US3584134A

US3584134A - Shielded air vents

Info

Publication number: US3584134A
Application number: US777680A
Authority: US
Inventors: Frederick J Nichols; James C Senn
Original assignee: LECTRO MAGNETICS Inc
Current assignee: LECTRO MAGNETICS Inc
Priority date: 1968-11-21
Filing date: 1968-11-21
Publication date: 1971-06-08
Anticipated expiration: 1988-06-08

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.

Description

United States Patent 2,230,261 2/1941 Peterset al.

Inventors Frederick J. Nichols Los Angeles;

James C. Senn, Westlake Village, both of, Calif.

Nov. 21, 1968 June 8, 1971 Lectro Magnetics Inc.

Los Angeles, Calif.

Appl. No. Filed Patented Assignee SHIELDED AIR VENTS l 1 Claims, 3 Drawing Figs.

US. Cl

References Cited UNITED STATES PATENTS 2,925,457 2/l960 Lindgren l74/35(.4) FOREIGN PATENTS 768,996 2/1957 GreatBritain l74/35(.4)

OTHER REFERENCES Ventilation & Air Conditioning of Shielding Enclosures," Bulletin No. 8365, published by Shielding, Inc., Riverton, N..l., (4pp.) Copy received US. Patent ffice March 12, 1958. Copy in Group 215,Class 174- 35(.4)

Primary Examiner-Darrell L. Clay Att0rneySmyth, Roston & Pavitt 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.

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ATTORNVjf SHIELDED AIR VENTS 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 exitfrom, 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 straightthrough 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-frequeney 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 straightthrough 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 [0 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 toflow 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 ll 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 and 15b are respectively inserted in the inner and outer groove. The shield elements of the type employed here are designated with numerallS 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 [6 between the two elements 150 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 150 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.c., 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 shields 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 la-l5b is the shield ing 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 a-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 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 straightthrough 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.

We claim:

1. Shielding air vent ofa 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 duets 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 forthin claim I, 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.

l0. 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 ofa length defining the thickness of the shield.

1]. 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.

Claims

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 waveguidetype metallic ducts with a cutoff frequency above the range of frequencies requiring shielding.

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.