Variable impedance matching networks for stretchable antennas

One example device having a variable impedance matching network includes a flexible substrate configured to flex in at least an X dimension and a Y dimension, wherein the X and Y dimensions are orthogonal and coplanar with a surface of the flexible substrate; an antenna formed on the flexible substrate, the antenna deformable in at least one of the X or Y dimensions, wherein an inductance (“LA”) of the antenna varies based on a flexing of the flexible substrate in the at least one of the X or Y dimensions, an impedance matching network formed on the flexible substrate and coupled to the antenna, the impedance matching network deformable in the at least one of the X or Y dimensions, and wherein a capacitance (“CM”) or inductance (“LM”) of the impedance matching network is configured to vary (i) based on the flexing of the flexible substrate in the at least one of the X or Y dimensions and a separation between two portions of the impedance matching network, and (ii) in proportion to the varying of LA to maintain a substantially constant impedance of the combination of the impedance matching network and the antenna.

FIELD

The present disclosure relates to variable impedance matching networks for stretchable antennas.

BACKGROUND

Wireless devices can use antennas to transmit and receive radio frequency (“RF”) information to other devices. To efficiently communicate using an antenna, an RF transmitter (or receiver) may be connected to the antenna via electronic components arranged to match an impedance between the RF transmitter and the antenna. Such arrangements may be referred to as “matching networks” or “impedance matching networks.” If a matching network is not used, or the RF transmitter's impedance does not otherwise match the antenna's impedance, some of the power transmitted by the RF transmitter to the antenna may be lost or signal reflection may occur, resulting in reduced performance.

SUMMARY

Various examples are described for variable impedance matching networks for stretchable antennas. One example device includes a flexible substrate configured to flex in at least an X dimension and a Y dimension, wherein the X and Y dimensions are orthogonal and coplanar with a surface of the flexible substrate; an antenna formed on the flexible substrate, the antenna deformable in at least one of the X or Y dimensions, wherein an inductance (“LA”) of the antenna varies based on a flexing of the flexible substrate in the at least one of the X or Y dimensions, an impedance matching network formed on the flexible substrate and coupled to the antenna, the impedance matching network deformable in the at least one of the X or Y dimensions, and wherein a capacitance (“CM”) or inductance (“LM”) of the impedance matching network is configured to vary (i) based on the flexing of the flexible substrate in the at least one of the X or Y dimensions and a separation between two portions of the impedance matching network, and (ii) in proportion to the varying of LAto maintain a substantially constant impedance of the combination of the impedance matching network and the antenna.

One example method for providing a variable impedance matching network may include determining an inductance (“LA”) for an antenna; determining an expected range of strains for the antenna based on predetermined a flexible substrate, the flexible substrate configured to flex in at least an X dimension and a Y dimension, wherein the X and Y dimensions are orthogonal and coplanar with a surface of the flexible substrate; determining changes in L based on the expected range of strains on the flexible substrate; determining a configuration of an impedance matching network based on the determined changes in L and the expected range of strains, wherein a capacitance (“CM”) or an inductance (“LM”) of the impedance matching network is configured to vary (i) based on a flexing of the flexible substrate in the at least one of the X or Y dimensions, and (ii) in proportion to the determined changes in LAto maintain a substantially constant impedance of the combination of the impedance matching network and the antenna; forming the antenna on a flexible substrate; forming the impedance matching network on the flexible substrate, the impedance matching network having the determined configuration; and coupling the antenna to the impedance matching network.

These illustrative examples are mentioned not to limit or define the scope of this disclosure, but rather to provide examples to aid understanding thereof. Illustrative examples are discussed in the Detailed Description, which provides further description. Advantages offered by various examples may be further understood by examining this specification.

DETAILED DESCRIPTION

Examples are described herein in the context of variable impedance matching networks for stretchable antennas. Those of ordinary skill in the art will realize that the following description is illustrative only and is not intended to be in any way limiting. Reference will now be made in detail to implementations of examples as illustrated in the accompanying drawings. The same reference indicators will be used throughout the drawings and the following description to refer to the same or like items.

