WO2011042699A1

WO2011042699A1 - Rf element

Info

Publication number: WO2011042699A1
Application number: PCT/GB2010/001875
Authority: WO
Inventors: Neil Collings; William Alden Crossland; Yang HAO
Original assignee: Cambridge Enterprise Limited
Priority date: 2009-10-09
Filing date: 2010-10-06
Publication date: 2011-04-14
Also published as: GB0917847D0

Abstract

An optically transparent radio frequency or microwave device has first and second optically-transparent conductive substrates. Each carries a respective optically- transparent conductive electrode, and an optically- transparent liquid crystal layer forming a dielectric between the electrodes. The dielectric properties of the liquid crystal layer are controlled by application of a variable bias between the electrodes, to vary a resonant frequency of the device.

Description

RF Element

The present invention is in the field of microwave and rf elements; some

embodiments relate to antennas.

It has long been recognised as desirable to provide microwave and rf elements that are optically transmissive and capable of being tuned, in either frequency or direction, or both.

US-A- 6, 177,909 discloses a reconfigurable photoconducting antenna on a semiconductor substrate. At equilibrium, the semiconductor is semi-insulating, and therefore appears as a dielectric. Illuminating a region of the substrate results in the generation of free carriers in the substrate and allows the creation of a conductive region (semi-metallic) in the substrate. This conductive region functions as the radiating element of the antenna. Controlling the pattern of the illuminated region directly controls the pattern of the radiating antenna. By using a digital micromirror device to control the pattern of the light, a desired antenna design may be placed on the semiconductor substrate. The pattern can be dynamically adjusted simply by changing the position of the individual mirrors in the array. The device operates through a standardized digital interface and can be switched between patterns in a period of approximately 20 microseconds. The pattern can therefore be readily and easily controlled through the use of a digital control system.

The NASA Glenn Research Center devised a prototype frequency agile antenna based on thin ferroelectric film and a semiconductor substrate that was no transparent to visible light A 350-nm

film was grown by pulsed laser ablation on 300-mm-thick silicon using a 50-nm Bi₄Ti₃O)₂/50 nm yttria-stabilized zirconia buffer layer. The antenna measured 0.675 by 0.45 cm and resonated at about 9.5 GHz in the TMoi mode. . When a voltage is applied to the antenna such that the radiator has a positive bias with respect to the ground plane, electrons are swept from the silicon to the ferroelectric/semiconductor interface. This sea of electrons, or plasma, forms a virtual ground resonance plane near the interface. An electron density of about 10^{l 9}/cm³ extinguishes the antenna. When the switch is opened, the radiator behaves more or less as a conventional patch antenna.

The measured far-field E- and H-plane radiation patterns were normal. By reversing the polarity, electrons were swept to the natural ground plane and a depletion region formed under the patch in the silicon. In this configuration, the antenna was tuned about 25 MHz with 80 V applied. Operating the antenna in a higher order odd mode produced a second useful frequency of operation, and the ferroelectric effect was used for vernier tuning. The tuning range is ultimately limited by the formation of an inversion layer, or generation of higher order modes because of the electrical thickness of the stratified device.

Other tunable antennas are disclosed: in "Liquid crystal tunable microstrip patch antenna", L. Liu and R.J. Langley, ELECTRONICS LETTERS 25th September 2008 Vol. 44 No. 20; in "ELECTROMAGNETIC-WAVE MODULATING, MOVABLE ELECTRODE, CAPACITOR ELEMENTS" WO 93/24922 (1993); and in R. Jackson and R Ramadoss, "A MEMS-based electrostatically tunable circular microstrip patch antenna" J. Micromech Microeng. 17 (2007) 1 -8.

The first shows liquid crystal tuning, but not using nanoparticle suspensions for enhancement, and the structure is neither transparent nor conformable. The second gives an electromechanical tuning mechanism. This would be equivalent to the electromechanical down-tilt tuning of mast antennas. The third concerns

electromechanical tuning using MEMS device to adjust the air gap in the antenna. This requires large voltages (165V) which make it impractical for a commercial device. In addition it is neither transparent nor flexible.

