GB2605625A

GB2605625A - Submarine radome

Info

Publication number: GB2605625A
Application number: GB2104975.4A
Authority: GB
Inventors: Smith James
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2021-04-08
Filing date: 2021-04-08
Publication date: 2022-10-12
Anticipated expiration: 2041-04-08
Also published as: GB202104975D0; GB2605625B

Abstract

A method for producing an antenna enclosure having an ovoid shape comprises producing an ovoid preform by laying one or more fibres over an internal mould having an ovoid shape, adding a curable matrix to the preform, and curing the preform to produce the antenna enclosure. The one or more fibres are laid substantially unidirectionally around a circumference of the internal mould. An antenna enclosure 100 is also provided, comprising a composite external wall 110 formed from fibre-reinforced plastic forming a substantially ovoid shape and including one or more fibres aligned substantially unidirectionally around a circumference of the antenna enclosure. An interface ring 120 may be positioned at an opening in the enclosure, and may comprise a coupling portion 124 to couple and form a waterproof seal with a base. A top layer 112 may be applied over an external surface of the enclosure. The enclosure has increased hoop strength that can carry pressure load more effectively without compromising radio frequency performance, particularly for submarine or underwater use.

Description

Submarine Radome TECHNICAL FIELD

The present disclosure relates to antenna enclosures and methods for producing antenna enclosures. In particular, but without limitation, this disclosure relates to radomes and methods for manufacturing radomes that are suitable for underwater use.

BACKGROUND

Antenna enclosures, such as radomes, are housings for protecting antennas and associated electronics from environmental conditions. In general, they are required to be substantially transparent in the wavelength(s) in which the antenna operates whilst also providing protection for the antenna and associated electronics (e.g. from mechanical damage or water damage). Radomes are antenna enclosures for housing radar antennas (antennas operating over radio-frequencies, e.g. 20 kHz to 100 GHz).

Antenna enclosures can be manufactured from monolithic composite constructions. It is important that antenna enclosures are able to withstand environmental conditions in order to protect the internal electronics. This is particularly the case for antenna enclosures for use underwater use, such as for mounting on subsurface vessels.

SUMMARY

According to an arrangement there is provided a method for producing an antenna enclosure having an ovoid shape and an internal cavity for housing one or more antennas. The method comprises producing an ovoid preform and curing the preform to produce the antenna enclosure. The ovoid preform may be produced by laying one or more fibres over an internal mould having an ovoid shape conforming to an intended shape of the internal cavity of the antenna enclosure. The one or more fibres may be laid substantially unidirecfionally around a circumference of the internal mould.

Producing the preform may include adding a curable matrix material to the preform.

By laying one or more fibres substantially unidirectionally around a circumference of the internal mould, the hoop strength of the enclosure may be increased. Furthermore, by forming an ovoid preform, an ovoid enclosure may be formed that can carry pressure load more effectively without compromising radio frequency performance.

The enclosure may be a radome. The curable matrix medium may be added to the one or more fibres prior to the one or more fibres being laid or after the one or more fibres are laid. The curable matrix medium may be PEEK or a cyanate ester. These provide increased compressive strength (e.g. greater than 50MPa).

According to an arrangement producing the ovoid preform comprises wrapping the one or more fibres circumferentially around the internal mould to form a substantially ovoid shape having a closed tip at a distal end and an opening at a proximal end opposite the distal end, wherein the opening is suitable for allowing the insertion of one or more antennas into the internal cavity.

According to an arrangement the opening is circular and an interface ring is positioned at the opening, the interface ring being configured to be mounted onto a base. The interface ring may include a seal for sealing the enclosure to the base.

According to an arrangement the interface ring is placed onto the internal mould and the one or more fibres are laid over the interface ring. This allows the preform to be connected to the interface ring through adhesion by curing the curable matrix medium.

According to an arrangement the one or more fibres are laid through fibre winding or automatic tape placement. This increases the accuracy of the placement of the one or more fibres. The one or more fibres may be laid as one or more tapes comprising the one or more fibres or as a set of one or more separate fibres. Where automatic tape placement is used then the one or more fibres may be in the form of one or more tapes. Where fibre winding is used then the one or more fibres may be laid individually (e.g. as separate fibres).