Wearable devices, including wearable or implantable medical devices, such as continuous glucose monitors or neural stimulators, may include one or more antennas to enable wireless communications between the wearable device and another device, such as a smartphone or handheld reader device. To provide a wearer with a more comfortable wearable device, a portion of the device that contacts the wearer's skin may be at least partially constructed of a flexible material that can flex as the wearer goes about their day. In some cases, an antenna may be affixed to such a flexible material and thus may also flex and deform in shape as the flexible material flexes and deforms.

However, because antenna performance can be sensitive to changes in the antenna's shape and size, the antenna's performance may be affected when it is flexed. In particular, the shape and size of the antenna may affect its inductance, and consequently its impedance. Thus, as the antenna changes shape and size, its impedance may change as well, and an impedance matching network used to match the antenna's impedance with other RF circuitry may be rendered partially ineffective as result.

To overcome this problem, an antenna formed on, or otherwise affixed to, a flexible material, such as a flexible printed circuit board (“PCB”) material, may be connected to other RF circuitry via an impedance matching network that is also formed on the flexible material. In this example, the antenna is a loop antenna and is connected to an impedance matching network that includes an interdigital capacitor (“IDC”). An IDC is formed of two opposing wire traces, each of which having complementary protrusions to create complementary comb-like shapes. The IDC's capacitance is based on the size, shape, and thickness of the two wire traces and protrusions, as well as the gap between them. In this example, both the antenna and the IDC are formed on the flexible material. The IDC is formed on the flexible material by depositing two wire traces, including respective protrusions, on the flexible material. One of the wire traces connects the IDC to the antenna, while the other connects the IDC to other circuitry, such as a wireless transceiver.

When the flexible material later flexes, the antenna's shape changes, causing a change in its impedance as discussed above. However, because the IDC is formed directly on the flexible material, its shape will change as well. Because the IDC's capacitance is based on its shape, including the lengths of one or more of the protrusions and the spacing between the protrusions and respective wire traces, as the IDC changes shape, its capacitance changes as well. Thus, the IDC can be used as a variable capacitance that changes based on the amount of flex applied to the flexible material. Since both the IDC and the antenna are formed on the flexible material, their respective changes in shape, and consequent changes in electrical response, may substantial cancel each other, thereby maintaining a substantially constant impedance. As a result there is less variation in the antenna's efficiency as it flexes.

This illustrative example is given to introduce the reader to the general subject matter discussed herein and the disclosure is not limited to this example. The following sections describe various additional non-limiting examples and examples of systems and methods for variable impedance matching networks for stretchable antennas.

Referring now toFIG. 1,FIG. 1shows an example antenna110formed on a flexible substrate100, such as a flexible PCB. In this example, the antenna110is a loop antenna with a radius of 5 millimeters (“mm”) and a wire trace width of 1 mm using wires 1 mm thick, though any suitable antenna may be used. Because the loop antenna110is two-dimensional, it is susceptible to deformation in two degrees of freedom, labeled with the x- and y-axes. Deformation in a third dimension, the z-axis, can also occur as a result of stretching such that the antenna wire itself stretches and get thinner, though in many cases this deformation can be ignored. The orientation of these two (or three) axes is arbitrary so long as they are all orthogonal to each other. A loop antenna's inductance varies according to a change in the area within the loop. When the antenna110flexes, its shape and size, and therefore the area, changes, changing its inductance. The flex may happen in either or both degrees of freedom, resulting in various changes in the antenna's inductance.FIG. 2shows a graph illustrating an example loop antenna's inductance changing as a function of strain in response to an applied flexing of the antenna. As can be seen, the antenna's inductance increases as the strain increases. A change in the antenna's inductance causes a corresponding change to the impedance presented to other circuitry, potentially reducing the antenna's efficiency, as discussed above.

