EMC Question of the Week: January 1, 2018
Twisting a pair of parallel wires,
- increases the characteristic impedance of the transmission line
- decreases the characteristic impedance of the transmission line
- increases the inductance per unit length of the transmission line
- decreases the inductance per unit length of the transmission line
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%.