Sensitive and accurate measurements are often affected by RF noise (EMI) due to the increase of environmental electromagnetic pollution. Because of this, locations such as communication and data centers, calibration and EMC test facilities, and Antenna Test Ranges for Telecom and Military systems all require electromagnetic shielding.
When faced with the purchase of a new EMC chamber, there are many aspects to consider. One question should be: what type of construction method should I use? A solution to this question is that two main styles of panels are used for shielded-room construction: pan-type panels and wood core panels. Pan-type panels are constructed from a single sheet of folded metal; wood core panels are multiple materials laminated together. Either style can meet the specified compliance tests; however, pan-type panels provide several distinct advantages over the laminated type.
Wood core panels are composed of an insulative substrate. Both sides are then covered with 0.5mm zincplated steel, as shown in Figure 1.
This method leads to several problems due to the limitations of a compression fit. See Figure 2 and 3 for an illustration.
Structural integrity is an essential factor influencing the long-term shielding effectiveness of the chamber. Wood Core compression fittings are susceptible to shear forces that diminish the reliability of the shield seam, as seen in Figure 3. Pan-type design is resistant to any disruption of its shielding effectiveness due to sliding or flexing forces.
Pan-type panels are composed of a single piece of 2 mm thick plated steel. The edges are precision cut and rolled over to form a U channel, as shown in Figure 4.
Figure 5 shows how the corners are welded together to provide maximum shielding by eliminating any seams.
The U channel design of pan-type panels provides exceptional strength and stability. The mating of the two panels forms an I-beam style of structural support, seen in Figure 6.
Wood core panels consist of a wood core and 2 sheets of steel. Both materials have a different expansion coefficient in reaction to fluctuations in temperature and humidity. Pan-style shielding geometry does not change over time because it is made of a singular material instead of 2 different materials. This factor makes pan-style shielding a superior choice for high humidity environments.
The foremost reason to choose pan-type shielding is the overall advantage provided to shielding effectiveness (SE). For the purpose of this document, shielding effectiveness is defined as the ratio of the magnitude of the incident electric field to the magnitude of the electric field on the other side of the chamber wall. In summary, all energy is reflected or absorbed. Undeniably, the weakest SE part of any chamber is the seams, penetrations, and doors. Every panel must maintain an SE of 360° around its edges. Corners are especially prone to poor SE. See Figure 7.
Since pan-type shielding has a skin depth of 2mm, which is approximately 4 times greater than the 0.5mm of wood core shielding, it provides far more magnetic shielding. 2mm Pan-type shielding provides more than ≥80dB at 10 kHz (H-field). See figure 10 for the shielding effectiveness values for pan-style shielding.
In addition, the mating surface area of a pan-type panel is considerably larger than that of a wood core panel. The folded and welded design assures that the mating surface remains uniformly flat, allowing for a better RF seal. The assembly of the pan-type panels uses a bolt-up connection that does not penetrate the shield, as it would with a wood core shield. The pan type shield also includes a double-row gasket that creates an RF seal on either side of the bolts used. Corner seams are overlapped, ensuring a tight RF seal. See Figure 8.
Wood Core chamber design is inherently poor at meeting SE requirements. Malformation of the sheet at its edges and corners lowers SE performance. Wadding and copper tape are frequently used to overcome their failures, as seen in Figure 9. This process usually involves multiple attempts to disassemble the panel to meet the SE requirement, significantly increasing installation time and final acceptance.
|Shielding effectiveness according to EN 50147-1 March 1996|
Wood Core panels require custom cutting, resulting in significant dust and noise on-site during the installation. As previously mentioned, re-assembly is highly probable, causing overall assembly time to increase and result in additional costs.
The modular design of pan-type panels allows for a clean and rapid installation, resulting in rare postinstallation SE mitigation. The corners of the shielded panels are cut, folded, welded, and galvanized during manufacturing. Between the flanges of the RF shielded panels, a special high-performance gasket is installed. Assembly is completed in the field with special fasteners M10 on a pitch of 150mm. These fasteners are installed with a pre-defined torque to ensure precise RF gasket compression and life-long RF shielding attenuation. This construction is based on decades of perfected practices and provides the highest RF shielding performance for military, commercial, and industrial applications. The 2mm, pan-type shielding system ensures a quick installation on-site without the need for welding, cutting, or sawing. The modular construction of pan-style shield rooms provides a flexible design tailored to each customer’s specific requirements and expectations. It is also ideally suited for relocations and future modifications.
There is also a significant difference between the two types of shield designs when referring to the preparation of the panels for assembly. On a wood core shield, all eight surfaces that contact one another need to be cleaned; however, the pan-type panels have only two surfaces to clean for reliable electrical continuity. This process represents additional time in installation for wood core shields.
Upon customer request, a pan-style shield room can be lined with plasterboard walls and a suspended ceiling making a pleasant working environment for the test engineer and their customer. The floor can also be constructed as a raised computer floor for easy cable routing.
Since the substrates of wood core panels have differing expansion coefficients and can be prone to moisture absorption, causing distortion and reduced SE, if an enclosure needs to be disassembled and moved, the edges of wood core panels will often get nicked or dinged, requiring complete panel replacement. Depending on the age and condition of the chamber, the rule of thumb for wood core chamber relocation is 5% to 10% scrap of the panels. This consideration means the chamber relocation contractor is adding at least 5% to the cost for a wood core chamber relocation. Seams are degraded over time due to copper tape used to seal ‘leaky’ seams corrosion. Pan-type shielding panels are plated with state-of-the-art materials, resulting in deficient oxidation. Their single-piece design maintains its form and durability over time. Re-assembly is as simple as the installation. Panels can then be reused, with only the gasket material and possibly the hardware needing replacement.
Purchasing a new EMC chamber can include considering many factors to ensure correct installation and overall success in the chamber’s means of use. Construction methods are first and foremost in the process and should never be compensated. After looking at the two main styles presented in the above sections, it’s clear that either style of pan-type or wood core meets the specified compliance tests; however, after breaking down the pros and cons of each, the overall advantages weigh heavily on pan-type shielding as being the superior method and choice. Its construction from a single sheet of folded metal provides more magnetic shielding, easier installation, and a longer lifespan—all of which are crucial details in the chamber’s construction.