Electromagnetic Field Standards and Exposure Systems
2: EM Environment Protection Lab, University of Wroclaw, Wroclaw, Poland
Electromagnetic Field Standards and Exposure Systems addresses the need to establish measurement standards for electromagnetic fields for the conditions and accuracy of results. This book provides a review of fundamental properties of electromagnetic fields and the simple antennas that can generate and receive such fields. The main focus of this book is dedicated to the analysis of the accuracy of measurements and field standards using a range of radiating structures. Electromagnetic Field Standards and Exposure Systems covers the broader fields of measurements in telecommunications, radio navigation, radio astronomy, bioscience, and free ranging EM radiation and helps to develop the following measurement standards: proper calibration of the measuring instrument; external environmental factors that affect accuracy; competence and training of the instrument operator.
Inspec keywords: telecommunication transmission lines; standards; loop antennas; electromagnetic fields; horn antennas; dipole antennas; electric potential
Other keywords: loop antennas; accuracy analysis; transmission line segment; standard EMF generation; dipole antennas; exposure systems; electromagnetic field standards; horn antennas
Subjects: Electrostatics; Transmission line links and equipment; General electrical engineering topics; Single antennas
- Book DOI: 10.1049/SBEW515E
- Chapter DOI: 10.1049/SBEW515E
- ISBN: 9781613531778
- e-ISBN: 9781613531815
- Page count: 216
- Format: PDF
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Front Matter
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1 Introduction
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Nowadays civilization may be characterized by the rapid growth of applications of the number and power of electromagnetic field (EMF) sources in, for instance, telecommunications, industry, science, medicine (ISM), and domestic uses. This growth has been accompanied by the necessity of measuring the electric field (E), magnetic field (H), and power density (S) in three main areas: Measurements of free propagating electromagnetic (EM) waves in telecommunications, radiolocation, radio navigation, radio astronomy, geophysics, and other areas. Here may include measurements relating to antennas (radiation pattern, gain, calibration). Electromagnetic interference (EMI) measurements to assure undisturbed coexistence of devices and systems in an EM environment that are subjects of electromagnetic compatibility (EMC). These measurements are related to the operation of the above-mentioned services as well as to many other sources of EMI, including electric motors and combustion engines, overhead power lines, information networks and devices, atmospheric discharges, and others. Biosphere exposure measurements relating to labor safety, especially in close proximity to EMF-generating devices and systems, as well as environmental protection and, first of all, general public protection.
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2 EMF of an arbitrary structure
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Generation of a standard EMF requires the application of standard transmitting and receiving antennas. Simple antennas are generally used in primary standards because their parameters can be calculated precisely. In this chapter, the parameters for simple antennas will be calculated theoretically, based on basic assumptions and simplifications. In particular, this chapter will consider a half-wave, symmetrical dipole antenna of large slenderness ratio and an electrically small loop antenna with uniform current distribution.
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3 Methods of the standard EMF generation
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The previous chapter referred to two calibration methods: the standard transmitting antenna method (STA) or standard field method, and standard receiving antenna method (SRA) or substitution method. Let's characterize them briefly. 1. The STA method is based upon EMF generation with the use of a transmitting antenna whose parameters (radiation pattern, efficiency) are known to a required accuracy and established either theoretically or experimentally. The EMF at an observation point is calculated taking into account the parameters of the antenna, its excitation, and the geometry of propagation. 2. The SRA method requires a receiving antenna of well-known parameters and a system that makes it possible to measure current in the antenna or the voltage at its input. The SRA is placed in an EMF generated by an arbitrary source and then replaced by an antenna (meter, probe, device) under test. Two assumptions are made in this procedure: the EMF is stable enough to not change during the replacement, the antenna under test is immersed in the same field as the SRA and makes identical EMF deformations to the SRA. In the previous discussions we took into account EMF standards with dipole or loop antennas. However, an almost identical approach may be adopted when other types of standards are in use. For instance, in guided wave standards, the EMF is established on the ground of excitation measurement of a system of known parameters, which is similar to the standard field method. In the case of different types of chambers the substitution method is more appropriate.
