Abstract: Discussed the electromagnetic screen technology, including electromagnetic screen technical principle, shielding material performance and application situation, shield technology matters needing attention, shield potency examination as well as privileged sites shield measure.
Key word: Electromagnetic screen; Shielding material; Shield potency
Introduction
In the last few years, along with the electromagnetic compatibility work’s development, the electromagnetic screen technology applies more and more widely. To have a more thorough understanding to the electromagnetic screen technology, must to the shielding material performance and the application situation, the shield technology matters needing attention, the shield potency examination as well as the privileged sites shield measure and so on carries on a more thorough discussion.
1 electromagnetic screen technical principle
The electromagnetic screen is one of electromagnetic compatibility technology key measures. Namely seals up with the metallic shield material the electromagnetism noise source, causes its exterior electromagnetism field intensity to be lower than the permissible value one measure; Or seals up with the metallic shield material the electromagnetism sensitive electric circuit, causes its internal electromagnetism field intensity to be lower than the permissible value one measure.
1.1 electric shields
Will bring the positive electricity conductor with the complete metallic shield body to surround, will be shielding body’s inside to induce with the charged conductor isometric negative charge, the flank appears with the charged conductor isometric positive charge, if metallic shield body earth, then the flank positive charge will flow in the earth, the flank will not have the electric field existence, namely the belt positive electricity conductor’s electric field is shielded in metallic shield in vivo.
1.2 alternating electrical field shield
In order to reduce the alternating electrical field to the sensitive electric circuit’s coupling disturbing voltage, may establishes the electrical conductivity good metallic shield body in noise source and between the sensitive electric circuit, and metallic shield body earth. The alternating electrical field is decided to the sensitive electric circuit’s coupling disturbing voltage size by the alternating electrical field voltage, the coupling capacity and product of the metallic shield body earth resistance. So long as tries to cause the metallic shield body good earth, can cause the alternating electrical field becomes to the sensitive electric circuit’s coupling disturbing voltage very small. The electric field shield reflects primarily, therefore shields body’s thickness not to need to be oversized, but take structural strength as main consideration factor.
1.3 alternating magnetic field shield
The alternating magnetic field shield has high frequency and the low frequency division. The low frequency magnetic screen is uses the high magnetic conductivity the material to constitute the low magnetic resistance circuit, causes the majority of magnetic fields to concentrate is shielding in vivo. Shields body’s magnetic conductivity to be higher, thickness is bigger, the magnetic resistance is smaller, magnetic screen’s effect is better. Certainly must be coordinated with equipment’s weight. The high frequency magnetic field’s shield uses the turbulent flow reversing magnetic field which the high conductivity the material produces to counter-balance disturbs the magnetic field to realize.
1.4 alternation electromagnetic field shield
Generally uses the conductivity high material to do shields the body, and will shield the body earth. It is uses the shield body to produce the reversed direction turbulent flow magnetic field and the self-magnetic field counter-balance under the high frequency magnetic field function weakens the high frequency magnetic field the disturbance, realizes the electric field shield because of the shield body earth. Shields body’s thickness not to need to be oversized, but take skin depth and structural strength as main consideration factor.
2 shield potency computation
Shield potency (SE) the definition is: The identical place is unshielded after the electromagnetic field the electromagnetism field intensity and Canada shield the body’s ratio of electromagnetism field intensity. The commonly used decibel counts (dB) to indicate.
SE=A R B (1)
In the formula: A is the absorption loss;
R is the reflection loss;
B is many times the reflection loss.
2.1 electromagnetic wave reflection loss
Because the air and the shield metal’s electromagnetism wave impedance is different, causes the incident electromagnetic wave to have the reflex action. But the air electromagnetism wave impedance is dissimilar in the different field source and the field area, calculates as follows separately.
In magnetic field source near field’s reflection loss R(dB) is
In the formula: μr is the relative permeability;
σr is the relative conductivity;
f is the electromagnetism wave frequency (Hz);
D is the radiant to shields body’s distance (cm).
In electric field source near field’s reflection loss R(dB) is
In electromagnetic field source distant field’s reflection loss R(dB) is
R=168-10log10 (μrf/σr) (4)
2.2 electromagnetic wave absorption loss
When enters in the metallic shield the electromagnetic wave in the shield metal biography time transmission, as a result of weakens has the absorption. Absorption loss A(dB) is
In the formula: d is shielding material thickness (mm).
