IE20140303A1 - Fenestration system for radio frequency attenuation - Google Patents

Abstract

The present invention relates to a fenestration system that acts in a manner to attenuate electromagnetic radiation across a spectrum of 500 MHz to 6 GHz. Attenuation of a minimum of 28 dB is achieved using a combination of a conductive metallic oxide coating, a conductive tape, a conductive gasket, a frame assembly and an earth bonding strap connected to an earthing point. The advantage of the present invention is that radio frequency attenuation can be achieved without sacrificing light transmission and is therefore suitable as a structural element in areas where a high level of security is required in tandem with aesthetic appeal. The fenestration system provides security and protection from unwarranted sources of electromagnetic energy and serves a duality of function. In the first instances, radio frequencies across a broad spectrum from GPRS to Wi-Fi and beyond can be attenuated in a localised manner from being received. In the second instance, the transmission of the afore mentioned spectrum can be attenuated to such an effect as to render transmission impossible. Such a duality of function provides commercial appeal to end-users such as prison authorities, military applications and government/private commercial entities tasked with the protection of electronic data from unwarranted interception.

Description

The present invention relates to a fenestration system that can achieve a radio frequency attenuation of at least 28 dB within a frequency range of 500 MHZ ~ 6 GHZ.

Background to the Invention In building terminology, fenestration generally refers to the presence of openings in a building.

Such openings cover a multitude of applications such as doors, windows, curtain walls, shop~ fronts, Iouvres, vents, skylights and wall panels. Fenestration systems allow for light transmission, clear air circulation and aesthetic appeal.

It is advantageous to have such openings protected from the environment. This can be achieved by using a framing system with some form of transparent protection such as glass. It is also advantageous to have such openings protected from the penetration of radio frequencies for security purposes. Unfortunately, the traditional paradigm of glass offers little protection from Radio Frequency (RF) or electromagnetic radiation.

By way of example, it is necessary to enable light and fresh air to permeate a prison cell but it may not be advantageous to enable RF energy to permeate such a space. This is because the detained prisoner may communicate with counterparts beyond the prison area using prohibited traditional mobile phone communication technology. It is often the case that such communications made by detainees occur not in the open common areas of detention centres, but in private and secluded areas such as individual cells. A requirement therefore exists to attenuate such electromagnetic radiation in the aforementioned circumstance.

By way of an additional example, it may be necessary to shield private communications from unwarranted eavesdropping or snooping technologies. When a communication is required to be private, it is possible for sophisticated commercially available technologies to effectively listen into private communications ad nauseam. Such clandestine operations may occur without the consent of the parties who are communicating. Scenarios include private government, military and security meetings or company meetings where sensitive information is being disclosed.

By way of a further additional example, it may be necessary to protect computer processing facilities such as server rooms and/or data centres from unwanted third party communications but still retain a visual inspection method of such rooms for maintenance, security and aesthetic purposes. Traditional fenestration glazing systems fail to adequately mitigate the threat of third party interception of such electronic devices and are therefore considered inadequate as a deterrent.

By way of a further additional example, the current art relating to attenuating RF and electromagnetic radiation in localised areas is to use a blocker or scrambling device. Such devices block all radio transmission within a defined radio frequency range. This can have unintended consequences, for example-in a detention centre; such a blanket attenuation of the transmission spectrum will block mobile phone transmission but also attenuates the private communication system that the detention officers rely on to communicate important operational messages. Such affected communication devices include two-way radios such as walkie-talkies etc. Therefore, RF scramblers and blockers are oftentimes not suitable attenuating solutions.

Preceding this disclosure, an intensive course of research was conducted on radio frequency attenuation technologies in the range of 500 MHZ - 18 GHZ. A notable summary review is presented in US 5488371 A.

The present disclosure relies on the inherent attenuating properties of metallic coated glass substrates. Such coatings are traditionally used for blocking long infra-red light to aid with heating/cooling of glass clad buildings. Metallic coatings may be applied by a number of methods including vacuum deposition, magnetron sputtering and sol-gel dipping. The metallic coating is thermally diffused with the glass substrate by a conventional tempering process and therefore becomes indistinguishable from the substrate itself. By carefully selecting the correct coating combination, a strong yet invisible barrier layer to electromagnetic radiation can be formed. One of the less well known properties of such conductive metallic coatings is the ability of said coatings to dissipate RF signals towards the periphery of a surface. This effect follows the principals of a Faraday ’s law.

