Objectives(Antennas and Wave Propagation)

SHORT ANSWER QUESTIONS (ANTENNAS AND WAVE PROPAGATION) SECTION – A GENERAL 1 The first antenna was built by: (a) J. D. Kraus

(b) Guglielmo Marconi

(c) Heinrich Hertz

(d) R. J. Marhefka

2 The regular transatlantic message service started in: (a) 1899

(b) 1900

(c) 1901

(d) 1902

3 Broadcasting began and the word radio was introduced in about: (a) 1901

(b) 1910

(c) 1920

(d) 1930

4 If L, M, t, I, T and i represent Length, mass, time, electric current, temperature and luminous intensity respectively the fundamental dimensions include: (a) L, M and t only

(b) L, M, t and I only

(c) L, M, t, I and T only

(d) All L, M, t, I, T and i

5 The unit of electric flux density is: (a) Coulombs per cubic meter

(b) Coulombs per square meter

(c) Coulombs per meter

(d) Coulombs

6 The wavelength of 2-GHz wave is: (a) 15 cm

(b) 15 mm

(c) 1.5 cm

(d) 1.5 mm

7 The relevant human dimension in terms of frequency is: (a) KHz

(b) Hundreds of KHz

(c) MHz

(d) Hundreds of MHz

8 The vector magnetic potential can be defined where: (a) charge density is zero

(b) current density is zero

(c) flux density is zero

(d) for all time variant fields

9 The relation E = -V is not adequate for time varying fields and need to be modified to the form E = - V + N, where N equals; (a) – v / t

(b) – A / t

(c) – 2v/ t2

(d) – 2A/ t2

10 The inadequacy of the relation E = -V is realized on the application of the curl operation to each side, since curl of the gradient is identically zero. But, from Faraday's’ Law E is not generally zero. To effect an improvement the equation is modified to E = (a) -V - D/t

(b) -V - /t

(c) -V - B/t

(d) -V - A/t

11 Let the axial ratio (AR) of a polarization ellipse is defined in terms of the ratio of electric field intensity E. If E2 is the value of E along major axis and E1 along minor axes AR = E2/E1. Circular polarization is an extreme case of elliptic polarization which corresponds to: (a) E1 = E2 and AR = 1

(b) E1 = 0 and AR = 1

(c) E1 = E2 and AR = 

(d) E1 = 0 and AR = 

12 Let the axial ratio (AR) of a polarization ellipse is defined in terms of the ratio of electric field intensity E. If E2 is the value of E along major axis and E1 along minor axes AR = E2/E1. Linear polarization is an extreme case of elliptic polarization which corresponds to: (a) E1 = E2 and AR = 1

(b) E1 = 0 and AR = 1

(c) E1 = E2 and AR = 

(d) E1 = 0 and AR = 

13 The polarization loss factor F for perfect match is: (a) 1

(b) 10

(c) 

(d) 0

14 The polarization loss factor F for total mismatch is: (a) 1

(b) 10

(c) 

(d) 0

15 If in a network Si is the input signal Ni is the input noise, So is the output signal and No is the output noise the noise figure is given by: (a) Si/So

(b) Si+Ni / So+No

(c) So/Si

(d) So+No / Si+Ni

16 In general cosmic noise decreases with the increase in frequency and is of considerable importance in: (a) LF and MF bands

(b) MF and HF bands

(c) HF and lower VHF bands

(d) VHF and lower UHF bands.

17 The brightness “B” is related to the brightness temperature TB by the Rayleigh-Jeans formula given by: (a) B = (2 k TB) /  2 (b) B = 2 / (2 k TB) (c) B =  / (2 k TB)

(d) B = (2 k TB) / 

18 Which of the following represents one of the Maxwell’s equations in correct form? (a)  E  dl = - [B/ t]  dv (c)

§ D  ds =  dv

(b) § H  dl = I + § [D/ t]  ds (d)  B  ds = J dv

OR

=0

19 If ES is the field intensity vector identified as a phasor by its subscript s, and k0 is the wave number, the equation 2 ES = - k02 ES is called: (a) Poisson’s equation

(b) Coulomb’s gauge condition

(c) Vector Helmholtz equation

(d) Diffusion equation

20 Identify the correct equation: (a) A = -  2v/ t2

(b) A = -  v/ t

(c) V = -  +  2v/ t2

(d) V = - E – A2 / t2

21. The Lorentz gauge condition is given by A =: (a) -2V/t2

(b) -V/t

(c) -V

(d) 0

22. The Coulombs gauge condition is given by A =: (a) -2V/t2

(b) -V/t

(c) -V

(d) 0

23. The free space wave number k0 is equal to: (a) 0(00)

(b) 0/(00)

