Avionics Mcqs - Set 13

1)   What is the load on a transmission line?

a. 0 Ω
b. Infinity
c. Some value between 0 and infinity
d. Some value between0 and 1
Answer  Explanation 

ANSWER: Some value between 0 and infinity

Explanation:
In practice, however, the load on a transmission line is neither infinite nor 0Ω; rather, it is typically some value in between. The load may be resistive or may have a reactive component.

2)   The transmission line in which the resistive impedance is equal to the characteristic impedance is called _____

a. Matched lines
b. Paralleled lines
c. Balanced lines
d. Unbalanced lines
Answer  Explanation 

ANSWER: Matched lines

Explanation:
Ideally, a transmission line should be terminated in a load that has a resistive impedance equal to the characteristic impedance of the line. This is called a matched line.

3)   How would the graph of wavelength vs voltage look like if the transmission line is matched (neglect resistive loss)?

a. Linearly increasing
b. Linearly decreasing
c. Exponentially decreasing
d. Constant
Answer  Explanation 

ANSWER: Constant

Explanation:
If a voltmeter is moved down a matched line from the generator to load and the rms voltage values are plotted, the resulting wavelength versus voltage line will be flat.

4)   In an unmatched line the actual signal on the line is ___________

a. The sum of forward and reflected signals
b. The difference of forward and reflected signals
c. The product of forward and reflected signals
d. The modulus of forward signal
Answer  Explanation 

ANSWER: The sum of forward and reflected signals

Explanation:
In an unmatched line, standing waves are formed which is a combination of forward and reflected signal. The signal actually on a line is simply the algebraic sum of the forward and reflected signals.

5)   What is the Standing wave ratio if a 75Ω antenna load is connected to a 50Ω transmission line?

a. 1
b. 2
c. 1.5
d. 1.43
Answer  Explanation 

ANSWER: 1.5

Explanation:
Standing wave ratio = SWR = Load impedance/ Characteristic impedance = 75/50 = 1.5.

6)   The ratio of the incident voltage wave Vi to the reflected voltage wave Vr is called the reflection coefficient.

a. True
b. False
Answer  Explanation 

ANSWER: False

Explanation:
The ratio of the reflected voltage wave Vr to the incident voltage wave Vi is called the reflection coefficient. The reflection coefficient provides the current and voltage information on the line.

7)   What is the reflection coefficient If a line is terminated in its characteristic impedance?

a. 0
b. Infinity
c. 2
d. 0.5
Answer  Explanation 

ANSWER: 0

Explanation:
The reflection coefficient of a line that is fully terminated in its characteristic impedance is 0. This is because there is no reflected voltage on the line.

8)   What percentage of power is reflected if the reflection coefficient is 0.5?

a. 50%
b. 75%
c. 25%
d. 12%
Answer  Explanation 

ANSWER: 25%

Explanation:
No explanation is available for this question!

9)   What is the resistive load if SWR= 3.05 and Zo =75Ω?

a. 1.23Ω
b. 51.23Ω
c. 254.2Ω
d. 24.59Ω
Answer  Explanation 

ANSWER: 24.59Ω

Explanation:
No explanation is available for this question!

10)   How does a shorted transmission line appear to a generator whose length is a quarter of the wavelength?

a. Very high impedance
b. Very low impedance
c. Short circuited line
d. RC tuned circuit
Answer  Explanation 

ANSWER: Very high impedance

Explanation:
At the load end of the shorted one quarter wavelength line, voltage is zero and current is maximum. But one-quarter wavelength back, at the generator, the voltage is maximum and the current is zero. To the generator, the line appears as an open circuit, or at least very high impedance.

11)   When a shorted line does looks like a series resonant circuit to the generator?

a. Length of line is one quarter wavelength
b. Length of line is one half wave lengths
c. Sorted line
d. Open line
Answer  Explanation 

ANSWER: Length of line is one half wave lengths

Explanation:
When the length of the transmission line is one half of the wavelength, a standing wave pattern is produced where the current and voltage at the generator and the end of the transmission line is same. In this case, the line looks like a series resonant circuit to the generator.

