Patent Publication Number: US-7592968-B2

Title: Embedded antenna

Description:
FIELD OF THE INVENTION 
   The invention relates to antennas, especially, but not exclusively, electrically small planar antennas for use in portable wireless devices such as mobile (cellular) telephones, personal digital assistants (PDAs) and audio-visual entertainment devices. 
   BACKGROUND TO THE INVENTION 
   There is a general trend towards miniaturisation of portable electronic devices, including portable wireless devices. As a result, antennas compete for space with the other device components (e.g. battery, display, keypad, printed circuit board). 
   In addition, modern wireless systems demand increasingly greater bandwidths in order to accommodate higher data rates. This is particularly true of video and audio applications that use the Ultra-Wideband (UWB) protocols being standardised by the IEEE. However, the goals of reduced physical size and increased bandwidth are not normally compatible. Further, reducing the physical size of the antenna normally tends to reduce the radiation efficiency of the antenna. There are fundamental theoretical performance compromises for electrically small antennas between required bandwidth, radiation efficiency and physical volume of the near-fields around the antenna (at a given centre frequency). Recent advances in small antenna design have attempted to achieve the highest bandwidth and radiation efficiency for a given volumetric size and operating frequency. 
   A key challenge in small antenna design is to provide adequate VSWR (voltage standing wave ratio) bandwidth and radiation performance for a given product application and physical volume requirement. 
   It would be desirable, therefore, to provide an antenna which, physically, is relatively small while satisfying relatively large bandwidth requirements and radiation efficiency requirements. 
   To this end, United States patent application US2005248488 (Modro), discloses a planar antenna folded to preserve or enhance the near-field resonant modes of the structure. It would be desirable, however, to improve on the antenna of US2005248488. 
   SUMMARY OF THE INVENTION 
   Accordingly, a first aspect of the invention provides an antenna comprising a resonant structure having a first portion disposed in a first plane, and at least one second portion disposed in a plane non-parallel with said first plane, wherein the resonant structure is embedded in a non-conductive material, and wherein said at least one second portion comprises at least one electrically conductive via. 
   Typically, said resonant structure comprises a third portion, the third portion being spaced apart from, and substantially parallel with, said first portion, said at least one second portion being disposed between said first and third portions. 
   Said at least one second portion may electrically connect said first and third portion and, in typical embodiments, extends between respective edges of said first and third portions. 
   Conveniently, said second portion is substantially perpendicular with said first portion. Said first portion normally comprises a layer or lamina of electrically conductive material and may for example be substantially rectangular in shape. A respective second portion is typically provided at opposite edges of said first portion. Said third portion may comprise a layer or lamina of electrically conductive material. 
   The second portion typically comprises at least two vias. The vias may be mutually spaced-apart or contiguous with one another. In preferred embodiments, the vias are arranged in a row and are aligned in a substantially coplanar manner. 
   In typical embodiments, the resonant structure is embedded in layers of embedding material, said first plane being substantially parallel with said layers and wherein said at least one via passes through at least one layer of embedding material. The embedding material may comprise a multi-layer substrate of non-conductive material, for example a dielectric material. 
   In some embodiments, the first portion comprises a layer or lamina of electrically conductive material and is shaped to define at least one slot, the at least one slot being open-ended at an interface between the first portion and at least one of said at least one second portions, wherein said at least one second portion includes a respective via aligned with a respective edge of said at least one slot, said respective vias defining a gap therebetween that is substantially aligned with the open end of said at least one slot. The gap is preferably substantially the same width as said at least one slot. 
   A second aspect of the invention provides a method of manufacturing an antenna comprising a resonant structure having a first portion disposed in a first plane, and at least one second portion disposed in a plane non-parallel with said first plane, the method comprising embedding the resonant structure in layers of non-conductive material; forming said at least one second portion by forming at least one electrically conductive via through at least one layer. 
   The present invention enables the size of antennas to be reduced while utilizing existing well-proven manufacturing technology. 
