Which values define the recognised coax impedance?

• A. 25 ohm, 50 ohm
• B. 60 ohm, 75 ohm
• C. 50 ohm, 75 ohm
• D. 40 ohm, 90 ohm

Answer: (C)

Coaxial cables have a characteristic impedance that defines how signals propagate and how reflections behave when the line meets equipment. In practice, two impedance values are standard and widely recognized: 50 ohms and 75 ohms.

Fifty-ohm coax is common for RF and wireless-type applications, where power handling and low loss over longer runs are important. Seventy-five-ohm coax is the standard for video and CATV systems, where different signal characteristics come into play. Other numbers like 25 ohms, 60 ohms, 40 ohms, or 90 ohms aren’t generally used as the standard coax impedance in typical installations, so they aren’t seen as the recognized values.

Choosing the correct impedance helps ensure maximum power transfer and minimizes reflections that can distort the signal. That’s why the commonly recognized coax impedances are 50 ohms and 75 ohms.