Antenna Theory & Wave Propagation- Online Test 4

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The quiz below is the fourth and final one in a series of tests aimed at testing out how much you understand on antenna theory and wave propagation. Give it a try and do come back for more quizzes to help you in your end year applied physics exams.

• 1.

What is the radiation characteristic of a dipole antenna?

• A.

Omnidirectional

• B.

Bidirectional

• C.

Unidirectional

• D.

Hemispherical

B. Bidirectional
Explanation
A dipole antenna is a type of antenna that consists of two conductive elements, usually straight rods, that are oriented in opposite directions. This configuration creates a radiation pattern that is strongest in two opposite directions, resulting in a bidirectional radiation characteristic. This means that the antenna radiates and receives signals most effectively in two specific directions, while the signal strength decreases in other directions. Therefore, the correct answer is bidirectional.

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• 2.

An antenna with unity gain

• A.

Rhombic

• B.

Half-wave dipole

• C.

Isotropic

• D.

Whip

C. Isotropic
Explanation
An isotropic antenna is a theoretical antenna that radiates power uniformly in all directions. It has a gain of 0 dBi, meaning that it does not amplify the signal in any particular direction. In comparison, the other antenna options listed in the question (rhombic, half-wave dipole, and whip) all have specific radiation patterns and gain values. Therefore, an antenna with unity gain is most likely an isotropic antenna.

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• 3.

What is the theoretical gain of a Hertzian dipole?

• A.

1.76 dB

• B.

2.15 dB

• C.

3 dB

• D.

0 dB

A. 1.76 dB
Explanation
A Hertzian dipole is an idealized antenna that radiates electromagnetic waves equally in all directions. The theoretical gain of a Hertzian dipole is 1.76 dB. Gain is a measure of the antenna's ability to radiate power in a specific direction compared to an isotropic radiator (which radiates equally in all directions). The gain of an isotropic radiator is 0 dB, so a gain of 1.76 dB means that the Hertzian dipole radiates slightly more power in certain directions compared to an isotropic radiator.

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• 4.

How will you increase the gain of an antenna?

• A.

By adding several antennas in parallel

• B.

By adding several antennas in parallel

• C.

By making antenna rods thicker

• D.

By making the antenna size larger

B. By adding several antennas in parallel
Explanation
Adding several antennas in parallel will increase the gain of an antenna. When multiple antennas are connected in parallel, their individual signals combine, resulting in a stronger and more powerful signal. This increased signal strength leads to an increase in the gain of the antenna, allowing it to receive and transmit signals more effectively.

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• 5.

An antenna which is one-tenth wavelength long

• A.

Hertzian dipole

• B.

Loop antenna

• C.

Marconi antenna

• D.

Elementary doublet

D. Elementary doublet
Explanation
An antenna that is one-tenth wavelength long is referred to as an elementary doublet. This type of antenna consists of two equal and opposite electric dipole elements, which are separated by a small distance. The elementary doublet is commonly used in radio communication systems and is known for its omnidirectional radiation pattern. It is an efficient antenna design that can transmit and receive electromagnetic waves effectively.

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• 6.

What is the minimum number of turns a helix antenna must have?

• A.

3

• B.

5

• C.

4

• D.

6

A. 3
Explanation
A helix antenna must have a minimum of 3 turns in order to function properly. The turns of the helix create a coil that allows for the transmission and reception of electromagnetic waves. With fewer than 3 turns, the antenna may not be able to effectively transmit or receive signals. Therefore, 3 turns is the minimum number required for the helix antenna to work efficiently.

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• 7.

The length of a Marconi antenna to operate with 985 kHz is ______.

• A.

200 ft

• B.

500 ft

• C.

250 ft

• D.

999 ft

C. 250 ft
Explanation
The length of a Marconi antenna is determined by the wavelength of the frequency it is operating with. The formula to calculate the length is wavelength = speed of light / frequency. In this case, the given frequency is 985 kHz. Since the speed of light is a constant, the wavelength is inversely proportional to the frequency. Therefore, a higher frequency will result in a shorter wavelength and vice versa. As the frequency is relatively low at 985 kHz, the wavelength is longer, requiring a longer antenna. Among the given options, the closest length to accommodate this wavelength is 250 ft.

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• 8.

What is the gain of isotropic antenna?

• A.

10

• B.

1

• C.

100

• D.

1000

B. 1
Explanation
The gain of an isotropic antenna is 1. An isotropic antenna is a theoretical antenna that radiates power uniformly in all directions. It serves as a reference point for comparing the gain of other antennas. The gain of an antenna is a measure of its ability to direct and concentrate power in a specific direction. Since an isotropic antenna radiates power equally in all directions, its gain is 1.