   Further advantageous aspects of the invention will be apparent to those ordinarily skilled in the art upon review of the following description of preferred embodiments and with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention are now described by way of example and with reference to the accompanying drawings in which: 
       FIG. 1  shows, in perspective view, a prior art planar antenna folded around the outer surface of a substrate; 
       FIG. 2  shows, in perspective view, an antenna embodying the invention, the antenna being embedded in non-conductive or dielectric material; 
       FIG. 3  shows, in perspective view, the conductive components of a preferred antenna embodying the invention; and 
       FIG. 4  shows, in perspective view, the antenna of  FIG. 3  embedded in non-conductive or dielectric material. 
   

   DETAILED DESCRIPTION OF THE DRAWINGS 
   Referring now to  FIG. 1 , there is shown a folded, rectangular slot-loop antenna  10  which may be the same or similar to the antenna of FIG. 2 of United States patent US2005248488. The antenna  10  comprises a layer, or lamina, of electrically conductive material  12 , typically metal, e.g. copper, provided on a substantially cuboid substrate  15 , typically comprising non-conductive or dielectric material. The substrate  15  has a width Wb, a length Ws and a thickness a. The antenna  10  has a substantially rectangular obverse face  40  and a substantially rectangular reverse face  42  joined by four substantially rectangular side faces  44 ,  46 ,  48 ,  50 . The obverse face  40  and reverse face  42  are substantially parallel and oppositely disposed with respect to one another, the side faces  44 ,  46 ,  48 ,  50  being substantially perpendicular to the obverse and reverse faces  40 ,  42 . The conductive layer  12  is provided on the substrate layer  15  such that a central region  32  of the conductive layer  12 , including a feed portion  20 , is located on the obverse face  40 , and that end regions  34 ,  36  of the conductive layer  12  are located partly on opposing side faces  46 ,  50  and partly on the reverse face  42 . 
   A quantity of the conductive material is removed from layer  12  to define a substantially rectangular loop-shaped slot  14  (which may be referred to as a slot-loop) through which the substrate  15  is exposed. The slot  14  divides the conductive layer  12  into a lamina  16  and a ground plane member  18 . The slot  14  substantially surrounds the lamina  16  but is open ended to provide the feed portion  20  of conductive material by which electrical signals (typically electromagnetic signals such as radio frequency (RF) or microwave signals) may be fed to and received from the lamina  16 . A coupling device in the form of a conductive feed line  21 , for example a coplanar waveguide, is provided for supplying signals to, and/or receiving signals from, the lamina  16  via the feed portion  20 . The feed line  21  is electrically isolated from the ground plane  18  by feed line slot portions  22 . The lamina  16 , ground plane  18  and slot  14  may together be referred to as the resonant structure of the antenna  10 . 
   The slot  14  is generally loop shaped and comprises a first slot portion  24  which is oppositely disposed with respect to the feed portion  20 ; a second slot portion  26  which is oppositely disposed with respect to the first slot portion  14  and is interrupted by the feed portion  20 ; and third and fourth slot portions  28 ,  30  which are oppositely disposed with respect to one another and which join the first and second slot portions  24 ,  26  at respective ends. In the preferred embodiment, the slot  14  is generally rectangular, the first and second slot portions  24 ,  26  being generally parallel with one another and the third and fourth slot portions  28 ,  30  being generally parallel with one another. Hence, the lamina  16  is also generally rectangular. 
   The slot  14  is folded around the substrate layer  15  so that the portions of the slot  14  which, during fundamental resonance mode, are associated with a significant electric or magnetic field (in particular the electromagnetic near-fields, i.e. the fields that are present adjacent the antenna) are located on the obverse face  40 , while the portions of the slot  14  which, during fundamental resonance mode, are associated with negligible or substantially zero electric or magnetic field are located mainly on the reverse face  42 . 
   The close proximity of the end regions  34 ,  36  and their respective slot portions  28 ,  30  on the reverse face  42  of the antenna  10  does not cause mutual interference because the slot portions are associated with little or no magnetic current/electric field during use. 
