METAMATERIALS, AND ASSOCIATED METHODS AND APPARATUSES

Abstract

 An apparatus 150 comprising a control system 200 and a metamaterial 201 electronically controlled by the control system 200, the control system including an electrical network 224 and a plurality of independently controllable impedance elements 223a, 223b, 223c, 223d, wherein each of the plurality of independently controllable impedance elements 223a, 223b, 223c, 223d is arranged to be switched between a first state in which at least one of the plurality of independently controllable impedance elements 223a, 223b, 223c, 223d is electrically connected to the electrical network 224 and a second state in which at least one of the plurality of independently controllable impedance elements 223a, 223b, 223c, 223d is electrically disconnected. This provides a plurality of discrete impedance values in the metamaterial 201 according to the combination of impedance elements in the first or second state. Also disclosed are related control systems, methods of control and vehicles.

Description

Field of the Invention

[0001] The present invention concerns the digital control of metamaterials. More particularly, but not exclusively, this invention concerns an apparatus comprising a control system and a metamaterial electronically controlled by the control system, and associated methods and further apparatuses.

Background of the Invention

[0002] Metamaterials are often based on resonant structures. The properties of some metamaterials can be changed by electronically controlling the impedance of these resonant structures, by controlling (either alone, or in combination) their inductance, capacitance or resistance.

[0003] The capacitance of a metamaterial can be controlled by varactor diodes. Varactor diodes change their capacitance value in response to an applied voltage. More specifically, a typical varactor diode takes advantage of the voltage-dependent capacitance of a reverse-biased p-n junction. The lower the bias voltage applied to the varactor diode, the narrower the depletion zone of the p-n junction and the higher the capacitance of the varactor diode. By including a varactor diode in an electrical network that is controlling a resonant metamaterial, the impedance of the metamaterial itself can be changed as the capacitance of the varactor diode changes.

[0004] Fig. 1(a) is a schematic diagram of a typical prior-art apparatus 50 comprising a control system 100 and a metamaterial 101. The control system 100 includes a digital voltage controller 102 that is a source providing digital voltage control signals. A varactor diode 103 is electrically in contact with the metamaterial 101. A voltage bus 104 conveys control signals from the digital voltage controller 102 to a digital-to-analogue converter 106, which then provides an analogue signal 107 to the varactor diode 103. Together, digital voltage controller 102, the voltage bus 104 and digital-to-analogue converter 106 can be considered collectively to be a signal generator 109. The voltage signal received by the varactor diode 103 alters the properties of its p-n junction such that its capacitance is raised or lowered. Raising or lowering the capacitance of the varactor diode 103 alters the impedance of the metamaterial 101. Fig. 1(b) is a schematic diagram of a high-impedance-surface-type metamaterial 101' controlled by the control system shown in Fig. 1(a). A high-impedance-type metamaterial may be particularly suited to reflection of incident RF frequency waves. A high-impedance surface is well-known to exhibit the perfect magnetic conductor condition within a fixed frequency range and, for this reason, high-impedance surfaces are often referred to as artificial magnetic conductors. A high-impedance-surface-type metamaterial may be a Sievenpiper surface having a periodic structure characterized a plurality of vertical vias 114 extending from a ground plane 119 and terminating at a frequency selective surface 118; such a surface is shown in more detail in Fig. 1(d) and Fig. 1(e) and described below.

[0005] The varactor diode 103 itself is electrically connected to the structure of the high-impedance-surface-type metamaterial 101'such that changing the capacitance of the varactor diode 103 as described above changes the electronic properties of the high-impedance-surface-type metamaterial 101'.

[0006] Fig. 1(c) is a schematic of a split-ring-resonator-type metamaterial 101" controlled by the control system shown in Fig. 1(a). The varactor diode 103 itself is electrically connected to the structure of the split-ring-resonator-type metamaterial such that changing the capacitance of the varactor diode as described above changes the electronic properties of the split-ring-resonator-type metamaterial 101". A split-ring-resonator comprises at least a metallic ring and a gap. Each ring may be substantially square or circular in shape. Each split-ring-resonator 101" has a gap etched into it. In the example shown, the varactor diode 103 is mounted in the etched gap such that it can act as an electrical bridge across the gap. By changing the capacitance of the capacitor in the gap between the ring(s), the resonant frequency of the split ring can be increased (when a smaller capacitance value is provided) or decreased (when a larger capacitance value is applied). Some example split-ring resonator designs comprise a pair of concentric metallic rings, each ring of the pair having a gap and being controlled by their own varactor diode. Examples of a split-ring resonator are shown in WO2018021973A2 and US2012236895A1.

[0007] Fig. 1(d) is a more detailed view of a high-impedance type design metamaterial as shown in Fig. 1(b), showing how the control system 100 shown in Fig. 1(a) is connected to the metamaterial 101'. The signal generator 109 is controlled such that the digital voltage controller 102 generates voltage control signals that are passed via the bus 104 to the Digital-Analogue-Converter (DAC) 106 and then passed along a single control line via 114, which in this embodiment is a through via 114, to the varactor diode 103. Thus, the varactor diode 103 can receive an analogue control signal from the digital voltage controller 102 which in turn changes the varactor diode's 103 capacitance and thus changes the electronic properties of the metamaterial 101'. The surface is a Sievenpiper high-impedance surface (see Fig. 1(e)) characterized as having a periodic structure being an array of regularly repeating vias 114 through a dielectric substrate 116. Such a surface usually has a capacitive frequency selective surface 118 mounted upon the dielectric substrate. A ground plane having a ground plane surface 119 may be arranged on the lower surface of the substrate. The capacitive frequency-selective surface 118 may comprise a plurality of flat conductive electrodes 120 arranged in a single layer of rows and columns on an upper surface of the substrate. The conductive electrodes 120 may optionally be spaced vertically from said ground plane by a distance t less than the wavelength of an operating RF frequency. Each of the electrodes 120 may be arranged in a checkerboard pattern on the top surface. The high-impedance surface may also comprise a first array of conductors connecting the electrodes 120 to the ground plane surface 119. Examples of a high-impedance type design metamaterials are shown in WO2008140544A1, and WO2007123504A1 and US8134521B2.

[0008] The skilled person is well aware of high-impedance surface-type metamaterials and split-ring resonator-type metamaterials. How these metamaterials are designed, and how they would be controlled by known control systems is well known in the state of the art. For that reason, detailed specific features of such metamaterials are not described in herein.

