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K.S.R. COLLEGE OF ENGINEERING (Autonomous)
SEMESTER - VI
16EC611 ANTENNA AND WAVE PROPAGATION
Objective(s):
- To gain knowledge about antenna fundamentals and radiation properties.
- To understand the concept of antenna arrays and special antennas
- To learn the various radio wave propagation.
UNIT – I ELECTROMAGNETIC RADIATION AND ANTENNA FUNDAMENTALS09 Hrs Review of electromagnetic theory: Vector potential -Retarded case - Hertizian dipole - Half-wave dipole - Quarter-wave monopole - Antenna characteristics: Radiation pattern - Beam solid angle - Directivity- Gain - Input impedance - Polarization - Bandwidth - Reciprocity - Effective aperture -Effective length - Antenna temperature.
UNIT – II ANTENNA ARRAYS 09 Hrs Expression for electric field from two, three and N element arrays - Linear arrays: Broad-side array and End-fire array - Method of pattern multiplication - Binomial array - Phased arrays, Frequency scanning arrays - Adaptive arrays.
UNIT - III ANTENNA TYPES 09 Hrs Loop antennas: Radiation from small loop and its radiation resistance -Helical antenna: Normal mode and axial mode operation-Yagiuda antenna- Log periodic antenna – Rhombic antenna- Horn antenna- Reflector antennas and their feed systems - Micro strip antenna.
UNIT – IV SPECIAL ANTENNAS AND ANTENNA MEASUREMENTS 09Hrs Antenna for special applications: Antenna for terrestrial mobile communication systems - GPR - Embedded antennas - UWB - Plasma antenna - Smart antennas. Antenna measurements: Radiation pattern, Gain, Directivity, Polarization, Impedance, Efficiency.
UNIT – V WAVE PROPAGATION 09 Hrs Ground wave propagation: Attenuation characteristics - Calculation of field strength- Space wave propagation: Reflection from ground for vertically and horizontally polarized waves - Reflection characteristics of earth- Resultant of direct and reflected ray at the receiver- Duct propagation - Sky wave propagation: Structure of the ionosphere- Effective dielectric constant of ionized region- Mechanism of refraction- Refractive index- Critical frequency- Skip distance-Maximum usable frequency
- Fading - Diversity reception. Total hours: 45
Text Books:
- Prasad K. D “Antennas and Wave Propagation”- Tech India Publications, 2009.
- John D Kraus and Ronalatory Marhefka, Ahmed S Khan “Antennas and Wave Propagation “Tata McGraw-Hill Book Company 4th^ Edition, Reprint 2010. References:
- Ballanis "Antenna Theory” John Wiley & Sons 3rd^ Edition 2012
- E.C.Jordan and Balmain"Electro Magnetic Waves and Radiating Systems" PHI Reprint 2003.
- R.E.Collins 'Antennas and Radio Propagation “McGraw-Hill 1987.
- D.Ganeshrao, B.Somanathan Nair, Deepa Reghunath “Antennas and Radio-Wave Propagation” Sanguine technical publishers I st^ Edition, 2007
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K.S.R. COLLEGE OF ENGINEERING (Autonomous), TIRUCHENGODE – 637 215 DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING COURSE / LESSON PLAN SCHEDULE
NAME : R.SATHEESKUMAR Class: III-ECE (A, B SECTION) SUBJECT: 16EC611 - ANTENNA AND WAVE PROPAGATION TEXTBOOKS:
- Prasad K. D “Antennas and Wave Propagation”- Tech India Publications, 2009.
- John D Kraus and Ronalatory Marhefka, Ahmed S Khan “Antennas and Wave Propagation “Tata McGraw-Hill Book Company 4th^ Edition, Reprint 2010.
REFERENCES:
- Ballanis "Antenna Theory” John Wiley & Sons 3rd^ Edition 2012
- E.C.Jordan and Balmain "Electro Magnetic Waves and Radiating Systems" PHIReprint2003.
- R.E.Collins 'Antennas and Radio Propagation “McGraw-Hill 1987.
- D.Ganeshrao, B.Somanathan Nair, Deepa Reghunath “Antennas and Radio-Wave Propagation” Sanguine technical publishers 1 st^ Edition, 2007.
C). LEGEND:
L - Lecture BB - Black Board Tx - Text OHP - Over Head Projector pp - Pages Rx - Reference Sl. No.
Lecture Hour
Topics to be covered Teaching Aid Required
Book No. / Page No.
UNIT-I ELECTROMAGNETIC RADIATION AND ANTENNA FUNDAMENTALS
1. L 1
Review of electromagnetic theory, Vector potential, Retarded case
BB
Tx 1/pp 274-277, 415-416, Rx 2/pp90, Rx 4/pp
- L 2 Hertizian dipole OHP
Tx 1/pp 423-438, Rx 2/pp 321, Tx2/pp 70 - 85
- L 3 Half-wave dipole antenna, quarter wave and Monopole antenna.
