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Lecture 17-20: Radar Antennas

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Lecture 17-20: Radar Antennas RADAR Antennas R A D A R S YS YS TS TE ME 2 2 MS 4 PtG R max nE S 4 3 kT BF (S / N) L L L i 0 1 t r p Lecture 17-20 DR Sanjeev Kumar Mishra Antenna: R • An antenna is A • an electromagnetic radiator, D • a sensor, A • a transducer and R • an impedance matching device • For Radar Application, A directive antenna which concentrates the S energy into a narrow beam. Y • Most popularly used antennas are: Parabolic Reflector Antennas S T • Planar Phased Arrays E • Electronically steered Phased array M antennas S • A typical antenna beamwidth for the detection or tracking of aircraft might be about 1 or 2°. R • An antenna is defined by Webster’s Dictionary as “a usually metallic A device (as a rod or wire) for radiating or receiving radio waves.” D A • The IEEE Standard Definitions [IEEE Std 145–1983]: Antenna (or R aerial) “a means for radiating or receiving radio waves.” S YS YS TS TE ME MS S E & H Fields surrounding an Antenna Antenna as a transition device R A D A R S YS YS Transmission-line Thevenin equivalent of antenna in transmitting mode TS Z A RA jX A TE (R R ) jX ME r L A MS Where ZA : antenna impedance R : Antenna resistance S A Rr : radiation resistance RL :loss resistance (i.e. due to conduction & dielectric losses) XA : equivalent antenna reactance ANTENNA PARAMETERS R • Circuit Parameters A • Input Impedance D • Radiation Resistance A R • Antenna Noise Temperature • Return Loss S • Impedance bandwidth YS • Physical Quantities • Electromagnetic Parameters YS TS • Size • Field Pattern (Beam Area, TE • Weight Directivity, Gain) ME • Profile • Radiated power MS • Efficiency S • Shape • Effective Length and effective area • Polarization (LP/CP/EP) TYPES OF ANTENNA R A D A R S YS YS TS TE ME MS S TYPES OF ANTENNA R • Structural classification: A • Wire Antennas D • Aperture Antennas A • Microstrip Antennas R • Array Antennas S • Reflector Antennas YS • Frequency dependency classification: YS • Frequency Dependent Antennas TS TE • Frequency Independent Antennas ME MS S Wire Antennas R A D A R S YS Dipole antenna Circular (Square) loop antenna YS TS TE ME MS S Helix antenna Aperture Antennas R A D A R S YS Horn antennas Conical Horn antennas YS TS TE ME MS S Slotted Waveguide antennas R A D A R S YS Pyramidal Horn antennas YS TS TE ME MS S Radiation pattern of a antenna Microstrip Antennas R A D A R S YS YS TS TE ME Rectangular patch antennas Circular patch antennas MS S Array Antennas R A D A R S Yagi-Uda antenna YS YS TS TE ME MS S Slotted waveguide array antenna Microstrip array antenna Reflector Antennas R A D A R Parabolic reflector antenna with front feed S YS YS TS TE Parabolic reflector antenna with cassegrain feed ME MS S Corner reflector antenna R A D A R S YS YS TS TE ME MS S Frequency independent antennas R A D A R S Log periodic antenna Planar Log periodic slot antenna Log-spiral antenna YS YS TS TE ME MS Various versions of Biconical antennas – Infinite S Discone antenna Biconical antenna, Finite Biconical antenna, a cone with finite ground, a cone with a stem and discone FUNDAMENTAL PARAMETERS OF ANTENNA R • Radiation pattern A D • Radiation power density A • Radiation intensity R • Antenna impedance • Beamwidth • Antenna temperature S • Directivity YS • Brightness Temperature • Antenna efficiency YS • Antenna Factor TS • Gain TE • Bandwidth ME MS • Group Delay S • Polarization Radiation pattern R A graphical or mathematical representation of the radiation properties of A an antenna such as amplitude, phase, polarization etc as a function of D A the angular space coordinates θ and Φ. R S YS YS TS TE ME MS S Polar pattern Linear pattern Radiation pattern R A D A R S YS YS TS TE Omni-directional radiation pattern ME Directional radiation pattern MS S R A D A R S YS YS TS TE ME MS • Same power is radiated S • Radiation intensity is power density over sphere (watt/steradian) • Gain is radiation intensity over that of an isotropic source Field regions of an antenna R (a) Reactive near field region A (b) Radiating near field (Fresnel) region D (c) Far field (Fraunhofer) region A R S YS YS TS TE ME MS S Field regions of an antenna R A D 3 2 A [0.62 D / R 2D ] R S YS YS TS TE ME MS S Radiation power density R The time average Poynting vector (average power density) can be 1 A written as W x, y, z ReE H * D av 2 A Where W = Radiation power density (W/ m2) R E = radiated electric field intensity (V/ m) H = radiated magnetic field intensity (A/ m) S YS YS TS TE ME MS S Directivity (D)/ Directive Gain R It can be defined as “the ratio of the radiation intensity in a given direction A from the antenna to the radiation intensity averaged over all directions” D U U 4U A D R U0 Prad / 4 Prad D(dB) 10log10[D(dim ensionless)] S YS Where, D = directivity (dimensionless) YS U = radiation intensity (W/ unit solid angle) TS U0= radiation intensity of isotropic source (W/ unit solid angle) TE Prad= total radiated power (W) ME MS S R 4 4 D0 A A 1r2r D Where, D = directivity (dimensionless) A ΩA = beam solid angle) R θ1r= HPBW in one plane (radian) S θ2r= HPBW in a plane at a right angle to other (radian) YS If beamwidth in degrees, equation can be written as: 4 4 YS D 0 1r2r TS 1d ( )2d ( ) TE 180 180 2 ME 4 180 41253 MS 1d2d 1d2d S For a planar arrays, a better approximation is 32400 D0 1d2d R A D A R S YS YS TS TE ME MS S Radiation pattern for a particular paraboloid reflector antenna Antenna efficiency R A D A R e0 ereced S e e 2 YS r cd e (1 )e YS 0 cd TS Where, e0 = total antenna efficiency (dimensionless) TE ecd = antenna radiation efficiency (dimensionless) ME : used to relate the gain and directivity MS e = reflection (mismatch) efficiency (dimensionless) S r ec = conduction efficiency (dimensionless) ed = dielectric efficiency (dimensionless) R A D A R S Z Z YS L C YS Z L ZC TS TE Where, Z = Antenna impedance ME L Z = characteristic impedance MS C S Gain (G)/ Power Gain R radiation int ensity Gain A total input(accepted ) power D 4U, (dim ensionless) A P R in 4U, 4U, Gain Prad ecd Pin S Pin Prad / ecd YS 4U, e e D(dim ensionless) YS cd cd Prad TS 4U, TE RelativeGain (dimensionless) Pin (isotropicsource) ME Where, D = directivity (dimensionless) MS U = radiation intensity (W/ unit solid angle) S Pin= total input power (W) Prad= total radiated power (W) ecd= antenna radiation efficiency (dimensionless) R The relationship between the gain and the beamwidth of an antenna A depends on the distribution of current across the aperture. D A For a "typical" reflector antenna the following expression is sometimes R used: 20000 G 1d2d S YS Where, θ1d = HPBW in one plane (degree) YS θ = HPBW in a plane at a right angle to other (degree) TS 2d TE ME MS S Effective Aperture (Aeff) R 4Aeff 4 A A G e 2 2 D A Where, = wavelength R A= Physical area of the antenna = antenna aperture efficiency S e YS YS TS TE ME MS S Antenna Input Impedance R • Antenna can be modeled as an impedance A D • Ratio of voltage to current at feed port A • Design antenna to maximize power transfer from transmission line R • Reflection of incident power sets up standing wave • Input impedance usually defines antenna bandwidth S YS YS TS TE ME MS S Bandwidth (2.1) R Bandwidth of the antenna is defined as the range of frequencies within A D which the performance of the antenna provides desired characteristics. A • Generally, Impedance BW when S11 -10dB [VSWR 2] R The frequency bandwidth of an antenna can be expressed Absolute Bandwidth (ABW) ABW f f S H L f f YS Fractional Bandwidth (FBW). FBW 2 H L f f YS H L TS Where, fH and fL denote the upper edge and the lower edge of the antenna TE bandwidth, respectively. ME MS For broadband antennas, the bandwidth can also be expressed as the S ratio of the upper to the lower frequencies, where the antenna performance is acceptable R A D A R S YS YS TS TE ME MS S R A D A R S YS YS TS TE ME MS S Polarization (2.1) R Polarization is defined as “the electric field vector of an antenna oriented A in space as a function of time”. D A R S YS YS TS TE ME MS Electromagnetic Wave S (2.1) R The polarization of a radiated wave is the property of an electromagnetic A wave describing the time varying direction and relative magnitude of the D A electric-field vector at a fixed location in space, and the sense in which it R is traced, as observed along the direction of propagation. There are three classifications of antenna polarization: S YS • Linear polarization, YS • circular polarization and TS • Elliptical polarization. TE ME #Circular and linear polarizations are special cases of elliptical polarization MS S R A D A R S YS YS TS TE ME MS S (a) Rotation of plane electromagnetic wave and (b) its polarization ellipse at z =0 as a function of time R A D A R S YS YS TS TE ME MS S Polarisation states for a z-directed plane wave Polarization Loss Factor R A D A R S YS YS TS TE ME MS S Note : Both the PLF and pe lead to the same answers Antenna Factor R A • The antenna factor is defined as the ratio of the electric field strength D to the voltage V (units: V or µV) induced across the terminals of a A antenna.
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