U.S. patent number 3,584,134 [Application Number 04/777,680] was granted by the patent office on 1971-06-08 for shielded air vents.
This patent grant is currently assigned to Lectro Magnetics Inc.. Invention is credited to Frederick J. Nichols, James C. Senn.
United States Patent |
3,584,134 |
Nichols , et al. |
June 8, 1971 |
SHIELDED AIR VENTS
Abstract
Shielding for vents of a shielded rooms is disclosed; two
different, air-permeable radiation attenuators are arranged at
right angles. The attenuator for high frequencies is a dual shield,
each one thereof being of the multiwaveguide type. The attenuator
for the lower frequencies is constructed of a plurality of overlaid
corrugated wire cloth layers.
Inventors: |
Nichols; Frederick J. (Los
Angeles, CA), Senn; James C. (Westlake Village, CA) |
Assignee: |
Lectro Magnetics Inc. (Los
Angeles, CA)
|
Family
ID: |
25110945 |
Appl.
No.: |
04/777,680 |
Filed: |
November 21, 1968 |
Current U.S.
Class: |
174/383; 454/275;
454/237 |
Current CPC
Class: |
H05K
9/0041 (20130101) |
Current International
Class: |
H05K
9/00 (20060101); H05k 009/00 () |
Field of
Search: |
;174/35.4 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
"Ventilation & Air Conditioning of Shielding Enclosures,"
Bulletin No. 8354, published by Shielding, Inc., Riverton, N.J., (4
pp.) Copy received U.S. Patent Office March 12, 1958. Copy in Group
215, Class 174--35(.4).
|
Primary Examiner: Clay; Darrell L.
Claims
We claim:
1. Shielding air vent of a shielded room wherein the air vent
includes a bend dividing the air vent into first and second
portions comprising:
a first shield extending transversely across the first portion of
the vent and with the first shield having a large plurality of air
passageways defined by electrically and magnetically conductive
means oriented at different directions, essentially at random, the
several air passageways having essentially angled or curved
configuration anywhere within the shield; and
a second shield extending transversely across the second portion of
the vent and with the second shield disposed transverse to the
first shield and being defined by a plurality of waveguide-type
metallic ducts with a cutoff frequency above the range of
frequencies requiring shielding.
2. Shielding air vent as disclosed in claim 1, the ducts of the
plurality of waveguide-type ducts having length exceeding width
dimensions of the individual ducts.
3. Shielding air vent as set forth in claim 1 and comprising a
third shield in the vent essentially similar to the second shield
and arranged at a particular distance therefrom.
4. Shielding air vent as set forth in claim 3, the duct lengths of
the second and third shield as respectively defining the thickness
of the shields, being essentially similar and the distance between
the second and third shields being essentially similar to the
thickness of each of said second and third shields
5. Shielding air vent as set forth in claim 1, the first shield
including a plurality of metallic wire cloth layers, superimposed
at random orientation of the wires of the cloth layers.
6. Shielding air vent of a shielded room and wherein the air vent
includes a bend dividing the air vent into first and second
portions comprising:
first means extending transversely across the first portion of the
air vent and with the first means defining a plurality of waveguide
ducts having dimensions so that the lower cutoff frequency is above
the frequency of the waves for which the shield is to be effective,
the ducts being longer in length than in width diameter; and
second means including a plurality of overlaid metallic wire cloths
arranged transversely across the second portion of the air vent and
with the second means transverse to the first means and constructed
to provide a random pattern of variable direction venting paths
through the second means.
7. Shielding air vent as set forth in claim 6, the wire pattern of
the cloths of the plurality as superimposed being arranged at
random.
8. Shielding air vent as set forth in claim 6, the first means
having a predominant extension transverse to the predominant
extension of the second means.
9. Shielding as set forth in claim 6, the wire cloth layers being
corrugated to define a wave pattern so that different incremental
cloth portions are oriented to face in different directions.
