Patent Publication Number: US-2022221660-A1

Title: Feed-through

Description:
FIELD OF THE INVENTION 
     The present inventive concept relates to a revolver-type feed-through for TEMPEST-grade electromagnetic shielding where one or more non-electric signal leads pass through a wall of a cabinet or other enclosure. 
     BACKGROUND OF THE INVENTION 
     Such a feed-through can be used to prevent emission of electromagnetic waves, for example to avoid electromagnetic interference or the leakage of critical information from electronic equipment, such as a network switch or server, placed in a cabinet or enclosure. For this application, standards, usually referred to using the term “TEMPEST”, are established that require attenuation up to the order of 100 dB at frequencies up to 10 GHz at the feed-through. 
     To achieve such attenuation characteristics, such a feed-throughs may comprise one or more waveguides operating below cut-off frequency. 
     Revolver-type feed-throughs, wherein a plurality of waveguides are distributed around the interface between a housing and a plug member pushable into the housing, are known in the prior art. Examples of such feedthroughs are disclosed in documents U.S. Pat. No. 4,849,723 and US20170090120A1. 
     There is always a need to improve such feed-throughs. 
     SUMMARY OF THE INVENTION 
     According to the present inventive concept, and, according to a first aspect, there is provided a revolver-type feed-through for TEMPEST-grade electromagnetic shielding where one or more non-electric signal leads pass through a wall of a cabinet or other enclosure, comprising a housing having a conical inner surface, the housing being attachable to the wall through a first attachment means; and a plug member having a conical outer surface matching the conical inner surface of the housing, the plug member being attachable to the housing through a second attachment means, wherein the conical inner surface of the housing and/or the conical outer surface of the plug member have recesses forming, when the plug member is attached to the housing, a plurality of waveguides. Each of the first attachment means and the second attachment means are operable by one or more operating members located, with the feed-through in installed position, exclusively on one side of the wall, so that the first attachment means and the second attachment means are exclusively operable from that one side of the wall. 
     Typically, the said one side of the wall is the inside of the cabinet of enclosure. In this way, a tamper- and breakin-proof feed-through is provided. Using such a feed-through eliminates or mitigates the need for additional security measures external to the feed-through, saving space and cost. 
     Preferably, the first attachment means and the second attachment means are removable. This allows for flexibility of installation. 
     A recess of the conical outer surface may have a slot for a cover, wherein, with the plug member attached to the housing and the cover installed in the slot, the housing covers the slot, preventing the cover from being removed from the slot. In typical use, a waveguide will be covered by a slot if it is unused, i.e., no signal passes through it. This provides additional security and tamper-proofing, since it eliminates the possibility of tampering with the operating members through the waveguides. 
     The housing may have a non-circular cross-section, which, with the housing in installed position at the wall, may fit into a corresponding non-circular opening in the wall, preventing the housing from rotating with respect to the wall. Alternatively, or additionally, the one or more operating means of the first attachment means may be operable without rotating the housing. This provides additional security and tamper-proofing, since it eliminates the possibility of tampering with or loosening the housing from the wall by rotating it. 
     The first attachment means may comprise an outer thread on the housing and a nut, comprising the operating member, having an inner thread matching the outer thread of the housing. This provides a simple yet secure mechanism for attaching the housing to the wall. 
     The second attachment means may, in a first alternative, comprise a bolt provided with an outer thread; a threaded bore in the plug member with an opening located, with the feed-through in installed position, on said one side of said wall, the threaded bore matching the outer thread of the bolt; and an elongated crossbar; wherein the bolt, with the plug member attached to said housing, pulls against said plug member through the threaded bore and pushes against the housing through the crossbar. This provides a simple yet reliable mechanism for attaching the plug member to the housing. 
     Using an elongated crossbar, which may fit into a pair of slots on the housing, instead of a more elaborate plate structure which must have large recesses matching the waveguides, simplifies manufacturing while still providing a second attachment means with higher strength and reliability that also is easier to install. 
     In a second alternative, the second attachment means may comprise a transversal slot through the plug member; a pair of openings in the housing, matching the transversal slot; and a locking rod provided with a convex surface, wherein the locking rod, with the plug member attached to the housing, extends through the transversal slot and the pair of openings and at a first end is pivoted at a pivot point fixed in position with respect to the housing and at a second end is pushed so that the convex surface abuts against an inner surface of the transversal slot, pushing the plug member in a direction into the housing. 
     This provides a mechanism that is easier to install, yet secure, if space behind the feed-through, usually on the inside of the cabinet or enclosure, is tight. Further, the locking rod provides a lever arm which simplifies removal of the second attachment means. 
     The locking rod may have a second convex surface, opposite the convex surface, where, during removal of the second attachment means, the second convex surface abuts a second inner surface of the transversal slot. This aids removal of the second attachment means using the locking rod as a lever arm. 
     The pivot point and the pushing may be provided by a frame arrangement mountable around the housing. The frame, in installed position, may abut against a shoulder of the housing. 
     Alternatively again, the feed-through may comprise second attachment means according to both the first alternative and according to the second alternative above. The alternative actually used at the point of installation may then be selected during installation of the feed-through, increasing flexibility. For example, the components may be provided as a kit. 
     The feed-through may further comprise a weather seal frame, comprising an upper part and a lower part adapted to fit around the housing; and a plurality of resilient sealing blocks adapted to be compressed between the upper part and the lower part of said weather seal, sealing around the one or more signal leads. Advantages and embodiments are identical to those described below in conjunction with the sixth aspect. 
     The plurality of waveguides may comprise four waveguides arranged in a rectangular pattern. This allows the waveguides to line up with the resilient blocks of the weather seal, allowing such a seal to be used together with the revolver-type feed-through. 
     The said plug member may comprise a transversally extending pin matching a transversal slot in the housing. During insertion of the plug member into the housing, the slots will rotationally guide the plug member. Further, with the plug member inserted into and attached to the housing, the slot and pin will prevent the plug member from rotating with respect to the housing. This provides a simple and robust mechanism that simplifies installation, as the plug member is ensured to always have the correct rotational configuration with respect to the housing. 
     According to a second aspect, there is provided a system, comprising the feed-through according to the first aspect, one or more of signal leads, and a cabinet or enclosure. 
     Advantages and embodiments of this second aspect are at least the same as and/or compatible with those described above in conjunction with the first aspect. 
     According to a third aspect, there is provided a method of installing a revolver-type feed-through for TEMPEST-grade electromagnetic shielding through a wall of a cabinet or other enclosure, comprising: attaching a housing to the wall through a first attachment means, the housing having a conical inner surface; pulling one or more non-electric signal leads through the housing; attaching a plug member to the housing through a second attachment means, an outer conical surface of said plug member matching the conical inner surface of the housing, the conical inner surface and/or the conical outer surface having recesses forming a plurality of waveguides enclosing said one or more signal leads; said method being characterized by said first attachment means and said second attachment means being exclusively operated from one side of said wall. 