[0009] State of the art control systems for metamaterials require varactor diodes in their control signal path. This leads to a low signal accuracy in the control signal path due to a mismatch between the assumed properties of the varactor diode and the actual properties of the varactor diode due to temperature variation of the varactor diode in use when controlling a metamaterial such as a split-ring resonator or a high-impedance surface metamaterial. The signal path is vulnerable to changes in temperature due to the temperature dependency of the capacitance of the varactor diode. Moreover, a varactor diode requires relatively large voltages (e.g., 10s of volts) to enable fine control of the metamaterial. The rate at which a varactor diode's capacitance can change is also not sufficiently rapid to be able to respond to the control signals required by many high-speed applications. This is because the charging/discharging current needs of a varactor diode increase with the frequency of the switching signal applied. Moreover, varactor diodes are typically limited to a maximum tunability ratio of 25:1, which limits their application further.

[0010] The present invention seeks to mitigate the above-mentioned problems. Alternatively, or additionally, the present invention seeks to provide metamaterials, and associated methods and apparatuses.

Summary of the Invention

[0011] According to a first aspect, the present disclosure provides a control system and a metamaterial electronically controlled by the control system, the control system including an electrical network and a plurality of independently controllable impedance elements, wherein each of the plurality of independently controllable impedance elements is arranged to be switched between a first state in which at least one of the plurality of independently controllable impedance elements is electrically connected to the electrical network and a second state in which at least one of the plurality of independently controllable impedance elements is electrically disconnected from the electrical network thereby providing a plurality of discrete impedance values in the metamaterial according to the combination of impedance elements of the plurality of independently controllable impedance elements in the first or second state.

[0012] According to a second aspect of the invention, there is provided a control system for controlling a metamaterial, the control system comprising an electrical network and a plurality of independently controllable impedance elements, wherein the electrical network is configured such that each of the plurality of independently controllable impedance elements are switchable between a first state in which at least one of the plurality of independently controllable impedance elements is electrically connected to the electrical network thereby altering the impedance of the electrical network, and a second state in which at least one of the plurality of independently controllable impedance elements is electrically disconnected from the electrical network thereby altering the impedance of the electrical network, thereby providing a plurality of discrete impedance values according to the combination of impedance element of the plurality of independently controllable impedance element in the first or second state.

[0013] According to a third aspect of the invention, there is provided an antenna ground plane comprising an apparatus in accordance with the first aspect of the invention.

[0014] According to a fourth aspect of the invention, there is provided a tunable RF frequency absorption structure comprising an apparatus as claimed in accordance with the first aspect of the invention.

[0015] According to a fifth aspect of the invention, there is provided a method of using an apparatus as provided in the first aspect of the invention, or using the control system as provided in the second aspect of the invention, the method comprising switching the state of the at least one impedance element of the plurality of independently controllable impedance element, such that the overall impedance value of the electrical network changes, thereby changing the impedance of the metamaterial.

[0016] According to a sixth aspect of the invention, there is provided a vehicle comprising the apparatus according to the first aspect of the invention, or a control system according to the second aspect of the invention.

[0017] It will be appreciated that features described in relation to one aspect of the present invention can be incorporated into other aspects of the present invention. For example, an apparatus of the invention can incorporate any of the features described in this disclosure with reference to a method, and vice versa. Moreover, additional embodiments and aspects will be apparent from the following description, drawings, and claims. As can be appreciated from the foregoing and following description, each and every feature described herein, and each and every combination of two or more of such features, and each and every combination of one or more values defining a range, are included within the present disclosure provided that the features included in such a combination are not mutually inconsistent. In addition, any feature or combination of features or any value(s) defining a range may be specifically excluded from any embodiment of the present disclosure.

Description of the Figures

[0018] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

Fig. 1(a) is a prior art apparatus comprising a metamaterial and a control system for controlling the metamaterial;

Fig. 1(b) is a high-impedance-type design metamaterial controlled by the varactor diode shown in Fig. 1(a);

Fig. 1(c) is a split ring resonator type design metamaterial controlled by the varactor diode shown in Fig. 1(a);

Fig. 1(d) is a more detailed view of a high-impedance-type design metamaterial as shown in Fig. 1(b) showing how the control system shown in Fig. 1(a) is connected to the metamaterial;

Fig. 1(e) is a perspective view of a high-impedance-type design metamaterial as shown in Fig. 1(b) and Fig. 1(d).

Fig. 2(a) is a metamaterial and a control system for controlling a metamaterial in accordance with an embodiment of the invention;

Fig. 2(b) is a more detailed view of a high-impedance-type design metamaterial showing how the control system shown in Fig. 2(a) can be connected to a high-impedance-type design metamaterial to control the metamaterial;

Fig. 3 is a schematic drawing of a plurality of independently controllable capacitors that are switchable into and out of an electronic network in accordance with an embodiment of the present invention.

Fig. 4 is a graphic representation of the reflectance phase change response of an apparatus in accordance with an embodiment of the present invention.

Fig. 5 is a graphic representation of the reflectivity response of an apparatus in accordance with an embodiment of the present invention.

Fig. 6 is a flow chart setting out the steps of a method of controlling a metamaterial using the control system as described in accordance with an embodiment of the invention.

Fig. 7 is a schematic drawing of a plurality of independently controllable resistors that are switchable into and out of an electronic network in accordance with an embodiment of the present invention.

Detailed Description

[0019] Embodiments are described herein in the context of approaches to improve control systems for controlling metamaterials by replacing the analogue variable capacitors of prior-art control systems with an electrical network with adjustable impedance that comprises a plurality of independently controllable impedance elements. Each impedance element is an electrical device which, when included in an electrical network, changes the impedance of said electrical network. Those of ordinary skill in the art will realize that the following detailed description is illustrative only and is not intended to be in any way limiting. Other embodiments of the present invention will readily suggest themselves to such skilled persons having the benefit of this disclosure. Reference will be made in detail to implementations as illustrated in the accompanying drawings. The same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or like parts.

[0020] According to the first aspect, the present disclosure provides an apparatus comprising a control system and a metamaterial electronically controlled by the control system, the control system including an electrical network and a plurality of independently controllable impedance elements, wherein each of the plurality of independently controllable impedance elements is arranged to be switched between a first state in which at least one of the plurality of independently controllable impedance elements is electrically connected to the electrical network and a second state in which at least one of the plurality of independently controllable impedance elements is electrically disconnected from the electrical network thereby providing a plurality of discrete impedance values in the metamaterial according to the combination of impedance elements of the plurality of independently controllable impedance elements in the first or second state.

[0021] As used herein, the term "metamaterial" refers to any material engineered to have a property that is never or rarely observed in naturally occurring materials. In particular, the metamaterials described herein refer to assemblies of components that form a structure with properties that can be changed by electrical or electromagnetic signals that are applied to it. The metamaterial may, for example, be a resonant structure. The properties of the resonant structure that can be changed may be the impedance, inductance, capacitance or resistance/conductivity of the structure.

[0022] As used herein, the term "impedance element" refers to an electronic element that can changes the impedance of an electrical network or circuit when it is present in a network or circuit.

