Despite the continuous emergence of new wireless communication systems, short-wave, the oldest and traditional communication method, is still widely valued worldwide. Today, with the rapid development of satellite communication and mobile communication, short-wave communication has not only been eliminated, but is still developing rapidly. The reasons are: the short-wave communication distance is long, the invulnerability and autonomous communication ability are strong, and the operating cost is low.
Shortwave communication technology development status
In recent years, short-wave communication technology has made great progress worldwide, and many new radio stations, new equipment and new technologies have appeared. Its main features are:
1. Shortwave radio
Shortwave single sideband radios are getting smaller and smaller, with more and more functions, better and better performance, and stronger and stronger compatibility. Digitization is the inevitable development trend of shortwave radio.
2. Shortwave antenna
Shortwave antennas are mainly developed in the direction of broadband, omnidirectional, no "blind zone" and high gain. The volume is getting smaller and smaller, and the efficiency is getting higher and higher. A variety of new base station antennas and car antennas are now available.
3. Frequency selection
In terms of frequency selection, in addition to the widely used ASAPS frequency measurement system and ALE adaptive frequency selection method, a short-wave full-band real-time adaptive frequency selection system and frequency management system are also introduced.
4. Noise elimination
In terms of anti-noise, a variety of noise suppression and noise reduction methods have been introduced, especially the ADSP2 single-ended noise canceller recently launched by SGC in the United States, which can be connected in series to the receiving audio amplifier circuit of any radio station or made into a noise canceling speaker , Eliminate the background noise in the channel, so that the listening quality of short-wave stations can reach or approach the listening level of ultra-short-wave stations.
5. Network communication
In terms of networking communication, in addition to the selective call networking in the adaptive (ALE) function, foreign countries have launched the CCIR493 digital selective call system, which allows each station to be assigned a unique ID code (4 ~ (6 digits), through which it can form a 10,000-level large network. Now the shortwave radio stations produced in Australia and some shortwave radio stations produced in Europe and the United States have been fixed in the whole machine as a regular function. CCIR493 digital selective call system can realize single call, group call, group call, send and receive short messages, transmit GPS positioning signals, transmit alarm signals, and realize short-wave / local telephone network two-way automatic dialing and other functions.
Short-wave communication blind zone and its solution
1. Shortwave propagation
Radio broadcasting, radio communication, television, radar, etc. must be realized by the propagation of radio waves.
The propagation process of radio waves on various media and media interface, due to reflection, refraction, scattering and diffraction, its propagation direction has undergone various changes. Due to diffusion and media absorption, its field strength continues to weaken. In order to make the receiving point have sufficient field strength, it is necessary to grasp the propagation path, characteristics and laws of radio waves to achieve a good communication effect.
Common transmission methods are:
Ground wave (surface wave) propagation, direct wave (line-of-sight) propagation, sky wave propagation, and scattering propagation.
Ultrashort wave is suitable for direct wave propagation method to communicate.
There are two basic propagation methods of shortwave:
A. Ground wave (surface wave) propagation.
B. Sky wave propagation. Skywave propagation is the main transmission method of shortwave communication.
1. Ground wave propagation
Electric waves propagating along the interface between the earth and air are called ground waves or surface waves, or simply ground waves. The ground wave propagation path is shown in Figure 1-1. Its propagation path mainly depends on the electrical characteristics of the ground. During the propagation of the ground wave, part of the energy is absorbed by the ground and is quickly attenuated. The shorter the wavelength, the faster the attenuation, so the propagation distance is not far. However, ground waves are not affected by climate and have high reliability. Generally, ultra-long wave, long wave, and medium wave radio communications utilize ground wave propagation.
2. Sky wave propagation
Sky wave refers to the radio wave that is radiated by the antenna to the high altitude and returned to the ground after being reflected or refracted by the sky ionosphere. The propagation route is shown in Figure 1-2.
Sky waves are the main propagation path of short waves. After the short wave signal is sent by the antenna, it is reflected back to the ground through the ionosphere, and then back to the ionosphere from the ground, which can be reflected many times, so the propagation distance is very long (ten thousand kilometers), and it is not blocked by ground obstacles. But the biggest weakness of sky wave propagation is that the signal is very unstable, and poor processing will affect the communication effect. With the continuous emergence of new radio communication technologies, the impact of sky-wave propagation weaknesses on short-wave communications is gradually being overcome.
3. Communication dead zone
The two propagation modes of ground wave and sky wave have been introduced above. Generally speaking, the propagation distance of the ground wave can reach 20-30 kilometers, and the shortest distance of the sky wave from the ionosphere for the first reflection landing (first hop) is about 80-100 kilometers, which can be seen from 20 to 100 kilometers. Both ground and sky waves are out of reach, forming a "silent zone" for short-wave communication, also known as a blind zone, as shown in Figure 1-3. Communication in the blind zone is mostly difficult. Due to the limitation of antennas, vehicle-mounted stations all have communication blind spots.
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