Patent Publication Number: US-11641062-B2

Title: Dual-polarized planar ultra-wideband antenna

Description:
CLAIM OF PRIORITY 
     This patent application is a continuation of U.S. patent application Ser. No. 15/780,483, filed on May 31, 2018, which is the United States national stage entry application of International Application Serial No. PCT/EP2016/079268, filed on Nov. 30, 2016, which in turn claims priority from European Patent Application Serial No. 15197294.0, filed on Dec. 1, 2015. Each of the above identified applications is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an antenna, more specifically to a compact and planar antenna operable in the GHz range as used for example in wireless communication. 
     DESCRIPTION OF RELATED ART 
     A theoretical monopole antenna includes a monopole arranged perpendicular to a nominally infinite or nearly infinite ground plane. There are also approximately planar monopole antennas known where the nominally infinite ground plane is arranged coplanar to a monopole, both mounted onto the surface of a (dielectric) substrate. The driven or active element of the monopole antenna is linked to other parts of a transmitting and/or receiving device by a signal feeding line which can be implemented as a planar waveguide with the central conductor or signal feeding line shielded on both sides by ground feeding lines. In many designs the driven element of a monopole antenna has an increased width compare to the width of the signal feeding line connecting it to the rest of the antenna components. For example the driven element of a monopole antenna could flare into a triangular shape or widen into a circular, rectangular, or other shape from a feeding point of the antenna. This widening is normally created for the purpose of having wider bandwidth, see for example “Compact Wideband Rectangular Monopole Antenna for Wireless Applications” by S. M. Naveen et al, Wireless Engineering and Technology, 2012, 3, 240-243 http://dx.doi.org/10.4236/wet.2012.34034 Published Online October 2012. 
     Further antenna designs are described for example in: “Coplanar Waveguide Fed Ultra-Wideband Antenna Over the Planar and Cylindrical Surfaces” by from R. Lech et al. as published in The 8th European Conference on Antennas &amp; Propagation, 2014 (EuCAP 2014), Hague, Netherlands, 6-11 Apr. 2014, pp. 3737-3740. 
     It should be understood that the above referenced documents show only some examples of known designs and a great variety of others are described in the published literature. But whilst the general principles of designing such antenna are known it continues to be an objective to derive more compact and more capable antenna to satisfy for example the demand for smaller mobile and stationary communication devices, such as phones, routers, relay station and the likes. It is further seen desirable to design new compact antennas to support MIMO (multiple in/multiple out) communication modes. 
     BRIEF SUMMARY OF THE INVENTION 
     A wideband compact antenna is provided suited for MIMO communication and other purposes, substantially as shown in and/or described in connection with at least one of the figures, and as set forth more completely in the claims. 
     These and other advantages, aspects and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which: 
         FIG.  1    shows a top view of an antenna of the prior art; 
         FIG.  2    shows a cross-section II-II of  FIG.  1   ; 
         FIG.  3    shows a cross-section III-III of  FIG.  1   ; 
         FIG.  4    shows an exemplary top view of an antenna according to an example of the invention; 
         FIG.  5    shows a bottom view of the antenna of  FIG.  4   ; 
         FIG.  6    shows a detail of  FIG.  5   ; and 
         FIGS.  7 A , B show bottom views of antennas according to further examples of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A typical planar antenna  10  is shown in  FIG.  1    to  FIG.  3   .  FIG.  1    shows a top view, while  FIG.  2    shows the cross-sectional view II-II and  FIG.  3    shows the cross-sectional view III-III. The ground plane in this arrangement is formed by a circular ring-shaped ground conductor  2  surrounding an inner area. A circular monopole conductor  1  is mounted onto the substrate  3  within the inner radius r 2  of the ground conductor  2 . Both are arranged coplanar on the same side  31  of a substrate  3  while the opposite side  32  of the substrate is free of conducting structures. 
     The circular monopole conductor  1 , which may be considered to form the driven or active element of the antenna  10 , may be electrically coupled to transmit/receive circuitry (not shown) via the signal feeding line  4  and a central pin  8  of a coaxial connector  6 . The ground conductor  2  is similarly electrically coupled to ground of the transmit/receive circuitry by the ground feeding lines  5  and the shielding  7  of the coaxial connector  6 . The ground conductor  2  and the ground connector lines  5  shield the signal feeding line  4  coupled to the monopole conductor  1  arranged in the opening of the ring-shaped ground conductor  2 . The antenna characteristics depend mainly on the separation distance between the ground conductor  2  and the monopole conductor  1 , particularly on the following geometrical parameters: the radius r 1  of the monopole conductor  1 , the outer radius r 3  and the inner radius r 2  of the ring-shaped ground conductor  2 , the distance Df of the feeding point  101  of the monopole conductor  1  to the inner border  21  of the ring-shaped ground conductor  2  and the distance Dg between the signal feeding line  4  to the ground connector lines  5  on both sides. The feeding point  101  of the monopole conductor  1  is defined as the point at which the monopole conductor  1  begins to widen from the (e.g. constant) width of the signal feeding line  4 . In other words, the feeding point  101  can be understood as the point at which there is the transition from the signal feeding line  4  into the monopole conductor  1 , since the feeding line  4  and the monopole conductor  1  are often one physical conductor/component. 
