With the progress of science and technology, the application of electromagnetic technology has created a great material civilization for mankind, but it has also brought people into an environment full of artificial electromagnetic radiation. As early as 1975, experts and scholars predicted that with the development of urban economy and the growth of population, a large number of electronic, communication, computer, automobile and electrical equipment will enter the home, and the artificial electromagnetic energy in urban space will increase by 7%- 14% every year, that is to say, the environmental electromagnetic energy density will increase by 26 times at most after 25 years, and by 70 times after 50 years.
Electromagnetic radiation refers to "the phenomenon that energy is emitted from a source into space in the form of electromagnetic waves". The electromagnetic environment is "the sum of all electromagnetic phenomena existing in a given place". The deteriorating electromagnetic environment not only does serious harm to the communication, computer and various electronic systems on which human life is increasingly dependent, but also poses a threat to human health. Therefore, countries all over the world attach great importance to the increasingly complex electromagnetic environment and its extensive influence, and electromagnetic environment protection and electromagnetic compatibility technology have become a rapidly developing new subject field.
In the past two decades, China's economy and urbanization have developed rapidly, and the electromagnetic environment in urban airspace has become more complicated, resulting in many new phenomena and problems. Mainly: (1) Due to the development and expansion of the city, some large and medium-sized radio, television and radio communication transmitting stations are surrounded by newly developed residential areas, and some residential areas form strong areas; (2) Mobile communication technology (including mobile phone communication, paging communication and trunking professional network communication) has developed rapidly. Hundreds of mobile communication base stations have been set up in high-rise buildings in urban areas. Although the power of each of these electromagnetic radiation sources is low (below 100W), the electromagnetic field intensity in urban areas has increased, and some high-rise buildings have been polluted. In addition, there are many microwave directional antennas, satellite antennas and short-wave antennas on high-rise buildings in cities. (3) With the increase of urban electricity consumption and the implementation of urban power grid reconstruction project, 1 10KV and 220KV high-voltage substations enter the central area of the city, indoors or outdoors, or above or underground, causing panic and complaints from the surrounding people; (4) The rapid development of urban transportation systems (automobiles, trams, subways, light rails and electrified railways) makes urban electromagnetic noise on the rise; 5. With the increase of personal radio communication means and household appliances, the electromagnetic energy density in small family environment increases, indoor electromagnetic environment and outdoor electromagnetic environment are integrated, and the total amount of urban electromagnetic environment is increasing.
Analysis of radiation characteristics of typical electromagnetic radiation equipment in cities
The radiation characteristics of ten hot issues of urban electromagnetic environment in China are analyzed as follows:
Radio and television transmission tower located in the urban area
Radio and TV towers are mostly built in densely populated urban centers, which makes the service area with high field strength. The frequency range of TV and FM broadcasting in China is: 48.5 MHz-960 MHz, which belongs to ultrashort wave and decimeter wave band, and electromagnetic waves propagate in a straight line in space. In our country, TV and FM equipment are installed on the same launch pad, and most of them are installed on the mast of the same tower.
The electromagnetic radiation acceptance of Beijing Central Radio and Television Tower, Shanghai Oriental Pearl Tower and Tianjin Sky Tower shows that the environmental power density of electromagnetic radiation around the ground and upper floors of tall TV towers is several μW/cm2 in cities with a height of more than 400 meters and a total emission power of more than 200KW, which are all less than the national standard of 40μW/cm2 in this frequency band. The measurement results show that although the radio and television tower built in the urban area has high transmission power, the antenna area on the tower is at a high mast (about 300 meters), and the main lobe radiation of electromagnetic waves emitted by the antenna far exceeds the top of surrounding buildings, so as to obtain a larger service area (service radius of 70-90 kilometers). The surrounding ground and some high-rise buildings are located in the weak sidelobe area of TV tower. However, the city planning department should examine and control the building height around the TV tower to prevent the top of the high-rise building from entering the sidelobe area of the TV tower with strong radiation and causing electromagnetic radiation pollution.
It is worth noting that the maximum radiation field strength of TV antenna with low gain and low power but low erection height may be greater in some occasions.
