1、 Rep. ITU-R BS.2037 1 REPORT ITU-R BS.2037 Evaluating fields from terrestrial broadcasting transmitting systems operating in any frequency band for assessing exposure to non-ionizing radiation (Question ITU-R 50/6) (2004) TABLE OF CONTENTS Page 1 Introduction 3 2 Characteristics of electromagnetic f
2、ields 3 2.1 General field characteristics . 3 2.1.1 Field components 3 2.1.2 Far field. 4 2.1.3 Near field 6 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 605kHz) 13 2.2.2 HF bands (3-30 MH
3、z). 13 2.2.3 VHF/UHF bands. 14 2.2.4 SHF (3-30 GHz), 0.1-1 m) 14 2.3 Mixed frequency field. 16 2.4 EMF inside buildings 17 3 Calculation 17 3.1 Procedures. 17 3.1.1 Closed solutions 17 3.1.2 Numerical procedures. 18 2 Rep. ITU-R BS.2037 Page 4 Measurements. 21 4.1 Procedures. 21 4.1.1 LF/MF bands 21
4、 4.1.2 HF bands. 21 4.1.3 VHF/UHF bands. 21 4.1.4 SHF bands. 22 4.2 Instruments . 22 4.2.1 Introduction. 22 4.2.2 Characteristics of the measurement instruments for electric and magnetic field 23 4.2.3 Narrow-band instrument types and specifications 24 4.3 Comparison between predictions and measurem
5、ents . 25 5 Precautions at transmitting stations and their vicinity 25 5.1 Precautions to control the direct health effects of RF radiation . 25 5.1.1 Employee (occupational) precautionary measures . 26 5.1.2 Precautionary measures in relation to the general public . 27 5.2 Precautions to control th
6、e indirect RF radiation hazards 28 Appendix 1 Examples of calculated field strengths near broadcasting antennas . 29 Appendix 2 Comparison between predictions and measurements 42 Appendix 3 Limits and levels . 63 Appendix 4 Additional evaluation methods 72 Appendix 5 Electromedical devices 77 Append
7、ix 6 References . 78 Rep. ITU-R BS.2037 3 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 used to protect humans from undesirable effects. Studies in many countries by di
8、ffering agencies 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 Report is intended to provide a single basis for the derivation and estimation of the values of electromagnetic radi
9、ation from a broadcast station that occur at particular distances from the transmitter site. Using such information, responsible agencies can then develop appropriate standards that may be used to protect humans from undesirable exposure to harmful radiation. The actual values to be applied in any r
10、egulation will naturally depend on decisions reached by responsible health agencies, domestic and worldwide. It is noted that this ITU-R Report and ITU-T Recommendations cover similar material, but with an emphasis on different aspects of the same general subject. For example, ITU-T Reommendations K
11、.51 (Guidance on complying with circuits for human response to electromagnetic fields) and K.61 (Guidance to measurement and numerical prediction of electromagnetic fields for compliance with human limits for telecommunication installations) provide guidance on compliance with exposure limits for te
12、lecommunication systems. Appropriate reference information is included in Appendix 6. 2 Characteristics of electromagnetic fields 2.1 General field characteristics This section gives an overview of the special characteristics of electromagnetic (EM) fields that are relevant to this Report, especiall
13、y 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 section concludes by defining the terms polarization and interference patterns. 2.1.1 Field components The EM field radiated fro
14、m an antenna comprises various electric and magnetic field components, which attenuate with distance, r, from the source. The main components are: the far field (Fraunhofer), also called the radiation field, in which the magnitude of the fields diminishes at the rate of 1/r; the radiating near field
15、 (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 criteria have been established and are commonly used to specify this behaviour; the reactive near field (Rayleigh), also called quas
16、i-static field, which diminishes at the rate of 1/r3. 4 Rep. ITU-R BS.2037 As the inductive and quasi-static components attenuate rapidly with increasing distance from the radiation source, they are only of significance very close to the transmitting antenna in the so-called near-field region. The r
17、adiation 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 transmitter to a distant receiver. 2.1.2 Far field In the far-field region, an electromagnetic field is predominant
18、ly 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 fields are situated at right angles to one another, lying in a plane, which is perpendicular to the direction of p
19、ropagation. It is often taken that far-field conditions apply at distances greater than 2D2/ where D is the maximum linear dimension of the antenna. However, care must be exercised when applying this condition to broadcast antennas for the following reasons: it is derived from considerations relatin
20、g 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. 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 th
21、e 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, this expression can be simplified because the electric and magnetic fields, and the direction of propagation, are all mutually orthogonal.