(c) 0(0/0)

(d) 0/(0/0)

24 If E is a function of  & t and has only Er component B will have (a) Only Br component

(b) only B component

(c) only B component

(d) B & B components

25 The induction and radiation fields of an oscillating electric dipole become approximately equal at a distance r, where r =: (a) /6

(b) /4

(c) /3

(d) /2

26 An antenna can be assumed to have sinusoidal current distribution provided its length is: (a)  /10

(b)  /5

(c)  /2

(d)  

27 If the radiated power of a quarter wave mono-pole is given by (1/2 )  0.609  Im2 (eff) / 2 the radiation resistance (in Ohms) of a half wave dipole is obtained to be: (a) 36.5 (b) 18.25 (c) 73 (d) 146 28 A dipole antenna is a straight radiator, usually fed in the center. It produces a maximum of radiation: (a) in the plane parallel to its axis

(b) in the plane normal to its axis

(c) at the place of feed

(d) at its extreme ends

29. The vertical radiation pattern of a center fed vertical dipole shown in fig. (A) is for the dipole length: (a) 2

(b) 3/2

(c) 

(d) 3/4

30. The combined radiation pattern of two non-directional radiators with separation d, fed with equal currents and with phase shift , (shown in FIG. A) belongs to: (a) d = /2,  =00

(b) d = /2,  = -900

(c) d = /4,  = -900 (d) d = /4,  =00

FIG. A

FIG. B

31. The combined radiation pattern of two non-directional radiators with separation d, fed with equal currents and with phase shift , (shown in FIG. B) belongs to: (a) d =,  =00

(b) d = /2,  = -900

(c) d = /4,  = -900 (d) d = /4,  =00

32. An ungrounded antenna near the ground: (a) acts as a single antenna of twice the height (b) acts as an antenna array

(b) is unlikely to need an earth mat (d) must be horizontally polarized.

33. The standard reference antenna for the directivity is: (a) Infinitesimal dipole (c) isotropic antenna

(b) elementary doublet (d) half wave dipole

34 A pure sinusoidal continuous variation results in: (a) Infinite bandwidth

(b) Wide bandwidth

(c) Narrow bandwidth

(d) Zero bandwidth

35 If Rr is the radiation resistance, T is the antenna temperature, RP indicates the radiation pattern and f denotes the frequency of operation, identify the correct statement. (a) All parameters Rr, T and RP are functions of f.

(b) Only Rr and T are functions of f.

(c) Only Rr and RP are functions of f.

(d) Only RP and T are functions of f.

36 An ungrounded antenna near the ground acts as: (a) a point source

(b) a single antenna of twice of its actual length

(c) an antenna array

(d) a single antenna of half of its actual length

37 The directive gain may have a numerical value between: (a) 0 to 1

(b) 0 to 10

(c) 0 to 

(d) –1 to +1

38 The quality factor of an antenna is (a) Directly proportional to bandwidth (b) Directly proportional to square of bandwidth (c) Inversely proportional to bandwidth (d) Inversely proportional to square of bandwidth 39 Identify the correct statement: (a) Time changing current radiates but accelerated charge does not radiate (b) Time changing current does not radiate but accelerated charge radiates (c) Both, time changing current and accelerated charge radiate (d) Both time changing current and accelerated charge do not radiate 40 From the circuit point of view an antenna appears to the transmission line as: (a) input resistance

(b) radiation resistance

(c) mutual impedance

(d) coupling impedance

41 If S(, ) is the Poynting vector and S(, )max represents its maximum value the normalized power pattern is given by: (a) S(, ) / S(, )max

(b) S(, )max / S(, )

(c) S(, )max - S(, )

(d) S(, )max + S(, )

42 The Poynting vector is given by S(, ) = [E2() + E2()] / Z0 where Z0 is: (a) input impedance of the Tx- line (c) intrinsic impedance of the space

(b) input impedance of the antenna (d) combined impedance of Tx-line and antenna

43 If the (total) beam area A (or beam solid angle) consists of the main beam area M plus the minor-lobe area m (i.e. A = M + m) beam efficiency is given by: (a) A / M

