GB2573311A

GB2573311A - An antenna assembly, a method of mounting an antenna assembly, a high impedance surface and a method of fabricating a high impedance surface

Info

Publication number: GB2573311A
Application number: GB1807227.2A
Authority: GB
Inventors: Okon Ernest; Snell Daniel; Bean Kevin
Original assignee: Thales Holdings UK PLC
Current assignee: Thales Holdings UK PLC
Priority date: 2018-05-02
Filing date: 2018-05-02
Publication date: 2019-11-06
Anticipated expiration: 2038-05-02
Also published as: GB2573311B; GB201807227D0; GB2573311B8

Abstract

An antenna assembly, or methods of its mounting or use, which is mounted on a conductive structure 3, where the antenna assembly comprises: an antenna 2 and a high impedance surface 1 where there is a separation between the antenna 1 and the high impedance surface 2 and where the high impedance surface is mounted on a first part of the conductive structure 3 such that, in use, a component of the antenna operational signals, that would otherwise be impacting upon the said first part, now impacts upon the high impedance surface 1. The surface areas requiring high impedance surface areas are identified by computer modelling or by experiment. Also disclosed is a high impedance surface, or a method of making a high impedance surface, comprising a non-conductive substrate with opposing surfaces; a conductive coating region on a first surface encompassing a plurality of through holes between the said surfaces; and wherein at least two of the through holes are each encompassed by separate coating regions formed on the second surface and where the inner surface of the holes has a conductive coating which electrically connects the coated region on the first surface with the coated regions on the second surface.

Description

FIELD OF THE INVENTION

Embodiments of the present invention relate generally to antenna assemblies, methods of mounting antenna assemblies, high impedance surfaces and methods of fabricating high impedance surfaces. In particular, embodiments of the present invention may relate to high impedance surfaces used as reflectors for antenna pattern enhancement.

BACKGROUND

In the field of microwave and radio antennae, destructive interference effects on antenna radiation are commonplace. Such destructive interference effects can be observed in antenna radiation patterns. An antenna radiation pattern refers to a representation of the relative field strength of the waves emitted or received by the antenna at different angles and locations. Destructive interference can lead to loss of coverage for communication links or radar systems, and can be observed in the form of deep nulls in the pattern.

A number of methods can be used to mitigate the effects of such destructive interference on the antenna pattern and improve the performance of microwave and radio antennas. Some examples of such methods include redesigning the antenna or placing corrugated surfaces as reflectors in a predetermined location in relation to the antenna.

However, redesigning of the antenna within the installed environment can be difficult, and is not feasible in some cases due to various constraints of the pattern coverage requirements, the immediate environment of the antenna, and the interference.

Whilst corrugated surfaces may be used where redesigning of the antenna is not possible, corrugated surfaces are bulky, heavy and costly. This is particularly detrimental where the antenna system comprising the antenna and the corrugated surface is installed on a mobile structure for example, such as an unmanned aerial vehicle (UAV), where it is advantageous for the structure to be light and aerodynamically efficient. Corrugated surfaces can be bulky and non-conformal to the airframe leading to drag on the structure during flight. Furthermore, corrugated surfaces can lead to a narrow bandwidth effect, with fractional bandwidth in some cases of less than 1%.

As such, the present invention sets out to mitigate any destructive interference of an antenna pattern without the need to redesign the antenna, whilst being light, aerodynamic and cost efficient.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided an antenna assembly mounted on a conductive structure, the antenna assembly comprising:

an antenna; and a high impedance surface reflector;

wherein the antenna and the high impedance surface reflector are mounted on the conductive structure with a separation between them, and wherein the high impedance surface reflector is mounted on a first part of the conductive structure such that a component of the radiation emitted or received by the antenna in use that would otherwise impact the first part of the conductive structure impacts the high impedance surface reflector.

In this specification, the term “high impedance” refers to an impedance greater than the impedance of free space, being around 377 Ohms.

In an embodiment, the high impedance surface reflector is a low profile reflector with a high input impedance due to the ratio of electric to magnetic fields on the surface of the reflector.

In an embodiment, the antenna is a microwave or radio antenna. The high impedance surface (HIS) reflector is used for antenna pattern enhancement. In an embodiment, the antenna operates in a frequency band, wherein the frequency band is a band located between 300MHz to 300GHz. This may be referred to as the antenna operating frequency band. In an embodiment, the antenna operates in a frequency band located between 300 GHz and 3 kHz.

The HIS reflector is a separate structure from the antenna, i.e. they are not attached to each other, but mounted separately on the conductive structure. In an embodiment, the antenna is substantially perpendicular to the conductive surface. In an embodiment, the antenna is substantially perpendicular to the HIS reflector.

In an embodiment, the high impedance surface (HIS) reflector has a positive reflection coefficient for a working frequency band, such that a destructive interference effect, that would otherwise be caused by a component of radiation with a frequency within the working frequency band emitted or received by the antenna in use reflecting from the first part of the conductive structure and interfering destructively with other components of the antenna radiation, is mitigated. The antenna and the high impedance surface reflector are configured such that the antenna operating frequency band overlaps with the HIS reflector working frequency band. In an embodiment, the working frequency band is a band located between 300MHz to 300GHz. In an embodiment, the working frequency band has a fractional bandwidth of 5% -10%, the fractional band width being the absolute bandwidth divided by the centre frequency (expressed in this case as a percentage).

