Radio-frequency energy harvesting

Abstract

A tuneable radio frequency (RF) energy harvesting apparatus comprises an antenna and a set of electrodes, the antenna including a first conductive element 602 arranged parallel to and spaced apart fro

Description

FIELD OF THE INVENTION The invention relates to a radio-frequency energy harvesting apparatus. BACKGROUND TO THE INVENTION Terms such as “wireless power transfer,” “wireless power transmission,” “electromagnetic power transfer” or the like typically refer to the transmission of electrical energy without requiring use of wire as a physical link. In such systems, an electrically powered transmitter device typically generates a time-varying electromagnetic field via which electromagnetic power is transmitted to a receiver device. The receiver device in turn extracts power from the electromagnetic field and supplies it to an electrical load. By obviating the need for wires, mobility, convenience and / or safety can be improved. The principles of wireless power transfer can be applied to energy harvesting, also called power harvesting or energy scavenging, for the conversion of ambient electromagnetic energy from the environment to electric power. The ambient energy may for example be harvested from stray electric and / or magnetic fields or radio waves from nearby electrical equipment. Power transfer efficiency in wireless power transfer is improved when an antenna of an energy harvesting apparatus is tuned to the frequency spectrum of the time-varying electromagnetic field via which the power is to be transferred (or from which the power is to be harvested). Generally, wireless power transfer systems operate with narrow bandwidths. Further, the operating frequency band of wireless power transfer systems is typically not capable of being tuned in the field, limiting in-field application to predefined and typically narrow frequency bands. These shortcomings can be particularly pronounced in energy harvesting applications, where one does not necessarily have control over bandwidth or frequency spectrum of the electromagnetic field to be harvested. While they can be addressed to some extent, for example, by using multiple harvesters of different sizes, such solutions have their drawbacks too. There is accordingly scope for improvement. The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge in the art as at the priority date of the application. SUMMARY OF THE INVENTION In accordance with an aspect of the invention there is provided an apparatus comprising an antenna and a set electrodes, the antenna including a first conductive element spaced apart by a distance from and arranged parallel to a ground plane, wherein the set of electrodes is arranged such that a first electrode of the set of electrodes is spaced apart from a second electrode of the set of electrodes and such that a first zone of each of the first and second electrodes extends from a plane in which the ground plane locates to a plane in which the first conductive element locates, wherein the first and second electrodes are arranged such that the first conductive element extends between the first electrode and the second electrode, and wherein, in use, a potential difference applied across the first zones of first electrode and the second electrode is adjustable in turn to adjust a resonant frequency of the antenna. The first conductive element may be shaped so as to curve over on itself to define an opening, and wherein the first electrode locates within the opening defined by conductive element and the second electrode locates outside of the conductive element. Ends of the first conductive element may be co-planar and spaced apart from each other to define a gap therebetween. The set of electrodes may include the first electrode and a plurality of second electrodes spaced apart from each other around and outside of the first conductive element. Alternatively, the second electrode may be in the form of a hollow cylinder which surrounds the first conductive element. The first conductive element may be in the form of an annulus, wherein the annulus is split along a radius thereof to define a gap between two radially extending edges thereof, wherein the ground plane is in the form of a disc and wherein the disc is centred on the centre of the annulus. The apparatus may include one or more further conductive elements, wherein each further conductive element is spaced apart from and parallel to the other conductive elements. Each conductive element may be of the same shape and size. Alternatively, each conductive element may be of different shape and / or size. Each electrode in the set of electrodes may include a further zone for each further conductive element. Each further zone of each electrode may extend between respective planes in which adjacent conductive elements locate. The zones of each electrode may be electrically isolated from each other such that, in use, different potential differences can be applied across different zones of the electrodes. A substrate may be disposed between the ground plane and the first conductive element. A substrate may be disposed between each of the conductive elements. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figure 1 is a chart showing a change in frequency response due to a change in the thickness of the substrate; Figure 2 is a chart which represents the frequency response graph for different resonators of different dimensions; Figure 3 a schematic diagram which illustrates an example embodiment of an apparatus according to aspects of the present disclosure; Figure 4 is a chart which illustrates the relationship between applied potential and effective substrate thickness; Figure 5 is a three-dimensional view of an example embodiment of an apparatus according to aspects of the present disclosure; Figure 6 is a top down view of the apparatus of Figure 5; Figure 7 is a three-dimensional view of another example embodiment of an apparatus according to aspects of the present disclosure; Figure 8 is a top down view of the apparatus of Figure 7; Figure 9 is a three dimensional view of an antenna arrangement for an apparatus according to aspects of the present disclosure; Figure 10 is a chart which illustrates the frequency response of the antenna