Precautions for Using Shielding Enclosures
A shielding enclosure is a metallic structure made of conductive or magnetic materials in various shapes, designed to confine electromagnetic energy within a defined space and suppress radiated interference. A typical shielding enclosure consists of a conductive fabric layered over foam plastic, with a silver-plated woven material bonded onto it to form a soft gasket that occupies most of the loose space. This type is primarily used for civilian applications and is suitable for foam gaskets in equipment cabinets and door panels.
When signal lines or control lines enter or exit the enclosure, they must pass through appropriate filters. Multi-pin connectors with filtered pins are suitable for such applications. Attention must be paid to the degree to which overall shielding effectiveness is degraded due to cables penetrating the shielded enclosure. Typically, an unfiltered wire passing through a shield can reduce shielding effectiveness by more than 30 dB.
All power lines entering the enclosure must pass through a filter block. It is preferable for the input side of the filter to extend outside the shielded enclosure. If the filter’s construction does not allow it to protrude through the enclosure, a dedicated compartment should be provided at the point where the power line enters the enclosure specifically for the filter. It must be noted that inserting a metal shaft or conductor through a waveguide-below-cutoff aperture will severely degrade shielding effectiveness. Metal caps should be added to components such as fuses and sockets.
When high requirements exist for shielding, ventilation, and mechanical strength—but weight is not critical—honeycomb panels should be used for ventilation openings. Welding is preferred to maintain continuous electrical contact and prevent leakage. When indicators/displays cannot be shielded from the rear and their leads cannot be filtered, the front of the indicator/display should be shielded using a metal mesh or conductive glass that maintains continuous electrical connection with the enclosure. Whenever possible, add shielding behind the indicator/display and filter all leads using feedthrough capacitors.
When a ground-isolated metal control shaft is required, a short concealed control shaft may be used. When not in adjustment, it should be covered with a screw cap or a metal gasket-equipped elastic cap. Any metal control shaft penetrating the shield should be grounded using metal contact fingers, grounding nuts, or RF gaskets. Alternatively, instead of using a grounded metal shaft, a non-grounded shaft can be implemented using a round tube whose waveguide cutoff frequency is higher than the operating frequency to serve as the control shaft. Shielding enclosures are generally used in conjunction with couplers.
