Patent Publication Number: US-2016249494-A1

Title: Voltage dividing shielded door seal

Description:
REFERENCES CITED 
     U.S. Patent Documents 
       
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 3,589,070 
                 June 1971 
                 Hansen 
               
               
                   
                 4,069,618 
                 January 1978 
                 Geiss 
               
               
                   
                 4,677,251 
                 June 1987 
                 Merewether 
               
               
                   
                 5,017,736 
                 May 1991 
                 Yarger et al. 
               
               
                   
                 5,569,878 
                 October 1996 
                 Zielinski 
               
               
                   
                 5,736,671 
                 April 1998 
                 Perala 
               
               
                   
                 5,786,547 
                 July 1998 
                 Zielinski 
               
               
                   
                 7,117,640 B2 
                 October 2006 
                 Hurzeler 
               
               
                   
                   
               
            
           
         
       
     
     BACKGROUND 
     Sensitive electronic equipment must be protected from interference or damage by harmful electromagnetic radiation from nearby radio or TV transmitters, radar, nearby lightning strokes and the electromagnetic pulse from a nuclear burst. To provide this protection the equipment is housed in a shielded room (or Faraday cage), an enclosure with continuous metallic walls, floor and ceiling. 
     The best enclosures are formed from continuously welded metal sheets. When the frequency is high enough that the metal sheets are several skin depths thick, the interior fields are entirely due to leakage at penetrations: air duct filters, power filters, signal line filters, data filters and doors. At low frequencies the magnetic shielding is due to the currents induced in the shield to cancel out the incident field and so the interior fields are related to the inductance of the current path around the inside of the enclosure and the resistance of the metal sheets. It is to be noted that the magnetic shielding effectiveness will naturally decrease with frequency. 
     As Faraday noted, static electric fields are completely eliminated in the enclosure because charge is redistributed to cancel them out, but the earth&#39;s magnetic field is still observable inside the enclosure as there is no canceling direct current included in the shield. 
     It is evident that the interior fields due to door leakage are determined by the voltage across the door seal on the inside surface of the door panel. All door seal designs seek to reduce that interior voltage. 
     The simplest seal is a braided wire gasket attached to the outer edge of the door panel to make contact with the door jamb. The edge of the door and the door jamb must both be bare metal, usually solder tinned steel. The problem with this design is primarily that the gasket eventually takes a set leaving gaps in the seal. Secondarily the metallic contact surfaces corrode with time increasing the contact resistance of the gasket. 
     The prior art seal described in U.S. Pat. No. 3,589,070 issued to Hansen employs a knife edge on the door panel (or the frame) that slides into a channel compressing beryllium copper or phosphor bronze spring fingers on each side of the knife edge. This design reduces the corrosion problem because the spring fingers scrub the oxidization from the contact area. This seal provides very effective shielding at most frequencies but suffers at very high frequencies due to the inductance of the tines of the spring fingers and the small gaps between them. 
     The knife edge design is incorporated into many subsequent seal designs. The prior art seal described in U.S. Pat. No. 4,069,618 issued to Geiss incorporates the knife edge seal but adds a woven wire gasket placed in the bottom of the spring finger channel. This provides another path for current in parallel with the two rows of spring fingers at low frequencies and attenuation through the gasket at high frequencies. The problems with this design are that it has the same shortcomings as the gasket seal—the gasket takes a set after continued use and there is a loss of effectiveness due to corrosion of the contact surfaces. 
     The prior Art of U.S. Pat. No. 4,677,251 issued to Merewether utilizes a knife edge seal but introduces a high frequency impedance between the two rows of spring fingers. This is achieved by placing a small gap in one of the contact surfaces of the spring finger channel. Behind this gap is a cavity filled with a lossy dielectric material. 
     When an unwanted high frequency electromagnetic field is impressed on the outside of the door much of that field is reflected by the low impedance of the first set of spring fingers. The leakage current that passes the first set of spring fingers must pass through the gap in the contact surface before reaching the interior row of spring fingers. The voltage drop across that gap is in series with the inside row of spring fingers. This results in a reduction in the voltage across the inside row of spring fingers. This voltage dividing action is very effective at high frequencies resulting in interior voltage reductions of 10 to 100 times better than results obtained with the knife edge alone. 