2. The medium-wave broadcasting transmitting station located in the suburbs.
The frequency range of medium wave broadcasting is 531-1602khz. Most transmitting antennas use tow towers to radiate vertically polarized waves. Generally speaking, one tower is called a non-directional antenna, two towers are weak directional antennas, and four towers and eight towers are strong directional antennas. Most of the energy travels along the ground, and a small part travels to the sky (mainly by ground waves). In order to ensure a better radiation service radius, the launch pad is generally built in the suburbs of cities. Due to the development of cities, many medium-wave transmitting antennas built in the suburbs in the early days of the People's Republic of China were surrounded by expanded urban areas, and such incidents occurred in Beijing, Jiangsu, Fujian, Xinjiang, Guangzhou, Wuhan, Hangzhou and other cities. On the one hand, it affects the transmission of electromagnetic wave signals, on the other hand, it makes the area near the antenna become a strong field area, and the field strength ranges from tens of v/m to hundreds of v/m, which exceeds the national standard of 40 v/m in this frequency band. Residents' telephones can't get through, TV viewing is disturbed, and some indoor light bulbs and tubes glow red without pressing the power supply. As a result, it not only consumes the useful transmitting power of radio stations, but also causes environmental pollution.
Short-wave broadcasting and communication transmitting station located in the outer suburbs
The frequency range of short wave transmission is 3-30MHz. Short-wave broadcasting and communication are long-distance radio propagation. The maximum emission direction of the antenna in the vertical plane has a certain elevation angle. Radio waves are reflected by the ionosphere in the air, covering hundreds to thousands of kilometers, commonly known as sky wave service. Electromagnetic waves traveling along the ground consume quickly and travel a short distance, which is the sidelobe of radiation waves. The calculated value of the main lobe is close to the measured value, while the calculated value of the side lobe is quite different from the measured value.
The short-wave antenna field covers a large area, and most of it is built on rural fields (only the right to occupy antenna towers is purchased). If the antenna area is adjacent to the village, similar problems will occur in the medium wave transmitting station. In some areas, the rural economy has developed. The old village is transformed from the original bungalow into a building with 2-6 floors, and the electromagnetic radiation to the population will increase. According to the field intensity monitoring results, the field intensity in the adjacent area can reach about 10v/m, and it increases with the height. The environmental standard of this frequency band is 12-40 v/m (varying with frequency).
Mobile communication base station located on the roof of high-rise residential buildings in urban areas
In recent years, urban wireless mobile communication has developed rapidly. At present, mobile communication networks mainly include: (1) mobile telephone network (commonly known as mobile phone); (2) wireless paging network; (3) Wireless trunking professional network. The same characteristics of this radio communication network are:
The working frequency of (1) is ultrashort wave and decimeter wave, and electromagnetic wave propagates as direct wave in space. Each communication network consists of a central station, a base station and a user station (user machine);
⑵ The greater the coverage of each wireless communication network, the greater the power of the transmitting equipment of each base station, or the more base stations there are. In order to improve the communication quality, more and more base stations are built in high-rise buildings in urban areas, and base station antennas have been built in high-rise houses in many areas; Due to the continuous division of cellular network, the antenna height of mobile phone base station is reduced from about 50 meters to about 20 meters, making the cell smaller.
⑶ The base station built in hot spots has many users in the service area, and the base station occupies multiple carrier frequencies, which increases the radiation of electromagnetic waves emitted by the antenna.
In the urban radio communication network, the mobile phone network ("mobile phone" network) is the most widely reflected and influential by the masses.
The monitoring results of electromagnetic radiation of mobile phone base station show that installing antennas in a standardized and scientific way will not pollute the surrounding buildings, because the main lobe and strong sidelobe of electromagnetic waves will pass through the top or gap of the surrounding buildings, but the following situations should be prevented:
(1) When the directional antenna of the base station is lower than the surrounding buildings and the main direction of the antenna is close to sensitive buildings;
⑵ When the base station antenna is installed at the edge of a high-rise building and has an inclination angle, the vertical lobe of the electromagnetic wave radiated by the antenna may hit the residential window below the antenna of the building;
(3) When there are multiple websites (multiple mobile phone antennas or multiple paging and trunking communication antennas) on the roof of a residential area, the electromagnetic wave has a compound frequency field strength.