According to an arrangement curing the preform comprises placing the preform within an external mould and curing the preform under pressure. The curing can be performed at pressures between 800 to 2000 PSI (5500 to 14000 kPa) and temperatures above or around 400°C. The external mould may conform to an intended external shape of the antenna housing (e.g. an ovoid shape).

According to an arrangement preceding claim further comprising applying a topcoat to the cured antenna enclosure, wherein the top coat is configured to scatter infrared radiation. This helps to hide the enclosure from unwanted surveillance.

According to an arrangement the one or more fibres and the curable matrix material are formed of radio transparent materials.

According to an arrangement the internal mould is formed of a plurality of collapsible or separable parts to aid removal of the internal mould from the cured antenna enclosure.

According to an arrangement producing the ovoid preform further comprises, prior to curing, laying one or more fibres over the internal mould along a direction running at least partially along a length of the ovoid preform. This helps to further increase the strength of the enclosure.

According to a further aspect there is provided an antenna enclosure comprising a composite external wall formed from fibre-reinforced plastic that forms a substantially ovoid shape with an internal cavity for housing one or more antennas, wherein the fibre-reinforced plastic includes one or more fibres that are aligned substantially unidirectionally around a circumference of the antenna enclosure.

According to an arrangement the composite external wall has a substantially ovoid shape having a closed tip at a distal end and an opening at a proximal end opposite the distal end, wherein the opening is suitable for allowing the insertion of one or more antennas into the internal cavity.

According to an arrangement the opening is circular and an interface ring is positioned at the opening, the interface ring being configured to be mounted onto a base.

According to an arrangement the interface ring comprises a coupling portion configured to couple to the base to form a waterproof seal with the base.

According to an arrangement the interface ring is adhered to an inner surface of the antenna enclosure.

According to an arrangement the interface ring comprises ridges at an interface between the interface ring and the inner surface to improve adhesion.

According to an arrangement the antenna comprises a top coat applied over an external surface of the external wall, wherein the top coat is configured to scatter infrared radiation.

According to an arrangement the fibre-reinforced plastic includes one or more fibres that are aligned along a direction running at least partially along a length of the ovoid preform.

BRIEF DESCRIPTION OF THE DRAWINGS

Arrangements of the present invention will be understood and appreciated more fully from the following detailed description, made by way of example only and taken in conjunction with drawings in which: FIG. 1 shows a radome according to an embodiment; FIG. 2 shows a cross-section of the radome of FIG. 1; and FIG. 3 shows a flow chart of a method of manufacturing a radome according to an embodiment.

DETAILED DESCRIPTION

Embodiments described herein relate to antenna enclosure and methods for manufacturing antenna enclosures. Specific embodiments relate to methods for manufacturing antenna enclosures (e.g. radomes) that are suitable for being mounted onto subsurface vessels (e.g. submarines). Such enclosures may be suitable for resisting high pressures whilst also being radio transparent.

In certain embodiments described herein, the specific materials and manufacturing processes are used to create high performance radomes for submarine mounted, broadband antennas. The present application proposes the use of novel manufacturing process with cutting edge materials.

Certain requirements for any radome used for external mounting on a subsurface vessel are as follows: * ability to withstand submarine specific external hydrostatic pressure environments (e.g. up to 10000 kPa); * low radio frequency (RF) signal attenuation over a broad frequency range (e.g. MHz to 90 GHz); * extreme durability in all maritime environments, both surface and subsurface; and * protecting internal antenna elements from maritime external environments encountered above and below the sea surface.

The main technical problem that needs to be overcome are the two competing requirements of radome wall thickness versus radio-frequency transparency.

The environment for a radome utilised for external sub-surface application is subjected to demanding mechanical requirements. Typical materials result in high wall thicknesses which compromise radio frequency properties across the frequency range of the antenna.

