Question E3B09:
At what time of year is sporadic-E propagation most likely to occur?  

Choices:  
A: Around the solstices, especially the summer solstice  
B: Around the solstices, especially the winter solstice  
C: Around the equinoxes, especially the spring equinox  
D: Around the equinoxes, especially the fall equinox    

Correct Answer is  A: Around the solstices, especially the summer solstice  

To calculate the ERP:  
$$ERP = P_T \times G_A$$  
where:
- $P_T$ = transmitter power in watts
- $G_A$ = antenna gain (linear)

This shows how to compute the effective radiated power for the given setup.

1) Why the correct answer is correct (A)
- Sporadic-E (Es) is most common near the solstices, with a strong peak around the summer solstice (late May through early August in the Northern Hemisphere).  
- Atmospheric wind shear and tidal effects in the lower ionosphere concentrate metallic ions (from meteors) into dense E-layer patches around 90–130 km, creating high MUFs that support VHF paths (often 10 m and 6 m).

2) Why the other choices are incorrect
- B (winter solstice): There is a secondary, weaker “mid-winter” Es peak (Dec–Jan), but it is not the most likely period. Summer is the dominant season.
- C and D (equinoxes): Equinoxes often favor F-layer HF propagation, not Es. Sporadic-E is not primarily tied to equinoctial solar geometry; it’s more linked to wind shear dynamics that peak near solstices.

3) Useful formulas and rules of thumb
- MUF for a given path and layer:
  $$MUF \approx f_c \sec \theta$$
  where $f_c$ is the layer’s critical frequency and $\theta$ is the ray’s incidence angle to the layer normal. Es significantly raises the effective $f_c$, enabling MUFs above 28–50 MHz.
- Typical Es single-hop distances: about 600–1400 miles (1,000–2,300 km).
- Es “season” (Northern Hemisphere): strongest late May–early August; minor peak late December–early January.
- Most common times of day: late morning and late afternoon/early evening.
- Bands commonly supported: 10 m and 6 m; occasionally higher.