     Advantages and embodiments of this third aspect are at least the same as and/or compatible with those described above in conjunction with the first aspect. 
     According to a fourth aspect, there is provided a revolver-type feed-through for TEMPEST-grade electromagnetic shielding where one or more non-electric signal leads pass through a wall of a cabinet or other enclosure, comprising a housing having a conical inner surface, the housing being attachable to the wall through a first attachment means; and a plug member having a conical outer surface matching the conical inner surface of the housing, the plug member being attachable to the housing through a second attachment means, wherein the conical inner surface of the housing and/or the conical outer surface of the plug member have recesses forming, when the plug member is attached to said housing, a plurality of waveguides, the feed-through being characterized by further comprising a weather seal frame comprising an upper part and a lower part adapted to fit around and seal against the housing; and a plurality of resilient sealing blocks adapted to be compressed between the upper part and the lower part of said weather seal, sealing around the one or more signal leads. 
     With weather seal should be understood a seal sealing against one or more of, but not limited to, weather, moist, dirt, oil, splashing water, which may be fresh or salt water, or similar. 
     This allows weather sealing to be provided directly by the feed-through, eliminating the need for additional measures to provide weather sealing, for example the need to place the cabinet or enclosure in a weather-sealed room. This saves space. This, for example, is useful in marine applications. 
     The upper part and the lower part may be identical in design, which simplifies manufacturing and installation. 
     Advantages and embodiments described above in conjunction with the first aspect are compatible with this fourth aspect. 
     According to a fifth aspect, there is provided use of a revolver-type feed-through for TEMPEST-grade electromagnetic shielding where one or more non-electric signal leads pass through a wall of a cabinet or other enclosure, the feed-through comprising: a housing having a conical inner surface, the housing being attachable to the wall through a first attachment means; and a plug member having a conical outer surface matching the conical inner surface of the housing, the plug member being attachable to the housing through a second attachment means, wherein the conical inner surface of the housing and/or the conical outer surface of the plug member have recesses forming, when the plug member is attached to the housing, a plurality of waveguides, in combination with a weather seal comprising a weather seal frame comprising an upper part and a lower part adapted to fit around and seal against the housing; and a plurality of resilient sealing blocks adapted to be compressed between the upper part and the lower part of the weather seal frame, sealing around the one or more signal leads. 
     Advantages and embodiments of this fifth aspect are at least the same as and/or compatible with those described above fourth aspect. Advantages and embodiments described above in conjunction with the first aspect are compatible with this fifth aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as additional objects, features and advantages of the present inventive concept, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein: 
         FIGS. 1 a  and 1 b    are exploded views, from two different respective angles, of a revolver-type feedthrough; 
         FIGS. 2 a , 2 b   ;  3 ;  4 , and  5   a ,  5   b ,  5   c ,  5   d  show a typical assembly sequence of the feed-through, wherein  FIGS. 2 a , 2 b   ;  3 ;  4 ; and  5   a ,  5   b  are perspective views and  5   c ,  5   d  are cross-sectional views; 
         FIGS. 6 ;  7 ;  8 ;  9   a ,  9   b ; and  10  show a typical assembly sequence of an alternative second attachment means of the feed-through, wherein  FIGS. 6 ;  7 ;  8 ;  9   a  are perspective views and  FIGS. 9 b    and  10  are cross-sectional views; 
         FIG. 11  is a cross-sectional view showing removal of the alternative second attachment means. 
         FIGS. 12, 13, 14, 15, and 16  are perspective views showing assembly of an optional weather seal for use with the feedthrough; 
         FIG. 17  schematically illustrates a protected cabinet or other enclosure; and 
         FIG. 18  schematically illustrates a waveguide operating below-cutoff frequency. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 17  illustrates a protected cabinet or other enclosure  201 , which is a context where a feed-through  2  (cf.  FIGS. 1 a  and 1 b   ) according to the present disclosure can be used. Such a cabinet or other enclosure can be used in a sensitive environment where processing devices  203  in the cabinet  201 , such as servers, electronically process unencrypted or encrypted sensitive information. The cabinet  201 , made from an electrically conductive material, which may be thick sheet metal, functions as a Faraday cage, containing within its enclosure any electromagnetic radiation from the processing devices  203 , thereby preventing the leakage of sensitive information. The processing devices  203  communicate with nodes  213  outside the cabinet  201  via non-electric signal leads  205 , such as optical fibers, which are terminated by means of connectors  207 ,  209  at each if their ends. The signal-leads must be non-electric in order not to spoil the functioning of the shielding, as an electric signal lead would act as an antenna, spoiling the shielding of the inside of the cabinet or enclosure. The signal leads may convey less sensitive information or the information carried by them may be encrypted. Furthermore, the conveyed optical signals as such usually give negligible leaked radiation, preventing interception of such signals, unless the fibers are tampered with. 
     The feed-through where the optical fibers extend out of the cabinet  201  requires special attention so that electromagnetic radiation does not escape the cabinet, risking interception by a third party nearby. 
     In addition to the shielding situation described above, the feed-through may be useful in other situations where considerable attenuation at a connection into a Faraday cage is needed. For instance, as the effect provided is more or less reciprocal, the feed-through could protect sensitive equipment in a cabinet from external electromagnetic interference, such as electromagnetic pulses with high energy. Further, a shielded room used for sensitive measurements could be protected from external electromagnetic interference, etc. 
     Additionally, the cabinet or enclosure  201  may be subject to requirements of weather proofing and/or tamper proofing and/or security against break-in. Normally such feed-throughs do not provide such functionality by themselves, requiring such functionality to be provided external to the cabinet or enclosure  201 , for example by the cabinet or enclosure being located in a secured and/or weather sealed room. With the present inventive concept, such functionality may be provided directly by the feed-through itself. This is particularly advantageous in environments where space is at a premium. 
     As is well known per se, an attenuating feed-through can be obtained by means of a waveguide below-cutoff, WBCO, of which one example is schematically illustrated in  FIG. 16 . In this example the waveguide  212 , made from a conductive material, has a cavity  214 , open at both ends, with a circular cross section with diameter D and a length  1 . The waveguide extends through a wall in a Faraday cage  201 . 
     Electromagnetic waves can propagate through a waveguide in a number of different modes, corresponding to different solutions to the Maxwell equations. These modes are distinguished by different configurations of the electric and magnetic fields. Each of these modes has a cut-off frequency, below which no substantial propagation in that mode is possible. Below the lowest cut-off frequency of all possible modes, no substantial propagation of electromagnetic waves is possible at all. Instead, signals suffer exponential attenuation. For example, in a circular waveguide, the mode with the lowest cut-off frequency is the TE 11  (transverse electric) mode. The cut-off frequency f c  of that mode can be shown to be, to three significant figures, 
     
