WO2012131376A1

WO2012131376A1 - Apparatus and methods

Info

Publication number: WO2012131376A1
Application number: PCT/GB2012/050706
Authority: WO
Inventors: Christopher Lowe; Adrian Stevenson; Kalyan Sarma; Daniel Joseph O'NEILL
Original assignee: Camtech Investments Limited
Priority date: 2011-04-01
Filing date: 2012-03-29
Publication date: 2012-10-04
Also published as: CN103703613A; GB201105569D0; GB2489524A; EP2695239A1

Abstract

This invention relates to electroacoustic antennas. We describe an electromagnetic wave antenna, an electromagnetic wave antenna, the antenna comprising: a bar of piezoelectric material having a length and a cross-section perpendicular to said length wherein said length is at least five times a lateral dimension of said cross-section of said bar; first and second electrodes deposited on said bar of piezoelectric material, spaced apart along said length of said bar; and wherein said bar of piezoelectric material has an inherent polarisation in a longitudinal direction along said length of said bar.

Description

Apparatus and Methods

FIELD OF THE INVENTION This invention relates to electroacoustic antennas. BACKGROUND TO THE INVENTION

An antenna made of piezoelectric material is described in WO2009/081089. However there is a prejudice in the art towards using piezoelectric material in the form of a generally planar plate. Whilst Figure 8b of '089 shows a yagi-type antenna the piezoelectric rods provide resonant elements and are not optimally driven.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is therefore provided an electromagnetic wave antenna, the antenna comprising: a bar of piezoelectric material having a length and a cross-section perpendicular to said length wherein said length is at least five times a lateral dimension of said cross-section of said bar; first and second electrodes deposited on said bar of piezoelectric material, spaced apart along said length of said bar; and wherein said bar of piezoelectric material has an inherent polarisation in a longitudinal direction along said length of said bar.

In embodiments providing a pair (or more) of electrodes spaced apart along the length of the bar facilitates a differential drive and efficient operation of the antenna for either transmission or reception of electromagnetic signals. In some preferred embodiments the spacing of the electrodes is chosen to substantially match to an impedance of a circuit to which the antenna is coupled, for example approximately 50 ohms or 75 ohms (the skilled person will appreciate that by varying separation between the electrodes the effective voltage across the electrodes may well also be varied).

Optionally in embodiments 3 or more electrodes may be employed to couple to the piezoelectric bar, again for improved coupling/efficiency. In embodiments because of a phase delay between these electrodes one or more of the electrodes may be provided with a phase matching element (for example a delay line) and/or the electrodes may be coupled to different taps of a transformer or balun. Such arrangements facilitate phase matching which can be difficult with a high Q antenna.

In embodiments the aspect ratio of the bar of piezoelectric material (that is the ratio of the length of the bar to a maximum lateral cross section dimension) is at least 10:1 , 20:1 or 50:1 . In general the cross section of the bar may be of any shape, but in embodiments is generally square, rectangular, circular or oval. (The skilled person will appreciate that here "bar" includes shapes such as rods, wires and whiskers). In some embodiments a plurality of pairs of electrodes may be disposed at intervals, preferably regular intervals, along the length of the bar. In embodiments electrodes of opposite polarity are interleaved. This facilitates differential driving/signal amplification.

In some preferred embodiments a layer of bi-electric material is provided on one or more longitudinal surfaces or faces of the piezoelectric bar, for example a layer of polymer such as PMMA. This effectively withdraws charge to the surface of the piezoelectric bar and thus facilitates efficient coupling of the antenna to electromagnetic radiation. Optionally an additional layer of metallisation may be provided at one or both ends of the bar to produce a sharper (acoustic) impedance transition, again for increased efficiency. In embodiments the antenna is supported at one or more locations along the bar at which acoustic vibrations are substantially at a minimum.

In embodiments the high aspect ratio of the piezoelectric bar naturally enhances the Q of the antenna. The high Q enables the antenna to be employed in a circuit with little or no RCL resonant components, that is in a receiving and/or transmitting circuit in which the resonant frequency of operation or tuning of a circuit is controlled or dominated by the resonant frequency of the antenna. Potentially any piezoelectric material may be employed. Examples of suitable materials include, but are not limited to: quartz, aluminium nitride, zinc oxide, diamond and others.

