Patent Publication Number: US-2021195813-A1

Title: Electromagnetic interface shield

Description:
The present invention relates to an enclosure for electromagnetic shielding of equipment and more particularly to an enclosure which contains an aperture. 
     When designing enclosures for devices it is important that the components are protected from electromagnetic radiation which may harm or influence the way the devices operate. This is typically achieved by isolating the device from external electromagnetic radiation by shielding. Shielding often includes a conductive enclosure used to attenuate electromagnetic radiation and is often referred to as a Faraday cage. The level of attenuation depends on a number of factors including the material used, material thickness, and the size of any apertures in the shielding. It is also desirable to shield equipment in order that electromagnetic radiation generated by the equipment cannot be measured by external devices as this may allow sensitive information related to the equipment to be accessed. 
     A purpose of the invention is to mitigate the risk of electromagnetic radiation generated by equipment exiting an enclosure via an aperture. 
     According to a first aspect of the invention there is an enclosure arranged to define a cavity for electromagnetic shielding of equipment to be housed within the cavity, the enclosure comprising: an aperture arranged to allow access to the enclosure; a conductive membrane arranged to close the aperture; and a waveguide element arranged between the conductive membrane and the cavity; wherein the waveguide element and conductive membrane are configured to attenuate electromagnetic radiation originating from the equipment to be housed within the enclosure thereby to inhibit the transmission of electromagnetic radiation from the enclosure via the aperture. In this manner the arrangement may allow an aperture to be present in the enclosure whilst not compromising the electromagnetic shielding provided by the enclosure. The waveguide element and enclosure may be substantially or in part a conductive or dielectric material. 
     The enclosure may further comprise a faceplate and a cooperating back-box arranged to define the cavity. In this manner the arrangement may allow greater access to the equipment within the cavity when the equipment is not in use. 
     The aperture may be aligned with a reset switch for equipment to be housed within the cavity thus allowing change of state of the electronic equipment without having to dismantle the enclosure. This may be desirable if the reset switch is infrequently used and requires some level of protection from accidental operation. The arrangement may serve the purpose that the state of the electronic equipment can be changed rapidly in the case of an emergency. 
     The aperture may be of sufficient diameter to ensure the reset switch is accessible by a user&#39;s digit. The size of the aperture will be dependent on its function. 
     The conductive membrane may be formed from foil. In use the foil may be pierced by the inserting of a digit or other device that may be used to actuate the reset switch. The pierced foil provides the added benefit that it indicates whether the reset switch has been actuated, and may warn a user that the electromagnetic shielding has been compromised. 
     The conductive membrane may comprise a mesh structure. The structure and configuration of the waveguide element may mean that the conductive membrane may not need to be pierced in order for the reset switch to be actuated. A deformable mesh may still allow a digit to actuate the reset switch directly, or a deformable mesh could move the waveguide element to allow the switch to be actuated. In another embodiment the mesh may be pierced and may provide an indication that the reset switch has been actuated. 
     The conductive membrane may be a frangible material. The frangible material may indicate whether the reset switch has been actuated, and may warn a user that the electromagnetic shielding has been compromised. 
     The length of the waveguide element may be substantially twice that of the wavelength of the predicted maximum frequency output from the equipment to be housed within the cavity. It will be understood that the calculations to determine the length of the waveguide element to attenuate electromagnetic radiation originating within the cavity thereby inhibiting the transmission of electromagnetic radiation from the enclosure via the aperture are known to the person skilled in the art. 
     The waveguide element may be tubular, and if configured to align with the aperture and conductive membrane will allow access to the cavity defined by the enclosure. 
     One end of the waveguide element may be arranged to abut against the internal surface of the conductive membrane, aiding effective attenuation of electromagnetic radiation from the equipment housed within the enclosure ensuring that the conductive membrane can effectively inhibit electromagnetic radiation from exiting the enclosure. 
     The waveguide element may be fixed to the interior of the enclosure and arranged such that the central axis of the waveguide element and the centre of the aperture are substantially aligned along a common access, preferably co-axially aligned, ensuring effective inhibiting of electromagnetic radiation from the equipment housed within the enclosure. This alignment has the further benefit of allowing access to the reset switch through the aperture. 
     The waveguide element may be fixed to the interior of the enclosure by adhesive, providing a secure bond between the waveguide element and the enclosure. The adhesive is preferably an electrically conductive adhesive. 
     The waveguide element may be releasably connected to the interior of the enclosure and arranged such that the central axis of the waveguide element and the centre of the aperture are substantially aligned along a common axis. This connection enables replacement of the waveguide element depending on the amount of electromagnetic attenuation required. This connection has the further benefit of enabling the conductive membrane to be replaced once it has been pierced. 
     The waveguide element may be releasably connected to the interior of the enclosure by a threaded connection enabling the waveguide element to be removed and replaced as required. 
     The waveguide element may be releasably connected to the interior of the enclosure by a bayonet connection enabling the waveguide element to be removed and replaced as required. 
     The waveguide element may be releasably connected to the interior of the enclosure by a sprung bush enabling the waveguide element to be removed and replaced as required. 
     According to a second aspect of the invention there is provided a method of manufacturing an enclosure arranged to define a cavity for electromagnetic shielding of equipment to be housed within the cavity, comprising the steps of: forming an aperture in the enclosure, arranged to allow access to the enclosure; providing a conductive membrane arranged to close the aperture; and providing a waveguide element arranged between the conductive membrane and the cavity; wherein the waveguide element and conductive membrane are configured to attenuate electromagnetic radiation originating from the equipment to be housed within the enclosure thereby inhibiting the transmission of electromagnetic radiation from the enclosure via the aperture. 
     The configuration of the waveguide and the conductive membrane enables the waveguide to attenuate the electromagnetic radiation such that when said radiation encounters the conductive membrane said radiation is at a sufficiently reduced intensity that it is inhibited from transmission from the enclosure via the aperture. This may be achieved by abutting or fixing a waveguide element to the interior of the enclosure arranged such that the central axis of the waveguide element and the centre of the aperture are substantially aligned along a common access, preferably co-axially aligned, ensuring effective inhibiting of electromagnetic radiation from the equipment housed within the enclosure. This alignment has the further benefit of allowing access to the equipment through the aperture. The tubular waveguide element and enclosure may be manufactured substantially or in part from a conductive or dielectric material. 
    
