Deep Dive: G3A10
The correct answer is C: Rotation of the Sun's surface layers around its axis. What causes HF propagation conditions to vary periodically in a 26- to 28-day cycle is the rotation of the Sun's surface layers around its axis. The Sun rotates, bringing active regions (sunspots, flares) into and out of view, creating a cycle. For amateur radio operators, this explains why propagation varies on a monthly cycle. Understanding this helps predict propagation patterns.
Why Other Answers Are Wrong
Option A: Incorrect. Long-term oscillations in the upper atmosphere aren't the cause - the cycle is related to solar rotation, not atmospheric oscillations. Option B: Incorrect. Cyclic variation in Earth's radiation belts isn't the cause - the cycle is related to solar rotation, not radiation belts. Option D: Incorrect. The Moon's orbital position isn't the cause - the cycle is related to solar rotation (26-28 days), not the Moon (28-day cycle is coincidental).
Exam Tip
26-28 day propagation cycle = Sun's rotation. Think '2'6-28 days = 'S'un 'R'otation 'C'ycle. Rotation of Sun's surface layers around its axis brings active regions into view, creating 26-28 day propagation cycle. Not atmosphere, radiation belts, or Moon - just Sun rotation.
Memory Aid
26-28 day propagation cycle = Sun's rotation. Think '2'6-28 days = 'S'un 'R'otation. Rotation of Sun's surface layers around its axis brings active regions into view, creating periodic propagation variations. Key cause of monthly propagation cycles.
Real-World Example
A sunspot group appears on the Sun. As the Sun rotates (about 27 days), the sunspot group moves across the face of the Sun, affecting propagation. When it's facing Earth, propagation is better. When it rotates away, conditions may decline. This solar rotation creates the 26-28 day propagation cycle.
Source & Coverage
Question Pool: 2023-2027 Question Pool
Subelement: G3A
Reference: 2023-2027 Question Pool · G3 - Radio Wave Propagation
Key Concepts
Verified Content
Question from the official FCC General Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the G3A topic.