1、 Rec. ITU-R BS.1698 1 RECOMMENDATION ITU-R BS.1698 Evaluating fields from terrestrial broadcasting transmitting systems operating in any frequency band for assessing exposure to non-ionizing radiation (Question ITU-R 50/6) (2005) Scope This Recommendation is intended to provide a basis for the deriv
2、ation and estimation of the values of electromagnetic radiation from a broadcasting station that occurs at particular distances from the transmitter site. Using such information, responsible organizations can then develop appropriate standards that may be used to protect humans from undesirable expo
3、sure to harmful radiation. The actual values to be applied in any regulation will naturally depend on decisions reached by responsible health agencies, domestic and worldwide. The ITU Radiocommunication Assembly, considering a) that radio-frequency energy may have unsafe effects on the human body; b
4、) that radio-frequency energy may induce harmful electric potentials in conducting material; c) that radio-frequency energy may have harmful effects on apparatus (such as radiocommunication apparatus, navigation instruments, cardiac pacemakers, scientific or medical equipment, etc.); d) that radio-f
5、requency energy may lead to unintentional ignition of inflammable or explosive material; e) that determination of hazardous radiation levels and electric potentials, in terms of spectrum content, intensity, cumulative effects, etc., are being made by competent authorities; f) that determination of a
6、reas where radio-frequency fields and electric potentials exceed safe levels are being made by competent authorities; g) that persons not associated with such systems may be exposed inadvertently to such radiation (including travellers by air) or to such electric potentials; h) that persons operatin
7、g and maintaining radio transmitting systems may be required to work in close proximity to the source of such radio-frequency exposures, recommends 1 that Annex 1 to this Recommendation should be used to evaluate the electromagnetic fields generated by terrestrial broadcasting transmitting systems o
8、perating in any frequency band, for assessing exposure to non-ionizing radiation. 2 Rec. ITU-R BS.1698 Annex 1 Evaluating fields from terrestrial broadcasting transmitting systems operating in any frequency band for assessing exposure to non-ionizing radiation CONTENTS Page 1 Introduction 3 2 Charac
9、teristics of electromagnetic fields 4 2.1 General field characteristics . 4 2.1.1 Field components . 4 2.1.2 Far field 4 2.1.3 Near field 7 2.1.4 Polarization 7 2.1.5 Modulation. 7 2.1.6 Interference patterns. 13 2.2 Field-strength levels near broadcasting antennas . 13 2.2.1 LF/MF bands (150-1 605
10、kHz). 13 2.2.2 HF bands (3-30 MHz) 13 2.2.3 VHF/UHF bands (30 MHz-3 GHz) . 14 2.2.4 SHF (3-30 GHz). 14 2.3 Mixed frequency field. 16 2.4 EMF inside buildings 16 3 Calculation 17 3.1 Procedures. 17 3.1.1 Closed solutions . 17 3.1.2 Numerical procedures 18 4 Measurements. 20 4.1 Procedures. 20 4.1.1 L
11、F/MF bands 20 4.1.2 HF bands 21 Rec. ITU-R BS.1698 3 Page 4.1.3 VHF/UHF bands 21 4.1.4 SHF bands 21 4.2 Instruments . 21 4.2.1 Introduction 21 4.2.2 Characteristics of the measurement instruments for electric and magnetic field. 22 4.2.3 Narrow-band instrument types and specifications . 23 4.3 Compa
12、rison between predictions and measurements . 24 5 Precautions at transmitting stations and in their vicinity 24 5.1 Precautions to control the direct health effects of RF radiation . 24 5.1.1 Employee (occupational) precautionary measures 25 5.1.2 Precautionary measures in relation to the general pu
13、blic 26 5.2 Precautions to control the indirect RF radiation hazards 26 Appendix 1 to Annex 1 Examples of calculated field strengths near broadcasting antennas 27 Appendix 2 to Annex 1 Comparison between predictions and measurements. 41 Appendix 3 to Annex 1 Limits and levels. 62 Appendix 4 to Annex
14、 1 Additional evaluation methods. 71 Appendix 5 to Annex 1 Electromedical devices. 75 Appendix 6 to Annex 1 References 76 1 Introduction For many years the subject of the effects of electromagnetic radiation has been considered and attempts have been made to quantify particular limits that could be
15、used to protect humans from undesirable effects. Studies in many countries by various organizations have resulted in various administrative regulations. It is noteworthy and understandable that no single standard has emerged from all the efforts in this regard. This Recommendation is intended to pro
16、vide a basis for the derivation and estimation of the values of electromagnetic radiation from a broadcast station that occur at particular distances from the transmitter site. Using such information, responsible organizations can then develop appropriate 4 Rec. ITU-R BS.1698 standards that may be u
17、sed to protect humans from undesirable exposure to harmful radiation. The actual values to be applied in any regulation will naturally depend on decisions reached by responsible health agencies, domestic and worldwide. It is noted that this ITU-R Recommendation and ITU-T Recommendations cover simila