(b) M / A

(c) A / m

(d) m / A

44 The directivity in terms of beam area A can be written as: (a) A / 4

(b) 4 / A

(c) A / 2

(d) 2 / A

45 If G is the gain k is the efficiency factor and D is the directivity of an antenna these are related by:

(a) G = k2 D

(b) G = D / k2

(c) G = D / k

(d) G = k D

46 The directivity D of antenna and the number N of the point sources in the sky that can be resolved by the are related by the equation: (a) D = N2

(b) D = N

(d) D = 1 / N2

(c) D = 1/ N

47 The directivity D is given in terms of the antenna aperture Ae by the following relation: (a) D = 4Ae/ 2

(b) D = 42/Ae

(c) D = Ae/42

(d) D = Ae/4

48 The field around an antenna may be divided into two principal regions called the near field or Fresnel zone and far field or Fraunhofer zone. The boundary between the two regions may be arbitrarily taken at a radius R for an antenna of maximum dimension L, where R and L are related by: (a) R = 2L2/ 2

(b) R = 2L/2

(c) R = 2L2/

(d) R = 2L/

49 Antennas act as reflection-less transducers over wide range frequencies if the discontinuities are: (a) large and abrupt gradual

(b) small and abrupt

(c) large and gradual

(d) small and

50 The ratio of the distance between antenna and point of observation of field to the physical size of an antenna is the deciding factor for considering an antenna to be a point source. Thus an antenna may be regarded as a point source if this ratio is: (a) > 1

(b) >> 1

(c) 2

74 Side lobes in a broad side array will be entirely eliminated provided the spacing between adjacent antennas does not exceed (a)  / 4

(b)  / 2

(c) 3 / 4

(d) 

75 The directional pattern of an end fire array using isotropic radiators is substantially independent of the spacing of the antenna radiators provided this spacing does not exceed (a)  / 8

(b)  / 4

(c) 3 / 8

(d)  / 2

76 Choose the correct statement: (a) Both Binomial and uniform amplitude distributions are special cases of DolphTchebyscheff distribution. (b) Both edge and uniform amplitude distributions are special cases of DolphTchebyscheff distribution. (c) Both Binomial and edge distributions are special cases of the Dolph-Tchebyscheff (D-T) distribution. (d) All the three distributions listed in (a, b & c) above are special cases of DolphTchebyscheff distribution.

SECTION – C DIFFERENT TYPES OF ANTENNAS 77 The parabola reflects the wave originating from a source at the focus and transforms: (a) A plane wave front from the feed at focus into spherical wave front (b) A plane wave front from the feed at focus into cylindrical wave front (c) Any curved wave front from the feed at focus into a plane wave front (d) A cylindrical wave front from the feed at focus into a spherical wave front 78 For large parabola of many  aperture a practical choice for feed can be corner reflector with a corner angle of (depending on F/D ratio of parabola): (a) 0 - 450

(b) 450 -900

(c) 600 -1200

(d) 900 - 1800

79 Beam widths for corner reflector are approximately equal in both principal planes provided corner angle  =: (a) 1200

(b) 900

(c) 600

(d) 450

80 When the field across the mouth of the parabola is everywhere of the same phase the beam generated (a) is omni-directional

(b) is sharply unidirectional

(c) has main beam with two minor side lobes (d) is bifurcated into two major beams. 81 Antennas commonly used for microwave links are: (a) Loop antennas (c) Rhombic antennas

(b) Log periodic antennas (d) Paraboloidal dishes

82 Identify the correct statement: (a) The feed pattern is called primary pattern and the pattern of reflector as secondary pattern. (b) The pattern of reflector is the primary pattern and that of feed is termed as secondary pattern. (c) Only rear feed pattern is called primary pattern. (d) Only front feed pattern is called primary pattern. 83 Zoning is used with a dielectric antenna mainly to: (a) Increase the bandwidth of the antenna (c) Permit pin pointed focusing short horn

(b) Reduce the bulk of the lens

(d) Correct the curvature of the wave front from a

84 The delay lenses wherein the electrical path length is increased or wave is retarded by the lens medium include: (a) E-plane metal lenses (c) Dielectric and H-plane metal lenses lenses.