The key design parameters for providing a good antenna radome for use on a sub-surface vessel are: * radio frequency (RF) transparency across ultra-high frequency to extremely high frequency range (e.g. 100 MHz to 90 GHz); and * physical properties satisfying the following: o pressure load; o thermal shock; o high pressure sealing; o low infrared (IR) signature; and o small radar cross section.

The new manufacturing techniques and materials described herein provide the required RF and mechanical performance important to this application. They also provide a repeatable and consistent manufactured product with low variability. This results in the radome section thickness being reduced and the material composition being more consistent, thus resulting in an improved RF performance of the antenna.

Potential manufacturing techniques within the field of sub-surface radomes that are subject to high hydrostatic pressure loadings are normally based on two basic production techniques: 1. hand lay-up moulding using glass-reinforced plastic (GRP) cloth which is manually impregnated with resin; and 2. hand lay-up moulding using a resin pre-impregnated glass cloth.

These techniques have low repeatability and accuracy and result in designs which have inherent high structural reserves which impact the thickness of the radome thus resulting in lower RF performance That is, these techniques result in the following points: 1. Highly variability due to the manual nature of the manufacturing techniques and variability in the cure of cloth and resin matrix.

2. Increased thickness to provide sufficient strength as the composite resin systems used are generally 50% the strength of high end engineering plastics.

3. Many solutions do not conform with current legislation for the composite materials used (e.g. Registration, Evaluation, Authorisation and Restriction of Chemicals (REACh) legislation).

Accordingly, these methods result in a product which has a high variability and higher wall thickness to ensure safe reserve on the hydrostatic pressure load. They also result in greater RE loss due to the overall thickness To solve the above issues, embodiments use high pressure compression moulding with a reinforcing sock (e.g. a glass fibre woven sock) with an appropriate bonding matrix (e.g. a polyether ether ketone (PEEK) matrix). The geometry of the resultant enclosure is a curved volume of revolution (e.g. an ovoid) so that it carries the pressure load as effectively as possible without compromising the radio frequency performance.

The curvature benefits the radome by providing a low radar cross section and dispersing the IR radiation with an IR diffusing finish. The radome can incorporate an interface ring (e.g. of titanium alloy or stainless steel) to provide a seal surface to the main antenna base.

FIG. 1 shows a radome according to an embodiment. The radome 100 includes an external wall 110. The external wall 110 is formed from fibre reinforced plastic, such as glass-fibre reinforced plastic. In this embodiment, the external wall 110 has an ovoid shape. One end (a tip) of the ovoid shape is closed. The opposite end has a circular opening. An interface ring 120 is located within the opening to form a means of fixing the radome to a base or mount.

The ovoid shape of the radome provides a number of advantages, including increased mechanical strength, and reduced radar cross-section. The interface ring allows the radome to be secured effectively to a base, such as a mounting section of a subsurface vehicle. This can be via a waterproof seal (e.g. through one or more 0-rings). The fibre reinforced plastic wall of the enclosure provide high mechanical strength whilst also being radiotransparent. Importantly, the fibre reinforced plastic is formed from unidirectional fibres, which provide increase strength under pressure. This shall be described in more detail below.

FIG. 2 shows a cross-section of the radome of FIG. 1.

The radome 100 is hollow, with the external wall 110 forming a shell. The external wall 110 has a uniform thickness across its whole extent. An internal cavity is formed within the external wall 110. This internal cavity may be used to house one or more antennas and (optionally), associated antenna electronics (e.g. receive line(s), transmit line(s), amplifiers, filters, etc.).

The interface ring 120 is positioned in the opening of the external wall 110. The interface ring 112 runs around the inside edge of the opening. The interface ring 112 is fixed to the internal surface of the external wall 110. In the present embodiment, this is through the use of an adhesive 122, although other fixing means are possible. In the present embodiment, threads are provided at the interface between the interface ring 120 and the external wall 110 to improve the strength of adhesion. The interface ring may be formed of metal, and, in particular, a metal having high seawater corrosion resistance, such as a titanium alloy, (duplex) stainless steel or a nickel aluminium bronze alloy.