arrangement shown in Figure 9; and, Figure 11 is a chart which illustrates the relationship between the frequency range and size or dimension of a conductive element according to aspects of the present disclosure. DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS Aspects of the present disclosure relate to a radio-frequency (RF) energy harvesting apparatus, which may also be termed a “harvester” herein. In particular, the present disclosure relates to the front-end of an RF energy harvesting apparatus. The apparatus may for example include an antenna and a set electrodes. The antenna includes conducting elements, which include a ground plane and at least a first conductive element (or “resonator”) which is spaced apart by a distance from and arranged parallel to and face-to-face with the ground plane. In some embodiments, the apparatus includes one or more further conductive elements. Each of the further conductive element is spaced apart from and parallel to the other conductive elements. In other words, the conductive elements (including the first and further conductive elements) are stacked one on top of the other, albeit with a gap or spacing being left between adjacent conductive elements. The distance between respective conducting elements may be the same or may differ for each set of adjacent conducting elements. In some embodiments, each of the conductive elements is of the same shape and size. In other embodiments, each of the conductive elements is of different shape and / or size. The conductive element(s) (i.e. including the first conductive element and some embodiments one or more further conductive elements) may be shaped so as to curve over on itself to define an opening. In some embodiments, the conductive element(s) may be in the form of annuluses. The term “annulus” should be broadly construed to include any shape defined by an elongate conductive trace or strip or material which substantially encircles or surrounds an area. Each annulus can therefore be of any suitable shape, such as circular, rectangular, pentagonal, hexagonal or the like. In some embodiments, the annulus is of a regular polygon shape. Each annulus may be split so as to take the form of a conductive trace or strip of material that circles back on itself such that the beginning of the trace or strip locates proximate to, but does not connect with, the end of the trace or strip. The annulus may thus be split to define a gap between a beginning and an end thereof. In the case of a circular annulus, for example, the annulus may be split along a radius thereof to define a gap between two radially extending edges thereof. The set of electrodes is arranged such that a first electrode of the set of electrodes is spaced apart from a second electrode of the set of electrodes. The first electrode locates in a centre of the annulus and the second electrode locates outside of the annulus. In one embodiment, the set of electrodes includes the first electrode and a plurality of second electrodes circumferentially or peripherally spaced around and outside of the annulus. In another embodiment, the second electrode is in the form of a hollow cylinder which encircles or surrounds the annulus. The first and second electrodes are arranged such that the conductive elements (including the first conductive element as well as any further conductive elements) extend between the first electrode and the second electrode (i.e. with an electrode on either side), such that a plane extending between the two electrodes is intersected by the first conductive element. In some embodiments, the set of electrodes is further arranged such that a first zone of each of the first and second electrodes extends substantially from a plane in which the ground plane locates to a plane in which the first conductive element locates. In other words, the first zone of each electrode extends through or along the distance that separates the ground plane and the first conductive element. Further, in some embodiments each electrode in the set of electrodes includes a zone for each conductive element. Each zone of each electrode extends between respective planes in which adjacent conducting elements locate. For example, in addition to the first zone which extends through the distance that separates the ground plane and the first conductive element, the electrodes may include further zones for each of the further conductive elements, each of which may extend through the distance that separates conductive elements that are adjacent the zone. The zones of each electrode are electrically isolated from each other such that, in use, different potential differences can be applied across different zones of the electrodes. In this manner, in use, a potential difference can be applied across the one or more zones of the electrodes. By adjusting the potential difference, a resonant frequency of the antenna is in turn adjusted. The RF harvesters disclosed herein may include multiple resonators which can be tuned to the same or different frequencies depending on the requirements. The resonators may be connected to an RF electronics front-end. The resonators may be connected to the RF electronics front-end through metallic vias, wires, conductive traces or other suitable means. The apparatus may include a control unit and a power source. The control unit may be connected to the electrodes and configured to control the potential difference across the one or more zones of the set of electrodes. The control unit may be configured to receive an input frequency value relating to a desired resonance frequency of a resonator and to convert the input frequency into a potential difference value, for example using the principles described herein. The power source may be connected to each of the zones of the set of electrodes such that a potential difference for each zone across the electrodes is individually controllable. The control unit may be configured to control the power source to apply the potential difference determined based on the input frequency so as to adjust the resonance frequency of that resonator of the apparatus. In embodiments in which the set of electrodes includes a plurality of second electrodes, the control unit may be configured to control each zone of the