One or more reflectors comprising a HIS can be placed in proximity to the antenna to improve the performance of the antenna by mitigating the effects of destructive interference on the antenna pattern caused by the conductive structure. This may be particularly useful where the antenna cannot be redesigned for example. By placing the HIS reflector on a surface close to the antenna which causes destructive interference, it is possible to mitigate destructive interference, since the reflection coefficient is altered from that of the conductive surface to that of the HIS reflector, as explained below.

The reflection coefficient of a conductive surface is around -1. In an embodiment, the reflection coefficient of a surface with high impedance is greater than zero within the working frequency band. In an embodiment, the reflection coefficient of a surface with high impedance is around +1 within the working frequency band. Thus by mounting the HIS reflector on the conductive surface, the reflection coefficient of around -1 is replaced by a reflection coefficient of greater than zero, for example around +1 for the working frequency band. A reflection coefficient of around +1 corresponds to a substantially unchanged phase for an impinging wave with a frequency within the working frequency band (whereas a reflection coefficient of -1 corresponds to a 180 degree phase change for the reflected wave, causing destructive interference with the incoming wave). The HIS reflector thus provides a way of mitigating any destructive interference effect of the surrounding structure of the antenna, without the need to redesign the antenna.

In an embodiment, the reflector comprises multiple high impedance surface layers. In an embodiment, the reflector has a thickness of greater than or equal to 0.5mm. Using multiple layers or a layer of thickness greater than or equal to 0.5mm increases the bandwidth.

The multiple layers may comprise two or more layers having different centre frequencies, such that together, the layers act to increase the effective bandwidth for example. The surfaces may be stacked one on top of the other to produce the multiple layers for example.

Two or more surfaces having different centre frequencies may alternatively or additionally be placed side by side for example.

In an embodiment, the HIS reflector has a planar structure. The reflector may be malleable to the shape of its surrounding, such as the structure that it is being mounted upon. Thus the HIS reflector may be low profile.

In an embodiment, the HIS reflector comprises a parallel combination of resistance, capacitance and inductance elements. The high impedance surface reflector may comprise a first conductive layer mounted on the conductive structure and a plurality of conductive regions spaced apart from the first conductive layer and each being electrically connected to the first conductive layer through an inductive element. The plurality of conductive regions together with the first conductive layer form the parallel combination of capacitors.

The plurality of conductive regions may be planar and may be in the same plane. Thus the HIS reflector may be low profile.

The plurality of conductive regions may be uniform in shape and size. The HIS reflector may have a periodic structure. The plurality of conductive regions may be arranged regularly to form a grid of conductive regions.

The central frequency and width of the working frequency band of the HIS depends on the capacitance and the inductance of the elements. Thus in an embodiment, the properties of the HIS reflector depend on:

• the shape and/or size of the plurality of conductive regions, • the spacing between them, • the spacing between the first conductive layer and the conductive regions, • the cross-sectional size of the inductive elements (e.g. diameter of hole or post), and • the dielectric constant of the material between the first conductive layer and the conductive regions.

The selection of these properties corresponds to a specific frequency band for which the HIS reflector alters the reflective properties - referred to as the working frequency band. This frequency band is selected to overlap with the operational frequency band of the antenna. In an embodiment, the reflection coefficient is substantially constant over the working frequency band, with some deviation at the band edges.

In an embodiment, in which the first conductive layer and the plurality of conductive regions are spaced apart by a layer of air, the area of each of the plurality of conductive regions is around the square of half a wave length in air at the operating frequency of interest of the antenna. In an embodiment, the conductive regions are square shaped and the length of a side of each of the plurality of conductive regions is around half a wave length in air at the operating frequency of interest of the antenna.

In an embodiment, the length of a side of each of the plurality of conductive regions is between 45 to 55% of a wave length in air at the operating frequency of interest of the antenna. In an embodiment, the shortest distance between the plurality of conductive regions is around one twentieth of a wave length in air at the operating frequency of interest of the antenna. In an embodiment, the shortest distance between the plurality of conductive regions is between 4.5 and 5.5% of a wave length in air at the operating frequency of interest.

Where the first conductive layer and the plurality of conductive regions are spaced apart by a different medium, the wavelength in the medium is that of interest.

The inductive elements may be conductive posts. In an embodiment, a cross sectional area of any one of the posts is smaller than a surface area of the corresponding conductive region. In an embodiment, an axial center of any one of the posts is situated in the center of the corresponding conductive region.

In an embodiment, the high impedance surface reflector comprises:

a non-conductive substrate having first and second opposing surfaces, wherein the first surface is mounted on the conductive structure;

a conductive coating region on the first surface encompassing a plurality of through holes between the first and second surfaces; and a plurality of conductive coating regions on the second surface, wherein at least two of the through holes encompassed by the conductive coating on the first surface are each encompassed by separate conductive coating regions on the second surface; wherein the inner surface of the at least two of the through holes comprises a conductive plating which electrically connects the conductive coating on the first surface and the conductive coating region on the second surface encompassing the through hole.

In an embodiment, the substrate is a laminar substrate and the first and second surfaces are planar.

The conductive coating region thus forms the first conductive layer mounted on the conductive structure, and the plurality of conductive coating regions on the second surface form the plurality of conductive regions spaced apart from the first conductive layer. The plated through holes form the inductive elements.

