What is electromagnetic shielding and shielding effectiveness?
Electromagnetic radiation consists of coupled magnetic fields (H field) and electric fields (E field) components. For many purposes these magnetic and electric fields are required to be excluded or contained within an environment or area such as a shielded enclosure. Effective Uses of Electromagnetic Shielding would include telecommunication equipment racks, MRI, EMC testing chambers as found at EMC Bayswater.
Containment of RF electromagnetic radiation
The containment aspect of why electromagnetic radiation is required to be shielded could be due to the generation of RF for testing purposes etc that could cause interference to nearby equipment and RF services. This can help to avoid interference with more sensitive equipment that maybe in the vicinity.
Exclusion of RF electromagnetic radiation
The exclusion aspect of why electromagnetic radiation is required to be shielded could be due to the requirement to measure in a controlled electromagnetic environment. A good example of this is EMC testing where ambient emissions from transmitters such as FM radio, TV broadcast, cellular and other data transmission services etc. These intentional signals are required to be attenuated to a level that allows the determination of compliance of a product against the required EMC product or generic standard.
How does electromagnetic shielding work and what affects the shielding effectiveness?
Shielded enclosures can fall under the banner of many naming conventions such as ‘Faraday cage’, ‘screened room’, ‘shielded enclosure’, ‘indoor open area test site (iOATS)’, ‘EMC chamber’. They are all essentially a means to eliminate or attenuate an electromagnetic source between the inner and the outer of the enclosure.
What affects the shielding effectiveness?
Several factors limit the shielding capability of these enclosures. Such factors include the build quality ensuring good continuity between panels especially at any mechanical joints. Although less flexible welded chambers can sometimes have an advantage in this area over modular built chambers. Any break in the surface of the chamber is a potential ‘Leak’ with smaller chambers the actual position of the leak can sometimes be difficult to locate due the RF reflecting constructively and destructively within the enclosure/cabinet itself. Included in high risk areas for leaks or weaknesses within an enclosure or chamber is any door or entry including air vents and cable penetration panels.
Shielding effectiveness of chamber doors
Doors come in many forms with various different mechanisms for closing and making contact between the two metal faces. The most common door used is a knife edge door that uses 1 or more channel on the chamber/enclosure wall. The door way that is lined with RF gasket (usually Beryllium Copper and Stainless Steel Gaskets called finger stock) and a on the door a corresponding number of ‘knife edges’ or strips that fit into the channels when the door is closed. Another older method is a compression door that utilizes the compression of the door and the door frame of the chamber wall and the RF finger-stock. This method is not usually suitable for high frequency chambers and are more prone to damage and require more maintenance on the whole.
Shielding effectiveness of openings such as penetration panels, air vents, wave-guides and holes
Another common weak are for shielding effectiveness is any penetration including panels for cables, wave-guides, air vents etc. The size of any opening either round, square of slot will have an effect upon the chamber performance the is dependent upon the size of the opening itself. The larger the openings the more of the RF spectrum will be allowed to pass. The lower cut-off frequency based upon using the equations and theory as described on page 90 of “Ministry of Defence Standard 59-411 Part 5, Issue 1 Publication Date 23 January 2007”.
- For round apertures: fc = (6900*25.4)/length = 175260/length (MHz)
- For square apertures: fc = (5900*25.4)/length = 149860/length (MHz)
How do we test the shielding effectiveness of a chamber or enclosure?
Shielding effectiveness testing is performed using a signal source a measuring instrument such as a spectrum analyser a transmit antenna and receive antenna. An over simplified explanation would be as follows.
- The first stage is to measure a reference in free space conditions with the antennas spaced apart and directed at each other.
- This is followed by a measurement with one antenna in the enclosure and the other antenna outside the enclosure maintaining the spacing and directivity of the antennas.
- The difference in the values would be the shielding effectiveness of the chamber or enclosure.