plurality of second electrodes to have the same or different potential difference as each other. The potential difference may be decided based on the input or desired resonance frequency of a harvester. The resonance frequency is dependent upon the thickness of substrate. V d = — ET Where d is substrate thickness, V is the potential across resonator and Eyis the total electric field between resonator and ground. The electric field due to the V is as follows: £ = ^^gi(fcx-wt+01) While the electric field due to the external potential is given as: E2 = 42ei(fc^wt+02) E2 can be changed by the external potential. By determining the suitable value of E2 for the required resonance frequency of harvester, control unit decides the potential across different zones of harvester. Figure 1 is a chart showing a change in frequency response due to a change in the thickness of the substrate (which may be equivalent to the distance between the resonator and the ground plane). For a first thickness, the resonance frequency (203) is centred on fi’. In response to a change in substrate thickness from, the resonance frequency (205) is centred on fi’+Af. Figure 2 is a chart showing the change in normalized frequency response due to a change in resonator dimensions. Each resonator or harvester may be designed for a particular frequency range which may be termed an operating or resonance frequency. In this context, two different dimension resonators are shown with distinct operating frequencies (fs and fg) due to the size difference. A resonator with a smaller width has an operating frequency of fs on which a first frequency response curve (303) is centred, whereas a resonator with a larger width has an operating frequency of fg on which a second frequency response curve (305) is centred. The resonator with the larger width and corresponding second frequency response curve (305) is shifted at fg which is lower than the first frequency response curve (303) of resonator with the smaller width. Figure 3 schematically illustrates the underlying concept of the apparatus of the present disclosure. A first conductive element (702) is spaced apart from and arranged parallel to and face-to-face with a ground plane (704). An associated electric field (706) exists between the first conductive element and the ground plane. A set of electrodes including first and second electrodes (708, 710) are provided and arranged such that the first electrode (708) is spaced apart from the second electrode (710). The set of electrodes is further arranged such that a first zone or part of each of the first and second electrodes (which zone or part in the illustrated embodiment includes the whole of each of the electrodes) extends substantially from a plane in which the ground plane (704) locates to a plane in which the first conductive element (702) locates. In other words, the first zone of each electrode extends substantially through the distance that separates the ground plane and the first conductive element. The first and second electrodes are arranged such that the first conductive element extends substantially between the first electrode and the second electrode, such that a plane extending between the two electrodes is sandwiched between the first conductive element and the ground plane. In use, a potential difference can be applied across the first zones of first electrode and the second electrode to induce an electric field (712). By virtue of the orientation of the set of electrodes relative to the conductive element, as described above, the electric field (712) induced by way of the potential difference applied across the electrodes intersects and is perpendicular to the electric field (706) induced by way of the potential difference between the conductive element and the ground plane. The potential difference and polarity applied across the electrodes (708, 710) determine the magnitude and direction of the electric field (712) therebetween. Greater the potential difference, stronger will be the electric field (712), and hence the interference may occur due to the presence of multiple fields. This interference due to the fields (706, 712) may increase or decrease the effective substrate thickness present between the conductive element (702) and the ground plane (704), which in turn shifts the resonant frequency of the antenna. In other words, the constructive or destructive interference between the plane gives the same effect as the effect achieved with different substrate thicknesses. The electric fields described above may be represented as follows: £ _ ^^Kkx-wt + 0^ E2 = edkx-wt+02') where Er is the electric field (706) between the conductive element (702) and the ground plane (704) having amplitude and phase angle 0t and where E2 is electric field (712) between the first and second electrodes (708, 710) having amplitude A2 and phase angle 02. After interference, the total electric field may be represented as given below: Et = ^Redkx-wt+0R) where: AR = Al + A2 + 2A1A2cos (0X — 02) and where: A! sin^) + A2sin(02) K A2 cos^) + A2cos(02) The constructive and destructive interference depends upon the difference of 0X and 02. When the difference between phase angle of two waves is near to zero, constructive interference may occur to increase the amplitude of resultant wave. Conversely, when the difference of phase angles is 180°, the amplitude of resultant wave decreases due to destructive interference. Therefore, by changing the values of 4^^2.01 and 02, the electric field of resultant wave can be controlled. In the present disclosure, A1 and 0^ (due to antenna field) are kept constant while the values of A2 and 02 (due to electrode field) are maneuverable. Due to the interference of E± and Ez, the resultant electric field ET may change causing the effective substrate thickness to vary, according to: V — = d Ey where d is the distance between the conductive element (702) and the ground plane (704). From the above, it can be inferred that the resonance frequency, fr, is inversely proportional to the virtual substrate thickness. By changing the virtual substrate thickness, the resonance frequency can shift accordingly: Figure 4 is a chart which illustrates the relationship between applied potential and effective substrate thickness. The exponential