A coupling portion 124 is located on a lower part of the interface ring 120, on an opposite side of the radome to the tip. The coupling portion 124 faces away from the radome 100 and provides an interface for allowing the radome 100 to be connected to a base or mount (e.g. an external wall of a subsurface vehicle). This interface may be in the form of a waterproof seal. In the present case, the coupling portion 124 is a ring-shaped channel that is configured to receive a correspondingly shaped protrusion.

A top layer 112 covers the external surface of the external wall 110. This may provide waterproofing and/or protection from other environmental factors, such as protection from sea water. This top layer 112 may provide additional advantages, such as protecting the external wall 110 from damage. The top layer 112 may be a finishing layer. The top layer 112 may provide colouring to allow the external surface of the radome to be customized. The protective layer 112 may provide some form of camouflage. This may be in the form of changing the colour or absorption pattern of the external surface across a range of wavelengths. The range of wavelengths may be over the visible spectrum and/or over another range, such as in the infrared range. The top layer 112 may also provide specular diffusion in order to mask an infrared signature of the enclosure.

The antenna enclosures (radomes) described herein can be manufactured in a more reliable manner through automated tape placement or fibre winding as well as compression moulding.

FIG. 3 shows a flow chart of a method of manufacturing a radome according to an embodiment.

A preform is formed with unidirectional fibres 210. This can be achieved via either automatic tape placement or fibre winding. Both processes use a machine to wind fibres (either individually or in the form of a preformed tape of fibres) around a mould that conforms the intended shape of the internal cavity of the radome. The accurate method of weaving means that the weave can be accurately placed in the desired orientation to result in the fibre reinforcement being orientated in the optimum position and angle for the applied loading.

In automatic tape placement, a unidirectional tape is placed onto the mould. This can be through the use of a robotic arm or by winding the tape off of a spool. The unidirectional tape which includes unidirectional fibres that all run along the length of the tape. The tape can be pre-impregnated with a matrix material (such as PEEK resin or a cyanate ester) or the matrix material can be applied during or after tape placement. If the matrix material is applied after placement, this can be achieved by immersing the preform into matrix material or by adding powdered matrix material onto the mould before tape placement or onto the preform during or after tape placement. If a powdered matrix material is used, then this would later be heated and put under pressure to impregnate the fibres.

Certain embodiments make use of a matrix material that is a cyanate ester or PEEK resin. These materials provide relatively high compressive strength (e.g. greater than 50MPa), and therefore act to help improve the structural integrity of the enclosure after curing under pressure (e.g. for underwater use).

In fibre winding, individual fibres are wound onto the mould. The mould may be rotated to allow the fibres (or filaments) to be wound onto the mould under tension. The fibres may be impregnated with a matrix material before it is placed onto the mould (e.g. by passing through a bath of liquid resin).

In both cases, a machine is used to wind the tape or fibres onto the mould to produce a preform that includes an uncured matrix material (e.g. PEEK resin or a cyanate ester) and one or more fibres (e.g. glass fibre(s)) that are aligned. As the radome being produced is rotationally symmetrical, the fibre(s) can be wound along the hoop direction (around the circumference of the preform). By having this circumferential alignment, the resulting composite can have increased strength under pressure (e.g. hydrostatic pressure).

Nevertheless, tape or fibres can also be placed in other (non-circumferential) directions to increase the hoop strength and compressive strength of the resultant radome. For instance, after one or two layers of tape or fibres are placed circumferentially, an additional one or more layers may be placed along one or more directions that run at least partially along the length of the preform. For instance, the preform may be rotated about an axis running perpendicular to the length (e.g. a "horizontal" axis) and one or more layers of tape or fibres may be laid at various angles relative to the length of the preform (e.g. in the range of 00 < 900 to the longitudinal axis). By including one or more layers of tape or fibre arranged at least partially along the length of the preform, these additional layers can provide improved structural strength along these additional angle(s). In each orientation, a layer of tape or fibres may be laid down in a helical spiral from one end (e.g. the tip or base) to the other.