       
         
           
             
               
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     where v is the propagation speed of the waveguide dielectric, i.e., the non-conductive material forming the bulk of the waveguide cavity. In air, v is to a good approximation 3×10 8  m/s. Below this cut-off frequency, over a distance l, an electromagnetic wave with frequency f suffers a total attenuation of again to three significant figures, 
     
       
         
           
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     It can thus be seen that to maximize attenuation of signals of a certain frequency requires as high a cut-off frequency as possible, and thus as narrow a waveguide as possible, as well as a long a waveguide as possible. It can also be seen that the larger the diameter a waveguide has, the longer the waveguide needs to be to achieve target attenuation. 
       FIGS. 1 a  and 1 b    show exploded views, from two different angles, of a revolver-type feed-through  2  providing TEMPEST-grade electromagnetic shielding where one or more non-electric signal leads  18  (cf.  FIG. 15 ) pass through a wall  16  of a cabinet or enclosure (cf.  101 ,  FIG. 15 ), for example in the context described above with reference to  FIGS. 17 and 18 . 
     The feed-through  2  is intended to be attached to the wall  16  and comprises a housing  4  intended to be attached to the wall  16  and a plug member  6  intended to be attached to the housing  4 . The wall  16  has a first side, typically corresponding to the outside of the cabinet or enclosure, towards the housing  4  and plug member  6 , as depicted in  FIGS. 1 a  and 1 b   , and a second side, typically corresponding to the inside of the cabinet or enclosure, towards the nut  24  and the bolt  38 , again as depicted in  FIGS. 1 a    and  1   b.    
     The housing  4  has a conical inner surface  8  matching a conical outer surface  12  of the plug member  6  so that a tight fit may be achieved between the conical inner surface  8  and the conical outer surface  12  (cf.  FIG. 5 c   ). 
     The plug member has a plurality of longitudinal recesses  20 , so that, with the plug member  6  inserted into and attached to the housing  4 , the conical outer surface  12  of the plug member  6  meets the conical inner surface  8  of the housing  4 , each recess  20  forming a waveguide  22  (cf.  FIG. 5 d   ) through the feed-though  2 . 
     In the depicted feed-through  2 , only the plug member  6  has recesses  20 . However, it is equally possible for the wave-guide-forming recesses  20  to instead be located on the conical inner surface  8  of the housing  4 , or on both the conical inner surface  8  of the housing  4  and the conical outer surface  12  of the plug member  6 . 
     In order to properly form the respective waveguides  22 , a tight fit is needed between the housing  4  and the plug member  6 . The housing  4  and the plug member  6  must thus be formed from material with good electrical conductivity, typically a metal such as steel or brass. Typically, they are made from machined, such as lathed, parts in order to achieve the required tolerances needed for a tight fit between the two parts. 
     The depicted plug member has four recesses  20  evenly distributed around the outer surface  12  so to form a quadratic, i.e., rectangular pattern. Thus, the waveguides  22  formed when the plug member  6  is inserted into the housing  4  will also form a quadratic/rectangular pattern. 
     Each of the recesses  20  of the outer conical surface  12  of the plug member  6  has a slot  28  located close to the outer end of the plug member  6 , in installed position on the first side of the wall. Before the plug member  6  is inserted into the housing  4 , a cover  30  may optionally be inserted into a respective such slot  28 . With the plug member  6  then inserted into the housing  4 , the wall  66  close to the outer end of the housing  4  will cover the cover  30  so that the cover  30  cannot be removed from the slot, preventing an intruder from gaining access to the opposite, second, side of the wall through the waveguide  22  located behind the cover  30 . 
     The plug member  6  comprises two transversally extending and diametrically opposite pins  68  located close to the outer end of the plug member  6 , matching two respective slots  70  in the housing  4 . During insertion of the plug member  6  into the housing  4 , the plug member  6  will be guided rotationally by the slots  70 . Further, with the plug member  6  inserted into and attached to the housing  4 , the slots  70  and pins  68  will prevent the plug member  6  from rotating with respect to the housing  4 . 
     The feed-through  2  further comprises a first attachment means  10  for attaching the housing  4  to the wall  16  of the cabinet or enclosure. The second attachment means is in the form of an outer thread  34  on the housing  4  and a nut  24  with an inner thread  36  matching the outer thread  34  of the housing  4 . Further, a washer  62  protects the housing  4  and will, in installed position, be located between the wall  16  and a shoulder  64  on the housing  4 , located next to the outer thread  34 . The washer  62  ensures electrical contact between the housing  4  and the electrically-conducting wall  16  and is preferably made of a material softer than the housing and/or the wall, such as copper. The nut  24  acts as an operating member  10  for the first attachment means  10  and is, with the housing  4  in installed position, entirely located on the second side of the wall  16 . Thus the first attachment means  10  is only operable from that second side of the wall  16 . The nut  24  can be rotated without rotating the housing  4  with respect to the wall  16 . Thus, the operating means  24  of the first attachment means  10  is operable without rotating the housing  4 . 
     The part of the housing  4  which in installed position extends through the wall  16  has a non-circular cross-section, as evident by two diametrically opposed flat sections  72 . This is matched by a corresponding non-circular cross section of the hole  32  in the wall  16 , including two straight sections  74  matching the flat sections  72  of the housing  72 . This geometric configuration prevents the housing, in installed position attached to the wall, from rotating with respect to the wall  16 . 
