When atmospheric duct propagation (super-refractive or trapping layers) exists over the path, how do the usual “line-of-sight” and “diffraction zone” concepts apply to radio links?

Introduction / Context:
Atmospheric ducting occurs when refractivity decreases sharply with height, creating a “duct” that traps and guides radio waves over long distances. This phenomenon extends coverage far beyond the optical horizon and invalidates many standard assumptions of free-space, line-of-sight links.

Given Data / Assumptions:

• Presence of a surface or elevated duct (super-refraction).
• Frequencies typically in VHF/UHF/microwave bands where ducting is relevant.
• Standard diffraction models (e.g., knife-edge) assume no trapping.

Concept / Approach:

In ducts, rays are bent back toward Earth, undergoing repeated reflections/refractions inside the layer. The path is no longer limited by geometric line-of-sight, and field strength predictions based on simple diffraction from obstacles are unreliable. Propagation becomes guided, resembling over-water or waveguide-like behavior with mode structure.

Step-by-Step Solution:

Verification / Alternative check:

Field reports show anomalous long-distance reception during ducting events; empirical models (e.g., ITU-R recommendations) treat such cases separately from standard line-of-sight or diffraction calculations.

Why Other Options Are Wrong:

• Unchanged applicability: contradicts guided behavior.
• “Only diffraction” or mixed statements: do not capture trapping mechanism.

Common Pitfalls:

Applying free-space path loss and Fresnel-zone clearance rules to ducted paths; ignoring strong frequency selectivity and mode interference.

Final Answer:

Both line-of-sight and diffraction-zone concepts become less applicable because energy is trapped and guided beyond the optical horizon