Abstract:
There is described herein a low profile dipole antenna element. A pair of these elements can be arranged in a crossed manner to provide two orthogonal polarized radiators. The antenna element may be combined with an electrically conductive surface and a feed cable and connected to a feed source.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is the first application filed for the present invention. 
     TECHNICAL FIELD 
     The present invention relates to the field of wireless communications systems and other systems utilizing radiating electromagnetic fields. In particular the present invention relates to antenna elements suitable for both transmission and reception of electromagnetic radiation as a sole element or as part of an array of elements. 
     BACKGROUND OF THE ART 
     Traditionally, antenna elements have been designed using perfect electrical conductors often placed above a perfect electrically conducting ground-plane. A dipole element is typically utilized and spaced one quarter wavelength above the ground-plane. A perfect electrical conductor has the property that when an electromagnetic wave impinges on the surface it is reflected with a 180 degree change in phase. Thus if the dipole element is one quarter wavelength corresponding to a 90 degree phase shift then the reflected component has a 360 degree total phase change and is hence in phase with the radiating signal reinforcing radiation away from the ground-plane reflector. Small variations of the one quarter wavelength spacing are used to adjust the effective radiating beam-width. This requirement for one quarter wavelength separation between the ground-plane and the radiating element limits the thickness of the antenna. 
     There is often a need to design low profile antennas. In some cases this can be met by using alternative elements such as patches. These elements do not always provide the necessary radiation patterns or other required characteristics. Therefore, alternative designs are desired. 
     SUMMARY 
     There is described herein a low profile dipole antenna element. A pair of these elements can be arranged in a crossed manner to provide two orthogonal polarized radiators. The antenna element may be combined with an electrically conductive surface and a feed cable and connected to a feed source. 
     In accordance with a first broad aspect, there is provided a planar dipole antenna element. The element comprises a substrate with a dielectric material having a first side and a second side; a first dipole element comprising a first conductive area on the first side of the substrate and a second conductive area on one of the first side and the second side of the substrate; a first transmission line on the first side of the substrate, the first transmission line having a first end connected to the second conductive area and a second end adapted for connection to a feed source; and a first sleeve on the second side of the substrate. The first sleeve comprises a third conductive area connected to the first conductive area at a first position and adapted for connection to a ground of the feed source at a second position, the distance between the first position and the second position corresponding to substantially one quarter wavelength, the first sleeve being substantially aligned on the second side of the substrate with the first conductive area on the first side of the substrate to provide a radiating function. 
     In accordance with a second broad aspect, there is provided a planar dipole antenna system. The system comprises a first antenna element comprising a substrate with a dielectric material having a first side and a second side; a first dipole element comprising a first conductive area on the first side of the substrate and a second conductive area on one of the first side and the second side of the substrate; a first transmission line on the first side of the substrate, the first transmission line having a first end connected to the second conductive area and a second end adapted for connection to a feed source; and a first sleeve on the second side of the substrate, the first sleeve comprising a third conductive area connected to the first conductive area at a first position and adapted for connection to a ground of the feed source at a second position, the distance between the first position and the second position corresponding to substantially one quarter wavelength, the first sleeve being substantially aligned on the second side of the substrate with the first conductive area on the first side of the substrate to provide a radiating function. The system also comprises an electrically conductive surface spaced from the antenna element and a first feed cable having a first end connected to the first antenna element at the second end of the first transmission line and grounded at the second position of the first sleeve, and a second end connected to the feed source. 
     Although the terms top and bottom sides are used throughout the description, the board may be mounted either way up, the utility of which will become apparent when a system comprising the antenna is described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which: 
         FIG. 1  is a schematic illustration of a dipole element as per the prior art; 
         FIG. 2   a  is a schematic illustration of an examplary dipole element with an independently tunable sleeve where the two monopole elements are on opposite sides of a board; 
         FIG. 2   b  is a schematic illustration of an examplary dipole element with an independently tunable sleeve where the two monopole elements are on a same side of a board; 
         FIG. 3   a  is a side view of an examplary feed network for the dipole element with an independently tunable sleeve as per  FIGS. 2   a  and  2   b;    
         FIG. 3   b  is a front view of an examplary feed network for the dipole element with an independently tunable sleeve as per  FIGS. 2   a  and  2   b;    
         FIG. 4  is a schematic of an examplary antenna element with two dipoles on a same board; 
         FIG. 5   a  is a side view of an examplary feed network for the dipole element with an independently tunable sleeve as per  FIG. 4 ; 
         FIG. 5   b  is a front view of an examplary feed network for the dipole element with an independently tunable sleeve as per  FIG. 4 ; 
         FIG. 6  is a schematic illustration of an examplary dipole element with an independently tunable sleeve with a balancing sleeve; 
         FIG. 7   a  is a side view of an examplary feed network for the dipole element with an independently tunable sleeve with a balancing sleeve; 
         FIG. 7   b  is a front view of an examplary feed network for the dipole element with an independently tunable sleeve with a balancing sleeve; and 
         FIG. 8  is a schematic illustration of an examplary dipole element with an independently tunable sleeve of  FIG. 6  with an additional pair of symmetrical balancing sleeves. 