    
     
       An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which:— 
         FIG. 1  shows a three dimensional view of an enclosure for electromagnetic shielding of equipment according to the present invention. 
         FIG. 2  shows a partially exploded three dimensional view of the component parts present inside the cavity defined by the enclosure according to the present invention. 
         FIG. 3  shows a three dimensional view of the conductive membrane and waveguide element in situ according to the present invention. 
         FIG. 4  shows a three dimensional view of the conductive membrane and waveguide element in situ according to an alternative embodiment of the present invention. 
         FIGS. 5 to 7  show plan views of releasable waveguide elements according to alternative embodiments of the present invention. 
     
    
    
     Referring to  FIG. 1 , there is provided a three dimensional view of an electrically conductive enclosure  10  comprising a multiple sided hollow back-box  14  which is open at one end (not shown). The open end is substantially closed by a faceplate  12  which when abutted against the open end of the back-box  14  defines a cavity (not shown) for electromagnetic shielding of equipment housed within the cavity.  FIG. 1  also shows an aperture  16  in the faceplate  12  of the enclosure  10  to enable access to a reset switch (not shown) which enables a function of the equipment within the cavity. In order to protect the reset switch from accidental use and to indicate to a user whether or not the reset switch has been actuated, the aperture  16  is shown as being closed by an electrically conductive membrane  18  indicating in this instance that the reset switch has not been actuated. 
       FIG. 2  shows a partially exploded three dimensional view of the elements of the invention. In  FIG. 2  the back-box  14  of  FIG. 1  has been removed to aid the description.  FIG. 2  shows the aperture  16  in the faceplate  12  of the enclosure  10  and the conductive membrane  18  which closes the aperture  16 .  FIG. 2  also shows a waveguide element  20 , the proximal end of which is aligned with the conductive membrane  18  and the aperture  16  in the faceplate  12  of the enclosure  10  along axis A-A. In this embodiment the distal end of the waveguide element  20  is aligned with a switch  22  which will preferably activate a function of electrical or electronic equipment (not shown) housed within the cavity. In order to activate the switch  22  a user may use a digit to pierce the conductive membrane  18 . In the embodiment shown the waveguide element  20  is tubular and the user&#39;s digit will allow direct access to the switch  22 . After the switch  22  has been actuated by the user it will be clear that the electromagnetic shield has been compromised due to the visible break in the conductive membrane  18 . It will also be clear that the functionality initiated by the actuation of the switch  22  has been initiated. 
     The length of the waveguide element  20  is defined by known equations that relate to the anticipated range of frequencies of the electromagnetic radiation that is emitted by the equipment housed in the cavity of the enclosure  10  of  FIG. 1 . The length of the waveguide element is preferably two wavelengths in length, and is made from a metal or metallised material. 
       FIG. 3  shows a three dimensional view of the elements of the invention that inhibits the transmission of electromagnetic radiation from the enclosure  10  of  FIG. 1  via the aperture  16  in situ. The conductive membrane  18  which closes the aperture  16  of  FIGS. 1 and 2  is abutted against the inner surface  32  of the faceplate  12  of the enclosure  10  of  FIG. 1 .  FIG. 3  also shows the waveguide element  20  abutted against the conductive membrane  18  on the inner surface  32  of the faceplate  12  of the enclosure  10 . The waveguide element is fixed to the inner surface  32  of the faceplate  12  by metallised adhesive.  FIG. 3  shows the conductive membrane  18  as being larger in diameter than the diameter of the waveguide element  20  and the diameter of the aperture and the diameter of the aperture  16  which it closes. This is to illustrate the position of the conductive membrane  18  and waveguide element  20  in relation to each other, and whilst it is preferable that the conductive membrane  18  is larger in diameter than the aperture  16 , it is only required that the conductive membrane  18  closes the aperture  16 . The centre of the aperture  16  in the faceplate  12  of the enclosure  10 , the centre of the conductive membrane  18  and the longitudinal axis of the waveguide element  20  are all substantially aligned along axis A-A. The waveguide element  20  shown in  FIG. 3  is tubular and if the conductive membrane  18  is pierced the interior of the enclosure  10  can be accessed through the bore  34  of the waveguide element  20 . 