In effect, the metallic glass coating creates a Faraday Cage on the surface of the glass substrate.

The electromagnetic charge that builds up on the specific coating is dissipated through a conductive path provided by the frame assembly or a bonding strip that links the glass to an earthing/ground point. The cumulative sum of the coated glass, the frame assembly and the conducting tape and gasket provides a direct electrical path between the surface coating and ground/earth. Therefore; the electromagnetic radiation is directed away from the fenestration system and is dissipated accordingly.

Of particular interest to the present invention are the inherent properties of Transparent Conductive Oxides (TCO’s). The main relevant intrinsic property of TCO thin films is the co- existence of electrical conductivity and optical transparency. TCO films have been extensively used by the semiconductor sector and the solar photovoltaic industries.

The most prevalent TCO’s mentioned and used in recent research generally come from Z1102, 171203 and 51102. These are binary oxides. However; doping any of these oxides with another carefully selected material creates TCO materials preferable for the current invention. For instance, Sn doped 171203 creates Indium Tin Oxide (ITO). The application of such coatings is covered in detail in disclosures such as US 5965246, US 6048621 and US 7090921.

Such commercial manufacturers of TCO/ITO coatings include, amongst others, Pilkingtonm, Saint GobainTM and Guardian”. In particular, Nippon Sheet Glass TECTM range is particularly well suited to such applications.

US 20040149472 Al described a method to attenuate electromagnetic radiation in general and more particularly through a transparent window that is filled with molten salt. The molten salts used with this invention are transparent to ultraviolet and visible light, highly conductive, and thermally stable to the extremes of high and low temperatures of the external environment.

However, the addition of such a molten salt limits the commercial appeal of the system.

A laminated wall structure was proposed in US 8029881 B2 to improve radio frequency wave attenuation compared to normal building materials. The laminated structure relies on one or more layers of a viscoelastic material and one or more layers of electrically conducting materials.

In this invention, walls and ceilings are typically constructed using panels of the laminated structure having different suitable materials as outer layers. The commercial appeal of this invention is more suited to the internal applications within a building envelope as distinct from a fenestration application.

Previous attempts have been made to embed fine wire mesh with defined spacing into various laminated materials to dissipate or redistribute a surface charge. Unfortunately, such wires detract from the aesthetic appeal of the fenestration system and are therefore deemed to be unsuitable for commercial applications. Similar attempts have been made to laminate clear _120 polymer based metallic coated filmed sheets into glass laminates but the commercial appeal of this approach is limited given that such laminations cannot be used as a structural elements of a building.

Within the context of the current disclosure, the shielding eflecriveness (SE) defines the degree of attenuation. It is expressed in decibels (dB). It represents the logarithm of the ratio between the power density before and after the shield. As the ratio value increases, the attenuation effect is improved. For instance, a shielding effectiveness of 20 dB means an attenuation of 99% of signal is being achieved.

For the present invention, attenuation within the frequency range of 500 MHZ and 6 GHZ, which corresponds to wavelength range between 600 mm and 50 mm, is considered suitable. This range includes, GPRS, 3G, 4G, Wi-Fi and Bluetooth compatible devices.

Accordingly, there exists a requirement to attenuate RF dependent transmissions in a localised manner while still retaining the positive benefits associated with traditional fenestration systems that rely on a frame and glass as a means of protection. The fenestration system is intended for use in prison, detention centres, military installations, private industries and government installations and buildings.

Summary of the Invention According to the invention there is provided, as set out in the appended claims, a fenestration system comprising, a specific conductive metallic coated glass component, a frame assembly, conductive gaskets and tape arranged in a manner so as to provide an electrical conductive path for electromagnetic energy to flow from the glass surface to an electrical ground/earth and a suitably modified building envelope/facade with additional RF attenuating characteristics so as to supplement the RF attenuating characteristics of the overall proposed fenestration system.

