Shortwave radio is radio transmission using radio frequencies in the shortwave bands (SW). There is no official definition of the band range, but it always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (approximately 100 to 10 metres in wavelength). It lies between the medium frequency band (MF) and the bottom of the VHF band. Radio waves in the shortwave band can be reflected or refracted from a layer of electrically charged atoms in the atmosphere called the Ionosphere. Therefore, short waves directed at an angle into the sky can be reflected back to Earth at great distances, beyond the horizon. This is called skywave or "skip" propagation. Thus shortwave radio can be used for communication over very long distances, in contrast to radio waves of higher frequency, which travel in straight lines and are generally limited by the visual horizon, about 64 km.
Shortwave communication can transmit information data, and can quickly and accurately transmit voice, pictures and other information. At present, shortwave communication is used in water conservancy, military, government, civil air defense and other fields. In civil air defense emergency communication and long-distance command, shortwave communication plays a particularly important role. Shortwave communication has obvious advantages in application, which can be summarized as follows.
(1) It can work in complex transmission environments. In traditional signal transmission methods, the transmission environment has a great impact. Shortwave communication solves this drawback and avoids the limitations of the transmission environment and terrain to the greatest extent. Especially in harsh environments such as remote mountainous areas and oceans, shortwave communication can well achieve high-quality communication transmission.
(2) It has extremely strong anti-destruction capabilities and autonomous communication capabilities. In information transmission, it is inevitably affected by the limitations of relays and network cables. Shortwave communication can adapt to these influencing factors to achieve data transmission and communication.
(3) Low operating power. Shortwave communication can be well applied in long, medium and short distance signal transmission, and can achieve communication transmission under low power.
The introduction of shortwave communication in the mobile communication system of civil air defense plays a critical role in improving the anti-destruction capability and anti-interference capability. The shortwave radio can accurately locate the interference direction, and the electromagnetic interference can be processed and avoided by combining adaptive wires and directional wires. As an important transmission technology of shortwave radio, FM transmission technology effectively improves the anti-interference ability of the radio. Shortwave radio needs to analyze the interference intensity of the band in communication, determine the frequency of the radio wave radio through this parameter, determine the band by real-time frequency fluctuations, and achieve communication electromagnetic avoidance on the channel with better transmission. In order to meet the development needs, shortwave communication technology continues to innovate with the help of new science and technology. As an emerging technology, automatic channel switching is applied to shortwave communication systems. When strong electromagnetic interference is detected in shortwave radio communication, the signal will be automatically switched to avoid electromagnetic interference. The communication band compression function of shortwave radio also ensures the stable operation of the radio.
The civil air defense mobile communication system has also been continuously developed, introducing new technologies, including the introduction of emerging high-frequency adaptive technology, error control technology, frequency modulation technology, modulation and demodulation technology, and frequency modulation technology to improve the quality and efficiency of civil air defense work. The application of shortwave radio in the civil air defense mobile communication system has formed a multifunctional cluster system, which has ensured the safety of the people and effectively ensured the smooth progress of national defense work.
In the application process of shortwave radio, such as the parking position of the communication vehicle, the antenna installation method, weather and hydrological conditions must be considered. When shortwave radio is used in the civil air defense communication system, these factors must also be considered comprehensively to effectively ensure the realization of the communication function in the final civil air defense communication system.
Parking location of communication vehicle
Due to the different coverage of provincial and municipal civil air defense mobile command systems, the parking range of mobile command vehicles is also different. Generally, the parking range of mobile command vehicles in municipal civil air defense mobile command communication systems is within 100 km. Shortwave radio stations mainly realize communication transmission in the system through ground waves and electromagnetic reflection electromagnetic waves. In shortwave communication transmission, obstacles will affect the transmission quality, resulting in low transmission quality. To this end, it is necessary to ensure the quality of signal transmission and reduce the impact of obstacles on the signal. Generally, in shortwave communication, the signal transmission direction and emission direction are adjusted to avoid interference sources and large obstacles such as high-voltage lines and forests, and communication transmission is carried out along open areas to ensure signal quality.
Antenna installation
There are three main types of shortwave radio communication antennas: dipole antenna, oblique antenna and whip antenna. Dipole antennas and oblique antennas require tall objects as supporting auxiliary erection. You can either set up stable support poles for support, or you can directly select the number of tall objects as support poles. Dipolar antennas are mainly used in fixed communications. The bipolar wire mode has a certain directionality. In fixed communications, the direction of the dipole antenna cannot be changed at will. Compared with the other two antenna modes, the installation process of the dipole antenna is more complicated. Inclined antennas are mainly used for fixed communications and temporary communications. However, there are restrictions on fixed communication directions during the communication process. Due to the different states of the whip antenna, the antenna selection and installation are also different. When it is a vertical whip antenna, the antenna chooses the omnidirectional antenna erection mode, and the shortwave radio station uses the radio wave signal transmission mode. When it is a tilted casual antenna, the antenna chooses a directional antenna erection mode, which is different from the vertical whip antenna. For the application of shortwave radio stations in civil air defense communication systems, the tilt of the antenna and the communication position of the communication vehicle will affect the quality of shortwave communication. In order to ensure the quality of shortwave communication, it is necessary to reasonably plan and control the tilt of the antenna and the parking position of the communication vehicle. The signal transmission direction of the shortwave radio station is opposite to the tilt direction of the antenna, and the central axis of the mobile command and communication vehicle needs to be the same as the communication direction. In addition, the position of the communication vehicle and the tilt direction of the antenna need to be appropriately adjusted based on the shortwave communication transmission distance.
Shortwave communication has the outstanding advantages of flexibility, simple equipment, rapid circuit establishment and low price. Compared with new communication methods such as optical fiber communication, it still has a wide range of application space and is one of the typical communication methods at present. Introduce shortwave radio in the civil air defense mobile communication system, and make full use of the advantages of shortwave radio and shortwave communication to ensure good communication. In the application of shortwave radio, in order to ensure good communication signal quality, it is necessary to reasonably set the position of the communication vehicle and the antenna installation method in combination with factors such as weather, hydrology, and electromagnetic influence to ensure normal civil air defense mobile communication.