Referring now toFIGS. 3A-3B,FIG. 3Ashows an example loop antenna310formed on (or otherwise affixed to) a flexible substrate300. The antenna310is coupled to an impedance matching network that includes a capacitor320that is also formed on the flexible substrate300. InFIG. 3A, both the antenna310and the capacitor320are at rest. The term “at rest” is used to refer to the antenna310and capacitor320when the flexible substrate300is not experiencing any deformation, and thus the antenna310and the IDC320are not deformed.

In this example, the capacitor320has been sized to match the antenna's impedance with other RF circuitry. Thus, at rest, each of these components has its designed at-rest inductance or capacitance, respectively.

FIG. 3B, however, illustrates the antenna310and the capacitor320when the flexible substrate300has been stretched in the X axis, thereby deforming both the antenna310and the capacitor320. As can be seen, the size of the antenna310has changed due to the stretching, which has thereby changed the antenna's inductance. However, because the capacitor320is also formed on the same flexible substrate300, the capacitor320also experiences a deformation in the X-axis, which increases the gap between the two wires of the capacitor320, thereby reducing its capacitance. Thus, while the antenna's inductance increases, the capacitor's capacitance has decreased a similar amount, thereby helping to compensate for the inductance change and to reduce the impact on the circuit's impedance. In some examples, the capacitor may be designed to substantially eliminate any impedance change, though in some examples, the capacitor320may be designed to reduce an amount of impedance change below a predefined tolerance threshold.

While the example shown inFIGS. 3A-3Billustrate a stretch in one dimension using a single capacitor, other examples may be employed to compensate for stretching in another dimension or in multiple dimensions, including stretching, bending, or and flexing.

Referring now toFIG. 4A,FIG. 4Ashows an example antenna410(having the same configuration as the antenna110ofFIG. 1) coupled to impedance matching that includes an IDC420, both of which are formed on a flexible substrate400. The antenna410is a loop antenna in this example, and the IDC420provides a capacitance, which in combination with the antenna's inductance, provides an impedance to other radio frequency (“RF”) circuitry, such as a transceiver. In general, the impedance of the combination of the antenna410and the IDC420(at rest) will be selected to match an impedance of such RF circuitry to provide efficient power transfer and reduce signal reflections within the antenna410or circuitry. In other words, the impedance matching enables efficient use of the antenna410by the RF circuitry.

Once a desired impedance is determined, the antenna's inductance and the IDC's capacitance can be designed accordingly. The IDC's capacitance can be established using known design techniques, and can be based on the length and number of protrusions within the IDC420as well as a gap between the two portions of the IDC420, as well as by the substrate material, or other dielectric material, within the gap. However, because both the antenna410and the IDC420are expected to deform as the flexible substrate400flexes, the expected amount of deformation of each can be used to appropriately design the IDC420to counter deformation of the antenna410, including the size (width and length) and number of protrusions, the gap between the protrusions, etc. Specifically, as the antenna's inductance increases due to flexing of the substrate400, the IDC's capacitance should decrease.

FIG. 4Bshows the IDC420shown inFIG. 4A. This example IDC420includes two wire traces422a-b, each with five protrusions424(or “fingers”). The wire traces, including the protrusions, have a width of 1.1 millimeters (“mm”) The protrusions424are each 5 mm long and the gap426is 2 mm.FIG. 5illustrate the changes in capacitance of the example IDC420shown inFIGS. 4A-4Bas the IDC420responds to deformation. As can be seen, and in contrast to the inductance change shown inFIG. 2, the capacitance decreases as the IDC is deformed. For example the deformation may increase the size of the gap426between protrusions424, or may increase the length of the wire traces422a-bor protrusions424, thereby reducing the capacitance. This is in contrast to a matching network constructed from discrete electronic components soldered to a PCB whose electrical properties do not change as the flexible substrate flexes.