BB
Tx 1/pp 452-461, Rx 1/pp 162, Tx 2/pp 157
4. L 4
Antenna characteristics: Radiation pattern, Beam solid angle, Directivity, Gain
BB
Tx 1/pp530-534,536,544- 548,539,572-574, Tx 2/pp 09-23, Rx 1/pp28,39,
- L 5 Input impedance, Polarization, Bandwidth, BB
Tx 1/pp578, 572-574, Rx 1/pp 73,64, Tx 2/pp 447,
- L 6 Reciprocity BB
Tx 1/pp 556-559, Tx 2/pp 259,655, Rx 1/pp 63,
- L 7 Effective aperture BB
Tx 1/pp 548 – 553, Rx 1/pp 81-84, Rx 2/pp 44, 517
- L 8 Effective length, Antenna temperature. BB
Tx 1/pp 554-556, 579-582, Rx 2/pp518, Tx 2/pp 25 – 30,44 – 45
- L 9 Beam width ,Problems BB Tx 1/pp 574, Rx 4/pp 13 UNIT-II ANTENNA ARRAYS
L 10,
L 11
Expression for electric field from two ,three and N element arrays
BB
Tx 1/pp 606-611,614-626, Tx 2/pp 100-107,115-121, Rx 1/pp 250-
L 12,
L 13
Linear Array :Broad side & End fire array BB
Tx 1/pp 602-604, Rx 1/pp 262-266, Rx 4/pp 89-
36. L 38
Space Wave Propagation: Reflection from ground for vertically and horizontally polarized waves
BB
Tx 1/pp 1152-1157, Tx 2/pp 799-
37. L 39
Reflection characteristics of earth- Resultant of direct and reflected ray at the receiver
BB
Tx 1/pp 1157-1159, Tx 2/pp 803-
- L 40 Duct propagation BB Tx 1/pp 1160-1162, Rx 4/pp 271-
- L 41
Sky wave propagation: Structure of the ionosphere
BB
Tx 1/pp 1112,1115-1116, Tx 2/pp 815-
- L 42
Effective dielectric constant of ionized region, Mechanism of refraction
OHP
Tx 1/pp 1117-1119,1121, Tx 2/pp 816-
41. L 43
Refractive index, Critical frequency, Skip distance, Maximum usable frequency
BB
Tx 1/pp 1119-1121,1122- 1123,1144-1145,1139-1143, Tx 2/pp822-
- L 44 Fading, Diversity reception. BB
Tx 1/pp 1147-1148, Tx 2/pp 812-813, Rx 4/pp 324
- L 45 Whistlers, Problems BB
Tx 1/pp 1149, Rx 4/pp 320
K.S.R. COLLEGE OF ENGINEERING (Autonomous), TIRUCHENGODE – 637 215 DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING 16EC611 ANTENNA & WAVE PROPAGATION QUESTION BANK
UNIT-I (CO1)
ELECTROMAGNETIC RADIATION AND ANTENNA FUNDAMENTALS
1. Define an antenna. (Remembering) Antenna is a transition device or a transducer between a guided wave and a free space wave or vice versa. Antenna is also said to be an impedance transforming device. 2. What is vector potential? State the expression for vector potential for time varying field. (Remembering) The vector potential is a quantity with magnitude and a direction. This is useful to find the magnetic flux density B and intensity H. The vector potential can be expressed as,
= ∫ r
Idl
A
Where, μ = Permeability Idl = current element r = distance between point “P” to the current element
3. What are retarded Vector potentials?Give its expression. (Remembering) (AU-June 2014) (AUT-June 2015) There is a finite time delay for propagation of an electromagnetic wave from a source point to the observation point.The effect of this propagation delay is introduced in vector and scalar potentials by simply substituting the time variable ‘t’ by a variable {t-r/c}.The potentials at the observation point are delayed or retarded by this time delay are known as retarded potentials. The expression for retarded vector potential is,
[A] = dv
4. What do you understand by retarded current? (Remembering) Since, the short electric dipole is so short, the current which is flowing through the dipole is assumed to be constant throughout its length. The effect of this current is not felt instantaneous at a distance point but only after an interval equal to the time required for the wave to propagate over the distance “r” is called the retardation time. 5. Show an expression for retarded current. (Understanding) [I]=I (^) m sinω[t-r/c] Where r =distance travelled c=velocity of propagation [I]=Retarded current and the bracket indicate that it is retarded current t-r/c=retarded time as phase of the wave at point P is retarded with respect to the phase of the current in the element by an angle ωr/c. 6. Define retardation time?(Remembering) It is the time for the wave to propagate over the distance r. It is given by r/c. 7. What are wire antennas and give its applications? (Remembering) Dipole antenna, loop antenna, helix antenna are some of the wire antennas, It can be used on automobiles, buildings, ships, aircrafts, spacecraft etc... 8. What is a Short Dipole? (Remembering) A short dipole is one in which the field is oscillating because of the oscillating voltage and current. It is called so because, the length of the dipole is short and the current is almost constant throughout the entire length of the dipole. It is also called as Hertzian Dipole, which is a hypothetical antenna and is defined as a short isolated conductor carrying uniform alternating current. 9. How radiations are created from a short Dipole? (Remembering) The dipole has two equal charges of opposite sign oscillating up and down in a harmonic motion. The charges will move towards each other and electric filed lines were created. When the charges meet at the midpoint, the field lines cut each other and new field are created. This process is spontaneous and so more fields are created around the antenna. This is how radiations are obtained from a short dipole. 10. Why a short dipole is also called an elemental dipole? (Remembering) A short dipole that does have a uniform current will be known as the elemental dipole. Such a dipole will generally be considerably shorter than the tenth wavelength maximum specified for a short dipole. Elemental dipole is also called as elementary dipole, elementary doublet and Hertzian dipole.