2.3 many times reflection losses
Electromagnetic wave in shielding layer multiple reflection loss B(dB) is
In the formula: Zm is the shield metal electromagnetism wave impedance;
Zw is the air electromagnetism wave impedance.
When A>10dB, generally may not count many times the reflection loss.
2.4 shield potency computation example
The field source is away from the different material the shield body (thickness 0.254mm) the 30cm far shield potency (dB) the computed result to see Table 1. Table 1 near field and the distant field threshold is λ/2π, Lamda is the electromagnetic field wave length.
Table 1 source is away from the different material the shield body (thickness 0.254mm) 30cm far shield potency dB
|
Frequency /Hz |
Copper |
Iron |
Aluminum |
||||||
|
Magnetic field near field |
Electric field near field |
Distant field |
Magnetic field near field |
Electric field near field |
Distant field |
Magnetic field near field |
Electric field near field |
Distant field |
|
|
60 |
3.46 |
Spatial |
Spatial |
3.22 |
Spatial |
Spatial |
Spatial |
Spatial |
Spatial |
|
1k |
24.89 |
Spatial |
Spatial |
14.66 |
Spatial |
Spatial |
Spatial |
Spatial |
Spatial |
|
10k |
44.92 |
212.73 |
128.73 |
51.50 |
217.50 |
134.00 |
Spatial |
Spatial |
Spatial |
|
150k |
69.40 |
190.20 |
130.40 |
188.0 |
308.0 |
248.00 |
Spatial |
Spatial |
Spatial |
|
1M |
97.60 |
185.40 |
141.60 |
391.0 |
479.0 |
435.00 |
88.00 |
176.0 |
- |
|
15M |
205.0 |
245.0 |
225.0 |
1102.0 |
1143.0 |
1123.0 |
174.0 |
215.0 |
- |
|
100M |
418.0 |
426.0 |
422.0 |
1425.0 |
1434.0 |
1430.0 |
342.0 |
350.0 |
- |
3 shield matters needing attention
3.1 shield integrities
If shields the body not to be incomplete, will cause electromagnetic field divulging. Specially the electromagnetic field shield, it uses shields the body to produce the reversed direction turbulent flow magnetic field and the self-magnetic field counter-balance under the high frequency magnetic field function weakens the high frequency magnetic field disturbance. If shields the body not to be incomplete, the turbulent flow effect reduces, namely the shield effect sells at a discount greatly.
3.2 shielding material shield potency and application situation
The electromagnetic screen technology’s progress, urges the shielding material the form to develop unceasingly, but no longer limits to the single-layer metal dull pattern, the shield potency also unceasingly enhances. When application wants the special attention different shielding material to have the different shield potency and the application situation.
3.2.1 metal plates
The electronic installation uses the metal plate to make the engine case, both firm durable, and has the electromagnetic screen function. Its electromagnetic screen potency and the metal dull material nature, the electromagnetic field source nature, electromagnetic field parameters and so on source and metal plate’s distance, shield body earth condition concern. Each metallic shield material’s performance see Table 2.
Table 2 each metallic shield material performance
|
Metallic shield material |
Is opposite in the copper conductivity (σCu=5.8×107Ω/m) |
time f=150kHz relative permeability |
time f=150kHz absorption loss/(dB/m) |
|
Silver |
1.05 |
1 |
52 |
|
Copper |
1.00 |
1 |
51 |
|
Gold |
0.70 |
1 |
42 |
|
Aluminum |
0.61 |
1 |
40 |
|
Zinc |
0.29 |
1 |
28 |
|
Brass |
0.26 |
1 |
26 |
|
Cadmium |
0.23 |
1 |
24 |
|
Nickel |
0.20 |
1 |
23 |
|
Phosphor bronze |
0.18 |
1 |
122 |
|
Iron |
0.17 |
1000 |
650 |
|
Steel #45 |
0.10 |
1000 |
500 |
|
Permalloy |
0.03 |
80000 |
2500 |
|
Stainless steel |
0.02 |
1000 |
220 |
3.2.2 shield thin films
Now many electronic installations use the engineering plastics to make the engine case, because engineering plastics’s processing shop characteristics are good, causes the engine case both the handsome appearance, and the cost is low, the quality is light. But engineering plastics non-electromagnetism barrier property. The shield thin film is uses processing technologies and so on spray coating, vacuum deposition, galvanization and glue, covers conductive coating in the engineering plastics and the organic medium’s surface, thus plays the plate shield the role. Generally conductive coating thickness is smaller than the electromagnetic wave in its internal dissemination wave length 1/4.