     The feed-through  2  of  FIGS. 1 a  and 1 b    further comprises a second attachment means  26  for attaching the plug member  6  to the housing  4 , comprising the following. A bolt  38  is provided with an outer thread  40 . The bolt may be inserted into an elongated crossbar  46 . The thread  40  matches an inner thread of a threaded bore  42  (cf.  FIG. 5 c   ) in the plug member. The threaded bore  42  has an opening located, with the plug member  6  inserted into and attached to the housing  4 , on the inner side of the wall  16 . 
     A head  26  of the bolt  38  may be rotated by a user to drive the bolt  38  into the plug member  6 , thereby pulling the plug member  6  into the housing  4 . Thus, the head  26  acts as an operating member for the second attachment means and is entirely located, in installed position, on the same, second, side of the wall  16  as the operating member of the first attachment means. Thus the second attachment means  14  is only operable from that second side of the wall  16 . 
     In installed position, the bolt  38  pulls against the plug member  6 , keeping it attached to the housing  4  through the thread of the threaded bore  42  of the plug member  6  (cf.  FIG. 5 c   ), while pushing against the housing though the crossbar  46 , which abuts two slots  76  located at the inner end of the housing  4 . 
       FIGS. 2 a , 2 b   ;  3 ;  4  and  5   a - d  show a typical assembly sequence of the feed-through  2 . 
     First, with reference to  FIGS. 2 a  and 2 b   , the housing  4  is pushed from the first side of the wall  16  into the hole  32  of the wall  16  (cf.  FIGS. 1 a  and 1 b   ), as indicated by an arrow A, the non-circular cross-section of the housing  4  lining up with the non-circular cross section of the hole  32  of the wall  16 , with the washer  62  located on the first side of the wall  16  between the wall  16  and the shoulder  64  of the housing  4 . 
     Then, the first attachment means  10  (cf.  FIGS. 1 a  and 1 b   ) is operated, on the second side of the wall  16 , by operating the operating means, by tightening the nut  24  by rotating it, as indicated by an arrow B. With the nut  24  tightened, the housing  4  is now attached to the wall  16  through the first attachment means  10 . 
     Then, with reference to  FIG. 3 , one or more non-electric signal leads  18 —typically optical fibers or fiber-optic cables—are pulled through the housing  4  and thus through the wall  16 . Covers  30  may be placed into the slots  28  corresponding to recesses  20 /waveguides  22  (cf.  FIGS. 1 a  and 1 b   ) not intended to carry signal leads  18 . 
     Then, still with reference to  FIG. 3 , the plug member  6  is pushed into the housing  4 , from the first side of the wall  16 , as indicated by an arrow C, the pins  68  being guided by the slots  70 . The waveguides  22  (cf.  FIGS. 5 a , 5 b   ) are formed. 
     Then, still with reference to  FIG. 3 , the bolt  38  is pushed, from the second side of the wall  16 , through the hole of the crossbar  46  and into the opening  44  (cf.  FIG. 1 b   ) of the plug member and the crossbar  46  placed into the two slots  76  (cf.  FIGS. 1 b   ,  4 ) of the housing  4 , as indicated by an arrow D. 
     Then, with reference to  FIG. 4 , the bolt  38  is tightened, on the second side of the wall  16 , by engaging the head  26  with a suitable tool and rotating the head  26 , as indicated by an arrow E. 
     With reference to  FIGS. 5 a , 5 b , 5 c , and 5 d   , with the bolt  38  tightened, the plug member  6  is now attached to the housing  4  through the second attachment means  14  (cf.  FIGS. 1 a , 1 b   ). The signal leads  18  run through waveguides  22  formed between the housing  4  and the plug member  6 , while waveguides having no signal leads are covered and blocked by the covers  30  placed into the slots  28 , with the covers  30  being prevented from being removed from their respective slots  28  in the plug member  6  by the wall  66  of the housing  4 . 
     As is best seen in  FIG. 5 c   , the threads of the threaded bore  42  are exclusively located close to the inner end of the threaded bore  42 . This ensures that the plug member  6 , when attached to the housing  4 , is mainly subjected to a compressing force, which makes the arrangement less sensitive to tensions caused by, e.g., temperature variations. 
     As is best seen in  FIG. 5 d   , the waveguides  22  follow the cone shape of the plug member  6 , diverging transversally outwards from the second side of the wall to the first side of the wall. 
       FIGS. 6 ;  7 ;  8 ;  9   a ,  9   b ; and  10  show assembly of an alternative second attachment means  14 , based on the plug member  6  having a transversal slot  48  (cf.  FIGS. 1 a , 1 b   ) extending through the same and the housing having two openings  50  (cf.  FIGS. 1 a , 1 b   ) matching, i.e., lining up with the transversal slot  48  of the plug member  6  when the plug member  6  has been inserted into the housing  4 . In installed position, a locking rod  52  extends through the transversal slot  48  and the openings  50 , pushing the plug member  6  in a direction into the housing  4 , as will be explained below. 
     With reference to  FIG. 6 , starting with the plug member  6  pushed into the housing  4  (cf. the procedure above disclosed in conjunction with  FIGS. 2 a , 2 b   , and  3 ), a frame arrangement  58  is mounted around the housing  4  on the second side of the wall  16 . The frame arrangement comprises a first part  58   a  and a second part  58   b  joined by bolts  78 . Both parts  58   a ,  58   b  of the frame arrangement  58  abut a shoulder  65  of the housing  4 . 
       FIG. 7  shows the two parts  58   a ,  58   b  of the frame arrangement  58  thus joined by the bolts  78 . 
     Then, with reference to  FIG. 8 , the locking rod  52  is inserted, as indicated by an arrow F, into the openings  50  and the transversal slot  48  (cf.  FIG. 9 b   ). The locking rod  52  has an elongate shape and is substantially flat in a vertical direction. At a first end of the locking rod  52 , a two-prong fork-like structure  60  is located, extending longitudinally and horizontally from the locking rod  52 . Further, a convex surface  54  extends transversally and horizontally close to the middle of the locking rod  52  along one of its long edges. A slot  80  is located close to the other, second, end of the locking rod  52  opposite that of the fork-like structure and on the opposite long edge to that of the convex surface  54 , the function all of which will be explained in the following. Alternatively, the slot  80  may be replaced by a shoulder (not depicted) extending all the way to the second end of the locking rod. 
