Assertion–Reason on reciprocity and S-parameters Assertion (A): For a reciprocal microwave junction, the scattering matrix S is symmetric. Reason (R): In a symmetric matrix, S_ij = S_ji by definition.

Introduction / Context:
Scattering parameters (S-parameters) describe how multiport microwave networks reflect and transmit signals. Reciprocity is a physical property of many passive networks composed of linear, bilateral materials. Understanding how reciprocity constrains the S-matrix is core to network analysis and measurements.

Given Data / Assumptions:

• Linear, time-invariant, passive, and bilateral network (no magnetized ferrites or active nonreciprocal elements).
• Standard S-parameter definitions referenced to equal port impedances.

Concept / Approach:
For a reciprocal network, the transmission from port i to port j equals that from port j to port i when referenced properly. In matrix form, this implies S = S^T (i.e., S_ij = S_ji), so the S-matrix is symmetric. The reason statement merely defines what a symmetric matrix means; it does not itself link reciprocity to symmetry. Therefore A is true, R is true, but R does not explain why reciprocity enforces symmetry—R only restates the property of symmetry.

Step-by-Step Solution:

Verification / Alternative check:
Microwave textbooks prove symmetry using Lorentz reciprocity theorem applied to fields and ports. The diagonal terms are not constrained by reciprocity, but off-diagonals must match pairwise.

Why Other Options Are Wrong:

• A: True; B captures that R does not provide the causal link.
• C/D: Incorrectly deny the truth of either the assertion or the reason.

Common Pitfalls:
Assuming nonreciprocal devices (circulators, isolators) also have symmetric S; they do not because reciprocity is violated.

Final Answer:
Both A and R are correct but R is not correct explanation of A