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Book Review |
Reinaldo Perez, Associate Editor |
Measurements are an essential part of EMC and I thought this book could be of use to many of us. Though most EMC standards address measurements in the far field, it turns out that many of the diagnostics done in EMC and other disciplines (e.g antennas, radars, etc.) is done in the near field. I have come across other texts that deal with EM measurements in the near field but this is a new book that addresses such near field measurements from a general approach.
The book is divided into ten chapters and their titles are as follows: Chapter 1: Introduction, Chapter 2: The Principles of Near Field EMF Measurements, Chapter 3: EMF Measurements Methods, Chapter 4: Electric Field Measurements, Chapter 5: Magnetic Field Measurement, Chapter 6: Power Density Measurements, Chapter 7: Directional Pattern Synthesis, Chapter 8: Other Factors Limiting Measurements Accuracy, Chapter 9: Photonic EMF Measurements, and Chapter 10: Final Comments.
EMF measurements in the far field (fraunhofer zone) are one of the less accurate measurements of physical quantities. Hazardous exposure to EMF requires field measurements in the primary and secondary field sources as well as fields disturbed by the presence of materials and the transmission media. The attention of this book is in the near field (fresnel region). The near field conditions cause further degradation of the near field EMF measurements accuracy as compared to those in the far field. These difficulties bring frustration to the designers of EMF measurement equipment. The problem with EMF standards is to address the accuracy of field measurements. According to the author, the accuracy of a good EMF standard does not exceed +/- 5%. The book is devoted to the specific problems of EMF measurements in the near field and to the analysis of the main factors limiting the measurement accuracy, especially in the near field. Chapter 1 addresses the numeric limits and frequencies in some of the known standards. Chapter 2 provides essential information for practical metrology, including a brief summary of the near field properties as well as the basic equations and formulas related to fields generated by simple radiating sources.
In order to determine the best method for EMF measurement in the near field, it is first necessary to determine which quantities best characterize the field. These quantities will then be the subject of measurements. Chapter 3 addresses the measurements of E, H and S in the near field. From the point of view of shielding, absorbing, or EMF attenuating materials, investigations of the E, H and S measurements are sufficient. For protection from unwanted EMF, the measurement of specific absorption rate (SAR) is added to the needed measurements. The SAR is calculated based on temperature rise measurements. The chapter also addresses the measurements of induced currents in the body. Electric field measurements are discussed in Chapter 4. The basic method of electric field measurements, within a wide frequency range, involves the use of a charge induction in a body illuminated by the field. Field averaging by a measuring antenna is discussed in the chapter and error of E-field measurement resulting from averaging of the measured field by the measuring antenna is calculated. The possibility of the field spatial distribution measurement, especially under conditions of multipath propagation and interference, and the measurement of the maximal and minimal magnitudes of the field strength requires the use of probes equipped with antennas whose sizes are much less than the wavelengths. Dipoles are normally used for such measurements and the measurement error versus Kh of the dipole antenna is calculated. It is important to know the precise frequency response of E-field measuring probes. In order to achieve this, the schematic representation of an E-field probe is studied (for low and high frequencies), as well as the schematic representation of such probes with different types of filtering. Chapter 4 also addresses the interactions of the measuring antenna and the field source and the sources of errors that such interactions introduce. This work is also extended to include errors from antenna input impedance changes. The chapter also addresses the changes to the probe's directional pattern. The magnitudes of the calculated directional pattern irregularities versus Kh are presented. This consideration is related to the electrical length of the dipoles, which is of special concern when resonant antennas are used (for shorter antennas, the error vanishes). The chapter ends with comparison of E-field probes.
Chapter 5 is very similar to Chapter 4 in its approach as it addresses magnetic field measurements. This chapter discusses probe properties for RF magnetic field measurements and in particular, the factors limiting measurement accuracy. The approach is limited to a probe consisting of a circular loop antenna loaded with a detector of a shaped frequency response. The majority of the presented estimations are fully applicable for hall-cell probes, magneto-optic probes, magneto-diode probes, and for other designs, especially when considering the averaging of the measured field upon the surface of the measuring antenna. As was done in Chapter 4, the frequency response of the probe is modeled using circuit analysis by representing the probe in its electrical schematic representation. The error in the accuracy of the measurement is made as a function of the distance between the source of radiation and the antenna. The effect of antenna input impedance is also addressed in the calculations.
Power density measurements are discussed in Chapter 6. The power density measurements in the near field (especially in proximity to electrically small sources) using the electric or magnetic field measurement is burdened with an error whose value is dependent upon the type and the structure of the source and the measured EMF component as well as the distance between the source and a point of observation. Based upon error measurements as a function of kR, as well as assuming the smallest distances from a field source in which the measurements should be performed, it is possible to estimate the minimal frequency at which the measurement may be applied without the necessity of using additional correction factors (because of the deterministic character of the error, the factors may be analytically estimated for a known source type, measured EMF component, propagation geometry and distance). Widely applied methods of the electromagnetic power density measurement are presented and analyzed in this chapter. The considerations are purely theoretical in character and they are concerned with the fields' relations only. The source of the accuracy limitation of the measurement (error of the method) is demonstrated in the chapter and the magnitude of the error for different combinations of source and measured is estimated. Then, a certain improvement of the accuracy, as a result of simultaneous measurements of E and H fields, and calculation of an arithmetical or geometrical mean is proposed. Chapter 7 addresses the detailed considerations of the polarization problem resulting from the need to understand and apply the omni-directional probes as well as their design and construction. These needs are the result of: the specificity of the polarization intricacies, especially in the near field; the necessity of understanding the polarization phenomena in order to select optimal procedures when measurements are planned and for interpretation of their results; and the development of the ability to select an appropriate probe and meter for specific measurements conditions.
In the previous chapter, the most typical and frequent factors limiting EMF measurement accuracy were discussed. They resulted mainly from the type of antenna applied in the EMF probe, its size, and mutual coupling between the antenna and the radiation source. In Chapter 8, a brief discussion is presented on the influence of thermal drift upon the probe parameters, the role of its dynamic characteristics, and the deformation of the measured field by a person performing the measurements, the meter, resonant phenomena, and other factors.
Chapter 9 addresses the issue of photonic measurements. There is considerable literature devoted to the field of opto electronics elements use as sensors for a variety of physical measurements. Several publications discuss the use of opto electronic transducers for EMF measurements. The principle of a photonic transducer may be conveyed to the modulation of an optical beam and the subject of the modulation may be the signal's phase (which is a function of the light velocity of propagation in an electro-optic medium), its frequency, amplitude or polarization. The type of modulation as well as that of the electro-optic crystal is selected in such a way as to obtain the device's maximal sensitivity. Sensitivity is still a measure issue with optical detectors. In Chapter 9, two approaches are followed; the direct interaction of the measured field onto the electro-optical crystal, and a voltage induced by the field in an auxiliary antenna (playing the role of the measured field concentrator) impressed to the modulator. Apart from technological differences, the factors limiting measurement accuracy are, in the case of the photonic probes, similar or identical to those discussed in Chapter 8.
Chapter 10 is dedicated to some final comments by the author. Again, this is a new and useful book which I recommend. EMC