23. Identify the expression for the effective aperture of a short dipole. (Applying) The effective aperture of a short dipole is given by Ae = 0.119λ^2 24. What is a dipole antenna? (Remembering) A dipole antenna may be defined as a symmetrical antenna in which the two ends are at equal potential relative to the midpoint. The dipole is usually fed at the centre having maximum current at the centre. That is maximum radiation is in the plane normal to the axis. 25. What is a half wave dipole? (Remembering) A half wave antenna is the fundamental radio antenna of metal rod or tubing or thin wire which has a physical length of half wavelength (λ/2) in free space at the frequency of operation. It is one of the simplest antenna and is frequently employed as an element of the antenna array. 26. Show the expression for the effective aperture of a Half wave Dipole. (Understanding) The effective aperture of a half wave dipole is given by Ae = 0.13λ 27. What is the radiation resistance of a half wave dipole? (Remembering) The radiation resistance of a half wave dipole is given by Rr=73 ohm 28. What is a monopole antenna? (Remembering) A monopole antenna consists of one half of a dipole (usually a short vertical) antenna mounted above the earth or group (reflecting plane). It is also called as Marconi antenna or Quarter wave monopole. It is fed by a coaxial cable connected to its base. 29. How half wave dipole antenna differs from monopole dipole antenna? (Remembering) The only difference between a λ/2 antenna and a λ/4 antenna is that the dipole radiates power more or less in all directions where as monopole radiates power in a hemisphere surface (Half of the sphere) 30. What is a quarter wave monopole? (Remembering)(AU-Dec 2013, June 2014) One-quarter wavelength long .it is also called as Marconi antenna. A quarter wave monopole antennas is half of a dipole antenna placed over a grounded plane 31. Calculate the physical height of a half wave dipole (λ/2) having antenna Q of 30 and bandwidth of 10 MHz. f = Q X BW=30 X10MHz=300MHz. λ= c / f ; **ans: 0.5 m
- What is the effective area of a half wave dipole operating at 1GHz? (Remembering)** Given : f = 1 x 10^9 Hz Effective area, Ae = 0.1309 λ^2 = 0.1309 (c/f)^2 = 0.1309(0.3) = **0.
- What are the parameters that determine the overall pattern of an antenna** array?(Remembering) Gain, Directivity, Input impedance, Band width, Polarization and Antenna temperature 34. Define radiation pattern. (Remembering) (AU-June 2014) A graph which shows the variation in actual field strength of electromagnetic field at all points which are at equal distance from antenna otherwise it is a mathematical function or graphical representation of the radiations from an antenna. The energy radiated in a particular direction by an antenna is measured in terms of Field Strength. (E Volts/m) 35. List out the different types of radiation pattern of an antenna. (Analyzing) There are two types of radiation pattern, they are, Field radiation pattern: If the radiation of the antenna is expressed in terms of the field strength E (in v/m), then it is called field strength pattern (or) Field radiation pattern. Power radiation pattern: When the radiation in a given direction is expressed in terms of power per unit solid angle, the pattern is called power radiation pattern. 36. List out the types of field radiation pattern. (Analyzing) There are two types of field radiation pattern. They are, E-plane pattern or vertical pattern: When the magnitude the normalized field strength is plotted versus Ѳ with constant Φ, the pattern is called E-plane pattern or vertical pattern. H-plane pattern or horizontal pattern:
When the normalized field strength is plotted versus Φ with constant Ѳ=π/2, the pattern is called H-plane pattern or horizontal pattern
37. Define power density. (Remembering) The power density Pd (Ѳ, Φ) is defined as power flow per unit area and is a function of the direction (Ѳ, Φ). The power density can be expressed in terms of the magnitude of the electric field intensity as,
Pd (Ѳ, Φ) = O
E
2
2
E
38. Define Beam area or Beam solid angle. (Remembering) (AUT-Dec 2016) It is the solid angle through which all the power radiated to the free space. It is a three dimensional angle formed by the major lobe. Mathematically, Beam area ΩA is defined as the integral of the normalized power pattern over a sphere.
ΩA Where, Pn(Ѳ,Φ) = normalized power ΩA = beam solid angle dΩ = Solid angle = r^2 sin Ѳ dѲdΦ steradian
39. Compare radian and Steradian. (Evaluating) Radian – One radian is defined as the plane angle with its vertex at centre of circle with radius “r” that is subtended by an arc whose length is also r. Steradian - One Steradian is defined as a solid angle with its vertex at the centre of the sphere with radius “r” that is subtended by a spherical surface area equal to that of a square with each side equal to r. 40. How is the 'STERADIAN' defined and how is it used? (Remembering) Steradian are a measure of the angular 'area' subtended by a two dimensional surface about the origin in three dimensional space, just as a radian is a measure of the angle subtended by a one dimensional line about the origin in two dimensional (plane) space. Steradian are equivalently referred to as 'square radians. It is used to measure the solid angle. 41. Motive radiation intensity (Analyzing) The radiation intensity of an antenna does not depend on the distance from the radiation (or) antenna. It is denoted by “U”. The radiation intensity is defined as “power radiated per unit solid angle”, expressed in W/S (^) r (i.e., watts/steradian).U (Ѳ, Φ) = r^2 Pd(Ѳ, Φ) 42. Show the expression for average radiation intensity. (Remembering) The radiation intensity U (Ѳ, Φ) is expressed in watts per steradian and is defined as time average power per unit solid angle. The average value of the radiation intensity is given by,
Uavg =
P rad
Where Prad = ∫∫ U (θ , φ) d ΩdΩ = sin ѲdѲdΦ
43. Show the expression for average power radiated. (Remembering) The expression for average power radiated is given by,
Pavg = W/m 2
44. Define Directivity? (Remembering) (AUT-Dec 2016) The directivity of an antenna is defined as the ratio of the maximum power density to the average power radiated.