FIG. 6shows a graph illustrating how the antenna's resonant frequency changes as a result of deformation. The first plot610illustrates the impact on resonant frequency as the antenna410shown inFIG. 4Aexperiences increasing amounts of strain. As can be seen, the resonant frequency decreases from approximately 402 megahertz (“MHz”), the frequency at which communications are to be conducted, to approximately 325 MHz with increasing strain, a change of approximately 19%. This will likely result in significantly reduced efficiency.

The second plot620illustrates the change in resonant frequency of the antenna410shown inFIG. 4Ain conjunction with the IDC420as both experience strain. As can be seen, the change in resonant frequency is significantly less due to the change in capacitance of the IDC420as it experiences strain. In this example, the resonant frequency increases from approximately 402 MHz to a maximum of approximately 435 MHz, a change of approximately 8%. In addition, as can be seen in the first plot610, the change in resonant frequency is approximately linear, thus, as strain increases, resonant frequency continues to decrease at a substantially constant rate. However, in the second plot620, the resonant frequency increases to approximately 430 MHz at 0.5 strain, approximately 7%, but then only slightly increases to a maximum of 435 MHz at approximately 0.875 strain, an additional increase of only about 1%, before beginning to decrease again. Thus, the example shown inFIG. 4Ais both less affected by flexing overall than the system inFIG. 1, but also tolerant of significant flex or deformation (e.g., when strain >0.5).

Referring now toFIG. 7,FIG. 7shows an example antenna710and impedance matching network720formed on a flexible substrate600. In this example, the antenna is again a loop antenna. However, the impedance matching720in this example includes two capacitors722a-bformed on the flexible substrate. Each capacitor722a-bis formed by two wires separated by a gap. However, the capacitors722a-bare formed such that the wires forming one capacitor722aare orthogonal to the wires forming the other capacitor722b. Thus, each capacitor722a-bis designed so that the gap between its wires changes in response to flex in a direction orthogonal to the other, thereby allowing the impedance matching to change its capacitance, due to a change in the width of the gap for a respective capacitor, based flexing of the substrate in one or two degrees of freedom. Capacitance also changes based on the length of the respective wires, thus, each capacitor722a-bwill change capacitance irrespective of the direction of the flexing; however, because each of these quantities can be accounted for, sizing the respective capacitors for a particular application can be accomplished according to known equations defining the capacitance of a parallel-plate capacitor.

Similar principles as those discussed above can be used to accommodate flexing of other types of antennas. For example,FIG. 8shows a substantially one-dimensional antenna with a length significantly greater than its width. In this example, a single parallel-plate capacitor820is formed on the flexible substrate to compensate for changes in the length of the antenna. It should be appreciated, however, that the capacitor820may be oriented to compensate for changes in the width of the antenna, or an impedance matching having two capacitors820, such as shown inFIG. 6, may be employed instead.

Referring now toFIG. 9A,FIG. 9Ashows an example antenna910and impedance matching network920formed on a flexible substrate900. In this example, the impedance matching network920includes an inductor930having a flattened coil shape. The inductor930is formed on the flexible substrate substantially described as above. As discussed above with respect to capacitors and antennas, the inductance of the inductor930may change as the flexible substrate flexes, thereby changing the size and shape of the inductor930. In this example, the coil shape of the inductor930allows for stretching or flexing in two DOFs, though as discussed above, the thickness of the metals trace defining the inductor may change as the inductor is flexed or stretched, further affecting its inductance.

An inductance of such a coil inductor may be established based on various physical parameters of the inductor. Physical parameters of the inductor930are shown inFIG. 9B. The inductance (“L”) of the inductor930may be determined according to the following equations:

Thus, as the inductor experiences strain, the value of davgwill increase, increasing the inductance, L, of the inductor930.

While in the example shown inFIG. 9A, the inductor930is formed as a coil, any suitable shape may be employed. Further, while only one inductor930is employed in this example, any suitable number of inductors may be employed according to different examples. Thus, as can be seen inFIG. 9, impedance matching networks according to this disclosure may employ inductors rather than capacitors to provide impedance matching between an antenna and RF circuitry using the antenna to transmit or receive RF signals.

