Ham Radio General Class Practice Test

What causes HF propagation conditions to vary periodically in a 28-day cycle?

1. Long term oscillations in the upper atmosphere

2. Cyclic variation in the Earth's radiation belts

3. The Sun's rotation on its axis

4. The position of the Moon in its orbit

The correct answer is: The Sun's rotation on its axis

The variation of HF propagation conditions on a 28-day cycle is influenced primarily by the Sun's rotation on its axis, which leads to periodic changes in solar activity. The Sun has a rotation period of approximately 27 days, and as it rotates, various solar phenomena such as sunspots, solar flares, and coronal mass ejections occur in a cyclical manner. These events have direct effects on the ionosphere, which is crucial for HF radio wave propagation. During times of increased solar activity, the ionosphere becomes more ionized, enhancing the propagation of HF signals. Conversely, during times of low solar activity, there may be less ionization, resulting in poorer propagation conditions. Therefore, as the Sun rotates, the changing atmosphere due to solar activity introduces these periodic variations in HF propagation. The other options do not appropriately account for the primary drivers of the 28-day cycle in HF conditions. Long-term oscillations in the upper atmosphere and the position of the Moon have different cycles and influences, while cyclic variation in the Earth’s radiation belts relates to different aspects of space weather but does not specifically correlate to the consistent 28-day cycle seen linked to solar activity and the Sun's rotation.