Research on electromagnetic radiation of mobile phones and base stations

At present, mobile phones have gradually become a necessity of modern life, and reports on electromagnetic radiation problems of public mobile phone base stations and mobile phones have gradually increased, and more and more people have begun to pay attention to the health effects of electromagnetic radiation.

Generally, people think that the electromagnetic radiation of public mobile phone base stations is large, and they are worried that they will affect their health. For small mobile phones with small power and no obvious antennas, they will think that the electromagnetic radiation intensity is smaller than that of base stations. So, which one is more harmful to the electromagnetic radiation intensity of cell phones and public mobile phone cellular base stations? This article analyzes their electromagnetic radiation by introducing national and international standards for mobile phone and base station electromagnetic radiation and specific test data.

In general, electromagnetic radiation (electromagneTIcradiaTIon) is defined as a phenomenon in which energy propagates through space in the form of electromagnetic waves.

Electromagnetic radiation occurs in various frequency bands of radio: there are very low to very high frequency radio waves, as well as higher frequency infrared, visible, ultraviolet, X-ray, γ-ray and cosmic rays. The amount of energy derived from electromagnetic radiation depends on the radio frequency. The higher the frequency, the greater the energy. Generally speaking, electromagnetic radiation can be divided into "ionizing radiation" and "non-ionizing radiation".

For example, the energy generated by X-rays, γ-rays and cosmic rays is enough to destroy molecules of human tissue structure, and even ionize atoms and molecules. This radiation is also called "ionizing radiation".

Most of our common electromagnetic radiation is generated by radio transmission equipment or industrial, scientific, medical and electronic products with a frequency of 9kHz to 300GHz. Its emission frequency is low and the energy is also weak. It is far from the energy to break down molecules. . So this type of radiation is also called "non-ionizing radiation".

2.1SAR value

Internationally, FCC, ICNIRP (International Non-Ionizing Exposure Protection Committee), IEEE and other agencies have successively formulated technical standards for measuring the impact of electromagnetic radiation on the human body, usually using SAR (SpecificAbsorpTIonRate, specific absorption rate) to express substances per unit mass per unit time Absorbed electromagnetic radiation energy. There are currently two common standards, one is the European standard 2W / kg, and the other is the American standard 1.6W / kg. According to the requirements of the International Telecommunication Union and the International Health Organization for measuring the SAR value of the technical standard for measuring mobile phone radiation, the electromagnetic radiation of GSM and narrowband CDMA mobile phones must be within the permitted range certified by the international authoritative health organization.

In the standard of "Electromagnetic Radiation and Protection" (GB8702-88) implemented in China, the public exposure standard is: within 24 hours of a day, the average body specific absorption rate should be less than 0.02W / kg for any continuous 6 minutes; occupational exposure standard For: within 24 hours of a day, the average body specific absorption rate (SAR) should be less than 0.1W / kg for any consecutive 6 minutes. It can be seen from the comparison of Table 1 that China's current standards are far stricter than other countries.

2.2 Electric field strength and power density

The basic limit of mobile phone radiation is measured by the SAR value, and the SAR value is measured in the shielded room. The space radio waves we live in are far more complicated than shielded rooms, which makes it more difficult for people to accept the concept of SAR value.

China's "Environmental Electromagnetic Wave Sanitation Standards" (GB9175-88) defines the microwave frequency band electromagnetic health standards.

First-level safety zone: refers to the area where all people who live, work and live for a long time under this environment electromagnetic wave intensity will not be affected by any harmful effects, and the electromagnetic radiation intensity is required to be <10μW / cm2. Secondary intermediate zone: refers to the area where all people who live, work and live for a long time under this environment electromagnetic wave may cause potential adverse reactions. The allowable electromagnetic radiation intensity is <40μW / cm2.