GDmax G (^) Dmax = Umax /Uavg = 4π Umax /Prad
45. Define gain (Remembering) The ratio of maximum radiation intensity in given direction to the maximum radiation intensity from a reference antenna produced in the same direction with same input power. i.e Maximum radiation intensity from test antenna Gain (G)= ------------------------------------------------------------------------------------------------------ Maximum radiation intensity from the reference antenna with same input power 46. Show the relationship between Gain and Directivity? (Understanding) G=kD. Where, G=gain, k=Efficiency factor (0 to 1) and D=Directivity
58. Show the important applications of reciprocity theorem? (Understanding) Equality of directional patterns Equality of directivities Equality of effective lengths Equality of antenna impedances 59. What is meant by effective aperture or effective area? (Remembering) Area over which an antenna extracts electromagnetic energy from the incident electromagnetic waves is called effective area (or) effective aperture. Effective aperture is defined as the ratio of power radiated (or) received by the antenna to the power density of the incident wave. **Effective aperture Ae = power received/power density = Pr /S
- Define effective length(Remembering)** It represents the effectiveness of an antenna as radiator or collector of electromagnetic energy. The distance through which an antenna can radiate or receive effectively is called as Effective length. 61. Classify the fields around the antenna or in dipole antenna. (Analyzing) Near field or Fresnel Zone Far field or Fraunhofer zone 62. What are the approaches used for finding potentials for electromagnetic field?(Remembering) Maxwell approach Heuristic approach 63. Define Beam efficiency? (Remembering) The total beam area (WA) consists of the main beam area (WM)plus the minor lobe area (Wm) Thus WA = WM+ Wm. The ratio of the main beam area to the total beam area is called beam efficiency. Beam efficiency = SM = WM / WA. 64. What is meant by antenna beam width? (Remembering) (AUT-Dec 2017) Antenna beam width is a measure of directivity of an antenna. Antenna beam width is an angular width in degrees, measured on the radiation pattern (major lobe) between points where the radiated power has fallen to half its maximum value .This is called as “beam width” between half power points or half power beam width.(HPBW). 65. What are the different types of Effective aperture? (Remembering) Scattering aperture Loss aperture Collecting aperture Physical aperture. 66. Define different types of aperture?(Remembering) Scattering aperture (As.) It is the ratio of the reradiated power to the power density of the incident wave. Loss aperture (Al). It is the area of the antenna which dissipates power as heat. Collecting aperture (Ac). It is the addition or summation of above three apertures. Physical aperture (Ap). This aperture is a measure of the physical size of the antenna. 67. Define scattering aperture ratio. (Remembering) The ratio between scattering aperture to the effective aperture is called as scattering aperture ratio. **Scattering aperture ratio β = As /Ae
- Define absorption ratio. (Remembering)** The ratio between maximum effective apertures to the physical aperture is called absorption ratio Absorption ratio =A (^) em/Ap Where, Aem = Maximum effective aperture Ap = physical aperture 69. Define Aperture efficiency? (Remembering) The ratio of the effective aperture to the physical aperture is the aperture efficiency. **i.e Aperture efficiency hap = Ae/ Ap (dimensionless).
- What is meant by effective length of an antenna?(Remembering) (AU-Dec 2013)** It represents the effectiveness of an antenna as radiator (or) collector of electromagnetic energy. An antenna can radiate of receiver effectively to that of particular distance called as effective length. It is denoted by L (^) e. Mathematically, it is the ratio between open circuit voltages to the incident field strength.
