Explain the meaning and technical standards of medium wave, short wave and frequency modulation.

Medium wave and short wave are frequency modulated according to wavelength, and high frequency is used to modulate low frequency, so that signals can spread. Far-ordinary medium wave broadcasting (MW) adopts amplitude modulation, and unconsciously, MW and AM are equated. In fact, microwave is just a kind of broadcast using amplitude modulation. For example, the modulation mode used by international short-wave broadcasting at high frequency (3-30MHz) is AM, and even aviation navigation communication with higher frequency than FM broadcasting (116-136 MHz) uses AM, but the AM band we refer to in our daily life refers to medium-wave broadcasting (MW medium-wave propagation). When a radio wave touches a conductor, it will induce a current in the conductor, thus losing some energy. This phenomenon of weakening the energy of radio waves is called radio wave absorption. The earth is a conductor and absorbs medium waves very strongly, so medium waves propagating in the form of ground waves will not travel very far (about two or three hundred kilometers). During the day, due to sunlight, the density of the ionosphere increases, making the ionosphere a good conductor. A small part of the medium wave propagating in the form of sky wave is strongly absorbed when it enters the ionosphere, and it is difficult to return to the ground. In addition, the medium wave propagating in the form of ground wave will be absorbed by the earth and will not travel far, which makes it difficult to receive the distant medium wave radio station during the day. At night, the atmosphere is no longer irradiated by sunlight, and the electrons and ions in the ionosphere compound with each other and increase significantly, so the ionosphere becomes thinner, the density becomes smaller, the conductivity becomes worse, and the absorption of radio waves is greatly weakened. At this time, the medium wave can spread far away by sky wave. Therefore, the medium-wave radio station receiving at night has more China medium-wave frequency range: 526.5kHz~ 1606.5kHz, and medium-wave transmission bandwidth: 9kHz short wave/view/26475.htmfm/view/42022.htm.