How is characteristic impedance defined in a transmission line context?

• A. The inherent impedance of the cable that should remain constant along the length to prevent reflections.
• B. The impedance measured at the connector only
• C. The impedance of the power supply
• D. The resistance of the conductor at DC

Answer: (A)

This question hinges on understanding what characteristic impedance means for a transmission line. It’s the intrinsic impedance that a uniform section of cable presents to a traveling signal, determined by the line’s geometry and the dielectric between conductors. Because it depends only on the physical construction, it should remain constant along the length of a uniform line to avoid reflections.

When the source and load impedances are matched to this characteristic value, the traveling wave sees a continuous impedance and reflections at any point are minimized, preserving signal integrity. In practical terms, the characteristic impedance is a property of the cable itself (not of the power supply or a single connector) and it is an AC concept, not the DC resistance of the conductor.