Introduction. Concept of shielding

Transcription

1 Shielding

2 Introduction Concept of shielding

3

4 Shielding of a metal shield (theory) For E field SE.. 2log E E i t For H field SE.. 2log H H i t Do they be equal?

5 Shielding of a metal shield (theory) It depends. For plane wave, they are equal But for near field, they are not.

6 Shielding of a metal shield (theory) 1. Plane wave assumption 2. Continuity of E and H at each boundary E E i t 2 ( ) [1 ( ) 2 t / j2 t ] t / jt where e e e e e 4 j j :Skin depth j t How to derive?

7 11.1 Shielding effectiveness. (S.E.) a. Definition Ei 1 for electric firld : S. E. 2log E Hi 2 for magnetic field : S. E. 2log H note: 1. If the incident field is an uniform plane wave,and the medium on each side of the barrier are identical, 1 and 2 definitions are identical. 2. For near fields and/or different medium, 1 and 2 are not equivalent. t t 3. Definition 1 (for E field) is taken as standard for 2 case.

8 C. Causes of shield (1) Re flection. (2) Absorption : e - z (3) Multiple reflection ( : attenuation) 1 Note : (1) & (2) will increase the S. E. of the barrier, but (3) will decrease the S. E. of barrier. S. E. R A M db db db db

9 11.2 Shielding effectiveness: far field sources (1) : - j z i x i j z i i y j z r r x j z r r y z x Exact solution For plane wave E E e a E H e a E E e a E H e a E E e a

10 E 1 - z H1 e ay where E E e a - z 2 2 x H - E e a 2 - z 2 y - j z t t x E E e a E j - j z y j t H t e a j j j Exact solution (2) E is known, and to determine the remaining amplitudes. i Er, E1, E2, and Et.

11 B. C. at Z Ei Er E1 E2 Ei Er E1 E2 Hi H r H1 H 2 Ei Er E1 E 2 at Z t E1 E2 Et - t E1 e - t E2 e - t Et e H1 H 2 H t E1 - t E2 - t E - - t t e e e t ( j t t ) e jt - j t e e e e 4 exact solution. where 1/, j in barrier metal.

12 4 Simplifications: 1. assume the barrier is a "good conductor" skin depth barrier thickness t. -t - t -t - jt - jt 1. e e e e e Ei E t t - t 4 4 e e

13 5 t S. E 2 log 2 log e M. db 1 1 db 4 P S R e A t.. : 2 log :, 2 log :. 1 db 1 db 4 6 The Multiple - reflection term is the middle term of (3). - -2t - j 2t M 2 log 1 - e e db 2-2t - j2t 2 log 1- e e ( if t ) where 1 for good conductor.

14 Approximate solution Assumption : 1 ( a good conductor) E E i. 2 E t 2 E t a. Re flection Loss ( E field ) t E

15 3 In the absence of attenuation E t Et E1 i 1 i E E E 4 2 the same as exact solution. E 4 2 log i R 1 2 log 2 log db E 4 4 t 2 Note: The transimission coefficient is very small 2 at the Boundary 1, and is approximately 2 at the Boundary 2. very little of E field is transmitted through the B.1.

16 E1 H1 E H i Ei Ei Et Ht Et 2 2 H 1 E1 E 1 3 H H H 4 t t t 2 i i i H H H if if

17 Note: E 1. E t i H t i H But primary transmission of H field occurs at the boundary 1,whereas the primary transmission of the E field occurs at the boundary It means that "thick" boundary has more effect on shielding against H field than to the E field. C. Absorption loss E 1 is attenuated in the conductor. Absorption factor A e t A db 2log e t

18 d. Multiple Reflection Loss When t >>δ the multiple reflection loss may be important. (1) for magnetic field 1. in in E in H t H H T H T H 2 1

19 Ĥ in Ĥ t1 Ĥ t2 boundary 1 boundary 2

20 2. The reflected wave is 2 E E e e rt 2 H in e rt H in at rt H in at boundary 1 boundary 2 3. rt t T H E e H H in

21 4. H t H H H t1 t1 t1 H t 2 rt 2 rt 4 1 rt e e let e (1 E 2 ) H, t3 E 1 E 5. H H t t1 2 4 SE 2log 2log 2log 1 db i H H i

22 log 1 2log 1 t db t j tβ ς η η M e η η η η e e η η same as exact solution

23 11.3 Shielding effectiveness - near field sources a. In the far field: (1) E E (2) (3) d and H 3 H are orthogonal

24 b. Near field for Hertzian dipole (1) Z w j j j 2 3 r r r j 1 2 r r E H j 3 r r r

25 (2) 6 2 r r Z w e (3) E H near field far field electric dipole is a high impedance source 3 r

26 c. Near field for magnetic dipole (loop) (1) Z w E 1 1 H r9 j 1 2 r r j r r r 2 3

27 (2) (3) E H Z w m near field 2f far field 2369 magnetic dipole impedance source in the near field is a low 3

28 d. Reflection loss for Electric source (1) we know for plane wave R db 2log (2) 2log E E 4 i t 2 2log 4 near field using Z w j r Z w r RdB 2 log log r f r

29 Shielding of a metal shield (Near Field) Electric dipole Z w 6 r Magnetic dipole Z w 2369 r

30 Shielding of a metal shield (Near Field) For electric dipole R e r 322 1log( ) 3 2 f r r For magnetic dipole R m 2 fr r log( ) r

31 Shielding of a metal shield (Near Field) 1. Reflection Loss increase as frequency decrease for E field 2. Reflection Loss decrease as frequency decrease for H field Note: The Absorption loss also Decrease as frequency decrease for H-field. How to solve the SE of magnetic field?

32 An example of S.E. for copper plate

33 Low Frequency, Magnetic Shielding Two approaches

34 Low Frequency, Magnetic Shielding Two factors that may degrade the ferromagnetic material: 1. Increasing frequency. 2. Increasing the field strength. Larger than 4KHz, the u is the same with the steel That s why we use the steel as The magnetic shielding material for Switch-mode Power Supply (2-1KHz

35 Low Frequency, Magnetic Shielding Phenomenon of the saturation of ferromagnetic materials saturation When increase the H-field

36 Low Frequency, Magnetic Shielding Solution: Using multi-layer to reduce the effect of the saturation

37 The effect of aperture In practice, the S.E. is limited by the necessary apertures and discontinuities. The simplest formula SE 2log 2 d The modified formula Ex: 2dB SE for 1GHz then, d < 1.6cm

38 The effect of mesh and Honeycomb SE can reach 1dB Why?

39 The effect of Seams Poor Better Best

40 The effect of aperture orientation Which one is better? By this trick, the SE can be improved up to 1dB

41 The effect of image plane

42 Enclosure resonance F k l m h n w ( / ) ( / ) ( / ) MHz F ~ 212/l ~ 212/h ~ 212/w for equal square enclosure

43 Gasket and Contact Strip Choosing concern: 1. Conductivity 2. Ease of mounting

44 Measurement of SE Method 1: EN5147-1

45 b. Measurement of S. E.

46 Measurement of SE Method two: Coaxial transmission line model