Assertion (A): A lossless open-circuited transmission-line stub of length slightly greater than λ/4 behaves as an inductive reactance at its input. Reason (R): A lossless short-circuited line of length slightly greater than λ/4 behaves as a capacitive reactance at its input.

Introduction / Context:
Transmission-line stubs are routinely used as reactive elements in matching networks. Whether a stub looks inductive or capacitive depends both on its termination (open or short) and its electrical length relative to a quarter wavelength. This assertion reason pair probes whether you recall the sign changes around the quarter-wave point.

Given Data / Assumptions:

• Lossless line with characteristic impedance Z0 and propagation constant β.
• Open and short terminations considered independently.
• Electrical length slightly greater than λ/4 so βl is a little larger than π/2.

Concept / Approach:

For a lossless line: Z_in(open) = −j Z0 cot(βl), and Z_in(short) = j Z0 tan(βl). Near βl = π/2, the signs of cot and tan change, flipping the reactive sense. Just past λ/4, an open stub becomes inductive while a shorted stub becomes capacitive. These are both true facts, but they do not explain each other because they refer to different boundary conditions.

Step-by-Step Solution:

Verification / Alternative check:

A Smith chart shows an open stub crossing from capacitive to inductive at exactly λ/4, while a shorted stub crosses from inductive to capacitive at the same point. This geometric visualization confirms the algebraic reasoning.

Why Other Options Are Wrong:

• A only or R only: both statements are standard results from the lossless line equations.
• Claiming R explains A: incorrect because explanation requires analysis of the open stub itself.
• Both wrong: contradicts basic transmission-line theory used in every RF textbook.

Common Pitfalls:

Forgetting which sign applies to open versus short terminations, and overlooking that the sign flips exactly at the quarter-wave length. Another common mistake is to think losses create resistance at these lengths; in the ideal lossless case, input impedance is purely reactive.

Final Answer:

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