Short wave is the only long-distance communication mode that is not limited by network hub and active relay system. In the event of war or disaster, all kinds of communication networks may be destroyed and satellites may be attacked. No matter which communication mode, its survivability and autonomous communication ability are incomparable to short wave;

Second, in mountainous areas, Gobi, oceans and other areas, ultrashort wave coverage is not enough, mainly relying on short wave;

Third, compared with satellite communication, short-wave communication does not need to pay telephone bills, and its operating cost is low.

Because the height and density of ionosphere are easily influenced by factors such as day and night, season and climate, short-wave communication is unstable and noisy. At present, it is widely used in telegraph, telephone, low-speed fax communication and broadcasting.

Although new radio communication systems are constantly emerging, short wave, an ancient and traditional communication method, is still widely valued all over the world, not only has it not been eliminated, but also has developed rapidly.

The working frequency band of short wave is determined as 1.6MHz-30MHz.

1.4 short wave propagation path

There are two basic modes of short wave propagation: one is ground wave and the other is sky wave.

As mentioned above, the ground wave propagates along the earth's surface, and its propagation distance depends on the characteristics of the surface medium. The conductive characteristics of the sea surface medium are most conducive to the propagation of electric waves, and short-wave ground wave signals can travel about 1000 km along the sea surface; The conductivity of land medium is poor, and the radio wave attenuation is large. The radio wave attenuation of different land medium is different (the attenuation of wet land surface is small, and the attenuation of dry sand surface is large). Short-wave signals can only travel along the ground for dozens of kilometers at most. Ground wave propagation does not need to change the working frequency frequently, but it is different from sky wave propagation because of the obstruction.

The main transmission route of short wave is sky wave. After the short-wave signal is sent by the antenna, it is reflected back to the ground through the ionosphere, and then reflected back to the ionosphere from the ground. It can be reflected many times, so it travels far (hundreds to tens of thousands of kilometers) and will not be blocked by ground obstacles. But the sky wave is very unstable. In the process of sky wave propagation, factors such as path attenuation, time delay, atmospheric noise, multipath effect and ionospheric fading will cause signal attenuation and distortion, which will affect the effect of short-wave communication.