Referring now toFIG. 10,FIG. 10shows an example antenna910and impedance matching network1020formed on a flexible substrate1000. In this example, the impedance matching network1020includes both an inductor1030and an IDC1040formed on the flexible substrate1000. Thus, as the flexible substrate flexes, the impedance of the impedance matching network1020may change to maintain a substantially constant impedance for the RF circuitry using the antenna1010to transmit or receive RF signals generally as discussed above. The embodiment shown inFIG. 10illustrates another example impedance matching network1020that combines both capacitors and inductors. And while this example includes a coil inductor1030and an IDC1040, any suitable type and number of inductors or capacitors may be employed.

Referring now toFIG. 11,FIG. 11shows a method1100for variable impedance matching networks for stretchable antennas. The discussion of the example method1100will be made with respect to the example antenna410and IDC420shown inFIGS. 4A-4B, but it should be appreciated that the method may be employed with respect to any suitable antenna and variable impedance matching network according to this disclosure, such as shown inFIGS. 3A-3B, or7-10.

At block1110, an inductance (“LA”) for the antenna410is determined. In this example, the inductance is determined while the antenna is at rest, though the inductance may also be measured at various amounts of strain, as will be discussed in more detail below. Antenna inductance may be determined according to any suitable technique. For example, the inductance may be determined mathematically according to a design, e.g., based on a size and shape, of the antenna410. In some examples, the antenna may be measured using an electronic device, such as an impedance meter. In some examples, antennas410may be coil antennas having diameters between substantially 1 mm and 1 centimeter (“cm”). However, any suitable size or shape may be employed for an antenna.

At block1120, a range of strains for the antenna410is determined based on a predetermined flexible substrate. In this example, the flexible substrate400is a flexible PCB having a polyimide composition, but may be any suitable flexible substrate may be employed, such as silicones, polyesters, PEEKs (polyether ether ketone), etc. Such flexible substrates400have well-known flex characteristics that can be modelled or measured based on a particular application. A range of strains may be determined based on an expected range of flexing of the flexible substrate in a particular application. For example, a medical device affixed to a wearer's chest may experience flexion of 10-20% in two dimensions as the wearer breathes. In some examples, the substrate may experience more significant amounts of flexion, including stretching up to 100% in one or more DOFs. Such range of strains may be determined based on an expected application for the antenna and associated device.

At block1130, change in the antenna's inductance (“ΔL”) over the range of strains is determined. The change in inductance may be measured as the antenna is stretched in one or more DOFs. In some examples, the change in inductance may be modelled. For example, the graph shown inFIG. 2illustrates a modelled change in inductance over a range of strains.

At block1140, a configuration for an impedance matching network is determined based on the change in the antenna's inductance. In this example, the impedance matching network is configured to vary in capacitance (“C”) based on flexion of the flexible substrate and to vary in proportion to the determined ΔL to maintain a substantially constant value for L*C as the substrate flexes. In some examples, “substantially constant” may be established within a tolerance threshold. For example, a tolerance of a 10% variation in the value of L*C as the flexible substrate may be established in some examples. However, the tolerance may be established based on system requirements, such as efficiency requirements, a maximum transmit power, a communications range, etc.

In one example, a type of capacitor or capacitors is determined. For example, one or more parallel-plate style capacitors may be employed. An example parallel-plate capacitor320is shown inFIGS. 3A-3B. As shown in the example ofFIG. 7, multiple parallel-plate capacitors722a-bmay be employed, each oriented along one DOF. In this example, the configuration for the impedance matching network is the IDC420shown inFIGS. 4A-4B.

