This set of Antennas Multiple Choice Questions & Answers (MCQs) focuses on “Frequency Independent Antenna – Helical Antenna”.
1. Which of the following modes does the condition πD << λ is satisfied in helical antenna?
a) Axial mode
b) Normal mode
c) Conical mode
d) Axial, Normal and Conical
View Answer
Explanation: The circumference of the helical antenna C= πD varies with the wavelength in different modes as follows
In Normal mode: πD << λ
Axial mode: C ≈ λ
Conical mode: C ≈n λ
2. What is the pitch angle of the helical antenna to become a loop antenna?
a) 90
b) 0
c) 45
d) 60
View Answer
Explanation: C is the circumference and L is the length, s is the spacing between turns.
When pitch angle α=0, the separation becomes zero and length will be equal to circumference of the loop. Hence it acts as a loop antenna.
3. What is the pitch angle if the separation of turns is at 2 cm and the circumference is of 4 cm?
a) 26.56
b) 62.65
c) 30
d) 25.34
View Answer
Explanation: From the figure, \(α=tan^{-1}\frac{S}{C}=tan^{-1}\frac{2}{4}=tan^{-1}\frac{1}{2}=26.56\)
4. What is the necessary condition for a helical antenna to operate in the axial mode?
a) πD << λ
b) πD ≈ λ
c) πD ≈ nλ
d) πD=0
View Answer
Explanation: In Normal mode: πD << λ
Axial mode: C ≈ λ
Conical mode: C ≈n λ
D=0 then there is no loop and is no longer helical.
5. What is the pitch angle of the helical antenna to become a linear antenna?
a) 90
b) 0
c) 45
d) 60
View Answer
Explanation: C is the circumference and L is the length, s is the spacing between turns.
When pitch angle α=90, the separation between turns becomes parallel to length (tending to infinity pitch angle) hence it acts as a linear antenna.
6. What is the Input impedance of the axial feed helical antenna with circumference per unit wavelength is 1.2?
a) 168Ω
b) 186Ω
c) 198Ω
d) 158Ω
View Answer
Explanation: Input impedance of helical antenna with axial feed is given by \(Z=140 \frac{C}{λ}\)
\(Z=140 \frac{C}{λ}=140×1.2=168\Omega\)
7. What is the total axial length of the helical antenna with 5 turns and circumference per wavelength is 1.2 and separation between turns is 2 cm?
a) 10cm
b) 20cm
c) 6cm
d) 12cm
View Answer
Explanation: The total axial length of the helical antenna with N turns and separation distance between the turns S is given by A= NS
A=NS=5 * 2 = 10cm
8. The Axial ratio of the helical antenna with 3 turns is _____
a) 1.11
b) 0.89
c) 6
d) 0.98
View Answer
Explanation: The axial ratio of helical antenna is given by \(AR = \frac{2N+1}{2N} \)
\(AR = \frac{2N+1}{2N} = \frac{7}{6} = 1.11\)
9. Find the number of turns in the helical antenna with directivity 120 and Cλ=1 and pitch angle 6°.
a) 100
b) 50
c) 120
d) 12
View Answer
Explanation: Directivity of helical antenna D = 12CλnSλ
Pitch angle \(α=tan^{-1}\frac{S_λ}{C_λ}\)
⇨ Sλ=Cλ tan α=1*0.1=0.1
D = 12CλnSλ
120=12×1×n×0.1
n=100 turns
10. Which of the following type does helical antenna belongs to?
a) Wire type
b) Aperture
c) Reflector
d) Array
View Answer
Explanation: Helical antenna belongs to wire type antenna. It is in the form of the helix and operates in the VHF and UHF. It is mainly a circular polarized antenna.
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