       FIGS. 9 a  and 9 b    show the locking rod  52  thus inserted, extending through the transversal slot  48  and the pair of openings  50 . The fork-like structure  60  surrounds a pin  56  provided by the frame arrangement  58  and functioning as a pivot point for the locking rod  52 . The slot  80  is aligned with a bolt  82  provided by the frame arrangement  58   
     The bolt  82  is then tightened, the locking rod  52  pivoting around the pin  56 , which remains fixed in position with respect to the housing  4  due to the frame arrangement  58  being supported by the housing  4  through the shoulder  65 . Simultaneously, the convex surface  54  of the locking rod  52  pushes the inner surface of the transversal slot  48 , pushing the plug member  6  into the housing  4  and fixing it in place. 
     Thus, with reference to  FIG. 10 , the second attachment means according to the present alternative attaches the plug member  6  to the housing  4 . The operating members, the locking rod  52  and the bolt  82 , are exclusively operable from the second side of the wall  16 , i.e., the same side as the operating member of the first attachment means, i.e., the nut  24  (cf.  FIG. 2 a , 2 b   ,  3 ). 
       FIG. 11  illustrates removal of the alternative second attachment means  14  (cf.  FIGS. 6 ;  7 ;  8 ;  9   a ,  9   b ; and  10 ). Due to the tight fit between the plug member  6  and the housing  4 , some force will be required to separate the two. 
     First, the bolt  82  is untightened, releasing the locking rod  52 . 
     Then, a force, such as a “knock”, is applied at the free end of the locking rod  52 , for example using a hammer, as illustrated by a vertical arrow G. The locking rod  52 , pivoting around the pin  58 , acts a lever arm, with a second convex surface  55 , opposite to the first convex surface  54  (cf.  FIG. 9 b   ) abutting a second inner surface of the transversal slot  48 , pushing the plug member  6  out of the housing  4 . 
     Finally, the locking rod  52  may be removed from the transversal slot  48  and the frame  58  may be removed from the housing  4 . 
     The feed-through may be delivered as a kit comprising both the bolt  38  and the crossbar  46  (cf.  FIG. 1 a   ) and the frame arrangement  58  and the locking rod  52 , so that the choice of the first or the second alternative for the second attachment means may be made according to need at the point of installation. 
       FIGS. 12, 13, 14, 15, 16  show assembly of an optional weather seal for use with a revolver-type feedthrough. 
     Starting with an assembled feed-through as described above in conjunction with  FIGS. 5 a - d    or  FIG. 11 , with reference to  FIG. 12 , resilient substantially cube-shaped blocks  104  are attached to the signal leads outside of the housing  4  on the first side of the wall  16 . As depicted, the cube-shaped blocks  104  may have one or several holes of different sizes to match the outer dimensions of the signal leads. If no signal lead is present, the corresponding block  104  may have no hole at all (not shown). The blocks  104  are made from a resilient material, such as natural or synthetic rubber. 
     Then, with reference to  FIG. 13 , the lower part  106   a  of a frame  106  (cf.  FIG. 14 ) is mounted, as indicated by an upward-pointing arrow H. The lower part  106   a  of the frame has a shape matching the resilient blocks  104  and the housing  4 , in particular the cylindrical shape of the housing  4 , including a cylindrical protrusion. Further, the lower part  106   a  comprises a lower semicircular resilient part  108  designed to seal around the housing  4 . A flat section  116  (not visible but identical to the flat section  116  visible on the top of the housing  4  matches a flat protrusion  114  of the lower part  106   a , preventing the lower part  106   a  from rotating around the housing  4 . 
     Then, with reference to  FIG. 14 , an upper part  106   b  of the frame  106  is mounted, as indicated by a downward pointing arrow I. Just like the lower part  106   a , the upper part  106   n  of the frame  106  has a shape matching the resilient blocks  104  and the housing  4 . Further, the upper part  106   b  comprises an upper circular-shaped resilient part (not visible) designed to seal around the housing  4 . The flat section  116  on the upper part of the housing  4  matches a flat protrusion  114  on the upper part  106   b  of the frame (not visible, but identical to the flat section visible on the lower part  106   a , cf.  FIG. 13 ), preventing the upper part  106   b  from rotating around the housing  4 . 
     The lower part  106   a  and the upper part  106   b  are made from non-resilient material, such as a thermoplastic, such as ABS. They may be identical in design, which simplifies manufacturing and installation. 
     Then, with reference to  FIG. 15 , two respective U-shaped reinforcement plates  110  are added to the upper surface of the upper part  106   b  and to the lower surface of the lower part  106   a , by sliding, as indicated by horizontal arrows J. The reinforcement plates  110  are preferably made of metal, such as steel. 
     Finally, with reference to  FIG. 16 , the upper part  106   b  and the lower part  106   a  are joined by bolts  112  through both of the reinforcement plates  110 , holes on the reinforcement plates  110  lining up with holes in the upper part  106   b  and lower part  106   a , compressing the resilient blocks  104 , ensuring sealing action of the resilient blocks  104  around the signal leads  18  and sealing around the housing  4  by the semicircular resilient parts. Further resilient parts may seal between the upper part  106   a  and the lower part  106   b.    
     The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims. 
     The following are examples. 
     1. A revolver-type feed-through ( 2 ) for TEMPEST-grade electromagnetic shielding where one or more non-electric signal leads ( 18 ) pass through a wall ( 16 ) of a cabinet or other enclosure, comprising:
         a housing ( 4 ) having a conical inner surface ( 8 ), said housing being attachable to said wall through a first attachment means ( 10 ); and   a plug member ( 6 ) having a conical outer surface ( 12 ) matching said conical inner surface ( 8 ) of said housing ( 4 ), said plug member ( 6 ) being attachable to said housing ( 4 ) through a second attachment means ( 14 ),       