rising curve (806) depicts that effective substrate thickness increases as the applied potential across electrodes. The interference phenomenon results in the increment in effective substrate thickness (804) due to the applied potential (802). One example embodiment of an apparatus according to aspects of the present disclosure is illustrated in Figures 5 and 6. The apparatus includes an antenna and a set electrodes. The antenna includes a first conductive element (602) and a plurality of further conductive elements (604, 606, ..., 608). The first conductive element (602) is spaced apart by a distance from and arranged parallel to and face-to-face with a ground plane (618). Each of the further conductive elements are spaced apart from and parallel to the other conductive elements. The conductive elements are thus spaced apart and stacked one above another. Each conductive element is shaped so as to curve over on itself to define an opening. Ends of each conductive element are co-planar, proximate each other and are spaced apart from each other to define a gap therebetween. In the illustrated embodiment, the conductive elements are in the form of annuluses and are arranged concentrically on the same axis. In the illustrated embodiment, the annuluses are circular in shape, although other embodiments may employ other shapes. In the illustrated embodiment, the annuluses are split along their respective radiuses to define, for each annulus, a gap between two radially extending edges thereof. The annuluses may therefore be split ring annuluses, ends of which do not overlap and which do not touch. In the illustrated embodiment, each conductive element is of the same shape and size, although in other embodiments they may be of different shapes and / or sizes. The ground plane is in the form of a disc centred on the centre of the annuluses (i.e. being concentric with the annuluses). In other embodiments the ground plane may have different shapes which may correspond with the shapes of the annuluses. A radius of the disc is larger than a radius of the annuluses. The apparatus includes a set of electrodes including a first electrode (906) and a second electrode (904). The electrodes are arranged such that the first electrode is spaced apart (903) from the second electrode. The first electrode (906) extends through the opening defined by the conductive elements, in the illustrated embodiment through the centres thereof (i.e. the first conductive element is concentric with the conductive elements). The second electrode locates outside of the conductive elements. In the illustrated embodiment, the second electrode (904) is in the form of a hollow cylinder which encircles the ground plane and the conductive elements. The first and second electrodes are thus arranged such that the conductive elements extend between the first electrode and the second electrode. The cylinder has a height that extends between the ground plane and the farthest conductive element. A substrate (not shown) may be disposed between the ground plane and the first conductive element and between each of the further conductive elements. In use, a potential difference can be applied across the first electrode and the second electrode and can be adjusted in turn to adjust a resonant frequency of the antenna. An example embodiment in which multi-layered and same-dimensioned resonators are contained within a cylindrical-shaped electrodes (904, 906) is thus provided. The resonators (602, 604, 606, 608) with the same dimensions are stacked above each other. A large hollow cylindrical electrode (904) is placed outside of the resonators and a small cylindrical electrode (906) is placed inside of the resonators. A potential difference is applied across inner and outer electrodes (904, 906). As the potential difference changed, an effective substrate thickness between the resonators (602, 604, 606, 608) changes too. Due to the variation in effective substrate thickness, the frequency response may change, thus providing an electronically controlled tuneable RF harvester. Another example embodiment of an apparatus according to aspects of the present disclosure is illustrated in Figures 7 and 8. The embodiment described with reference to Figures 7 and 8 differs from the embodiment described with reference to Figures 5 and 6 in configuration of the set of electrodes. In the embodiment of the apparatus illustrated in Figures 7 to 8, the set of electrodes includes a first electrode (1000) and a plurality of second electrodes (1002, 1004, 1006, 1008, 1010, 1012, 1014, 1016). The set of electrodes is arranged such that the first electrode (1000) is spaced apart from each of the plurality of second electrodes (1002,1004,1006,1008,1010,1012,1014,1016). The first electrode (1000) locates in a centre defined by the conductive elements. Each of the plurality of second electrodes locates outside of the conductive elements. The second electrodes may be spaced apart around the periphery of the conductive elements. The second electrodes may be equally spaced apart from each other. The second electrodes may be arranged such that the distance (1102) between any of two electrodes (1002, 1004, 1006, 1008, 1010, 1012, 1014, 1016) is greater than A / 4 (where A refers to wavelength of a signal for which the resonator is designed) to avoid the RF cage shielding effect. The first electrode and the second electrodes are thus arranged such that the conductive elements extend between the first electrode and each of the second electrodes. The electrodes may be cylindrical in shape and may act as a detuned faraday cage by virtue of the distance between adjacent electrodes. In the embodiment illustrated in Figures 7 to 8, the electrodes in the set of electrodes are partitioned into different zones (1202, 1204, 1206,1208). The zones are arranged such that each zone extends between points locating approximately within planes in which adjacent-most conducting elements of the antenna locate. In other words, a first zone (1202) of the set of electrodes extends between a plane in which the ground plane locates and a plane in which the first conductive element locates. A next zone (1204) of the set of electrodes extends between the plane in which the first conductive element locates and a