The plated through holes form electrical connections between the coating region on the first surface and the coating region on the second surface. This results in the reflector exhibiting the characteristics of a HIS. The plated through holes form hollow electrical connection structures between the coating region on the first surface and the coating regions on the second surface, resulting in a light weight structure. Such light weight structures may be particularly useful where the HIS structure is to be used for an antenna system installed on a mobile structure for example.

The HIS reflector may be a printed structure comprising an array of regular shaped patches (e.g. rectangular patches) printed on a substrate such as RT/Duroid 5880. Plated through via holes are drilled through the center of the patches. The structure is backed by a ground plane.

The conductive coating regions on the second surface may be uniform in shape and size. The conductive coatings on the second surface may be arranged regularly to form a grid of conductive coating regions with non-coated areas of the second surface in between the conductive coating regions.

In an embodiment, a cross sectional area of any one of the plated through holes is smaller than a surface area of the corresponding conductive coating region on the second surface. In an embodiment, an axial center of any one of the plated through holes is situated in the center of the corresponding conductive coating region on the second surface.

The antenna may comprise an integrated reflector configured to direct radiation emitted by the antenna in use, and wherein the high impedance surface reflector is mounted on a first part of the conductive structure such that a component of the radiation emitted by the antenna and directed by the integrated reflector in use that would otherwise impact the first part of the conductive structure impacts the high impedance surface reflector. The antenna may be a fan-beam antenna and the integrated reflector a paraboloid reflector for example.

The conductive structure may be a part of an aircraft fuselage. In an embodiment, the antenna is installed on a UAV platform.

In an embodiment, the separation distance between the antenna and the HIS reflector is greater than 100mm. In an embodiment, the separation distance is 250mm or more.

According to another aspect of the present invention, there is provided an aircraft, comprising the antenna assembly mounted on the aircraft fuselage, wherein the aircraft fuselage comprises the conductive structure. In an embodiment, the aircraft is a UAV.

According to another aspect of the present invention, there is provided a method of mounting an antenna assembly on a conductive structure, comprising:

mounting an antenna and a high impedance surface reflector on the conductive structure with a separation between them, wherein the high impedance surface reflector is mounted on a first part of the conductive structure such that a component of the radiation emitted or received by the antenna in use that would otherwise impact the first part of the conductive structure impacts the high impedance surface reflector.

The location of the high impedance surface reflector relative to the antenna may be determined by electromagnetic modelling or by measurement of the installed antenna performance at various locations for example.

The HIS reflector is a standalone device and is not an integral part of the antenna. As such, it is not necessary for the HIS reflector to be formed and installed at the same time as the antenna. The HIS reflector may be installed as a supplementary structure after the antenna has already been installed. The reflector may be a standoff reflector, i.e. a reflector located a distance away from the antenna, for mitigating destructive interference near an antenna.

According to another aspect of the present invention, there is provided a computer implemented method of determining a location for mounting a high impedance surface reflector on a conductive structure on which an antenna is mounted, the method comprising:

electromagnetic modelling to determine the part of the conductive structure for which a component of radiation emitted or received by the antenna in use reflecting from the part of the conductive structure causes the maximum destructive interference effect with other components of the antenna radiation;

identifying the location for mounting the high impedance surface reflector as the location for which the component of the radiation emitted or received by the antenna in use that would otherwise impact the part of the conductive structure impacts the high impedance surface reflector.

According to another aspect of the present invention, there is provided a carrier medium comprising computer readable code configured to cause a computer to perform the method of determining the location.

Since some methods in accordance with embodiments can be implemented by software, some embodiments encompass computer code provided to a general purpose computer on any suitable carrier medium. The carrier medium can comprise any non-transient storage medium such as a floppy disk, a CD ROM, a magnetic device or a programmable memory device, or any transient medium such as any signal for example an electrical, optical or microwave signal.

According to another aspect of the present invention, there is provided a high impedance surface comprising:

a non-conductive substrate having first and second opposing surfaces;

a conductive coating region on the first surface encompassing a plurality of through holes between the first and second surfaces; and a plurality of conductive coating regions on the second surface, wherein at least two of the through holes encompassed by the conductive coating on the first surface are each encompassed by separate conductive coating regions on the second surface, wherein the inner surface of the at least two of the through holes comprises a conductive plating which electrically connects the conductive coating on the first surface and the conductive coating region on the second surface encompassing the through hole.

In an embodiment, the high impedance surface is configured as a reflector.

According to another aspect of the present invention, there is provided a method of fabricating a high impedance surface, the method comprising the steps of:

providing a non-conductive substrate having first and second opposing surfaces;

forming a plurality of through holes between the first and second surfaces;

forming a conductive coating region on the first surface encompassing two or more of the through holes;

forming a plurality of conductive coating regions on the second surface, wherein at least two of the through holes encompassed by the conductive coating on the first surface are each encompassed by separate conductive coating regions on the second surface;

plating the inner surface of the at least two of the through holes so as to electrically connect the conductive coating on the first surface and the conductive coating region on the second surface encompassing the through hole.

In an embodiment, the high impedance surface is configured as a reflector.

The properties of the HIS may be altered by changing the shape and/or size of the plurality of coating areas for example, since the shape and size correspond to a specific frequency band for which the HIS alters the reflective properties. Data representing the relationship between the shape and size of the plurality of coating areas and the specific frequency band for a particular substrate configuration may be obtained through alteration of the shape and size and testing or through modelling for example. Such data may then be used to select shape and size of the plurality of coating regions on the second surface according to the band of frequency of interest of the antenna.

