ANTENNAS AND WAVE PROPAGATION EC602

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1 ANTENNAS AND WAVE PROPAGATION EC602 B.Tech Electronics & Communication Engineering, Semester VI INSTITUTE OF TECHNOLOGY NIRMA UNIVERSITY 1

2 Lesson Planning (L-3,P-2,C-4) Chapter No. Name Hours 1. Basic antenna concepts and theorems 2. Point sources and Arrays Electric dipole and thin layer antennas 4. Loop Antenna 4 5. Yagi-Uda Antenna 1 6. Helical antenna 2 7. Reflector Antennas

3 Chapter No. Name 9. Slot horn & complementary antennas Hours 10. Patch Antenna Antennas for special applications Antennas measurement Radio wave propagation 3 2 3

4 Text/Reference Book 1. J. D. Krauss - Antennas, McGraw Hill 2. K.D. Prasad - Antennas & Wave Propagation, Satyaprakash Publications 3. Jordan & Balmain - Electromagnetic wave & radiating systems, PHI Publication 4. C. A. Balanis- Antenna Theory, Analysis & Design, Wiley India Pvt. Ltd. 4

5 CH:-1 BASIC ANTENNA CONCEPTS AND THEOREMS Aarti Gehani Asst. Prof. Institute of Technology, Nirma University 5

6 Topics Introduction How does an antenna radiate? Various definitions Antenna parameters Transmission formula Various theorems 6

7 INTRODUCTION 7

8 INTRODUCTION (CONTI.) An antenna is considered as a region of transition between a transmission line and space. Antenna converts electrons to photons, or vice versa. Antenna radiate/couple/concentrate/direct electromagnetic energy in the desired/assigned direction. No hard and fast rule for selecting antenna Antenna Isotropic/Omni-directional/Non-directional Anisotropic/Directional 8

9 INTRODUCTION (CONTI.) Some application (e.g., radars, mobile), same antenna for transmission and reception Some application (e.g., radio, television), separate antenna for transmission and reception No difference in selection factors for transmitting and receiving antenna Cost, size and shape, etc. important High efficiency and high gain- basic requirement for transmitting antenna Low side lobes and large SNR- basic requirement for receiving antenna 9

10 BASIC ANTENNA PARAMETERS Radiation is produced by accelerated or decelerated charge. Basic radiation equation: IL=Qv (A m s -1 ) I = time changing current, As -1 L = length of current element, m Q = charge, C v = time change of velocity which equals the acceleration of the charge, m s -2 So, time changing current and accelerated charge radiates 10

11 HOW DOES ANTENNA RADIATES? 11

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14 Electric field comes into picture i.e. change in voltage is only possibility So, voltage rise at OC. Current starts flowing back After some time, voltage also starts flowing back. If line is SC, voltage = 0 and vice versa In case of perfect OC or SC, theoretically perfect reflection Waves takes some time to change their direction Meanwhile some energy leaks in to the space This leakage is called radiation. 14

15 15 The more is the opening, the more time wave takes to change the direction, thus more energy leakage in to the space. Maximum radiation when ends are flared to make 180 angle Thus, an antenna is a transition device, or transducer, between guided wave and a free space wave or vice versa. Antenna is a device which interfaces circuit and space. Circuit point of view, antenna is a resistance Rr, called the radiation resistance. Not a physical resistance, but a resistance coupled from space to antenna terminal

16 PATTERN 16

17 PATTERN (CONTI.) Radiation pattern or Antenna pattern is defined as a mathematical function or a graphical representation of the radiation properties of the antenna as a function of space coordinates. Radiation pattern is determined in far field region. Radiation properties include power flux density, radiation intensity, field strength and polarization. Pattern are of two types: 1. Power Pattern 2. Field Pattern 17

18 PATTERN (CONTI.) Power Pattern: normalized power vs. spherical coordinate position [3] OR Power Pattern: Trace of received power at constant radius [2] Field Pattern: normalized E or H vs. spherical coordinate position [3] OR Field Pattern: Graph of spatial variation of the electric (or magnetic) field along a constant variation [2] 18

19 PATTERN (CONTI.) Various parts of a radiation patterns are called as lobes, which are further classified as major or main, minor, side and back lobes. Major lobe: The radiation lobe containing the direction of maximum radiation Minor lobe: Lobe excepting major lobe Minor lobe = Side lobe + Back lobe 19

20 PATTERN (CONTI.) Side lobe: Radiation lobe in any direction other than the intended lobe Back lobe: Radiation lobe whose axis makes an approximately 180 with respect to main lobe Minor lobes represent the radiation in undesired directions and they should be minimized. Level of minor lobes is usually expressed as a ratio of power density in the lobe in question to that of the major lobe. 20

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23 HPBW & FNBW 23

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25 HPBW & FNBW (CONTI.) 25

26 EXAMPLE-1 26

27 RADIAN, STERADIAN & BEAM AREA/ BEAM SOLID ANGLE Ω A The measure of a plane angle is a radian. One radian is defined as the angle with its vertex at the center of a circle of radius r that is subtended by an arc whose length is r. Since the circumference of a circle of radius r is 2πr, there are 2π rad (2πr/r) in a full circle. 27

28 RADIAN, STERADIAN & BEAM AREA/ BEAM SOLID ANGLE Ω A (CONTI.) The measure of solid angle is steradian. One steradian is defined as the solid angle with its vertex at the center of a sphere of radius r that is subtended by a spherical surface area equal to that of a square with each side of length r. Since the area of a sphere of radius r is A=4πr 2, there are 4π sr (4πr 2 /r 2 ) in a closed sphere. 28