L (^) e = Where V = Open circuit incident voltage E = Incident field strength
71. Show the expression for effective length of an antenna. (Understanding)
Le =
Z
2 AeRl
Where, Ae = effective aperture Rl = load resistance, Z = impedance
72. Construct the relationship between effective aperture and directivity. (Applying) The relationship between effective aperture and directivity is given by,
D == (^2) max
Ae
λ
π
Where D = directivity Aemax = effective aperture
73. Define antenna temperature. (Remembering) The antenna temperature is a parameter that depends on the temperature of the regions the antenna is ‘looking at’. Both the antenna temperature (T (^) A) and radiation resistance (Rr) are single valued scalar quantities. The expression for antenna temperature is,
K
SA
T A = e
Where S = Power density per unit bandwidth Ae= effective aperture, K = Boltzman’s constant = 1.38*10-
74. Show the relationship between antenna temperature and solid angle. (Understanding)
TA = (^) S A
S T
Where ΩA = antenna beam solid angle in steradian ΩS = Source solid angle in steradian TA = Antenna noise temperature TS = source temperature in °K
75. What is meant by equivalent noise temperature of an antenna (Te)? (Remembering) It is defined as that fictional temperature at the input of the network which would account for the noise ΔN at that output. (ΔN----additional noise introduced by the network itself) 76. Show the expression for noise figure of an antenna. (Understanding) The noise figure (F) related with effective noise temperature is
F = 1 +
To
Te
T (^) e = T (^) o (F-1) Where F = Noise figure (no dimension) T (^) o = 290° K
77. What is meant by radiation resistance? (Remembering) (AUT-Dec 2017) The antenna is a radiating device in which the power is radiated in to space in the form of electromagnetic waves. It is defined as the fictitious resistance which when inserted in series with the antenna will consume the same amount of power as it is actually radiated. The antenna appears to the transmission line as a resistive component nd this is known as the radiation resistance. 78. What is meant by front to back ratio? (Remembering) It is defined as the ratio of the power radiated in desired direction to the power radiated in the opposite direction. i.e FBR = Power radiated in desired direction / power radiated in the opposite direction. 79. Define antenna efficiency?(Remembering) The efficiency of an antenna is defined as the ratio of power radiated to the total input power supplied to the antenna. Antenna efficiency = Power radiated / Total input power 80. Show the expression for radiation efficiency. (Remembering) The expression for radiation efficiency is given by,
16 MARKS
- Derive the expression for vector potential in detail. (Applying) (AUT-Dec 2017)
- Derive the expression for the retarded vector potential for sinusoidal current element. (Appling) (AUT-June 2016,Dec 2017)
- Derive the electric and magnetic field quantities radiated from a Hertizian dipole antenna. (Remembering) (AUT-June 2016, June 2017)
- Define radiation resistance. Derive the Expression for power radiated and find the radiation resistance of Hertizian dipole antenna. (Remembering)
- Derive the Expression for far field component of a dipole antenna whose length is λ/2. (Applying)(AUT-June 2015, Dec 2015, June 2016)
- Derive radiation resistance for a half wavelength dipole. (Creating) (AUT-Dec 2015, June 2016,Dec 2017)
- Derive the Expression for power radiated and find the radiation resistance of an quarter wave and monopole dipole antenna. (Applying) (AUT-June 2015)
- Explain the following parameters of an antenna: (Understanding) (i) Beam solid angle (AUT-June 2015, Dec 2015) (ii) Radiation pattern (AUT-June 2017) (iii) Gain (iv) Polarization (AUT-June 2017) (v) Bandwidth
- Explain in detail about Effective aperture and derive its equation in different conditions. (Evaluating) (AUT-June 2017)
- Explain the following parameters of an antenna: (Understanding) (i) Antenna Temperature (AUT-June 2017) (ii) Input impedance (AUT-June 2015) (iii) Directivity (AUT-June 2017)
- State and prove the reciprocity theorem with respect to antenna. Also mention its application. (Evaluating) (AUT-June 2015)
- Derive the relationship between effective aperture (AUT-Dec 2017) and directivity of an antenna. (Applying) (AUT-Dec 2015)
- What is the effective length of an antenna? Determine the effective length of a quarter wave monopole. (Remembering) (AUT-June 2016)
- Derive the expression for the gain of half wave dipole antenna. (Creating) (AUT-June 2016)
- Calculate the physical height of a half wave dipole with antenna quality factor Q of 28 and bandwidth of 10 MHz. (Evaluating) (AUT-June 2017)
- Problems and exercises.
- An antenna has radiation resistance of 72Ω and loss resistance of 8Ω and power gain of 12dB. Estimate the antenna efficiency and directivity. (AUT-Dec 2017)
UNIT-II (CO2) ANTENNA ARRAYS
1. What is point source? (Remembering) It is the waves originate at a fictitious volume less emitter source at the center ‘O ’of the observation circle. 2. What is meant by antenna array? (Remembering) It can be achieved by combining the individual antenna radiations in desired direction and canceling the radiation in undesired direction. Such a system is called an antenna array. It may also be defined as “A radiating system consisting of several spaced and properly phased radiators”. 3. List out the salient features of antenna array? (Remembering) An antenna is a system of similar antennas oriented similarly to get greater directivity in a desired direction. It will also provide high gain in a desired direction of an antenna. 4. What is meant by uniform linear array? (Remembering) An array is linear when the elements of the array are spaced equally along the straight line. If the elements are fed with currents of equal magnitude and having a uniform progressive phase shift along the line, then it is called uniform linear array. 5. What are the types of antenna array? (Remembering) Broad side array. End fire array Collinear array. Parasitic array
6. What is Broad side array?(Remembering) (AUT-June 2015,Dec 2017) Broad side array is defined as an arrangement in which the principal direction of radiation is perpendicular to the array axis and also the plane containing the array element. For Broad side array the phase difference adjacent element is d = 0. Elements are placed parallel to each other and perpendicular to array axis and elements is fed with current of equal magnitude and same phase. 7. Define End fire array(Remembering) (AU-Dec 2013) (AUT-Dec 2017) End fire array is defined as an arrangement in which the principal direction of radiation is coincides with the array axis. For End fire array δ = - βd Where β = 2π/λ d = Distance between the elements 8. What is collinear array? (Remembering) In this array the antenna elements are arranged coaxially by mounting the elements end to end in straight line or stacking them one over the other with radiation pattern circular symmetry. **Eg.Omni directional antenna.
