What happens when an electromagnetic wave strikes a metallic barrier?

In the past, EMC has been an issue which was regarded almost as a ‘black-art’ by many people who did not have a direct technical involvement in the subject. Even highly qualified engineers would ‘leave the details to the boffins’. It is our view that EMC is rapidly becoming a topic for all involved parties to understand and appreciate particularly as the marketplace for shielding materials has totally changed over recent years from a military dominated specialize market to a commercial product center where price, quality and service now define both methods of shielding and the materials employed. The days of special metal particle loaded elastomers costing tens of pounds per meter for use in tank laser rangefinders for the military manufacturer have been superseded by simple and cost-effective knitted mesh and sponge rubber elastomer seals for use in large quantities by rack and enclosure manufacturers for telecommunications and Cable TV applications. The objective of this introduction is to describe basic terms the reasons for shielding and how effective shielding can result in a high level of attenuation of unwanted EMI.

To now answer the question, basically, the incoming wave has two components, an Electric (E) Field and a Magnetic (H) Field. These are at right angles to each other and the direction of travel. Both fields, on contact with the barrier, are subjected to absorption and reflection losses, i.e., part of the field is absorbed by the metal barrier and part is reflected away from the barrier.

The strength of the residual or emergent field may be expressed in relation to the original or incident field, thus providing an indication of the effectiveness of the shielding. This is generally expressed in decibels of attenuation where, for example, a simple screen or barrier may produce 20 dB of attenuation whereas a well-designed and effective shield could produce over 100dB attenuation.

Attenuation can be conceptually likened to the ability of a sponge to soak up water or blotting paper to pick up ink. The levels of attenuation in decibels may be calculated using mathematical equations to prove the point with a high degree of accuracy. In simple terms we should perhaps consider equating decibels of attenuation to a percentage

As shown, every 20 dB increase represents a ten-fold increase in shielding effectiveness.

The expression of shielding effectiveness derived from the correlation of Absorption loss, Reflection loss and a factor for correction due to multiple reflections within thin screens defines, in itself, merely the characteristics of the metallic barrier which in this example is the enclosure.

Penetrations through the shield will also have their own individual effect on the overall effectiveness of the shielding, and the shielding integrity is only good as its weakest link. The weakest part of any shield can vary according to enclosure design and may be where two mating surfaces still permit a slot or narrow gap to exist. However, in most cases the weakest link is a display window or, due to space restrictions, a ventilation grille with an insufficient aspect ratio in its construction or slots instead of holes. Perforations.

What is the relevance of the size and shape of open penetrations through the shield?

In short, open penetrations are only likely to exist where apertures for the entry of services (cables etc.) are created without mating connectors, for aur ventilation or where similar deliberate gaps are necessary.

Where apertures do exist, their effect on the shield is related to their physical size, the thickness of the barrier and the wavelength of the electromagnetic energy. All individual penetrations are treated separately although in some instances multiple penetrations immediately adjacent to each other can cause a cumulative degradation of the shielding effectiveness.

If the wavelength is longer than the lateral dimensions of the aperture in the shield, very little of the signal will pass through the barrier as it will be ‘operating beyond cut-off’. Where the wavelength is below cut-off, the thickness of the metallic barrier is also important as an aspect ratio of 3 or 4:1 (metal thickness to hole diameter) is critical to achieving acceptable attenuation. If, for ventilation purposes a matrix of small perforations can be provided instead of the more usual slots found in enclosures, the shielding effectiveness will be greatly enhanced.

What is created is effectively a waveguide and this is often the term employed by manufacturers to also describe and EMI honeycomb vent panel or ‘attenuation’.