       FIG. 4  shows a three dimensional view of an alternative embodiment of the elements of the invention that inhibit the transmission of electromagnetic radiation from the enclosure  10  of  FIG. 1  via the aperture  16  in situ. In this embodiment the conductive membrane is a deformable mesh  42  which closes the aperture  16  of  FIGS. 1 and 2  is abutted against the inner surface  32  of the faceplate  12  of the enclosure  10  of  FIG. 1 .  FIG. 4  shows a solid waveguide element  44  fixed to the deformable mesh  42  by metallised adhesive to the inner surface  32  of the faceplate  12  of the enclosure  10 .  FIG. 4  shows the deformable mesh  42  as being larger in diameter than the diameter of the solid waveguide element  44  and the diameter of the aperture  16  which it closes. This is to illustrate the position of the deformable mesh  42  and solid waveguide element  44  in relation to each other, and whilst it is preferable that the deformable mesh  42  is larger in diameter than the aperture  16 , it is only required that the deformable mesh  42  closes the aperture  16 . The centre of the aperture  16  in the faceplate  12  of the enclosure  10 , the centre of the deformable mesh  18  and the longitudinal axis of the solid waveguide element  44  are all substantially aligned along axis A-A. When the user wishes to actuate the switch  22  (not shown), the user presses a digit against the deformable mesh  42  which deforms under user applied pressure and moves the solid waveguide element  44  further inside the cavity away from the inner surface  32  of the faceplate  12  along axis A-A to actuate the switch  22  (not shown). 
       FIG. 5  shows a plan view of an alternative embodiment of the invention wherein the waveguide element  20  is releasably connected to the inner surface  32  of the faceplate  12 . The waveguide element  20  comprises a male thread  52  which matches a female thread (not shown) on a nut  54  which is fixed to inner the surface  32  of the faceplate  12 . In this alternative embodiment the conductive membrane  18  may be positioned at the male threaded end of the waveguide element  20 , the female threaded end of the nut  54  or in the outer surface  56  of the faceplate  12 . 
       FIG. 6  shows a plan view of another alternative embodiment of the invention wherein the waveguide element  20  is releasably connected to the inner surface  32  of the faceplate  12 . The waveguide element  20  comprises at least one slot  62  which enables the waveguide element to be deformed. The slotted end of the waveguide element  20  comprises a flange (not shown), which is configured to rest in a recessed portion (not shown) of a housing member  64  when the waveguide element  20  is not deformed. In order to insert or release the waveguide element  20  from the housing member  64  the waveguide element  20  is pinched by applying pressure at diametrically opposite points substantially at the slotted end in order to reduce the diameter of the waveguide element  20 . Once the waveguide element  20  is inserted into the housing member  64 , or released from the housing member  64  as desired, the pressure on the slotted end of the waveguide element  20  is released and the waveguide element  20  returns to its original diameter. In this alternative embodiment the conductive membrane  18  may be positioned at the slotted end of the waveguide element  20 , at the recessed portion (not shown) of the housing member  64  or in the outer surface  56  of the faceplate  12 . 
       FIG. 7  shows a plan view of another alternative embodiment of the invention wherein the waveguide element  20  is releasably connected to the inner surface  32  of the faceplate  12 . The waveguide element  20  comprises male bayonet members  72  which cooperate with reciprocal female bayonet mountings  74  fixed to the inner surface  32  of the faceplate  12  to retain the waveguide element  20  with respect to the faceplate  12 . In this alternative embodiment the conductive membrane  18  may be positioned at the proximal end of the waveguide element  20 , at the female bayonet mounting  74  or in the outer surface  56  of the faceplate  12 .