[0023] The impedance elements may be capacitive elements, and/or inductive elements, and/or resistive elements.

[0024] The impedance elements may be capacitive elements and the discrete impedance values may be capacitance values, or the impedance elements may be inductive elements inductive elements and the discrete impedance values may be inductance values, or the impedance elements may be resistive elements and the discrete impedance values may be resistance values.

[0025] The discrete impedance values may result from a combination of values that are any of resistance, capacitance and/or inductance values.

[0026] As used herein, "capacitive elements" refers to any element that alters the capacitance of an electrical network or circuit when it is included into the network or circuit. A capacitive element may be, for example, a capacitor.

[0027] As used herein, "inductive elements" refers to any element that alters the inductance of an electrical network or circuit when it is included into the network or circuit. An inductive element may be, for example, an inductor.

[0028] As used herein, "resistive elements" refers to any element that alters the resistance of an electrical network or circuit when it is included into the network or circuit. A resistive element may be, for example, a resistor.

[0029] It may be that at least some of the impedance elements are a combination of at least two of capacitive elements and/or resistive elements and/or inductance elements. In such a case it may be that the discrete impedance values correspond to a combination of at least two of: capacitive values and/or resistive values and/or inductance values.

[0030] As used herein, "independently controllable", means that each of the impedance elements can be independently connected or disconnected from the electrical network.

[0031] The fact that the plurality of independently controllable impedance elements are each switchable between two states means that the impedance (e.g., capacitance) of the electrical network can be controlled digitally, by binary digital signals. This allows for a more environmentally stable method of controlling a metamaterial than the pre-existing varactor diode systems previously described, because the voltage-capacitance response of a varactor diode is susceptible to fluctuations of temperature. In contrast, the overall impedance value of the electrical network remains stable at a higher range of temperatures. The skilled person would understand that by changing which impedance elements are included into the electrical network, the impedance of the electrical network can be changed, thus changing the properties of the metamaterial.

[0032] The electrical network may be in a parallel-type configuration, wherein each of the impedance elements is arranged such that the voltage across each impedance element is the same as the voltage across each of the other impedance elements. Alternatively, the electrical network may be in a series-type configuration.

[0033] The metamaterial may, for example, be of a high-impedance surface design or a split-ring resonator design. The metamaterial may include a digital line suitable for receiving a digital signal from a varactor diode. Such a digital line may be used as a means of conveying a digital voltage signal from the digital bus to an independently controllable impedance element. The metamaterial may include further routing means (for example, one or more through vias) that can convey digital voltage signals from the digital bus to the remaining of the plurality of independently controllable impedance elements. Alternatively, routing means may be located on a separate chip that forms part of the control system.

[0034] Advantageously, the voltage required to change the impedance of the electrical network is only the sum of the voltages required to switch the plurality of independently controllable impedance elements into and out of the electronic network. The voltage required to switch any one of the plurality of independently controllable impedance elements between the first state and the second state may be on the order of 10s of millivolts. The voltage required to switch any one of the plurality of independently controllable capacitors between the first state and the second state may be between 0.5 Volts and 10 Volts. The voltage required to switch any one of the plurality of independently controllable capacitors between the first state and the second state may be less than 10 Volts, optionally less than 5 Volts and optionally less than 1 Volt.

[0035] The apparatus may include having an electrical network that can have at least three discrete impedance values, where the at least three discrete impedance values comprise:

a. a minimum impedance value of the electrical network, preferably an impedance value of substantially zero, or;

b. an impedance value when all of the plurality of independently controllable impedance elements are electrically connected to the electrical network;

c. an impedance value when at least one of the plurality of independently controllable impedance elements is electrically connected to the electrical network and at least one, different impedance element of the plurality of independently controllable impedance elements is electrically disconnected from the electrical network.

[0036] This allows for discretized and predictable control of the properties of the controlled metamaterial. An impedance value of (substantially) zero (a) and an impedance value where all of the capacitors are included in the circuit (b) allows the metamaterial to have the maximum range of change in impedance (e.g., capacitance), for a given arrangement of independently controllable impedance elements.

[0037] The skilled person would understand that having at least two impedance elements, each of the elements having different impedance values associated with each element, allows for the controlled metamaterial to have at least four different states, each of the four states causing a different overall discrete impedance value in the metamaterial. The number of states the control system and/or the metamaterial may take the form of the expression:

where n is the number of independently controllable impedance elements and S is the total number of states. Alternatively, having at least two impedance elements, each of the elements having the same impedance value, allows for the controlled metamaterial to have at least three states having different overall impedance values.

[0038] The control system may comprise a digital voltage source and a digital bus, and each of the plurality of independently controllable impedance elements may be configured to independently receive digital voltage signals from the digital voltage source via the digital bus, wherein the digital voltage signals received by each impedance element switch said impedance element between the first state and the second state. Such an arrangement allows for the entire control system to be digital. This means that no digital-to-analogue signal conversion is required. Thus, it may be that there is no analogue converter in the path between the digital source and the tunable impedance element. This further contributes to the ability to use lower voltage in controlling the properties of the metamaterial, contributes to the stability of the signal transmitted to the metamaterial, and increases the switching speed at which the overall impedance can be changed.

[0039] Each of the plurality of independently controllable impedance elements may have a corresponding switch in the electrical network configured to independently switch each of the plurality of independently controllable impedance elements between the first state and the second state.

[0040] Each corresponding switch may comprise at least one of a FET-type switch, a PIN diode switch, or a MEMS-type switch. The skilled person will appreciate that the apparatus may comprise any number of FET-type switches, PIN diode switches, or MEMS-type switches, in any combination. Other types of known electronic switches may be used, if appropriate.

[0041] The skilled person will appreciate that the number of impedance elements in the electrical network determines the resolution by which the overall impedance value can be changed. Moreover, as each impedance element has only two states, either being in the electronic network, or outside the electronic network, each impedance element can be thought of as providing one binary digit (bit) of control over the control system. Thus, as used herein, a system with four individually controllable impedance elements will have four bits of control, corresponding to 16 discrete states of operation that the apparatus can take.

[0042] The plurality of independently controllable impedance elements may comprise 10 or fewer impedance elements. The plurality of independently controllable impedance elements may comprise eight or fewer impedance elements. The plurality of independently controllable impedance elements may comprise six or fewer impedance elements. The plurality of independently controllable impedance elements may comprise four or fewer impedance elements. The plurality of independently controllable impedance elements may comprise two or more impedance elements. The plurality of independently controllable impedance elements may comprise four impedance elements.

[0043] The control system may have 20 or fewer bits of control, optionally 18 or fewer bits of control, optionally 16 or fewer bits of control, optionally 12 or fewer bits of control, optionally 10 or fewer bits of control, or optionally eight or fewer bits of control. The control system may have two or more bits of control, optionally four or more bits of control, optionally six or more bits of control and optionally eight or more bits of control.