The method may comprise a step of modifying the shape and/or size of the conductive coating regions by laser trimming. It is possible to modify or fine tune the reflective properties of the HIS easily by modifying the shape and/or size of the printed coating region by laser trimming.

The conductive coating regions may be formed by printing.

The coating regions on the second surface may be uniform in shape and size. The coating regions on the second surface may be disposed regularly to form a grid of coated patches with non-coated areas of the second surface in between the coated patches.

According to another aspect of the present invention, there is provided a method of use of a high impedance surface reflector to mitigate antenna interference caused by a conductive structure on which the antenna is mounted, wherein the high impedance surface reflector is mounted on a first part of the conductive structure with a separation between the antenna and the high impedance surface reflector, such that a component of the radiation emitted or received by the antenna that would otherwise impact the first part of the conductive structure impacts the high impedance surface reflector.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, embodiments are described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic illustration of a corrugated surface;

Figure 2 is a schematic illustration of an antenna assembly comprising an antenna and a HIS reflector mounted on a conductive structure, in accordance with an embodiment of the present invention;

Figure 3(a) is a schematic illustration of an elevated perspective view of an example HIS structure;

Figure 3(b) is a schematic illustration of a cross sectional view of the HIS structure;

Figure 4(a) is a schematic illustration of an alternative example of a HIS structure in accordance with an embodiment of the present invention;

Figure 4(b) is a schematic illustration of an elevated perspective of the alternative HIS structure;

Figure 4(c) is a schematic illustration of an example HIS structure;

Figure 4(d) is a model of an example HIS structure;

Figure 5(a) is an illustration of an elevated perspective of a HIS structure when installed onto a UAV fuselage;

Figure 5(b) is an illustration of a HIS structure when installed on a fuselage;

Figure 6 is a flow diagram of a process of manufacturing a HIS structure;

Figure 7 is a flow diagram of a process of determining a location for mounting a HIS reflector.

DETAILED DESCRIPTION

Figure 1 shows a schematic illustration of a corrugated surface structure. The corrugated surface may be installed on a conductive surface in proximity to an antenna. The structure mitigates the effects of destructive interference of the conductive surface on the antenna pattern and thus improves the performance of the antenna. However the structure is bulky, heavy and expensive.

Figure 2 shows an antenna assembly mounted on a conductive structure 3 in accordance with an embodiment of the present invention. The antenna assembly comprises an antenna 2 and a high impedance surface reflector 1. The antenna 2 and the high impedance surface reflector 1 are mounted on the conductive structure 3 with a separation between them. The antenna may be substantially perpendicular to the conductive structure 3 and the HIS reflector 1. The high impedance surface reflector 1 is mounted on a first part of the conductive structure 3 such that a component of the radiation emitted or received by the antenna 2 in use that would otherwise impact the first part of the conductive structure 3 impacts the high impedance surface reflector 1. The antenna 2 and the HIS reflector 1 are not integrated, and are mounted separately on the conductive structure 3.

The HIS reflector 1 has a positive reflection coefficient in the working frequency range such that a destructive interference effect, that would otherwise be caused by a component of radiation having a frequency within the working frequency range emitted or received by the antenna in use reflecting from the first part of the conductive structure and interfering destructively with other components of the antenna radiation, is mitigated.

The HIS reflector 1 may be a standoff reflector which is installed at a predetermined location in relation to the antenna 2 to improve the performance of the antenna 2. This is achieved by placing the HIS reflector 1 on a conducting surface 3 of the surrounding structure of the antenna 2. By placing the HIS reflector 1 on the conducting surface 3, instead of the reflection coefficient of the conductive surface of around -1, a postive reflection coefficient, for example of a surface with high impedance is provided within the working frequency range, akin to an open circuit of around +1. In other words, the high impedance surface has a positive reflection coefficient within the working frequency range instead of the negative reflection coefficient of the conductive surface 3. Where the HIS reflector 1 is placed at a location where deep nulls in antenna radiation patterns are observed, it is possible to mitigate the effects of destructive interference, thus improving the performance of the antenna 2. Therefore, the performance of an antenna may be improved without the need to make alteration to the antenna design. The antenna 2 has an operating frequency range which overlaps with the working frequency band of the HIS reflector 1.

The location of the HIS reflector 1 may be determined in an embodiment by using electromagnetic modelling to determine the part of the conductive structure for which a component of radiation emitted or received by the antenna 2 in use reflecting from the part of the conductive structure causes the maximum destructive interference effect with other components of the antenna radiation. The HIS reflector 1 is then mounted so that the component of the radiation emitted or received by the antenna in use that would otherwise impact the determined part of the conductive structure 3 impacts the high impedance surface reflector 1 instead. Alternatively, experimental testing may be used to determine a location for the HIS reflector. Although placing the HIS reflector at the location at which maximum interference occurs is discussed, the HIS reflector may be placed at any location on the conductive structure at which destructive interference occurs, in order to mitigate the impact on the antenna interference pattern. Thus mitigation of interference effects may be provided simply by placing the HIS reflector on the conductive structure in the proximity of the antenna, without determining the optimum positioning.