     Moderately low frequency currents must also cross that gap. In addition to the high resistance path through the lossy dielectric, there is also the DC path through connections between the inner and outer shield surfaces of the door panel. In drawings this path is denoted as a continuous metal surface, but in most door designs there are only the connections due to interior structural elements and penetrating bolts for hinges and latches. The resistance of this path is still very small but not negligible compared to the contact resistance of the interior row of spring fingers. Consequently the voltage dividing action is still present even at low frequencies. 
     SUMMARY OF THE INVENTION 
     The present invention is directed at improving the magnetic shielding effectiveness of the “voltage dividing door seal” of U.S. Pat. No. 4,677,251 at low frequencies by providing one or more rows of spring fingers in parallel with the outside row of spring fingers to reduce the DC resistance of the seal. The voltage dividing action expected is still present, so the shielding effectiveness is larger than that provided by a knife edge seal alone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the configuration of the typical shield room door assembly as used in a high quality shielded enclosure. 
         FIG. 2  is a cutaway view of one embodiment of the prior art door seal of U.S. Pat. No. 4,677,251 for comparison proposes. It shows the relationship of the knife edge and receiver members. 
         FIG. 3  is a similar cutaway view of one embodiment of the present invention. This drawing can be directly compared to  FIG. 2 . 
         FIG. 4  is the equivalent circuit of the present invention as shown in  FIG. 3 . 
         FIG. 5  is a cutaway view of one embodiment of the present invention as a seal for the astragal of a shielded double door. 
         FIGS. 6 and 7  are cutaway views of the present invention where more than one knife edge/receiver combination is used to decrease the low frequency resistance even further for better shielding effectiveness at low frequencies. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows the configuration of the typical shield room door assembly  10 . It is to be noted that the shield door frame  20  includes the door threshold since a door seal must be provided all around the periphery of the door panel  12 . The frame is often constructed of thick tubular steel welded together and capped to assure that there is no path into the enclosure through the door frame. The door panel should provide the same shielding protection as the walls. It is usually two layers thick so each layer can be thinner than the wall sheets. The door core can be a wood core or a filled metal frame. The hinges  14  must be strong enough to support the weight of the door vertically and strong enough to resist the outward pressure of the door seal upon closure. The number and placement of the hinges depends on the size of the door. The closer  16  is more complicated than a door knob, as it takes considerable force to engage the door seal. The door seal  18  must reflect or absorb the incident energy at high frequencies. At low frequencies  18  must provide a low impedance path for current between the door panel  12  and the frame  20 . 
       FIG. 2  shows the cut away view  18  of one embodiment of the prior art of U.S. Pat. No. 4,677,251 for comparison purposes. 
     The knife edge  22  is usually an extrusion of a brass or bronze alloy because extrusions of these materials have less corrosion than steel or copper, and these materials have a low contact resistance with the beryllium copper or phosphor bronze spring finger rows  32  and  33 . The knife edge  22  is riveted to the frame  20  but silver soldered in place to eliminate leakage under the knife edge between points  25  and  38 . The contact surfaces  44  and  27  are usually brass or bronze extrusions as well, silver soldered to the panel sheets  28  and  29 . When the door is closed the outside row of finger stock  32  is compressed by the outside surface of the knife edge  22  and the inside surface  44  of the outside panel  29 . The inside row of spring fingers  33  is compressed by the inside surface  26  of the knife edge  22 . 
     When an unwanted electromagnetic field is impressed on the outside of the door a voltage is impressed across the outside gap  24  and  25 . That voltage is reduced significantly by the low impedance of the first row of spring fingers  32 . That reduced voltage travels over the top of the knife edge  22  to be impressed between surfaces  26  and  27 . 
     The gap  34  in the contact surface  27  allows communication with the cavity filled with a lossy dielectric material  36 . As the frequency is increased the losses in the cavity increase the impedance across the gap  34 , reducing the voltage across the interior row of spring fingers  33  thereby reducing the voltage across the interior gap between points  38  and  28 . 
