EMC Question of the Week: January 1, 2018

Four Twisted Wire Pairs

Twisting a pair of parallel wires,

  1. increases the characteristic impedance of the transmission line
  2. decreases the characteristic impedance of the transmission line
  3. increases the inductance per unit length of the transmission line
  4. decreases the inductance per unit length of the transmission line

Answer

The best answer is "c". As long as the spacing between the wires remains the same, twisting a wire pair has no significant impact on the characteristic impedance. The inductance and capacitance of the wire pair are also unchanged by twisting; however twisting a pair of wires reduces the end-to-end length of the transmission line. Therefore, the inductance per unit length and the capacitance per unit length of a wire pair are increased slightly by twisting the wires together. The "twist factor" used to adjust parallel wire calculations of per-unit-length quantities to twisted wire per-unit-length quantities is typically less than 2%.

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