plane in which a next conductive element in the stack of conductive elements locates, and so on such that for each space defined between adjacent conducting elements there is a corresponding zone of the set of electrodes. The zones of the electrodes within the set of electrodes correspond to each other. In other words, the different electrodes have the same zones. The zones of each electrode are electrically isolated from each other such that, in use, different potential differences can be applied across different zones of the electrodes. A multi-layered RF harvester with same-dimensioned resonators, along with the arrangements of cylindrical electrodes, is thus provided. In this embodiment, the outer cylindrical electrode of the embodiment described above with reference to Figures 5 to 6 is replaced with multiple cylindricalshaped electrodes (1002, 1004, 1006, 1008, 1010, 1012, 1014, 1016), which may be arranged circularly. The outer electrodes (1002, 1004, 1006, 1008, 1010, 1012, 1014, 1016) and inner electrode (1000) may have the same or different potential differences. Due to the potential difference, the electric field may increase or decrease the effective substrate thicknesses. The advantage of using a plurality of rods instead of a single large cylindrical electrode is that electromagnetic energy (1018,1020) can also enter from the sides. Although the electrodes are illustrated as having a cylindrical shape, any other suitable shape may be employed. Further, each of the electrodes may be divided into n-number of zones or sections, where n is equal to the number of conductive elements such that there is a zone for each conductive element. Each electrode may have multiple zones or sections (1202, 1204, 1206, 1208), which may have the same or different potentials. Adjusting the potential of each section (1202, 1204, 1206, 1208), a single band may be achieved, and all the resonators can be tuned at a desired frequency. Hence the bandwidth of the harvester can be increased in a certain frequency range. As mentioned, in some embodiments each of the different conductive elements of the antenna may be of a different shape and / or size compared to that of the other conductive elements. An example embodiment of an apparatus in which different conductive elements have different sizes is illustrated in Figure 9. In the illustrated embodiment, conductive elements (402, 404, 406, 408) are in the form of annuluses and are arranged concentrically on the same axis. In the illustrated embodiment, the conductive elements are circular in shape, although other embodiments may employ other shapes. In the illustrated embodiments, the conductive elements are split along their respective radiuses to define, for each annulus, a gap (410, 412, 414) between two radially extending edges thereof. The conductive elements may therefore be split ring annuluses, ends of which do not overlap and which do not touch to define a slot or gap. The slots or gaps of respective conductive elements may be aligned. Each conductive element in the stack of conductive elements has a size that increases with increasing axial displacement from the ground plane. The ground plane is in the form of a disc centred on the centre of the conductive element. A radius of the ground plane is larger than a radius of the largest conductive element. In the exemplary embodiment of Figure 9, a harvester with multi-layered and multi-dimensional resonators is disclosed. The multiple resonators (402, 404, 406, 408) are stacked above each other at a certain distance. Slots (410, 412, 414, 416) are defined in the resonators (402, 404, 406, 408). A common ground (418) in the form of a disc-shaped ground plane is used for all the resonators (402, 404, 406, 408). The dimensions of a common ground plane (418) are greater than the largest resonator of the harvester. The dimensions of resonators from top to bottom may increase or decrease based on application. Number of resonators (402, 404, 406, 408) may be increased or decreased according to the required number of bands. The resonators (402, 404, 406, 408) may have a circular ring, rectangular ring, pentagonal ring, hexagonal ring shape or a combination of one or more of these or other shapes. Figure 10 is a chart which illustrates the frequency response of the antenna arrangement shown in Figure 9. Each resonator of the apparatus may have a distinct frequency response (4002,4004, 4006, 4008) with different operating frequencies fa, fb, fc, fn respectively based on the top to the bottom dimensional configuration of resonators (402, 404, 406, 408). Figure 11 is a chart which illustrates the relationship between the frequency range (502) and size or dimension (504) of a conductive element. As the frequency (502) increases from MHz to GHz, a harvester's size (504) decreases. A curve (506) shows the decrease in size while moving from MHz to GHz on the frequency scale. The size of RF components become smaller for increasing frequency of operation. Hence, the change in size of resonator affects its frequency response. Frequency may be changed either mechanically or electrically. In mechanical methods, mostly switches are integrated with the resonator. Upon switching, different frequency response may be achievable. Another method is through electrical tuning, as described herein. Here, applied potential across electrodes determines the frequency response. Hence, there is no physical change in the resonator while changing the frequency response. Aspects of the present disclosure provide an apparatus in the form of a tuneable RF harvester having at least one resonator; at least one substrate and a ground plane; and tuning setup which comprises two or more electrodes. The one or more resonators may have one or more slots which comprise one or more capacitors to attain single or multiband tunning. The slot may have rectangular, square, circular, ring or elliptical shapes. The resonators may have combinations of the different shaped slots. The multiple resonators with different dimensions can share a same substrate and a ground plane. The tuning setup is introduced to interfere the associated electric field of resonators. This interference may change the virtual thickness of substrate. The tuning setup may include two or more