- What is parasitic array? (Remembering)** In this array the elements are fed parasitically to reduce the problem of feed line. The array of antennas in which the parasitic elements get the power through electromagnetic coupling with driven element which is in proximity with the parasitic element is known as parasitic array. **Eg.Yagi-uda antenna.
- What is meant by similar Point sources? (Remembering)** Whenever the variation of the amplitude and the phase of the field with respect to the absolute angle for any two sources are same then they are called similar point sources. The maximum amplitudes of the individual sources may be unequal. 11. What is meant by identical Point sources? (Remembering) Similar point sources with equal maximum amplitudes are called identical point sources. 12. What is meant by array factor? (Remembering) It is ratio of the magnitude of the resultant field to the magnitude of the maximum field.
Array factor =
E max
E T
= E max = 2E o
The factor by which the array increases the field strength over that of single element radiating the same total power.
13. Define beam width of major lobe? (Remembering) It is defined the angle between the first nulls (or) it is defined as twice the angle between the first null and the major lobe maximum direction. **Beam width (BW) = 2 X angle between first null and maximum of major lobe BW = 2 γ
- What are the different conditions can be applied to the antenna array? (Remembering)** Two point sources with currents of equal magnitudes and with same phase Two point sources with currents of equal magnitudes and with opposite phase Two point sources with currents of unequal magnitudes and with opposite phase 15. Distinguish between broad side and End fire array. (Analyzing) (AUT-June 2016, Dec 2016) BROAD SIDE ARRAY END FIRE ARRAY In this number of identical antennas are set up along a line drawn perpendicular to their respective axis.
In this number of identical antennas are spaced equally along a line parallel to their respective axis. Individual elements are fed with current of equal amplitude and phase
Individual elements are fed with current of equal amplitude and out of phase Radiates perpendicular to the axis Radiates parallel or along the axis. Narrow beam width Wide Beamwidth
16. What is mean by Half Power Point Direction (HPPD)? (Remembering) At half power points, power is ½ (or) voltage and currents is 1/√2 times the maximum value. **i.e.,, E = ± 1/√
- List out the various properties used in BSA and EFA. (Analyzing)** Major lobe Magnitude of major lobe Nulls
1. Antenna is fed in phase d = 0 Antenna elements are fed with ±βd 2. Maximum radiation is perpendicular along the direction of array axis
Maximum radiation is along the array axis
Beam width of major lobe is twice the reciprocal of array axis,+2l/nd
Beam width is greater than that for that of a broad side array with increased directivity
31. What is mean by phased array?(Remembering) The array in which the phase and the amplitude of most of the elements is variable, provided that the direction of maximum radiation and pattern shape along with the side lobe is controlled. 32. What is mean by frequency scanning array?(Remembering) (AU-Dec 2013) The array in which the phase change is controlled by varying the frequency. The is found to be the simplest phased array as at each element separate phase control is not necessary. 33. What is mean by retro array?(Remembering) The array which automatically reflects an incoming signal back to the source.It acts as a retro reflector similar to the passive square corner reflector. 34. What is mean by adaptive array?(Remembering) An array which automatically turn the maximum beam in the desired direction while turn the null in the undesired direction. The adoptive array adjust itself in the desired direction with awareness of its environment. 35. What are the conditions to obtain end fire array pattern? (Remembering) End fire array is defined as an arrangement in which the principal direction of radiation coincides with the array axis. For end fire array, δ = - βd Where, δ = Phase difference of the current fed between the sources of the end fire array d = Distance between the elements 36. List the features of adaptive arrays. (Analyzing)(AUT-June 2015) An adaptive antenna is type of smart antenna, its “smart” because it improves on the traditional antenna by adjusting for traffic patterns at a given time to **increase signal strength and quality.