In embodiments because the piezoelectric material is relatively electrical non- conductive, many antennas may be located close to one another, for example within a distance of one wavelength of the electromagnetic radiation, without substantially mutually affecting one another by de-tuning or in some other disadvantageous way. Thus for an increased overall signal level output a plurality of the antennas may be arranged in a two- or three-dimensional array. In embodiments multiple to the arrays may be stacked with different, preferably substantially orthoganol orientations of the antennas in different layers. Additionally or alternatively multiple orthoganol piezoelectric bar antennas may be combined for receiving/transmitting multiple polarisations. In embodiments the length of the bar of piezoelectric material is an integral number of path wavelengths of a wavelength of an acoustic wave travelling longitudinally in the bar of piezoelectric material and having a frequency of the transmitted and/or received electromagnetic wave. Depending upon the material, the wavelength for a 100 MHz electromagnetic (EM) wave may be of order 50 μηι, but to facilitate fabrication the length of the antenna may be n half wavelengths where n is at least 5, 10, 20, 30, 40, 50, 100 or more. Thus the length of the antenna may be of order mm, substantially facilitating fabrication of the antenna whilst nonetheless being much smaller than a conventional metal quarter wave dipole, and whilst also retaining other advantages of the antenna including, for example, the extremely high Q.

Thus in another aspect the invention provides a method of receiving or transmitting an electromagnetic (EM) wave, the method comprising coupling said electromagnetic wave to a bar of piezoelectric material such that said EM wave induces longitudinal acoustic vibrations in said bar of piezoelectric material, and wherein a length of said bar of piezoelectric material is substantially equal to an integral number of half wavelengths of said longitudinal acoustic vibrations at a frequency of said EM wave.

In embodiments the longitudinal axis is selected such that it defines a direction of substantially maximum inherent polarisability of the piezoelectric material. In embodiments the length of the bar is at least 0.01 mm, 0.1 mm or 1 mm. At a resonant frequency of operation the antenna may have a Q of at least 10³, preferably at least In a further aspect the invention provides a method forming an electroacoustic wave antenna using a piezoelectric material, the method comprising: exciting a surface wave pattern of charge in the piezoelectric material, said pattern of charge extending laterally in two dimensions; and using said pattern of charge to form said electromagnetic wave antenna.

The invention also provides a piezoelectric antenna, the antenna comprising: a substrate of piezoelectric material; at least one set of electrodes for exciting a surface wave pattern of charge in said piezoelectric material, said pattern of charge extending laterally in two dimensions; and means for coupling a received or transmitted electromagnetic wave signal into or out of said antenna using the same or additional said electrodes, wherein said pattern of charge forms said antenna.

In some preferred embodiments the surface wave pattern of charge is a standing wave pattern of charge, in particular a substantially stationary pattern of positive and negative charge distributed over the 2D surface of the piezoelectric material. In some embodiments the charge distribution forms stripes in one direction on the surface, but in others the surface standing wave is spatially periodic in the two lateral dimensions of the surface. In embodiments the standing wave pattern of surface charge may be generated by interdigitated electrode fingers and the configuration of these electrodes may be controlled to control the pattern of surface charge, for example apodizing an electrode and/or controlling an electrode width. The skilled person will be aware of various types of electrode configuration which may be employed, for example a dispersive configuration used for a chirp filter. The means for coupling a received or transmitted electromagnetic wave signal into or out of the antenna may comprise, for example, a radiofrequency coupling element such as a capacitor connected to an electrode driving acoustic vibrations in the piezoelectric material; the skilled person will be aware of many alternatives. In embodiments the electrodes excite acoustic waves from one or a pair of (adjacent) lateral edges, to excite acoustic waves in one direction or a pair of, in embodiments orthogonal, directions to generate a 2D surface pattern of charge or, in effect, plasma (the excited charge may be viewed as a plasma). As previously mentioned the acoustic wave length defining the separation of charged regions may be of order 10⁵ -10⁵ times smaller than the electromagnetic wave wavelength, as can be appreciated from velocity = frequency x wavelength. Thus standing waves may be generated in either an X or Y direction (these being directions in a lateral surface plane of the device), interference between the standing waves generating "carpet" patterns of charge. (A standing wave may be generated by launching a travelling wave in the surface in a particular direction).