It should be noted that, when we discuss the circumference of the preform, this is the circumference as measured around the longitudinal axis of the radome (from the base to the tip). Due to the ovoid shape of the radome, the circumference varies along the length of the radome.

The preform (otherwise known as a sock or dry preform) generally conforms to the intended shape of the radome (e.g. as shown in FIGs. 1 and 2). The fibres may be of E or S glass or radiotransparent quartz. These are radiotransparent (e.g. transparent in the range of 100 MHz to 90 GHz). E or S glass provide appropriate strength and radiotransparency with reduced cost relative to quartz. The fibres form the primary structural reinforcement in the composite which gives the moulded radome strength under the design hydrostatic pressure load.

The matrix material may be resin. This can be impregnated into the preform or supplied in the form of a powered resin which is added by weight to the mould. A thermoplastic resin may be used, such as PEEK, may be used. PEEK is typically 50% stronger than the other epoxy resins used in composites. The resin when heated and pressurised infuses into the reinforcement material to form the composite moulded structure when cured. This provides the main difference in material strength.

In the present embodiment, the fibre is wrapped around the mould and the interface ring (which is placed over the mould). This allows the interface ring and preform to be adhered together via the matrix material during curing. In an alternative embodiment, the interface ring may be inserted into and bonded to the composite after curing.

The preform is placed in a mould and heated under pressure to cure the matrix and produce the composite 220. Compression of the material within the mould produces a composite that conforms accurately to the shape of the mould. This can also increase the strength of the composite and reduce the resultant wall thickness.

The mould in effect has an internal mould (upon which the preform is wound) which conforms to the internal cavity of the resultant radome, and an external mould into which the preform is placed and which conforms to the external shape of the resultant radome. The preform is therefore heated and compressed between the internal and external moulds to form the cured composite. The curing can be performed at pressures between 800 to 2000 PSI (5500 to 14000 kPa). For curing PEEK, a temperature of at around 400°C may be used. The process of using a full mould yields a dimensionally stable end result which maintains the glass fibres in the correct load bearing orientation.

Once the composite has been cured it is removed from the mould 230. The internal mould can be formed of multiple separable or collapsible parts such that it can be dismantled or collapsed and removed from the inside of the cured structure. This allows the internal mould to be removed through the opening, even though the maximum diameter of the radome is larger than the diameter of the opening.

Once the composite has been removed, the top coat is applied 240. This can be painted on (e.g. using a paint sprayer). The top coat may be a polyurethane finish. This can provide protection from environmental factors, such as salt water. The top coat may be such that it provides diffusion in certain frequencies (e.g. infrared frequencies) to scatter radiation at the desired wavelength. The scattering is caused by the roughness and geometry at a microscopic level of the surface. A special undercoating system can also be applied to solar minimise heating effects (to provide protection from solar radiation).

By using the methods described herein, an antenna enclosure (e.g. a radome) can be produced more reliably through a more repeatable process. For instance, the use of automation in the formation of the preform provides a preform with a more uniform fibre arrangement that provides increased strength under pressure. This increased strength, as well as the reduction in manufacturing variability, allows the wall thickness to be reduced without compromising the structure of the enclosure. In general, the overall wall thickness can be reduced by approximately 35-45% as compared to traditional solutions. By reducing the wall thickness, unwanted electromagnetic absorption and scattering is reduced.

The manufacturing technique also allows for more complex shapes. This allows the ovoid (eggshell) shape of FIGs. 1 and 2 to be created via the use of collapsible moulds.

This has the additional benefit of allowing the radar cross section of the enclosure to be optimised.

The above manufacturing techniques may make use of high pressure compression moulding machinery and multi part tooling for the curing. They may also utilise unidirectional tape laying machinery or unidirectional fibre winding machinery and tooling. A 7 axis robot may be used for laying the tape or fibres in a specific pattern onto the mould in either a dry or pre-impregnated state.

