola • Axial symmetry – parabolic reflector Property of R. Struzak 52 How to Make Parabolic Reflectors Cheaply Water Steel tube Explosive Thin metallic sheet over parabolic surface Flat metallic sheet Air Parabolic surface Sand (fixed) Concrete/iron block 15 Jan 2003 Property of R. Struzak 53 e.i.r.p. • Equivalent Isotropically Radiated Power (in a given direction): e.i.r. p. PGi • The product of the power supplied to the antenna and the antenna gain (relative to an isotropic antenna) in a given direction 15 Jan 2003 Property of R. Struzak 54 Antenna Effective Area • Measure of the effective absorption area presented by an antenna to an incident plane wave. • Depends on the antenna gain and wavelength 2 Ae G( , ) [m ] 4 2 Aperture efficiency: a = Ae / A A: physical area of antenna’s aperture, square meters 15 Jan 2003 Property of R. Struzak 55 Power Transfer in Free Space PR PFD Ae GT PT 2 4 r GR 4 2 PT GT GR 4r 15 Jan 2003 2 • : wavelength [m] • PR: power available at the receiving antenna • PT: power delivered to the transmitting antenna • GR: gain of the transmitting antenna in the direction of the receiving antenna • GT: gain of the receiving antenna in the direction of the transmitting antenna • Matched polarizations Property of R. Struzak 56 Linear Polarization • In a linearly polarized plane wave the direction of the E (or H) vector is constant. • Two linearly polarized waves produce one elliptically polarized wave – the resultant E vector has direction varying in time – its tip draws an ellipse. 15 Jan 2003 Property of R. Struzak 57 Elliptical Polarization LHC Ex = cos (wt) Ey = cos (wt) Ex = cos (wt) Ey = cos (wt+pi/4) Ex = cos (wt) Ey = -sin (wt) RHC Ex = cos (wt) Ey = -cos (wt+pi/4) 15 Jan 2003 Property of R. Struzak Ex = cos (wt) Ey = cos (wt+3pi/4) Ex = cos (wt) Ey = sin (wt) 58 Ex Ey Polarization ellipse M N 15 Jan 2003 • The superposition of two plane-wave components results in an elliptically polarized wave • The polarization ellipse is defined by its axial ratio N/M (ellipticity), tilt angle and sense of rotation Property of R. Struzak 59 Polarization states LHC UPPER HEMISPHERE: ELLIPTIC POLARIZATION LEFT_HANDED SENSE (Poincaré sphere) LATTITUDE: REPRESENTS AXIAL RATIO EQUATOR: LINEAR POLARIZATION 450 LINEAR LOWER HEMISPHERE: ELLIPTIC POLARIZATION RIGHT_HANDED SENSE RHC LONGITUDE: REPRESENTS TILT ANGLE POLES REPRESENT CIRCULAR POLARIZATIONS 15 Jan 2003 Property of R. Struzak 60 Comments on Polarization • At any moment in a chosen reference point in space, there is actually a single electric vector E (and associated magnetic vector H). • This is the result of superposition (addition) of the instantaneous fields E (and H) produced by all radiation sources active at the moment. • The separation of fields by their wavelength, polarization, or direction is the result of ‘filtration’. 15 Jan 2003 Property of R. Struzak 61 Antenna Polarization • The polarization of an antenna in a specific direction is defined to be the polarization of the wave produced by the antenna at a great distance at this direction 15 Jan 2003 Property of R. Struzak 62 Polarization Efficiency (1) • The power received by an antenna from a particular direction is maximal if the polarization of the incident wave and the polarization of the antenna in the wave arrival direction have: – the same axial ratio – the same sense of polarization – the same spatial orientation . 