# Antennas Questions and Answers – Construction of Yagi – Uda

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This set of Antennas Multiple Choice Questions & Answers (MCQs) focuses on “Construction of Yagi – Uda”.

1. The length of the director compared to the driven element is ______
a) Greater
b) Smaller
c) Independent to each other
d) Depends on the type of driven element

Explanation: The length of the director is less than the driven element. It is shorter than half wavelength of the dipole. Director is used to increase the gain of antenna.

2. The length of the reflector compared to the driven element is ______
a) Greater
b) Smaller
c) Independent to each other
d) Depends on the type of driven element

Explanation: Reflector is used to reflect all the energy towards the radiation direction. It increases the front to back lobe ratio. So always the reflector length is greater than the driven element length.

3. The distance between directors of Yagi-Uda antenna is ____
a) 0.2λ
b) 0.49λ
c) 0.52λ
d) 0.62λ

Explanation: The distance between directors of Yagi-Uda antenna is 0.2λ. The directors will increase the gain.
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4. What is the approximate distance between directors for a Yagi-Uda antenna operating at frequency 150MHz?
a) 0.4m
b) 1.6m
c) 2m
d) 4.8m

Explanation: λ = $$\frac{c}{f} = \frac{3×10^8}{150MHz}=2m$$ The distance between directors of Yagi-Uda antenna is 0.2λ
⇨ D=0.2λ=0.2(2)=0.4m

5. If length of the reflector, driven element, director islr, lde, ld, respectively then the order in the increasing order of their lengths is _____
a) lr, lde, ld
b) ld, lde, lr
c) lde, lr, ld
d) lr, lde, lde

Explanation: The lengths of reflector, driven element, director is in the orderlr > lde > ld.
So, the increasing order is ld, lde, lr .

6. For a feeder length of 0.5λ, which among the following would be the reflector length?
a) 0.525λ
b) 0.425λ
c) 0.23λ
d) 0.35λ

Explanation: The reflector length is approximately 5% greater than the driven element length.
So, l = 0.525λ is correct as it is the only option which is greater than 0.5λ.

7. If the input impedance of a half wave dipole Yagi-Uda is 73Ω, then input impedance of a half wave folded dipole as driven element is ____
a) 73Ω
b) 146Ω
c) 36.5Ω
d) 292Ω

Explanation: Half wave folded dipole impedance is 4 times the half wave dipole.
⇨ Z = 4 × 73 = 292Ω.

8. What is the reflector length (in feet) for a three element Yagi-Uda array, operating at a frequency 500MHz?
a) 2
b) 0.2
c) 1
d) 0.1

Explanation: For a 3 element Yagi-Uda array,
Reflector length (feet) = 500/f (MHz) = 500/500 = 1 feet

9. What is the driven length (in feet) for a three element Yagi-Uda array, operating at a frequency 500MHz?
a) 0.95
b) 0.2
c) 1
d) 2

Explanation: For a 3 element Yagi-Uda array,
Driven element length (feet) = 475/f (MHz) = 475/500 = 0.95 feet.

10. What is the Director length (in feet) for a three element Yagi-Uda array, operating at a frequency 500MHz?
a) 0.91
b) 1
c) 2
d) 0.5

Explanation: For a 3 element Yagi-Uda array,
Director element length (feet) = 455/f (MHz) = 455/500 = 0.91 feet.

11. For a feeder length of 0.5λ, which among the following would be the director length?
a) 0.525λ
b) 0.455λ
c) 0.493λ
d) 0.55λ

Explanation: The director length is approximately 5% less than the driven element length.
⇨ 0.5-(0.5(5/100)) = 0.475
So, l = 0.455λ is correct as it is the only option which is less than 0.475λ.

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