High-Frequency Line Parameters: Inductive Reactance vs AC Resistance Assertion (A): At high frequencies, the line's inductive reactance is very large compared to its ac resistance. Reason (R): Due to skin effect, the ac resistance of a conductor is higher than its dc resistance.

Introduction / Context:
Distributed-parameter lines exhibit frequency-dependent reactance and resistance. This item tests understanding of how inductive reactance and skin-effect resistance scale with frequency and whether one explains the other.

Given Data / Assumptions:

• Inductance per unit length L is approximately constant with frequency over a wide range.
• Inductive reactance X_L = ω * L increases linearly with frequency.
• AC resistance R_ac rises with frequency due to skin effect.

Concept / Approach:

At high frequencies, X_L commonly exceeds R_ac, so the line is more inductive than resistive. Skin effect makes R_ac > R_dc, but that fact does not cause X_L to be large; rather, X_L is large because ω is large. Thus A is true, R is true, but R does not explain A.

Step-by-Step Solution:

Verification / Alternative check:

Measurements on coax or twin-lead at RF show reactance dominating series resistance in many bands, despite rising R_ac.

Why Other Options Are Wrong:

Option A: Incorrect causal link. Option C/D/E conflict with correctness of A and R.

Common Pitfalls:

Attributing frequency behavior of reactance to resistive mechanisms; ignoring that both R_ac and X_L increase with frequency but for different physical reasons.

Final Answer:

Both A and R are correct but R is not correct explanation of A