Some of the basic concepts of electromagnetic radiation
1. Common electromagnetic radiation sources
Generally speaking, radar systems, television and broadcasting transmission systems, radio frequency induction and medium heating equipment, radio frequency and microwave medical equipment, various electrical processing equipment, communication transmission stations, satellite earth communication stations, large power generation stations, transmission and transformation equipment, high-voltage and ultra-high voltage transmission lines, subway trains and electric trains, as well as most household appliances, can generate various forms, frequencies, and intensities of electromagnetic radiation sources.
2. Division of electromagnetic radiation field zones
The electromagnetic radiation field is generally divided into far-field and near-field.
Near field and characteristics
A region within a wavelength range centered on the field source is commonly referred to as a near-field or an induced field. The near-field usually has the following characteristics:
1) In the near-field, there is no definite proportional relationship between the strength of the electric field and the strength of the magnetic field. Namely: E1377H. In general, for field sources with high voltage and low current (such as transmitting antennas, feeders, etc.), the electric field is much stronger than the magnetic field. For field sources with low voltage and high current (such as molds of certain induction heating equipment), the magnetic field is much larger than the electric field.
2) The electromagnetic field intensity in the near field is much greater than that in the far field. From this perspective, the focus of electromagnetic protection should be on the near-field.
3) The electromagnetic field intensity in the near-field varies rapidly with distance, resulting in significant non-uniformity in this space.
Far field and characteristics
The spatial range with the field source as the center and a radius beyond one wavelength is called the far-field or radiation field. The main characteristics of the far-field are as follows:
1) In the far-field, almost all electromagnetic energy is 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.
2) In the far-field field, there is a relationship between electric field strength and magnetic field strength as follows: in the international system of units, E=377H, the direction of electric field and magnetic field operation is perpendicular to each other, and both are perpendicular to the direction of electromagnetic wave propagation.
3) The far-field is a weak field, and its electromagnetic field intensity is relatively small.
