# Antennas Questions and Answers – Frequency Independent Antenna – Helical Antenna

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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

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

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

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

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

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

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

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

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

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