Note: Descriptions are shown in the official language in which they were submitted.
543~8
A-1623 ¦ BACKGROUND OF THE INVENTION
._~
The prior art is replete with various devices for
ensuring that radio frequency or electromagnetic waves do not
penetrate into or out of a sealed enclosure. Early prior art
devices were designed to try to protect highly sensitive
computers by preventing radio fre~uency and electromagnetic
waves from entering a sealed enclosure contain.ing the computer
equipment. Due to the classified nature of the programs which
various computers utilize, and the use to which they are
directed, ].ater patents directed to shielding a particular
environment were drawn to various methods and devices for
preventing electromagnetic or radi.o ~Erequency waves from being
transmitted from the shielded enclosure to the outside
environment. These devices were important because highly
sophisticated methods were being developed to eavesdrop on
highly confidential governmental communications and operations.
Rec~ntly, a new problem has presented itself in
the form of an extremely high frequency electromagnetic pulse
which is produced by the detonation of a nuclear blast high
in the atmosphere. This problem is documented in the
~anuary/February 1983 issue of Science magazine (pp. 41-49
wherein the deleterious effects of this electromagnetic
pulse are described wi-th respect to the problems which are
imposed upon communications systems and shielded enclosures.
These high frequency electromagnetic waves could play havoc
with the classified material contained ln compute:rs which is
to be protected in the enclosed environment.
~>' I
A-1623 1¦
he majority of prior art devices, which have dealt
with ensuring tha-t radio frequency or electromagnetic waves
do not p~netrate into or exit from -the sealed environment,
generally protect the environment from relatively low
frequencies of radiation, but does not afford comparable
a-ttenuation levels at microwave frequencies. As the
¦ computational speed o modern digital computers has increased
so has the frequency content of electromagnetic emanations
from these computers and diyital communica-tions systems.
Therefore, a door seal that is more effective at microwave
frequencies is required to protect the classified material
being processed within a shielded enclosure.
t has been established -tha-t the voltage drop
¦ across the seal on -the inside surface of all shlelded enclosure
or equipment access doors is an important consideration when
designing a seal which would protect a shielded enclosure
from the egress of very high frequency electromagnetic waves
through the shielded door. In this context, all of these
doors require extremely good metal-to-me-tal contact around
their periphery.
Prior art patents which have dealt with this problem
I of providing an adequate door seal employ beryllium copper or
phosphor bronze spring fingers within the door seal. Such
¦ prior art door seals are described in U.S. Patents 3,589,070
issued to Hansen and ~,069,618 issued to Geiss Both of these
patents describe door seals used in shielded enclosures.
These patents employ a knife edge provided on a first closure
member which coopera-tes with spring fingers provided on a
second closure member to form a door seal for screen rooms
.
.
I
-1~23 ¦ and shielded enclosures. While it has been shown that the
door seal configurations shown in the Hansen and Geiss
patents are quite effective in forming a good closure at
low frequencies, at very hiyh frequencies in excess of .5 GHz,
the inductance of the tines of the spring fingers increases
and the leakage throuyh the slots formed in the door seal
increases, thereby decreasing ~he effectivenessof the door
seal.
The patent to Geiss also uses a woven me-tallic RF
gasket in the area of cooperation be-tween the knife edge and
the spring fingers. This gasket introduces ano-ther low
frequency current path in parallel wi-th the two rows of spring
fingers thereby reducing the voltage drop across the door
seal. Secondly, the presence of this yasket increases the
microwave attenuation of the door seal by acting as ano-ther
barrier to the microwave energy. This material does not have
the resilience of the beryllium copper spring fingers and
ultimately it will l'take a set" and added attenuation of the
waves will be los-t.
While differen-t RF absorbing materials can be used
between the rows of spring fingers to improve the microwave
at-tenuation of the door seal, the limi-ted space ~e~ween ~he
spring fingers of approximately one centimeter limits the
effectivenessof the door seal.
Consequently, it is seen that there is a continuing
need for a more effective door seal which would block both
low frequency elec-tromagnetic w~ves as well as very hiyh
frequency electromagnetic waves.
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~L2.~
70015-~
SU~ARY OF TH~ INVENTION
The present invention is direc-ted to improving -the
sealing effectiveness of a door seal for a shielded enclosure
b" introducing a high frequency impedance between the two
coacting surfaces of the door seal. The door seal is provided
wi-th two cooperating members whish move rela-tive -to one another.