10. Shielding air vent as set forth in claim 6, the first means
including a pair of multiwaveguide shields, each having a plurality
of waveguide elements of a length defining the thickness of the
shield.
11. In a shielding arrangement of the character described, a
plurality of overlaid wire mesh cloths with random arrangement of
the wires of the several cloths in relation to each other, the
cloths being corrugated with alignment of the corrugation grooves
and ridges of the several cloths.
Description
The present invention relates to shielding for air vents of
shielded rooms or the like, the shielding to be effective with
regard to electromagnetic waves over a wide range of frequencies. A
shielded room, in general, is an enclosure constructed and designed
to be impermeable to electromagnetic waves over a large spectrum
including low RF frequencies and extending into the microwave
range. These rooms are needed for various purposes; at times they
serve as an enclosure into which such radio frequencies must not
enter from the outside; at other times or at a different occasion,
specific equipment is to be used, operated, tested, etc., inside of
the room, thereby emitting radio waves which could, but should not,
create interferences, and would also amount to unlicensed
broadcasting.
A completely shielded enclosure, having walls constructed for
blocking off radio waves at and from all sides, must nevertheless
be vented. There must be an air intake and an air outlet to provide
ventilation. Therefore, there exists the problem of providing an
air passage which at the same time is still impermeable to
electromagnetic radiation within the desired frequency range and to
the required degree, the wave attenuation and rejection must reach
a minimum level. It is an object of the invention to provide
electromagnetic wave shielding while permitting the passage of air,
i.e., a passageway for air must be constructed, attenuating any RF,
etc., radiation tending to pass through the passageway. Moreover,
such a shielded passageway must be economical and preferably also
easy to clean.
Difficulties are created and enhanced, in particular, because in a
given environment the mode of propagation of an electromagnetic
wave in the gigacycle range is very different from propagation of a
very low-frequency radio wave. The wavelengths of interest extend
over a range of six or more orders of magnitude. In particular,
some of the wavelengths are comparable or even small in relation to
the physical dimensions of rooms accessible to people and also in
relation to air vents and air passageways. Other wavelengths within
the range of interest are considerably larger than any of these
dimensions. Thus, propagation, transmission, reflection and
diffraction processes differ to a substantial degree for the
different wavelengths.
The shielding for the air vent in accordance with the invention
includes substantially two portions, one to be effective for
shorter wavelengths, the other one for longer wavelengths. The
shield to be effective in and as an air duct, as well as attenuator
for higher frequencies, includes a plurality of waveguide-type
ducts constructed to have a cutoff frequency above the frequency
range of interest. While such a duct attenuator can be constructed
to operate as attenuator for all waves, just by using high magnetic
permeability material and by making the ducts very long, as
compared to the shortest wavelength of interest, a more economic
solution as suggested here provides for a separate filter for the
longer waves. Moreover, it was found of particular advantage to
bipart this waveguide air duct shield. One can think of each
elemental waveguide as being divided along its length in about half
with the two portions placed one behind the other, but preferably
without indexing alignment and at a distance about equal to the
duct length of each portion. The resulting attenuation is
significantly larger than in case of undivided waveguide ducts.
These shields are placed, for example, into the venting opening of
the shielding room itself and serve as an entrance to (or exit
from, depending on the air flow path) a venting chamber through
which the shielded room is connected to the air-conditioning and/or
heating and venting system.
The venting chamber has a second opening preferably arranged at
right angles to the opening in which the first-mentioned shield or
shields is (are) provided. That second opening is covered by a
second-type shield which, in the most general sense, is provided by
and with electrically conductive, air permeable means establishing
random curved or angle flow paths for air, i.e., flow paths of
random orientation with essentially no straight through flow path
anywhere. This prevents the development of coherent wave fronts on
the other side of the shield when electromagnetic radiation is
directed toward one side thereof. In the preferred form, the second
shield includes a plurality of overlaid wire mesh or cloth; the
wires of the several cloths being arranged at random as far as
overlaying is concerned, so that the several cloths together define
a wall impermeable for low-frequency electromagnetic waves but not
for air. Shielding is significantly enhanced if the wire cloth has
undulated or corrugated configuration to change direction of
orientation of any elemental surface portion of the cloth, as that
reduces the probability of many straight-through airpaths. The
intersecting wires of the mesh-type cloths are firmly bonded
together to establish metallic, conductive areas in which apertures
are small in relation to the wavelength of the waves to be
attenuated so as to short circuit the electric field vector of the
radiation without generating local reradiating dipoles.