A periodic distribution of charge on the surface may thus be defined and controlled, for example by controlling electrode configuration and/or electrode stimulation. This pattern of charge may therefore be fixed or variable/controllable. In embodiments the standing wave pattern of charge defines a phased array antenna, and in this way the antenna may be provided with one or more directional responses controlled by the pattern of charge generated. Broadly speaking the pattern of charge may be considered as an image of an antenna distribution. In preferred embodiments the surface waves comprise a surface wave mode of the piezoelectric material, but in other embodiments the surface wave may be generated in a membrane of piezoelectric material - the desirable result is that the energy should be primarily confined at the surface of the piezoelectric material because in effect energy in the volume of the material is wasted.

In an alternative approach the surface wave pattern may comprise a temporally varying 2D distribution of charge, with a frequency which substantially matches that of an electromagnetic wave to be transmitted or received. This may be achieved for example by employing long wavelength waves or by launching waves from a lower surface or by exciting the piezoelectric material from behind. This temporally varying charge distribution may be considered as a piston which drives or receives an electromagnetic wave. The width of the distribtution is, broadly speaking, defined by the width of an underlying electrode; the thickness of the material may be arranged to match resonance modes. In embodiments the material may be provided with a ground plane, for example around the piezoelectric material, or alternatively this may be provided by the environment in which the piezoelectric antenna is employed. The size of the pattern may be larger or smaller than the acoustic wavelength but in general will be smaller than the electromagnetic wavelength. A range of shapes of the excited pattern may be employed including, but not limited to, a circular, square or rectangular or oval pattern. In embodiments the surface of the piezoelectric material may be curved, for example concave or convex, to direct the radiation transmission/reception pattern. The antenna may be used to transmit electromagnetic radiation or to receive an electromagnetic wave (example detecting by detecting a change or modification in the spatial distribution of charge on the piezoelectric material).

DESCRIPTION OF DRAWINGS AND PREFERRED EMBODIMENTS These and other aspects of the invention will now be further described, by way of example only with reference to the accompanying Figures in which:

Figure 1 : Centre 'fed' signal coupling method/geometry. This is the simplest iteration of the antenna. Its basic structure consists of a piezoelectric whisker with a sinusoidal radio frequency voltage fed to two metal electrodes separated by a gap. The gap determines the impedance of the antenna, and therefore the transfer of energy from the transmitter into a longitudinal (or torsional or at lower frequencies flexural) electro- acoustic wave, which in turn becomes the radiated electromagnetic wave or vice versa. Figure 2: Multiple phase signal coupling method/geometry. This is a more complex structure where rather than applying a sinusoidal radio frequency to two metal electrodes, several other electrodes in a "zebra" like pattern are added in order to adjust impedance and bandwidth. Here we consider external electronic adjustment to the phase of the applied signals in order to achieve constructive interference of an electroacoustic standing wave induced in the whisker.

Figure 3: Transformer coupling method;/geometry. This is an example of a more traditional coupling structure where multiple tapping points of the secondary windings of a transformer a re connected to multiple electrode points of a "zebra" electrode pattern applied to a piezoelectric whisker. This configuration and other similar transformer/balun arrangements allow us to reduce the impedance of the antenna according to the turns ratio of the primary and secondary coils.

Figure 4: Single phase signal coupling method/geometry. The "zebra" electrode pattern on the piezoelectric whisker is arranged so that there is a 180° phase difference between each electrode. This means a signal can be tapped every 360° from one group of electrodes in phase, and used with another group of the same spacing, but shifted relative to the first group by 180°. the net result is a minimal count single phase feed can be used to couple to the structure while maintaining a high Q factor.