12)   When does a shorted transmission line look like an inductor to the generator?

a. Length is one quarter wavelength
b. Length is between one quarter and one half of wavelength
c. Length is more the one half of the wavelength
d. At all times
Answer  Explanation 

ANSWER: Length is one quarter wavelength

Explanation:
If the line length is less than one-quarter wavelength at the operating frequency, the shorted line looks like an inductor to the generator. If the shorted line is between one quarter and one-half wavelength, it looks like a capacitor to the generator.

13)   The characteristics of an open transmission line are just the opposite of shorted transmission line with respect to what the generator sees.

a. True
b. False
Answer  Explanation 

ANSWER: True

Explanation:
To the generator, an open one-quarter wavelength line looks like a series resonant circuit and a one-half wavelength line looks like a parallel resonant circuit. This is just the opposite of a shorted line.

14)   When does an open transmission line look like an inductor to the generator?

a. Length is one quarter wavelength
b. Length is between one quarter and one half of wavelength
c. Length is more the one half of the wavelength
d. At all times
Answer  Explanation 

ANSWER: Length is between one quarter and one half of wavelength

Explanation:
If the open transmission line is less than one-quarter wavelength, the generator sees a capacitance. If the line is between one-quarter and one-half wavelength, the generator sees an inductance. These characteristics repeat for lines that are some multiple of one-quarter or one-half wavelengths.

15)   The impedance of a line is zero when ___________

a. It acts as inductor
b. It acts as capacitor
c. It acts as a resistor
d. Series resonant circuit
Answer  Explanation 

ANSWER: Series resonant circuit

Explanation:
If the line acts as a series resonant circuit, its impedance is zero. If the line is of such a length that it acts as a parallel resonant circuit, its impedance is near infinity. If the line is some intermediate length, it is reactive.

16)   The special transmission lines constructed on PCBs to act as tuned circuits is called as?

a. Stripline
b. Loop lines
c. Printed lines
d. Transmission lines
Answer  Explanation 

ANSWER: Stripline

Explanation:
At UHF and microwave frequencies the values of inductance and capacitance become so small that it is difficult to realize them physically with standard coils and capacitors. Special transmission lines constructed with copper patterns on a printed circuit board (PCB), called microstrip or stripline, can be used as tuned circuits, filters, phase shifters, reactive components, and impedance-matching circuits at these high frequencies.

17)   Which of the following is not true with respect to microstrip?

a. Used for transmission in UHF and microwave region
b. Copper is etched on an insulating base material
c. Point to point communications with wire are eliminated
d. Microstrip is ever onequarter or one-half wavelength long.
Answer  Explanation 

ANSWER: Microstrip is ever onequarter or one-half wavelength long.

Explanation:
Microstrip is a flat conductor separated by an insulating dielectric from a large conducting ground plane. The microstrip is usually onequarter or one-half wavelength long and is used for transmission in the UHF and microwave region.

18)   What is the characteristic impedance of microstrip with the dimensions h = 0.0625 in, w = 0.1 in, t = 0.003 in, and ε= 4.5?

a. 84.35Ω
b. 42.6Ω
c. 53.9Ω
d. 65.21Ω
Answer  Explanation 

ANSWER: 53.9Ω

Explanation:
No explanation is available for this question!

19)   What is the transmission line in which a flat conductor is sandwiched between two ground planes?

a. Microstrip
b. Stripline
c. Coaxial line
d. Balanced line
Answer  Explanation 

ANSWER: Stripline

Explanation:
Stripline is a conductor sandwiched between two ground planes. It is more difficult to make than microstrip; however, it does not radiate as microstrip does.

20)   Microstrip and striplines can be made by using monolithic, thin-film, and hybrid IC techniques to produce ________

a. Micro circuits
b. Microwave integrated circuits
c. Nano circuits
d. Embedded circuits
Answer  Explanation 

ANSWER: Microwave integrated circuits

Explanation:
Even tinier microstrip and striplines can be made by using monolithic, thin-film, and hybrid IC techniques. When these are combined with diodes, transistors, and other components, microwave integrated circuits (MICs) are formed.

