Magnetic shielding theory and its primary shielding formulas are based on the perfect shielding geometries of a sphere or an infinitely long cylinder. As these geometries are typically not practical from a fabrication standpoint, it is important to understand how physical characteristics influence the effectiveness of your shield design.
From our years of experience in designing, fabricating, and testing shields, we know that the following parameters are essential to developing the best “strategy” for shield design and for predicting how the magnetic shield will perform in attenuating magnetic fields.
Geometry
We base most magnetic shielding formulas and principles on the optimal geometry of a sphere or an infinitely long cylinder. As these shapes are not generally practical in the real world, we need to subjectively degrade values for a material’s permeability based on the differences between a given shield’s geometry when compared with that of a sphere or infinitely long cylinder.
Shape
Creating rounded shields such as cylinders or boxes with rounded corners is beneficial because it is difficult for magnetic flux lines to turn 90 degrees. Gentle radii provide a better path for magnetic flux lines than sharp corners. Some percentage of magnetic flux lines that are already entrapped within the skin depth of a material will tend to leave the material whenever they encounter a sharp corner. To contain and redirect flux that is already entrapped, designs should generally include gentle radii. When designing your shield, it is a good idea to keep the shape simple, always envisioning a “path of least resistance” upon which the magnetic flux can travel.
Size
Shield size is a significant factor in its overall performance. All things being equal, smaller shields result in better performance, which means that it should always be your goal to design a shield that will envelop the component or space you are attempting to shield as closely as possible. Additionally, because materials are a major cost component in shield design, smaller shields will yield better performance at a lower cost.
Magnetic Continuity
Magnetic continuity is necessary for proper flux diversion and is best achieved by developing single-piece shields free of surface interruptions. When conditions make single-piece shields impossible, we can maintain continuity at corners and transitions either mechanically with good overlapping contact or through welds using parent material. Maintaining continuity between surfaces enhances overall shield design and ensures that the magnetic flux will be able to continue along the lowest reluctance path.
Closure
Whenever possible, a shield should be closed on all sides. This configuration, even if rectangular, most closely approximates a sphere and creates a closed “magnetic circuit.” Additionally, complete closure provides shielding in all axes thus guaranteeing the highest shield performance. Removable covers, lids, and doors are often required to achieve closure. In these instances, it is critical to ensure continuity through mechanical connections to avoid compromising shield performance.
Length to Diameter Ratios and the Impact of Openings
When you are unable to close one or both ends of a shield, or if the shield must have holes, it is important to consider the impact that penetrations will have on the performance of your shield. Generally, magnetic fields can travel into an opening up to five times the diameter of that opening. This means that for shields with open ends, the ratio of the shield’s diameter to its length should be increased as much as possible to improve performance. By increasing the length of a shield while maintaining its diameter, we approximate an infinitely long cylinder — a configuration that improves the shielding performance at a region of increasing distance from the opening. Similarly, we can add tubulations around openings to protect shields with large holes and penetrations. The length of the tubulation should be proportionate to the diameter of the opening that it is protecting, coming as close to five times the length of the diameter as possible to avoid a total degradation of the attenuation at that location.