       FIGS.  4  and  5    are schematic illustrations of an embodiment of antenna  10  according to an example of the invention.  FIG.  4    shows a top view of the embodiment of the antenna  10 , while  FIG.  5    shows the corresponding bottom view of the same antenna  10 . Conducting areas of antenna  10  are shown as hatched when visible in the respective view and as outlined with a dashed line when located on the (hidden) side in the respective view. 
     The antenna  10  of  FIGS.  4  and  5    comprises a substrate  13  with a first side  131  and a second side  132 . On the first (top) side  131  there is shown a first driven element or monopole conductor  11  with a first signal feeding line  14  merging into or coupled to monopole conductor  11  at the feeding point  141 . Further shown on the first side  131  is a connection to the ground potential of the antenna  10 , referred to as the second ground conductor  16 , which may be a strip of conducting material along or parallel to one edge of the first side  131 , e.g. to the left or the right of the monopole conductor  11  extending to an inner border  160 . Also indicated on side  131  are the inner circumference d 1  and the outer circumference d 2  of a first ground conductor as dashed lines as the first ground conductor  12  is mounted onto the other (bottom) side  132  of the substrate  13 . 
     Together with the first ground conductor  12  there is mounted on the second side  132  of the substrate  13 , a second signal feeding line  15  connecting to the first ground conductor  12  at a feeding point  151 . Also connected to the first ground conductor  12  is a ground connector  125 , which may by a strip of conductive material connecting the ground conductor to an edge of the substrate (and further via connectors or pins not shown to the ground potential of the antenna  10 ). 
     A feeding point, be it the first feeding point  141  or the second feeding point  151  may denote the approximate area where the signal feeding lines  14 ,  15  merge/widen into the monopole conductor  11  and into the area first ground conductor  12 , respectively. 
     The substrate  13  is generally made of a dielectric material. The substrate  13  and its dimensions, particularly its thickness, are chosen depending on the desired application. The electromagnetic properties of the substrate  13 , especially its permittivity, influence also the characteristics of the antenna  10 . Therefore, the properties of the substrate  13  must be considered when choosing other design parameters of the antenna. The substrate  13  in the example may be a thin planar rectangular cuboid or parallelepiped, such as a flat sheet or board, with facing main sides or faces  131 ,  132 . Preferably, the first side  131  and the second side  132  are parallel to each other and/or flat. However, the substrate  13  may also be a curved shape for specific applications. In the illustrated embodiment, the substrate  13  may be a rigid plate, for example with a constant thickness. However, the substrate  13  may also be a flexible material like a foil and/or could be of varying thickness. The thickness of the substrate  13  refers to the separation distance between the first side  131  and to the second side  132 . 
     As indicated in  FIGS.  4  and  5   , the first driven element or monopole conductor  11  on side  131  may be an extended area covered with a solid or at least a continuous layer of conducting material. In particular, the monopole conductor  11  may be a solid approximately disk-shaped area as shown, but other shapes may be contemplated. In should be noted that the term “monopole” is used herein not exclusively as a strict technical term but as a term to encompass all types of compact driven antenna elements of which monopoles have the most wide spread usage. Compact dipole or more complicated antenna elements with more parasitic satellites may also be used as the monopole conductor  11 . 
     Hence, the shape of the monopole conductor  11  is not limited to circular, as will be clear to a person skilled in the art. It can be ellipsoidal, triangular, rectangular, multi-angular, fractal, or any other shape. For example, the outer circumference d 0  of the monopole conductor  11  can be shaped similar to one of the outer circumference d 2  and/or the inner circumference d 1  of the first ground conductor  12 . The shape of the monopole conductor  11  may also differ from the ground conductor  12 . The area of the first monopole conductor  11  and thus the size of its outer circumference d 0  is best chosen such that it falls within the projection of the inner circumference d 1  of the first ground conductor  12 . 