     wherein said conical inner surface ( 8 ) of said housing ( 4 ) and/or said conical outer surface ( 12 ) of said plug member have recesses ( 20 ) forming, when said plug member ( 6 ) is attached to said housing ( 4 ), a plurality of waveguides ( 22 ), said feed-through ( 2 ) being characterized by each of said first attachment means ( 10 ) and said second attachment means ( 14 ) being operable by one or more operating members ( 24 ,  26 ) located, with said feed-through in installed position, exclusively on one side of said wall ( 16 ), so that said first attachment means ( 10 ) and said second attachment means ( 14 ) are exclusively operable from said one side of said wall ( 16 ). 
     2. The feed-through ( 2 ) of example 1, wherein a said recess ( 20 ) of said conical outer surface ( 12 ) has a slot ( 28 ) for a cover ( 30 ), wherein, with said plug member ( 12 ) attached to said housing ( 4 ) and said cover ( 30 ) installed in said slot ( 28 ), said housing ( 4 ) covers said slot ( 28 ), preventing said cover ( 30 ) from being removed from said slot ( 28 ).
 
3. The feed-through ( 2 ) of any one of examples 1-2, wherein said housing ( 4 ) has a non-circular cross-section, which, with said housing in installed position at said wall, may fit into a corresponding non-circular opening ( 32 ) in said wall ( 16 ), preventing said housing ( 4 ) from rotating with respect to said wall.
 