[0044] The apparatus may be configured to provide 20 or fewer possible discrete impedance values in the metamaterial, optionally 18 or fewer possible discrete impedance values in the metamaterial, optionally 16 or fewer possible discrete impedance values in the metamaterial, or optionally 14 or fewer possible discrete impedance values in the metamaterial. The apparatus may be configured to provide 12 or fewer, optionally 10 or fewer, or optionally eight or fewer, possible discrete impedance values in the metamaterial. The apparatus may be configured to provide two or more possible discrete impedance values in the metamaterial, optionally four or more possible discrete impedance values in the metamaterial, or optionally six or more possible discrete impedance values in the metamaterial. The apparatus may be configured such that there are eight discrete impedance values possible in the metamaterial.

[0045] As described above, a prior-art method of controlling metamaterials relies on varactor diodes. In the present invention, it may be that the discrete impedance values comprise a plurality of non-zero capacitance values that correspond to capacitance values that would fall within the normal operational range of capacitance values exhibited by a varactor diode in normal use as a means for controlling a metamaterial circuit. However, it is envisioned that such capacitance values would be reached at a lower magnitude of applied voltage than required to obtain the same capacitance value from a varactor diode.

[0046] When the impedance elements are capacitive elements, the discrete capacitance values may comprise a number of capacitance values that fall within the range 0.01pF and 100pF. The discrete capacitance values may comprise a number of capacitance values that fall within the range 0.1pF and 10pF.

[0047] When the impedance elements are capacitive elements, the capacitance element may be configured such that their overall capacitance value (and thus the overall capacitance value of the electrical network) varies substantially proportionally to the square root of the magnitude of an applied voltage, wherein the applied voltage corresponds to the voltage sum of the independently received digital voltage signals. This allows the electrical network to have a voltage-capacitance response that is similar to that of a varactor diode (albeit at lower control voltages).

[0048] The metamaterial may be a varactor diode controllable metamaterial. the control system may be suitable for retrofit to existing metamaterial systems, such as those controlled by varactor diodes. For example, at least one of the digital voltage signals may be configured such that it can be routed through an existing signal line on a metamaterial.

[0049] The metamaterial may be a high-impedance-surface-type metamaterial or a split-ring-resonator-type metamaterial. A high-impedance-surface-type metamaterial may have a periodic structure characterized a plurality of vertical vias and terminating at a frequency selective surface. A split-ring-resonator-type metamaterial comprises at least a metallic ring and a gap in said metallic ring. Further features of a high-impedance-surface-type metamaterial and a split-ring-resonator-type metamaterial are described in the Background of the Invention section of this document. The control system may be configured to alter the reflectance, absorbance, or resonance of the metamaterial that it is controlling.

[0050] The apparatus may be configured to interact with an applied RF frequency, such that: in a first mode of operation, a first combination of the plurality of independently controllable impedance elements is in the first state such that the reflected signal is phase shifted relative to the applied RF frequency, and in a second mode of operation, a second combination of the plurality of independently controllable impedance elements is in the second state, such that the reflected signal is phase shifted relative to the applied RF frequency, wherein the phase shift of the reflected signal is different in the first and second mode of operations.

[0051] When in the first mode of operation, for an applied RF frequency within a first operational frequency range between a first RF frequency and a second, higher RF frequency, the reflected signal may be phase shifted relative to the applied RF frequency by between +90° phase shift and -90° phase shift, and/or when in the second mode of operation, for an applied frequency within a second operational frequency range between a third RF frequency that is higher than the first RF frequency and a fourth, yet higher RF frequency, the reflected signal may be phase shifted relative to the applied RF frequency by between +90° phase shift and -90° phase shift.

[0052] It may be that the range of phase shift across the first operational frequency range spans the entirety of between +90° phase shift and -90° phase shift. It may be that the range of phase shift across the second operational frequency range spans the entirety of between +90° phase shift and -90° phase shift.

[0053] The first operational frequency range and second operational frequency range may at least partially overlap. The first operational frequency range and the second operational frequency range may at least partially overlap such that the third frequency is closer to second frequency than the first or fourth frequencies. The first mode of operation may cause incident RF frequencies within the first operational range to be phase shifted relative to the applied frequency. The phase shift relative to the applied frequency may be between +90° and -90°. The second mode of operation may cause incident RF frequencies within the first operational range to be phase shifted relative to the applied frequency. The phase shift relative to the applied frequency may be between +90° and -90°. The first mode of operation and second mode of operation may be configured such that there is minimal overlap of the first and second operational frequency ranges. This helps increase the range of incident RF frequencies that the control system (and thus the metamaterial that it controls) can operate over, for a given number of independently controllable impedance elements.

[0054] There may be a third mode of operation, the control system is configured such that a third independently controllable impedance element of the plurality of independently controllable impedance elements is also in the first state, such that when the applied RF frequency falls within a third operational range between a fifth RF frequency and a sixth, higher RF frequency, the reflected signal is phase shifted relative to the applied RF frequency.

[0055] The second RF frequency and the third RF frequency may be substantially the same RF frequency. The fourth RF frequency and the fifth RF frequency may be substantially the same RF frequency. This allows for there to be as little overlap as possible between the operational ranges of each mode of operation of the apparatus.

[0056] There may be a further mode of operation, in which the control system is configured such that a further independently controllable impedance element of the plurality of independently controllable impedance elements is also in the first state, such that when the applied RF frequency falls within a further operational range between a RF frequency and a higher RF frequency, the reflected signal is phase shifted relative to the applied RF frequency.

[0057] Having overlapping operation ranges where the phase shift of incident RF waves can be shifted between +90° phase shift and -90° phase shift as described in the paragraphs above ensure that there is enough digital resolution such that the metamaterial can be controlled digitally to modulate the phase change of an applied RF frequency over a range of applied RF frequencies. In this, or further aspects of the invention, in the case where the impedance element is a capacitance element and the impedance elements are capacitors, there may be sufficient digital resolution to simulate an analogue voltage-to-capacitance conversion (such as that performed by a varactor diode).

[0058] The apparatus may be arranged such that the metamaterial has a baseline reflectivity value. In a first mode of operation, a first combination of the plurality of independently controllable impedance elements may be in the first state in which the reflectivity of the metamaterial is lower than the baseline reflectivity value for an applied RF frequency within a first operational range between a first RF frequency and a second, higher RF frequency. In a second mode of operation, the control system may be configured such that a second combination of the plurality of independently controllable impedance elements are in the first state in which the reflectivity of the metamaterial is lower than the baseline reflectivity value for an applied RF frequency within a second operational range between a third RF frequency higher than the first RF frequency and a fourth, yet higher RF frequency.

[0059] The magnitude in the decrease in reflectivity of the metamaterial from the baseline may be greatest when the applied RF frequency is at the center of the first operational range, and/or at the center of the second operational range.