Satellite earth stations around the city.
The uplink transmission power of satellite earth station is generally between several hundred W and several KW, and the antenna elevation angle is generally between several degrees and several tens of degrees. Due to the high transmission power, the electromagnetic wave radiation power density is high at a certain distance in front of the antenna and in the surrounding space; According to the antenna pattern, there are also quite high stray radiation doses in the front and rear areas of the antenna, which are caused by electromagnetic sidelobes. Electromagnetic sidelobe refers to the lobe outside the main beam.
The monitoring results of ground electromagnetic radiation power density near several satellite earth stations in Beijing are as follows: the effective emission power of the antenna is about 70dBW. When the antenna diameter is 9-18m and the antenna frame height is 8-10m, the maximum value in front of the antenna appears between 20-70m, and the power density is 4.8- 16 μ w/cm2. The maximum back leakage of the antenna is between 10-30m, and the power density is about 3-30μ W/cm2.
6. EHV power lines and substations are gradually approaching the city center.
China's power grid is 500 kV -220 kV (or 330 kV)-10 kV -35 kV-10 kV series. Before the 1980s, high-voltage power lines (above 1 10KV) were mainly distributed outside the city (or outside the moat). Since 1990s, with the increase of urban electricity consumption, the power sector has increased the construction of urban power equipment, and 1 10KV and 220KV high-voltage cables have been introduced into urban areas. Shanghai, Beijing and Tianjin have respectively built 1 10KV and 220KV high-voltage substations in urban areas.
High-voltage power lines and substations transmit 50Hz power frequency current, which will not produce high-frequency electromagnetic radiation. Its main harm is that when the transmission voltage is high, power frequency electric field and power frequency magnetic field are generated around its conductor or near the substation, which is easy to cause harm to human body. Secondly, the generated electromagnetic noise (mainly below 30MHz) will interfere with broadcasting and radio communication when it is strong. The monitoring results show that under (1) high voltage power lines, the1/0kV double-tower is several hundred to 2,000 V/m, and the 220KV double-tower is about 3,000 V/m, both of which are less than the limit of 4 kV/m; Under 500KV line, it is about 5000V/m, which is greater than the limit of 4 kV/m; (2) High-voltage substation, in the open air of1/0kV and 220KV main transformer1.5m, the power frequency field intensity is as high as 2050-3200 V/m; For indoor and underground substations, due to the shielding effect of buildings, the power frequency field strength values of outdoor and ground substations are basically the same as the environmental background values (below 10V/m). ⑶ The electromagnetic noise value of indoor and underground substations 20 meters away is basically less than the limit of 55 dB μ V/m. ..
(7) Industrial, scientific and medical high-frequency equipment widely distributed in urban areas.
"Equipment or devices designed to generate and locally use RF energy according to the requirements of industry, science, medical care, household or similar purposes" are called engineering medical high-frequency equipment.
Engineering medical high-frequency equipment needs to use electromagnetic energy, but it doesn't want to emit electromagnetic waves. This kind of equipment has a strong electromagnetic oscillation source. Theoretically, there should be a fixed working frequency. However, in order to simplify the equipment and reduce the cost, the frequency spectrum quality of its oscillation source is very poor (not specific working frequency), and electromagnetic radiation (or electromagnetic wave leakage) with wide frequency will occur, which will cause electromagnetic radiation harm to the surrounding environment and operators.
The monitoring results of high-frequency induction heating and high-frequency medium heating equipment show that (1) two kinds of high-frequency heating equipment have high field strength at 0.5- 1 m, which means that the workers operate or adjust the position of the machine, indicating that the electromagnetic radiation pollution in the working environment is serious; ⑵ With the increase of the distance from the measuring point, the field strength decays to several V/m at about 5 meters, and it is generally not detected at 10 meters or outside the workshop, indicating that the field strength of industrial high-frequency equipment decays quickly. It shows that the electromagnetic radiation of industrial high-frequency equipment has little influence on the radiation of people outside the factory; ⑶ The impact of industrial high-frequency equipment on the environment is mainly electromagnetic noise interference. The distance test shows that the interference field strength of different frequencies can reach 56-90 dB μ V/m at the distance of 30-80 meters.
