Electromagnetic shield and earth

The electromagnetic radiation (the both the electric field and the magnetic field) invades and it will not leak and an electromagnetic shield is done. In a word, they are a buildings, an electric equipments, electric parts, and the partitions, etc. installed from a certain part to other parts to prevent an unnecessary electromagnetic radiation from going into and leaking.

Electromagnetic shield (electric wave shield)

Because the electromagnetic radiation has both the electric field and the magnetic field characters, it might be not so effective to have done only only an electrostatic shield and a magnetic shield as follows. If the neither is given, it is not complete.

Electric field shield (electrostatic shield)

If the object is enclosed with the conductor by ..electric field shield (electrostatic shield).. one to cover the electric field and the conductor is grounded (earth), it is possible ..the purpose.. to reach. There is no space and it is necessary to enclose it with the conductor to intercept it to the electrostatic field completely.

Magnetic field shield (magnetic shielding)

The magnetic shielding encloses the object with the magnetic substance with an effective thickness by the one to cover the magnetic field. An enough thickness is necessary for the magnetic substance to cover the quietness magnetic field.

Material of electromagnetic shield

The iron plate, the metallic mesh board, the conductive coating material, and plating, etc. are typical materials used for an electromagnetic shield. The plating of the conductive coating material and the insulator might be effective only though might be effective of the electrostatic shield as an electromagnetic shield.

The magnetic shielding is possible for the electromagnetic radiation even in case of the nonmagnetic material such as copper and the aluminum sheets. This is because the electric wave is covered because the eddy current with the magnetism of the electromagnetic radiation flows to the conductor.

The hole that opens in the conductor or the mesh should be very small compared with the wavelength of the electromagnetic radiation. Otherwise, the electric wave passes the barrier through this hole.

When the electric wave absorption material is used together with the place where the shield was given, it is effective. This is the same idea as the use of the acoustic material in the anechoic room of the sound.

Example of applying electromagnetic shield

Coaxial cable (shield line) and earth

Coaxial cable (Shielded Cable) shields electromagnetic with a braided wire (wire mesh) that surrounds the wick line at the center and a metallic foil. This electromagnetic shield prevents the signal from the wick line from leaking outside, and prevents the signal from the outside from invading internally.

It is general to connect an outside conductor with steady potential, and to ground the shield line for the low frequency. This is because the obstruction cannot often be excluded if it doesn't ground it in a low frequency.

In this case, whether it grounds in the one side of the shield line or it grounds it on both sides might become a problem. It is basic to ground it only in one side in the low frequency. The obstruction current might flow to the shield line when grounding it on both sides and the disturbance voltage be generated in the wick line.

Because the impedance matching cannot be taken if both ends of the coaxial cable are not connected in the high frequency, the hull in the shield line will be inevitably grounded.

Electric wave darkroom (radio anechoic chamber and shield room)

There is something that is called an electric wave darkroom in the electromagnetic radiation like the anechoic room of the sound. It is used for the measurement of an unnecessary radiation of an electric equipment and the measurement of the influence of the external electromagnetic radiation that an electric equipment receives, etc.

It is general to make the room enclosed with the iron plate and the net of iron. The interval of meshes of a net is chosen by thinking about the frequency used. Going into and the outflow of the electromagnetic radiation from the power supply drawn in internally are considered.

The earth (earth) is given to the entire shield room. Therefore, because there is a shock hazard in the shield room, one using the insulation transformer (separates transformer) is safe, and the insulation transformer becomes a kind of filter. I also was making the company work age like that.

Wireless LAN electric wave leakage measures of office

There is a problem that the electric wave of wireless LAN can be received outside the building in the office etc. made broadband. There is a possibility that the falsification of unlawful computer access and data that misuses the leakage and the security of the information data etc. happen when there are these kind of things.

Then, it is possible to make the entire office a simple electric wave darkroom. The galvanized sheet iron, the copper foil, and the aluminum foil are used for the roof, the wall, and the floor as a material of an electromagnetic shield, and the electromagnetic shield glass is used for the window. The noise filter is turned on to the electric wiring, and the electric wire tube is made the iron tube of the magnetic substance.

Door of microwave oven

There is usually a punching metal in a front window of the door of the microwave oven. The electric wave never leaks to the outside because the hole of this punching metal is very small if it sees from the micro wave (wavelength about 12cm and frequency 2.45GHz) compared with the wavelength of the electric wave of the microwave oven. However, because this hole is very large compared with wavelength, the visible ray (It is wavelength 400nm-700nm by a kind of the electromagnetic radiation) can see the inside by easily passing.

Case of television and electronic equipment (cabinet)

The device that doesn't pass an electromagnetic radiation unnecessary as for the case of a television and an electronic equipment is done. There are a lot of one made of plastic in respect of safety, weight, and the cost in the television though there is no problem if it is a metallic box. The conductive body is mixed, and in this case, because the electromagnetic radiation passes, the conductive coating material might be painted, and the aluminum foil be pasted internally.