electrodes at different potentials. The change in potential changes the electric field and hence the effective thickness of substrate varies which may tune the harvester. The harvester may include a stack of multi-dimensional resonators with a common ground plane and each resonator has one or more variable capacitors in one or more slots. In one embodiment, the tuning setup may comprise the hollow cylindrical electrode outside the stack of resonators and a cylindrical electrode inside the stack. In another embodiment, the tuning setup has multiple cylindrical electrodes outside the stack. The electrodes may be arranged in the form of a circle. The multiple cylindrical electrodes outside the stack may have same or different potential whereas the cylindrical electrode present inside the stack may have different potential compared to each of the outside electrodes. The tuning setup may have multiple electrodes which may have rectangular, curved rectangular structures outside the stack. The electrodes may have same or different distances from each other. The electrodes may have multiple sections and may have the same or different potential to provide the different magnitudes of interference to each resonator present in the stack. The electrodes may act as a detuned faraday cage. The change in the potential difference of cylindrical electrodes, present outside and inside of stack, may be utilized for single or multiple bands tuning. The stack of resonators may have one or more combinations of circular, rectangular, pentagonal, or hexagonal ring shapes. Embodiments of a tuneable RF harvester are thus described in which the tunability is achieved by changing an “effective” or “electrical” substrate thickness between a resonator (e.g. provided by a first and / or further conductive elements) and the ground. The terms “effective substrate thickness,” “effective thickness of the substrate” and the like mean the electrically effective substrate thickness derived from changing electric field characteristics and a potential difference applied across the electrodes. Changes in effective substrate thickness may result in the variation of a resonance frequency. Further, two or more different dimensional conductive elements with the same ground plane and which resonate in two or more distinctive operating frequencies can be employed. Each resonator may be configured for a certain frequency corresponding to its dimensions. Aspects of the present disclosure represent the RF harvester comprising multilayered and multi-dimensional resonators with a common ground. Due to the multi-dimensional characteristic, each resonator may have its resonance frequency providing multi-band. Variation of the effective thickness of the substrate may be effected by changing an electric field strength through the substrate. By varying the potential across two electrodes on either side of the resonator, the effective thickness of the substrate may be increased or decreased as the infinity point of the electric field is reformed. The effective thickness of a substrate is changed as a function of applied voltages on the electrodes of a tuning setup. Hence, the substrate thickness is adjusted electronically, and the harvester is tuned as per requirement. Infinity point for any electric field may be considered as a point where electric field eventually becomes zero. If the field is travelling for an infinite amount of distance, it will eventually die off. But this infinity point can be reformed by applying external electric field. If a destructive interference between internal and external electric field occurs, it makes electric field zero thus changing its infinity point. In this context, the effect of constructive or destructive interference on substrate thickness is described with the help of infinity point. The harvester with multi-layered and same-dimensioned resonators may include one or more cylindrical electrodes inside and outside the harvester. By applying the potential across electrodes, the infinity point of resonators may be changed using constructive or destructive interference. This method may be used to tune the harvester comprised of multi-layered, same, or different-dimensioned resonators. To avoid the RF or Faraday cage problem, multiple same-dimensioned cylindrical-shaped electrodes are placed outside the harvester. The distance between two electrodes should be greater than A / 4. The outer electrodes of the tuning setup may have cylindrical, square, rectangular, trapezoidal, or curved rectangular structures. In use, under the effect of applied potential, the electrodes will produce a strong electric field that will interfere (constructively or destructively) with the resonators. In embodiments of the present disclosure, multiple electrodes interface with one or more resonators through an electric field generated between inner and outer electrodes. The resonator is placed between the inner and outer electrodes. The electric field due to the resonator is interfered with the electric field between the inner and outer electrodes. The constructive or destructive interference between two fields determines an increase or decrease in virtual or effective substrate thickness. The apparatus described herein harvests energy from electromagnetic waves using resonators. Instead of using electromechanical parts or particular types of dielectric material to achieve a shift in resonance frequency, the apparatus described herein uses electrical interference for tuning. The resultant effect mimics the effect that can be achieved with change in the thickness and hence phenomenon is named as effective thickness of the substrate. Finally, the tunability of the apparatus described herein can be used to capture electromagnetic waves of different frequency ranges. The foregoing description has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the 5 above disclosure. The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the invention be limited not by 10 this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims. Finally, throughout the specification and accompanying claims, unless the context requires 15 otherwise, the word ‘comprise’ or variations such as ‘comprises’ or‘comprising’ will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