   The portions of the conductive layer  12  on the side faces  46 ,  50  of the substrate  15  comprise conductive strips. Depositing conductive material on the sides  46 ,  50  of the substrate  15  as well as on the obverse and reverse faces  40 ,  42  complicates the manufacturing process. Moreover, it is found that the electrical and magnetic fields generated around the antenna  10  during use, i.e. the near-fields, are not symmetrical and exhibit irregularities (especially around the slot  14  at the interface between the obverse/reverse faces  40 ,  42  and the side faces  46 ,  50 ) that can adversely affect the performance of the antenna  10 . It is important that the electric and magnetic near-fields associated with adjacent portions (e.g. the end regions  34 ,  36  and their respective slot portions  28 ,  30 ) of the resonating structure of the antenna  10  do not appreciably destructively interfere after folding. It is now considered that antenna structures with some degree of symmetry of their near-field distribution (which is often associated with a symmetrical geometry of the antenna) are more amenable to folding in this way. 
   Accordingly, it is proposed to embed a folded antenna within electrically non-conductive, or insulating, material, e.g. a dielectric material. Advantageously, the non-conductive material also exhibits a relatively high magnetic permeability, for example a magnetic permeability of at least 2.5 and preferably at least 3. The embedding material may be said to comprise high contrast material, or high electromagnetic contrast material. Normally, such material has a dielectric constant or magnetic permeability that is greater than 1 (in a vacuum). The material in which the antenna is embedded (hereinafter referred to as the embedding material) surrounds at least those portions of the antenna that create, or are associated with, electrical and magnetic near-fields during use. In the example of a folded slot-loop antenna the same or similar to the antenna  10  of  FIG. 1 , the embedding material surrounds at least the slot  14 . In practice, it is convenient to embed the whole of the antenna, or at least the whole of the conductive layer together with any slots formed therein, in embedding material. In preferred embodiments, the embedding material covers, or substantially covers, the resonant structure of the antenna. Typically, this includes conductive layer(s) or lamina(s) and any slots formed therein, or any other component that resonates during use when electromagnetic signals are received by or emitted from the antenna. Clearly, one or more connection or feed points are exposed so that signals may be sent to and received from the antenna. 
   In  FIG. 1 , the conductive layer  12  is located on the outer surface of the dielectric block  15 —no dielectric material is present on the outer side of the conductive layer  12 . In contrast,  FIG. 2  shows an embedded antenna  110  comprising a resonant structure which may be the same or similar to the resonant structure of the antenna  10  and so like numerals are used to indicate like parts. The conductive layer  112  and the slot  114  are embedded in a non-conductive or dielectric material  115  which, advantageously exhibits a high magnetic permeability. 
   Embedding the antenna improves the symmetry of the near-field of the antenna and so improves the performance of the antenna. Further, the embedding material  115  reduces the effective length of the resonating structure or resonator with the result that, for given operating frequency band(s), the antenna may be smaller than if it were not embedded. In the particular example of  FIG. 2 , where the antenna  110  is a folded slot-loop type of antenna, by embedding the peripheral slot  114  in dielectric material, the near-field is more symmetrical around the length of the slot  114  than in the case where the conductive layer  112  is located only on the outer sides of a dielectric block (see  FIG. 1 ). In the latter case, dielectric material is only present on one side of the slot  14  at any point along the slot length, whereas in the embedded case ( FIG. 2 ) the slot-line is loaded on both sides, resulting in a more symmetrical field distribution (when viewed in a cross-sectional plane perpendicular to the slot line  14 ). Since there is more dielectric material  115  adjacent to the slot-line  114  (i.e. on all sides), the effective dielectric constant of the volume of space that surrounds the slot-line  114  is greater and the required length of the resonator is decreased compared with the asymmetric slot-line case of  FIG. 1 . In addition, near-field discontinuities which can arise at the junctions of mutually perpendicular sections of the slot-line  14 ,  114  (e.g. at the interface between horizontal and vertical sections of slot-line  14 ,  114  as viewed in  FIGS. 1 and 2 ), are reduced since the slot  114  is embedded in dielectric. 