     
    
    
     It will be noted that throughout the appended drawings, like features are identified by like reference numerals. 
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of a typical dipole element representing the prior art. The element comprises an etched circuit board  100  mounted perpendicularly to a large ground-plane  110  upon which a conductive area  120  of the form shown by the shading has been placed, commonly obtained by etching away the unwanted copper cladding on a layer of dielectric material. This copper area is connected to both the ground-plane  110  and an outer conductor of a coaxial cable feed  130 . These elements are provided on a top layer of the etched circuit board. The second side (or bottom layer) of the element is shown where the centre conductor  150  of the coaxial cable feed is connected to a conductive area  140 . Conductive area  140  acts as a balun to convert the unbalanced nature of the coaxial feed to the balanced nature of the dipole radiating element formed by conductive area  120 . By adjustments to the length, width and position of the dipole shape  120  together with the balun  140 , the characteristic impedance of both the dipole element and coaxial feed may be matched. This design requires a height from the ground-plane usually slightly in excess of one quarter wavelength. 
       FIGS. 2   a  and  2   b  illustrate exemplary embodiments of an antenna element with a balun arrangement that is implemented such that the element can be parallel to the ground-plane rather than orthogonal. This balun arrangement allows for some reduction in height when used in isolation. The dipole element comprises of an etched copper circuit board  200 . As per  FIG. 2   b , on one side of this circuit board conductive areas  210  and  220  are etched to form a dipole element. The conductive part  240  of area  220  from the centre-line to its connection point to the element feed  230  is nominally a one quarter wavelength long transmission line section of suitable impedance to match the dipole to a feed network when adjusted for the dielectric constant of the supporting circuit board  200 . 
     The line may be considered to be of micro-strip form. The dielectric loading of this micro-strip line section means that the quarter wavelength section is significantly shorter than the quarter wavelength in free space used to determine the element dimensions. The exact dimensions are adjusted to achieve the desired performance characteristics. The distance from the top edge of conductive area  210  to the bottom edge of conductive area  220  may be nominally one half wavelength in free space. The second side of the circuit board  200  comprises a grounded conductive area  250  somewhat less than the quarter wavelength of conductive area  210 . This area  250  may be connected to conductive area  210  using connection points  260 , such as vias, and serves as a sleeve. This sleeve has two purposes. Firstly, it acts as a ground-plane for the transmission line  240 . With this ground-plane in place, the transmission line  240  now acts as a balun to connect the balanced nature of the dipole element to the unbalanced nature of the feed network. The second function of sleeve  250  is to act as a radiating sleeve and expand the bandwidth of the radiating element comprising of areas  210  and  220  forming a dipole radiator. The length of the sleeve  250  from the connection points  260  can be varied to adjust the antenna bandwidth as desired within limits providing it is always longer than the dielectrically loaded quarter wavelength required for the balun. Ground points  270  may be provided on the sleeve. Connection feed-point  260  may be connected to the centre conductor of a coaxial cable feed, the outer conductor of which is connected to ground-points  270 . Alternatively the appropriate selection of conductor diameters and spacing of the parallel line feed can be implemented to connect the antenna to a feed network. 
     As per  FIG. 2   a , conductive area  220  may also be provided on an opposite surface to conductive area  210 . In this embodiment, a first monopole is present on surface one and a second monopole is present on surface two. Together, the first monopole and the second monopole form the dipole. The sleeve  250  is provided on the same surface as the second conductive area  220 , in an overlapping relation with respect to conductive area  210 . In both embodiments illustrated, the sleeve  250  is independent of the dipole and can be tuned to obtain an increased bandwidth for a given match level. 