1. An apparatus comprising an antenna and a set electrodes, the antenna including a first conductive element spaced apart by a distance from and arranged parallel to a ground plane, wherein the set of electrodes is arranged such that a first electrode of the set of electrodes is spaced apart from a second electrode of the set of electrodes and such that a first zone of each of the first and second electrodes extends substantially from a plane in which the ground plane locates to a plane in which the first conductive element locates, wherein the first and second electrodes are arranged such that the first conductive element extends between the first electrode and the second electrode, and wherein, in use, a potential difference applied across the first zones of first electrode and the second electrode is adjustable in turn to adjust a resonant frequency of the antenna.

2. The apparatus as claimed in claim 1, wherein the first conductive element is shaped so as to curve over on itself to define an opening, and wherein the first electrode locates within the opening defined by conductive element and the second electrode locates outside of the conductive element.

3. The apparatus as claimed in claim 2, wherein ends of the first conductive element are coplanar and spaced apart from each other to define a gap therebetween.

4. The apparatus as claimed in any one of claims 1 to 3, wherein the set of electrodes includes the first electrode and a plurality of second electrodes spaced apart from each other around and outside of the first conductive element5. The apparatus as claimed in any one of claims 1 to 3, wherein the second electrode is in the form of a hollow cylinder which surrounds the first conductive element.

6. The apparatus as claimed in any one of claims 1 to 5, wherein the first conductive element is in the form of an annulus, wherein the annulus is split along a radius thereof to define a gap between two radially extending edges thereof, wherein the ground plane is in the form of a disc and wherein the disc is centred on the centre of the annulus.

7. The apparatus as claimed in any one of the preceding claims, including one or more further conductive elements, wherein each further conductive element is spaced apart from and parallel to the other conductive elements.

8. The apparatus as claimed in claim 7, wherein each conductive element is of the same shape and size.

9. The apparatus as claimed in any one of claims 7 or 8, wherein each electrode in the set 5 of electrodes includes a further zone for each further conductive element, wherein each further zone of each electrode extends between respective planes in which adjacent conductive elements locate, and wherein the zones of each electrode are electrically isolated from each other such that, in use, different potential differences can be applied across different zones of the electrodes.1010. The apparatus as claimed in any one of the preceding claims, wherein a substrate is disposed between the ground plane and the first conductive element.

11. The apparatus as claimed in any one of claims 7 to 10, wherein a substrate is disposed 15 between of the further conductive elements.

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