The antenna 2 may be a fan-beam antenna. A fan-beam antenna comprises an integrated truncated paraboloid reflector or a circular paraboloid reflector as well as the antenna itself, for example. The integrated reflector may be formed from a metallic dish for example. The antenna 2 may be configured to emit and receive narrow band data. In an embodiment, narrow band refers to within 10% or less of the operating frequency. The high impedance surface reflector 1 is mounted on the conductive structure 3 such that a component of the radiation emitted by the antenna 2 and directed by the integrated reflector in use that would otherwise impact the part of the conductive structure 3 impacts the high impedance surface reflector 1.

Figures 3(a) and 3(b) show a schematic illustration of a HIS reflector 1 that may be mounted on the conductive structure 3 as shown in Figure 2 in accordance with an embodiment of the present invention. A HIS can be thought of as a metamaterial, which is formed of assemblies of multiple elements arranged in repeating patterns. The electrical properties of a HIS are derived not only from the properties of the base materials forming the HIS, but from the characteristics of the assemblies. Such characteristics include the shape, geometry, size and arrangement.

The high impedance surface reflector 1 comprises a first conductive layer 4, which is mounted on the conductive structure 3 and a plurality of conductive regions 5 spaced apart from the first conductive layer 4 and each being electrically connected to the first conductive layer 4 through an inductive element 6. In this case, the inductive elements 6 are solid conductive posts. The posts may all be of the same length, such that the plurality of conductive regions 5 are all spaced the same distance apart from the first conductive layer 4.

Figure 3(a) is an elevated view of the HIS reflector 1, and Figure 3(b) is a cross sectional view of the HIS reflector 1. The first conductive layer 4 may be a metal sheet for example. It may be flat or malleable. The plurality of conductive regions 5 may be metal plates for example. The plates may be smaller than the wavelength of the frequency of interest of the antenna. The plates may be mounted in the same plane such that together they form a discontinuous planar surface.

In general, the electromagnetic properties of an HIS reflector 1 can be modelled as a circuit of capacitors and inductors. Each conductive region 5 together with the first conductive layer 4 provides a capacitance, and the element 6 linking them together provides an inductance. As such, the protrusions in the array, comprising the conductive region 5 and element 6, together with the first conducting surface behave as a parallel resonant LC circuit. The circuits are resonant at a frequency determined by the properties of the reflector, specifically:

• the size, i.e. area, of the conductive regions 5, • the shape of the conductive regions 5 • the spacing between the conductive regions 5, • the spacing between the first conductive layer 4 and the conductive regions 5, • the cross-sectional size (i.e. diameter for a circular element) of the elements 6, and • the dielectric constant of the material between the first conductive layer 4 and the conductive regions 5, as will be described below. The working frequency band is centred on the resonant frequency. The working frequency band may be determined by measuring the reflection phase from the structure at different frequencies. A reflection phase of around +1 indicates the frequency is within the operating frequency band.

The high electromagnetic impedance originates from this resonance, in other words, the impedance will be high at the resonant frequency of the reflector. The impedance referred to here is the input impedance in the direction into the HIS at normal or off angle incidence, i.e. a direction other than that parallel to the surface. The impedance is the ratio of electric to magnetic field on the surface of the HIS.

The size of the conductive regions 5 determines the capacitance C, where the capacitance C for each conductive region 5 is:

A

C £_r£Q ~ a

where A is the area of the conductive region and d is the spacing between the first conductive layer 4 and the conductive regions 5. The height and diameter of the inductive element 6 determines the inductance L, wherein the inductance L for each inductive element 6 is: L = μ₀ ^Aele™^ent where μ₀ is the permeability of free space, A_ei_ement is the cross section area of the inductive element (i.e. the post or hole) and I is the length of the element (i.e. the post or hole). In fact, I and d will be the same, since the spacing is defined by the length of the inductive element.

The resonant frequency f₀ of each element depends on the capacitance C and inductance L:

^fo= 2^7

The resonant frequency mainly depends on the ratio of the area of conductive regions 5 to the length of the inductive elements 6. For a fixed length of the inductive elements 6, a larger area of conductive regions 5 means a lower resonant frequency and a smaller area means a higher frequency. Similarly, for a fixed area, longer inductive elements 6 means a lower frequency whilst shorter means higher frequency.

The conductive regions 5 may be flat plates, therefore the HIS reflector 1 may have a low profile. The HIS reflector may have a small thickness, making it light and meaning the HIS reflector 1 is close to the conductive surface. The HIS reflector may conform to the shape of the structure on which the reflector is to be installed. These features are particularly useful where the antenna system comprising an antenna 2 and the HIS reflector 1 is to be installed on a mobile structure, as it can reduce the weight and the space required for the system. The mobile structure may be a UAV for example, in which case the HIS 1 being closely matched to the surface of the UAV also increases the aerodynamic efficiency of the UAV and the antenna system. A low profile also results in low aerodynamic resistance.

In the figure, the conductive regions 5 are formed as hexagonal conductive patches of the same size. In other embodiments, the conductive regions 5 may be formed of different shapes, such as rectangles, triangles or squares. The size and shape may be regular. The spacing between the conductive regions 5 may also be regular. The regions 5 may be metal or a metal alloy for example.

A schematic illustration of a HIS according to an embodiment of the present invention is shown in Figures 4(a) and (b). Figures 4(a) and (b) show a schematic illustration of a HIS that may be mounted as a reflector 1 on the conductive structure 3 as shown in Figure 2 in accordance with an embodiment of the present invention.

