RADAR Antennas R A D A R R A D A R S Y S T E M S S Y S T E M S. Lecture DR Sanjeev Kumar Mishra. 2 max

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Y T E M Y T E M anjeev Kumar Mishra Lecture 17-20

1 Y T E M Y T E M anjeev Kumar Mishra Lecture ntennas i p r t t ne L L L N kt BF PG max 4 ) / ( 4 2

2 Y T E M ntenna: n antenna is an electromagnetic radiator, a sensor, a transducer and an impedance matching device For adar pplication, directive antenna which concentrates the energy into a narrow beam. Most popularly used antennas are: Parabolic eflector ntennas Planar Phased rrays Electronically steered Phased array antennas typical antenna beamwidth for the detection or tracking of aircraft might be about 1 or 2.

3 n antenna is defined by Webster s ictionary as a usually metallic device (as a rod or wire) for radiating or receiving radio waves. The IEEE tandard efinitions [IEEE td ]: ntenna (or aerial) a means for radiating or receiving radio waves. Y Y T M E & H Fields surrounding an ntenna ntenna as a transition device

4 Y Y T M Transmission-line Thevenin equivalent of antenna in transmitting mode Z r jx Where Z : antenna impedance : ntenna resistance r : radiation resistance L :loss resistance (i.e. due to conduction & dielectric losses) X : equivalent antenna reactance L ( ) jx

5 Y Y T M NTENN PMETE Circuit Parameters Input Impedance adiation esistance ntenna Noise Temperature eturn Loss Impedance bandwidth Physical Quantities Electromagnetic Parameters ize Field Pattern (Beam rea, Weight irectivity, Gain) Profile adiated power hape Efficiency Effective Length and effective area Polarization (LP/CP/EP)

6 TYPE OF NTENN Y Y T M

7 TYPE OF NTENN Y Y T M tructural classification: Wire ntennas perture ntennas Microstrip ntennas rray ntennas eflector ntennas Frequency dependency classification: Frequency ependent ntennas Frequency Independent ntennas

8 Y Y T M Wire ntennas ipole antenna Helix antenna Circular (quare) loop antenna

9 Y Y T M perture ntennas Horn antennas Conical Horn antennas lotted Waveguide antennas

10 Y Y T M Pyramidal Horn antennas adiation pattern of a antenna

11 Y Y T M Microstrip ntennas ectangular patch antennas Circular patch antennas

12 Y Y T M rray ntennas Yagi-Uda antenna lotted waveguide array antenna Microstrip array antenna

13 Y Y T M eflector ntennas Parabolic reflector antenna with front feed Parabolic reflector antenna with cassegrain feed Corner reflector antenna

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15 Y Y T M Frequency independent antennas Log periodic antenna Planar Log periodic slot antenna iscone antenna Log-spiral antenna Various versions of Biconical antennas Infinite Biconical antenna, Finite Biconical antenna, a cone with finite ground, a cone with a stem and discone

16 Y Y T M FUNMENTL PMETE OF NTENN adiation pattern adiation power density adiation intensity ntenna impedance Beamwidth ntenna temperature irectivity Brightness Temperature ntenna efficiency ntenna Factor Gain Bandwidth Group elay Polarization

17 Y T E M Y T E M adiation pattern graphical or mathematical representation of the radiation properties of an antenna such as amplitude, phase, polarization etc as a function of the angular space coordinates θ and Φ. Polar pattern Linear pattern

18 Y Y T M adiation pattern irectional radiation pattern Omni-directional radiation pattern

19 Y Y T M ame power is radiated adiation intensity is power density over sphere (watt/steradian) Gain is radiation intensity over that of an isotropic source

20 Y T E M Y T E M Field regions of an antenna (a) eactive near field region (b) adiating near field (Fresnel) region (c) Far field (Fraunhofer) region Field regions of an antenna

21 [ / 2 2 ] Y Y T M

22 Y Y T M adiation power density The time average Poynting vector (average power density) can be written as 1 * W av x, y, z e E H 2 Where W = adiation power density (W/ m 2 ) E = radiated electric field intensity (V/ m) H = radiated magnetic field intensity (/ m)

23 Y Y T M irectivity ()/ irective Gain It can be defined as the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions U U 0 P rad ( db) 10log U / U P rad [ (dim ensionless )] Where, = directivity (dimensionless) U = radiation intensity (W/ unit solid angle) U 0 = radiation intensity of isotropic source (W/ unit solid angle) P rad = total radiated power (W)

Y T E M Y T E M r r 2 1 0 4 4 Where, = directivity (dimensionless) Ω = beam solid angle) θ 1r = HPBW in one plane (radian) θ 2r = HPBW in a plane at a right angle to other (radian)

24 Y T E M Y T E M r r Where, = directivity (dimensionless) Ω = beam solid angle) θ 1r = HPBW in one plane (radian) θ 2r = HPBW in a plane at a right angle to other (radian) If beamwidth in degrees, equation can be written as: d d d d d d r r ) 180 ( ) 180 ( 4 4 For a planar arrays, a better approximation is d d