[0060] Whether in the first mode of operation or the second mode of operation, the reflectivity of the metamaterial may be the same for an applied RF frequency at the first RF frequency, the second RF frequency or the third RF frequency.

[0061] The apparatus may be configured such that the first operational range of the metamaterial in the first mode of operation and the second operational range of the metamaterial in the second mode of operation overlap, such that at the boundary of the first operational range and the second operational range, the reflectivity of the metamaterial is the same, and optionally corresponds to a desired reflectivity goal.

[0062] The apparatus optionally comprises a third mode of operation, wherein the control system is configured such that a third combination of the plurality of independently controllable impedance elements is in the first state such that the reflectivity of the metamaterial is lower than the baseline reflectivity value for an applied RF frequency within a third operational range between a fifth RF frequency higher than the third RF frequency and a sixth, yet higher RF frequency.

[0063] The apparatus optionally comprises a further mode of operation, wherein the control system is configured such that a further combination of the plurality of independently controllable impedance elements is in the first state such that the reflectivity of the metamaterial is lower than the baseline reflectivity value for an applied RF frequency within a further operational range between another RF frequency higher than the third RF frequency and a yet another, higher, RF frequency.

[0064] The entire control system may be located on a chip or on a printed circuit board. The chip or printed circuit board may be configured to interface with a metamaterial. The metamaterial may be integrated with, and/or form, a part of the chip and/or the printed circuit board.

[0065] According to the second aspect, the present disclosure provides a control system for controlling a metamaterial, the control system comprising an electrical network and a plurality of independently controllable impedance elements, wherein the electrical network is configured such that each of the plurality of independently controllable impedance elements are switchable between a first state in which at least one of the plurality of independently controllable impedance elements is electrically connected to the electrical network thereby altering the impedance of the electrical network, and a second state in which at least one of the plurality of independently controllable impedance elements is electrically disconnected from the electrical network thereby altering the impedance of the electrical network, thereby providing a plurality of discrete impedance values of according to the combination of impedance element of the plurality of independently controllable impedance element in the first or second state.

[0066] According to the third aspect of the invention, there is provided an antenna ground plane comprising an apparatus in accordance with the first aspect of the invention.

[0067] According to the fourth aspect of the invention, there is provided a tunable RF frequency absorption structure comprising an apparatus in accordance with the first aspect of the invention.

[0068] According to the fifth aspect of the invention, there is provided a method of using an apparatus as provided in the first aspect of the invention, or using the control system as provided in the second aspect of the invention, the method comprising switching the state of the at least one impedance elements of the plurality of independently controllable impedance elements, such that the overall impedance value of the electrical network changes thereby changing a property of the metamaterial.

[0069] The method of using the apparatus may also comprise the following steps:

a. generating digital control signals from the digital control source,

b. transmitting the digital control signals from the digital control source to at least one of the independently controllable impedance elements,

c. the digital control signals thus switching the state of the at least one of the impedance elements, such overall impedance value of the electrical network changes such that a property the metamaterial changes in turn.

[0070] According to the sixth aspect of the invention, there is provided a vehicle comprising the apparatus as provided in the first aspect of the invention or a control system as provided in the second aspect of the invention. The vehicle may be configured to carry an explosive payload. The vehicle may comprise a plurality of apparatuses as provided in the first aspect of the invention. The plurality of apparatuses may be distributed over the vehicle. The vehicle may be an aerial vehicle. The aerial vehicle may be an Unmanned Aerial Vehicle (UAV). Alternatively, the aerial vehicle may be a missile. The missile may be, for example, a cruise missile, surface-to-air missile, air-to-air missile, or a guided bomb.

[0071] Fig. 2(a) is a schematic apparatus 150 comprising a control system 200 and a metamaterial 201, the control system 200 controlling the metamaterial 201 in accordance with an embodiment of the invention. The control system 200 comprises a digital voltage controller 202, which is a source for digital voltage signals, a voltage bus 204 configured to provide control signals to an adjustable impedance device 213, the adjustable impedance device in this embodiment having adjustable capacitance and thus also being an adjustable capacitance device. Said adjustable impedance device element is "digital" because the overall capacitance of the adjustable impedance device 213 and thus electrical network can take one of a number of discrete values. In Fig. 2(a), the digital adjustable impedance device 213 is directly mounted to the metamaterial 201. Those skilled in the art would appreciate that the adjustable impedance device 213 may be indirectly mounted to the metamaterial, e.g., by being mounted on a separate surface remote from the metamaterial, while still being electrically connected to the metamaterial. The bus 204 comprises a plurality of individual connections 214, which connect to the adjustable impedance device 213. The total voltage supplied by the individual connections 214 is on the order of single digit volts.

[0072] The skilled person would immediately appreciate how the adjustable impedance device 213 can be coupled to a metamaterial 201 such as the high-impedance surface 101' shown in Fig. 1(b) or the split ring resonator 101" of Fig 1(c). For example, the adjustable impedance device 213 can, instead of the varactor diode 103, be coupled to the high-impedance surface 101' or split ring resonator 101" so the control system 200 can control the electrical properties of high-impedance surface 101' or split ring resonator 101".

[0073] As mentioned above in the background of invention, the skilled person is well aware of high-impedance surface-type metamaterials and split-ring resonator-type metamaterials. How these metamaterials are designed, and how they would be controlled by known control systems is well known in the state of the art. For that reason, detailed specific features of such metamaterials are not described in detail herein, as the skilled person would be able to implement the various embodiments described herein using the description provided in combination with their common general knowledge.

[0074] Fig. 2(b) is a more detailed view of a high-impedance-type design metamaterial 201' showing how the adjustable impedance device 213 is connected to a high-impedance-type metamaterial 201'. The high-impedance-type metamaterial 201' is substantially the same as the high-impedance-type metamaterial 101' shown in Fig. 1(b). Digital voltage controller 202 generates voltage control signals that are passed along individual control lines 214a, 214b, 214c, 214d to the adjustable impedance device 213. A first control line 214a carries the output on a control line that is equivalent to the path 114 in Fig. 1d. The remaining control lines 214b, 214c, and 214d are routed through a through via 218 drilled into the high-impedance-type metamaterial 201' and are also connected to the adjustable capacitance device 213. Thus, the adjustable impedance device 213 can receive independent voltage control signals from each of the separate control lines 214a, 214b, 214c and 214d, which together form part of the bus 202.

