# FORMATIVE TEST 2 (ENG. ELECTROMAGNETICS)

1. With reference to Figure 1, for a time varying electromagnetic field the boundary  condition for the tangential component of the electric field at the surface of a perfect  conductor is

u

(a) D2n =

Medium 2: Dielectric Medium 1: Conductor

u

Figure 1

Wrong. D2n is the normal component of the electric flux density.

(b) E2t =

Correct.

(c) D2n = σ

Wrong. This is correct for the normal component of the electric flux density. (d) B1n =

Wrong. This is correct for the normal component of the magnetic flux density.

1. The magnitude of the magnetic component of an electromagnetic uniform plane wave  propagating in vacuum is 0.001 A/m. The magnitude of its electric component is (a) 0.001 /(120π ) V / m

Wrong. Remember the voltage-current relationship.

(b) 0.001 V / m

Wrong. The two components cannot have the same numerical value.

(c) 0.12π V / m

Correct.

(d) 0

Wrong. If the electric component vanishes, no wave can exist.

1. The electric component of a sinusoidal uniform plane wave is given  by x

j z E ( )e u β = 120π . The magnetic component of this wave is

(a) x j z H e u − β

Wrong. The phase sign and the unit vector are wrong.

j z H e u − β

(b) y

Wrong. The phase sign and the unit vector direction are wrong.

j z H e u − β

(c) z

Wrong: The phase sign and the unit vector are wrong.

(c) j z He uy

β

Correct:

Page 2

1. The time-averaged power density and the direction of power flow for a sinusoidal  uniform plane wave with the electric component given by x E = (120π )cos(βz )u is (a) 2 1 W / m , z u

Wrong. Check the relationship between the electric and magnetic components and  also check your calculations.

(b) 2 60π W / m , z u

Correct.

(c) 2 60π W / m , z u

Wrong. The power density cannot have a negative value

(d) 2 60π W / m , z u

Wrong. The direction sign is not correct

1. A circularly polarised wave can be assumed as a combination of two linearly  polarised waves. If the electric field of one of the waves is given as y E = cos(βz )u , an expression for the electric field of the circularly polarized wave is:

(a) y y E = cos(βz )u + cos(βx )u

Wrong. The second term does not show a propagating wave in the z direction. Also,  its unit vector is wrong.

(b) y x E = cos(βz )u + cos(βz ± π / 2 )u

Correct.

(c) y y E = cos(βz )u + cos(βz ± π / 2 )u

Wrong. This is a linearly polarized wave.

(d) y x E = cos(βz )u + cos(βz )u

Wrong: This is a linearly polarised wave.

1. A transmission line with characteristic impedance of 50 ohm is terminated in a load of  50 + j50 ohm. The magnitude of the reflection coefficient is

(a) 1/

Correct.

(b) 1

Wrong. This value occurs when the line is terminated in short or open circuit. (c)

Wrong. This is the matched condition (ie, the load and the line impedance are the  same).

(d) 2

Wrong. The magnitude of the reflection coefficient is always between 0 and 1.

1. In a transmission line terminated in a load, the reflection coefficient is 0.5 + j0.5.On  this line, the ratio of the voltage maximum to the voltage minimum is

(a) 1

Wrong. This is the matched condition. The ratio must be bigger than one. (b) j2

Wrong. The ratio cannot be a complex or an imaginary number.

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(c) 0

Wrong. The answer suggests that there is no voltage maximum. It is physically  impossible.

(d) 5.7

Correct.

1. When a transmission line is terminated in an open circuit, then the value of VSWR is  (a) 0

Wrong. The value of the VSWR is between 1 and infinity.

(b) ∞

Correct.

(c) 1

Wrong. This is the matched condition.

(d) -1

Wrong. The VSWR cannot be negative.

1. A half cycle rotation around the Smith chart is equivalent to

(a) one wave-length

Wrong.

(b) half of a wave-length

Wrong. This corresponds to a full cycle rotation.

(c) quarter of a wave-length

Correct.

(d) one-eighth of a wave-length

Wrong. This is equivalent to a quarter cycle rotation.

1. On the Smith chart the part circles represent

(a) the loci of reactances (susceptances)

Correct

(b) the loci of reflection coefficients

Wrong.

(c) the loci of resistances (conductances)

Wrong.

(d) the loci of reactances and resistances (susceptances and conductances) Wrong.

1. In the direction of propagation, a TE mode of a waveguide has  (a) an electric component

Wrong.

(b) neither an electric nor a magnetic component

Wrong.

(c) both an electric and a magnetic components

Wrong.

(d) a magnetic component

Correct.

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1. In a hollow waveguide, the group velocity represents

(a) the phase velocity

Wrong.

(b) the signal velocity

Correct.

(c) the light velocity

Wrong.

(a) the sound velocity

Wrong.

1. The cutoff frequency of the TE10 mode of a hollow rectangular waveguide with  dimensions a=20 mm and b=10 mm is

(a) 15 GHz

Wrong. This is the cutoff frequency of the TE01mode.

(b) 10 GHz

Wrong. Check your calculations.

(c) 7.5 GHz

Correct.

(d) 5 GHz

Wrong. Check your calculations.

1. Generally, the efficiency of the short dipole antenna (whose length << wavelength) (a) increases with the antenna length

(b) decreases with the antenna length

(c) does not change with the antenna length

(d) decreases with the antenna diameter

1. The overall resistance of an antenna is related to

(a) the antenna far field

Wrong. The antenna radiation resistance is related to the far field.

(b) the antenna near field

Wrong. The antenna reactance is related to the near field.

(c) the antenna ohmic losses

Wrong. Antenna ohmic losses are generated by the current flow in the antenna. (d) the antenna far field and ohmic losses

Correct.