Chapter 6 Shielding. Electromagnetic Compatibility Engineering. by Henry W. Ott

Transcription

1 Chapter 6 Shielding Electromagnetic Compatibility Engineering by Henry W. Ott 1

2 Forward A shield is a metallic partition placed between two regions of space. To maintain the integrity of the shielded enclosure, noise voltages should be filtered from all cables that penetrate the shield. Cable shields that penetrate a shielded enclosure must be bonded to that enclosure. 2

3 Near Fields and Far Fields The characteristics of a field are determined by the source (the antenna), the media surrounding the source, and the distance between the source and the point of observation. 3

4 Near Fields and Far Fields The ratio of E to H is the wave impedance. In the near field E and H must be considered separately. 4

5 Characteristic and Wave Impedance Zw E H Z0 j j For any conductor, in general, Z S 7 r r f

6 Shielding Effectiveness S E log 10( ) 10 E1 H1 S H 20log ( ) In the design of a shielded enclosure, there are two prime considerations: 1. S of the shield material itself. 2. S resulting from discontinuities and apertures in the shield. At high frequencies, it is S of the aperture that determines the overall S of a shield, not the intrinsic S of the shield material. S varies with frequency, geometry of shield, measurement position, type of field, angle of incidence, and polarization. 6

7 Shielding Effectiveness The S results of the plane sheet calculations are useful for estimating the relative S of various materials. S ARB db. A: absorption or penetration loss is the same in either the near or the far field and for E or H fields. R: reflection loss is dependent on the type of field, and the wave impedance. B: correction factor accounting for multiple reflections in thin shields. B can be neglected if A > 9; and can also be neglected for E and plane waves. 7

8 Absorption Loss t E1 E0e t H1 H0e f r r m. in. A E t t 20log ( ) 20log ( e ) 8.69 db E1 3.34t f db. ( t is in inch) r r 8

9 Absorption Loss 9

10 Reflection Loss Z1 Z2 H 2Z H When a wave passes through a shield, it encounters two boundaries 10

11 Reflection Loss For thick shields, the total transmitted wave is E t 4ZZ 1 2 ( Z Z ) E 0 4Z E 4ZZ 4Z H Ht H Z ( Z1 Z2) 1 Z1 0 If Z Z 1 2 The largest reflection occurs when the wave enters the shield (1st boundary) for E; The largest reflection occurs when the wave leaves the shield (2nd boundary) for H. Because the primary reflection occurs at the 1 st surface for E, even very thin materials provide good reflection loss. For H, the primary reflection occurs at the 2 nd surface. The multiple reflections within the shield can significantly reduce S. 11

12 R Reflection Loss E Z Zw 20log ( ) 20log ( ) 20log ( ) db. E 4Z 4 Z If the wave approaches at other than normal incidence, then R increases with the angle of incidence. S R to Plane Waves r R log 10( ) db. f r 12

13 Reflection Loss R in the Near Field 1. R for a practical source lies between the electric field lines and the magnetic field lines. 13

14 Reflection Loss Electric Field Reflection Loss 1. Zw e R e 1 2 fr when r log ( ) db. r r f r 2. An actual electric field source, however, has some small magnetic field component in addition to the electric field. It therefore has a R somewhere between the electric field line and the plane wave line. Magnetic Field Reflection Loss 1. Z 2 fr when r 2. w m R m 2 frr log 10( ) db. r 14

15 Reflection Loss 2. Most real magnetic field sources have a small electric field component in addition to the magnetic field. It therefore has a R somewhere between the magnetic field line and the plane wave line. 3. When the distance to the source is not known, the near field magnetic R can usually be assumed to be zero at low frequencies. Multiple Reflections in Thin Shields 1. 15

16 Reflection Loss 2. This can be neglected in the case of a thick shield, because A is high. 3. For E, most of the incident wave is reflected at the 1 st boundary, and only a small percentage enters the shield. Therefore, multiple reflections within the shield can ne neglected for E. 4. For H, most of the incident wave passes into the shield at the 1 st boundary. The effect of multiple reflections inside the shield must be considered. 5. B 2t 20log 10(1 e ) db. (See Appendix C) 16

17 Reflection Loss 17

18 Composite Absorption and Reflection Loss Plane Waves B is normally neglected in calculating S for plane waves, because R is so high and the corretion term is small. 18

