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Elektromagnetik etkileşimin özlenmesi için ekranlama düzenlerinin tasarımı

Başlık çevirisi mevcut değil.

  1. Tez No: 75127
  2. Yazar: HAKAN YAŞAR
  3. Danışmanlar: PROF. DR. ERCAN TOPUZ
  4. Tez Türü: Yüksek Lisans
  5. Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1998
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Elektronik ve Haberleşme Mühendisliği Ana Bilim Dalı
  12. Bilim Dalı: Elektronik ve Haberleşme Mühendisliği Bilim Dalı
  13. Sayfa Sayısı: 106

Özet

ÖZET Bu çalışmada, cihazların ışınım yoluyla etkileşiminin önlenmesi için ekranlama düzenlerinin tasarımı incelenmiştir. Elektronik cihazları ve sistemleri, elektromagnetik interferansa karşı korumanın ana yollarından birisi ekranlamadır. Elektromagnetik emisyonlar ile elektromagnetik çevrenin kirliliğinin artması ve teknolojideki ilerlemeyle bir çok elektronik sistemin elektromagnetik duyarlılığının artmasından dolayı, elektriksel sistemlerdeki elektromagnetik etkileşimin etkileri sorun olmakta ve elektromagnetik uyumluluğun önemi artmaktadır. Ekranlama etkinliği, ekranlanan bölgeye giren veya çıkan alanın şiddetindeki desibel olarak azalma ile karakterize edilebilir. Ekranlama etkinliği, sadece ekranlamanın yapıldığı malzemeye ve kalınlığına bağlı kalmayıp, aynı zamanda frekansa, ekranın kaynağa olan uzaklığına ve ekran süreksizliklerinin şekline ve miktarına da bağlıdır. Uygulamada, ekranlardaki açıklıklardan kaçınmanın mümkün olmadığı bir çok durum vardır. Bunlar, giriş-çıkış bağlantıları, kontrol panelleri ve dahili elektronik elemanların ısınmasından dolayı havalandırılmasını sağlayan fan ve havalandırma kanalları gibi değişik sebeplerden dolayı mevcuttur. Bu delikler, ekranın verimini azalttığından dolayı önem taşırlar. Bu tezde, çeşitli tip ekranların ( kapalı, tabakalı, ve bazı bölgelerinde delik bulunan ) ekranlama etkinliği incelenmiş ve tabakalı ekranlar için sayısal değerlendirmeler yapılmıştır. XI

Özet (Çeviri)