15 Jan 2003 Property of R. Struzak 63 Polarization Efficiency (2) • When the polarization of the incident wave is different from the polarization of the receiving antenna, then a loss due to polarization mismatch occurs Polarization efficiency = = (power actually received) / (power that would be received if the polarization of the incident wave were matched to the receiving polarization of the antenna) 15 Jan 2003 Property of R. Struzak 64 Polarization Efficiency (3) LCH A: POLARIZATION OF RECEIVING ANTENNA W: POLARIZATION OF INCIDENT WAVE W 2 A Polarization efficiency = cos2 450 LINEAR H RCH 15 Jan 2003 Property of R. Struzak 65 How to Produce Circularly-Polarized EM Field y x Ixcos(t+x) 15 Jan 2003 • Radio wave of elliptical (circular) polarization can Iycos(t+y) be obtained by superposition of 2 linearly-polarized waves produced by 2 crossed dipoles and by controlling the amplitude- ratio and phase-difference of their excitations. Property of R. Struzak 66 Reflection & Image Theory • Antenna above perfectly conducting plane surface • Tangential electrical field component = 0 – vertical components: the same direction – horizontal components: opposite directions • The field (above the ground) is the same if the ground is replaced by the antenna image 15 Jan 2003 Property of R. Struzak + - 67 Polarization Filters Wall of thin parallel wires (conductors) |E1|>0 |E1|>0 |E2| = 0 |E2| ~ |E2| Vector E wires Vector E wires • At the surface of ideal conductor the tangential electrical field component = 0 15 Jan 2003 Property of R. Struzak 68 e.i.r.p. & PFD: Example 1 • What is the PFD from a TV broadcast GEO satellite at Trieste? 1.8 10 2 103 PFD 4 (3.8 10 103 103 ) 2 – EIRP: 180 kW – Distance: ~38'000 km – Free space 15 Jan 2003 Property of R. Struzak 1.8 105 1.8 1016 1 10 11 Wm -2 110 dB(Wm 2 ) 70 e.i.r.p. & PFD: Example 2 • What is the PFD from a WLAN transmitter? 1.8 10 1 PFD 4 (3.8) 2 1 1.8 10 2 1.8 10 1 10 3 Wm -2 – EIRP: 180 mW – Distance: 3.8 m? – Free space 30 dB(Wm -2 ) In this example, WLAN produces thousand millions times stronger signal than the satellite! 15 Jan 2003 Property of R. Struzak 71 Power Transfer: Example 1 • What is the power received from GEO satellite (=0.1m, PT =440 W, GT=1000) at Trieste (distance ~38'000 km, GR=1)? • Free space 15 Jan 2003 PR PT GT GR 4r 2 0.1 2 3 4.4 10 10 6 4 38 10 4.4 105 10 2 4.4 1018 1 10 15 W 150 dB(W) Property of R. Struzak 72 2 Power Transfer: Example 2 • What is the power from a transmitter (=0.1m, PT=44 mW, GT=1) received at distance of 3.8 m (GR=1)? • Free space 15 Jan 2003 PR PT GT GR 4r 2 0.1 3 4.4 10 1 1 4 3.8 2 4.4 10 3 4.4 10 2 10 5 W 50 dB(W ) Property of R. Struzak 73 Mismatch Effects SWR 15 Jan 2003 Gain Reduction Gain Reduction 1.0 0 0 1.5 4% 0.2 dB 2.0 11% 0.5 dB 3.0 25% 1.3 dB 5.0 44% 2.6 dB 10 67% 4.8 dB Property of R. Struzak 74 2 Identical Antennas r • Excitation: I1 = I; I2 =Iej rr r • Ant#1 field-strength: E’= C*D(, ) 2 d 1 r = d*cos • Ant#2 field-strength: E” = [C*D(, )]*ej(r+) 15 Jan 2003 Property of R. Struzak 75 2 Identical Antennas - AAF • Resultant field-strength E = E’ + E” • E = E’ *[1+ej(r+)] = C*D(, )*[1+ej(r+)] = C*D(, )*F(, ) Pattern multiplication • AAF(, ) = | F(, ) |2 = Antenna array factor = Gain of array 15 Jan 2003 Property of R. Struzak 76 2 Antenna Array Factor (1) • F() = 1+ej(r+) ; (r+) = x • F() = 1+ejx = 2[(1/2)(e-jx/2 +ejx/2)]ejx/2 = 2[cos(x/2)]ejx/2 • |F()| = 2cos(x/2) = 2cos[(d/2)cos + /2) = 2cos[(d/)cos + /2] • |F()|2 Antenna Array Factor = gain of 2 isotropic antennas 15 Jan 2003 Property of R. Struzak 77 2 Antenna Array Factor (2) • |F()|2 = {2cos[(d/)cos + /2]}2 • Gain: Max{|AAF()|2} = 4 (6 dBi) when (d/)cos + /2 = 0, , …, k • Nulls: when (d/)cos + /2 = /2, …, (k + 1)/2 • Relative gain = |AAF()|2 / Max{|AAF()|2} = {cos[(d/)cos + /2]}2 Array2ant simulation 15 Jan 2003 Property of R. Struzak 78 Isotropic Antenna Over Conducting Plane 2AntOverPlane simulation 15 Jan 2003 Property of R. Struzak 79 Linear Array of n Antennas • equally spaced • F = 1+ejx+ej2x+ej3x+…+ej(N-1)x antennas in line = (1-ejNx) / (1-ejx) • currents of equal magnitude • |F| = |(1-ejNx) / (1-ejx)| • constant phase = [sin(Nx/2) / sin(x/2)] difference between = F() array factor adjacent antennas • numbered from 0 to (n-1) • x/2 = (d/)cos + /2 Array_Nan simulation 15 Jan 2003 