Which type of cable provides better protection against EMI?

• A. Shielded twisted pair
• B. Coaxial
• C. Fiber optic
• D. Twisted pair

Answer: (B)

EMI protection comes from creating a barrier that blocks external electromagnetic fields from reaching the signal and keeps the cable from radiating interference. Coaxial cable does this very effectively: a central conductor is surrounded by a dielectric, and an outer metallic shield (foil or braid) sits under the outer jacket. That shield acts like a continuous Faraday cage around the signal, so external noise induces currents on the shield instead of the inner conductor, and when the shield is grounded those noise currents are diverted away. The result is strong suppression of both incoming and emitted interference and a stable impedance for the carried signal. Shielded twisted pair also reduces EMI, but the shielding encloses two conductors and can have seams or gaps that let interference through, so its protection isn’t as uniform as coaxial. Twisted pair, especially unshielded, offers the least EMI protection among these options. Fiber optic avoids electrical interference entirely by transmitting light, but in conventional EMI-talk of metallic cabling, coaxial’s shielded, single-conductor setup is the most robust choice.