       FIGS. 3   a  and  3   b  illustrate an exemplary embodiment for a feed network to be used with the antenna element of  FIG. 2   a  or  2   b . The side view of  FIG. 3   a  shows the circuit board  200  mounted nominally one quarter wavelength above an electrically conductive ground plane  310 . The precise spacing is determined by the required radiation pattern in manners well known to practitioners of the art. Space  320  may be left unfilled, or alternatively, it may be filled with dielectrics such as foam. Whilst other dielectrics with higher dielectric constants may be used, they are usually precluded by surface mode effects degrading the radiation pattern and or efficiency. A centre conductor  330  of the coaxial cable  340  is connected to the circuit board  300  at point  240  shown in  FIGS. 2   a  and  2   b . The front view of  FIG. 3   b  shows conductors  350  which may be used to connect the outer conductor of the feed coax connected to conductive area  250  at the points  260 . The diameter of these connecting conductors together with their spacing is adjusted to match the characteristic impedance of the feed cable using methods well known to practitioners of the art. 
     In an alternative embodiment, the conductive ground-plane  310  is replaced with a Perfect Magnetic Conductor (PMC) or Electromagnetic Band-Gap (EBG) surface. An EBG reflector exhibits a frequency dependant reflection phase passing through zero degrees at the band-gap centre. This enables the space  320  to be considerably narrowed. Whilst in theory the spacing could be reduced to zero, in practice the spacing is often chosen to be around one tenth to one fifteenth of a wavelength or less. Using the dipole elements illustrated in  FIGS. 2   a  and  2   b , this enables a reduction in the depth of the antenna using air or foam spacing. In addition, the provision of an EBG surface can significantly reduce the transmission of surface waves, thus improving the front to back ratio of the radiated pattern for a given size ground-plane. Alternatively, the size of the ground-plane may be reduced for any given performance required, thus improving both radiation patterns and radiated efficiency. Solid dielectric may be substituted for the air or foam in this embodiment. In some cases the spacing between the dipole and the PMC surface is minimized to ensure the suppression of surface wave propagation which, has been shown to reduce the element gain by 3 dB or more. A spacing of 1/120 wavelengths has been shown to have minimal gain loss when compared with an element ¼ wavelength above a PMC ground-plane element. 
       FIG. 4  illustrates another embodiment for the dipole element with independently tunable sleeve, whereby two orthogonal polarized elements are provided within the same space. The first dipole element comprises an etched copper circuit board  400 . On a first side of this circuit board, conductive areas  410  and  420  are etched to form a dipole element. The conductive part  440  of area  420 , from the centre-line to its connection point at the element feed  430 , is a nominally one quarter wavelength long transmission line section of suitable impedance to match the dipole to a feed network. In some embodiments, the line may be of micro-strip form. The exact dimensions are adjusted to achieve the desired performance characteristics. The distance from the top edge of conductive area  410  to the bottom edge of conductive area  420  being nominally one half wavelength in free space. 
     The second side of the circuit board  400  comprises a grounded conductive area  450  somewhat less than one quarter wavelength. This area is connected to conductive area  410  using vias  460  and represents the sleeve, which acts as a ground-plane for the transmission line  440 . With this ground-plane in place, the transmission line now acts as a balun to connect the balanced nature of the dipole element to the unbalanced nature of the feed network. The sleeve also acts as a radiating sleeve to expand the bandwidth of the radiating element comprising of areas  410  and  420  forming a dipole radiator. The length of this conductive area  450  from the connection points  460  can be varied to adjust the antenna bandwidth as desired within limits, providing it is always longer than the dielectrically loaded quarter wavelength required for the balun. Ground points  470  are provided on the sleeve. Connection feed-point  460  can be connected to the centre conductor of a coaxial cable feed, the outer conductor of which is connected to ground-points  470 . 
     Alternatively, by appropriate selection of conductor diameters and spacing, a parallel line feed can be implemented to connect the antenna to the feed network. A second dipole element is also etched on the copper circuit board  400 , orthogonal to the first dipole element. On the first side of the circuit board  400  conductive areas  415  and  425  are etched to form the second dipole element. The conductive part  445  of area  425 , from the centre-line to its connection point at the element feed  435 , is a nominally one quarter wavelength long transmission line section of suitable impedance to match the dipole to the feed network. The distance from the left edge of conductive area  415  to the right edge of conductive area  425  may be nominally one half wavelength in free space. 
