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I have a portable radio which serves for FM / MW / SW bands. At nights I can listen to 4-6 international channels from different countries on MW. However, I am unable to tune them at daytime. Is it about my location as a result of earth's spin or there are other reasons that I don't know? Shutting down the radio station at nights comes to my mind but all the stations are from different (meridians) countries.

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During the day, the ionosphere is strengthened by direct exposure to the solar wind, making it thicker and lower relative to the Earth's surface. This prevents any sort of skip propagation at MW frequencies, limiting MW propagation to ground wave only, which only covers a limited range from the transmitter.

At night, the ionosphere is higher and more diffuse, allowing skip propagation to much longer distances.

These effects have been known for a long time, which is why most MW stations are required to reduce power at night in order to reduce interference with distant stations on the same frequency. Only designated "clear-channel" stations are permitted to continue to operate at full power.

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  • $\begingroup$ Or in some cases inverse of the last paragraph is true: Some stations do not broadcast during daytime at all and instead only run at night for regional coverage, leaving daytime coverage to short-range VHF stations. $\endgroup$ – AndrejaKo Sep 3 '16 at 13:47
  • $\begingroup$ This is really interesting. Thank you for sharing this information Dave $\endgroup$ – Karel Capek Sep 3 '16 at 19:16
  • $\begingroup$ I am not familiar with terms like "skip propagation". Can you please tell me what does it exactly mean? And do these signals are sent directly from the location where the stations are based (country etc.)? Or there are radio towers that are interconnenected? $\endgroup$ – Karel Capek Sep 3 '16 at 19:33
  • $\begingroup$ @KarelCapek Comments are not really meant for asking further questions not directly related to the original question that you asked. However, you are welcome to ask it as a separate question. I recommend reading Wikipedia's article on radio propagation first. Amateur radio study material should also discuss how radio waves in different parts of the RF spectrum propagate and which propagation modes produce which results. $\endgroup$ – user Sep 5 '16 at 20:32

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