18、r material, but with an emphasis on different aspects of the same general subject. For example, ITU-T Recom-mendations K.52 (Guidance on complying with limits for human exposure to electromagnetic fields) and K.61 (Guidance to measurement and numerical prediction of electromagnetic fields for compli
19、ance with human limits for telecommunication installations) provide guidance on compliance with exposure limits for telecommunication systems. Appropriate reference information is included in Appendix 6. 2 Characteristics of electromagnetic fields 2.1 General field characteristics This section gives
20、 an overview of the special characteristics of electromagnetic (EM) fields that are relevant to this Recommendation, especially the distinction between the near field and the far field. Simple equations are derived for calculating the power density and the field strength in the far field, and the se
21、ction concludes by defining the terms polarization and interference patterns. 2.1.1 Field components The EM field radiated from an antenna is comprised of various electric and magnetic field components, which attenuate with distance, r, from the source. The main components are: the far field (Fraunh
22、ofer), also called the radiation field, in which the magnitude of the fields diminishes at the rate of 1/r; the radiating near field (Fresnel), also called the inductive field. The field structure of the inductive field is highly dependent on the shape, size and type of the antenna although various
23、criteria have been established and are commonly used to specify this behaviour; the reactive near field (Rayleigh), also called quasi-static field, which diminishes at the rate of 1/r3. As the inductive and quasi-static components attenuate rapidly with increasing distance from the radiation source,
24、 they are only of significance in the vicinity of the transmitting antenna in the so-called near-field region. The radiation field, on the other hand, is the dominant element in the so-called far-field region. It is the radiation field which effectively carries a radio or television signal from the
25、transmitter to a distant receiver. 2.1.2 Far field In the far-field region, an electromagnetic field is predominantly plane wave in character. This means that the electric and magnetic fields are in phase, and that their amplitudes have a constant ratio. Furthermore, the electric fields and magnetic
26、 fields are situated at right angles to one another, lying in a plane, which is perpendicular to the direction of propagation. It is often taken that far-field conditions apply at distances greater than 2D2/ where D is the maximum linear dimension of the antenna. Rec. ITU-R BS.1698 5 However, care m
27、ust be exercised when applying this condition to broadcast antennas for the following reasons: it is derived from considerations relating to planar antennas; it is assumed that D is large compared with . Where the above conditions are not met, a distance greater than 10 should be used for far field.
28、 2.1.2.1 Power density The power density vector, the Poynting vector S, of an electromagnetic field is given by the vector product of the electric, E, and magnetic, H, field components: S = E H (1) In the far field, in ideal conditions where no influence of the ground or obstacles is significant, th
29、is expression can be simplified because the electric and magnetic fields, and the direction of propagation, are all mutually orthogonal. Furthermore, the ratio of the electric, E, and magnetic, H, field strength amplitudes is a constant, Z0, which is known as the characteristic impedance of free spa
30、ce1and is about 377 (or 120 ). Thus, in the far field, the power density, S, in free space is given by the following non-vector equation: S = E2/Z0= H2 Z0 (2) The power density at any given distance in any direction can be calculated in the far field using the following equation: S = P Gi /(4 r2) (3
31、) where: S : power density (W/m2) in a given direction P : power (W) supplied to the radiation source, assuming a lossless system Gi: gain factor of the radiation source in the relevant direction, relative to an isotropic radiator r : distance (m) from the radiation source. The product PGiin equatio
32、n (3) is known as the e.i.r.p. which represents the power that a fictitious isotropic radiator would have to emit in order to produce the same field intensity at the receiving point. For power densities in other directions the antenna pattern must be taken into account. In order to use equation (3)
33、with an antenna design whose gain Gais quoted relative to a reference antenna of isotropic gain Gr, such as a half-wave dipole or a short monopole, the gain factor Gi must be replaced by the product of Gr Ga, as in equation (4). The relevant factor Gr is given in Table 1. S = P GrGa/(4 r2) (4) 1Gene