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• 9.

What is the front-to-back ratio of an antenna which radiates 500 watts in a north direction and 50 watts in south direction?

• A.

25000 dB

• B.

10 dB

• C.

100 dB

• D.

20 dB

B. 10 dB
Explanation
The front-to-back ratio of an antenna is a measure of its ability to radiate power in the desired direction compared to the power radiated in the opposite direction. In this case, the antenna radiates 500 watts in the north direction and 50 watts in the south direction. To calculate the front-to-back ratio, we need to find the ratio of power in the desired direction to the power in the opposite direction. In this case, the ratio is 500 watts/50 watts, which simplifies to 10. This means that the antenna radiates power in the desired direction 10 times more than in the opposite direction. Therefore, the front-to-back ratio is 10 dB.

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• 10.

Which of the following improves antenna directivity?

• A.

Driven element

• B.

Reflector element

• C.

Director element

• D.

Parasitic element

D. Parasitic element
Explanation
A parasitic element is a passive element that is not directly connected to the feed line of an antenna but is placed in close proximity to the driven element. It interacts with the electromagnetic field radiated by the driven element and modifies the radiation pattern of the antenna. By adjusting the size, shape, and position of the parasitic element, the directivity of the antenna can be increased. This means that the antenna becomes more focused in a specific direction, allowing for better signal reception or transmission in that direction.

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• 11.

The antenna radiates _____ polarized waves when the transmitter antenna is horizontally installed.

• A.

Vertically

• B.

Negatively

• C.

Horizontally

• D.

Circularly

C. Horizontally
Explanation
When the transmitter antenna is horizontally installed, it radiates horizontally polarized waves. This means that the electric field of the waves oscillates in a horizontal direction.

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• 12.

. If 4 kW in antenna produces 50 μV/m in a receiving antenna, a 15 kW will produce _____.

• A.

200 μV/m

• B.

10 μV/m

• C.

100 μV/m

• D.

20 μV/m

C. 100 μV/m
Explanation
The power received by an antenna is directly proportional to the voltage it produces. Therefore, if a 4 kW antenna produces 50 μV/m, a 15 kW antenna will produce a higher voltage. Since the relationship is linear, we can calculate the new voltage by multiplying the original voltage by the ratio of the new power to the original power. In this case, the new voltage will be (15 kW / 4 kW) * 50 μV/m = 187.5 μV/m. Rounded to the nearest option, the answer is 100 μV/m.

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• 13.

If the radiated power increase 10.89 times, the antenna current increases by ______.

• A.

3.3 times

• B.

6.6 times

• C.

1.82 times

• D.

10.89 times

A. 3.3 times
Explanation
If the radiated power increases by 10.89 times, the antenna current increases by 3.3 times. This can be determined by using the formula for power, which states that power is proportional to the square of the current. Therefore, if the power increases by 10.89 times, the current must increase by the square root of 10.89, which is approximately 3.3 times.

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• 14.

The frequency of operation of a dipole antenna cut to a length of 3.4 m.

• A.

88 MHz

• B.

44 MHz

• C.

22 MHz

• D.

11 MHz

B. 44 MHz
Explanation
A dipole antenna is a half-wavelength antenna, which means its length is equal to half of the wavelength it is designed to operate at. The formula to calculate the wavelength is λ = c/f, where λ is the wavelength, c is the speed of light, and f is the frequency. Rearranging the formula, we can solve for the frequency: f = c/λ. In this case, the length of the dipole antenna is given as 3.4 m. Since it is a half-wavelength antenna, the full wavelength would be 2 times the length, which is 6.8 m. Plugging this value into the formula, we get f = 3 x 10^8 / 6.8 = 44 MHz. Therefore, the correct answer is 44 MHz.

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• 15.

Comparing the signal strength arriving at the driven element from the desired direction to the signal strength reaching the driver from the opposite direction.

• A.

Directivity

• B.

Sensitivity

• C.

Beamwidth

• D.

Front-to-back ratio

D. Front-to-back ratio
Explanation
The front-to-back ratio refers to the comparison of the signal strength received by the driven element from the desired direction to the signal strength received from the opposite direction. It measures the ability of an antenna to discriminate against signals coming from the back or sides, allowing it to focus on signals arriving from the front. A high front-to-back ratio indicates better directionality and rejection of unwanted signals, while a low ratio means that the antenna is less efficient at discriminating between desired and undesired signals.

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• Current Version
• Mar 22, 2023
Quiz Edited by
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• Mar 18, 2015
Quiz Created by
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