SUMMARY ANALYSIS AND DESIGN OF ELECTROMAGNETIC SHIELDING SYSTEMS In this study, the analysis and design of electromagnetic shielding systems are investigeted. Utilizing certain shielding teqniques, it is possible to minimize the destructive effects of electromagnetic interference between electronic devices and systems. Electromagnetic compatibility (EMC) is the ability of devices or systems, subsystems, circuits and components to function as designed without error, malfunction or unacceptable degradation of performance, due to electromagnetic interference (EMI) within their intended electromagnetic environment. Because of the increased pollution of the environment with electromagnetic emissons and because of the rising susceptibility of system components, the effects of electromagnetic interactions in electrical systems are a growing matter of concern. Ever increasing density of electronic devices result in an electromagnetic environment that is becoming increasingly more susceptibile to interference. Whenever a new device or element is added in environment, the probability of electromagnetic interference increases. For this reasons, the importance of electromagnetic compatibility in the design of electric and electronic devices is continously increasing. The sources of electromagnetic interference can be classified as natural, such as lightning and electrostatic discharge or man-made, such as the intense electromagnetic signals from a nearby transmitters (e.g. radar, radio and television), power switching devices, electric motors, welding devices and unwanted RF emissions from the operation of other electric/electronic devices. The sources of electromagnetic interference are unwanted emissions which reach the device under consideration either via conduction or radiation. These noise sources can be either a conducted voltage or current, or an electric or magnetic field propagated. When the source of interference cannot be controlled, or does not require control of outer interaction, a properly designed shield prevents coupling of undesired radiated electromagnetic energy into the device or equipment. In most cases, the effectiveness of an electromagnetic shield is determined by apertures that exist in the shield. In a lot of practical situations, apertures are necessary for the provision interconnection of the device with other subsystems/devices, and for cooling/ventilation purposes. XllThe organization of the thesis is as follows: In the first Chapter, the purpose and the content of the study is introduced. Chapter 2 deals with electromagnetic compatibility, and introduces the basic concepts of electromagnetic interference. The theory of shielding that is the principal means of protection against interference is presented in Chapter 3. In this chapter, plane wave shielding theory is discussed for a single layer of shield, and for doubly layered and laminated shields. For design practice, the basic expressions are given together with their numerical evaluations utilizing (a normalized universal) parameter set. Finally, in Chapter 4, the problem of radiated emission from apertures in metalic enclosures are investigeted. The degradation of the shielding efficiency, due to the presence of apertures of various sizes and dimentions are investigeted. For electrically small apertures, which are of importance in many practical problems, the radiated/intercepted fields can be described in terms of electric and magnetic polarizabilities of these apertures. This case is investigeted in the thesis and analitic expressions for the insertion loss of the resulting shields are given. The thesis ends with a summary of main conclusions reached in the thesis and a brief discussion of the problem areas for further research. The derivation of electromagnetic field distributions inside a rectangular cavity, utilizing the Green's function approach is given in Appendix A. In Appendix B, the mechanism of coupling through apertures in rectangular cavities and waveguides is described on hand of two field equivalence principles. The approach adopted in the thesis for the analysis of electromagnetic shielding and main conclusions are described below: The analysis of an electromagnetic shield can be based upon transmission line equation, when transverse electromagnetic waves are considered (Fig. 1.). Shielding effectiveness or insertion loss is defined as the ratio of the field strength at a point external to the shield, before and after insertion of the shield and is expressed in dB. Shielding effectiveness S is expressed as in equation (3.68): S=A+R+B (1) In the expression for shielding effectiveness given in equation (1), all terms can be expressed in terms of functions of shield parameters cr, pi, which are commonly expressed relative to the parameters of copper, and the frequency/ The absorption loss A is A = \.3\4jfröd dB (2) X1UH x=0 3C=1 x=0 xH dE. - = -jw\iH ax dH f. Mr - = -(a + yw8)£ ax ax dl_ dx = -YV (a) (b) Figure 1 Transmission line analogy a) Transverse electromagnetic wave expressions for planar sheet b) Transmission line equavalance where d : thickness of shield, cm ]+0.0535D(/a/u)1/2+ 0.354} dB (3) for a plane wave source : £ = 168-201ogO/a)1/2 dB (4) for a high impedance source £ = 362 -20 log {^'vTd dB (5) XIVwhere, D : the distance from source to shield, cm a : conductivity relative to copper H : permeability relative to copper ?E|« Eu^ti EWHa (a) ^4^4 (b) 'Ej2>^2 E«rH« Figure 2 Reflection of waves (a) at single interface (b) at double interface If penetration loss A is equal to or more than 15 dB, the effect of the internal reflections can be ignored and correction term B can be neglected. When this is not the case then, correction term B which is given by B(dB) = 201og|l - XlO~{An0). (cosO.230^ - /sin 0.230^ (6) has to be taken account, lvalues which apply to (6) are given in Table 3.1. Except for low frequency shielding against low impedance fields, one can ascert X=\. For perforated sheets, screening and waveguide arrays shielding effectiveness Sa is given by equation (3.102). The fields inside a perfectly conducting rectangular cavity excited by a center-driven thin dipole (Fig. 3.) and a square loop (Fig. 4.) are expressed as in equation (4.5), (4.6) and (4.10), (4.1 1) respectively. When apertures are electrically small, the fields in the vicinity of the hole can be represented approximately by the fields at the wall without an aperture, plus the fields of electric and magnetic dipoles located at the center of the aperture. XVFigure 3 A center-driven thin dipole inside a rectangular cavity The field transmitted to the other side of the conducting wall may be considered dipole field and can be calculated from electric and magnetic dipole moments. These dipole moments are given by P = aeeQE0 M = -amH0 (7) (8) ae : electric polarizability scalar am : magnetic polarizability tensor d z Figure 4 A square loop in a rectangular cavity For an open cavity, for the same source (see Fig. 4.3, Fig 4.4) the fields are expressed by (4.16), (4.17) and (4.22), (4.23) respectively. When the antenna-aperture distance is greater than 0.1a, where a is the typical dimension of enclosure, the effect of the aperture on antenna is negligible. Insertion-loss expressions is given for a cavity with small apertures and an open cavity as follows: XVIFor a dipole antenna with constant current at the antenna terminals: Electric field, (I-L.),n = hh (r\ k\M\ \r') Magnetic field, jt-1- r 1 Ih ( r\ jJc~~ (IX.). = ^ ir] k\M\ \r J jk - +. r jkr where r' = r + d'. For a dipol antenna with constant voltage at the antenna terminals: (I.LVo=4(I.L)/o Z, = input impedance of the antenna inside the cavity Z, = input impedance of the antenna in free space For a loop antenna with constant voltage at the antenna terminals Electric field, CL h° M“ L'J.. 1 jk-- r Magnetic field, 4LD2 (r^ jk ”'+-'~'2 W/.-^-^ M. r' + jkr' r jkr~ where r' = r + (d - d'). (9) (10) (ID (12) (13) xvu

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