The history of environmental electromagnetics began in the 19th century with the need for radio anti-interference. 1881 Heaviside published the first "On Interference" article, which did not attract much attention at the time, and with the occurrence of interference in the electrical transportation, the Berlin Electrical Association set up a committee on interference in 1887, with the members of the Hermholtz and Siemens. In 1889, the United Kingdom studied the problem of communication interference, and the fifth and sixth research groups of the International Telecommunication Union (ITU) still take this as their main research topic. In information security and bioelectromagnetics has made great progress.
3.4.3.1 Environmental electromagnetic field sources and their interference
Strong and weak power facilities, all over the society of the various occasions, constituting a lot of electromagnetic environmental field sources, it is difficult to list, such as according to the spectral division, can be roughly divided into six categories:
(1) frequency interference (50 Hz)
Including power transmission and distribution systems and power traction systems.
(2) Very Low Frequency Interference (below 3 kHz)
(3) Carrier Frequency Interference (10 to 300 kHz)
Includes harmonics generated by high-voltage DC transmission, harmonics generated by AC transmission, and harmonics generated by AC electric railroads.
(4) radio frequency, video interference (300 kHz ~ 300 MHz)
Including medical equipment (ISM), transmission line corona discharge, high-voltage equipment and power traction system spark discharge; internal combustion engines, electric motors, household appliances, lighting appliances and so on belong to the sources of interference in this type of frequency.
(5) microwave interference (300 MHz ~ 300 GHz)
Including ultra-high frequency, ultra-high frequency, very high frequency generated by the interference.
(6) lightning and nuclear electromagnetic pulse interference
If the characteristics of the radiated electromagnetic energy from the point of view, can be divided into two categories, the first category is specialized in radiating electromagnetic energy devices; the second category of electromagnetic energy is the side effects of radiation devices.
The first type of radiated electromagnetic energy devices are radio transmitters: radio, television, radio communications, wireless navigation and radar. This type of device is not only a large number, but also the power of a single device is getting bigger and bigger. As early as 1975, Geneva's International Frequency Registration Board (IFRB) registered more than 2 million radio transmitters. And the rate of growth is 6% per year. In the same year, the Longwave and Airwave Radio Administrative Conference reported more than 10,000 transmitters with a total power of 540 MW. higher frequency transmitters, in 1976, there were more than 2 million mobile and base station transmitters in the United States alone. There are also millions of amateur radio stations worldwide and growing at 20% per year. Military radio stations cannot be counted.
The second category is devices that unintentionally radiate electromagnetic energy during operation, which are numerous and varied, including radio and television receivers, electric motors, high-voltage power transmissions, automobiles, medical equipment, etc., as well as a wide range of household appliances. A very large number of electromagnetic sources co-existing at the same time, their electromagnetic energy radiation is random, a wide range of frequencies, or a high-power emission mixed with which constitute the peak.
As a city in various regions of the environment electromagnetic field distribution and its spectral characteristics, the most reliable data is the measured method of obtaining. Figure 3.4.4 shows experimental data from the International Radio Advisory Committee (CCIR), which is the result of examples conducted in 103 urban areas in the United States during the period 1966-1971. The data is divided into four groups: group A represents urban centers, group B is suburban areas, group C is far-flung suburbs, and group D is the lowest level of rural areas. According to industry estimates of man-made radio radiation density of 7% to 14% growth every year, 10 years, a 1-fold increase. This shows that the worst electromagnetic environment, or electromagnetic pollution, is found in urban areas where the population is concentrated and where there are a large number of electromagnetic radiation devices, as well as in areas adjacent to wireless transmitters and ultra-high-voltage transmission lines.