In accordance with the actual measurement methods required by the National Standards for Electromagnetic Radiation Protection (GB8702-88) and other national standards, we use EMR-300 radiation measuring instrument in a relatively shielded shielded room (absolutely shielded room, mobile phone has no downlink signal) and other conditions It is possible to make the mobile phone work in the state of maximum output power, and use the analog comparison method and the simulation method for various types of GSM, CDMA, PHS mobile phones produced by different manufacturers and different types, respectively 5cm ~ 8cm, 12cm ~ 14cm from the mobile phone , 20cm has been tested many times. The measured values ​​at 12-14 cm and 20 cm are relatively small, which meets the relevant limit requirements. What we are more concerned about is the distance between 5 and 8 cm, which is in line with people's habits in using mobile phones.

The actual measurement results show that: under this experimental condition, the radiation field strength of different types of mobile phones produced by different manufacturers has certain differences, but the electromagnetic radiation intensity far exceeds the national standard.

We have carried out related measurements on a mobile base station.

The power density of the test points within a radius of 5 meters from the base station's transmit antenna is basically more than 10μW / cm2 (more than the national standard); while the measured values ​​10 meters away are relatively small.

When people use a mobile phone, the mobile phone transmits radio waves to the transmitting base station, and the base station also transmits radio waves to the mobile phone. The mobile phone antenna and receiver are close to the ear, the antenna is only 2 to 5 cm away from the head, the head is in the near-field radiation area of ​​the mobile phone, and the electromagnetic radiation it receives is relatively large; while the general public mobile phone base station is about the distance from the citizen activity area From 15 meters to 50 meters, as far as the person in its radiation field is concerned, it belongs to the far-field radiation range, and the electromagnetic radiation is integrally or omnidirectionally distributed to the human body.

The farther away from the antenna, the lower the electromagnetic radiation intensity. In an ideal situation, the electromagnetic radiation intensity (expressed by power density) of a point is inversely proportional to the square of the distance from the point to the antenna, that is: at a distance of 1 cm from the antenna The intensity of electromagnetic radiation is 100 times greater than the intensity at 10 cm. Assuming that the power of an ordinary mobile phone is 1 watt and is 10 centimeters away from the head to talk, in theory, the electromagnetic radiation intensity received at this time is equivalent to the electromagnetic radiation intensity of 10 meters away from the base station with a power of up to 10,000 watts. In fact, most of the radiated power of public mobile phone base stations is below 20 watts. In this case, the electromagnetic radiation intensity of the public mobile phone base station is less than 5% of that of the mobile phone, which is negligible.

The measured results of electromagnetic radiation intensity of mobile phones and public mobile phone base stations are also in good agreement with the above theoretical analysis. At a distance of 10 meters from the mobile phone base station antenna, the radiation intensity is relatively small, all below 10 μW / cm2, and under the experimental conditions described herein, the radiation intensity at a distance of 5 cm from the mobile phone antenna is almost above 50 mW / cm2.

In fact, the transmission power of the base station depends on the communication distance between the mobile phone and the base station and the number of users. The existing GSM and CDMA systems all use microcell coverage, and thousands of low-power base stations are installed. The intensity of electromagnetic radiation also decreases accordingly. At present, the public mobile communication network is very mature, and the coverage radius of some cell base stations is only tens of meters; at the same time, this microcell base station coverage system reduces the electromagnetic radiation intensity of mobile phones, because today's GSM, CDMA, PHS and other mobile phones With the power adjustment function, the better the signal found by the mobile phone and the closer it is to the base station, the lower its transmission power. The test made in this article is conducted under the condition of the maximum output power of the mobile phone, so in the process of actually using the mobile phone, its radiation intensity is much smaller than the measured value.

Through the above field test and analysis, we can draw the following conclusions:

(1) Any radio transmitting device will generate electromagnetic radiation. From this perspective, an absolute "green phone" does not exist.

(2) The current mobile phone radiation limit specified in the standard is generally 50 times smaller than the radiation dose that may cause danger. Therefore, when using mobile phones, try to choose a standard that meets the standard

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