Lower voltage operation is clearly desirable. In embodiments switching has been achieved in liquid crystal patch antennas with an applied voltage of 5V. Transparent antennas have been investigated by NASA based on thin silver coatings on PET film as the transparent conductive layers. Another transparent antenna device has been made by a joint team involving Cambridge University and Queen Mary University of London - see "Optically transparent UWB antenna" Electronics Letters; 7/2/2009, Vol. 45 Issue 14, p722-723.

Embodiments allow an r.f. element structure, for example an antenna, that is transparent and can be frequency tuned.

In one aspect, there is provided an optically transparent rf device comprising a optically-transparent conductive substrate, an optically- transparent conductive electrode, and an optically- transparent liquid crystal layer forming a dielectric between the electrode and the substrate, the dielectric properties of the liquid crystal layer being controllable by application of a variable bias between the substrate and the electrode, whereby the resonant frequency of the antenna device may be varied.

In one family of embodiments the substrates are of rigid material, such as glass. This enables- for example - structures such as windows or windscreens to be made that incorporate the LC material within their structure.

The antenna device may be dimensioned to be active in a frequency region between 2 and 110 GHz in its off state.

The material of the liquid crystal layer may be such that application of the bias can vary the resonant frequency by more than 10 per cent.

An exemplary class of materials is the cyano-biphenyl compounds, but the skilled person will be able to determine many other suitable materials. Reference is directed to the following published patent documents for a non-exhaustive list of LC compounds JP2005120208, JP2004285085, US2005/0067605 and US 6,838,017 The liquid crystal layer may include a suspension of anisotropic nano-particles. in one embodiment the nano particles are carbon nanotubes.

In another embodiment metal coated organic microtubes (see US Patent 5184233) are used.

In yet other embodiments metallic nanowires or metal/semiconductor nanowires are used.

Embodiments of the invention aim to allow an r.f. element structure, for example an antenna, to be transparent, frequency tunable and conformable.

The substrate and electrode in these embodiments are conformable whereby the antenna device may be conformed. To achieve this, the substrate materials can be of plastics, such as PET. Other plastics materials will occur to the skilled person.

In a second aspect, there is provided an antenna array comprising plural antenna devices according to the one aspect.

Antenna arrays can thus be directionally variable, by coupling the antenna devices together electrically and varying the relative delays of the signals from each of the patches.

In a third aspect an optically transparent element has a transparent conductive substrate, a transparent conductive electrode, and a liquid crystal layer forming a dielectric between the electrode and the substrate, the dielectric properties of the liquid crystal layer being controllable by application of a variable bias between the substrate and the electrode, whereby a resonant frequency of the element may be varied.

The element may be active in a frequency region between 2 and 110 GHz in the off- ie unbiased -state. The material of the liquid crystal layer may be such that application of the bias can vary the resonant frequency by 2 per cent using unoptimised liquid crystal materials

In a microwave element of the third aspect, the liquid crystal layer may include a suspension of anisotropic nano-particles.

In an embodiment, the substrate and electrode are conformable whereby the antenna device may be conformed.

The antenna device, microwave array or microwave element may be assembled with an optically transparent member, such as a vehicle window.

In the drawings:-

Fig 1 shows a plan view of a transparent rf element in the form of a patch antenna; Fig 2 shows a cross-section through the antenna of Fig. 1 taken along the line ΙΙ-ΙΓ Fig 3 shows a cross-section through the antenna of Fig. 1 taken along the line ΙΙΙ-ΙΙΓ; Fig 4 shows an exemplary array of rf elements;

Fig 5a and b show schematically the operation of the rf element of Fig 1 .

