Patent Application
20160249494
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'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.
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.
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.
This reduces the low frequency impedance of the present invention to be less than that of prior art shown in
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.
