Overall shielding design for electromagnetic radiation protection
The phenomenon of electromagnetic radiation refers to the harm of electromagnetic radiation generated by high-power transmitting equipment to the human body and the interference with the normal operation of electronic devices. Natural electromagnetic radiation comes from Earth's thermal radiation, solar thermal radiation, cosmic rays, and static electricity; Artificial electromagnetic radiation comes from a wide range of sources, including high-voltage lines, substations, radios, electronic instruments, medical equipment, laser photography equipment, and office automation equipment. It can also come from the use of household appliances such as microwaves, televisions, mobile phones, air conditioning, electric blankets, and radios.
The electromagnetic radiation field is generally divided into near field and far field. The area within one wavelength range centered on the field source is commonly referred to as the near-field; On the contrary, in the region beyond one wavelength range, it is usually referred to as the far-field. The two types of field have different characteristics: in the near field, there is no definite proportional relationship between the strength of the electric field and the strength of the magnetic field. Generally, for field sources with high voltage and low current, such as transmitting antennas and feeders, the electric field is much stronger than the magnetic field; For field sources with low voltage and high current, such as some induction heating equipment, the magnetic field is much larger than the electric field. In the far-field, all electromagnetic energy is essentially radiated and propagated in the form of electromagnetic waves, and the attenuation of radiation intensity in this field is much slower than in an induced field. In the far-field, there is a relationship between the electric field and magnetic field strength as follows: E=377H, the direction of operation of the electric field and magnetic field is perpendicular to each other, and both are perpendicular to the direction of propagation of electromagnetic waves. The far-field is a weak field, and its electromagnetic field intensity is relatively small.
The electromagnetic field intensity in the near field is much greater than that in the far field, and the impact and damage of electromagnetic radiation on the human body are mainly in the near field. This article intends to provide some ideas and practices for the overall shielding design of electromagnetic radiation protection within the nearby field range.
A certain radar antenna project has a transmission frequency of 1-3GHz. Staff need to work in the underground work hall for 24 hours, with an area of approximately 800-10000 meters. Require a fully shielded design for personnel safety protection. Design requirements: The electromagnetic shielding effectiveness within the shielding area shall not be less than 50dB; The microwave radiation power density shall not exceed 10I.LW/cm. Before design, the safety indicators and construction and testing standards to be adopted should be considered first.
The existing national standards include GB8702-1988 "Regulations on Electromagnetic Radiation Protection", GB10436-1989 "Hygienic Standards for Microwave Radiation in the Workplace", GJB/Z25-1991 "Design Guidelines for Grounding, Bonding, and Shielding of Electronic Equipment and Facilities", sJ31470-2002 "Construction and Acceptance Specification for Electromagnetic Shielding Room Engineering", and GB12190-1990 "Measurement Method for Shielding Efficiency of High Performance Shielding Rooms".
Case analysis: In fact, the fact that the work hall was located underground in this design case has already considered the issue of electromagnetic radiation damage to personnel. In order to minimize electromagnetic radiation, the owner proposed to design a fully shielded work hall. Our country has certain experience in the design and construction of small-scale shielding rooms, and the results can be guaranteed. However, such complex situations are relatively rare and quite difficult, as there are many workers entering and exiting, a large number of large-diameter inlet and outlet pipelines processing, and shielding efficiency requirements for large-area shielding areas in the cases.
Design concept: Adopting a multi-layer shielding design concept, the overall shielding design of the work hall is carried out to control the leakage prevention measures of various entry and exit interfaces. The inlet and outlet pipelines are relatively concentrated and processed using filtering, grounding and other technologies to ensure shielding effectiveness.