Referring to Figs 1 and 2, the first family of embodiments of the transparent element are antennas, such as antenna 1. The antenna 1 is a 10 GHz rectangular patch antenna that has substrates of first and second mutually-opposed sheets 10,12 of transparent plastics, in this embodiment PET, sometimes known as "Mylar". On the inside faces 1 0a, 12a of the two layers 10,12 are disposed respectively first and second thin silver layers 22,24, such as high-transmissive silver. The PET with silver conductive coating on one side is marketed as AgHT-4 (Trade Mark). These form the patch 2 itself. The thickness of the silver layers is such as to allow good transparency to visible light. In other embodiments materials other than silver are used; selection of the materials will be within the ability of the skilled person, bearing in mind any constraints inherent in the particular embodiment. Examples are gold, ITO, tin oxide, silver materials. The essential property that makes a transparent conductive material suitable is that it has low resistivity, eg 4 ohms per square for the AgHT-4.

In this embodiment, the first layer 22 has dimensions of 10 mm by 13 mm, the glue spacers are each 1 mm in lateral extent and the second layer 24 has dimensions of 10 mm by 13 mm. Given these dimensions, the embodiment has a resonant frequency of around 1 1.1 GHz in the "off state (i.e. no bias voltage) and around 10.8 GHz with a bias of 5 volts.

Spacers 13 around the edge of the silver layers 22,24 seal the PET sheets 10,12 together to form a liquid crystal cell 20, which contains a liquid crystal material 18. In this embodiment, the spacers 13 are formed of either thin plastic sheet or glass rods, however other materials will occur to the skilled person. The cell is relatively thick, depending on the materials used. The skilled person will be able to decide on a useful thickness dependent on application and materials. Typical embodiments may be between 100 and 500 microns thick.

The internal faces 10b, 12b of the silver layers 22, 24 have respective alignment layers 1 1,14 disposed on them.

The liquid crystal material in this embodiment is a nematic liquid crystal material 18 composed predominantly of cyanobiphenyl compounds, and containing a suspension of anisotropic nano-particles 26, such as carbon nanotubes, CNT. The liquid crystal material also includes -in this embodiment - ball spacers 16 suspended in the material, and these serve to maintain the separation of the two layers 10,12.

Roughly, the long dimension of the patch is about one half the wavelength of the microwave radiation. The velocity of the wave is lower in the ON cell, because the polarisation of the wave is along the LC director. Therefore the frequency is lower. Conversely, in the OFF cell, the velocity of the wave is higher and therefore the frequency is higher.

Referring to Figs 1 and 3, an embodiment of an antenna feed line 21 can be seen. It will be seen that the AgHT electrodes 22, 24 are extended inside the plastics layers 10, 12 to form feed line layers 22a, 24a, in this case from a non-peripheral region of the patch 1. The feed line layers 22a, 24a are disposed one directly above the other, and spaced apart by a spacer layer 15 of a thickness similar to that of the LC material 18. The width of the feed line layers is selected in concert with the spacer dimensions and material to provide a characteristic impedance of around 50 Ohm, so as to couple correctly to or from the antenna. The feed line continues to a suitable location for a conventional connector structure -for example an SMA connector, allowing coupling into, for example, a coax cable. For examples of dimensions, a 1.54 mm feed line with 0.5 mm spacing and a 0.3 mm feed line with 0.1 mm gap both give approx. 45 ohm impedances.

Other feed locations are of course possible.

A method of manufacture of the antenna 1 of this embodiment will now be described: i) The desired shape of the antenna is provided using from AgHT film to form the electrodes 22,24, for example by cutting, and secured to the inside faces 10a, 12a of two sheets 10,12 of PET. ii) A solution of Nylon 6 in formic acid is then spun onto the electrodes to form the respective alignment layers 11 ,14 iii) The coated plastic is then placed in an oven to evaporate residual formic acid. iv) The sheets 10, 12 are then washed several times with isopropanol to clean the nylon thoroughly. v) The alignment layers 1 1,13 are then brushed in order to promote a uniaxial alignment direction for the liquid crystal cell. vi) The nematic liquid crystal material 24, here as an example Merck BL037, is placed between the two sheets 10,12 of coated plastic and the sandwich is passed through a laminator. The LC material 24 contains a suspension of the CNT 26. The cell gap is assured by ball spacers 16 which are suspended in the liquid crystal prior to the lamination process. vii) After lamination the edges of the sandwich are sealed with a thin layer of glue to prevent moisture ingress.

