EM Shielding Dilemma: Understanding Transverse Wavelengths

In summary, the classical picture of an electromagnetic wave specifies the wavelength along the direction of travel, but with EM shielding using a grid, the spaces must be less than the wavelength in order to achieve shielding. This implies a transverse wavelength that is "blocked", which is not defined in the classical picture. The fields are transverse to the direction of propagation and can be cancelled out along the surface of a perfect conductor. A screen of vertical wires can act as a polarizer by blocking certain components of the electric and magnetic fields. The relationship between wavelength and grid hole size has to do with the boundary conditions, where the grid spacing determines the lowest mode that can be supported. If the wavelength is less than twice the grid spacing, there will not
  • #1
invisigo
10
0
In the classical picture of an electromagnetic wave, the wavelength is specified along the direction of travel. However, with EM shielding that is using a grid (microwave, chicken wire), I've heard that so long as the spaces are less than the wavelength, you will achieve electromagnetic shielding. This description implies that there is a transverse wavelength to a EM wave that is "blocked", but in our classical picture, we never defined a transverse wavelength.

Can anyone explain this dilemma or propose a physical picture that works?
 
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  • #2
The fields are transverse to the direction of propagation. The tangential electric field and the normal magnetic field are canceled out along the surface of a perfect conductor. If we have a screen of vertical wires (not a grid, but just along one direction), then what will happen is that the component of the electric field along the wires and the magnetic field perpendicular to the wires will not propagate through the screen as they will be canceled out. However, the component of the electric field normal to the wire and the magnetic field tangent will transmit through. Hence, you have a polarizer. So a mesh are two polarizers at right angles, each one will remove one of the two polarizations that the field can be decomposed into (the wave can be polarized in any direction in the plane perpendicular to the direction of propagation but it can always be decomposed into the summation of two polarizations) and thus it will prevent the transmission of an arbitrary field.
 
  • #3
The idea I'm more confused about is the relationship between wavelength and the grid hole size. One idea I just thought about is maybe it has to do with the minimum spot size of the radiation. If this is the case, then the rule of thumb for hole size does not hold exactly. I can get a beam with wavelength 1m into shielding with holes of 1m just by increasing my aperture.

Since spot size d = focal length*wavelength*3.83 / pi*aperture diameter

Then a focal length of 50meters and a aperture diameter of 100meters would allow me to get through your EM shielding designed to block 1m waves.
 
  • #4
It has to do with the boundary conditions. The grid spacing is indicative of the lowest mode (and wavelength) that can be supported in the grid. The grid will always have a finite amount of depth, and so you can do a very crude analysis by treating a single grid element as a rectangular waveguide. In this case, we know that the tangential electric field and the normal magnetic field must go to zero on the surface of the conductor (assuming PEC). All of the fields must be zero inside the conductor, past the surface. So the boundary conditions are the zeroing of certain components on the surface for wave solutions. The result is that the components of the wave in the plane parallel to the grid must be sinusoidal and thus the lowest mode is going to be of a wavelength twice the distance between the edges of the grid.

If the electromagnetic wave has a wavelength lower than around twice the grid spacing (normally we choose 1/4 wavelength spacing), then there will not be a supported propagating mode through the grid. The radiation will still transmit through because it will travel as an evanescent mode, but it will be severely weakened. The degree of this attenuation will be dependent upon the thickness of the grid's wire and the spacing of the grid in comparison to the incident wave.
 
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  • #5
Ah! Yes, thank you for the clear explanation! I never thought to think of shielding as a waveguide, but it makes perfect sense now! I am indebted to you for this insight.
 

1. What is EM shielding and why is it important?

EM shielding refers to the use of materials or techniques to block or reduce the amount of electromagnetic (EM) radiation that passes through a given area. This is important because EM radiation can interfere with sensitive electronic equipment and also pose health risks to humans.

2. What are transverse wavelengths and how do they relate to EM shielding?

Transverse wavelengths refer to the distance between two consecutive peaks or troughs in the electromagnetic wave. They play a crucial role in EM shielding as they determine the frequency and energy of the radiation, which in turn affects the effectiveness of shielding materials.

3. What are the different types of EM shielding materials?

There are various types of EM shielding materials, including metals (such as aluminum, copper, and silver), conductive fabrics, conductive paints, and metal-coated plastics. Each type has its own advantages and limitations, and the choice of material will depend on the specific application and level of shielding required.

4. Can EM shielding be 100% effective?

No, it is not possible for EM shielding to be 100% effective. This is because EM radiation can still penetrate through small gaps or openings in the shielding material. However, with proper selection and installation of shielding materials, it is possible to significantly reduce the amount of EM radiation that passes through a given area.

5. How can I determine the appropriate level of EM shielding for my needs?

The level of EM shielding required will depend on the specific application and the level of radiation that needs to be blocked. This can be determined through testing and measurement of the EM radiation in the area, as well as consulting with experts in the field of EM shielding. It is also important to regularly monitor and maintain the shielding to ensure its effectiveness over time.

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