22、 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 space1and 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
23、: S = E2/Z0= H2 Z0 (2) 1Generally, the characteristic impedance of a medium is given by )/( =z where is the magnetic permeability (=1.2566 106F/rn in free space), and is the permittivity (= 8.85418 1012H/rn in free space). HES rrr =Rep. ITU-R BS.2037 5 The power density at any given distance in any
24、direction can be calculated in the far field using the following equation: S = P Gi /(4 r2) (3) 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
25、 an isotropic radiator r: distance (m) from the radiation source. The product PGi in equation (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 ot
26、her directions the antenna pattern must be taken into account. In order to use equation (3) 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,
27、 as in equation (4). The relevant factor Gr is given in Table 1. S = P GrGa/(4 r2) (4) TABLE 1 Isotropic gain factors for different types of reference antenna Thus, 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 o
28、f 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) (6) where: Gm: gain of the antenna relative to a short monopole. Reference antenna type Isotropic gain factor GrTypical applications wher
29、e reference antenna type is 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 6 Rep. ITU-R BS.2037 2.1.2.2 Field strength Equations (2)-(10) as
30、sume 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 account of the directional characteristic of the radiation source, then equation (7) is obtained for the electr
31、ic field strength 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 fed to the radiation source (W), assuming a lossless system C: factor (0 C 1), which takes account of the directional
32、 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 than relative to an isotropic radiator, then the factors Gdor Gm, respectively, should be used in place of Gi, as sh
33、own in equations (8) and (9). ddPGrCCrPGZE 2.4964.140= (8) mmPGrCCrPGZE 90340= (9) In order to calculate the magnetic field strength in the far field of a 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), th
34、e 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 far field. In the near field, there is an arbitrary phase and amplitude relationship between the electric and magnetic field strength vectors, and
35、 the field strengths vary considerably from point to point. Consequently, when determining the nature of the near field, both the phase and the amplitude of both the electric and magnetic fields must be calculated or measured. In practice, however, this may prove very difficult to accomplish. Rep. I
36、TU-R BS.2037 7 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 relationship mentioned above. The E and H amplitudes, together with their phase relationship, must be measured or calculated separate
37、ly 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 for simple ideal radiators such as the elementary dipole. In the case of more complex antenna systems, other mathematical techn
38、iques 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 density and other relevant characteristics of the field, even in the complex near-field region. Measurement in the near field is
39、 even more difficult as no reference calibration method exists. The International Electrotechnical Commission is currently working on the issue of a measurement standard for high frequency (9 kHz to 300 GHz) electromagnetic fields particularly in the near field 1. In addition, EN 61566 (Measurements
40、 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 Polarization is defined as the direction of the electric field vector, referenced to the direction of propagation of the wave fr
41、ont. 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 surface of the Earth) although other types of polarization are used such as slant and elliptical. 2.1.5 Modulation Modulation i
42、s 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
43、into consideration when carrying out measurements or calculations to determine whether or not the limits are being exceeded. The modulation often results in a signal varying in both amplitude and frequency. For this reason temporal averaging is usually required in determining the values to be used i
44、n measurement and calculation. This requirement is also acknowledged in relevant Standards. 2.1.5.1 Characteristics of radio emission The Radio Regulations (RR) classify the emissions from radio transmitters according to the required bandwidths, and the basic and optional characteristics of the tran
45、smission. The complete classification consists of nine characters as follows: Characters 1-4 describe the bandwidth, using three digits and one letter; Characters 5-7 describe the basic characteristics, using two letters and one digit; Characters 8-9 describe any optional characteristics, using two
46、letters. 8 Rep. ITU-R BS.2037 Only the three basic characteristics are relevant to the consideration of RF safety considerations. These are: the type of modulation of the main carrier Character 5 the nature of the signal(s) which modulate(s) the main carrier Character 6 the type of information to be
47、 transmitted Character 7 Table 2, which is based on information given in the RR, lists the various characters which are used to classify the three basic characteristics of a radio emission. For sound and television broadcasts, the relevant characters are as follows: AM radio (LF, MF and HF double si
48、deband) A3E AM radio (HF single sideband, reduced/variable carrier) R3E AM radio (HF single sideband, suppressed carrier) J3E Television pictures C3F Television sound F3E or A3E FM radio F3E or F9E DVB G7F DAB G7E TABLE 2 Characters used to define the class of emission, based on information given in
49、 the RR Character 5 Type of modulation of the main carrier Character 6 Nature of the signal(s) modulating the main carrier Character 7 Type of information to be transmitted N Unmodulated 0 No modulating signal N No information transmitted A Amplitude modulation: double-sideband 1 Single channel containing: quantized or digital information not using a modulating sub-carrier A Telegraphy for aural reception H Amplitude modulation: single-sideband, full carrier 2 Single channel containing: quant
