Apertures

Apertures, slots and discontinuities in a practical shield limit its effectiveness at providing a uniform surface over which current can flow. These discontinuities arise in a real design because (a) there must be access through the shield, e.g. for ventilation or viewing, and (b) the shield is not a single item but is built of several panels which have mating joints, across which there may be no assured current path.

At each discontinuity the current flow is distorted from the optimum uniform flow pattern and is forced to follow a path around the break. This has consequences:

local magnetic field cancellation is reduced and may be eliminated, so that the magnetic shielding effect vanishes, at least nearby;

the separation of inner and outer surfaces due to the skin effect is compromised, so that current flow on one surface can contaminate the other, leading to field coupling across the barrier;

electric fields can leak through the break.

The effects of an aperture on field coupling are greatest in the region of the aperture and are directly proportional to its length rather than its width. So there are two rules that flow from this:

minimise the length of all discontinuities, if necessary using conductive gaskets

never place a sensitive or noisy circuit near a gap in the shield

There are different theories for determining shielding effectiveness (SE) due to apertures. The simplest assumes that SE is directly proportional to the ratio of longest aperture dimension L and frequency, with zero SE when L = /2: SE = 20log(/2L). Thus the SE increases linearly with decreasing frequency up to the maximum determined by reflection and absorption in the barrier material, with a greater degradation for larger apertures. This doesn't accord particularly well with observations, mainly because resonances and proximity to circuit components affect the coupling, although it is useful in pointing out significant contributory factors.

Seams and joints

An electromagnetic shield is normally made from several panels joined together at seams. Unfortunately, when two sheets are joined the electrical conductivity across the joint is imperfect. This may be because of distortion, so that surfaces do not mate perfectly, or because of painting, anodising or corrosion, so that an insulating layer is present on one or both metal surfaces.

Consequently, the shielding effectiveness can be reduced by seams as much as it is by apertures. The reduction of shielding effectiveness depending on the dimensions of the aperture applies equally to a non-conductive length of seam. The problem is especially serious for hinged front panels, doors and removable hatches that form part of a screened enclosure. It is lessened to some extent at very high frequencies (typically GHz) if the conductive sheets overlap, since this increases capacitance which provides a partial current path.

The effect of a joint discontinuity is to force shield current to flow around the discontinuity and thereby to create a field coupling path through the shield.

Gasketing

You can improve shielding effectiveness by reducing the spacing of contact points, such as fasteners, between different panels. But this is often impractical, and then the conductive path between two panels or flanges can be ensured by using any of the several brands of conductive gasket that are available.

The purpose of conductive gaskets is to remedy the problem of irregular contact at pressure points, by providing a continuous contact path across a joint in a conformal manner. There is then much less variation in joint impedance along the length of the joint and hence less diversion of current paths. The optimum situation is where the gasket provides the same impedance as the bulk impedance of the metal; different types of gasket approach this ideal more or less closely. Without a gasket, as increasing pressure is applied by fasteners or other means, the irregular high spots are flattened resulting in a greater number of contact points and more surface area. A gasket provides this effect more widely at lower closing pressure. The ideal gasket will bridge all irregularities within its designed deflection without losing its resilience, conductivity or stability.

Mechanical considerations, to ensure contact pressure, are important as is the environmental performance and the electrolytic (electrochemical) compatibility of the chosen materials, particularly in corrosive atmospheres. The crucial EMC requirement is to maintain a low-impedance current path through the joint; the quality of the gasket material and of the gasket-to-panel interface is what determines this. Continuity all along the length of the joint, and contact to bare metal at either side of the gasket, are essential. If an adhesive is used to hold the gasket in place, this must be conductive, and the surfaces mating with the gasket must be stripped of paint and conductively treated.

If you need both an environmental seal and an electromagnetic one, you can achieve this using a single gasket, typically a conductively loaded elastomer. But this exposes the gasket to a greater risk of contamination and subsequent corrosion, which reduces its effectiveness on both counts. Although it takes up more space along the joint, designing for two separate gaskets, with the environmental seal outside, is a better option.

The electrochemical series

This series, not to be confused with other relationships, lists common metals in order of their compatibility with respect to the potential that is created by coupling dissimilar metals together. The greater the potential difference, the more current flows when an electrolyte is present in the junction and the greater the corrosion that will occur. Therefore to minimise joint corrosion, mating surface metals should be chosen to be close together in this series.

Hatches and doors

For high performance enclosures (greater than 60 dB SE) continuous low resistance electrical contact must be maintained around the periphery of doors, covers and panels. Shielding of hatches and bay doors creates a very demanding operational role for RF door seals. Continued high shielding performance of a door, hatch or canopy RF seal is a difficult task. The RF seal itself is also subject to harsh environmental effects such as salt spray, water, oil contaminates, fungal attack, temperature cycling and vibration.

A simple conductive elastomer gasket is not a practical joint solution here. For this type of installation a conforming hollow gasket is more appropriate. In some hollow RF gasket sections it is possible to pressurise the hollow gasket to give a constant closure force. In all external hatch or door seal applications contamination of the joint is one of the most common causes of gasket failure.

The performance of RF gasket seals around bay doors and hatches that are opened and closed frequently cannot be guaranteed even though the number of compressions may be specified in the manufacturer’s data sheet. There are often inadequate maintenance procedures for this type of seal assembly. Due to the harsh environment this type of RF seal operates in, damage is inevitable and regular maintenance is necessary. This makes the knife-edge form of construction with double shrouded finger strip gaskets, despite its expense, more attractive. The gasket itself is protected from mechanical damage, and the wiping action maintains better continuity across the mating surfaces.

Waveguide below cutoff

For apertures in thin sheets, the hole size is much larger than the thickness. If the dimensions are the other way around, the aperture becomes a tube and it can then be characterized as a waveguide.

The cross-sectional dimensions and any dielectric filling influence the cut-off frequency. This frequency is the lower limit below which wave propagation is prevented, or put the other way, it defines the maximum wavelength that can be transported. There will be a cut-off wavelength, greater than which waves simply cannot propagate. For an air-filled rectangular waveguide the TE₁₀ mode (lowest) cut-off frequency occurs when the larger dimension is a half wavelength, i.e.

F_C           =          300/(2 · w) MHz,

where w is the larger dimension in metres

and for a circular waveguide the equivalent approximate formula is

F_C          =          176/d MHz,

where d is the inside diameter in metres

For frequencies well below F_C the shielding effectiveness (attenuation along its length) of a single rectangular tube of width w >> h and length L acting as a waveguide below cut-off is approximately 27.2 · (L/w) dB. For a circular waveguide the equivalent expression is approximately 32 · (L/d) dB where d is the diameter. From either of these expressions, a simple rule of thumb follows, which is that if the length of the hole is more than four times its maximum dimension, the shielding effectiveness of the hole remains above 100dB – which is usually more than enough for practical purposes.

If the hole is filled with a dielectric material of relative permittivity _r, the cut-off frequency is reduced by 1/_r compared to air; but provided that this is appreciated, the principle can easily be used to carry dielectric rods or tubes, such as control spindles or actuators, through a shield. Its most typical application is in "honeycomb panels" of multiple tubes bonded together, which give high airflow for good ventilation without compromising shielding. The outside of the tube(s) must be bonded to the main shielding barrier. The method cannot, under any circumstances, be used to carry a conductor through a shield, since this destroys the attenuation effect of the waveguide.

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