     In most door constructions there is no continuous metal barrier  40  at the back of this cavity. The DC path for current flow between the inside and outside surfaces of the door panel is often determined by the number and location of structural reinforcements, hinge bolts and latch bolts. 
       FIG. 3  shows the cut away view of one embodiment of the present invention door seal  18 . Here and in all subsequent drawings we have used the same identifying number for the same element of the seal so that a direct comparison with  FIG. 2  and  FIG. 4  is possible. In this embodiment the knife edge  22  is mounted on the inside surface of the outside sheet  29  of the door panel  12  and is pressed into another brass or bronze extruded channel  42  upon closure. The inside edge  26  of the channel  42  is fabricated with the same slope as the knife edge to compress the inside spring finger row  33 . At low frequencies the extra row of spring fingers  50  is in parallel with the outside row of spring fingers  32 . 
     This reduces the low frequency impedance of the present invention to be less than that of prior art shown in  FIG. 2 , thus increasing the shielding effectiveness of the seal at low frequencies without reducing the increase in the shielding effectiveness due to the voltage dividing effect. 
       FIG. 4  shows the equivalent circuit of the present invention of  FIG. 3 . Because we used the same ID number in both  FIG. 2  and  FIG. 3 , this is also the equivalent circuit for the prior art of  FIG. 2  except for the reversal of the voltage node notation  22  and  44  since in the present invention the knife edge is mounted on the door panel  12  and not the frame  20 . The open circuit voltage V oc  is that voltage that would be impressed by the electromagnetic field across the door seal between points  24  and  25  if the seal were a completely open circuit. The outside impedance Z out  is that impedance that would be observed by a voltage source impressed between points  24  and  25  with a completely open door seal. This is not the free space impedance of 377 ohms but it is a relatively high impedance. The inside impedance Z in  is the impedance seen by a voltage applied across the inside gap  38  to  28  when the door seal is a completely open circuit. This impedance may be close to the outside impedance in a large enclosure at high frequencies. At low frequencies, it is the resistance and inductance of the current path around the inside of the enclosure that control this impedance. Z in  can be quite small for a small enclosure. This makes it more difficult to obtain high levels of magnetic shielding and is the principle motivation for the present invention. The impedance observed across the gap Z gap  in surface  27  is between spring finger rows  33  and  50 . The sloped inside surface of item  42  compresses the inside spring finger row  33  and provides the grounding contact surface  26 . The first row of spring fingers  32  would have a low impedance Z 32 ; A one foot length of compressed spring fingers would have a contact resistance of a few milliohms. Two rows  32  and  50  as supplied in the present invention would have half the impedance of one row. While the present invention is intended to increase the shielding effectiveness of the seal at low frequencies, the present invention improves the shielding effectiveness of the prior art at all frequencies. 
       FIG. 5  shows one embodiment of the present invention applied to the astragal of a shielded double door. Here again we retained the same ID numbers used in the prior drawings. It is to be noted that there are now two of the lossy dielectric filled cavities  36  and two voltage dividing gaps  34 . The first gap is between the outside row of finger stock  32  and the added spring finger rows  50  and  51 . 
     The presence of this second cavity/gap  36 / 34  increases the shielding effectiveness at all frequencies. An extrusion  43  is used to compress the inside row of finger stock  33 . This seal could also be applied to a door frame/door panel seal. 
       FIG. 6  shows a configuration where both a knife edge extrusion  22  and a channel extrusion  42  are applied to the frame  20  and to the inside surface of the outside sheet  29  of the door panel  12 . The outside knife edge  22  on the frame  20  enters the extruded channel  42  mounted on the outside edge of the door panel  18 . This channel contains the outside row of spring fingers  32  and one of the added rows of spring fingers  50 . The inside knife edge  22  on the door panel enters the channel  42  on the frame which contains two added optional spring finger rows  51  and  52 , The frame mounted extrusion  42  compresses the inside row of spring fingers  33  between surface  26  and surface  27 . In this configuration either 1, 2 or 3 extra rows of spring fingers  50 ,  51  and  52  can be used to reduce the low frequency seal resistance. 
       FIG. 7  shows a configuration where two of the knife edge extrusions  22  are applied to the inside surface of she outside sheet  29  of the door panel  12  and two receiver channels  42  are applied to the door frame  20 . In this configuration three rows of finger stock  50 ,  51  and  52  could be used to lower the seal resistance at low frequencies. Comparing  FIG. 6  and  FIG. 7  reveals that only the location of the inside knife edge  22  and receiving channel  42  are critical to the design.