Property of R. Struzak 80 Phased Arrays • Array of N antennas in a linear or spatial configuration • The amplitude and phase excitation of each individual antenna controlled electronically (“software-defined”) – Diode phase shifters – Ferrite phase shifters • Inertia-less beam-forming and scanning (sec) with fixed physical structure 15 Jan 2003 Property of R. Struzak 81 Antenna Arrays: Benefits • Possibilities to control – – – – – Direction of maximum radiation Directions (positions) of nulls Beam-width Directivity Levels of sidelobes using standard antennas (or antenna collections) independently of their radiation patterns • Antenna elements can be distributed along straight lines, arcs, squares, circles, etc. 15 Jan 2003 Property of R. Struzak 82 Beam Steering Beam direction d 3 2 • BeamEqui-phase steering wave front using phase = [(2/)d sin] shifters at Radiating each elements radiating Phase 0 shifters element Power distribution 15 Jan 2003 Property of R. Struzak 83 4-Bit Phase-Shifter (Example) Bit #3 Bit #4 Input 00 or 22.50 00 or 450 Bit #1 Bit #2 00 or 900 00 or 1800 Output Steering/ Beam-forming Circuitry 15 Jan 2003 Property of R. Struzak 84 Switched-Line Phase Bit Delay line Input Output Diode switch 2 delay lines and 4 diodes per bit 15 Jan 2003 Property of R. Struzak 85 Switching Diode Circuit PIN diode PIN diode Tuning element Tuning element b a a: RF short-circuited in forward bias b: RF short-circuited in reverse bias 15 Jan 2003 Property of R. Struzak 86 Adaptive (“Intelligent”)Antennas • • • • • Array of N antennas in a linear or spatial configuration Used for receiving signals from desired sources and suppress incident signals from undesired sources The amplitude and phase excitation of each individual antenna controlled electronically (“softwaredefined”) The weight-determining algorithm uses a-priori and/ or measured information The weight and summing circuits can operate at the RF or at an intermediate frequency 15 Jan 2003 1 w1 wN N Weight-determining algorithm Property of R. Struzak 87 Direction Separation RECEIVER unwanted transmitter wanted transmitter U W 15 Jan 2003 Property of R. Struzak Adaptive antennas 88 Directive Antenna Effectiveness • An ideal directive antenna receives power coming only from within apical angle • It can eliminate (or attenuate) radiation coming from a limited number of discrete interferers (but cannot eliminate isotropic noise) 15 Jan 2003 Property of R. Struzak 89 Directive Antenna Effectiveness Rec J T Effective (T within antenna beam J outside) 15 Jan 2003 Rec Rec J J T Limiting case (T and J at edges) Property of R. Struzak T Not effective (T and J within antenna beam) 90 Directive Antenna Effectiveness Receiver 2 2 A R 2sin R R R h T 2 2 2 2 J A h R cos cot 2 A 15 Jan 2003 Property of R. Struzak 91 Directive Antenna Effectiveness Rc Plane Surface T T 2 R 2 ( sin 2 ) J A2 sin 2 2 sin 2 Volume (rotation around T - J ) h 2 hR arcsin1 arcsin R 3 4 2 2 2 2 2 2 h R h R h 3 2 15 Jan 2003 Property of R. Struzak 92 Various Antenna Types (Pictures) 15 Jan 2003 Property of R. Struzak 93 Antenna Summary • Antenna: substantial element of radio link • We have just reviewed – – – – 15 Jan 2003 Basic concepts Radio wave radiation physics Elementary radiators Selected issues relevant to antennas Property of R. Struzak 94 Antenna References • Scoughton TE: Antenna Basics Tutorial Microwave Journal Jan. 1998, p. 186-191 • Kraus JD: Antennas, McGraw-Hill Book Co. 1998 • Stutzman WL, Thiele GA: Antenna Theory and Design JWiley &Sons, 1981 • Johnson RC: Antenna Engineering Handbook McGraw-Hill Book Co. 1993 • Pozar D. “Antenna Design Using Personal Computers” • Li et al., “Microcomputer Tools for Communication Engineering” • Software – http://www.feko.co.za/apl_ant_pla.htm – www.gsl.net/wb6tpu /swindex.html (NEC Archives) 15 Jan 2003 Property of R. Struzak 95