At leas-t one of -the members contains a small cavity ~herein
which is filled with a dielectric or lossy ma-terial. The
periphery surrounding the cavity is closed, with t~e exception
of a small gap ensuring communlca-tion between the material
provided within -the cavity and a sealing surface between -the
two coopera-ting members. The combina-tion of this gap and the
dielectric material introduces a high frequency impedance be-tween
-the surfaces of -the seal. This impedance results in a voltage
dividing action and reduces the voltage across the protected
surface of the door seal,
According -to a broad aspect of the inven-tion
-there is provided an electromagne-tic shielded door seal
utilized wi-th a shielded enclosure adapted for US2 between a
door and a door frame, said door seal comprising:
firs-t closure seal member fixedly a-t-tached to said door
provided wi-th a pair of firs-t spaced wall members Eorming
a flrst slot therebetween;
second closure seal member fixedly attached -to said
door frame provided with a pair of second spaced w~ll members
forming a second slo-t -therebe-tween, one of said second wall mem-
bers adapted to be inser-ted in-to sald first slot and
operatively csopera-ting^ wi-th said firs-t clocure seal member -to
form an RF elec-tromagnetic frequency resistant seal;
a first cavity provided in said firs-t closure seal member,
said cavi-ty filled with an energy absorbent dielectric material,
70015-~32
said first cavi-ty substantially enclosed wi-th the exception
of a dis-tinct -First gap providecl in a surrounding periphery of
said first cavity, said first gap direct.l.y opening on-to said
Eirst slot; and
firs-t and seco.nd spring closure members provided respecti-
vely on each of said first pair of spaced wall members, said
firs-t and second spring closure members resiliently urged
agains-t the said second spaced wall member which is inserted in-to
said firs-t slo-t.
Alternatively, the closure members can be reversed,
i.e. the first closure member can he attached to the door
frame and the second slosure member can be a-ttached to the
door.
~ccording to another broad aspec-t of the invention
there is provided an electromagnetic door seal utilized with a
shielded enclosllre adapted fox use be-tween a door and a
door frame comprising:
first closure seal member fixedly at-tached to said door;
second closure seal member fixedly attached -to said door,~
frame operatively cooperating w th said first closure seal
member to form an RF or electromagne-ti.c frequency resistant
seal;
a cavi-ty provided in said first closure seal member
filled wi-th an energy absorben-t dielectric ma-te.rial,said
cavity substantially enclo~ed with the excep-tion of a dis-tinc-t
gap provided in a surrounding periphery of said cavi-ty, said
gap provided on -the portio~ of said periphery of said cavity
directly facing and opening onto said second clo~ure member; and
first and second sprin~ closure members provided on said
surrounding periphery of said first closure seal member on
either side of said gap.
-4a-
70015-32
Again, the closure members can be reversed.
~ ccording to another broad aspect of the invention
there is provided an electrcmagnetic door seal utilized with a
shielded enclosure adapted for use between a door and
a door frame comprising:
first closure seal member fi~edly attached to said door;
second closure seal member provided wi-th an L-shaped
sealing surface, said second closure means fixedly at-tached to
said door frame and operatively cooperating with said first
closure seal member to form an RF or electromagnetic frequency
seal;
a cavity provided in said first closure seal member
filled with an energy a~sorbent dielectric material, said
cavity substanti.ally enclosed wi-th the exception of a distinct
gap provided ln a surrounding periphery of said ca~ity, said
gap provided on the portion of said periphery of said cavity
directly facing and opening onto said second closure mem~er;
and
first spring closure member provided on one of the L shaped
legs of said L--shaped sealing surface and a second spring
closure member provided on the second leg o~ said L-shaped
se~ling surfase.
Again, the closure mem~erscan be reversed.
Other objects, advantages and novel features of the
presen-t invention will become apparent from the followi.ng
detailed description of the invention when considered in
conjunction with the accompanying drawings; wherein like
reference numerals identify like parts throughout. !
-~b-
70015-32
BRIEF DESCR~PTION OF THE D~WINGS
Figure l is a front view of the door which is used
in a shielded enclosure;
Figure 2 is a cu-taway view of one embodiment of
the door seal of the presen-t i.nvention showing t.he relation-
ship of -the first and second cloc.ure members;
-4c~
,7 A,
~2
.
