Feature Electronics Kemtron explains the advantages of honeycomb vents
ost electronic enclosures need some form of air ventilation to keep cool the electronics within. If this is a RFI/EMI shielded enclosure then the overall shielding effectiveness of the enclosure can be compromised. For many com- mercial applications perforated sheet, metal or metallised plastic will suffice but for high performance requirements waveguides provide superior shielding over a wide frequency range. Waveguides can be circular, rectan- gular or honeycomb shaped. The prin- ciple is that of “Waveguide beyond cutoff”. The honeycomb vent is series of tubes that acts as a waveguide, guid- ing electromagnetic waves into or out of the enclosure, but as the tubes are long enough then it attenuates those waves. Typically the tube should be at least 3 times as long as the diameter. Good practice is 4 times. Therefore a 3.18mm cell should be 12.7mm long.
The most cost effective is aluminium honeycomb because it offers high shielding performance, light weight and good airflow as it is 96% open area.
How to stay cool M
A typical shielded enclosure will use two honeycomb ventilation panels one as the inlet and one for the exhaust. Aluminium honeycomb vents are supplied in an aluminium frame with an EMI gasket to ensure a good electri- cally conductive bond with the enclo- sure. Many frame styles are available to suit different mounting methods. Kemtron's standard honeycomb vent uses two layers of 3.18mm cell x 6.4mm thick laid at 90˚, and is known as cross cell. It improves shielding performance by eliminating any polarizing effects. Kemtron’s standard finish (Kempass) for aluminium vent panels fully meets the RoHS directive and applies per- manganate passivation which is a chro-
Most electronic enclosures need some form of air ventilation to keep the electronics inside cool
mate free, inorganic and non toxic coat- ing. The process produces a dense, uni- form coating consisting of aluminium and reduced manganese oxides giving a light yellow colour to the surface. The surface finish is conductive with a low contact resistance. It also meets all requirements of MIL-C-5541E for corro- sion and electrical conductivity. Surtec 650, tin or nickel plating finishes are also available. A range of painted fin- ishes to match enclosure finishes or military colours is also offered. Protective kick plates can be incorpo- rated in the vent to prevent damage to the honeycomb.
Slant honeycomb can also be sup- plied to stop any moisture running back down the hon- eycomb into the enclosure
Other vents available are commercial grade standard size fan vents, steel, stainless steel and brass to special order.
Kemtron T:+44 (0)1376 348115
www.kemtron.co.uk
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