19 Composite Absorption and Reflection Loss Electric Fields B is normally neglected in calculating S for electric fields, because R is so high and the corretion term is small. Magnetic Fields If the shield is thick (A > 9 db), B can be neglected. If the shield is thin, then B must be included. The primary loss for magnetic fields is A. It is difficult to shield lowfrequency magnetic fields. 19

20 Composite Absorption and Reflection Loss 20

21 Shielding with Magnetic Materials If a magnetic material is used, and. Two effects: A increase. R decreases. In the case of low-f magnetic field, very little R occurs, and A is the primary shielding mechanism. Under these conditions, it is often advantageous to use a magnetic material to increase A. In the case of low-f electric field or plane waves, the primary shielding mechanism is R. Thus, using a magnetic material would decrease S. When magnetic materials are used as a shield, three often overlooked properties: 21

22 Shielding with Magnetic Materials 1. as f. 2. depends on field strength. 3. Machining or working high magnetic materials, such as mumetal, may change their magnetic properties. High materials are most useful as magnetic field shields at low f. 22

23 Shielding with Magnetic Materials In general, the higher, the lower is the field strength that causes saturation. 23

24 Shielding with Magnetic Materials Another advantage of multilayer shields is that increased R occurs from additional reflecting surfaces. 24

25 Experimental Data The measurements were made in the near field with the source and receptor 0.1 in apart. 25

26 Apertures In practice, most shields are not solid. There must be access covers, doors, holes for cables, ventilation, switches, displays, and joints and seams. All of these apertures considerably reduce S of the shield. As a practical matter, at high f, the intrinsic S of the shield material is of less concern than the leakage through the apertures. Apertures have more effect on the magnetic field leakage than on the electric field leakage. Accordingly, greater emphasis is given to methods of minimizing the magnetic field leakage. 26

27 Apertures The amount of leakage from an aperture depends mainly on the following 3 items: 1. The maximum linear dimension, not area, of the aperture. 2. The wave impedance of the EM field. 3. The f of the field. 27

28 Apertures Slots, even if very narrow, can cause considerable leakage if their lengths are greater than 1/10. For an aperture, S 20log 10( ) 2 Multiple Apertures More than one aperture will reduce the S of an enclosure. The amount of reduction depends on 1. the number of apertures, 2. f, 3. the spacing between the apertures. For a linear array of closely spaced apertures, the reduction in S: S 20log n

29 Apertures It is advantageous to distribute apertures around the surfaces of a product to minimize the radiation in any one direction. 29

30 Apertures Seams A seam is a long narrow slot that may or may not make electrical contact at various points along its length. It is necessary to guarantee electrical contact points at frequent intervals along a seam in order to reduce the length of the resulting antenna. A seam with periodic contact points can then be considered a linear array of closely spaced apertures. 30

31 Apertures Contact can be obtained by using (1) multiple fasteners, (2) contact buttons, (3) contact fingers, or (4) conductive gaskets. Low contact resistance between mating surfaces is primarily a function of the following two items: 1. Having a conductive surface or finish. 2. Providing adequate pressure. Transfer Impedance A better, more reliable, and more repeatable method of measuring the quality of electrical contact between mating parts of a shield is by measuring its transfer impedance,. Z T 31

32 Apertures S ZW 20log 10( ) Z T 32

33 Apertures R is a function of actual electrical contact between the mating parts, and it depends primarily on the surface finish and pressure. C depends on the spacing and the surface area of the two half s of the seam. Increasing the capacitance alone, however, is not sufficient to produce a lowimpedance seam without also providing direct electrical contact with sufficient pressure between the mating parts. 33

34 Waveguide below Cutoff Additional attenuation can be obtained from an aperture if the hole has depth. The cutoff f of waveguides: f c f c d Hz for a round waveguide. d is the diameter in inche. s Hz for a rectangular waveguide. is the largest dimension of the waveguide's cross section in inches. As long as f is much less than f c S t 32 db for a round waveguide. d 34

35 Waveguide below Cutoff t S 27.2 db for a rectangular waveguide. is the largest dimension of the hole's cross section in inches. The shielding determined from the above equations is in addition to that resulting from the size of the aperture, S 20log 10( ). 2 35

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