[0075] Fig. 3 is a schematic of a plurality of independently controllable capacitors 223a, 223b, 223c, 223d, that are switchable into and out of an electrical network 224, the electrical network 224 being part of the adjustable impedance device 213 of a control system 200 for controlling a metamaterial 201 in accordance with an embodiment of the present invention. In Fig. 3, there are four independently controllable capacitors, 223a, 223b, 223c, 223d, that are controlled individually by four respective switches 225a, 225b, 225c, 225d. The four switches 225a, 225b, 225c, 225d can each be controlled to switch each of the independently controllable capacitors 223a, 223b, 223c, 223d in to and out of electrical connection with the electrical network 224. Thus, the overall capacitance of the adjustable impedance device 213 is determined by the number of the independently controllable capacitors 223a, 223b, 223c, 223d that are electrically connected to the electrical network 224. The electrical network 224 is arranged as a parallel circuit, so the capacitance contributed to the total capacitance of the adjustable impedance device 213 by each of the independently controllable capacitors, 223a, 223b, 223c, 223d is the sum of the capacitances of the capacitors that are included in the electrical network 224.

[0076] In Fig. 3 each of the switches 225a, 225b, 225c, 225d is a FET-type switch. It should be appreciated that in other embodiments of the invention each of the switches 225a, 225b, 225c, 225d may be a PIN diode switch, or a MEMS-type switch. The skilled person will understand that in yet further embodiments of the invention each of the switches may be arranged such that any combination or number of the aforementioned switch types (e.g., PIN, FET, MEMS) may be used.

[0077] In Fig. 3 there are four capacitors 223a, 223b, 223c, 223d controlled by voltage signals being passed along control lines 214a, 214b, 214c, 214d to switches 225a, 225b, 225c, 225d, which control the capacitors by switching them into and out of electrical connection with the electrical network 224. In Fig. 3, each capacitor has only two states, being either in the electronic circuit, or outside the electronic circuit, and thus each capacitor can be thought of as providing one binary digit (bit) of control over the control system. Thus, as used herein, a system with four individually controllable capacitors will have four bits of control. Each bit of control corresponds to a discrete capacitance value the overall capacitance can take. In Fig. 3, there are four capacitors each having different capacitance values, and therefore four bits of control and thus 16 discrete capacitance values the overall capacitance can take. In other embodiments of the invention, there may be a different number of capacitors in the electrical network providing greater or fewer bits of control of the overall capacitance of the system. The skilled person will readily understand that increasing the number of capacitors capable of being switchably connected to the electrical network increases the number of bits of control, and decreasing the number of capacitors capable of being switchably connected to the electrical network decreases the number of bits of control. Each bit of control corresponds to a discrete capacitance value that corresponds to the overall capacitance value that the adjustable impedance device 213 takes in a certain arrangement of the electrical network 224 (i.e., depending on which capacitors 223a, 223b, 223c, 223d are connected to the electrical network 224).

[0078] In Fig. 3, one of the bits of control corresponds to a case where all of the capacitors 223a, 223b, 223c, 223d are disconnected from the electrical network 224 and thus the capacitance contributed by the capacitors 223a, 223b, 223c, 223d is (substantially) zero. One of the bits of control corresponds to a case where all of the capacitors 223a, 223b, 223c, 223d are connected from the electrical network 224 and thus the capacitance contributed by the capacitors 223a, 223b, 223c, 223d is the sum of capacitance value of each of the capacitors.

[0079] In Fig. 3 the capacitors 223a, 223b, 223c, 223d all have the same capacitance values, though it will be appreciated that in other embodiments of the invention at least some of the capacitors may have different capacitance values to the other capacitors.

[0080] In this example, the discrete capacitance values the overall capacitance of the adjustable impedance device 213 can take all fall within the range 0.5pF and 2pF.

[0081] Fig. 4 is a graphic representation of the reflectance phase change response of an applied RF frequency modulated by an apparatus comprising a metamaterial controlled by a control system in accordance with an example embodiment the present invention. The graph shows the frequency of an incoming RF signal on the X axis and the reflection phase of the RF signal after modulation is shown on the Y axis. Depicted in Fig. 4, with reference to the capacitors shown in Fig. 3, is a first reflectance phase change response 223a' of an RF frequency when a first capacitor 223a is connected to electrical network 224. The remaining capacitors 223b, 223c, 223d are not connected to the electrical network 224. This configuration corresponds to a first mode of operation of the apparatus. The RF signal undergoes a reflection phase change that is different depending on the RF frequency applied to the metamaterial. There is a maximum phase change value M1 and a minimum phase change value M2.

[0082] In the first capacitor configuration 223a', the metamaterial modulates applied RF frequency signals such that when an RF frequency between the frequency limits A1 A2 of a first range of operation is applied to the metamaterial, the reflected signal has a phase change B1, relative to the applied signal, of +90° at the lowest frequency A1 of the first range of operation and a phase change B2, relative to the applied signal, of -90° at the highest frequency A2 of the first range of operation . This corresponds to a first band of operation 233a.

[0083] A second capacitor configuration 223b' of the apparatus corresponds to when a first capacitor 223a is connected to electrical network 224 and a second capacitor 223b is also connected to electrical network 224, the remaining capacitors, 223c, 223d not being connected to the electrical network 224.

[0084] In the second capacitor configuration 223b', the metamaterial modulates applied RF frequency signals such that when an RF frequency between the frequency limits A2 A3 of a second range of operation is applied to the metamaterial, the reflected signal has a phase change B1 relative to the applied signal of +90° at the lowest frequency A2 of the second range of operation and a phase change B2 relative to the applied signal of -90° at the highest frequency A3 of the second range of operation. This corresponds to a second band of operation 233b.

[0085] A third capacitor configuration 223c' of the metamaterial corresponds to when a first capacitor 223a is connected to electrical network 224 and a second capacitor 223b and third capacitor 223c are also connected to electrical network 224, the remaining capacitor 223d not being connected to the electrical network 224.

[0086] In the third capacitor configuration 223c', the metamaterial modulates applied RF frequency signals such that when an RF frequency between the frequency limits A3 A4 of a third range of operation is applied to the metamaterial, the reflected signal has a phase change B1, relative to the applied signal, of +90° at the lowest frequency A3 of the third range of operation, and a phase change B2, relative to the applied signal, of -90° at the highest frequency A4 of the third range of operation. This corresponds to a third band of operation 233c.

[0087] Capacitor configurations 233a, 233b, 233c are various discrete overall capacitance values that adjustable impedance element 213 and thus the apparatus 200 can take. Digital resolution is only required to be sufficient to ensure adjacent bands touch with minimal overlap (as shown by each the shaded bands 233a, 233b, 233c).

[0088] The skilled person will understand that in some embodiments only the first band 233a and band 233b may overlap. The skilled person will understand that in some embodiments only the second band 233b and third band 233c may overlap. There may be in some embodiments a fourth, fifth, or sixth band, or any other number of bands corresponding to the number of capacitance configurations the adjustable impedance element 213 can take. This in turn corresponds to the number of discrete impedance values in the metamaterial 201 of the apparatus 200. There may only be a first and second band in other embodiments.