Medical high-frequency equipment mainly includes ultrashort wave physiotherapy instrument, microwave therapeutic instrument and high-frequency scalpel. The working object of electromagnetic energy is people, and it plays a certain therapeutic role by relying on electromagnetic energy. However, individual field strengths are high, which is harmful to the surrounding operators on the one hand, and interferes with broadcasting, television and communication on the other hand due to poor spectrum purity. The monitoring results show that: (1) its working frequency is wider than the industrial frequency band, from 0.3MHz to several GHz, and the power of the equipment is slightly lower than that of the industry, generally between tens and hundreds of W; ⑵ The radiation law of field strength of medical equipment is the same as that of industry, which decays rapidly with the increase of distance, and most of them can't be detected beyond 10 meter.
Eight Personal Communication Tools-Mobile Phone (Mobile Phone)
Mobile phone is an advanced communication tool for wireless and wired relay. This is a convenient and effective way of communication when an emergency or something important happens. By 1999, the number of mobile phone users in China reached 38 million.
The communication program of mobile phone is: the voice signal is converted into high-frequency electromagnetic wave, which is radiated into the air through the antenna of handheld phone. The electromagnetic wave carrying the voice signal reaches a certain channel of the nearest mobile phone base station (usually built on the roof of high-rise building) through space, and the signal received by the base station is transmitted to the central exchange station of mobile communication network through optical cable or microwave, and then the central station converts the signal and connects it to the local telephone network (or another base station), thus realizing wireless and wired (or wireless and wired).
Mobile phones transmit information by emitting electromagnetic waves. Although the transmission power has been reduced from 3-5W to 0.25 2W at present, the antenna of the mobile phone is too close to the brain (5- 10 cm) when talking, and it will be radiated by electromagnetic waves for a long time. According to the monitoring results of electromagnetic wave intensity near the antenna when some mobile phones are talking, it can be seen that most of the power density values are tens to hundreds of μw/cm2, and the longest talk time can reach 1000μw/cm2. According to the Regulations on Electromagnetic Radiation Protection formulated by the State Environmental Protection Agency, the average value of environmental electromagnetic radiation field parameters for any continuous 6 minutes within 24 hours a day should be less than 40μw/cm2. It can be seen that when using mobile phones, there are strong electromagnetic waves in the local environment that exceed the environmental standards. The simplest protection method is to match a pair of headphones.
Various household appliances have gradually entered people's family life.
Household appliances are household appliances, which can also be used in industrial and agricultural production and public utilities, so they are also called household appliances. Its development history is very short, only a few decades, but it has developed quite rapidly. According to statistics, there are more than 200 kinds of products and nearly 10,000 kinds of fancy designs in the earliest countries that produce household appliances.
Household appliances can improve the production environment, reduce housework, create favorable conditions for socialized service in daily life and improve people's living standards; On the other hand, if used improperly, it will worsen the electromagnetic environment, affect people's health or cause radio noise interference.
Common household appliances can be divided into air conditioning appliances, refrigeration appliances, kitchen appliances, cleaning appliances, heating appliances, beauty and fitness appliances, bakelite appliances, art film appliances and other appliances.
After years of research, the Environmental Protection Committee of the United States found that long-term living in extremely low frequency electromagnetic fields (such as power frequency 50Hz) will
It can cause some cancers in humans. With all kinds of household appliances entering thousands of households, people's chances of being exposed to extremely low-frequency magnetic fields generated by household appliances such as refrigerators and electric blankets are gradually increasing.
