Seam aperture leakage in aerospace enclosures.(EMC TEST)

When the discussions turn to severe aerospace applications, it's difficult to imagine one that is more severe than a launch vehicle/spacecraft combination. Here is a microwave systems platform operating at frequencies of 2,500 MHz and higher that has a rocket engine with 4,000[degrees]F exhaust temperatures at one end, -452.5[degrees]F (+4[degrees]K) at the other end, 800g shock, 20 to 600g random vibration, and sound pressure levels at the location of the electronics packages that exceed 140 dBspl.

The acoustic energy is so intense that, without sound protection, electronic parts just disintegrate. Then there are the rapid changes in temperature and pressure: +80[degrees]F to -452.5[degrees]F and pressure from sea level to near vacuum in approximately 300 seconds. All this plus the added constraint that it costs $ 10,000 per launch pound to get the platform off the ground. That's a great incentive to make things as lightweight as possible.

It's a given that the maximum shielding effectiveness of an electronic enclosure is determined by the attenuation provided by the skin material. Even then, differences in shielding effectiveness can result from nonhomogeneous effects caused by variations in material thickness, forming/bending/welding, nonlinear material behavior at different radio frequencies, field intensities, and source locations. Many of these mechanical/material properties also directly affect the structural integrity of the enclosure.

In aerospace applications, particularly for high-performance mobile platforms such as aircraft and launch vehicle/spacecraft, enclosures often are constructed from hogged-out aluminum billets. This approach to making a box means that five of the six surfaces are made from one continuous piece that assures homogeneity and reduces the number of seams that need to be protected. That's great for shielding, plus it allows the box to be hermetically sealed if necessary to reduce corrosion and prevent contamination.

Sealing must be capable of withstanding pressure changes. For example, a launch vehicle goes from sea level to near perfect vacuum in about 300 seconds. Unless the enclosure and fasteners are strong enough to withstand the internal pressure buildup, the enclosure will suffer permanent deformation. In the case of aircraft, the pressure variations from repeated flights not only deform the enclosure, they also serve as a pump to introduce water vapor into the enclosure and promote corrosion.

From a shielding perspective, hogging the enclosure from a solid billet assures that the walls have adequate thickness, the surfaces are flat, and the material stiffness is adequate to minimize or eliminate load deflection and all but does away with the nonlinear material behavior resulting from forming/bending/welding. And at 2,500 MHz, an aluminum box 1/8-inch thick has the potential to provide greater than 16,000-dB attenuation.

There's no way to measure it, but based on the calculation, you could immediately conclude that this approach describes the perfect shielded enclosure. Unfortunately, from the shielding perspective, all of the variations are minor when compared to the performance degradations caused by apertures in the enclosures needed to accommodate cables, switches, displays, and maintenance panels.

After handling the aperture problem, there are several additional problem areas in aerospace applications that must be addressed. While not typically considered problems in most other applications, large changes in temperature and pressure, cavity resonance, shock and vibration, and corrosion degrade the performance of a shielded enclosure. But first, aperture attenuation needs to be addressed since aperture shielding effectiveness determines the maximum attenuation of the enclosure. …

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