Several kind of spray coating crafts achieve the shield potency see Table 3.
Table 3 several kind of spray coating crafts achieves shield potency
|
Spray coating craft |
Thickness/μm |
Resistance/(ω/mm2) |
Shield potency /dB |
|
Zinc hot spraying |
25 |
4.0 |
50~60 |
|
Nickel base coating |
50 |
0.5~0.2 |
30~75 |
|
Silver base coating |
25 |
0.05~0.1 |
60~70 |
|
Copper base coating |
25 |
0.5 |
60~70 |
|
Graphite base coating |
25 |
7.5~20 |
20~40 |
|
Galvanization |
0.75 |
0.1 |
85 |
|
Chemical plating |
1.25 |
0.03 |
60~70 |
|
Vacuum deposition |
1.25 |
5~10 |
50~70 |
|
The ionization plates |
1.0 |
0.01 |
50 |
The different thickness’s copper thin film’s shield potency see Table 4.
Table 4 copper thin film shield potency
|
Thickness/μm |
Frequency /Hz |
A |
R |
B |
Shield potency /dB |
|
0.105 |
1M |
0.14 |
109 |
- 47 |
62 |
|
0.105 |
1G |
0.44 |
79 |
- 17 |
62 |
|
1.25 |
1M |
0.16 |
109 |
- 26 |
83 |
|
1.25 |
1G |
5.20 |
79 |
- 0.6 |
84 |
|
2.196 |
1M |
0.29 |
109 |
0.6 |
110 |
|
2.196 |
1G |
9.20 |
79 |
0.6 |
90 |
|
21.96 |
1M |
2.90 |
109 |
- 3.5 |
108 |
|
21.96 |
1G |
92 |
79 |
0 |
171 |
The meter either monitor’s shield, may realizes in the meter or monitor’s frontage establishment diaphanous conducting material. The diaphanous conducting material is covers conductive coating in the organic medium or the glass surface, causes its both diaphanous, and has certain shield potency. The different transmittance conductive glass’s shield potency see Table 5.
Table 5 different transmittance conductive glass shield potency
|
Transmittance |
1MHz |
10MHz |
100MHz |
1000MHz |
|
60% |
94 |
72 |
46 |
21 |
|
65% |
90 |
68 |
42 |
16 |
|
71% |
84 |
62 |
36 |
11 |
|
75% |
78 |
56 |
30 |
6 |
|
80% |
74 |
52 |
28 |
4 |
3.2.3 gauzes
When has ventilates, diaphanous, the watering, the survey and so on needs, must in the equipment outer covering the opening, to enhance equipment’s electromagnetic screen effect, should use gauze’s perforation shield. Or uses in the electronic installation shell’s connecting point, provides the effective electromagnetic screen. Perforation shield potency SE (dB) with electromagnetic wave parameters and so on frequency, perforation size and quantity concerns.
In order to enhance the perforation the shield potency to be possible to take the following measure:
1) covers the gauze on the heavy-caliber perforation, must make the silk screen and the shield stereoscopic contact is good;
2) changes the pocket the eyelet;
3) uses the wave guide attentuator type vent;
4) the cover has gauze’s shield glass on diaphanous and the survey hole;
5) in needs the water, on the gas seal hole the pad to include material and so on rubber gauzes.
Following introduces several commonly used gauze shielding material.
3.2.3.1 pure metal silk screen liner
The pure metal silk screen liner is one kind elastic, the electric conduction knitting metal backing silk screen strip, uses in the electronic installation shell’s connecting point, provides the effective electromagnetic screen. When application, the casting or the machining shell selects the rectangular section the pure metal silk screen liner, the sheet metal shell selects the circular section highly the pure metal silk screen liner, the compression quantity for originally 25% about. The pure metal silk screen liner’s shield potency see Table 6.
Table 6 pure metal silk screen liner’s shield potency dB
|
Material |
Magnetic field (100kHz) |
Electric field (10MHz) |
Plane wave |
|
|
1GHz |
10GHz |
|||
|
Silver-plating brass |
80 |
135 |
105 |
95 |
|
Tin-plating package of copper steel |
80 |
130 |
105 |
95 |
|
Tin-plating phosphor bronze |
80 |
130 |
110 |
100 |
|
Aluminum |
60 |
130 |
90 |
80 |
|
Constantan |
60 |
130 |
90 |
80 |
3.2.3.2 environment seal gauze liner
The environment seal gauze liner is becomes by the knitting gauze and the rubber union, the environment seal gauze liner besides can provide the effective electromagnetic screen, but may also provide the effective environment seal. May use in the electronic installation shell’s fixed connecting point or the active connecting point, for example crack in a door and so on. Generally compression quantity for originally high 25% about. And has the rubber core gauze liner’s shield potency to see Table 7.