A-1623 FIGS. 3, 4 and 5 are cutaway views of different
embodiments of the present invention showing the relationship
of the first and second closure members; and
FIG. 6 is the equivalent circuit for the present
invention with respect to the door seal shown in FIG. 2.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a typical door/door frame
combination 10 which can be utilized with the door seal
described in the present invention. Th~ combination contains
a door 12 provided with a door core constructed of virtually
any standard door core materials such as wood, aluminum,
s~eel or the like. The door core is surrounded by a layer
of 26 gauge galvanized steel and can be provided with a
color-coordinated decorative veneer or acoustical covering
provided on both the inside and outside of the door core.
A plurality oE knuckle ball bearing hinges 14 is provided
between the door 12 and door frame 20 to allow opening and
closing of the door. Although three knuckle ball bearing
hinges are shown in FIG. 1, it is understood that this number
iB merely illustrative of the number and positioning of the
hinges which are utili2ed. An alignment bearing 16 is
provided -to ensure that the door will easily be opened and
closed while ensuring that radiation cannot enter or leave
the shielded enclosure. A polished chrome door handle 18
is provided to allow access to the interior of the enclosure.
~ ~ .
-1623
A similar handle is provided on the inside door in the
enclosure. The door frame 20 can be constructed of standard
materials which are used to prevent the passage of radiation
into a sealed enclosure. The frame can be constructed with
a four inch depth tubing frame having shielded lapp joints,
but the invention is not to be construed to be limited to
such a construction.
FIG. 2 shows a cutaway view of one embodiment of
the present invention showing the various components of
the door seal in an open position. The door seal is provided
with a first closure member 23 secured to the door frame 20
and a second closure member 25 secured to the door 12. The
first closure member 23 provided in the door frame 20 contains
an L-shaped member having legs 22 and 2~. A second L-shaped
member having legs 26 and 28 is also provided. The legs 24
and 28 are opposed to one another and are arranged such -that
a slot 30 is provided therein. Although it is not important
for this invention, these L-shaped members can be constructed
of material such as hot-rolled steel. A ~ap 34 is provided
within the slot 30 and between legs 22 and 28 of the two
L shaped members. A cavity 36 is provided between the legs 22,
26 and 28 within which is provided a dieleckric or lossy
material such as polyurethane. ~dditionally, although it is
not imperative to -the operation of the present invention,
the dielectric or lossy material can include conductive
particles of copper or iron or any other ma-terial used to
attenuate RF or electromagnetic waves. Although the exact
dimensions of the gap 3~ are not inlportant, it has been found
that a dimension of between four and eight millimeters can be
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~1
'~ 8
A-1623
provided. I-t is important to note that the gap 34 must be
provided such that there is communication between the slot
30 and the dielectric material within the cavity 36. A
pair of spring fingers 32,33 is provided on the surface of
the opposed members 24 and 28, respectively. These spring
fingers can be constructed out of beryllium copper or phosphor
bronze or any other similar material.
The second closure member 25 is provided on the
door 12 and is provided with an L-shaped member 42 directly
joined to a knife edge 38. A slot 44 is provided between
L-shaped member 42 and the knife edge 38. The door core 40
is provided adjacent to the slot 44 and can be constructed
of various materials, as has been indicated ilereinabove.
In operation, the door 12 is swung closed such that
knife edge 38 is provided between the resilient spring fingers
32,33 to ensure that an effective seal has been made.
Although the door seal has been described as having a knife
edge provided on the door and spring fingers provided on the
door frame, the positioning of these elements can be
reversed. Additionally, either side of the seal can be
outside of the door. Furthermore, both the first and second
closure members 23,25 are provided along the entire periphery
of the door 12 at the door frame 20.
The operation of this seal can best be explained
using the Thevenin equivalent circuit shown in FIG. 6. For
illustrative purposes, consider that a plane wave field is
incident from the bot-tom of FIG. 2 upward toward the seal.
70015-32
~ithout any spring fingers in the assembly, a large voltage VOC
would appear between the knife edge 38 and leg 28, as shown
in FIGS. 2 and 6. With the spring fingers 32,33 in place, the
voltage between the knife edge 38 and the leg 28 is reduced
to only the voltage drop across the contacts of the spring
fingers. Since the impedance of the spring finger is much
lower than the equivalent impedance of the slot Z30, the output
voltage VO is much smaller than the voltage VOC. In a
shielded doorit isthis reduction that is responsible for the
effectiveness of the door seal.
