A ship at anchor has been communicating with a shore station approximately 350 miles distant on a frequency in the 4 MHz band through early afternoon. Toward the late afternoon and evening, what effect should be noticed?
⢠HF (High Frequency) propagation in the 3â30 MHz range ⢠How time of day (sunlight vs night) affects ionospheric layers and absorption ⢠The relationship between frequency, distance (350 miles), and skip distance for sky-wave communication
⢠Ask yourself: In early afternoon, is a 4 MHz signal over 350 miles more likely using ground-wave, sky-wave, or a mix? What happens to that mode after sunset? ⢠How does the ionosphere change from day to night, especially the D-layer (which causes absorption) and F-layer (which does most of the refraction)? ⢠For a path of about 350 miles, do lower HF frequencies generally work better during the day or at night, and does their performance tend to improve or deteriorate after sunset?
⢠Check how D-layer absorption changes from day to night and whether that makes lower HF (like 4 MHz) better or worse after sunset. ⢠Verify whether lower HF frequencies (around 3â5 MHz) are typically used for night-time or day-time NVIS/medium-range communication (~200â500 miles). ⢠Make sure the chosen option matches a gradual change in signal quality as afternoon turns to evening and night, not an abrupt change.
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