The Electromagnetic Spectrum Innovator DATA SHEET Shielding Theory and Introduction Shielding Eecff tiveness Shielding Theory Electromagnetic shielding is used to prevent electromag- netic signals such as radio signals from leaving or enter- ing a box or enclosure. Signals inadvertently emitted by an electronic device can cause distortion or interruption in normal radio communications in a localized area. This is the basis of most laws and regulations concerning electromagnetic interference. In addition, normal radio signals can cause unprotected electronic devices to mal- function. Depending on the devices function, a malfunc- tion in the device could be a minor inconvenience such as static on a radio, or life threatening such as the mal- function of a life support system at a hospital. Frequency (MHz) Shielding Theory The length of the hole and wavelength of the signal that needs to be shielded are the major factors determining The electromagnetic shield in most cases is the elec- the shielding effectiveness of an electronic enclosure. tronic housing itself. The housing/shield forms a metal The distance between spotwelds, or screws which hold cage around the electronic circuits in a device. Most a metal housing together count as long narrow holes. of the electromagnetic signal is absorbed with a small Higher frequencies (lower wavelengths) flow more easily portion (3 to 10 dB) of the signal reflected off the metal through smaller holes, and so the highest frequency housing. Most of the absorbed signal creates alternating needed to be shielded is the frequency of concern when currents at radio frequencies which travels on the surface designing shielding. of metal. This allows the electromagnetic shield to keep signals from outside the enclosure on the outside of the Aperture versus frequency charts can give a rough esti- shield and signals from inside signals on the inside of the mate of the shielding effectiveness of a metallic elec - shield. tronic housing. The shield will continue to function as long as there are Typical Shielding Eecff tiveness no holes in the electromagnetic shield which would allow the currents to flow from one side of the shield to the other. Holes are a necessity in an electronic enclo- sure. Connectors, wires, and cables are needed to trans- mit information to and from electronic devices. Doors and covers are needed to get access to components to ff maintenance, service, and keypads may also be required. The problem is that all of these items cause openings in the shield which reduce the performance of the shield. Special devices such as shielding gaskets, shielding ven- tilation panels, shielded filtered connectors, and shielded switches minimize the effect of a hole in the shield. Frequency (MHz) Test Methods: ASTM D-4935-89 Test Fixture: Flanged coaxial transmission line www. apitech.com 855.294.3800 info apitech.com www.linkedin.com/company/api-technologies-corp-/ 1 082020ED Shielding Eectiveness (dB) Aperture Length (Wavelength - Inches)The Electromagnetic Spectrum Innovator DATA SHEET Quietshield Fabric-over-Foam I/O & Waved Metal Gaskets Specifications Material: Beryllium Copper, CA 174 (per QQ-C-533) Finish: STD: Electro tin plate, 100 micro inches (per MIL-T-10727) For RoHS: Nickel - Change last 2 p/n digits - NI For Hi-Rel: Gold - Change last 2 p/n digits - AU Material Thickness: .005 (.13mm) compressed Weight Height: .030+.020/-.015 (.76+.51/-.38mm) Length increase when flattened : 0.008 (.20mm) per inch Waved Metal Gaskets (Select part number by filling in xxx: 572019-00xxx-70) Fabric-over-Foam I/O Gaskets Dimensions in inches (mm) www. apitech.com 855.294.3800 info apitech.com www.linkedin.com/company/api-technologies-corp-/ 2 082020ED