# Microwave Engineering Questions and Answers – Coupled Line Directional Couplers

This set of Microwave Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Coupled Line Directional Couplers “.

1. In coupled line directional couplers, power from one line to another is transmitted through a microstrip line running between them.
a) true
b) false

Explanation: In coupled line couplers, the power is transmitted between the 2 lines by coupling from one line to another due to the interaction of the electromagnetic fields. Hence, wireless power transmission occurs here.

2. The number of conductors used in the construction of coupled line couplers is fixed.
a) true
b) false

Explanation: Since the method of power transmission in coupled line couplers is wireless power transmission by the interaction of electromagnetic fields, any number of wires can be used. But as a standard, 3 lines are used in the construction of these couplers.

3. The mode of propagation of propagation supported by coupled line couplers is:
a) TM mode
b) TE mode
c) TEM mode
d) quasi TEM mode

Explanation: Coupled transmission lines are assumed to operate in TEM mode of propagation. TEM mode of propagation is mostly valid for coaxial and stripline structures while microstrip lines support quasi TEM mode of propagation.

4. Coupled line couplers are:
a) symmetric couplers
b) asymmetric couplers
c) in phase couplers
d) type of hybrid coupler

Explanation: Coupled line couplers are symmetric three line couplers. Symmetric here means that the lines are of equal width and thickness. Their position with respect to ground is identical.

5. For coupled line coupler, if the voltage coupling factor is 0.1 and the characteristic impedance of the microstrip line is 50 Ω, even mode characteristic impedance is:
a) 50.23 Ω
b) 55.28 Ω
c) 100 Ω
d) 80.8 Ω

Explanation: Even mode characteristic impedance of coupled line coupler is Z0√ (1+C) /√ (1-C).here C is the voltage coupling coefficient. Substituting the given values, even mode characteristic impedance is 55.28 Ω.
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6. If the coupling coefficient of a coupled line coupler is 0.1 and the characteristic impedance of the material is 50 Ω, then the odd mode characteristic impedance is:
a) 45.23 Ω
b) 50 Ω
c) 38 Ω
d) none of the mentioned

Explanation: Odd mode characteristic impedance of a coupled line coupler is Z0√ (1-C)/ √ (1+C). C is the voltage coupling co-efficient. Substituting the given values, odd mode characteristic impedance is 45.23.

7. Dielectric and conductor loss have no effect on the directivity of the coupled line coupler.
a) true
b) false

Explanation: Both dielectric loss and conductor loss reduce the directivity of the coupled line coupler. In the absence of loss under matched conditions, the directivity of a coupler could be up to 70 dB.

8. Multisection couplers have a very narrow operational bandwidth which is a major disadvantage.
a) true
b) false

Explanation: Multisection couplers have very high operational bandwidth. This high bandwidth can be achieved only when the coupling levels are low. In order to achieve these low coupling levels, stripline are used in their construction.

9. Three section binomial couplers have very low directivity as compared to other coupler designs.
a) true
b) false

Explanation: Three section binomial couplers have very low conductor and dielectric losses. This low loss can be achieved by efficient design. Since the losses are low for a binomial coupler, they have directivity greater than 100 dB.

10. The capacitance per unit length of broadside parallel lines with width W and separation d is:
a) ∈W/d
b) ∈d/W
c) dW/∈
d) none of the mentioned