EMC Question of the Week: April 14, 2025
The main EMC-related advantage of twisting a pair of parallel wires is that it reduces the
- far-field radiation
- near-field coupling
- inductance per unit length
- all of the above
Answer
The best answer is “b.” A twisted wire pair is much less susceptible to electric- and magnetic-field coupling than an untwisted wire pair with the same wire diameter and separation. That is because the voltages coupled from uniform incident fields to adjacent twist-loops tend to cancel each other. Likewise, the coupling from adjacent loops to other nearby circuits or objects tends to cancel. This can be particularly important for reducing the coupling between signals on wires sharing the same wiring harness.
Twisting a wire pair rarely has a noticeable effect on far-field radiation because significant radiated emissions are almost always due to the common-mode current on the wires. And twisting wires generally has very little impact on the common-mode currents.
Twisting wires does reduce the end-to-end length a little (typically less than a few percent). This means both the inductance per unit length and the capacitance per unit length are increased slightly. However, it has no impact on the characteristic impedance, and the slight increase in length is not inherently an EMC advantage or a disadvantage.
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