Antennas Multiple Choice Questions on “Baluns”.
1. What is a Balun?
a) It is used to balance the unbalanced systems
b) It unbalances the balanced systems
c) A twisted wire
d) Main beam of antenna with large beam width
Answer: a
Clarification: A Balun is a device which connects a balanced two –conductor line to an unbalanced coaxial line. It eliminates field mismatch. The current distribution is present in the inner conductor and is zero at the outer conductor.
2. A Balun is used to make the current along the outer side of the outer conductor along the coaxial cable _______
a) Zero
b) Maximum
c) Maximum or minimum depends on power supply
d) Infinity
Answer: a
Clarification: When a coaxial cable is connected to a half wave dipole, the current distribution along the outer side of the outer conductor may also present. It results in loss and field mismatch. So a Balun is used to make the current at the outer side of the outer conductor zero. Current distribution is present at the inner side of the outer conductor.
3. The process of forcing the current at the outer side of the outer conductor to be zero is called _____
a) Current distribution
b) Current chokes
c) Field effect
d) Impedance chokes
Answer: a
Clarification: A Balun is a device which connects a balanced two –conductor line to an unbalanced coaxial line. It eliminates field mismatch. It forces the current at the outer side of outer conductor to be zero. This is called current choke.
4. A Balun joins a balanced line and an unbalanced line.
a) True
b) False
Answer: a
Clarification: A Balun joins a balanced line which is usually a two conductor’s twisted cable with equal current distribution to an unbalanced line with one conductor with unequal current distribution and vice-versa. It acts like a transformer.
5. All Baluns provide impedance transformation.
a) True
b) False
Answer: b
Clarification: A 1:1 Balun doesn’t provide any impedance transformation. Some Baluns provide impedance transformation like 1: 4 or 9:1.
6. For a 4: 1 Balun, what is the unbalanced impedance if the balanced impedance is 2KΩ?
a) 8K
b) 0.5K
c) 4K
d) 2K
Answer: b
Clarification: A 4: 1 Balun implies its impedance ratio. (frac{Z_{bal}}{Z_{unbal}} = frac{4}{1} )
Unbalanced impedance = 2K/4=0.5K.
7. Among current Balun and voltage Balun, which works better?
a) Current Balun
b) Voltage Balun
c) Both work equally
d) Depends the power supply
Answer: a
Clarification: A current Balun offers better balance and can tolerate load impedances and balance variations better compared to Voltage Balun. In a current Balun the output terminal voltage can be of any value to make the currents equal in the feed line.
8. Which of the following is false regarding Transformer Balun?
a) It has a narrow band frequency
b) It has infinitely wide band frequency
c) Ideally it provides zero insertion loss
d) Impedance matching is adjusted by its turn ratio
Answer: a
Clarification: A transformer Balun has ideally infinite wide band frequency and provides zero insertion loss. By changing the number of turns we can modify the impedance only when the input and output impedances are only resistive.
9. The phase difference between the outputs of Balun in frequency domain is ____
a) 180°
b) 120°
c) 60°
d) 90°
Answer:a
Clarification: A Balun can be viewed as a three port device, with matched input and a differential output. The differential outputs are equal and opposite. So, they are 1800 out of phase with respect to each other.
10. For an unbalanced to balanced signal conversion, if the turn’s ratio in a Balun is 1:2 then the balanced impedance is ______ times the unbalanced impedance.
a) 4
b) 2
c) (frac{1}{2} )
d) (frac{1}{4} )
Answer: a
Clarification: A Balun can be viewed as a transformer. (frac{Z_{unbal}}{Z_{bal}} = (turns ,ratio)^2=(frac{1}{2})^2=frac{1}{4} )
⇨ Zbal = 4Zunbal
So the balanced impedance is four times the unbalanced impedance.