Figure 5: Omnidirectional whisker antenna. Here whiskers arranged orthogonally with wire connections to the antenna elements according to the schemes mentioned. In this guise the orthogonal antennas can receive waves from multiple polarisation and from different directions, making it largely omnidirectional. Note, to enhance directivity further wired or non wired whiskers can be placed in parallel to one of the elements below in a similar manner directional yagi antenna structures.

Figure 6: Mounted whisker antenna. These structures indicate how whiskers, especially those mms in length can be mounted to avoid damping of the electroacoustic wave and reducing antenna efficiency. Mounting points are close to the nodes of the selected electroacoustic wave resonance modes.

Figure 7: Planar electroacoustic (EA) antenna. Whisker designs can also be implemented as a planar version which is much more easily fabricated from piezoelectric wafers commonly used for SAW and QCR (Quartz Crystal Resonance) oscillators. Electrodes are formed using standard photo lithographic methods to place electrodes at a central feed position, and antenna elements cut from the wafer according to the relevant crystalline lattice. This form also provides an opportunity to guide/trap the electroacoustic wave - and hence the charge - to one surface so surface impedance can be better matched to free space in order to give maximum radiative coupling.

Figure 8: Suspended electroacoustic (EA) element. The suspended electroacoustic element develops the idea further so that instead of a large planar structure for an antenna we make use of photo lithographic techniques to back etch a silicon wafer coated with a piezoelectric film to give elements suspended by electrodes (or by other methods) so that the suspended film structure is free to vibrate along its length. The circuit coupling method is either via the zebra like pattern described earlier, an interdigitated electrode structure, or other inductive means. Figure 9: Single layer EA array. These suspended elements can be part of multiple suspended structures co-linear in 2D, with some or all of the elements coupled to a receiver or transceiver. This can be integrated into an RFID chip in order to produce a compact receiving and transmitting structure.

Figure 10: Integrated multilayer EA array. Stacking of the collinear arrays can produce a compact 3D antenna structure that can very efficiently pump/extract the radio energy connections as described earlier to the receiver front-end of the relevant RF chipset. Each layer can also be polarised in a different direction to maximize coupling in real world environments.

Electroacoustic antennas We will describe larger electroacoustic antenna forms that have micron to mm dimensions for operation in the KHz to GHz frequency range.

Referring to the accompanying Figures and to the above Figure descriptions, we will therefore describe key structural features that improve radiation efficiency, coupling efficiency and Q factor, to improve electroacoustic antenna performance.

Whisker antenna and zebra electrode

To describe these designs we begin with the simplest antenna form - a whisker based single dipole piezo-antenna (Fig 1 ) form.

The advantages include: o It has more radiation efficiency due to the larger radiating surface area relative to its volume. (We gain in the coupled electromagnetic radiation because the accelerating charge which would normally act as the source is no longer screened by being buried within the interior of the crystal)

o Similarly with the long aspect ratio we gain in dipole moment again increasing radiation efficiency. o Q factor is also improved due to the narrow width which better defines its frequency.

To benefit from this efficiency, the whisker should preferably also be matched for radio signal performance at the fundamental frequency and at the harmonics. To do this we place the electrodes around the centre point of the half wave (Fig 1 ), where the electrical impedance to surrounding circuits is much lower. If lower impedances are needed, multiple elements can also be mounted in a parallel array. For single elements an impedance transformer can be made from a single whisker. Here we make a "Zebra" pattern with several parallel electrodes that have a small phase differences (<45 degrees) between them, due to the distribution of the electroacoustic wave (Fig 2). This scheme lowers electrode impedance without the need to use multiple elements. To make use of these signals, their individual phase should be made coherent with a charge coupled device or a suitable network. A similar result can be achieved with a matching transformer or balun to transform the impedance (Fig 3).

If a narrow operating frequency range is needed, the spacings of the above multipoint electrode can be made to match the electroacoustic nodes and anti-notes of the electro-acoustic wave (Fig 4). This type of impedance transformer, because of the tight fit between the electroacoustic wave and the electrode positions leads to a high Q factor, which in turn improves selectivity and radiation performance. Of benefit may be adjustment of the whisker's surface impedance to match it to the free space impedance. This may need surface coatings, which can be achieved with a contiguous dielectric film(s), or a non-contiguous dielectric film pattern.