Capacitance (“C”) for an IDC may be calculated based on the lengths of the wire traces422a-band the protrusions424as well as the gap426between the protrusions424and wire trace422a-b. Such a capacitance may be determined or modelled as described in Igreja, Rui, and C. J. Dias. “Analytical evaluation of the interdigital electrodes capacitance for a multi-layered structure.”Sensors and Actuators A: Physical112.2: 291-301, 2004. Thus, a configuration of an IDC may be determined by selecting a number of protrusions, the width of the protrusions, a length of the wire traces or protrusions, a gap between protrusions and associated wire traces, etc. In addition, material properties may be determined, such as a conductive materials, e.g., copper, and a thickness of such a material. For example, a thickness of the conductive material may help determine an amount of shape change due to strain of the impedance matching network. Further, an IDC configuration may be selected based on the range of strains determined at block1130. For example, if ΔL in a first DOF is twice the ΔL in a second DOF based on the range of strains, the IDC may be designed to achieve a larger change in capacitance (“ΔC”) in the first DOF than the second DOF. Further, when stretched or flexed, the conductive material may stretch and get thinner at a particular location, which may provide a second or third DOF. Thus, a thickness of one or more portions of the IDC (or other capacitor types) may be selected based on the range of strains determined at block1130.

In some examples, as discussed above, the impedance matching network may include one or more parallel-plate capacitors. The capacitor(s) may be oriented to change in capacitance based on flexion of the flexible substrate. For example, a capacitor may be oriented such that flexion or stretching in a DOF increases a gap between the two plates by orienting the plates orthogonally to the DOF. Such an orientation may reduce the capacitance of the capacitor when stretched because capacitance decreases in proportion to the increase in the gap. Thus, to accommodate two DOFs, two capacitors may be employed, one oriented with respect to each of the respective DOFs, such as shown inFIG. 7. And while the example inFIG. 7employs two capacitors, any suitable number of capacitors may be employed. Thus, the capacitor(s) may vary in capacitance in two or more DOFs.

As discussed above with respect toFIGS. 9A-9B and 10, an impedance matching network may employ one or more inductors. Thus, at block1140, in some examples, an inductance (“LM”) may be configured to change based on a flexing or stretching of the flexible substrate to compensate for expected changes in the antenna's inductance to provide a substantially constant impedance to other RF circuitry. Further, the impedance matching network may include both capacitive and inductive elements, which may be selected as discussed above to compensate for expected changes in the antenna's inductance to provide a substantially constant impedance to other RF circuitry.

At block1150, the antenna410is formed on the flexible substrate. In this example, a wire trace is deposited on the flexible substrate according to the determined shape and size of the antenna. In some examples, the inductance of the antenna may be measured, as discussed above with respect to block1110, after it has been formed.

At block1160, the impedance matching network is formed on the flexible substrate. In this example, the wire traces and protrusions of the IDC420are formed by depositing a conductive metal, e.g., copper, on the flexible substrate. While a conductive metal is employed in this example, any suitable conductive material may be employed. Further, in this example, no dielectric material is deposited on the flexible substrate; however, in some examples, a dielectric material may be deposited within a gap between the electrodes of the capacitor, or a portion of the flexible material may be cut away, e.g., by laser cutting, to provide air as a dielectric material between the two conductors of the capacitor. While this example employs an IDC, any suitable capacitor (or capacitors) may be formed on the flexible material, including parallel-plate capacitors, such as those shown inFIGS. 7-8. Further, examples, such as shown inFIGS. 9A and 10may include one or more inductors that may be formed on the flexible substrate.

At block1170, the antenna is coupled to the impedance matching network. In this example, a metal trace is deposited on the flexible material to couple the antenna410to the IDC420. In other examples, a metal trace may be deposited on the flexible material to couple the antenna410to an impedance matching network formed of one or more capacitors or one or more inductors.

While the method1100shown inFIG. 11has been described as being performed in a particular order, not such order should be implied by the Figure or the description. Instead, the steps may be performed in any suitable order. For example, block1150may be performed before block1110such that the antenna is formed and then its inductance measured. Still further variations may be employed according to different examples.