     The first ground conductor  12  comprises an electrically conducting material deposited as a layer onto the second side  132  of the substrate  13 . The first ground conductor  12  on the opposite side  132  may be an extended area covered with a solid or at least a continuous layer of conducting material. As explained in detail below the area covered by the first ground conductor  12  may enclose a central or inner area free of conducting material. The first ground conductor  12  is approximately annular. It will be appreciated by a person skilled in the art that any other shape of the first ground conductor  12 , which substantially encloses a central area of the surface  132  can be used. The enclosed area could be an ellipsoidal, a triangular, a rectangular, a multi-angular or any other approximately or nearly closed shape. 
     In the shown embodiment, the first ground conductor  12  is defined by two concentric circular borders with an inner circumference d 1  and an outer circumference d 2 , respectively. Hence, the first ground conductor  12  may be essentially ring-shaped. When used as the driven element of the antenna  10 , the first ground conductor  12  may be regarded as a ring antenna element. 
     The monopole feeding line  14 , the second signal feeding line  15  and the ground connector  125  are made of electrically conducting material and are connected on their near end to the monopole conductor  11  and the first ground conductor  12 , respectively and on their far end to structures and elements beyond the elements of the antenna  10  as shown in  FIGS.  4  and  5   , in particular to signal ports and ground potential, respectively 
     The antenna  10  characteristics, for example the input impedance or the reflection coefficient, depend, among other things, on the thickness of the substrate  13 , the electromagnetic properties of the substrate  13  and the geometrical arrangement and shapes of the ground conductor  12  and the monopole conductor  11 . In the example shown, the parameters of the geometrical arrangement are, inter alia, d 0 , d 1  and d 2 . Electromagnetic properties of the substrate  13  include, for example, the permittivity, permeability, and loss tangent. 
     Whilst the various conductive elements or structures in  FIG.  4    and  FIG.  5    are mounted on both sides  131 ,  132  of the substrate  13 , certain constraints as to their placement relative to each other may be applied to optimize the performance of the antenna  10 . 
     One of such constraints may be that the first and the second signal feeding lines  14 ,  15  are oriented essentially perpendicular, at an angle of 80 to 100 degrees, or even at an angle of 85 to 95 degrees, in reference to their respective axis extending approximately from the centre of the monopole conductor  11  and the first ground conductor  12 , respectively. In other words, if one of the signal feeding lines, e.g. feeding line  14  is formed as a narrow strip of conductive material located essentially at the middle of one edge of the substrate  13 , the second signal feeding line  15  may be a similar strip located essentially at the middle of one of the two adjacent edges of the substrate (besides being located on the opposite side of the substrate). The feeding lines  14 ,  15  are essentially perpendicular in order to yield two orthogonal polarizations and thus achieve a desirable isolation between the two signal feeding lines  14 ,  15  (and hence signal input ports of the antenna  10 ). 
     Further, the first ground conductor  12  on the bottom side  132  of the substrate  13  may have an inner circumference d 1  enclosing an area free of parts of the first ground conductor  12  which fully encloses an outer circumference d 0  of the monopole conductor  11  located on the other (top) side  131  of the substrate  13 . 
     Another constraint may be that the second ground conductor  16  and the second signal feeding line  15  are located at the same edge of the substrate  13  (albeit on different sides). 
     Another constraint may be that the second ground conductor  16  may extend in direction from an edge of the substrate  13  towards the middle of the substrate  13  up to a border line  160  without however such border line  160  touching or overlapping with the outer diameter d 2  of the first ground conductor  12 , as projected onto the first side  131  and indicated by the dashed line in  FIG.  4   , for example. 
     Another constraint may be that the feeding point  141  is close to or even inside the inner diameter d 1  of the first ground conductor  12 , as projected onto the first side  131  and indicated by the dashed line in  FIG.  4   , for example. 
     For example, the input impedance at the feeding point  141  or at the feeding point  151  may be designed to match a desired impedance. The desired impedance is typically selected to match the transmitting and/or receiving circuitry (not shown). Values often used are, for example, 50 Ohm or 75 Ohm. 
     It may be desirable to operate antenna  10  as two essentially independent (sub-)antennas, particularly as two antennas with a mutually cross-polarized reception/transmission characteristics. The first of such (sub-)antennas may be formed by the first monopole conductor  11  with the first monopole feeding line  14  and the first ground conductor  12 . The second of such antennas may be formed by the first ground conductor  12  with the second monopole feeding line  15 , operating as a ring antenna with a parasitic element and the second ground conductor  16 . 
     In other word the above example describes a compact antenna which can be designed and operated as two (sub-) antennas with at least part of the ground of one (sub-) antenna acting as driven element of the second (sub-) antenna. 