21)   A microstrip transmission line is to be used as a capacitor of 4 pF at 800 MHz. The PCB dielectric is 3.6. The microstrip dimensions are h = 0.0625 in, w = 0.13 in, and t = 0.002 in. What is the characteristic impedance of the line?

a. 48.9Ω
b. 12.3Ω
c. 54.3Ω
d. 65.7Ω
Answer  Explanation 

ANSWER: 48.9Ω

Explanation:
No explanation is available for this question!

22)   All EM waves propagate at the speed of light irrespective of medium.

a. True
b. False
Answer  Explanation 

ANSWER: False

Explanation:
The speed of light depends on the medium through which it travels and it the same for EM waves as light can be thought of an EM wave. The speed of EM waves is maximum in a vacuum.

23)   In a current carrying conductor, what happens to the magnetic field produced if the supplied voltage is increased and current is maintained constant?

a. Field strength increases
b. Field radius increases
c. Filed strength decreases
d. Does not change
Answer  Explanation 

ANSWER: Does not change

Explanation:
The magnetic field produced by a current carrying conductor is only influenced by the current and not the voltage. Since the current is maintained constant, there is no change in the magnetic field strength or radius.

24)   Which of the following is the most necessary for an electric field to exist?

a. Current
b. Potential difference
c. Dielectric material
d. Metal conductor
Answer  Explanation 

ANSWER: Potential difference

Explanation:
The most important thing for an electric field to exist is a potential difference between two conductors. It is not that only metal conductors can produce an electric field, potential difference between fluids also produces electric fields.

25)   What happens to the electric field between two conductors when the permeability of the material between the plates is increased?

a. Field strength increases
b. Field radius decreases
c. Field strength decreases
d. Nothing happens
Answer  Explanation 

ANSWER: Nothing happens

Explanation:
The permeability of a material only affects the magnetic field and not the electric field. Electric field depends upon permittivity of the material between the two conductors.

26)   Which of the following is not true?

a. A conductor carrying alternating current radiates
b. A transmission line must not radiate energy
c. Antennas are transmission lines which are made to radiate energy
d. A parallel wire transmission, when left open, does not radiate
Answer  Explanation 

ANSWER: A parallel wire transmission, when left open, does not radiate

Explanation:
If a parallel-wire transmission line is left open, the electric and magnetic fields escape from the end of the line and radiate into space. This radiation, however, is inefficient and unsuitable for reliable transmission or reception.

27)   The radiation from an open line can be increased by bending to which of the following angle?

a. 0°
b. 180°
c. 90°
d. 53.76°
Answer  Explanation 

ANSWER: 90°

Explanation:
The radiation from a transmission line can be greatly improved by bending the transmission line conductors so that they are at a right angle to the transmission line. The magnetic fields no longer cancel and, in fact, aid one another. The electric field spreads out from conductor to conductor. The result is an antenna.

28)   What is the ratio of the electric field strength of a radiated wave to the magnetic field strength called?

a. Impedance of space
b. Dielectric constant
c. Permittivity
d. Permeability
Answer  Explanation 

ANSWER: Impedance of space

Explanation:
The ratio of the electric field strength of a radiated wave to the magnetic field strength is a constant. It is called the impedance of space, or the wave impedance.

29)   The fields in the Fresnel zone are radio waves that contain the information transmitted.

a. True
b. False
Answer  Explanation 

ANSWER: False

Explanation:
The near field describes the region directly around the antenna where the electric and magnetic fields are distinct. These fields are not the radio wave, but they do indeed contain any information transmitted. The near field is also referred to as the Fresnel zone.

30)   At what distance from the antenna does the far field start?

a. 2 wavelengths
b. 5 wavelengths
c. 10 wavelengths
d. 25 wavelengths
Answer  Explanation 

ANSWER: 10 wavelengths

Explanation:
The far field that is approximately 10 wavelengths from the antenna is the radio wave with the composite electric and magnetic fields. For example, at 2.4 GHz, one wavelength is 984/2400 = 0.41 feet. The far field is 10 times that, or 4.1 ft or beyond.

31)   The Far field is also known as ______________

a. Fresnel zone
b. Fraunhofer zone
c. Maxwell zone
d. Marconi zone
Answer  Explanation 

ANSWER: Fraunhofer zone

Explanation:
The far field is also called the Fraunhofer zone. It is named after the Bavarian physicist Joseph Ritter von Fraunhofer.