Figure 4(a) shows a top view of the reflector 1. The upper part of Figure 4(b) shows a cross sectional view of the HIS reflector 1 and the lower part of the figure shows a top view of part of the HIS reflector 1. The dotted lines demonstrate where the through holes 6 and the spacing between the conductive regions 5 correspond in the two different views.

The HIS reflector 1 comprises a non-conductive substrate 7 which may be substantially planar and has first and second opposing surfaces. The surfaces may be planar. The substrate comprises a plurality of through holes between the first and second opposing surfaces.

The high impedance surface reflector 1 comprises a first conductive layer 4, which is mounted on the conductive structure 3 in use, and a plurality of conductive regions 5 spaced apart from the first conductive layer 4 and each being electrically connected to the first conductive layer 4 through an inductive element 6. In this case, the inductive elements 6 are the plated through holes 6.

The first conductive layer 4 is a conductive coating on the first surface of the substrate 7, which may be a conductive planar sheet with a plurality of through holes 6. This region is labelled the “ground plane” in the figure, and in an embodiment, the first conductive layer 4 is grounded. It is not necessary for the first conductive layer 4 to be grounded however. The coating may be metal or a metal alloy for example.

The conductive regions 5 are a plurality of conductive coating regions on the second surface of the substrate 7, wherein each conductive region 5 encompasses one of the through holes 6. In the figure, the conductive regions 5 are formed as square conductive patches of the same size. However, in other embodiments, the conductive regions 5 may be formed of different shapes, such as rectangles, triangles or hexagons. The size and shape may be regular. The spacing between the conductive regions 5 may also be regular. The regions may be metal or a metal alloy for example. These regions are labelled “metal coating” in the figure.

The through holes 6 are plated, such that the conductive planar coating 4 and the conductive coating regions 5 are electrically connected through the plated through holes 6. Therefore, the HIS surface structure of this embodiment comprises an array of conductive regions 5 spaced apart from a first conductive surface 4, wherein the conductive regions 5 are connected to the first conductive surface 4 by hollow tube-like conductive structures 6.

The plated through holes 6 act as inductors and the conductive regions 5 spaced apart from the first conductive surface 4 act as capacitors. The combined effect of the array is of an LC circuit which is resonant at a chosen frequency, as for the HIS shown in Figure 3. This is despite the higher electrical impedance associated with the plated through holes 6 and the change in the current distribution in the structure when compared against a HIS structure with solid conductive posts such as shown in Figure 3. At microwave frequencies in particular, the skin depth of the plated through holes 6 (which determines the penetration of current in the conductor) is very small, and the thickness of the plating of the through holes 6 is sufficient to provide a conductive path for current with minimal attenuation. The skin depth d is determined as d =

.where ω is the transmission frequency (in this case the microwave frequency, where ω = 2π/ί), μ is the effective permeability of the plating material and σ is the conductivity of the plating material.

By replacing solid conductive posts with conductive tube like hollow structures that are formed by plated through holes 6, it is possible to decrease the weight of the HIS reflector even further. In addition, the HIS reflector with the plated through holes have an increased level of conformity to the mounting surface.

The effect of the plated through hole is the electrical equivalent of an inductor. The printed patches together with the ground plane are electrically equivalent to capacitors. Hence, the combined effect is akin to an LC circuit which is resonant at a particular frequency.

Figure 4(c) is a schematic illustration of an example HIS reflector showing dimensions. The example HIS reflector is 2.57 cm in height, 300 cm in length and 150 cm in width. The conductive regions 5 are square. Figure 4(d) shows a model of the HIS reflector.

The size of the holes and the thickness of the plating are selected for good performance at the operating frequency of interest of the antenna.

As described above, HIS works within a specific frequency band. The centre frequency is the resonant frequency of the reflector. This is determined by the factors described above, where the cross-sectional size of the element 6 corresponds to the diameter of the hole, and the dielectric constant is that of the substrate. The width of the working frequency band of the HIS reflector is determined by the substrate thickness. In the operating frequency band, a high impedance effect is observed in the reflector where the tangential magnetic field, i.e. on the surface of the reflector, is low. The operating frequency band for the HIS reflector should overlap with the operating frequency band for the antenna.

The frequency coverage depends on the dimensions of the conductive regions 5 as explained above. Thus the size of the conductive regions 5 is selected based on the required operating frequency and a given type and size of the substrate. Each HIS reflector 1 of a certain size and shape corresponds to a specific resonant frequency, and thus a frequency band centred on this resonant frequency for which it alters the reflective properties. A HIS reflector has a particular resonant frequency, at which it has high impedance. The reflective properties are altered for the working frequency band for which the impedance is high.

In an embodiment, the size of the conductive regions 5 is around half a wavelength in the substrate 7 for the frequency of interest of the antenna 2. For example, it may be 45% to 55% of the wavelength in the substrate for the frequency of interest. The shape may vary however since shape is not a major contributing factor to the working frequency - the area relative to the spacing between the conductive regions 5 and the first conductive layer 4 is the main factor, as discussed above. In an embodiment, the spacing between adjacent conductive regions 5 should be a fraction of a wavelength. For example, the spacing may be 1% to 10% of the wavelength in the substrate for the frequency of interest. The spacing may be 4.5% to 5.5% of the wavelength in the substrate for the frequency of interest. The substrate thickness determines the bandwidth of operation, where a larger thickness results in a wider working frequency band. In an embodiment, the substrate thickness is in the range 0.5mm to 10mm. Beyond a certain thickness, there is no increase in the working frequency bandwidth, however stacking multiple layers can be used to further increase the bandwidth.