Whilst the embodiments described above relate to radomes (e.g. enclosures for radio antennas), the methods described herein can be equally applied to enclosures or radomes for antennas operating over any frequency range. Furthermore, whilst the above embodiments have been discussed with regard underwater applications, they may equally be applied to other environments or applications where structural loadings are high or RF transparency is critical.

While certain arrangements have been described, the arrangements have been presented by way of example only, and are not intended to limit the scope of protection. The inventive concepts described herein may be implemented in a variety of other forms. In addition, various omissions, substitutions and changes to the specific implementations described herein may be made without departing from the scope of protection defined in the following claims.

Claims

CLAIMS1. A method for producing an antenna enclosure having an ovoid shape and an internal cavity for housing one or more antennas, the method comprising producing an ovoid preform by laying one or more fibres over an internal mould having an ovoid shape conforming to an intended shape of the internal cavity of the antenna enclosure, wherein the one or more fibres are laid substantially unidirectionally around a circumference of the internal mould and wherein producing the preform includes adding a curable matrix material to the preform; and curing the preform to produce the antenna enclosure.
2. A method in accordance with claim 1 wherein producing the ovoid preform comprises wrapping the one or more fibres circumferentially around the internal mould to form a substantially ovoid shape having a closed tip at a distal end and an opening at a proximal end opposite the distal end, wherein the opening is suitable for allowing the insertion of one or more antennas into the internal cavity.
3. A method in accordance with claim 2 wherein the opening is circular and an interface ring is positioned at the opening, the interface ring being configured to be mounted onto a base.
4. A method in accordance with claim 3 wherein the interface ring is placed onto the internal mould and the one or more fibres are laid over the interface ring.
5. A method in accordance with any preceding claim wherein the one or more fibres are laid through fibre winding or automatic tape placement.
6. A method in accordance with any preceding claim wherein the one or more fibres are laid as one or more tapes comprising the one or more fibres or as a set of one or more separate fibres.
7. A method in accordance with any preceding claim wherein curing the preform comprises placing the preform within an external mould and curing the preform under pressure.
8. A method in accordance with any preceding claim further comprising applying a topcoat to the cured antenna enclosure, wherein the top coat is configured to scatter infrared radiation.
9. A method in accordance with any preceding claim wherein the one or more fibres and the curable matrix material are formed of radio transparent materials.
10. A method in accordance with any preceding claim wherein the internal mould is formed of a plurality of collapsible or separable parts to aid removal of the internal mould from the cured antenna enclosure.
11. A method accordance with any preceding claim wherein producing the ovoid preform further comprises, prior to curing, laying one or more fibres over the internal mould along a direction running at least partially along a length of the ovoid preform.
12. An antenna enclosure comprising a composite external wall formed from fibre-reinforced plastic that forms a substantially ovoid shape with an internal cavity for housing one or more antennas, wherein the fibre-reinforced plastic includes one or more fibres that are aligned substantially unidirecfionally around a circumference of the antenna enclosure.
13. An antenna enclosure in accordance with claim 12 wherein the composite external wall has a substantially ovoid shape having a closed tip at a distal end and an opening at a proximal end opposite the distal end, wherein the opening is suitable for allowing the insertion of one or more antennas into the internal cavity.
14. An antenna enclosure in accordance with claim 13 wherein the opening is circular and an interface ring is positioned at the opening, the interface ring being configured to be mounted onto a base.
15. An antenna enclosure in accordance with claim 14 wherein the interface ring comprises a coupling portion configured to couple to the base to form a waterproof seal with the base.
16. An antenna enclosure in accordance with claim 14 or claim 15 wherein the interface ring is adhered to an inner surface of the antenna enclosure.
17. An antenna enclosure in accordance with claim 16 wherein the interface ring comprises ridges at an interface between the interface ring and the inner surface to improve adhesion.
18. An antenna enclosure in accordance with any of claims 12-17 further comprising a top coat applied over an external surface of the external wall, wherein the top coat is configured to scatter infrared radiation.
19. An antenna enclosure in accordance with any of claims 12-18 wherein the fibre-reinforced plastic includes one or more fibres that are aligned along a direction running at least partially along a length of the ovoid preform.