Which method is commonly used to achieve impedance matching on a transmission line?

• A. Using a wide range of frequencies
• B. Terminating the line with its characteristic impedance
• C. Increasing the source impedance indefinitely
• D. Relying on shielding alone

Answer: (B)

When a signal travels along a transmission line, reflections occur at any impedance mismatch between the line and what it’s connected to. The most reliable way to prevent those reflections is to terminate the line with its characteristic impedance. If the load equals Z0, the line presents a constant impedance to the source, so the incident wave is absorbed by the load rather than sent back as a reflected wave. This minimizes standing waves and delivers maximum power transfer into the load. The characteristic impedance is set by the line’s geometry and materials, and for a nearly lossless line Z0 is approximately sqrt(L/C). The reflection coefficient becomes zero when ZL equals Z0, which is why this termination method is the standard practice for impedance matching.

Other ideas don’t achieve this reliably. Simply increasing the source impedance without matching the load won’t remove the discontinuity at the end of the line. Shielding helps reduce electromagnetic interference but doesn’t alter the impedance that the line presents to the signal. Trying to cover a wide frequency range helps with bandwidth, but it doesn’t by itself solve the issue of reflections from a mismatched end at a given frequency.