As a fire-free, smokeless and fast cooking cooker, microwave oven is clean, hygienic and time-saving. The principle of microwave oven heating food is that the microwave radiated by the microwave generator (magnetron) in the microwave oven causes the friction between molecules in the instrument to generate heat. In order to prevent microwave leakage, microwave ovens generally have "triple protection measures": (1) multiple safety interlock micro-start devices; ⑵ The periphery of the oven door is designed as a diversion structure, which has the function of guiding the microwave to reverse phase, and the microwave will be offset by the opposite reflected wave, thus preventing the microwave from leaking from the periphery of the oven door; (3) There is a metal woven net in the glass interlayer of the oven door observation window, which can block the microwave penetration. The monitoring results of microwave ovens at home and abroad show that when the microwave oven is turned on, there is about 1.0-230μw/cm2 microwave leakage at 5- 10 cm. The microwave leakage of most products at 0.5m can be less than 1.4μw/cm2.
Electromagnetic noise on both sides of traffic trunk line
Engine ignition system is one of the strongest sources of broadband electromagnetic noise interference. The main reason of interference is still the intersection of current and arc phenomenon. Spark current pulse and arc with steep waveform front are generated during ignition. The peak value of spark current can reach several thousand amperes, and it is oscillating. Its frequency spectrum includes fundamental wave and its harmonics, and can be extended to X-band. In addition to the ignition system (the main interference source), there are also automobile electric horns, generator rectifiers, and batteries that are instantly turned on and off with high current. Even the motor of the electric window will produce narrow-peak electromagnetic interference.
According to the monitoring results of electromagnetic noise of traffic trunk lines in Beijing and Shenzhen, it has the following characteristics: (1) At traffic intersections during rush hours, the electromagnetic noise value is between 44-50 dBμV/m; ⑵ With the increase of distance, the interference field strength decays in a negative logarithmic relationship, and the attenuation law of each frequency with distance is almost the same, and the attenuation radian is the largest between 0 and 30 meters; ⑶ Observed from the measuring spectrometer, the frequency band of automobile spark interference is very wide, from several MHz to over 300 MHz, but the interference field strength is very large in the range of 10-25 MHz; (4) The spark interference of cars is small, while the spark interference of trucks and motorcycles is the biggest. Frequency is not inversely proportional to the field strength, and it is related to the ignition system type, driving speed, load capacity and vehicle wear degree of various vehicles. 5. From the spectrum analyzer, the peak of car interference within 50 meters is enough to interfere with the effect of radio and television viewing, so it is suggested that residential buildings should be more than 50 meters away from roads in urban planning.
Third, do a good job in urban electromagnetic environment protection.
2 1 century, mankind will enter the information society, and the electromagnetic environment of the city will be more complicated. It is suggested that urban electromagnetic environment protection should do the following work:
1. Widely publicize and implement the National Regulations on Electromagnetic Radiation Protection and the Administrative Measures on Environmental Protection of Electromagnetic Radiation. Construction projects related to electromagnetic radiation shall fulfill the environmental protection procedures of construction projects, conform to the relevant national environmental protection standards, and make the urban electromagnetic environment develop in an orderly manner.
Conduct popular science propaganda on electromagnetic radiation and electromagnetic environmental protection. Electromagnetic radiation is invisible, intangible, inaudible and inaudible, so it is difficult to be detected, and people are unfamiliar with it, so it is easy to have doubts. If there is an antenna near someone's house, residents will feel that they are living in radiation. In fact, if the antenna is properly erected, it will have no impact on the surrounding environment. Some reports about electromagnetic radiation are not objective and sensational, saying that electromagnetic radiation is harmful, but the intensity is not clearly written, which is easy to mislead the masses. It is necessary to publicize the popular science materials of electromagnetic radiation and let the masses know the basic knowledge and characteristics of electromagnetic waves, that is, they are not afraid of electromagnetic waves and can scientifically protect against electromagnetic radiation pollution.
Third, carry out scientific research on urban electromagnetic environment management and electromagnetic radiation protection. The energy density of urban electromagnetic waves is getting larger and larger, but there is no exact statement at home and abroad about how large the field strength and power density capacity of urban airspace are. The task of environmental protection management is to prevent electromagnetic radiation pollution, protect the environment and safeguard public health, and promote the development of legal practices accompanying electromagnetic radiation.