The electrodes 22,24 of the cell are connected to high frequency connectors for electrical biasing and testing.

An alternative technique to make the alignment layer uses photoalignment

The thickness of the two layers 10,12 is selected according to the application of the antenna, and bearing in mind the degree of flexibility needed. For many applications -for example where applied to a curved vehicle windshield, vehicle rear-window or the like- there is no need for full flexibility, but rather conformability to a gently curved glass or laminate substrate. For some applications, however, a much less rigid substrate is essential.

In some embodiments polymer walls are disposed inside the LC layer so that tight curvatures- for example less than 10 mm radius- do not disrupt layering in the LC. Antenna tunability in direction (rather than frequency tuning) can also be realised using an array of independently addressable elements.

Referring to Fig 4, an illustrative array 400 includes 4 patch antennas 401a-d, each with a respective feed line 402a-c, each antenna and feedline being optically transparent and the substrates 403 on which the antenna are mounted being optically transparent.

A method of operation is now described

In use, high frequency, e.g. terahertz, signals are applied to the patch 1 , and a lower frequency (e.g. dc) bias is applied to control the permittivity of the CNT-doped LC 18. The bias may be a constant dc or may be a varying potential, or example a pulsed dc, bearing in mind that the LC material response time is quite slow due to the relatively thick layer of liquid crystal employed..

Referring to Figs 5a and 5b,the anisotropic particles 26 dispersed within the liquid crystal 18 are CNT, in this embodiment. In other embodiments, dye doping is used. In yet other embodiments, the liquid crystal material is a PDLC material without CNT. The spacing s between the substrates is substantially less that the extent e of the substrates.

In Fig 5a, the anistropic particles 26 are caused to be aligned generally parallel to the substrates by the liquid crystal material 18..

In Fig 5b, when a field is applied between the AgHT electrodes 22,24 the molecules of the liquid crystal material 18 are caused to tilt, and to thereby draw the particles 26 into a rotated position to the plane of the substrates. 10,12. A change in the alignment of the anisotropic particles results in the permittivity between the AgHT electrodes 22,24 being varied, which thereby changes the resonant frequency of the patch 1. The invention has now been described with reference to embodiments thereof. The invention is not however restricted to the described features.

Claims

1. An optically transparent rf device comprising first and second optically-transparent conductive substrates, each carrying a respective optically-transparent conductive electrode, and an optically- transparent liquid crystal layer forming a dielectric between the electrodes, the dielectric properties of the liquid crystal layer being controllable by application of a variable bias between the electrodes, whereby a resonant frequency of the antenna device may be varied.

2. A device according to claim 1 wherein the substrates are of rigid material, such as glass.

3. A device according to claim 1 wherein the substrates are of conformable material whereby the device may be conformed.

4. A device according to claim 1, 2 or 3 wherein the device is dimensioned to be active in a frequency region between 2 and 1 10 GHz in its off state.

5. A device according to any preceding claim wherein the material of the liquid crystal layer is such that application of the bias can vary the resonant frequency by more than 2 per cent.

6. A device according to claim 5, wherein the material is a cyano-biphenyl compound.

7. A device according to any preceding claim, wherein the liquid crystal layer includes a suspension of anisotropic nano-particles.

8. A device according to claim 7, wherein the nano-particles comprise carbon nanotubes.

9. A device according to claim 7, wherein the nano-particles comprise one or more of metal coated organic microtubes, metallic nanowires and metal/semiconductor nanowires.

10. An antenna array comprising an element according to any preceding claim.