Table 7 belt rubber core gauze liner’s shield potency dB
|
Material |
Magnetic field (100kHz) |
Electric field (10MHz) |
Plane wave |
|
|
1GHz |
10GHz |
|||
|
Tin-plating package of copper steel |
80 |
130 |
105 |
95 |
|
Tin-plating phosphor bronze |
80 |
130 |
110 |
95 |
|
Constantan |
60 |
125 |
90 |
80 |
3.2.3.3 gauze shield glass
The gauze shield glass is presses the gauze between two glass, not can only provide the effective electromagnetic screen, but may also provide the effective diaphaneity. May use in electronic installation’s observation window, for example meter, digital or image monitor and so on. The gauze shield glass’s shield potency see Table 8.
Table 8 gauze shield glass shield potency dB
|
Material |
Magnetic field (100kHz) |
Electric field (10MHz) |
Plane wave |
|
|
1GHz |
10GHz |
|||
|
Blackening copper gauze, opening 60% |
55 |
120 |
60 |
40 |
|
Blackening copper gauze, opening 45% |
55 |
120 |
80 |
50 |
3.2.3.4 aluminum system honeycomb passes the air deflector
The aluminum system honeycomb passes the air deflector is constitutes by the aluminum frame’s in aluminum system honeycomb. Wave guide’s honeycomb not only has the electromagnetic screen potency, and has the high air conductivity. May use in electronic installation’s well ventilated window. The aluminum system honeycomb passes air deflector’s shield potency to see Table 9.
Table 9 aluminum system honeycomb passes air deflector’s shield potency dB
|
Material |
Magnetic field (100kHz) |
Electric field (10MHz) |
Plane wave |
|
|
1GHz |
10GHz |
|||
|
The monolayer plates the chromates |
40 |
80 |
60 |
40 |
|
Single-layer plate with cadmium |
75 |
125 |
105 |
85 |
|
Single-layer tin-plating |
70 |
125 |
105 |
85 |
|
Single-layer nickel plating |
80 |
135 |
115 |
95 |
|
The multi-layer plates the chromates |
65 |
110 |
95 |
85 |
3.2.4 conductive fibers
The conductive fiber divides into the following 5 kinds.
1) copper-plates or the nickel after the chemical fiber fabric makes the electric conduction cloth, may has the nimble shielding properties to high frequency and the microwave.
2) compound makes the electric conduction cloth and the resin the absorption electric conduction cloth, because selects can absorb the electromagnetic wave the resin, therefore the shielding properties are better.
(3) makes the electric conduction cloth with the electric conduction good metal or the carbon black textile fiber and the chemical fiber mix.
The above 3 kind of electric conduction fabric may make against static electricity and against electromagnetic radiation work clothes, does shields the window blind, the tent, the shroud, its shield potency generally in 50~60dB.
4) makes the resistance paper with the conductive fiber and the wood-pulp mix, may make the sensitive integrated circuit’s shield packing, its shield potency generally in 30~40dB.
5) synthesizes the directional tinsel packing silica gel which by many independent tinsel in the silica gel is made, can provide the effective electromagnetic screen and the environment seal, in the non-fixed slit, for example flange’s connection, its shield potency see Table commonly used 10.
Table 10 directional tinsel packing silicon solid core rubber shield potency dB
|
Material |
Magnetic field (100kHz) |
Electric field (10MHz) |
Plane wave |
|
|
1GHz |
10GHz |
|||
|
Tin-plating phosphor bronze |
75 |
130 |
110 |
100 |
|
Constantan |
80 |
130 |
115 |
100 |
3.2.5 electric conduction pellets
The electric conduction pellet shielding material is the glass granule which, the pure silver granule, the carbon black granule, the copper silver-plating granule, the nickel silver-plating granule, the aluminum silver-plating granule, the graphite nickel plating granule plates with silver mixes separately in the silicon or the fluorine silica gel, may squeeze out each kind of shape, uses in the electromagnetism and the water vapor seal. Their shield potency see Table 11.