Polarisation is also important to launching and receiving a radio signal. For example a cross pair design, or a whisker triplet (Fig 5) can provide an omnidirectional pattern that does not have directional sensitivity that can compromise RFID tags with frequent blackspots and dropouts.

Signal processing with multiple elements of different lengths, combined in parallel is also possible. This is similar to the effect of changing the periodicity of a surface acoustic wave transducer, also known for their signal processing capability. For example a band pass filter can be built into a multiple whisker antenna by varying the lengths of the elements according to a specific function. In all the above cases the antenna needs to be mounted appropriately so electrical connections can be made to the transceiver. Hence it is most desirable for mounting and electrical connections to take place at the stationary nodal points along the whisker. In summary, the whisker can vibrate if any of the following boundary conditions are satisfied: either bound ~ bound, bound ~ free or free - free (Fig 6). Clearly, for rigid mounting bound ~ bound is appropriate, however if the whisker is very short free ~ bound may also be suitable. A whisker that is 10 mm in length will operate at a fundamental frequency of approximately 100 kHz. Higher frequencies can be realised for the same length if the electrode spacing is altered to support say 10 wavelengths, which will resonate at a 1 MHz harmonic, or 1000 wavelengths which resonate at 1 GHz. But clearly adjustments can be made according to application constraints, which include directivity, frequency and size. Clearly the small size and possibility for very low operating frequency is attractive for RFID applications. At the other end of the scale, low frequencies are more easily accessed with these antenna configurations.

Evanescent wave antenna and zebra electrode

Another variant of the whisker is the evanescent wave form (Fig 7). This is rectangular strip rather than cylindrical form, and generates radio waves from a resonant evanescent wave such as a Love wave propagating in the surface. Similar to the whiskers, excitation is with an interdigitated electrode structure either at the centre, or distributed along its length (Fig 8). Unlike SAW devices these have end reflectors to create resonant waves, rather than standing waves. Benefits are easy preparation from a piezoelectric wafer, the antenna's self- supporting nature, and its easy integration with chip-based circuitry. From piezoelectric films resonant lamb and Rayleigh modes can also be constructed according to known techniques, but modified to ensure ends are free for their reflections to produce standing waves. A further advantage the formation of a multiplicity of elements, is that antenna gain can be realised by paralleling the antennas of together, to form 2D arrays (Fig 9), or stacked to extend radio capture over 3D (Fig 10). This can lead to high capture efficiency for very compact dimensions, increasing the potential performance of RFID tags or other communication products that are trending downwards in size.

Further enhancements to aspects and embodiments of the invention as described above are as follows. These may optionally be employed alone or in combination with any or all of the above described embodiments.

• using methods to trap the electroacoustic wave at the surface of the device in order to maximise radiation efficiency

• use of surface guided electroacoustic waves to bring charge to the surface for maximum radiation efficiency

• using a 2D piezoelectric sheet or 'panel' to radiate perpendicular to its surface, via a charge pattern over its surface

• control of the distribution of electro-acoustic waves in order to give periodic charge patterns in a surface to artificially synthesise antenna forms on/in a piezoelectric sheet

• control of an electroacoustic wave distribution to produce a periodic spatial charge pattern over a 2D surface

• control of an electroacoustic wave distribution to produce a periodic temporal distribution of charge · use of a temporal and/or spatial charge pattern in order to produce a steerable antenna array formed of modulated charge

• amplification of the charge wave with a semiconductor layer

• adjusting the skin depth in order to match the surface impedance to that of free space, hence boosting radiation efficiency using coatings of intermediate impedance to match the electroacoustic wave to free space using dipolar coupling between antenna elements to produced enhanced 3D structures coupling to antenna elements via direct electroding, inductive coils or microwave stripline methods

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

CLAIMS:

1 . An electromagnetic wave antenna, the antenna comprising:

a bar of piezoelectric material having a length and a cross-section perpendicular to said length wherein said length is at least five times a lateral dimension of said cross-section of said bar;

first and second electrodes deposited on said bar of piezoelectric material, spaced apart along said length of said bar; and

wherein said bar of piezoelectric material has an inherent polarisation in a longitudinal direction along said length of said bar.

