So I grew up listening to the radio, and had a transmitter that played stations on LW and SW as well. Over time I completely forgot about radio, until recently. So I searched for resources that allow to play SW stations online. And I noticed that some stations have multiple frequencies which shifts(?) over a particular time period. Why is it so?
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Different frequencies will reflect off of different levels of the atmosphere, which results in them reaching different points on the ground. Which levels reflect which frequencies varies with the position of the sun. So, a station will change frequencies to do one of two things: either continue broadcasting to the same region of the world as the day goes on (or changes to night), or, to reach a different region of the world with programming targeted at that region. (Because the sun is higher, and the days are longer, in the summer versus the winter, stations will also usually have completely different sets of frequencies they operate on during different seasons.)
As a broad rule, higher frequencies are used during the day, and lower frequencies are used during the night. Lower frequencies are also used more often during the winter, not just because of increased darkness but also because of decreased noise from thunderstorms. (Electrical discharges, such as lightning, technically generate noise across the entire spectrum of radio frequencies, but there is a rapid decline in intensity with increase in frequency.)
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