4. The feed-through of any one of examples 1-3, wherein said one or more operating means ( 24 ) of said first attachment means ( 10 ) is operable without rotating said housing ( 4 ).
 
5. The feed-through ( 2 ) of any one of examples 1-4, wherein said first attachment means ( 10 ) comprises an outer thread ( 34 ) on said housing ( 4 ) and a nut ( 24 ), comprising said operating member ( 24 ), having an inner thread ( 36 ) matching said outer thread ( 34 ) if said housing ( 4 ).
 
6. The feed-through ( 2 ) of any one of examples 1-5, wherein said second attachment means ( 14 ) comprises:
         a bolt ( 38 ) provided with an outer thread ( 40 );   a threaded bore ( 42 ) in said plug member ( 6 ) with an opening ( 44 ) located, with said feed-through ( 2 ) in installed position, on said one side of said wall ( 16 ), said threaded bore ( 42 ) matching said outer thread ( 40 ) of said bolt; and   an elongated crossbar ( 46 );   wherein said bolt ( 38 ), with said plug member ( 6 ) attached to said housing ( 4 ), pulls against said plug member ( 6 ) through said threaded bore ( 42 ) and pushes against said housing ( 4 ) through said crossbar ( 46 ).
 
7. The feed-through ( 2 ) of any one of examples 1-5, wherein said second attachment means ( 14 ) comprises:
   a transversal slot ( 48 ) through said plug member ( 6 );   a pair of openings ( 50 ) in said housing ( 4 ), matching said transversal slot ( 48 ); and   a locking rod ( 52 ) provided with a convex surface ( 54 ), wherein said locking rod ( 52 ), with said plug member ( 6 ) attached to said housing ( 4 ), extends through said transversal slot ( 48 ) and said pair of openings ( 50 ) and at a first end pivoted at a pivot point ( 56 ) fixed in position with respect to said housing ( 4 ) and at a second end is pushed so that said convex surface abuts against an inner surface of said transversal slot ( 48 ), pushing said plug member ( 6 ) in a direction into said housing ( 6 ).
 
8. The feed-through ( 2 ) of example 7, wherein said pivot point ( 56 ) and said pushing is provided by a frame arrangement ( 58 ) mountable around said housing ( 4 ).
 
9. The feed-through ( 2 ) of example 8, wherein said frame arrangement ( 58 ), in installed position, abuts against a shoulder ( 65 ) of said housing ( 4 ).
 
10. The feed-through ( 2 ) of any one of examples 1-5, comprising the second attachment means ( 14 ) of example 6 and the second attachment means ( 14 ) of any one of examples 7-9, selectable during installation of said feed-through ( 2 ).
 
11. The feed-through ( 2 ) of any one of the preceding examples, wherein said plug member ( 6 ) a transversally extending pin ( 68 ), matching a transversal slot ( 70 ) in said housing ( 4 ).
 
12. The feed-through ( 2 ) of any one of the preceding examples, further comprising:
   a weather seal frame ( 106 ) comprising an upper part ( 106   b ) and a lower part ( 106   a ) adapted to fit around said housing; and   a plurality of resilient sealing blocks ( 104 ) adapted to be compressed between said upper part ( 106   b ) and said lower part ( 106   a ) of said weather seal frame ( 106 ), sealing around said one or more signal leads ( 18 ).
 