In one embodiment, various processed glass arrangements may be selected, depending on the end use. Such arrangements include single glazed, double glazed triple glazed (insulated glass units), laminated glass using such interlayers as polyvinyl butyral or ethylene—vinyl acetate, composite glass consisting of laminated polycarbonate interlayers and other glass composite arrangements known to those skilled in the art. The individual glass components can range from 2mm to 25mm in thickness while the polycarbonate layers can range from 2mm to 20mm in thickness. The crosslinking action for lamination can be provided by an autoclave process or by a resin cure altemative such as ultra—violet light.

A framing assembly is designed to accommodate the insulated glass units and laminates. The assembly consists of an extruded or thermally joined material such as aluminium, steel, polyvinyl chloride or other suitable materials. The extruded material can be welded or joined using conventional swaged fittings to create a shaped frame. The frame can be thermally broken to increase its insulating performance. Frame size can vary from that required to act as a traditional fenestration frame for domestic purposes to an entire glass wall facade, depending on the intention of the end-user and the perceived threat from RF.

A conductive coated glass surface acts as an effective Faraday Cage and forces a redistribution of any polarised electromagnetic signals that accumulate on the surface towards the edges of the glass substrate. Indium Tin Oxide, in particular Sn doped M203 provides a suitable surface coating for attenuating purposes. Such a coating can be either applied by a hard coat or soft coat deposition system. The thickness of the coating is usually in the micron range and can be thermally cured using a conventional tempering process. The surface morphology of the coating can be characterised by atomic force microscopy and/or transmission electron microscopy. Other suitable alternative metallic coatings include the use of aluminium, gallium or indium doped zinc oxide and/or carbon nanotube technology. The coating can be applied to various sides of the glass, although the convention is that only one side of the glass is usually coated.

In one embodiment, the TCO metallic coating must have a sheet resistance of between 7 Ohms/square and 20 Ohms/square with 100hms/square being the preferred sheet conductivity.

The grounding for the fenestration system is achieved by employing conductive tapes and a conductive gasket. The gasket and conductive tapes are arranged in a way to withdraw any electric charges that reside on the surface of the glass as a result of RF waves passing through it.

A material such as copper tape, with a minimum shielding effectiveness value of 60-80 dB across a range of 500 KHz to 20 GHZ, is placed around the perimeter of the glass unit to be shielded. The tape can be mounted with conductive glue or by using a self-adhesive layer such as an acrylic adhesive layer. Such an adhesive layer must have an adhesion performance of at least 4.5 N/cm and a tensile strength of 40 N/cm to prevent tearing during application. Other conductive metallic tape products with a similar appropriate level of shielding effectiveness may also be suitable for this application.

The periphery/edge of the insulated glass units and laminated units, which are usually supported on plastic glass packing elements in a conventional framing system, are now supported by an incompressible electrically conductive gasket.

Such a gasket requires a high resistance to compression and a typical shielding effectiveness of 60-80 dB across a range of 500 KHZ to 20 GHZ. A gasket material manufactured from neoprene foam/rubber encased in a fine wire mesh of sufficient width to support an insulated glass or laminated glass provides a suitable conductive path to prevent the leakage of RF current from the periphery of the assembly.

In some instances, a conducting element can be laminated into the lay-up of glass/polycarbonate/glass composite to withdraw any current that builds up on the glass surface.

The conducting element can be manufactured of a suitable material such as a stannic oxide wire.

Such methods have been previously utilised in aircraft cockpit glass windows to favourably alter the fracture mechanics of such windows during flight.

To provide maximum effectiveness, the building facade into which the fenestration system is incorporated may need to be altered to maximise the shielding effectiveness of the overall building. Such suitable building alterations include the incorporation of foil-backed insulation, the incorporation of wire mesh into precast concrete systems and the electrical grounding of the fenestration frame to the fine wire mesh/structural steel elements that may form part of a pre-cast wall. Other arrangements known to those skilled in the art of pre-cast concrete or preformed non- concrete structural elements can also be used.

Brief Description of the Drawings The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:— Figure la & 1b is a 3D isometric View of the fenestration assembly in a prison window context illustrating an internal and external View of the assembly. The illustration of the assembly as illustrated in Figure la shows the internal View of the fenestration system in the context of a prison cell window shown from the prisoners side while the illustration presented in Figure 1b shows the external View of the fenestration system in the context of a prison cell window as visible from the outside of the prison.

