Patent Publication Number: US-11652291-B2

Title: Tri-frequency multi-polarisation omnidirectional antenna

Description:
FIELD OF THE INVENTION 
     The present invention relates to a tri-frequency multi-polarisation omnidirectional antenna. More particularly, but not exclusively, the present invention relates to a tri-frequency multi-polarisation omnidirectional antenna comprising a substrate having a plurality of electrically conducting loops arranged thereon the loops being dimensioned to excite horizontally polarised TE modes at a plurality of frequencies and a dielectric resonator arranged on the substrate, the dielectric resonator having an electrically conductive probe extending therethrough substantially normal to the substrate to excite vertically polarised TM modes in the dielectric resonator at a plurality of frequencies. 
     BACKGROUND 
     Wi-Fi coverage is improved when one employs a plurality of different frequencies and also a plurality of different polarisations. At present there is no single antenna which can simultaneously provide both a plurality of frequencies and polarisations. A plurality of antennas needs to be employed. 
     The present invention seeks to overcome the problems of the prior art. 
     STATEMENT OF INVENTION 
     Accordingly, the present invention provides a tri-frequency multi-polarisation omnidirectional antenna comprising:
         an electrically insulating substrate comprising first and second faces;   a first plurality of curved electrically conductive strips arranged on the first face and being arranged to form an outer loop with each strip being spaced apart from the adjacent strip of the loop;   a second plurality of curved electrically conductive strips arranged on the first face and being arranged to form an inner loop with each strip being spaced apart from the adjacent strip of the loop;   a third plurality of curved electrically conductive strips arranged on the first face and being arranged to form middle loop with each strip being spaced apart from the adjacent strip of the loop;   the three loops being concentric with each loop centered on a common symmetry point, the axis normal to the first face and passing through the common symmetry point being the symmetry axis;   the middle loop having a larger diameter than the inner loop and the outer loop having a larger diameter than the middle loop;   a first power divider connected to the strips of the outer loop;   a second power divider connected to the strips of the inner loop;   a dielectric resonator comprising a first face, the first face arranged on the first face of the substrate;   an electrically conductive probe being arranged at least partially within the dielectric resonator and extending at least part way along the symmetry axis.       

     The tri-frequency multi-polarisation omnidirectional antenna according to the invention can operate simultaneously at three different frequencies (for example 2.4, 5.2 and 5.8 GHz Wi-Fi bands) and also in both TE and TM modes for dual orthogonal polarisation. This improves Wi-Fi coverage. The antenna is also compact and simple to manufacture. 
     Preferably the loops are circular. This mitigates gain variation in the azimuthal plane and so increases uniformity of coverage. 
     Preferably the dielectric resonator is cylindrical and comprises first and second faces and a side wall extending therebetween, the resonator comprising a dielectric axis extending from the center of the first face to the center of the second face, the dielectric axis being coaxial with the symmetry axis. A cylindrical shape is preferable for obtaining uniform signal coverage. 
     Preferably the dielectric resonator comprises at least one step change in diameter part way along its length. 
     Preferably the dielectric resonator is dimensioned such that when excited by a microwave signal provided to it by the probe it excites vertically polarised TM modes in a lower band and an upper band, the lower band containing 2.4 GHz and the upper band containing 5.2 GHz and 5.8 GHz. 
     Preferably the dielectric resonator is glass. 
     Preferably the tri-frequency multi-polarisation omnidirectional antenna as further comprises a probe signal source connected to the probe, the probe signal source being configured to provide a microwave signal at at least one of 2.4 GHz, 5.2 GHz and 5.8 GHz. 
     Preferably the loops are circular. 
     Preferably the middle loop is arranged proximate to the inner loop such that the two are electrically coupled together to form a composite electrical structure. 
     Preferably the strips of composite electrical structure are dimensioned such that when excited by a microwave signal the composite electrical structure excites horizontally polarised TE modes in an upper frequency band, the upper frequency band containing 5.2 GHz and 5.8 GHz. 
     Preferably the tri-frequency multi-polarisation omnidirectional antenna further comprises a second signal source connected to the second power divider, the second signal source being configured to provide a microwave signal at at least one of 5.2 GHz and 5.8 GHz. 
     Preferably the strips of the outer loop are dimensioned such that when excited by a microwave signal the outer loop excite a horizontally polarised TE mode in a lower frequency band, the lower frequency band containing 2.4 GHz. 
     Preferably the tri-frequency multi-polarisation omnidirectional antenna further comprises a first signal source connected to the first power divider, the first signal source being configured to provide a microwave signal at 2.4 GHz. 
     Preferably the tri-frequency multi-polarisation omnidirectional antenna further comprises an electrically conductive ground plane on the second side of the substrate. 
     Preferably the tri-frequency multi-polarisation omnidirectional antenna further comprises a ring slot in the ground plane. 
     Preferably the tri-frequency multi-polarisation omnidirectional antenna further comprising a recess in the first face of the dielectric resonator arranged about the symmetry axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described by way of example only and not in any limitative sense with reference to the accompanying drawings in which 
         FIG.  1    shows, in perspective view, a tri-frequency multi-polarisation omnidirectional antenna according to the invention. 
         FIG.  2    shows the first face of the electrically insulating substrate of the antenna of  FIG.  1    in plain view from above; 
         FIG.  3    shows the tri-frequency multi-polarisation omnidirectional antenna of  FIG.  1    in vertical cross section; 
         FIGS.  4 ( a ) to  4 ( d )  show simulated and measured parameters of an antenna according to the invention; and 
         FIGS.  5 ( a ) to  5 ( f )  show simulated and measured normalised radiation patterns from the antenna. 
     
