Shortwave radio services may change frequency often as the ionosphere's reflectivity varies. Unlike domestic broadcast stations that stay on a single assigned frequency, shortwave broadcast stations move frequency to take advantage of hour-to-hour and season-to-season changes in the ionosphere. As the 11-year sunspot cycle waxes and wanes, shortwave stations the world around move to shorter wavelengths when there are more sunspots and to longer wavelength bands when sunspots are minimal.
Listening to shortwave radio requires more effort than istening to local domestic radio. The best frequencies to search change from one hour to the next throughout the day. Due to the effect of the sun, shortwave signals sometimes may disappear for days at a time, then reappear with astounding strength. Many shortwave stations do not broadcast at all hours of the day. In addition, a station must be targeting your part of the world specifically; otherwise the signal will probably be weak.
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