   In preferred embodiments, the depth to which the antenna  110  is embedded is substantially uniform around the outer surfaces of the antenna  110 . By way of example, the depth (e.g. measured from the surface of the embedding material of the embedding material to the surface of the conductive layer  112 ) may be at least approximately 50% of the thickness of the conductive layer  112  itself, when measured in the same direction, especially where the resonating structure comprises a slot-line or slot loop resonator. More generally, the depth or thickness of the embedding material is preferably such that, during use, it encloses or contains substantially all of the electromagnetic near-fields generated by the resonating structure. 
   Conveniently, the embedding material may be shaped to suit the required application. In the illustrated embodiment, the embedding material  115 , and therefore the antenna  110  as a whole, is substantially cuboid in shape. 
   In typical applications, the embedded antenna may be mounted on a surface or substrate such as a PCB (Printed Circuit Board). The dielectric or embedding material located between the underside of the embedded antenna and the PCB reduces the detuning of the antenna due to near-field interaction. In the particular example of the antenna  110 , the embedding dielectric material concentrates the near-fields close to the slot  114  and away from the surface of the surrounding dielectric block. The result is that the antenna pass band and radiation performance are more immune to variation due to circuit board proximity. 
   With some manufacturing processes, it can be difficult or inefficient to create a resonant structure, such as the one shown in  FIGS. 1 and 2 , in which portions of the conductive layer  12 ,  112  are non-parallel or perpendicular. This problem applies particularly when the resonant structure is embedded. Accordingly, in preferred embodiments, at least one portion of the resonant structure of the antenna, especially where the antenna is embedded, is formed from one or more electrically conductive connector or via. Typically, a plurality of discrete, spaced-apart connectors are used to provide a portion of the resonant structure. In particularly preferred embodiments, the, or each, connector takes the form of a via. A via is a connector or contact for creating an electrical connection between, typically two, but possibly more, layers of a multi-layer structure or substrate. Commonly, a via comprises an aperture or channel formed, e.g. by drilling, through one or more layers of a substrate, the aperture being filled, plated or coated with an electrically conductive material (usually a metal, e.g. copper) to provide a conductive pathway between layers for the purposes of layer-to-layer interconnection. 
     FIG. 3  shows the resonant structure, generally indicated as  211 , of an antenna  210  ( FIG. 4 ) in which portions of the resonant structure are formed from electrical connectors, and in particular vias. The resonant structure  211  of  FIG. 3  is that of a folded slot-loop antenna which may be the same or similar to the resonant structures of the antennas  10 ,  110  and so like numerals are used to indicate like parts and similar descriptions apply, as will be apparent to a skilled person.  FIG. 4  shows the resonant structure of  FIG. 3  embedded in embedding material  215 , as described for the embedded antenna  110 . 
   The resonant structure  211  includes a central portion  232  formed as a layer or lamina of metal or other conductive material and two end portions  234 ,  236  also formed as a layer or lamina of metal or other conductive material. Similarly, ground plane portions  218  are formed as strips or patches of metal or other conductive material. Unlike the structures of  FIGS. 1 and 2 , the central portion  232  and end portions  234 ,  236  are not formed from a common, folded conductive layer—each portion  232 ,  234 ,  236  comprises a separate lamina or piece of conductive material. It will be seen that the slot portions  224 ,  226  formed between the central portion  232  and the ground plane  218  are open ended and, in the case of slot portion  224  extends from side-to-side across the resonant structure  211 . In the case of the slot portion  226 , it may also be said to extend from side-to-side across the resonant structure  211 , but interrupted by the feed portion  220 . Similarly, the respective slot portions  228 ,  230  formed between the end portions  234 ,  236  and the ground plane  218  are open ended as shown. 
   Instead of the conductive strips used in the structures of  FIGS. 1 and 2 , the central portion  232  is connected to the end portions  234 ,  236  by means of a plurality of discrete electrical conductors in the preferred form of conductive vias  260 . The parallel ground plane portions  218  are similarly connected. Each via  260  comprises a length of electrically conductive material, typically metal, and is usually formed in the manner described above. The vias  260  are typically substantially cylindrical in shape but do not necessary need to be so. 
