Loaded transmission line: the overall attenuation–frequency characteristic most closely resembles which filter class?

Introduction:
“Loading” a line (e.g., adding series inductance at intervals) was historically used in telephony to equalize amplitude/phase, producing a passband up to a cutoff. This question checks recognition that the attenuation characteristic then resembles a low-pass filter.

Given Data / Assumptions:

• Periodic or lumped inductive loading on a transmission line.
• Objective: reduce distortion over voice band and set a finite cutoff.
• Small dielectric and copper losses assumed relative to loading effect.

Concept / Approach:

Loading increases effective series inductance L′, flattening group delay and reducing attenuation in the voice band, but introduces a cutoff frequency. Above this frequency, attenuation rises steeply, mirroring a low-pass filter response.

Step-by-Step Solution:

Verification / Alternative check:

Classical Campbell/Heaviside loaded-line theory maps directly to low-pass ladder sections with definable cutoff ωc.

Why Other Options Are Wrong:

• High-pass/band-pass/band-stop: do not match the monotonic low-frequency passband and high-frequency stop behavior of a loaded line.
• All-pass: does not model attenuation shaping.

Common Pitfalls:

Assuming loading “boosts highs” — it actually limits high-frequency transmission, acting as low-pass.

Final Answer:

low-pass filter