2. An electromagnetic wave antenna as claimed in claim 1 wherein said longitudinal direction defines a direction of substantially maximum inherent polarisability of said piezoelectric material.

3. An electromagnetic wave antenna as claimed in claim 1 or 2 wherein the said length is at least 0.01 mm, 0.1 mm or 1 mm.

4. An electromagnetic wave antenna as claimed in claim 1 , 2 or 3 wherein at a resonant frequency said antenna has a Q of at least 10³, preferably at least 10⁴.

5. An electromagnetic wave antenna as claimed in any preceding claim comprising at least three said electrodes spaced apart along said length of said bar.

6. An electromagnetic wave antenna as claimed in claim 5 further comprising a plurality of phase matching elements, or a transformer, coupled to said electrodes.

7. An electromagnetic wave antenna as claimed in any preceding claim comprising a plurality of pairs of said electrodes disposed at intervals along said length of said bar.

8. An electromagnetic wave antenna as claimed in any preceding claim further comprising a layer of dielectric material on at least one surface of said piezoelectric bar to enhance coupling of electromagnetic radiation to acoustic vibrations in said bar.

9. An electromagnetic wave antenna as claimed in any preceding claim further comprising a support, and wherein said antenna is mounted on said support substantially at a position of minimum acoustic vibration of said antenna.

10. A set of substantially mutually orthoganol electromagnetic wave antennas each as claimed in any of claims 1 to 9

1 1 . A substantially two-dimensional array of electromagnetic wave antennas each as claimed in one of claims 1 to 9.

12. A stack of arrays of electromagnetic wave antennas as claimed in claim 1 1 .

13. A stack as claimed in claim 12 wherein electromagnetic wave antennas in different layers of said stack are orientated substantially orthogonally to one another.

14. An electromagnetic wave antenna, set, array or stack as claimed in any preceding claim in combination with an antenna signal receiving or driving circuit, wherein said antenna controls a frequency of operation of said circuit.

15. A method forming an electroacoustic wave antenna using a piezoelectric material, the method comprising:

exciting a surface wave pattern of charge in the piezoelectric material, said pattern of charge extending laterally in two dimensions; and

using said pattern of charge to form said electromagnetic wave antenna.

16. A piezoelectric antenna, the antenna comprising:

a substrate of piezoelectric material;

at least one set of electrodes for exciting a surface wave pattern of charge in said piezoelectric material, said pattern of charge extending laterally in two dimensions; and

means for coupling a received or transmitted electromagnetic wave signal into or out of said antenna using the same or additional said electrodes, wherein said pattern of charge forms said antenna.

17. A method or antenna as claimed in claim 15 or 16 wherein said surface wave pattern is a surface standing wave pattern.

18. A method or antenna as claimed in claim 17 wherein said surface standing wave pattern is spatially periodic in two lateral dimensions.

19. A method or antenna as claimed in claim 17 or 18 further comprising exciting said wave pattern using interdigitated electrodes and configuring said electrodes to control said pattern of charge.

20. A method or antenna as claimed in any one of claims 17 to 19 wherein said exciting comprises exciting said piezoelectric material on two lateral edges, each said excitation generating a generally striped pattern of charge with stripes generally parallel to a said lateral edge, such that said striped patterns interact to establish a said 2D standing wave pattern of charge.

21 . A method or antenna as claimed in any one of claims 17 to 20 wherein said surface standing wave pattern of charge defines a phased array antenna, and wherein said antenna has a directional response controlled by said pattern of charge.

22. A method or antenna as claimed in claim 15 or 16 wherein said surface wave pattern comprises a temporally varying 2D distribution of charge, and wherein a frequency of said temporal variation substantially matches a frequency of said electromagnetic wave.

23. A method or antenna as claimed in any one of claims 15 to 22 wherein said piezoelectric material comprises a sheet or membrane of material.

24. A method antenna as claimed in any one of claims 15 to 23 wherein a surface of said piezoelectric material is curved to direct an electromagnetic wave received or transmitted by said antenna.