Electromagnetic propagation and antennas — match each concept with its best description. List I (Concept) A. Evanescent wave B. Skip distance C. Return loss D. Antenna array List II (Description) 1. Pattern multiplication principle (array factor × element pattern) 2. Occurs on a mismatched transmission line; measure of reflected power 3. Propagation in a lossy medium with attenuation constant 4. Short-wave (HF) ionospheric propagation parameter (no ground reception inside skip zone) 5. Propagation below cutoff in a guide; fields decay exponentially

Introduction:
This matching item spans wave propagation, transmission-line mismatch metrics, and antenna-array fundamentals. Mastering these associations is essential for RF design, microwave engineering, and antenna systems.

Given Data / Assumptions:

• Evanescent waves arise when a structure operates below cutoff or at total internal reflection beyond the critical angle.
• Skip distance is an HF (short-wave) ionospheric parameter indicating the nearest ground point of skywave return.
• Return loss quantifies mismatch using reflected/incident power ratio.
• Antenna arrays leverage the pattern multiplication principle.

Concept / Approach:

Choose the description that uniquely identifies each concept: exponential field decay without net power flow along the propagation direction for evanescence; HF skywave geometry setting the skip zone for skip distance; mismatch metric on lines for return loss; and multiplication of array factor with element pattern for arrays.

Step-by-Step Solution:

Verification / Alternative check:

Standard EM textbooks define evanescence as exponential attenuation in classically forbidden regions; antenna texts derive array patterns as product of element pattern and array factor; and transmission-line handbooks define return loss as −20 log10|Γ|.

Why Other Options Are Wrong:

• Relating return loss to HF skip (4) confuses propagation with line mismatch.
• Assigning arrays to lossy media (3) misstates the array principle.
• Evanescent ≠ lossy; attenuation here is geometric/boundary-induced, not purely material loss.

Common Pitfalls:

Mixing up return loss with VSWR or mismatch loss; they are related but not identical. Also, evanescent fields can exist in lossless media due to boundary conditions, not because the medium is lossy.

Final Answer:

A-5, B-4, C-2, D-1