While the specification concludes with claims particularly pointing
out and distinctly claiming the subject matter which is regarded as
the invention, it is believed that the invention, the objects and
features of the invention and further objects, features and
advantages thereof will be better understood from the following
description taken in connection with the accompanying drawings in
which:
FIG. 1 is a cross-sectional view through a shielded venting
arrangement for a shielded room in accordance with the preferred
embodiment of the invention;
FIG. 2 is a perspective view of one of the shields used in the
airflow path of the venting arrangement; and
FIG. 3 is a cross-sectional view through another shield, used also
in the airflow path of the venting arrangement.
Proceeding now to the detailed description of the drawings, in FIG.
1 thereof, there is illustrated an arrangement in accordance with
the preferred embodiment of the present invention. Reference
numeral 10 denotes metallic wall portions or panels of a shielded
room. The interior of the room is generally designated with letter
R, while E denotes the exterior. Wall portions or panels are
attached to frame elements 11 to provide a solid, preferably
self-supporting, self-contained and shielding structure for a
shielded room.
Shielding, of course, means shielding against passage of radio
waves and microwaves. This, in turn, requires gapless closing of
the entire room by metallic panels, etc. As was outlined above, the
immediate problem arises that the room has to be vented. There must
be at least one air intake and one exhaust air vent. In order to
avoid venting from becoming the "weak link" of the shielding
structure, a construction is suggested here for the air vent
exhaust or intake which permits air to flow between the exterior
and the interior of the shielded room but rejects and attenuates
longer radio waves as well as microwaves.
Between two of the posts 11 and within a particular wall panel
portion, there is provided an opening 12 through which the shielded
room communicates with a ventilating system including a venting
chamber 30 placed directly at opening 12. This opening is lined by
a frame element 13. The frame element has flanges defining three
closed grooves circumscribing the opening. In relation to the
interior R of the room, frame element 13 has an inner, a middle and
an outer groove. Shielding elements 15a and 15b are respectively
inserted in the inner and outer groove. The shield elements of the
type employed here are designated with numeral 15 in FIG. 2.
Element 15 has a heavy steel, honeycomb structure, defining a
plurality of small air ducts such as 17, extending for flow of air
transverse to the opening 12. The arrow 18 in FIGS. 1 and 2 denotes
generally the direction of airflow permitted through the
air-venting and multiple waveguide element 15. The ducts 17 each
are approximately 1 inch long so that each shield 15 is about 1
inch thick; the spacing 16 between the two waveguide-type shields
15a and 15b is also about 1 inch wide, i.e., it is about equal to
the length of each individual waveguide element 17 of the two
shields.
The shielding element 15 serves as a waveguide shield for plane
waves tending to propagate through a duct 17. The attenuation
results from the particular characteristics of such waveguide to
have a cutoff frequency below which it ceases to sustain any
principal transmission mode, and attenuates in proportion to the
ratio of length to cross section of the waveguide. The attenuation,
in turn, is effective from several megahertz up to just below the
waveguide cutoff frequency selected, for example, to be in the
gigacycle range. Waves of higher frequency pass through the
waveguide shield but are presumed not to occur.
Waves having frequency below the cutoff frequency may be produced
inside of the room and will enter, for example, the waveguides in
shield 15a but will be attenuated therein. Some wave energy will
enter the space 16 between the two elements 15a and 15b and in a
direction colinear with the airflow direction 18. Reflection occurs
inside of space 16, for example, at the exit side for wave shield
15a so that a portion of radiation is returned toward the source
and further attenuated between and within the shields 15a and
15b.
