physics : class12 Antenna Radiation Patterns : Radio Waves : Electromagnetic Waves MCQ Questions and Answers
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Antenna Radiation Patterns
MCQ questions and answers.
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MCQ Questions
1. A short dipole antenna has an electric-field pattern proportional to sin(theta), where theta is measured from the dipole axis. At what angle from the axis is the radiated power one-fourth of its maximum value?
A.
15 degrees
B.
30 degrees
C.
45 degrees
D.
60 degrees
Correct Answer:
30 degrees
Explanation:
Radiated power is proportional to the square of the electric field, so P/Pmax = sin^2(theta). Setting sin^2(theta) = 1/4 gives sin(theta) = 1/2, hence theta = 30 degrees.
2. In the far-field radiation pattern of an ideal short dipole, the antenna axis corresponds to which feature of the pattern?
A.
A direction of maximum radiation
B.
A null in radiation
C.
The half-power beamwidth boundary
D.
A direction of circular polarization
Correct Answer:
A null in radiation
Explanation:
For a short dipole, the electric field varies as sin(theta), where theta is measured from the dipole axis. At theta = 0 degrees and 180 degrees, the field and therefore the radiated power are zero, producing nulls along the axis.
3. A short dipole has a normalized power pattern P(theta) = sin^2(theta). What is the ratio of power radiated at theta = 60 degrees to that radiated at theta = 30 degrees?
A.
1
B.
3
C.
sqrt(3)
D.
1/3
Correct Answer:
3
Explanation:
The ratio is sin^2(60 degrees) divided by sin^2(30 degrees). This equals (3/4)/(1/4) = 3, so the radiation at 60 degrees is three times that at 30 degrees.
4. The three-dimensional radiation pattern of an ideal short dipole is best described as which shape?
A.
A sphere with uniform radiation in every direction
B.
A torus surrounding the dipole axis
C.
Two narrow cones directed along the dipole axis
D.
A paraboloid opening along the dipole axis
Correct Answer:
A torus surrounding the dipole axis
Explanation:
The short dipole radiates maximum power perpendicular to its axis and zero power along its axis. Rotating the two-dimensional figure-eight pattern about the dipole axis produces a toroidal, or doughnut-shaped, pattern.
5. For an ideal half-wave dipole, which statement about its principal radiation pattern is correct?
A.
It has maximum radiation along the wire and zero radiation perpendicular to it
B.
It has zero radiation along the wire and maximum radiation perpendicular to it
C.
It radiates equally in all directions in three-dimensional space
D.
It has two equal maxima exactly along the wire axis
Correct Answer:
It has zero radiation along the wire and maximum radiation perpendicular to it
Explanation:
The half-wave dipole has a pattern broadly similar to that of a short dipole in its principal plane: radiation is zero along the antenna axis and maximum in directions perpendicular to the wire.
6. Two identical short dipoles are oriented parallel to each other and driven with equal currents in phase. If their separation is much smaller than the wavelength, what happens to the far-field pattern compared with one short dipole?
A.
The pattern becomes isotropic
B.
The pattern develops a sharp null perpendicular to both dipoles
C.
The normalized pattern remains essentially that of a single dipole
D.
The maximum radiation shifts to the dipole axes
Correct Answer:
The normalized pattern remains essentially that of a single dipole
Explanation:
When the spacing is much smaller than the wavelength and the currents are in phase, the fields add with nearly the same phase in every observation direction. The total amplitude changes, but the normalized angular radiation pattern remains essentially the single-dipole pattern.
7. A short dipole has a maximum radiation intensity of 8 units and a normalized power pattern sin^2(theta). What is its radiation intensity at theta = 45 degrees from the dipole axis?
A.
2 units
B.
4 units
C.
5.66 units
D.
8 units
Correct Answer:
4 units
Explanation:
The normalized power at 45 degrees is sin^2(45 degrees) = 1/2. Therefore, the radiation intensity is 8 multiplied by 1/2, which equals 4 units.
8. For a short dipole, the electric-field amplitude at an observation direction is 0.5 times its maximum value. What is the corresponding power level relative to the maximum, expressed in decibels?
A.
-3 dB
B.
-6 dB
C.
-12 dB
D.
-20 dB
Correct Answer:
-6 dB
Explanation:
Power is proportional to the square of field amplitude. Thus, P/Pmax = (0.5)^2 = 0.25. The power ratio in decibels is 10 log10(0.25), which is approximately -6 dB.
9. An ideal short dipole is rotated about its own axis while the observation point remains fixed in the plane perpendicular to the dipole. How does the received radiation change, assuming the observation remains in the far field?
A.
It becomes zero after a 90-degree rotation
B.
It varies from maximum to minimum sinusoidally
C.
It remains at its maximum value
D.
It alternates between two equal nulls
Correct Answer:
It remains at its maximum value
Explanation:
The short-dipole pattern depends only on the angle between the observation direction and the dipole axis. In the plane perpendicular to the axis, this angle remains 90 degrees during rotation about the axis, so the radiation remains maximum.
10. A receiving antenna has a radiation pattern with a deep null in a particular direction. If a distant interfering transmitter lies exactly in that direction and the antenna pattern is reciprocal, what is the ideal effect at the receiver?
A.
The interference is received with maximum power
B.
The interference is suppressed because the antenna gain is zero in that direction
C.
The interference changes the main-lobe direction
D.
The interference is converted into circular polarization
Correct Answer:
The interference is suppressed because the antenna gain is zero in that direction
Explanation:
By reciprocity, the receiving pattern has the same directional characteristics as the transmitting pattern. A deep null corresponds to nearly zero receiving gain in that direction, so a signal arriving exactly along it is strongly suppressed.
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