Table 11 electric conduction pellet shielding material shield potency dB
|
Material Material |
Magnetic field (100kHz) |
Electric field (10MHz) |
Plane wave |
|
|
1GHz |
10GHz |
|||
|
Glass silver-plating conductive rubber 811 |
65 |
130 |
100 |
90 |
|
Pure silver conductive rubber 856,857 |
70 |
130 |
100 |
90 |
|
Carbon black conductive rubber 860 |
93 |
77 |
68 |
88 |
|
Copper silver-plating conductive rubber 871 |
75 |
120 |
115 |
110 |
|
Nickel silver-plating conductive rubber 891 |
75 |
120 |
110 |
100 |
|
Aluminum silver-plating conductive rubber 895 |
75 |
120 |
110 |
100 |
|
Graphite nickel plating conductive rubber 750 |
100 |
100 |
100 |
85 |
3.2.6 lead the bakelite
Leads the bakelite is in the silicon, the epoxy mixes in the fine metal granule, for example the silver, the nickel, the copper silver-plating, the aluminum silver-plating and so on, apply between each kind of shielding material, plays the caking, the shield and the seal role.
3.2.7 conductive coatings
The conductive coating is in the polypropylene and gathers in the ammonia fat to mix in the pure silver granule, may apply in the plastic body shields and needs the flexible shield on the equipment.
3.2.8 electric conduction foil belt
The electric conduction foil belt is spreads the electric conduction polypropylene rubber by one-sided back the copper tape or the aluminum belt is composed, may use in the shield seal which, the winding cable shielding the electronic installation joins and so on. Its shield potency generally in 55~60dB.
3.2.9 beryllium copper reeds
The beryllium copper reed has the elastic shielding material, may use in the electronic installation activity joint the shield, for example gate, window and so on. Its shield potency see Table 12.
Table 12 beryllium copper reed’s shield potency dB
|
Material Material |
Magnetic field |
Electric field |
Plane wave |
|
100kHz |
10MHz |
1GHz |
|
|
Standard reed |
110 |
100 |
90 |
|
Soft reed |
95 |
85 |
75 |
3.2.10 shield the duplicate plywood
Shields the duplicate plywood is by the metallic foil, the insulation substrate and presses the sensitive rubber to be composed, may use in the printed circuit, electronic installation’s shield. Its shield potency generally in 40~45dB.
3.2.11 purified cotton wash the electromagnetism material
The purified cotton washes the electromagnetism material is distributes evenly the copper atom in the cotton and kapok washes in the material, forms transparently both and has the electromagnetic screen function material, may apply in the screen beam radiation protection, the handset microwave radiation protection and so on. Its shield potency >50dB.
3.3 shield the body good earth
The metallic shield body good earth, speaking of the electric shield, will cause the shield body flank the induced charge inflow earth, but will not have the induction electric field existence. Speaking of the alternating electrical field shield, because the alternating electrical field is decided to the sensitive electric circuit’s coupling disturbing voltage size by the alternating electrical field voltage, the coupling capacity and product of the metallic shield body earth resistance, so long as tries to cause the metallic shield body good earth, can cause the alternating electrical field becomes to the sensitive electric circuit’s coupling disturbing voltage very small. Therefore, the metallic shield body’s earth is not good, will reduce the shield effect.
3.4 privileged sites special shield measures
3.4.1 joint processing
In shields body’s connecting point, because the union surface uneven, unclean, the welding quality not good, between the fastening screw has reasons and so on crevice, creates the slit in the connecting point, the cause shield body’s shield effect reduces. Should better use the continuous welding to the fixed joint. Before the welding, should be going to weld superficial the non-conducting material elimination to be clean. Must interrupted place carry on joining as far as possible to the complete outer covering. Should use and contract the conductive gasket to the non-fixed joint, enhances the joint the electromagnetism seal effect. The commonly used conductive gasket material has the metallic braid, to include the tinsel rubber and so on. To active joint, if on the doorframe, uses the elasticity to refer to the reed to enhance the joint the electromagnetic screen effect.
Conductive gasket’s fixed way has the trench to locate, the glue to be fixed and ways and so on rib piece fastening.