- Give the formula for First null beam width of a broad side array. (Remembering) (AUT-Dec** 2015) The first nulls occur at l=±1 and the beam width between first nulls (BWFN) for such an array is given by, 38. Illustrate beam forming. (Understanding) (AUT-Dec 2015) It is the method used to create the radiation pattern of the antenna array by adding constructively the phases of the signals in the direction of the targets/mobiles desired and nullifying the pattern of the targets/mobiles which are undesired/interfering targets. This can be done with a simple FIR tapped delay line filter. The weights of the FIR filter may also be changed adaptively and used to provide optimal beam forming and actual beam pattern formed. 39. What is meant by front to back ratio? (Remembering) (AUT-Dec 2015) The Front-to-Back Ratio is a parameter used in describing directional radiation patterns for antennas. If an antenna has a unique maximum direction, the front-to-back ratio is the ratio of the gain in the maximum direction to that in the opposite direction (180 degrees from the specified maximum direction). This parameter is usually given in dB. **FBR=Power radiated in desired direction/power radiated in the opposite direction
- Criticize the two types of point sources. (Evaluating) (AUT-Dec 2017)** Isotropic, non-isotropic point source
16 MARKS
- What is an antenna array? What are the types of antenna arrays, for each type explain it with array diagram and radiation pattern. (Remembering)
- Derive the expression for electric field of an array of 2-point sources. (Applying) (AUT-Dec 2015)
- Derive an expression for antenna array factor. (Applying) (AUT-Dec 2016)
- Derive and draw the field pattern of array of two point sources with equal amplitude and phase. (Applying) (AUT-June 2016, Dec 2016,Dec 2017)
- Draw the radiation pattern of array of two point sources with equal amplitude and opposite phase by calculating the maximum, minimum and half power point directions. (Creating)
- Derive the total far field pattern for uniform linear array with ‘n’ isotropic sources. (Remembering)
- A linear broadside array consists of 4 equal isotropic in phase point sources with λ/3 spacing. Find the directivity and Beam width. (Remembering) OR Derive the expression for maxima, minima and beam width of a broad side array. (Applying)
- Derive the expressions for field pattern of End-fire array of N sources of equal amplitude and spacing. (Remembering)(AUT-June 2015, June 2017)
- Two identical vertical radiators are spaced d=λ/4 meters apart and fed with currents of equal magnitude but with a phase difference ‘β’. Evaluate the resultant radiation for β=0° and thereby identify the direction of maximum and minimum radiation. (Evaluating)
- For a 2-element linear antenna array separated by a distance d = 3λ/4, derive the field quantities and draw the radiation pattern for the phase difference 45°. (Remembering)
- Explain the principle of pattern multiplication with suitable examples. (Evaluating) (AUT-June 2015, Dec 2015, June 2016, Dec 2016,Dec 2017)
- What is binomial array? Draw the pattern of 10 element binomial array with spacing between the Elements of 3 λ/4 and λ/2. (Remembering) (AUT-June 2016, Dec 2017)
- Illustrate binomial array and phased array antenna (AUT-June 2017) with example. (Understanding) (AUT-June 2015, June 2016)
- Explain the following: (Understanding) (i) Adaptive array (ii) Frequency scanning array (AUT-June 2015) 15. Prove that the directivity of increased end fire array is 1.789 times greater than the directivity of Ordinary end fire array. (Understanding) (AUT-June 2016)
- A uniform linear array consists of 16 isotropic point sources with a spacing of λ/4. If the phase difference is -90 degree, calculate the directivity, HPBW, Beam solid angle. (Evaluating) (AUT-June 2017)
- Compare broadside array with end fire array. (Understanding) (AUT-June 2017)
- Explain how to compare the different cases of the array containing two isotropic sources. (Understanding) (AUT-Dec 2017)
- Problems and exercises. **UNIT-III (CO3) ANTENNA TYPES
- What is a loop antenna? (Remembering) (AUT-Dec 2016)** A loop antenna is a radiating coil of any convenient cross-section of one or more turns carrying radio frequency current. It may assume any shape (e.g. rectangular, square, triangular and hexagonal). 2. Show an expression of radiation resistance of a small loop. (Understanding) Radiation resistance of a small loop is given by Rr=31,200 (NA/λ^2 ) 2 Ω , Where N- Number of turns. A – Area of the small loop 3. How to increase the radiation resistance of a loop antenna? (Remembering) The radiation resistance of a loop antenna can be increased by: Increasing the number of turns Inserting a ferrite core of very high permeability with loop antenna’s circumference which will raise the magnetic field intensity called ferrite loop. 4. What are the types of loop antennas? (Remembering) Loop antennas are classified into: Electrically small (circumference <l/10) Electrically large (dimension comparable to l) 5. What are Electrically Small loop antennas? (Remembering) Electrically Small loop antennas is one in which the overall length of the loop is less than one- tenth of the wavelength. Electrically Small loop antennas have small radiation resistances that are usually smaller than their loop resistances. They are very poor radiators and seldom employed for transmission in radio communication. 6. What are electrically large loop antennas? (Remembering) Electrically Large loop antennas is one in which the overall length of the loop approaches the wavelength. 7. Why loop antenna called as magnetic dipole? (Remembering) If the dimensions of the loop antenna are small as compared with wavelength, then the radiation pattern of the loop is similar to that of the elementary dipole. The loop is surrounded by a magnetic field at right angles to the loop everywhere. Hence small loop antenna is called magnetic dipole. 8. List out the uses of loop antenna. (Analyzing) (AUT-Dec 2017) Various uses of loop antenna are: It is used as receiving antenna in portable radio and pagers
It has narrow bandwidth. Efficiency is very small.
