Which of the following descriptions about the second Fresnel zone is correct? (Multiple choice)

Question

Options

• AOut of phase with point source
• BIn phase with point source
• CSmaller than the first-Fresnel zone
• DGreater than the first-Fresnel zone

• A
  70% (42)
• B
  12% (7)
• C
  18% (11)

Explanation

Fresnel zones are concentric ellipsoidal regions around the direct path between a source and receiver, where each successive zone adds an extra half-wavelength of path length. The second Fresnel zone's waves arrive approximately half a wavelength out of step with those from the first zone, meaning they are out of phase with the direct (point source) signal and cause destructive interference - confirming A. Each zone's outer radius scales as √n·r₁, so the second zone's outer boundary (r₂ ≈ 1.41·r₁) extends beyond the first, making the second zone greater in size - confirming D.

Why the distractors are wrong:

• B (in phase): Being in phase with the source is the property of odd-numbered zones (1st, 3rd, 5th…), not even ones. Even zones interfere destructively.
• C (smaller): Each successive zone grows outward; no Fresnel zone is smaller than the one inside it.

Memory tip: Think "ODD = OK (constructive), EVEN = Enemy (destructive)." The second zone (even) fights the source signal and sits outside the first, so it's both out of phase and larger.

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