Figure 2 is a cross—sectional View of the glass/frame/conductive gasket interface.

Figure 3 is a schematic of the testing regime illustrating the position of the network spectrum analyser, the vertical and horizontal transmitters/receivers and the fenestration system in situ in a prison cell mock-up.

Figure 4 is the shielding effectiveness results of the current invention graphed across a frequency range of 500 MHZ to 6 GHZ.

Detailed Description of the Drawings In the preferred example visible in Figure la & lb, the glazing content of the RF attenuating fenestration system consists of three glass units, two external 21 & 25 units and one internal unit . The preferred embodiment uses laminated security glass as the internal unit 10; comprising of a laminated structure using 4 mm toughened glass 29, 10 mm polycarbonate 26 and 4 mm toughened glass 29. Lamination of the composite is provided by a resin cure interlayer. The polycarbonate layer 26 acts as an anti-bandit layer in the context of a security or prison setting.

The polycarbonate layer 26 also provides a defence against manual attack. Such laminates must conform to standards such as EN356 and LPS 1270 as well as LPS1175.

In the preferred example, the frame 1 is designed to accommodate three glass units 10, 21 & 25.

The flame 1 is constructed from powder coated steel and is intended for use in detention centres and prisons. The frame system 1 is designed to retain the detainee while simultaneously enabling them access to clean air. The seam welded frame 1 has no bolts or retaining devices accessible fi*om the detainee’s side of the frame system and this lessens the likelihood of escape. Similarly, the internal unit 10 is comprised of a glass laminate as described above. This also reduces the likelihood of escape.

In the current example, an additional layer of security is provided by welding steel bars 2 behind the internal coated glass laminate 10 to the fenestration frame 1 in the cavity between the external unit 21 and the laminated glass unit 10.

The detainee can access fresh air using a finger-drive element which turns a lead screw which in- turn opens the small external insulated glass unit 8. The detainee’s side of external insulated glass unit used to access fresh air is secured by a perforated stainless steel sheet 9 that enables air to permeate. The spacing of such holes is small enough to attenuate any unwarranted electromagnetic radiation. If required, a blind can be incorporated in the space between the internal laminated glass and the external insulated glass units. Such a blind can be actuated by the detainee using a finger driven device 7 used to open/close the smaller external insulated glass unit. Electrical conductivity between the frame and the wall/facade can be achieved by using grounding or earthing strap 11.

The glass composite 10 is retained using a retaining bead constructed from steel 3. The larger of the external units 21 is positioned in a frame 22 and functions as a casement window. Such an arrangement provides access to the inner glass laminate 10 to enable maintenance and cleaning activities to be undertaken without jeopardising the integrity of the security. The additional smaller external insulated glass unit 21 is also is positioned in a frame 24 and functions as a casement window, but its range of opening limits the possibility of entry/exit.

The two external insulated glass units 21 & 25 are mounted into the fenestration frame seat 3 and secured in place with a conventional snap retaining head 28.

The toughened glass panes of the internal glass unit 10 are coated on a single side ITO glass. The ITO coated side of the glass is orientated towards the outside surfaces of the internal unit, away from the detainee’s side.

Sn doped 171203 ITO is used as the RF attenuating coating. Float glass is used as the substrate.

The coating was applied by chemical vapour deposition to the external facing side of the internal glass composite 10.

As illustrated in Figure 2, a single side adhesive copper conductive tape 22 is placed around the periphery of the glass unit in a U shape. In this instance, only the internal unit 10 was treated to the copper taping process, however; if the two outer units 21 & 25 were replaced with ITO coated substrates, then such a taping process would be required on those units also.

The insulated glass units 21 & 25 can be manufactured using a conventional aluminium spacer bar or a more thermally insulating spacer bar such as those manufactured by Swiss Spacer TM.

A compression resistant electro-conductive neoprene gasket enclosed in a fine wire mesh 27 is positioned around the periphery of its seating position 3 of the fenestration frame 1. The gasket 27 has the same width as the laminated glass unit, in this instance; 18 mm. To enable a clear conductive path between the conductive gasket and the frame, the powder coating is removed using a die-grinder to expose the bare metallic surface of the frame directly under the conducting gasket 3. The frame 1, as illustrated in Figure la & 1b, is metallic and uses a bonding strap 11 to ensure electrical conductivity between the fenestration system frame 1 and the building facade by connecting the frame 1 to the structural steel elements embedded in the cavity 5. If an aluminium frame or a PVC frame is to be used instead, then a bonding strap such as ll can be used to complete the electrical conductive path directly between the conducting gasket and an appropriate earth/ground source.