20. List out the advantages of helical antenna. (Analyzing) N-turn helix is an end fire array of “n” sources. Helix not only has a nearly uniform resistance input over a wide bandwidth but it also operates as a super gain end fire array over the same bandwidth. It is non critical with respect to conductor size and turn spacing, therefore can achieve circular polarization over a wide bandwidth. It is easy to use in arrays because of almost negligible mutual impedance. 21. List out the applications of helical antenna. (Analyzing)(AU-Dec 2013) The applications of helical antenna are: It became the workhouse of space communications for telephone, television and data, being employed both on satellites and at ground stations Many satellites including weather satellites, data relay satellites all have helical antennas It is on many other probes of planets and comets, including moon and mars, being used alone, in arrays or as feeds for parabolic reflectors, its circular polarization and high gain and simplicity making it effective for space application. 22. What is YagiUda antenna? (Remembering) It is an array of a driven element, a reflector and one or more directors. If three-element array are used then such a type of Yagiuda is referred to as beam antenna. This is the most common antenna used for TV reception. The gain of the antenna is around 7 db and its radiation pattern is very much directive in one direction(normally receiving direction). 23. What do you mean by parasitic element? (Remembering) The passive elements which are not connected directly connected to the transmission line but are electrically coupled are called as parasitic elements. 24. What do you mean by driven elements? (Remembering) Driven elements are an active element where the power from the transmitter is fed or which feeds the received power to the receiver. 25. Why folded dipole antenna is used in Yagi antenna? (Remembering) The folded dipole has high input impedance. If the distance between the driven and parasitic element is decreased, it will load the driven element so input impedance of driven element reduces. But this will be compensated. 26. What is the function or action of reflector in Yagiuda antenna? (Remembering) The radiation coming from the front at the reflector is absorbed and it retransmits the radiation towards the dipole in such a way that it adds with the incoming signal. For any radiations coming from the back side, reflector retransmits the radiation in such a way that it is out of phase with the direct radiation from back side at dipole and hence they cancel each other. 27. What is the function or action of director in Yagiuda antenna? (Remembering) For the radiation coming from the front, the director generates its own radiation in such a way that it adds with this radiation at dipole and increases signal strength. For radiations coming from the back side, director generates its own radiation such that it cancels the radiation from back at dipole. 28. List out the optimum specification of a three element of YagiUda antenna. (Analyzing) Length of driven element=0.45λ to 0.49 λ Length of director= 0.4 λ to 0.45 λ Length of reflector= 0.5 λ Spacing b/w reflector & driven element= 0.1 λ to 0.25 λ Spacing b/w director & driven element= 0.2 λ to 0.4 λ 29. List out the various design parameters in 3-element Yagi array antenna. (Remembering) Reflector length = 500/f (mHz) feet (or) 152/f(mHz) meters Driven element length = 475/f (mHz) feet (or) 143/f(mHz) meters Director length = 455/f (mHz) feet (or) 137/f(mHz) meters 30. List the merits & demerits of YagiUda antenna.(Analyzing) Merits: Light weight & low cost High directivity Unidirectional radiation Simple construction Demerits: It is sensitivity to frequency Bandwidth is reduced if we increase number of elements.
31. Show the various applications of YagiUda antenna.(Analyzing) Used in television reception Used as a transmitter for low frequency applications 32. Design a three element Yagi-Uda antenna to operate at a frequency of 200 MHA. (Creating) Given: f = 200MHZ λ = C/f
1. 50 m
6
8
The length of the driven element Ld = 478/f(MHZ) = 478/200 = 2.39 feet Length of the reflector Lr = 492/f(MHZ) = 492/200 = 2.46 feet Length of the director Ld = 461.5/f(MHZ) = 461.5/200 = 2.30 feet Element spacing S = 142/f(MHZ) = 142/200 = 0.71 feet
33. What is a frequency independent antenna? (Remembering) (AU-June 2014) If the structure of the antenna is defined in terms of angles only, then it comes under the category of frequency independent antenna. In any frequency independent antenna, the characteristic like impedance, radiation pattern and directivity are independent of frequency. **Eg, Log periodic antenna, Spiral antenna
- Why log periodic antenna is called so? (Remembering)** The log periodic antenna is called so because it has impedance and radiation characteristics that are regularly repetitive as a logarithmic function of frequency. 35. What is LPDA? (Remembering) LPDA means log periodic dipole array. It is defined as an antenna whose electrical properties repeat periodically with logarithm of the frequency. It is a broad band narrow beam antenna, and it is frequency independent antenna. 36. Define design ratio or scale factor in LPDA. (Remembering) The dipole lengths and the spacing between two adjacent dipoles are related through parameter called design ratio (or) scale factor denoted by τ. Thus the relationship between Sn and S (^) n+1 and Ln and L (^) n+1 is given by,
= = τ
n n
n
L
L
S
S
37. What are the different regions in log periodic antenna and how are they differentiated? (Remembering) Inactive transmission line region – L< λ/2 – high capacitive impedance Active region – L = λ/2 – offers resistive impedance Inactive reflective region – L>λ/2 – offers inductive impedance 38. List the design parameters of a Log periodic antenna. (Analyzing) (AUT-June 2017) Apex angle α Design ratio τ Spacing factor σ 39. Show the expression for α, τ, σ of Log periodic antenna. (Understanding) Design ratio: Relates the position of a dipole elements from the origin Spacing factor: Ratio of the separation b/w adjacent dipoles to the length of the dipole. σ=1- τ/4 tan α/2 or Ln /Ln+1=R (^) n /Rn+ Acute angle: α=2 tan-1^ (1- τ/4 σ) where τ=Design ratio= Sn/2L (^) n 40. What are the applications of log periodic antenna? (Remembering) HF communication Television reception All round monitoring 41. What is rhombic antenna? (Remembering) An antenna which consists of four straight wires arranged in the shape of diamond, suspended horizontally above the surface of the earth is called as a rhombic antenna. It is otherwise called as diamond antenna or traveling wave antenna. 42. List some special features of rhombic antenna. (Remembering) The terminating resistance used may have the value 600Ω to 800Ω. Terminating resistance is a non inductive with a negligible value of capacitance. For low power systems, the ordinary resistor can be used as the terminating resistor. The typical value of power loss is 35 to 50% in terminating resistor leading to a power gain of 50 to 60.