It should be pointed out that for best performance, individual
ducts of shield 15a should not be aligned with ducts in shield 15b.
Of course, some wave energy will still be permitted to pass shield
15b in direction colinear to the airflow, i.e., in the same
direction as before, but waveguide attenuation within the ducts of
this shield reduces the wave energy below the desired level.
It was specifically observed that the attenuation of waves in the
frequency range immediately below the waveguide cutoff frequency
was materially enhanced by using two spaced waveguide shields as
shown, to take advantage of the reflection between shields if
compared with attenuation by a single waveguide shield of double
thickness and duct length; good performance was observed when the
waveguide shields were spaced by a distance approximately equal to
the shield's thickness or duct length of the individual
waveguide.
As shields 15a and 15b each are relatively thin, they are quite
permeable to radio waves too long to permit waveguide consideration
to govern propagation. For attenuation of such lower frequencies, a
wire-mesh-type shield 20 is arranged in venting chamber 30 and at
right angles to opening 12 and shields 15a and 15b therein. The
venting chamber 30 has wall structure 31 and two openings, each
provided with air-permeable radiation shielding. The dual shield
15a-15b is the shielding in one of the openings; shield 20 covers
the other one. Their positions could be reversed. Even though
shield 20 is effective as attenuator predominantly for wavelengths
where the shield 15a-15b ceases to be effective and vice versa, it
is beneficial that direction of propagation of waves permitted to
pass through either shielding arrangement be oriented at right
angles to the respective other window; the curved air deflector 32
operates as a reflector to disperse and redirect wave fronts.
Low-frequency shield 20 is illustrated by way of example in FIG. 3.
The shield is constructed of wire mesh or cloth layers 21, 22,
etc., each shaped to have corrugated or undulated configuration.
There will be 10 or more such layers to form the shield. The wires
of the several layers are misaligned through random arrangement of
the layers as far as the cloth wire patterns are concerned, so that
hardly any straight-through ducts are provided anywhere in the
shield and in any direction. The layers are juxtaposed and
particularly corrugation grooves and ridges of the several layers
are aligned so that there is little probability of occurrence of
straight-through air paths at right angles or oblique to the
general extension of the shield 20 across the one opening of
chamber 30. Nevertheless there is sufficient passageway for air
along angled or curved paths, while, on the other hand, there are
essentially no straight-through transmission paths for wave energy
except, possibly, for a few random "pin holes."
The wire cloth shield serves as an attenuating and dispersing means
for lower frequency electromagnetic waves. Each cloth is made of
firmly interconnected electrically conductive wires. The impinging
undesired radiation induces currents in the conductive wires which,
in turn, generate opposing electromagnetic fields, thus operating
as cancellation for propagating electric and magnetic fields. The
wires of the different cloth layers shield each other; each cloth
increment tending to reradiate a reduced level and each wave
increment tending to pass through cloth apertures is shielded by
another wire cloth. The random arrangement of wire overlay of the
several cloths together with the corrugation pattern thereof,
particularly avoids the establishing of radiation "windows" through
which coherent waves could penetrate the different portions of the
shield and escape, regardless of the direction of the incoming
waves. The cloths thus operate as radiation-dispersing and
-attenuating means, preventing development of any coherent wave
fronts on the other side of the shield.
Current induced in the curved corrugations induce magnetic fields
short circuited in other cloth layers within the planned
predominant extension of the shield. Each cloth layer is, of
course, in metal-to-metal contact with a wall panel 10, as well as
with the wall 31 of the venting chamber so that neither layer nor
any portion thereof can serve as a radiating dipole. As the
shielding arrangement is effective for radiation in either
direction, it is basically immaterial which shield is closer to the
interior or to the exterior of the shielded room.
The invention is not limited to the embodiments described above but
all changes and modifications thereof not constituting departures
from the spirit and scope of the invention are intended to be
included.
* * * * *