     A possible operation of the antenna  10  as a system of two (sub-)antennas is further illustrated in  FIG.  6   .  FIG.  6    shows a detail of the feeding point area  151  of  FIG.  5   . The first ground conductor  12 , the feeding point  151 , and the second signal feeding line  15  may be substantially similar to those elements described in  FIGS.  4  and  5   . In  FIG.  6   , there is shown a section of the first ground conductor  12 , the second signal feeding line  15 , and the feeding point  151 . Further shown are currents I 0 , I 1 , I 2  which are generated by operation of the first (sub-) antenna formed by the monopole conductor  11  with the first signal feeding line  14  and the first ground conductor  12 . The current I 0  induced splits at the feeding point  151  in accordance to the impedance Z 1  in the first ground conductor  12  and the impedance Z 2  at the feeding point  151  of the second monopole feeding line  15 . 
     The above configuration may be operated desirably with the materials, locations and dimensions of the above described structure designed such that for any current I 0  flowing in the first ground conductor  12  as generated by operation of the first (sub-)antenna with the first ground conductor  12  acting as ground has a substantially higher impedance Z 2  for electrical current at the feeding point  151  through the signal feeding line  15  than the complex impedance Z 1  in the rest of the ground conductor  12 . The current I 0  is then effectively confined within the ground conductor  12  without leaking into the second monopole feeding line  15 . In other words the current I 2  is negligible compared to both the total current I 0  and the current I 1  after the node at the feeding point  151  with I 0 =I 1 +I 2 . For the signal applied to feeding line  15  the impedance is designed to be the nominal input impedance, e.g. 50 Ohm, while the magnitude of the impedance Z 1  can, for example, be around 0.01 Ohm. 
     When driving or feeding the first ground conductor  12  as a ring antenna via the second signal feeding line  15 , the second ground conductor  16  acts as ground for the second feeding line  15  and the first ground conductor  12 . The radius of the first ground conductor  12 , its dimensions and the position and dimensions of the ground connector  125  may be designed such that in the given operating frequency range the ground connector  125  appears as an open circuit, i.e. having an odd numbered multiple of a quarter of the wavelength of the RF wave at the location of the ground connector (in both directions around the first ground connector  12  as being effectively a ring antenna). 
     In addition, the second signal feeding line  15  is typically coupled capacitively or inductively to the interior monopole antenna  11  (on the other side  131  of the substrate  13 ). This coupling aids at shrinking the total size of the antenna or at partially removing the impact of the first ground conductor  12  on the monopole conductor  11  when exciting the first signal feeding line or signal input port  14  and thus achieving wider bandwidth. However, a small portion of induced current will flow through line  14 . The amount of current thus leaking through line  14  can be taken as indicator of the isolation between the two signal feeding lines or input ports  14 ,  15 . Depending on the general design parameter mentioned above, it is for example possible to achieve better than 30 dB isolation between the input ports within a wide bandwidth of around 2.0-2.7 GHz. For frequency ranges 1.7 GHz-2.0 GHz the isolation can still be better than 22 dB. 
     It was further found that isolation of signal input port  15  across a broader range of frequencies can be further improved by adding blind or parasitic conductive path extensions to the ground conductor  12  on the bottom side  132  of the antenna  10 . 
     In the examples of  FIGS.  7 A,  7 B  there is shown a first ground conductor  12  mounted on the second side  132  of a substrate  13 , a second signal feeding line  15  connecting to the first ground conductor  12  at a feeding point  151 . Also connected to the first ground conductor  12  is a ground connector  125 , which may by a strip of conductive material connecting the ground conductor to an edge of the substrate (and further via connectors or pins not shown to the ground potential of the antenna  10 ). In addition, the ground conductor  12  further includes a conductive path extension  126 . The conductive path extension  126  as shown in  FIG.  7 A  can be a strip of conductive material branching off the outer circumference of the ground conductor  12  as a blind extension or parasitic element. 
     The location at which the conductive path extension  126  is connected to the ground conductor  12  may be located essentially opposite of the feeding point  151 , e.g. within 160 to 200 degrees along the circumference of the ground conductor  12  from the feeding point  151 . 
     The conductive path extension  127  as shown in  FIG.  7 B  can be further extended compared to the conductive path extension  126  of  FIG.  7 A  by including a meandering strip of conductive material. 
     The path extension may also be realised internally within the ground conductor  12 , for example by giving sections of the ground conductor  12  a meandering form instead of the solid form shown. 
     The ground conductor  12  may further include an isolating gap (not shown) particularly at the location of the conductive path extension  126 ,  127 , with the gap splitting the ground conductor  12  into two branches. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not of limitation. Likewise, the various diagrams may depict an example structure or other configuration for the invention, which is done to aid in understanding features and functionality that can be included in the invention. Further, it should be understood that the various features, aspects and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in various combinations, to one or more of the other embodiments of the invention. In particular, where approximative terms such as “essential” are used it is understood that minor variations within for example 10 percent or less from a strict geometrical shape or orientation are included.