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4 Accuracy analysis of EMF standards with dipole antennas
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This chapter and the following ones are devoted to the problems of error estimation in measurements for the standard EMF generation and measurement. We show the complexity of the problems as well as sources of error identification and values of error estimations. This understanding leads to the possibility of estimating the uncertainty of a procedure and a class of standards considered. Every result of a measurement is subject to errors introduced by the limited accuracy of the procedure (method) applied, inaccuracy in the meters and other auxiliary equipment, imperfection of humans' minds and the possibility of mistakes, as well as varying conditions and circumstances during the measurements, etc.
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5 Accuracy analysis of EMF standards with loop antennas
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The accuracy of H-field generation is consider in this chapter. In these considerations, only “internal” factors will be taken into account, i.e., location of a transmitting and a receiving antenna, excitation measurement error, electrical sizes of the loop antennas, and others. Because of the specificity of the standard, it is possible to neglect here any “external” factors limiting accuracy of the standard, such as the influence of ground conductivity, the presence of other conducting objects in the neighborhood of the standard, multipath propagation, or interference caused by external EMF. This simplification is acceptable because of the fact that the antennas are much smaller compared to the wavelength and, as a result, are much less sensitive to the factors mentioned. For the same reason, the H-field standards may work in a less rigorously controlled environment, with no screened or anechoic chambers.
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6 Accuracy analysis of EMF standards with a segment of a transmission line
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As has already been mentioned, an EMF standard with a segment of a transmission line, working in conditions of full matching in both its sides, has many advantages compared to sets radiating EM energy into space. The main advantages include a simple design, a possibility to generate quite strong EMF with relatively low exciting power, insensitivity to external fields and very limited energy radiated outside of a set, an ability to work in a wide frequency range, and simple and frequency-independent relationship between power (voltage) exciting a set and EMF inside it. The most important disadvantages include very limited space inside a set and strong couplings between a line and an OUT. The most popular and the most widely applied solution consists of a segment of strip line with basic mode TEM; this is usually called a TEM cell.
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7 Accuracy analysis of the standards with horn antennas
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The accuracy of the power gain and the effective area of the horn antenna are of primary importance for the class of the EMF standard in which the antenna is applied. Horn antenna work above 1 GHz frequency range. Horn antennas are usually fed from a waveguide; the simplest “horn antenna”is an open end of a waveguide. Possible estimation errors and measurements are discussed in this chapter.
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8 Comparative analysis of the EMF standards
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Previous chapters presented and partially analyzed factors limiting the accuracy of EMF standards with dipole antennas, whip antennas, loop antennas, directional antennas, and guided waves. Several examples of accuracy estimations of the standards designed and used by the authors were introduced. Now we will try to present several methods that may be used when an estimation of separate factors limiting accuracy may be evaluated and the ways an estimation of a standard's accuracy may be verified. Although the role of the factors may be discussed without an unequivocal answer, intensive work has been done toward their full identification and evaluation. The first step here is the continuous care of the power sources, meters, directional couplers, and other auxiliary equipment. These devices must be stored in appropriate conditions that ensure their protection against electrical and mechanical failures, corrosion, and other unwanted damage that can degrade their electrical properties. Apart from this, the equipment should be subject to periodic testing and calibration. Some components, for instance, thermocouple heads or the diode detectors, may be tested and calibrated directly by their users. However, the most important devices must be calibrated in a specialized laboratory, for example, by the manufacturer or other authorized institution (as, for instance, with data shown in Tables 7.1a, 7.1b, and 7.2). Final results of the approaches, i.e., the accuracy of a standard, may be proven by a comparison with other devices. The comparisons may be performed within a single lab, with different standardization methods, or, much better, cross-comparison standards in different labs. The latter may be especially helpful in eliminating permanent errors that could neither be observed nor taken into account in a lab where they frequently appear in its routine measurements.
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9 Final comments
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This work briefly describes methods of standard electromagnetic field generation in terms of primary standards and different types of secondary standards including exposure systems. With some exceptions (as, for instance, exposure systems presented in section 3.5) we may summarize that there are only four approaches to standard EMF generation that are in general use, namely: standards with dipole antennas, standards with loop antennas, standards with horn antennas, standards based upon guided waves.
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Back Matter
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