32)   An Em wave is said to be vertically polarized when the angle between the electrical field and earth is _______

a. 50°
b. 20°
c. 90°
d. 180°
Answer  Explanation 

ANSWER: 90°

Explanation:
Polarization refers to the orientation of magnetic and electric fields with respect to the earth. If an electric field is parallel to the earth, the electromagnetic wave is said to be horizontally polarized; if the electric field is perpendicular to the earth, the wave is vertically polarized.

33)   Which of the following is not true?

a. Right circular polarized antennas can pick up left circular polarized waves due to propagation effects
b. Circular polarization has lesser attenuation in free space
c. Circular polarized wave can follow the curvature of earth
d. In circular polarization the electric and magnetic fields rotate as they leave the antenna
Answer  Explanation 

ANSWER: Circular polarized wave can follow the curvature of earth

Explanation:
In circular polarized wave the polarization angle of the electric field and the earth is continuously changing. This does not affect the transmission direction and hence circular polarized waves cannot bend with the curvature of earth, like any other EM wave.

34)   What happens when a vertical or horizontal polarized antenna receives a circular polarized wave?

a. Gain increases
b. Signal strength increases
c. Signal strength reduces
d. Cannot receive circular polarized waves
Answer  Explanation 

ANSWER: Signal strength reduces

Explanation:
A vertical or horizontal antenna can receive circular polarized signals, but the signal strength is reduced. When circular polarization is used at both transmitter and receiver, both must use either left- or right-hand polarization if the signal is to be received.

35)   Which of the following devices assist in using the same antenna for transmission and receiving?

a. Monoplexer
b. Multiplexer
c. Duplexer
d. Switch
Answer  Explanation 

ANSWER: Duplexer

Explanation:
An antenna can transmit and receive at the same time as long as some means is provided for keeping the transmitter energy out of the front end of the receiver. A device called a duplexer is used for this purpose.

36)   A dipole antenna is also called as?

a. Marconi antenna
b. Yagi antenna
c. Bidirectional antenna
d. Hertz antenna
Answer  Explanation 

ANSWER: Hertz antenna

Explanation:
One of the most widely used antenna types is the half-wave dipole antenna. This antenna is also formally known as the Hertz antenna after Heinrich Hertz, who first demonstrated the existence of electromagnetic waves.

37)   The impedance at the center of the antenna is known as?

a. Characteristic impedance
b. Radiation resistance
c. Transmission impedance
d. Recovery resistance
Answer  Explanation 

ANSWER: Radiation resistance

Explanation:
The transmission line is connected at the center. The dipole has an impedance of 73 V at its center, which is the radiation resistance. At the resonant frequency, the antenna appears to be a pure resistance of 73 V.

38)   What happens when the radiation resistance of the antenna matches the characteristic impedance of the transmission line?

a. No transmission occurs
b. No reception occurs
c. SWR is maximum
d. SWR is minimum
Answer  Explanation 

ANSWER: SWR is minimum

Explanation:
When the radiation resistance of the antenna matches the characteristic impedance of the transmission line, the SWR is minimum and maximum power reaches the antenna. This allows maximum power to be transmitted.

39)   The type of dipole antenna that has a higher band width is called as?

a. Conical antenna
b. Yagi antenna
c. Helical antenna
d. Marconi antenna
Answer  Explanation 

ANSWER: Conical antenna

Explanation:
A common way to increase bandwidth in the antenna is to use a version of the dipole antenna known as the conical antenna. The overall length of the antenna is 0.73λ or 0.73(984)/f = 718.32/f. This is longer than the traditional one-half wavelength of a dipole antenna, but the physical shape changes the necessary dimensions for resonance.

40)   The radiation pattern of a half-wave dipole has the shape of a ______

a. Doughnut
b. Sphere
c. Hemisphere
d. Circular
Answer  Explanation 

ANSWER: Doughnut

Explanation:
The radiation pattern of any antenna is the shape of the electromagnetic energy radiated from or received by that antenna. Typically that radiation is concentrated in a pattern that has a recognizable geometric shape. The radiation pattern of a half-wave dipole has the shape of a doughnut.

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