   When implementing a portion of the resonant structure using vias  260  it is preferred to use at least two vias  260 , one at or adjacent either end of the portion being implemented. It is more preferable to provide, if space allows, one or more additional vias  260  between said at least two vias  260 . The vias  260  are typically spaced-apart although they may be contiguous. One option is to provided as many vias  260  as the manufacturing technology allows. In respect of each portion being implement by vias  260 , the vias  260  are preferably arranged in a row, each via  260  in the row being orientated in substantially the same manner. Hence, the vias  260  in a row are preferably substantially parallel with on another. For example, to implement the portion of the resonant structure  211  between the central portion  232  and the end portion  234 , respective vias  260 A,  260 B are located at or adjacent the ends of the portion (and therefore also at the ends of the central and end portions  232 ,  234 ) and, preferably, a plurality of additional vias  260  are arranged to form a row therebetween. The row of vias  260  lies in the plane of the portion being implemented, for example in a plane that is substantially perpendicularly with the planes of the central and end portions  232 ,  234 . Alternatively, only vias  260 A,  260 B,  260 C and  260 D could be used to implement the vertical (as illustrated) portion(s) of the structure  211 , i.e. no additional vias  260  between vias  260 A and  260 B. 
   Slots in the resonant structure may be implemented by an appropriately dimensioned space or gap between adjacent vias  260 . For example, in the resonant structure  211  where there is a folded slot  214 , the portions of the slot to be present on the portion of the structure implemented by vias  260  is implemented by two appropriately spaced and positioned vias  260  (see for example the vias  260 C and  260 B in  FIG. 3  which implement the slot portion between slot portions  226  and  228 ). In general, in the case of folded slots, a respective via  260  is substantially aligned with an edge of the slot portion formed in the or each adjacent conductive layer or lamina. 
   In an alternative embodiment (not illustrated), a plurality of spaced apart or contiguous vias are provided between corresponding portions  218  of the ground plane. 
   The implementation of portions of the resonant structure using vias, or other connectors, is particularly suitable when the antenna is manufactured using multi-layer substrate technology, such as LTCC (Low Temperature Co-fired Ceramic) technology wherein the embedding material  215  comprises LTCC. With such technology, the embedding or dielectric material comprises multiple layers. Those portions of the resonating structure that are formed as or from a conductive lamina or layer (sometimes referred to as active conductive portions) may be formed in conventional manner by depositing, or otherwise providing, a layer of conductive material between adjacent layers of embedding material. Hence, such portions are substantially parallely disposed with the substrate layers. The other portions of the resonating structure may be formed using conductive vias that pass through the substrate layers (usually substantially perpendicularly with the substrate layers). Normally, the vias connect one conductive layer with another conductive layer (as shown in  FIGS. 3 and 4 ). In alternative embodiments, however, the vias may be used to implement any portion(s) of the resonant structure that are non-parallel with the substrate layer(s), irrespective of whether or not said portion(s) connect two or more other portions. 
   For embodiments where the resonant structure comprises a slot, the higher the dielectric constant and/or electromagnetic permeability, the shorter the total physical, or actual, slot length for a given operating frequency band. 
   It is preferred that the vias are solid rather than hollow, since solid vias create a lower impedance connection between the component parts of the resonant structure that they connect. It is further preferred to make the vias as thick as the fabrication technology will allow in order to minimize inductance. 
   The invention is not limited to use with resonant structures of the folded slot-loop type illustrated herein. The invention is particularly suited for use with antennas having a resonant structure with respective portions being disposed in non-parallel planes, especially, but not exclusively, where one or more of said portions includes at least one slot. For example, in an alternative embodiment, the resonant structure may comprise a patch or microstrip resonator, typically located in a spaced apart relationship with a ground plane. Moreover, the principles and techniques described herein can be applied to other, predominantly symmetrical, planar antenna structures where the field modes are understood. 
   The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.