29 29

30 RADIAN, STERADIAN & BEAM AREA/ BEAM SOLID ANGLE Ω A (CONTI.) The infinitesimal area da on the surface of a sphere of radius r, is given by da=(r dθ) (rsinθ dφ) = r 2 dω 30

31 RADIAN, STERADIAN & BEAM AREA/ BEAM SOLID ANGLE Ω A (CONTI.) 31

32 RADIAN, STERADIAN & BEAM AREA/ BEAM SOLID ANGLE Ω A (CONTI.) Beam area is the solid angle through which all of power radiated by the antenna would stream if P(θ,φ) maintained its maximum value over Ω A and was zero elsewhere. Thus the power radiated = P(θ,φ) Ω A watts. It can also be described approximately as Beam area Ω A θ HP φ HP (sr) 32

33 RADIATION INTENSITY

34 BEAM EFFICIENCY Total beam area = Main beam area + Minor lobe area Therefore, Ω A = Ω M + Ω m Ratio of main beam area to total area is called the beam efficiency ε M. Thus, Beam efficiency = ε M = Ω M / Ω A (dimensionless) Ratio of minor lobe area to the total beam area is called stray factor. ε m = Ω m / Ω A It follows that ε M + ε m =1

35 DIRECTIVITY D

36 DIRECTIVITY D (CONTI.)

37 DIRECTIVITY D (CONTI.) Thus, the directivity is the ratio of the area of a sphere (4π sr) to the beam area Ω A of the antenna. Smaller the beam area, larger the dirctivity D The idealized isotropic antenna (Ω A = 4π sr) has the lowest possible directivity D=1 Unit of directivity is dbi (decibels over isotropic i.e. wrt isotropic antenna)

38 GAIN G Gain G of an antenna is a quantity which is less than the directivity D due to ohmic losses in the antenna. In transmitting, losses involve power fed to the antenna which is not radiated but heats the antenna structure. Mismatch in feeding also reduces gain. Ratio of gain to directivity is called antenna efficiency factor. G = kd k = efficiency factor (0 k 1), dimensionless

39 GAIN G(CONTI.)

40 ANTENNA APERTURES The aperture of an antenna is the area that captures energy from a passing radio wave. P=SA (W) Effective aperture is not always equal to the physical aperture Aperture efficiency is given by ε ap = A e /A p (dimensionless) For horn and parabolic reflector antenna, aperture efficiencies are in the range of 50 to 80%. 40

41 ANTENNA APERTURES (CONTI.) 41

42 ANTENNA APERTURES (CONTI.) Equating both the equations and substituting E r = E a A e /rλ, we get λ 2 = A e Ω A (m 2 ) Aperture-beam-area-relation We also know that D = 4π/Ω A, so from the above equation we can say that D = 4π A e / λ 2 Directivity from Aperture

43 DIRECTIVITY

44 RADIATION RESISTANCE Rr is that part of an antenna s feed point resistance that is caused by radiation of electromagnetic waves from the antenna. Determined by the geometry and not the material Equivalent to a resistor in a circuit Caused by the radiation reaction of the conduction of electrons in antenna. Accelerated electrons produces EM waves. Waves carries energy taken from electrons. Loss of energy of the electrons appears as an effective resistance to movement of other electrons. 44

45 Antenna Field Zones Mainly two types: Near field or Fresnel zone and Far field or Fraunhofer zone. Boundary between the two is arbitrarily taken as R = (2L 2 )/λ (m); L = maximum dimension of the antenna (m) and λ = wavelength (m) 45

46 RADIO COMMUNICATION LINK 46

47 RADIO COMMUNICATION LINK (CONTI.) 47

48 SOME MORE DEFINITIONS Polarization: Orientation of E-field vector of a wave Axial Ratio: Ratio of major to minor axes of polarization ellipse Signal to Noise Ratio: Ratio of signal fed to the network to the noise Antenna Temperature: Fictitious temperature at the input of an antenna which would account for noise N at the output Front to Back Ratio: Ratio of energy in main lobe to that in the back lobe 48

49 SOME MORE DEFINITIONS (CONT.) Antenna Bandwidth: Range of frequency over which an antenna maintains its certain required characteristics, viz. gain, radiation resistance, polarization, FBR, etc. Driving Point or Terminal Impedance: Impedance measured at the input terminals of an antenna. Effective Height/ Length: Ratio of induced voltage at the terminal of the receiving antenna under an open circuit condition to the electric field intensity or strength. 49

50 Antenna Theorems 1. Equality of directional patterns: The directional pattern of a receiving antenna is identical with its directional pattern as transmitting antenna. 2. Equality of transmitting and receiving antenna impedance: The impedance of an isolated antenna when used for receiving is the same as when used for transmitting. 3. Equality of effective length: The effective length of an antenna for receiving is equal to its effective length as a transmitting antenna. 50

51 REFERENCES 1. J. D. Krauss - Antennas, McGraw Hill 2. C. A. Balanis- Antenna Theory, Analysis & Design, Wiley India Pvt. Ltd pdf 51

EC ANTENNA AND WAVE PROPAGATION

EC ANTENNA AND WAVE PROPAGATION EC6602 - ANTENNA AND WAVE PROPAGATION FUNDAMENTALS PART-B QUESTION BANK UNIT 1 1. Define the following parameters w.r.t antenna: i. Radiation resistance. ii. Beam area. iii. Radiation intensity. iv. Directivity.