25 adiation pattern for a particular paraboloid reflector antenna Y Y T M

Y Y T M ntenna efficiency e 0 e r e e r cd e c e d e 0 (1 )e c d Where, e 0 = total antenna efficiency (dimensionless) e cd = antenna radiation efficiency (dimensionless) : used

26 Y Y T M ntenna efficiency e 0 e r e e r cd e c e d e 0 (1 )e c d Where, e 0 = total antenna efficiency (dimensionless) e cd = antenna radiation efficiency (dimensionless) : used to relate the gain and directivity e r = reflection (mismatch) efficiency (dimensionless) e c = conduction efficiency (dimensionless) e d = dielectric efficiency (dimensionless) 2

27 Y Y T M Z Z L L Z Z Where, Z L = ntenna impedance Z C = characteristic impedance C C

28 Y Y T M Gain (G)/ Power Gain Gain Gain radiation int ensity total input ( accepted ) power in 4U, (dim ensionless ) P in 4U P, 4U, in P e 4U, ecd ecd (dim ensionless ) Prad 4U, elativegain (dim ensionless ) P ( isotropicsource ) Where, = directivity (dimensionless) U = radiation intensity (W/ unit solid angle) P in = total input power (W) P rad = total radiated power (W) e cd = antenna radiation efficiency (dimensionless) P rad / e cd rad cd P in

29 Y T E M Y T E M The relationship between the gain and the beamwidth of an antenna depends on the distribution of current across the aperture. For a "typical" reflector antenna the following expression is sometimes used: d d G Where, θ 1d = HPBW in one plane (degree) θ 2d = HPBW in a plane at a right angle to other (degree)

30 Y T E M Y T E M Effective perture ( eff ) 2 4 eff G 2 4 e Where, = wavelength = Physical area of the antenna e = antenna aperture efficiency

31 Y T E M Y T E M ntenna can be modeled as an impedance atio of voltage to current at feed port esign antenna to maximize power transfer from transmission line eflection of incident power sets up standing wave Input impedance usually defines antenna bandwidth ntenna Input Impedance

32 Y Y T M Bandwidth Bandwidth of the antenna is defined as the range of frequencies within which the performance of the antenna provides desired characteristics. Generally, Impedance BW when 11-10dB [VW 2] The frequency bandwidth of an antenna can be expressed bsolute Bandwidth (BW) Fractional Bandwidth (FBW). BW FBW 2 f f f H f L Where, f H and f L denote the upper edge and the lower edge of the antenna bandwidth, respectively. For broadband antennas, the bandwidth can also be expressed as the ratio of the upper to the lower frequencies, where the antenna performance is acceptable H H f f L L (2.1)

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35 Y Y T M Polarization Polarization is defined as the electric field vector of an antenna oriented in space as a function of time. Electromagnetic Wave (2.1)

36 (2.1) Y Y T M The polarization of a radiated wave is the property of an electromagnetic wave describing the time varying direction and relative magnitude of the electric-field vector at a fixed location in space, and the sense in which it is traced, as observed along the direction of propagation. There are three classifications of antenna polarization: Linear polarization, circular polarization and Elliptical polarization. #Circular and linear polarizations are special cases of elliptical polarization

37 Y Y T M (a) otation of plane electromagnetic wave and (b) its polarization ellipse at z =0 as a function of time

38 Y Y T M Polarisation states for a z-directed plane wave

39 Note : Both the PLF and p lead to the same answers Polarization Loss Factor Y Y T M

40 Y Y T M ntenna Factor The antenna factor is defined as the ratio of the electric field strength to the voltage V (units: V or µv) induced across the terminals of a antenna. For an electric field antenna, the field strength is in units of V/m or µv/m and the resulting antenna factor F is in units of 1/m: F= E incident /V received In a 50 Ω system, the antenna factor is related to the antenna gain G and the wavelength λ via: F= [9.73/ (λ*g 1/2 )]

41 Y Y T M

42 NTENN Most popularly used antennas are: Parabolic eflector ntennas Planar Phased rrays Electronically steered Phased array antennas Y Y T M

43 adar ntenna rchitecture Comparison Y T E M

44 rray adar Passive rray adar ctive rray adar Y T E M

45 ctive Phased rray adar Y T E M

46 igital rray adar rchitecture: igital on eceiver Y T E M Each active analog T/ module is followed by an / for immediate digitization Multiple received beams are formed digitally by the digital beam-former.

47 eference 1. C Balanis, ntenna Theory and esign, 3rd Edition, Wiley, G Kumar and K P ay, Broadband Microstrip ntenna, rctech Publication, K hevgaonkar, Electromagnetic Waves, 2006 Y T E M

Chapter 2. Fundamental Properties of Antennas. ECE 5318/6352 Antenna Engineering Dr. Stuart Long

Chapter 2. Fundamental Properties of Antennas. ECE 5318/6352 Antenna Engineering Dr. Stuart Long Chapter Fundamental Properties of Antennas ECE 5318/635 Antenna Engineering Dr. Stuart Long 1 IEEE Standards Definition of Terms for Antennas IEEE Standard 145-1983 IEEE Transactions on Antennas and Propagation