34、rally, the characteristic impedance of a medium is given by )/( =z where is the magnetic permeability (= 1.2566 106F/m in free space), and is the permittivity (= 8.85418 1012H/m in free space). HES rrr =6 Rec. ITU-R BS.1698 TABLE 1 Isotropic gain factors for different types of reference antenna Thus
35、 when the gain of the antenna Gd(Ga= Gd) is expressed relative to that of a half-wave dipole: S = 1.64 PGd /(4 r2) (5) where: Gd: gain of the antenna relative to a half-wave dipole. Similarly, when the gain of the antenna Ga= Gmis expressed relative to that of a short monopole: S = 3.0 PGm /(4 r2)
36、6) where: Gm: gain of the antenna relative to a short monopole. 2.1.2.2 Field strength Equations (2)-(10) assume plane wave (far-field) conditions and are not applicable to near-field calculations. If equation (2) is inserted into equation (3) to eliminate S, and a factor C is introduced to take ac
37、count of the directional characteristic of the radiation source, then equation (7) is obtained for the electric field strength (E) in the far field of a radiation source: iiPGrCCrPGZE 3040= (7) where: E : electric field strength (V/m) Z0= 377 , the characteristic impedance of free space P : power fe
38、d to the radiation source (W), assuming a lossless system C : factor (0 C 1), which takes account of the directional characteristic of the radiation source (in the main direction of radiation, C = 1). If the gain of the antenna is expressed relative to a half-wave dipole or a short monopole, rather
39、than relative to an isotropic radiator, then the factors Gdor Gm, respectively, should be used in place of Gi, as shown in equations (8) and (9). ddPGrCCrPGZE 2.4964.140= (8) mmPGrCCrPGZE 90340= (9) Reference antenna type Isotropic gain factor, GrTypical applications where reference antenna type is
40、relevant Isotropic radiator 1.0 Radar, satellite and terrestrial radio link system Half-wave dipole 1.64 Television, VHF and sometimes HF broadcasting Short monopole 3.0 LF, MF and sometimes HF broadcasting Rec. ITU-R BS.1698 7 In order to calculate the magnetic field strength in the far field of a
41、radiation source, equation (10) is used: H = E/Z0 (10) where: E : electric field strength (V/m) H : magnetic field strength (A/m) Z0= 377 (120), the characteristic impedance of free space. 2.1.3 Near field The field structure in the near-field region is more complex than that described above for the
42、 far field. In the near field, there is an arbitrary phase and amplitude relationship between the electric and magnetic field strength vectors, and the field strengths vary considerably from point to point. Consequently, when determining the nature of the near field, both the phase and the amplitude
43、 of both the electric and magnetic fields must be calculated or measured. In practice, however, this may prove very difficult to accomplish. 2.1.3.1 Power density and field strength It is not easy to determine the Poynting vector in the near field because of the arbitrary phase and amplitude relatio
44、nship mentioned above. The E and H amplitudes, together with their phase relationship, must be measured or calculated separately at each point, making the task particularly complex and time-consuming. Using analytical formulas, an estimation of the field strength in the near field is only feasible f
45、or simple ideal radiators such as the elementary dipole. In the case of more complex antenna systems, other mathematical techniques must be used to estimate field strength levels in the near-field region. These other techniques allow relatively precise estimations of the field strength, the power de
46、nsity and other relevant characteristics of the field, even in the complex near-field region. Measurement in the near field is even more difficult as no reference calibration method exists. The International Electrotechnical Commission is currently working on the issue of a measurement standard for
47、high frequency (9 kHz to 300 GHz) electromagnetic fields particularly in the near field 1. In addition, EN 61566 (Measurements of exposure to Radiofrequency electromagnetic field strength in the frequency range 1 kHz-1 GHz sub-clause 6.1.4) gives more information on this topic. 2.1.4 Polarization Po
48、larization is defined as the direction of the electric field vector, referenced to the direction of propagation of the wave front. In broadcasting, different types of polarization are used. The main types are vertical and horizontal (with respect to a wave front which is travelling parallel to the s
49、urface of the Earth) although other types of polarization are used such as slant and elliptical. 2.1.5 Modulation Modulation is a very special characteristic of the emission from a broadcasting transmitter. As certain effects of EM radiation are sensitive to the type of modulation used, it follows that the presence of modulation must be taken into consideration when making safety assessments. Modulation must also be taken into consideration when carrying out measurements or calculations to determine whether or not the lim