[0089] A metamaterial and control system as described above in relation to Fig. 4 is particularly suited for use as an antenna ground plane, particularly as a high-impedance surface for an antenna ground plane. In such an embodiment a phase shift of +90 degree and -90 degree describes the bandwidth of the antenna ground plane. For a given capacitance value (as described by the response traces 223a', 223b' and 223c' and the corresponding bands 233a, 233b, 233c) the ground plane will function for a range of frequencies, as shown in Fig. 4. The range of frequencies the antenna can operate at is directly related to the number of different bands 233a, 233b, 233c provided.

[0090] Fig. 5 is a graphic representation of the reflectivity response of a tuneable radio frequency absorber structure apparatus in accordance with an example embodiment of the present invention, said apparatus comprising a metamaterial and a control system, the metamaterial being controlled by the control system.

[0091] In a first operational state of the apparatus of Fig. 5, a first independently controllable capacitator of the plurality of independently controllable capacitors 223a, 223b, 223c, 223d is connected to the electrical network 224. A maximum value M1'corresponds to a highest possible reflectivity of the device, also known as the baseline reflectivity of the device. There is a reflectivity goal that is lower than the maximum reflectivity value M1'. When a frequency between the frequency limits C1 C2 of a first range of operation is applied to the metamaterial, the metamaterial undergoes a change in reflectivity as part of a first reflectivity response 223a" in which the reflectivity is decreased relative to the reflectivity goal. If the frequency applied is at the midpoint between of the frequency limits C1 C2 of the first range of operation, the reflectivity drops to a local minimum value M2'. The reflectivity response takes the form of a notch function across the first range of operation.

[0092] In a second operational state of the apparatus, a second independently controllable capacitator of the plurality of independently controllable capacitors 223a, 223b, 223c, 223d is connected to the electrical network 224. When a frequency between the frequency limits C2 C3 of a second range of operation is applied to the metamaterial, the metamaterial undergoes a change in reflectivity as part of a second reflectivity response 223b" in which the reflectivity is decreased relative to a the reflectivity goal. If the frequency applied is at the midpoint between of the frequency limits C2 C3 of the second range of operation, the reflectivity drops to the local minima value M2'. The reflectivity response takes the form of a notch function across the second range of operation.

[0093] In a third operational state of the apparatus, a third independently controllable capacitator of the plurality of independently controllable capacitors 223a, 223b, 223c, 223d is connected to the electrical network 224. When a frequency between the frequency limits C3 C4 of a third range of operation is applied to the metamaterial, the metamaterial undergoes a change in reflectivity as part of a third reflectivity response 223c" in which the reflectivity is decreased relative to a reflectivity goal. If the frequency applied is at the midpoint between of the frequency limits C3 C4 of the third range of operation, the reflectivity drops to the local minima value M2'. The reflectivity response takes the form of a notch function across the third range of operation.

[0094] The local minima value M2'occurs at a higher RF frequency in the second reflectivity response 223b' than the RF frequency at which the local minima value occurs in the first reflectivity response 223a". The local minima value M2'occurs at a higher RF frequency in the third reflectivity response 223c" than the RF frequency at which the local minima value M2' occurs in both the first reflectivity response 223a" and the second reflectivity response 223b".

[0095] The first reflectivity response 223a" of the metamaterial overlaps the second reflectivity response 223b" of the metamaterial. The second reflectivity response 223b" of the metamaterial overlaps the third reflectivity response 223c" of the metamaterial.

[0096] The point of overlap of the first reflectivity response 223a" and the second reflectivity response 223b" corresponds to the desired reflectivity goal of the metamaterial.

[0097] The skilled person will understand that in some embodiments only the first reflectivity response 223a" and second reflectivity response 223b" may overlap. The skilled person will understand that in some embodiments only the second reflectivity response 223b" and third reflectivity response 223c" may overlap. There may be in some embodiments a fourth, fifth, or sixth reflectivity response, or any other number of reflectivity responses corresponding to the number of discrete impedance values in the metamaterial. In other embodiments there may only be a first and second reflectivity response.

[0098] A metamaterial and control system as described above in relation to Fig. 5 is particularly suited for use as RF absorber structures. A RF absorber structure is provided accordingly as a further embodiment of the invention.

[0099] Fig. 6 is a graphic representation of a method of of controlling a metamaterial using the control system as described in accordance with an embodiment of the invention. The method 500 comprises:

Generating 510 digital control signals from the digital voltage source;

Transmitting 520 the digital control signals from the digital control source to at least one of the independently controllable capacitors;

Switching 530 the state of the at least one capacitor, such overall capacitance of the adjustable impedance device changes such that a property of the metamaterial changes in turn.

[0100] Fig. 7 is a schematic of a plurality of independently controllable resistors 623a, 623b, 623c, 623d, that are switchable into and out of an electrical network 624, the electrical network 624 being part of an adjustable resistance element (not shown) of a control system for controlling a metamaterial in accordance with an embodiment of the present invention. In this embodiment, there are four independently controllable resistors, 623a, 623b, 623c, 623d, that are controlled individually by four respective switches 625a, 625b, 625c, 625d. The four switches 625a, 625b, 625c, 625d can each be controlled to switch each of the independently controllable resistors 623a, 623b, 623c, 623d in to and out of electrical connection with the electrical network 624 by control signals received from individual connections 614a, 614b, 614c, and 614d connected to a digital voltage controller (the digital voltage controller being substantially the same as that described with reference to Fig. 2(a)). Thus, the overall resistance of the adjustable resistance element 613 is determined by the number of the independently controllable resistors 623a, 623b, 623c, 623d that are electrically connected to the electrical network 224. Since in this case the electrical network 624 is arranged in series, the resistivity contributed to the resistance of the adjustable resistance element 613 by each of the independently controllable resistors, 623a, 623b, 623c, 623d is the sum of the resistance of the resistors that are included in the electrical network 624.

[0101] The arrangement of resistors of Fig. 7 can be used as a digital impedance element in a control system 200, as shown in Fig 2(a) and Fig 2(d) and described above in relation to those figures. The arrangement of resistors as described in Fig. 7 has the same number of bits of control as the arrangement of capacitors shown in Fig. 3.

[0102] According to a yet further embodiment of the invention there is provided an aerial vehicle including a control system as described herein, a metamaterial as described herein, or configured to be controlled by the method as described herein. In some embodiments the aerial vehicle is configured to carry an explosive payload. In some embodiments the aerial vehicle is an Unmanned Aerial Vehicle (UAV). In some embodiments the aerial vehicle is a missile.

[0103] It will be appreciated by those of ordinary skill in the art that features of these example embodiments may be combined in other embodiments that fall within the scope of the present disclosure.