In the present design, the voltage can be reduced
further. It is apparent that the voltage between the knife
edge 38 and the leg 28 is the sum of the voltages between the
knife edge 28 and the leg 24 and the voltage between the leg 24
and the leg 28. According to the equivalent circuit in FIG. 6,
the voltage between the knife edge 38 and the leg 28 is applied
to the series combination of -the impedance Z32 of the spring
finger 32 and the inpedance Z34 across the gap 34.
Since the impedances Z33 and Z32 are small, the
impedance Z34 does not have to be very large to reduce -the
voltage across the knife edge 38 and leg 24 significantly.
Since this voltage drives the fields inside the enclosures, the
improved shield effectiveness is obtained by introducing the
gap 34 and the dielectric material wi-thin the cavity 36. Since
the dimensions of the cavity, and the electrical conductivity,
permittivity and permeability of the material contained in the
cavi-ty can be varied, the performance characteristics of the
present invention can be changed.
FIG. 3 shows an alternate embodiment of the present
invention wherein both the first and second closure members 23
~2~ 8
70015-32
and 25 respectively include gaps such that dielectric material
provided in each of the closure members can communicate with the
door seal. The first closure member 23 is similar to the
embodiment shown in FIG. 2 and the same reference numerals are
utilized. The second closure member 25 is provided on the door
frame 20 and includes an L-shaped member having legs 46 and 48
which are used to substantially enclose a cavity 54 having
dielectric material similar to the material contained in cavity
36 provided therein. Similar to the seal shown in FIG. 2, a
knife edge 56 is provided which will cooperate with the spring
fingers 32 and 33 in the manner which has been described herein-
above, and the slot 44 is provided between leg 48 and the knife
edge 56. A single spring member 50 is provided on the leg 48
which is to cooperate with the outside surEace oE the leg 24 of
the first closure member 23. Additionally, similar to the gap
34 which is provided in the first closure member, a gap 52 is
provided between leg 48 and the continuation of the knife edge
56 allowing communication between the door seal and the di-
electric material 54. As was true with respect to the embodiment
shown in FIG. 2, the reversal of the first and second closure
members on the door 12 itself and the door frame 20 is also
contemplated.
FIG. 4 shows a third embodiment of the present inven-tion
which can be applied to a wiping/compression finger stock door
seal having a Eirst closure member 60 provided on door frame 61
and a second closure member 62 provided on door 79. The first
closure member 60 is applied to the door frame 61 and sub-
stantially encloses a cavity 70 having dielectric material
therein. The cavity 70 is surrounded by leg members 64, 66 and
82, except for a small gap 68 provided in leg member 82,
~ 8~ 7~015-32
The second closure member 62 is provided with a leg 74 having a
spring finger 76 thereon, and a leg 72 having a spring finger
78 thereon. Conventional door core material is provided at 80.
In operation, the gap 68 would cooperate with a sealing surface
to ensure that the voltage dividing effect described herein-
above will also be present in this embodiment, As was true with
respect to the previous embodiments, the first and second closure
members can be reversed with respect to the positioning on the
door frame or the door itselE.
Yet another embodiment of the present invention is
shown in FIG. 5. This embodiment illustrates the voltage
dividing door seal applied to a compression door. The door
seal includes a first closure member 82 provided on door frame
83 and a second closure member 84 provided on door 99, The first
closure member contains legs 88, 90 and 92 which substantially
surround a cavity 86 created therein. The cavity contains
dielectric material similar to that whlch has been described
with respect to the other embodiments of the present invention.
The leg 92 is provided with a gap 94 such that the dielectric
material can communicate with the door seal, As was true with
respect to the gap provided in all of the embodiments of the
present invention, this gap can be dimensioned between four and
eight millimeters. Spring fingers 96 and 98 are each provided on
one side of the gap which would cooperate with a sealing surface
102 on the second closure member 84. A door core 100 is provided
which is constructed of conventional door core material, As
was true
--10--
~ 38
-1623
with respect to all of the o-ther embodiments of the presen-t
invention, the positioning of the firs-t closure member 82 and
the second closure member 84 can be reversed with respect to
the door frame and the door itself.
Other features, advantages, characteristics,
alternatives, modifica-tions and variations of the present
invention will be apparent to those skilled in the art, and
such may be made by those skilled in the art within the
spirit and scope of the present invention.
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