In order to modify the frequency band of a HIS reflector 1, further manufacturing processes such as laser trimming of the printed shapes can be used.

The location of the holes has an effect on the performance of the HIS and this is also a parameter that can be varied. The figures show a central location in the conductive regions 5. The location will have an effect on the impedance profile of the HIS with frequency, and hence the reflection phase of the structure. The location of the holes does not have a significant effect on the specific frequency of operation, i.e. the resonant frequency.

An example of a HIS reflector will now be described. The HIS reflector of the example comprises an R/T Duroid substrate, which is 1.57mm in thickness, with a rectangular planar dimension of 280mm x 266mm. The plating has a thickness of 0.017mm. The conductive regions 5 are 20mm x 20mm. There is a 2mm spacing between the conductive regions 5. The holes are 1mm in diameter.

Figure 5(a) is a schematic illustration of a HIS reflector 1 installed on an UAV 10, wherein the HIS reflector 1 is placed on the fuselage of the UAV with an antenna (not shown in the figure) installed on it. The optimal location of the HIS reflector 1 may be determined using electromagnetic modelling or by testing. Alternatively, it may be installed in any location in proximity to the antenna. The antenna may be installed on the rear fuselage for example. The fuselage comprises the conductive surface 3. The UAV may be a Thales Watchkeeper WK450 for example. The antenna may be a fan beam antenna installed on the rear fuselage of the Watchkeeper and used for a narrow band data link.

UAVs often implement narrowband data links and an antenna may be installed on the UAV 10 in order to implement the data link. Examples of antennae used may include fan beam antennae. However, an antenna installed on a UAV 10 may exhibit nulls in the antenna pattern due to destructive interference from the fuselage of the UAV 10.

By installing the HIS reflector 1 on the fuselage, the effect of the fuselage on the rear antenna pattern is mitigated. The HIS reflector alters the reflective properties of the fuselage, from a destructive interference effect to a constructive interference effect, for the working frequency band.

Whilst antenna systems for installing on a UAV 10 are discussed here, it will be appreciated by the skilled person that the HIS reflector 1 may be applied to any other structures where destructive interference is caused by a surface or feature of the antenna environment or the antenna itself. For example, destructive interference effects on antenna radiation patterns are often caused by antenna phase mismatches.

This interference can be observed in the form of deep nulls in antenna radiation patterns which can lead to loss of coverage for communication links or radar systems. In such cases, a HIS reflector may be installed at a predetermined location, calculated using electromagnetic modelling for example, to mitigate these effects.

Figure 5(b) shows a prototype HIS reflector on a Watchkeeper fuselage.

According to an embodiment of the present invention, a HIS reflector may be manufactured according to the following steps, which are shown in the flow diagram of Figure 6. The HIS reflector may be fabricated using printed circuit board techniques, using printing or wet or dry etching, for example.

The flow diagram of Figure 6 comprises initial step 601 of providing a substrate. The substrate is non-conductive and may be planar. An example of a substrate material is RT/Duroid 5880. In other embodiments, other non-conductive planar substrates that are suitable for drilling and printing conductive ink on may be used.

The next step of the flow diagram of Figure 6, step 602, is to coat a first surface of the substrate with a conductive material to form the first conductive surface 4 (ground plane). The conductive material may be copper or gold for example. Coating or deposition is a process whereby a metal is deposited on the substrate. This is followed by step 603, which is to print a plurality of areas of conductive material on a second surface of the substrate to form the conductive regions 5. Again, the conductive material may be copper or gold for example. In this embodiment, a technique of printing conductive material on the substrate by coating or deposition is used, as this is simple and cost effective. However, other methods may be used to form regions of conductive material on the substrate, for example by etching, in which the substrate is clad with the metal and then a pattern is created by etching away the metal from the surface in selected areas.

The plurality of areas of conductive material form conductive regions 5 on the second surface, which may have the same size and shape. The conductive regions 5 may be printed to form a repeating pattern, wherein the size and shape of the conductive regions 5 may be predetermined depending on the frequency of interest as described above. The separation distances between the patches may also be regular.

The above is followed by step 604, which is to drill a plurality of holes through the substrate, wherein a central axis of each hole is aligned with the centre of the corresponding conductive region 5 on the second surface. In other embodiments, the holes may be formed in such a way that the central axis of each hole is not aligned with the centre of the conductive region. Whilst the through holes are formed by drilling in the described method, other methods of providing holes may be used in other embodiments.

In step 605, the holes are plated, so as to form plated through holes 6. The holes may be plated with copper or gold for example. These plated through holes 6 provide electrical connection between the conductive regions 5 on the second surface and the first conductive surface 4 (ground plane), and a HIS reflector is formed as stated in step 606.

The ground plane, also referred to as first conductive surface 4, provides a conducting path linking the assemblies of multiple elements arranged in repeating patterns together. As the properties of the HIS reflector are derived not only from the properties of the base materials forming the HIS reflector, but from the characteristics of the assemblies, the shape, geometry, size and arrangement of the multiple elements can be varied in order to change the properties of the HIS reflector. In step 603, the positioning, size or shape of the printed areas of conductive material on the second surface of the substrate may be varied depending on the frequency of interest or the geometry of the antenna system. In some embodiments, an additional step of modifying the size of the printed coating region by laser trimming may be included.