13. The feed-through ( 2 ) of any one of the preceding examples, wherein said plurality of waveguides comprises four waveguides ( 22 ) arranged in a rectangular pattern.
 
14. A system, comprising the feed-through ( 2 ) of any one of the preceding examples, one or more said signal leads, and said cabinet ( 18 ) or enclosure.
 
15. A method of installing a revolver-type feed-through ( 2 ) for TEMPEST-grade electromagnetic shielding through a wall ( 16 ) of a cabinet or other enclosure, comprising:
   attaching a housing ( 4 ) to said wall ( 16 ) through a first attachment means ( 10 ), said housing ( 4 ) having a conical inner surface ( 8 );   pulling one or more non-electric signal leads ( 18 ) through said housing ( 4 );   attaching a plug member ( 6 ) to said housing ( 4 ), an outer conical surface ( 12 ) of said plug member ( 6 ) matching said conical inner surface ( 8 ) of said housing ( 4 ), said conical inner surface ( 8 ) and/or said conical outer surface ( 12 ) having recesses forming a plurality of waveguides ( 22 ) enclosing said one or more signal leads ( 18 );       

     said method being characterized by said first attachment means ( 10 ) and said second attachment ( 14 ) means being exclusively operated from one side of said wall ( 16 ). 
     16. A kit, comprising:
         the feed-through ( 2 ) of any one of examples 1-5 or 11-15, comprising the second attachment means of any one of examples 7-9; and   an alternative second attachment means ( 14 ),
 
said alternative second attachment means ( 14 ) comprising:
   a bolt ( 38 ) provided with an outer thread ( 40 );   a threaded bore ( 42 ) in said plug member ( 6 ) with an opening ( 44 ) located, with said feed-through ( 2 ) in installed position, on said one side of said wall ( 16 ), said threaded bore ( 42 ) matching said outer thread ( 40 ) of said bolt; and   an elongated crossbar ( 46 ),
 
wherein said bolt ( 38 ), with said plug member ( 6 ) attached to said housing ( 4 ), pulls against said plug member ( 6 ) through said threaded bore ( 42 ) and pushes against said housing ( 4 ) through said crossbar ( 46 ), and
 
wherein one of said second attachment means ( 14 ) and said alternative second attachment means ( 14 ) is selectable during installation of said feed-through ( 2 ).
 
17. A revolver-type feed-through ( 2 ) for TEMPEST-grade electromagnetic shielding where one or more non-electric signal leads ( 18 ) pass through a wall ( 16 ) of a cabinet or other enclosure, comprising:
   a housing ( 4 ) having a conical inner surface ( 8 ), said housing being attachable to said wall through a first attachment means ( 10 ); and   a plug member ( 6 ) having a conical outer surface ( 12 ) matching said conical inner surface ( 8 ) of said housing ( 4 ), said plug member being attachable to said housing ( 4 ) through a second attachment means ( 14 ),       

     wherein said conical inner surface ( 8 ) of said housing ( 4 ) and/or said conical outer surface ( 12 ) of said plug member have recesses ( 20 ) forming, when said plug member ( 6 ) is attached to said housing ( 4 ), a plurality of waveguides ( 22 ), said feed-through ( 2 ) being characterized by further comprising:
         a weather seal frame ( 106 ) comprising an upper part ( 106   b ) and a lower part ( 106   a ) adapted to fit around and seal against said housing; and   a plurality of resilient sealing blocks ( 104 ) adapted to be compressed between said upper part ( 106   b ) and said lower part ( 106   a ) of said weather seal frame ( 106 ), sealing around said one or more signal leads ( 18 ).
 
18. The feed-through of example 17, wherein said plurality of waveguides ( 22 ) comprises four waveguides ( 22 ) arranged in a rectangular pattern.
 
19. Use of a revolver-type feed-through ( 2 ) for TEMPEST-grade electromagnetic shielding where one or more non-electric signal leads ( 18 ) pass through a wall ( 16 ) of a cabinet or other enclosure, the feed-through comprising:
   a housing ( 4 ) having a conical inner surface ( 8 ), said housing being attachable to said wall through a first attachment means ( 10 ); and   a plug member ( 6 ) having a conical outer surface ( 12 ) matching said conical inner surface ( 8 ) of said housing ( 4 ), said plug member being attachable to said housing ( 4 ) through a second attachment means ( 14 ),       

     wherein said conical inner surface ( 8 ) of said housing ( 4 ) and/or said conical outer surface ( 12 ) of said plug member ( 6 ) have recesses ( 20 ) forming, when said plug member ( 6 ) is attached to said housing ( 4 ), a plurality of waveguides ( 22 ), 
     in combination with a weather seal comprising:
         a weather seal frame ( 106 ) comprising an upper part ( 106   b ) and a lower part ( 106   a ) adapted to fit around and seal against said housing ( 4 ); and   a plurality of resilient sealing blocks ( 104 ) adapted to be compressed between said upper part ( 106   b ) and said lower part ( 106   a ) of said weather seal frame, sealing around said one or more signal leads ( 18 ).