MAJR Products explains the EMI shielded window selection process
Helping engineers understand how to find the right EMI shielding window for their applications
- What’s the frequency?
- Who is it for?
- What type of busbar is needed?
- What are the Wire type, openings per square inch, light transmission and use environments?
Typical questions often asked by engineers when considering EMI contamination wherever there is a window in a hardened enclosure. Whether it’s in something as small as a handheld device or as large as a shipping container, any separation between the interior and exterior of a unit needs to fulfil two major requirements; transparency and EMI attenuation. EMI Shielded windows can protect against stray signals coming into or leakage coming from the unit itself.
In medical devices, for example, small units often use EMI windows to allow personnel to view inside and read data collected by the unit. Viewing transparency is critical but so is EMI attenuation to maintain levels of optimal performance and accuracy. In these cases, shielded windows can be quite small but they are no less important.
At the other end of the spectrum, shielded windows can be large panels needed in sensitive and secure DoD applications, such as manned military hardened enclosures. Where troop safety, comfort, exterior light and outward visibility are important, EMI windows still provide protection from unintentional or intentional electromagnetic sources
EMI shielded window build considerations
MAJR Products’ EMI shielded windows are built using a fine wire mesh and a proprietary process which embeds the optical wire between polycarbonate, acrylic and glass sheet stock allowing superior optical quality while still meeting demanding shielding requirements. As the mesh collects electromagnetic radio frequencies it passes these stray signals to a busbar. Sounds simple, but there are just about as many considerations that need to be taken into account as there are potential applications.
- Wire mesh openings per square inch (OPI): This can vary depending on the application of the shielded window. Common OPIs are 50, 80, 100, 145 and 230 with 100 being most common.
- Wire size and type: Wire thickness runs from .0012” to .0022”. Often made with stainless steel they also come in copper, again to suit various applications and conductivity. They are all silver blacked.
While wire type and OPI of the weave does not interfere with reflectance, which remains constant at 5-7%, it does change the amount of light transmission. An OPI of 50 and wire thickness of .0012” allows for 88% light transmittal, whereas an OPI of 230 and wire thickness of .0014” drops this to 34%. It is the openings per square inch that affect this the most. A thicker wire of .0022” but with 100 OPIs in the mesh still has a light transmission of 62%.
Another important consideration is the amount of desired shielding effectiveness. Desired effectiveness (dB) will vary greatly depending on the application and environment, and of course, military specifications can be quite different from those in the consumer market. The range of shielding effectiveness for an EMI shielded window for a 40 MHz environment is obviously quite different from those in a 1 GHz application.
The final large consideration is the shape of the busbar which is attached to the substrate/mesh assembly edge. Edge only, edge and one or two sides of the shielded window, or formed to step into a particular opening – all are options available to facilitate the finished unit’s effectiveness and easy assembly process.
Selecting shielded windows is never straight forward
Balancing shielding effectiveness with light transmission and other engineering requirements is a juggling act requiring that most shielded windows are custom made. Adding non-glare hard coats, custom substrate colours and coatings for abrasion and chemical resistance only adds to the mix of engineering considerations.
MAJR Products have expert engineers with the right knowledge to advise on choosing the right OPI, wire thickness and type plus substrate coatings. Regardless of the application - military, consumer, medical, aerospace or communications - MAJR can assist your engineers to calculate it all plus take into account the proper size and busbar for your next project. As the UK representatives for MAJR Products, the Wavelength team are your first port of call to discuss customised prototypes or large production runs.