Lower frequencies, especially AMbroadcasts in the mediumwave and longwave bands (and other types of radio even below that), travel most efficiently this way. This is because they are more efficiently diffracted by the figure of the Earth due to their low frequencies.
Ionospheric reflection is taken into consideration as well. The ionosphere reflects frequencies in a certain band, which often changes due to solar conditions. Because the solar wind "blows" the ionosphere toward the Earth on the day side, and away from it on the night side, this natural radio "mirror" is much closer to the surface during the day. This prevents skywave propagation from being very effective in daylight hours.
At night, mediumwave and shortwave transmissions travel better by skywave.
The use of aerial amplifiers is not always recommended as they tend to increase the amount of received noise or give you so much signal that they overload the input to your receiver, resulting in stations appearing in odd places and sometimes mixing together.
It should be noted that 'groundwave' signals only apply to the long and medium wave bands. Groundwave implies that a signal is travelling along the ground, or at least not much above rooftop level.
Groundwave signals propagate along the air-earth boundary to cover the intended service area, and are attenuated in direct relation to the operating frequency.
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