Avionics Multiple Choice Questions on “Antenna Fundamentals”.
1. All EM waves propagate at the speed of light irrespective of medium.
a) True
b) False
Answer: b
Clarification: 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.
2. 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: d
Clarification: 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.
3. 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: b
Clarification: 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.
4. 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: d
Clarification: 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.
5. 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: d
Clarification: 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.
6. 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: c
Clarification: 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.
7. 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: a
Clarification: 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.
8. The fields in the Fresnel zone are radio waves that contain the information transmitted.
a) True
b) False
Answer: b
Clarification: 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.
9. At what distance from the antenna does the far field start?
a) 2 wavelengths
b) 5 wavelengths
c) 10 wavelengths
d) 25 wavelengths
Answer: c
Clarification: 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.
10. The Far field is also known as ______________
a) Fresnel zone
b) Fraunhofer zone
c) Maxwell zone
d) Marconi zone
Answer: b
Clarification: The far field is also called the Fraunhofer zone. It is named after the Bavarian physicist Joseph Ritter von Fraunhofer.
11. 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: c
Clarification: 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.
12. 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: c
Clarification: 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.
13. 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: c
Clarification: 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.
14. Which of the following devices assist in using the same antenna for transmission and receiving?
a) Monoplexer
b) Multiplexer
c) Duplexer
d) Switch
Answer: c
Clarification: 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.