One, the computation of screen efficiency:

 

Screen has two objectives: The electromagnetism that is limitative shield interior is annoyed give some area; more 2 it is to prevent ab extra electromagnetism to disturb (annoy) enter the some area inside shield. Shield has solid core commonly model, blame solid core (for example, gauze) braid a few kinds of types such as the belt with the metal, latter basically uses as the screen of cable. The screen effect of all sorts of shields all expresses with the screen efficiency of this shield.

 

Screen efficiency behaved shield the attenuation degree to electromagnetic wave. Can go to the intensity attenuation of electromagnetic wave normally as a result of shield original 1% to extremely one of, because this connects commonly used decibel (DB) will state. The screen efficiency of general shield can amount to 40 DB, the screen efficiency of the shield of equipment of for military use can amount to 60 DB, the screen efficiency of the shield of TEMPEST equipment can amount to 80 DB above.

 

 

The evaluation to screen action can express with screen efficiency:

 

 

Screen efficiency SE is bigger, state screen effect has been jumped over.

 

Additional, still can use transmission coefficient (or transmission coefficient) TE shows screen result, the H0 of magnetic field intensity when the HS of perhaps points to the somewhere when existence shield than; magnetic field intensity of the E0 of electric field intensity when the ES of electric field intensity that TE is the somewhere when pointing to existence shield and nonexistent shield and nonexistent shield, namely:

 

 

Transmission coefficient (or transmission coefficient) concern for reciprocal with screen efficiency each other, namely

 

 

2, the screen efficiency of complete shield:

 

Complete shield is the screen structure that shows one closes completely, electromagnetism field crosses discrepancy of shield wall ability to should close only structure.

 

 

1. The return loss of electromagnetic wave

 

Electromagnetic wave transmission produces reflection and transmission to different medium interface

 

Electromagnetic wave crosses the reflex when shield and transmission:

 

 

2. The absorption of electromagnetic wave loss

 

Electromagnetic wave arrives at shield wear when appearing personally

 

 

From on type can see, it is when frequency F is taller, absorbing loss is quite big, watch 2-1 gives out the T of screen slab thickness that when stuff of a few kinds of commonly used metals is absorbing loss to be A=8.68 DB, 20 DB, 40 DB respectively, needs.

 

Express 2-1 μ of conductance of the electrical conductivity σ of a few kinds of metals, magnetism and place need screen ply T.

 

Can by the watch 2-1 see: ? ?

 

① is become when F ≥1 MHz, with the shield that a kind of any plate with thick 0.5 Mm make, can will field is strong abate for former field strong 1/100 is controlled. Accordingly, when choosing material and ply, sex of the mechanical intensity that answering to consider data again, stiffness, craft and moistureproof, anticorrosive wait for an element. 

 

② is become when F ≥10 MHz, the screen that with 0.1 Mm thick cupreous skin makes field of physical ability general is strong abate for former field strong 1/100 is lower even. Accordingly, at this moment shield can stick the insulating material that has Copper Foil to be made with the surface. ·

 

③ is become when F ≥100 MHz, shield can be made with cupreous layer or silver-colored layer in the plating on plastic housing or gush.

 

Watch 2-2 listed stuff of commonly used metal is opposite opposite electrical conductivity of copper and opposite magnetism conductance. According to the absorption of the requirement attenuation can beg the ply that gives shield, by type

 

 

Stuff of watch 2-2 commonly used metal is mixed to opposite electrical conductivity of copper opposite magnetism conductance

 

3. Of electromagnetic wave for many times return loss

 

When electromagnetic wave wears a shield, wearing appear personally produce reflection, this reflex wave is returned when entering an area, be reflexed again, relapse so, till its energy is absorbed to come negligible till.

 

 

3, shield is not complete the influence to screen effect:

 

Total meeting has all sorts of aperture crack such as the door, lid, appearance, switch on shield, and penetrate even the line, these different level land destroyed the integrality of screen.

 

 

Influencing factor: The largest linear size of starting a hole (be not an area) , the frequency of wave impedance, electromagnetic wave.

 

 

Actual aperture leak not only with seam wide, board thick about, and measure of as linear as its dimension, gap, frequency concern.

 

Ought to reduce the presence of the aperture on shield as far as possible, and the length of aperture is controlled as far as possible be under the 1/20 of electromagnetic wave wavelengh.

 

2. The influence of starting a hole

 

Wait to install switch, pushbutton, potentiometer, often need to open circle, square or rectangular hole on screen face plate.

 

 

4. The influence of wire network

 

Apply at needing the shield of natural and ventilated or inward peek.

 

 

General, inside 1~100MHz, SE=60~100dB of metallic screen net, SE=50~90dB of net of screen of vitreous interlining metal.

 

Its diaphaneity is poorer when making peek window with wire network.

 

5. Film reachs the influence of electric glass

 

On glass or organic medium film vacuum evaporates or an electric film regards spray as electromagnetism shield, can use the wire network structure that replaces vitreous interlining.

 

Pervious to light the gender is good, effective to the screen of heft of the electric field in electromagnetism field, and fainter to the screen of magnetic field heft.

 

6. The influence of screen cable

 

Screen line and screen cable are an electron use at join in equipment the most commonly used lead when two shield. To assure softness, bend easily, its are outer commonly used much money paid for shares belongs to shield to silk is braided and be become.

 

Screen efficiency and the material that weave shield, density direct and relevant, general monolayer braids the screen efficiency of screen to be about between 50~60dB, double deck braids screen to be able to amount to 80~90dB.