The fenestration system can be fitted into a conventional precast concrete wall as illustrated in Figure la & lb. The twin wall arrangement as illustrated in Figure la & lb has a double wall construction, however; other arrangements are possible.

The twin wall, as illustrated in Figure la & lb, uses precast concrete for wall 4 and wall 6 and a includes a layer of insulation positioned in the cavity 5 between wall 4 and wall 6. Insulating materials such as those commercially available from amongst others; Kingspanm or Rockwoolm is considered suitable. The front and back of the insulation is encapsulated by a thin layer of aluminium foil. Complete encapsulation in a thin metallic bag as with other insulation material such as Aspen Aerogelm is also possible.

A fine wire mesh with 5mm pitch spacing is also positioned within the cavity 5. Other fine wire mesh products with suitable pitch spacing can be selected and tuned to block particular frequencies. The remaining space in the cavity 5 is shuttered with concrete and structural steel as required. The external appearance of the fenestration system can be secured from the environment by using a traditional concrete sill 23.

Using a testing regime detailed in Figure 3 and carried out in accordance with MIL. STD. 285 (Attenuation measurements for enclosures & electromagnetic shielding for electronic test purposes), a Rhode & Schwarzm vector network spectrum analyser 33 was placed outside a specifically constructed prison mock-up cell 30. The transmitter 32 was placed outside the cell while the receiver 37 was placed inside the mock-up prison cell 30. RF energy 31 between 500 MHZ and 6 GHZ with both vertical and horizontal polarisation patterns was transmitted 34 and received 35 without the fenestration system 36 in place and afterwards with the fenestration system 36 in place.

The resulting shielding effectiveness of the experimental set—up is presented in Figure 4. Both horizontal and vertical polarisation shielding effectiveness values were recorded. The average minimum shielding effectiveness recorded was 28.8 dB, recorded at approximately 2.1 GHZ. The maximum shielding effectiveness recorded was 39.6 dB, recorded at a frequency of approximately 5.1 GHZ.

While the invention has been described herein with reference to several specially preferred embodiments, these embodiments have been presented by way of example only, and not to limit the scope of the invention. Additional embodiments thereof will be obvious to those skilled in the art having the benefit of this detailed description, especially to meet specific requirements or conditions. Further modifications are also possible in alternative embodiments without departing from the inventive concept.

The invention is not limited to the embodiments hereinbefore described but may be Varied in both construction and detail.

Claims (8)

Claims

1. A fenestration system that attenuates electromagnetic radiation in the range of 500 MHZ - 6 GHZ by 28 dB for use in the security sector characterised in that, the fenestration system comprises of:- a conductive metallic oxide coated glass surface a conducting tape element adhered to the edge of the coated glass a supporting frame system a conducting gasket between the framing element and the coated glass element a building envelope/facade suitable modified by the inclusion of RF attenuating materials to enhance the shielding effectiveness of the fenestration system an electro—conductive bonding element between the fenestration system and the building facade/envelope.

2. A fenestration system as claimed in Claim 1 that uses a permanent conductive metallic oxide coating that propagates electrical current towards the periphery of a glass substrate.

3. The use of a metallic adhesive tape to act as a conductor that provides an electrical path for the current to flow from the glass element to the adjacent fenestration frame.

4. The absence of an edge deletion from the coated glass substrate commonly associated with insulated glass or processed manufacturing.

5. A frame system that enables the conductive metallic coated glass to be structurally supported while maintaining a level of electrical conductivity

6. A process for supporting the glass element within the fenestration frame that uses a conductive ridged gasket of neoprene rudder encased in a fine wire mesh.

7. An electrical path to ground comprised of the use of bonded electrical connecting straps between the frame and an electrical ground so that conductivity between the metallic coated glass substrate and ground is constantly maintained. 12

8. A radio frequency/e1ectromagnetic radiation attenuating fenestration system substantially as described with reference to the drawings and/or the accompanying description 320