Division near field and far-field
The measurement method of electromagnetic radiation is usually related to the distance between the measuring point and the radiation source. In the case of far field and near field, the nature of the electromagnetic field is different. Therefore, it is clear that the far field and near field measurement.
1、Far field and near field of electromagnetic field
Alternating electromagnetic field generated by a source of electromagnetic radiation can be divided into the different nature of the two parts, which a part of the energy of the electromagnetic field between the radiation source to the surrounding space and radiation source periodically to flow back and forth, not on the outward emission, known as the induction field; another part of the electromagnetic energy from the radiator, in the form of electromagnetic wave radiated outward, known as the radiation field.
In general, the electromagnetic radiation field is divided into the far zone field (field) and the near field (radiation field) according to the difference of the induced field and the radiation field. As far field and near field division is relatively complex, according to different working environment and measurement purposes, generally speaking, the field source as the center, in the three wavelength range, is usually called near field, also known as the sensing field. The near zone field usually has the following characteristics:
In the near field, the electric field strength and the magnetic field strength are not determined by the size of the magnetic field strength. Namely: 377H E. Under normal circumstances, the high current source (such as transmitting antennas, feeders, etc.), the electric field is much stronger than the magnetic field, for the high voltage low current source (such as certain induction heating equipment of the mold), the magnetic field is much greater than the electric field.
The electromagnetic field intensity in the near field is much larger than that in the far zone. From this point of view, the focus should be in the near field of electromagnetic protection.
The electromagnetic field intensity of the near field is relatively fast, and is not uniform in this space.
The main features of the far zone field are as follows:
In the far field, all of the electromagnetic energy is radiated by electromagnetic wave, and the attenuation of the radiation intensity is much slower than that of the field.
In the far zone field, the electric field intensity and the magnetic field intensity are as follows: in the international system of units, E=377H, the electric field and the magnetic field direction are perpendicular to each other.
The significance of the division of the near field and the far zone: Normally, for a certain intensity of electromagnetic radiation source, the electromagnetic field intensity of the near field radiation is relatively large, so it should pay special attention to the protection of the electromagnetic radiation near field. The protection of the near field of electromagnetic radiation is the protection of workers and workers in the near field. For the far zone field, due to the electromagnetic field is smaller, usually less harmful to human.对我们The most frequently contacted 3000MHz bands ranging from short wavelength 30MHz to the micro band, its wavelength range from 10 m to 1 m.
2、Far field measurement
In the far zone field (radiation) can be introduced to the power density vector (Poynting vector) and is vertical to the direction of the electric field vector, magnetic field vector, Poynting vector and Poynting vector direction for electromagnetic wave propagation direction.
In numerical value, S=EH=E2/377, E=377H. The unit of the electric field strength E is (V/m), the unit of the magnetic field strength H is (A/m), the power density of the unit is (W/m2), all is the international unit system (SI).
The formula can be seen in the far field, the electric field and the magnetic field is not independent, it can only measure the electric field intensity, the magnetic field strength and power density of a project, the other two projects can be converted.
In general, the measurement of the far field and the near field can be simplified:
The national standard, when the working frequency of electromagnetic radiator below 300MHz, to respond to the workplace of the electric field strength and the magnetic field strength were measured. When the frequency of electromagnetic radiation is greater than 300MHz, can only measure the electric field strength.
The corresponding wavelength of 300MHz frequency is 1 m, and the /6 is 400px, and the radiation field is 400px. Such as the division of the 3, 3 meters away from the radiation source can be considered far field area.
General electromagnetic environment refers to a wide range of electromagnetic radiation from a variety of sources, through various channels of transmission of electromagnetic radiation background value, which belongs to the far zone field, the spectrum of radiation is very wide, the electromagnetic field intensity is small.
The measurement of the far zone radiation field is far from 1GHz, which can be used as a far field intensity meter, and also can be used to interfere with the field intensity meter.
3、Near field measurement
In the near field region, the electric field intensity E and the magnetic field strength H are not determined by the ratio of the size of the 377H E, which need to measure the magnitude of the electric field intensity E and the magnetic field strength H. For high voltage and low current source, the electric field is the main electric field in the sensing field. The field source (such as the induction coil) is the main field in the sensing field. For example, there is no connection on the wall of the wall power outlet, the current is basically zero, the voltage is not zero, the socket in the vicinity of a certain intensity of the power frequency electric field, but the power frequency magnetic field is zero. National standard limit value of 30MHz following the field intensity as a criterion, because the band wavelength is greater than 10 meters, the measurement point of the induction field often can not be ignored, the electric field strength H and the ratio of the magnetic field strength .
From the relationship between the front and the distance, we can know that the electric field intensity of the near field is very large (according to different equipment, the intensity of the electric field can reach several A/m), but the field intensity decreases rapidly with the increase of distance. Therefore, the range of the near field intensity meter should be large enough, and the measurement probe should be small enough to measure the results of the test points. Near field area monitoring is mainly for the work place monitoring.
Due to the near field, far from the other electromagnetic radiation source contribution may be neglected. Therefore, in field measurement does not use frequency instrument. Can be used to measure the near field strength, such as the Italy PMM company’s 8053 type of instrument, with better measurement accuracy and powerful data processing function. At present, the general electric field strength meter is different from the early near field intensity meter. The former is wider than the latter.
Example: the near field of the specific radiation source (the field region) and the far field (radiation field area).（ = c / f）
| Frequency (f) | Wavelength ( ) | Limit（3 ） |
| 50 / 60 Hz Power | 6000 / 5000 km | 18000 / 15000 km |
| 50 kHz Electric welding | 6 km | 18km |
| 27 MHz CB Broadcast, diathermy | 11.1 m | 33.3 m |
| 100 MHz FM Radio broadcast | 3 m | 9 m |
| 433 MHz Industrial application | 0.7 m | 2.1 m |
| 900 MHz Mobile phones, pagers | 0.33 m | 1 m |
| 2.45 GHzMicrowave, industry | 0.12 m | 0.36 m |
| 6 GHz Digital broadcasting | 0.05 m | 0.15 m |
| 20 GHz Satellite transmission | 0.015 m | 0.045 m |
Specific division of the field
Relationship between field strength and distance
R indicates that the distance between the measuring points and the radiation source is inversely proportional to the intensity of the sensing field at that point. The intensity of the radiation field is inversely proportional to the intensity of the R (the product of the intensity of the radiation field and the distance from the R is independent of R, called the product of the electric field intensity). In the area near the source of radiation, the intensity of the induced field increases with the decrease of the distance r.
Division of near field and far field
When measuring the distance r＝λ/2π≈λ/6, induction field intensity and radiation field intensity is. In the distance from the radiation source（r<λ/6）, the intensity of the sensing field is greater than that of the intensity of the radiation field, which is the opposite of the（r<λ/6）. The formulation of the near field and far field is widely used, but in different application fields, the boundary is not uniform. Also known as near field and far zone field.
When r is more than 3λ, the component of the induction field can be ignored.