In order to enhance the slit the shield potency to be possible to take the following measure:
1) increases the metal plate thickness, may the length realize through increase nearby;
2) reduces the junction plane slot width, may through increase the junction plane working accuracy, the welding or the unit cast realizes;
3) installs the conductive gasket, the commonly used conductive gasket has the knitting wire netting, the soft metal, the honeycomb reed, the conductive rubber and so on;
4) spreads the conductive coating in the connecting point, the commonly used conductive coating has leads the bakelite, the electric conduction fat and so on;
5) adjusts the tight nail spacing, causes it is smaller than λ/20 (Lamda for electromagnetic field wave length);
3.4.2 perforation shields
When has ventilates, the illumination, the watering, the survey and so on needs, is enhances equipment’s electromagnetic screen effect, should use the perforation shield. The perforation shield’s effect and electromagnetic wave parameters and so on frequency, perforation size and quantity concern.
3.4.3 braided shields
Because the electric cable needs to move and curving, its shield uses weaves belt’s form. Weaves belt’s shield effect to increase along with the braided density increases, increases along with the electromagnetic wave frequency reduces. Generally electric cable’s shielding layer is with does not lead magnetism the tinsel knitting, may realize the electric field shield. If must realize the magnetic screen, electric cable’s shielding layer should use leads magnetism the tinsel knitting.
3.4.4 honeycomb cell board shield
When equipment’s ventilation is high with the shield request, uses the honeycomb cell board shield to have the good effect. The honeycomb cell board shield is uses many compound hexagon metal tube welding to constitute together? . And each metallic pipe plays the wave guide attentuator the role, but the well ventilated wind pressure drop is not big. The honeycomb cell board’s electromagnetic screen effect is decided by wave guide’s weaken characteristic, namely concerns with wave guide’s geometry size.
3.4.5 kneading board hole shield
When the equipment needs to install the meter, the data or the graph monitor, deals with the kneading board hole to shield, guarantees the shield the integrity. The kneading board hole shield’s good method as establishes the screening can in the meter or monitor’s rear area. The screening can through the conductive gasket and the metal kneading board connection, enters the going beyond a line establishment through the screening can put on the heart electric capacity.
3.4.6 electric interlock shields
The choice protected electric interlock should have enough inserts the needle, for electric cable in each shielding layer in an electric interlock termination. For the guarantee shield’s integrity, must along electric cable one week, entire connects electric cable’s outside shielding layer and the electric interlock, should better be the welding; The electric interlock place maintains the good electrical connection through the conductive gasket and equipment’s metal shell; Electric interlock should also maintain the good electrical connection with the electric interlock place.
3.4.7 multiple shields
When the single-layer shield’s effect cannot meet the requirements, may use the multiple shield. Specially to the frequency band wide shield, separately uses the conductivity and the magnetic conductivity high several material composition multiple shield, may achieve to the high frequency electric field and the low frequency magnetic field has the good effect shield.
3.4.8 printed wiring board shield
1) establishes the conductor shielding in the electromagnetism noise source and to the electromagnetic interference sensitive accepting circuit, connects in parallel fashion to the circuit wafer datum electric potential.
2) as far as possible many retains the electric conduction line’s between spreading level, connects in parallel fashion to the circuit wafer datum electric potential.
3) peripheral (nearby electric interlock exception) establishes the grounding in printed wiring board’s three.
4) establishes the screening can separately to the electromagnetism noise source and to the electromagnetic interference sensitive accepting circuit, connects in parallel fashion to the circuit wafer datum electric potential.
5) establishes the screen between the printed wiring board, connects in parallel fashion to the circuit wafer datum electric potential.
4 shield potency examination
After shielding the body completes, needs to carry on the shield potency the examination.
4.1 shield potency check-out facility
The shield potency’s check-out facility has the frequency conversion supply oscillator, the radio frequency amplifier, the transmitting antenna, the electromagnetic field receiving antenna, the attentuator, to survey the receiver, the data recording instrument.
4.2 shield potency examination method
1) locates the measuring point;
2) calibrates the check-out facility;
3) surveys does not have when the launch ambient level H;
4) surveys is unshielded when in the survey accepts after checking receives transmitter’s electromagnetism field intensity W;
5) surveys has when the shield in the survey accepts after checking receives transmitter’s electromagnetism field intensity Y.
4.3 shield potency SE examination analysis
The shield potency SE formula is
SE=20log10 [(W-H)/(Y-H)] (7)
After the computation, will shield potency SE and the design requirements compares, looked whether to achieve the design requirements, whether the safety margin does satisfy requests, whether to have had the design. If cannot meet the requirements, wants the concrete study reason and improves, until satisfies the request. If has had the design, also wants the concrete study reason and performs in the later design to improve.
