EMC Question of the Week: June 18, 2018
The resistance per unit length of a balanced twisted-wire-pair transmission line is equal to
- the resistance per unit length of each wire
- twice the resistance per unit length of each wire
- half the resistance per unit length of each wire
- the characteristic impedance of the transmission line
Answer
The correct answer is "b." Since the current associated with any wave propagating in the wire pair must flow out on one wire and back on the other, the total resistance is the sum of the two wire resistances. This is also true for unbalanced 2-conductor transmission lines, such as coaxial cables. The transmission line's resistance per unit length will always be the sum of the resistances per unit length of each conductor in the pair.
In transmission lines with more than two conductors, the resistance of the wire(s) carrying current in one direction is added to the resistance of the wire(s) carrying current in the other direction. For example, a signal wire in a ribbon cable with return conductors on both sides would have a resistance per unit length equal to the resistance per unit length of the signal wire plus the half the resistance per unit length of one of the return conductors.
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