(b) H-plane metal lenses (d) E-plane and H-plane metal

85 The fast lenses wherein the electrical path length is decreased by the lens medium include: (a) Dielectric and E-plane metal lenses (c) H-plane metal lenses

(b) E-plane metal lenses (d) E-plane and H-plane metal

lenses. 86 Refraction in the lenses may involve one or two surfaces. The use of lenses having refraction through two surface is not very common but it offers better performance, prevents refocusing of energy into the feed and provides wide angle scanning when r is of the order of: (a) 3.5

(b) 2.5

(c) 1.5

(d) 0.5

87 Lenses with n > 1 are non-dispersive, their thickness decreases as n increases, mismatch between lens and free space increases and energy loss due to refraction increases. The bandwidth of such a lenses in terms of the frequency of operation is about: of the frequency of operation. (a) 50%

(b) 30%

(c) 20%

(d) 10%

88 The zoning makes the lens frequency sensitive, increases the energy loss, side lobe level and the shadowing effect. These effects can be minimized by using a design with: (a) Large f/D ratio keeping it = 1

(b) Large f/D ratio keeping it  1

(c) Less f/D ratio keeping it = 0.5

(d) Less f/D ratio keeping it < 0.5

89 Identify the correct statement: (a) In comparison to reflector gain of lens antenna is 1 or 2 dB less, but tolerance on surfaces is more. (b) In comparison to reflectors gain and tolerance on surfaces of lens antenna is little more. (c) In comparison to reflectors gain and tolerance on surfaces of lens antenna is little less. (d) In comparison to reflectors their gain is 1 or 2 dB more and also have less lenient tolerance on surfaces. 90 The small loop and short dipole (with loop axis parallel to the dipole) have identical field patterns for: (a) E

(b) H

(c) both E and H

(d) E and H interchanged

91 The circular loop of diameter d is generally regarded as small loops if: (a) d < /2

(b) d < /4

(c) d < /8

(d) d < /10

92 A small rectangular loop with area A satisfies the condition: (a) A < /10

(b) A < /100

(c) A < 2/100

(d) A < 2/10

93 A longitudinal slot in a cylinder has a radiation pattern is practically circular in plane perpendicular to the axis provided the cylinder diameter in terms of  is of the order of (a)  / 10

(b)  / 8

(c)  / 4

(d)  / 2

94 The slot and dipole have the same field patterns for: (a) E alone

(b) H alone

(c) E as well as H

(d) interchanged E and H

95 Identify the correct statement: (a) The Horizontal slots may result in horizontal polarization and vertical slots in vertical polarization. (b) The vertical slots may result in horizontal polarization and horizontal slots in vertical polarization. (c) Both vertical and horizontal slots may result in horizontal polarization.

(d) Both vertical and horizontal slots may result in vertical polarization. 96 A properly designed slot antenna may have bandwidth of about: (a)  10 % of center frequency

(b)  5 % of center frequency

(c)  2 % of center frequency

(d)  1 % of center frequency

97 The sharpest beam and highest gain for a given mouth size of a horn are obtained by (a) very small flare angles

(b) small flare angles

(c) large flare angles

(d) very large flare angles

98 A longitudinal slot in a cylinder has a radiation pattern is practically circular in plane perpendicular to the axis provided the cylinder diameter in terms of  is of the order of (a)  / 10 (b)  / 8 (c)  / 4 (d)  / 2 99 In an optimum horn the difference in path length along the edge and the center in E plane is: (a)  /4

(b)  /2

(c)  3/4

(d)  

100 Helical Antennas combine the geometry of a straight line, a circle and a cylinder. These antennas are: (a) Circularly polarized with high gain

(b) Circularly polarized with low gain

(c) Linearly polarized with high gain

(d) Linearly polarized with low gain

101 The axial (end fire) mode of helical antenna is most practical because it can achieve over a wide band: (a) Linear Polarization

(b) Circular Polarization

(c) Elliptical Polarization

(d) All the above

102 In mono-filar helices the term transmission mode is used to describe the manner in which the electromagnetic wave is propagated along: (a) A very short helix (c) A long helix

(b) A medium length helix

(d) An infinite helix

YAGI-UDA ANTENNA 103 A Yagi antenna may have: (a) A single director and multiple reflector directors

(b) A single reflector and multiple

(c) Multiple directors and multiple reflectors

(d) All the above combinations

WAVE PROPAGATION 104 With vertical polarization the magnitude of reflection coefficient for space wave is commonly: (a) quite large at moderately small angles of incidence (b) quite small at moderately small angles of incidence (c) quite small at moderately large angles of incidence (d) quite large at moderately large angles of incidence 105 The conductivity and dielectric constant of earth vary greatly with conditions. At broadcast band and lower frequencies, the earth can be regarded (approximately) as:

(a) pure capacitive R&C

(b) pure resistive

(c) pure inductive

(d) a combination of

106 At Brewster’s angle the reflection coefficient “Rv” for vertically polarized wave is: (a) >>1

(b) 1

(c)