Figure 3.4.4 Electromagnetic field noise power averages
In order to ensure error-free reception of broadcast signals, the transmitter's transmitter power density is about 1,000 times higher than the average natural electromagnetic field strength at the earth's surface. This is the level of interference of the transmitter, the level of harm. And due to the increasing level of background interference electromagnetic fields, and to further increase the power level of the transmitter. For example, 60 years ago, a 120 kW long-wave transmitter could cover the whole of Poland with its power intensity. Now, to achieve the same effect, the transmitter power has to be increased by 17 times to 2 MW.
3.4.3.2 Hazards of the electromagnetic environment on human beings
Radio transmissions of radio-frequency electromagnetic radiation can be divided into: long-wave, medium-wave, short-wave and microwave. The hazards to humans vary.
High-frequency electromagnetic field harm to the human body, as early as during the Second World War, has attracted attention and research. High-frequency electromagnetic field radiation effect, so that people's eyes cataracts, hair loss, on the nerves, skin, muscles have an impact. Electromagnetic waves through the depth of the organism, the frequency of different, in-depth distance varies greatly, in more than 100 GHz, in-depth up to the millimeter level, a few GHz can be up to a few centimeters, 10 MHz can be more than 1 m above.
Long-term in the medium and short-wave staff or long-term impact, its electromagnetic wave effect mainly caused by central nervous system dysfunction and vegetative nerve tension disorders. Expressed as neurasthenia, dizziness, insomnia, fatigue, memory loss, female workers' menstrual disorders.
The most prominent microwave radiation harm is caused by vegetative nerve disorders, mainly in the cardiovascular system, such as arrhythmia, sinus bradycardia. The eye is a vulnerable organ, the testicular damage is also relatively large. The effect on brain tissue and blood is also relatively large. Of particular note is the cumulative effect of microwave damage.
Foreign medical research has shown that the strong (transmission) power lines of the industrial frequency electromagnetic field on human tissue is also harmful. China's power sector made part of the survey, as shown in Table 3.4.3. German medical journal reported that residents living near transmission lines, blood and nervous system degeneration and even lead to death. A woman filed a lawsuit in court against the German Power Authority, claiming that since the high voltage transmission lines were erected near her farm, it had reduced the milk production of her cows. It was later determined that this was indeed the case.
Table 3.4.3 Major Symptoms of Chinese Operators at High Frequencies (10 to 300 kHz)
3.4.3.3 Establishing Safety Standards for Electromagnetic Fields
The cumulative amount of electromagnetic waves absorbed by the tissues of a biological organism is called the dose. In recent years electromagnetic wave dosimetry research in the theoretical aspects and experimental aspects have made great progress in the animal body to do a lot of measured data, and through the theoretical conversion to the human body tissues. For example, a person with a height of 1.75 m and a weight of 70 kg is exposed to an electromagnetic field of 1 mW/cm2, and when the field is parallel to the axis of the person, the maximum absorption is between 70 and 80 MHz, which is called the resonant frequency. According to the experimental results, the maximum absorption occurs in different frequency bands for different body structures, as mentioned above for monkeys, it occurs around 300 MHz, and for rats, it occurs at 2450 MHz. at 5 GHz, the electromagnetic energy is mainly absorbed on the surface of the human body; and at 30-300 MHz, it is mainly absorbed in the head and torso.
The development of standards for safe permissible doses for exposure is a very complex endeavor. However, in order to protect the hazards of electromagnetic radiation, countries have issued radiation safety standards. Table 3.4.4 shows the microwave radiation safety standards issued by the early countries.
Table 3.4.4 microwave radiation safety standards
China's Ministry of Electronics Industry in 1979 put forward the first microwave radiation interim standards, the provisions of a day of 8 h continuous irradiation of the field of the maximum radiation average power density shall not exceed 0.038 mW/cm2; intermittent irradiation of a day of 8 h shall not exceed 0.3 mW/cm2; more than 1 mW/cm2 places to use protective gear. In 1989, the health standard for microwave radiation in workplaces (GBIO436-89) was promulgated, stipulating that the average power density of exposure is 50 μW/cm2 for a continuous day of work of 8 h.