In a transmission line, what primarily determines the amount of reflection at a discontinuity?

• A. The mismatch between source impedance and line's characteristic impedance
• B. The length of the line
• C. The ambient temperature
• D. The temperature coefficient of the dielectric

Answer: (B)

The amount of reflection at a discontinuity on a transmission line is determined by impedance mismatch. When the impedance seen at the discontinuity (the load or termination) differs from the line’s characteristic impedance, part of the incident signal is reflected back. The strength of that reflection is quantified by the reflection coefficient Γ = (Z_L − Z0) / (Z_L + Z0). If Z_L equals Z0, Γ is zero and there’s no reflection; any mismatch yields a reflected wave, with the amount increasing as the mismatch grows.

The line’s physical length only affects the phase of the reflected wave (the delay it experiences as it travels back and forth along the line). In a single discontinuity without multiple reflections, the length doesn’t change how much energy is reflected. Temperature effects can slightly alter impedances but don’t determine the reflection magnitude by themselves—they would only matter insofar as they change the impedance mismatch.

So, the primary factor is the impedance mismatch between the line (Z0) and whatever is at the discontinuity (Z_L).