How far can sky wave reception in the medium frequency band reach at night?

In the medium frequency (MF) band, particularly for frequencies around 1 MHz to 3 MHz, sky wave propagation occurs due to the reflection of radio waves off the ionosphere. At night, the ionosphere becomes more conducive to sky wave propagation because the D layer, which absorbs lower frequency signals during the day, diminishes, allowing higher frequency signals to refract back to Earth over much longer distances.

While ground wave propagation is limited to a few tens of kilometers depending on the frequency and other conditions, sky wave reception can extend significantly beyond this range. Nighttime conditions can allow reception distances to reach thousands of kilometers.

The correct answer reflects the maximum potential distance for MF sky wave reception at night, primarily driven by the capabilities of the ionosphere to refract signals back to Earth. While distances like 400 kilometers and 800 kilometers fall within expected range during optimal conditions, and 24 kilometers pertains to ground wave propagation, it is 12,870 kilometers that argues for the theoretical maximum distance, which encompasses the global reach of radio wave propagation. This figure is a representation of high-performance scenarios potentially achievable under optimal ionospheric conditions, often referred to in radio communications literature.