For a transmission line, how does a load mismatch affect the standing wave pattern and reflected power?

• A. Reflections cause standing waves
• B. Reflections only affect impedance, not the standing pattern
• C. Load mismatch reduces radiation efficiency only
• D. Standing waves eliminate reflections

Answer: (A)

The main idea is that a load mismatch on a transmission line causes reflections, and those reflections create a standing wave pattern along the line. When the load impedance is different from the line’s characteristic impedance, part of the incident wave is reflected back toward the source. The forward and reflected waves interfere, producing standing waves with alternating voltage and current maxima (antinodes) and minima (nodes) along the line. How strong this pattern is depends on the reflection coefficient Γ = (ZL − Z0)/(ZL + Z0); a larger mismatch gives a larger Γ, so more power is reflected and the standing-wave pattern becomes more pronounced. The power delivered to the load drops by the reflected portion, while the energy not delivered to the load is reflected back toward the source (and can be radiated if the line is part of an antenna system). If the load is perfectly matched, there are no reflections and no standing waves.

That’s why the statement that reflections cause standing waves is the best description. It isn’t correct to say reflections only affect impedance without altering the standing pattern, and standing waves don’t eliminate reflections.