[0104] Alternatively or additionally, in other embodiments of the invention the impedance element may be an inductance element, where the impedance elements are inductors, and the impedance values are inductance values. Such a system would be achieved by replacing each of the resistors 623a, 623b, 623c, 623d described above in relation to Fig. 7 with an inductor.

[0105] Alternatively, in other embodiments of the invention the impedance elements may be a combination of inductors, capacitors or resistors, the electrical network having both some parallel characteristics (as shown in Fig. 3) and some series characteristics (as shown in Fig. 7). In such an embodiment, the electrical properties of the metamaterial are controlled by adjusting at least two properties of the impedance, capacitance, and resistance of the electrical network simultaneously. In yet further embodiments of the invention the impedance elements may be devices that are capable of adjusting two or more of the capacitive values and/or resistive values and/or inductance values of an electrical network when introduced to the electrical network.

[0106] Whilst in the foregoing description, integers or elements are mentioned which have known obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present disclosure, which should be construed as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the disclosure that are described as advantageous, convenient or the like are optional, and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the disclosure, may not be desirable and may therefore be absent in other embodiments.

Claims

1. An apparatus (150) comprising a control system (200) and a metamaterial (201) electronically controlled by the control system (200), the control system including an electrical network (224) and a plurality of independently controllable impedance elements (223a, 223b, 223c, 223d),
wherein each of the plurality of independently controllable impedance elements (223a, 223b, 223c, 223d) is arranged to be switched between a first state in which at least one of the plurality of independently controllable impedance elements (223a, 223b, 223c, 223d) is electrically connected to the electrical network and a second state in which at least one of the plurality of independently controllable impedance elements (223a, 223b, 223c, 223d) is electrically disconnected from the electrical network (224) thereby providing a plurality of discrete impedance values in the metamaterial (201) according to the combination of impedance elements of the plurality of independently controllable impedance elements in the first or second state.

2. An apparatus (150) as claimed in claim 1 wherein the impedance elements (223a, 223b, 223c, 223d) are capacitive elements, or the impedance elements are inductive elements, or the impedance elements are resistive elements.

3. An apparatus (150) as claimed in claim 1 or claim 2, wherein the control system further comprises a digital voltage source (202) and a digital bus (204), and wherein each of the plurality of independently controllable impedance elements (223a, 223b, 223c, 223d) is configured to independently receive digital voltage signals from the digital voltage (202) source via the digital bus (204), wherein the digital voltage signals received by each impedance element (223a, 223b, 223c, 223d) switch said impedance element between the first state and the second state.

4. An apparatus (150) as claimed in any preceding claim, wherein each of the plurality of independently controllable impedance elements has a corresponding switch (225a, 225b, 225c, 225d) in the electrical network (224) configured to independently switch each of the plurality of independently controllable impedance elements (223a, 223b, 223c, 223d) between the first state and the second state.

5. An apparatus (150) as claimed in claim 4, wherein each corresponding switch (225a, 225b, 225c, 225d) comprises at least one of a FET-type switch, a PIN diode switch, or a MEMS-type switch.

6. An apparatus (150) as claimed in any preceding claim, wherein the impedance elements (223a, 223b, 223c, 223d) are capacitive elements, wherein the discrete capacitance values comprise a plurality of capacitance values that fall within the range 0.1pF and 10pF.

7. An apparatus (150) as claimed in any preceding claim, wherein the apparatus is configured to modulate an applied RF frequency, such that:

in a first mode of operation, a first combination (223a') of the plurality of independently controllable impedance elements are in the first state such that the reflected signal is phase shifted relative to the applied RF frequency, and

in a second mode of operation, a second combination (223b') of the plurality of independently controllable impedance elements are in the second state, such that the reflected signal is phase shifted relative to the applied RF frequency,

wherein the phase shift of the reflected signal is different in the first and second mode of operations.

8. An apparatus (150) as claimed in claim 7, wherein,

when in the first mode of operation, for an applied RF frequency within a first operational range between a first RF frequency (A1) and a second, higher RF frequency (A2), the reflected signal is phase shifted relative to the applied RF frequency between +90° phase shift and -90° phase shift,

when in the second mode of operation, for an applied RF frequency within a second operational range between a third RF frequency (A3) that is higher than the first RF frequency and a fourth, yet higher RF frequency (A4), the reflected signal is phase shifted relative to the applied RF frequency between +90° phase shift and -90° phase shift.

9. An apparatus (150) as claimed in claim 8, wherein the first operational range and the second operational range at least partially overlap.

10. An apparatus (150) as claimed in any preceding claim, wherein the metamaterial (201) has a baseline reflectivity value; and

in a first mode of operation, a first combination of the plurality of independently controllable impedance elements (223a, 223b, 223c, 223d) are in the first state, in which the reflectivity of the metamaterial (201) is lower than the baseline reflectivity value (M1') for an applied RF frequency within a first operational range between a first RF frequency (C1) and a second, higher RF frequency (C2),

in a second mode of operation, a second combination of the plurality of independently controllable impedance elements (223a, 223b, 223c, 223d) are in the first state in which the reflectivity of the metamaterial (201) is lower than the baseline reflectivity value (M1') for an applied RF frequency within a second operational range between a third RF frequency (C3) higher than the first RF frequency and a fourth, yet higher RF frequency (C4).

11. An apparatus (150) as claimed in claim 10, wherein, whether in the first mode of operation or the second mode of operation, the reflectivity of the metamaterial is the same for an applied RF frequency at the first RF frequency, the second RF frequency or the third RF frequency.

12. A control system (200) for controlling a metamaterial (201), the control system comprising an electrical network (204) and a plurality of independently controllable impedance elements (223a, 223b, 223c, 223d),
wherein the electrical network is configured such that each of the plurality of independently controllable impedance elements (223a, 223b, 223c, 223d) are switchable between:

a first state in which at least one of the plurality of independently controllable impedance elements (223a, 223b, 223c, 223d) is electrically connected to the electrical network (204) thereby altering the impedance of the electrical network (204), and

a second state in which at least one of the plurality of independently controllable impedance elements (223a, 223b, 223c, 223d) is electrically disconnected from the electrical network (204) thereby altering the impedance of the electrical network (204), thereby providing a plurality of discrete impedance values according to the combination of impedance element of the plurality of independently controllable impedance elements (223a, 223b, 223c, 223d) in the first or second state.

13. An antenna ground plane or tuneable RF frequency absorption structure comprising an apparatus (150) as claimed in any of claims 1 to 11.

14. A method (500) of using the apparatus (150) of any of claims 1 to 11, the method comprising switching (530) the state of at least one impedance element of the plurality of independently controllable impedance elements, such that the overall impedance of the electrical network (204) changes, thereby changing the impedance of the metamaterial (201).

15. A vehicle comprising the apparatus as claimed in any of claims 1 to 11 or a control system as claimed in claim 12.

Drawing