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51.	Which of the following Maxwell equation is correct for a non-conducting and lossless medium?(a) ∇.D=ρv(b) ∇.D=0(c) ∇×D=ρv(d) ∇×E=0The question was posed to me during an online exam.My doubt stems from Radiation topic in division Radiation, Baluns & Polarization of Antennas
Answer» CORRECT ANSWER is (b) ∇.D=0 Explanation: SINCE it is given non-conducting medium, the charge density ρv=0 and current density J=0. The Maxwell equations are: Faradays Law: ∇×E=\(-\frac{\PARTIAL B}{\partial t}\) Amperes Law: ∇×H=\(\frac{\partial D}{\partial t}\) Gauss Law for electric FIELD: ∇.D=0 Gauss law for magnetic field: ∇.B=0

52.	In which of the following polarization the electric field components are perpendicular to each other and have unequal magnitudes?(a) Linear(b) Vertical(c) Circular(d) EllipticalThe question was posed to me in unit test.I need to ask this question from Types of Polarization in section Radiation, Baluns & Polarization of Antennas
Answer» Correct answer is (d) Elliptical To explain I would say: In linear COMPONENTS are in the same plane. Vertical and horizontal are types in linear polarization. Circular polarization has electric field components are perpendicular to each other and have EQUAL MAGNITUDE. In ELLIPTIC, the electric field components are perpendicular to each other and have UNEQUAL magnitude.

53.	What is a Balun?(a) It is used to balance the unbalanced systems(b) It unbalances the balanced systems(c) A twisted wire(d) Main beam of antenna with large beam widthI had been asked this question in an interview for job.Question is from Baluns in chapter Radiation, Baluns & Polarization of Antennas
Answer» Correct choice is (a) It is used to BALANCE the unbalanced systems For explanation I would say: A Balun is a device which CONNECTS a BALANCED two –CONDUCTOR LINE to an unbalanced coaxial line. It eliminates field mismatch. The current distribution is present in the inner conductor and is zero at the outer conductor.

54.	Which of the following Maxwell equation is obtained from Amperes law?(a) ∇×H=J(b) emf=\(-\frac{\partial \phi}{\partial t}\)(c) ∇×E=\(\frac{-\partial B}{\partial t}\)(d) ∇×D=ρvThe question was asked during an interview for a job.Origin of the question is Radiation topic in section Radiation, Baluns & Polarization of Antennas
Answer» Correct ANSWER is (a) ∇×H=J Explanation: Faradays Law: ∇×E=\(-\frac{\PARTIAL B}{\partial t}\) Amperes Law: ∇×H=J+\(\frac{\partial D}{\partial t}\) GAUSS Law for electric field: D=ρv Gauss law for MAGNETIC field: ∇.B=0

55.	Which type of polarization is found in parabolic reflector?(a) Right-hand Circular(b) Linear(c) Depends on the feed(d) Left-hand CircularI had been asked this question in a national level competition.I want to ask this question from Types of Polarization topic in chapter Radiation, Baluns & Polarization of Antennas
Answer» Right choice is (C) Depends on the feed For explanation: The parabolic antennas take the polarization of the feed. The linear polarization takes place in the MONOPOLE dipole. In the Dipole ALSO a linear polarization takes place. In helical, circular LOOP antennas we can find the circular polarization.

56.	Which of the following is false regarding Transformer Balun?(a) It has a narrow band frequency(b) It has infinitely wide band frequency(c) Ideally it provides zero insertion loss(d) Impedance matching is adjusted by its turn ratioI had been asked this question during an online exam.Enquiry is from Baluns topic in section Radiation, Baluns & Polarization of Antennas
Answer» The CORRECT OPTION is (a) It has a narrow band frequency To ELABORATE: A transformer Balun has ideally infinite wide band frequency and provides zero insertion loss. By CHANGING the number of turns we can modify the IMPEDANCE only when the input and output impedances are only resistive.

57.	A Balun joins a balanced line and an unbalanced line.(a) True(b) FalseThe question was posed to me at a job interview.My doubt stems from Baluns topic in chapter Radiation, Baluns & Polarization of Antennas
Answer» The correct option is (a) True To EXPLAIN I would say: A Balun joins a balanced line which is usually a two CONDUCTOR’s TWISTED cable with equal current distribution to an unbalanced line with one conductor with UNEQUAL current distribution and vice-versa. It ACTS like a transformer.

58.	Power radiated by a Hertzian dipole of length λ/30 and carrying a current 2A?(a) 0.87W(b) 3.51W(c) 2.51W(d) 8.77WThe question was posed to me during an interview.I want to ask this question from Radiation topic in division Radiation, Baluns & Polarization of Antennas
Answer» Right option is (B) 3.51W Explanation: POWER RADIATED by a HERTZIAN dipole Prad=Rrad I^2 Rrad = \(80\pi^2(\frac{L}{\lambda})^2=80\pi^2(\frac{\frac{\lambda}{30}}{\lambda})^2=0.877\Omega\) Prad = Rrad I^2=0.877×2×2=3.51W