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2 Rec ITU R BS 1698, Evaluating fields from terrestrial broadcasting transmitting systems. operating in any frequency band for assessing exposure to. non ionizing radiation,1 Introduction 3,2 Characteristics 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 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 LF MF bands 20,4 1 2 HF bands 21,Rec ITU R BS 1698 3. 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 Comparison 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 public 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 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 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 provide 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 used 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 similar 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. compliance 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 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 section 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 Fraunhofer 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 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. As the inductive and quasi static components attenuate rapidly with increasing distance from the. radiation source 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 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 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 must 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. 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. 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 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. space1 and is about 377 or 120, Thus in the far field the power density S in free space is given by the following non vector. 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,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 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. For power densities in other directions the antenna pattern must be taken into account. In order to use equation 3 with an antenna design whose gain Ga is 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 Gr Ga 4 r2 4, 1 Generally the characteristic impedance of a medium is given by z where is the magnetic.
permeability 1 2566 10 6 F m in free space and is the permittivity 8 85418 10 12 H m in free. 6 Rec ITU R BS 1698, Isotropic gain factors for different types of reference antenna. Reference antenna Isotropic gain Typical applications where reference antenna. type factor Gr 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. 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, Gd gain of the antenna relative to a half wave dipole. Similarly when the gain of the antenna Ga Gm is expressed relative to that of a short monopole. S 3 0 PGm 4 r2 6, 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. account 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. E C 30 PGi 7,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 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 Gd or Gm respectively should be used in place. of Gi as shown in equations 8 and 9,Z0 1 64 PGd C,E C 49 2 PGd 8. E C 90 PGm 9,Rec ITU R BS 1698 7, In order to calculate the magnetic field strength in the far field of a radiation source equation 10 is. 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 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 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 relationship 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 for. 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 density. 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 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, Polarization 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 surface 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 limits are being exceeded. 8 Rec ITU R BS 1698, 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 in 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 transmission 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 letters. Only the three basic characteristics are relevant to the consideration of RF safety considerations. 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 derivation and estimation of the values of electromagnetic radiation from a broadcasting station that

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