     The second side of the circuit board  400  comprises a grounded conductive area  455  somewhat less than one quarter wavelength. This area is connected to conductive area  415  using vias  465  and serves as the sleeve for the second dipole element. The sleeve acts as a ground-plane for the transmission line  445  and as a radiating sleeve to expand the bandwidth of the radiating element comprising of areas  415  and  425  forming the dipole radiator. The length of this conductive area  455  can be varied to adjust the antenna bandwidth as desired within limits, providing it is always longer than the dielectrically loaded quarter wavelength required for the balun. Ground points  475  are provided on the sleeve. Connection feed-point  465  can be connected to the centre conductor of a second coaxial cable feed, the outer conductor of which is connected to ground-points  475 . 
     Alternatively, by appropriate selection of conductor diameters and spacing, a parallel line feed can be implemented to connect the antenna to a feed network. In this implementation conductive area  425  has been separated from conductive area  445  by a crossover bridge comprising a conductive track  495  having the same width as conductive area  445  and printed on the second side of circuit board  400 . Conductive areas  425 ,  445  and  495  are connected using vias  485 . Alternatively the sides of the board used for creating this orthogonal dipole may be reversed, eliminating the need for the crossover bridge. This alternative embodiment requires that the two dipoles be individually adjusted to compensate for performance differences when mounted above a ground-plane, be it a perfect electrical or magnetic conductor. Also alternatively, the monopoles of each dipole may be provided on opposite sides of the board, as per the embodiment of  FIG. 2   a.    
       FIGS. 5   a  and  5   b  illustrate side and front views, respectively, of the antenna element of  FIG. 4  connected to a feed network. The circuit board  500  is mounted nominally one quarter wavelength above an electrically conductive ground-plane  510 . The precise spacing is determined by the required radiation pattern. Space  520  may be left unfilled or it may be filled with dielectrics, such as foam. Whilst other dielectrics with higher dielectric constants may be used, they are usually precluded by surface mode effects degrading the radiation pattern and/or efficiency. The centre conductor of the coaxial cable  540  is connected to the element balun  445  at point  475  as shown in  FIG. 4 . The outer conductor of the feed coax  540  is connected to dipole sleeve conductive area  455  at the points  435  as shown in  FIG. 4 . The diameter of these connecting conductors together with their spacing is adjusted to match the characteristic impedance of the feed cable. A second coaxial feed  540  is similarly connected to the second dipole element which is orthogonal to the first dipole element. Similarly to the feed network of  FIGS. 3   a  and  3   b , the conductive ground-plane may be replaced with PMC or an EBG with the appropriate space  520 . Also alternatively, the size of the ground-plane may be reduced for any given performance required, thus improving both radiation patterns and radiated efficiency. Solid or perforated dielectric may be substituted for the air or foam in this implementation. 
     In another embodiment, an additional conductive area is added to the dipole element, as illustrated in  FIG. 6 . Conductive area  610  balances the sleeve  250  and may be referred to as a balancing sleeve. The balancing sleeve  610  may be left floating as shown, or connected to the dipole element  220  using vias located at points  620 . This same modification can be applied to the embodiments illustrated in  FIGS. 2   a ,  2   b , and  4 . This modification may be particularly applicable when the elements described are to be used in an arrayed form. For a given return loss, the bandwidth may be further extended by the extra sleeve elements. Alternatively the additional sleeves may provide an improved return loss response for a given bandwidth. 
       FIGS. 7   a  and  7   b  show a dipole antenna element with an additional sleeve  720  incorporated into a feed network. The additional sleeve  720  is laid over the element from which it is separated by a spacer  710 . The spacer  710  may comprise of air, foam, perforated or solid dielectric. 
     In another alternative embodiment for the dipole element, a further additional balancing sleeve  810  may also be placed alongside the element  800 , as per  FIG. 8 . In this case, a pair of identical balancing sleeves  810  are used in addition to the first balancing sleeve  610 , to avoid squinting of the radiation pattern. Various other embodiments for having the low profile antenna with an independent sleeve will be understood by those skilled in the art. Such embodiments will allow the sleeve to be tunable in order to achieve a desired bandwidth. For example, only the pair of sleeves  810  are provided without balancing sleeve. 
     The size and spacing of the sleeve and balancing sleeves may be varied to set the filtering characteristics of the dipole antenna element as desired. In addition, the thickness of the board  200  may be varied to obtain a given coupling. The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.