A HIS reflector according to the present embodiment of the invention may be manufactured cost efficiently by implementing circuit printing methods.

Figure 7 is a flow chart describing a computer implemented method of determining a location for mounting a high impedance surface reflector on a conductive structure on which an antenna is mounted.

The method comprises electromagnetic modelling to determine the part of the conductive structure for which a component of radiation emitted or received by the antenna in use reflecting from the part of the conductive structure causes the maximum destructive interference effect with other components of the antenna radiation.

There are a number of electromagnetic modelling approaches which could be used for this step, such as the method of moments, described for example in R.F. Harrington, “Field computation by moment methods”, New York: Macmillan 1968 and incorporated herein by reference, or the finite difference time domain method, described for example in A. Taflove and S. C. Hagness, “Computational Electrodynamics: The FiniteDifference Time Domain Method” London: Artech House 3rd ed. 2005 and incorporated herein by reference.

The location for mounting the high impedance surface reflector is identified as the location for which the component of the radiation emitted or received by the antenna in use that would otherwise impact the part of the conductive structure impacts the high impedance surface reflector.

The location may alternatively be determined by measurements of the installed antenna performance for example.

Once the location of the HIS reflector has been determined, either by electromagnetic modelling or by other means, the HIS reflector is then mounted on the conductive structure within the vicinity of the antenna, at the determined location. The location of the HIS reflector is such that it mitigates any destructive interference effect of the surrounding structure on the antenna pattern.

A cover for the HIS reflector, such as a radome, may also be mounted to protect the HIS structure from the elements. This may be done for a HIS reflector mounted on a UAV for example.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed the novel apparatus and methods described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of methods and apparatus described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms of modifications as would fall within the scope and spirit of the inventions.

Claims

1. An antenna assembly mounted on a conductive structure, the antenna assembly comprising:

an antenna; and a high impedance surface reflector;

2. The antenna assembly of claim 1, wherein the high impedance surface reflector has a positive reflection coefficient for a first frequency range, such that a destructive interference effect, that would otherwise be caused by a component of radiation with a frequency within the first frequency range emitted or received by the antenna in use reflecting from the first part of the conductive structure and interfering destructively with other components of the antenna radiation, is mitigated.

3. The antenna assembly of claim 1 or 2, wherein the high impedance surface reflector comprises:

a first conductive layer mounted on the conductive structure;

a plurality of conductive regions spaced apart from the first conductive layer and each being electrically connected to the first conductive layer through an inductive element.

4. The antenna assembly of any preceding claim, wherein the high impedance surface reflector comprises:

5. The antenna assembly of claim 4, wherein the conductive coating regions on the second surface are uniform and are arranged regularly to form a grid of conductive coating regions with non-coated areas of the second surface in between the conductive coating regions.

6. The antenna assembly of claim 4 or 5, wherein the length of a side of each of the conductive coating regions on the second surface is around half a wave length in the substrate medium at the operating frequency of interest.

7. The antenna assembly of any of claims 4 to 6, wherein a shortest distance between the conductive coating regions on the second surface is around one twentieth of a wave length in the substrate medium at the operating frequency of interest.

8. The antenna assembly of any of claims 4 to 7, wherein a cross sectional area of any one of the plated through holes is smaller than a surface area of the corresponding conductive coating region on the second surface.

9. The antenna assembly of any of claims 4 to 8, wherein an axial center of any one of the plated through holes is situated in the center of the corresponding conductive coating region on the second surface.

10. The antenna assembly according to any preceding claim, wherein the antenna comprises an integrated reflector configured to direct radiation emitted or received by the antenna in use, and wherein the high impedance surface reflector is mounted on a first part of the conductive structure such that a component of the radiation emitted by the antenna and directed by the integrated reflector in use that would otherwise impact the first part of the conductive structure impacts the high impedance surface reflector.

11. The antenna assembly of claim 10, wherein the antenna is a fan-beam antenna and the integrated reflector is a paraboloid reflector.

12. The antenna assembly according to any preceding claim, wherein the conductive structure is a part of an aircraft fuselage.

13. The antenna assembly according to any preceding claim, wherein the separation distance is greater than 100mm.

14. An aircraft, comprising the antenna assembly of any preceding claim mounted on the aircraft fuselage, the aircraft fuselage comprising the conductive structure.

15. A method of mounting an antenna assembly on a conductive structure, comprising:

16. The method of claim 15, wherein the location of the high impedance surface reflector relative to the antenna is determined by electromagnetic modelling.

17. A computer implemented method of determining a location for mounting a high impedance surface reflector on a conductive structure on which an antenna is mounted, the method comprising:

18. A high impedance surface comprising:

a non-conductive substrate having first and second opposing surfaces;

19. A method of fabricating a high impedance surface, the method comprising the steps of:

providing a non-conductive substrate having first and second opposing surfaces;

forming a plurality of through holes between the first and second surfaces;

20. A method according to claim 19, wherein the conductive coating regions are formed by printing.

21. A method according to claim 19 or 20, further comprising a step of modifying the shape and/or size of the conductive coating regions by laser trimming.

22. Use of a high impedance surface reflector to mitigate antenna interference caused by a conductive structure on which the antenna is mounted, wherein the high impedance surface reflector is mounted on a first part of the conductive structure with a separation between the antenna and the high impedance surface reflector, such that a component of the radiation emitted or received by the antenna that would otherwise impact the first part of the conductive structure impacts the high impedance surface 5 reflector.