    
    
     Shown in  FIG.  1    is a tri-frequency multi-polarisation omnidirectional antenna  1  according to the invention. The tri-frequency multi-polarisation omnidirectional antenna  1  comprises an electrically insulating substrate  2  having first and second faces  3 ,  4 . Arranged on the first face  3  is a plurality of loops  8 ,  12 ,  16  and power dividers  9 ,  13  which are described in more detail with reference to  FIG.  2   . Also arranged on the first face  3  is a dielectric resonator  20  having an electrically conductive probe  27  arranged therein. This is described in more detail with reference to  FIG.  3   . 
     Shown in  FIG.  2    is the first face  3  of the electrically insulating substrate  2  in plain view. Arranged on the first face  3  is a first plurality of curved electrically conductive strips  7 . The strips  7  are arranged to form an outer circular loop  8  with each strip  7  separated from each adjacent strip  7  of the loop  8  as shown. Connected to the strips  7  of the outer loop  8  is a first power divider  9 . The first power divider  9  receives a microwave signal from a first port  10  and provides it to each of the curved strips  7 . The curved strips  7  of the outer loop  8  are dimensioned such that when excited by a microwave signal from the first port  10  the outer loop  8  excites a horizontally polarised TE mode in a lower frequency band, the lower frequency band containing 2.4 GHz. 
     Also arranged on the first face  3  is a second plurality of curved electrically conductive strips  11 . The strips  11  are arranged to form a circular inner loop  12  with each strip  11  separated from each adjacent strip  11  in the loop  12  as shown. Connected to the strips  11  of the inner loop  12  is a second power divider  13  which receives a microwave signal from a second port  14  and provides it to each of the curved strips  11 . 
     Also arranged on the first face  3  is a third plurality of curved electrically conductive strips  15 . The strips  15  are arranged to form a circular middle loop  16  with each strip  15  separated from each adjacent strip  15  in the loop  16  as shown. 
     The outer, inner and middle loops  8 ,  12 ,  16  are concentric with each loop  8 ,  12 ,  16  centered on a common symmetry point  17 . The axis  18  passing through the common symmetry point  17  normal to the first face  3  of the substrate  2  is termed the symmetry axis  18 . 
     The middle loop  16  has a larger diameter than the inner loop  12 . The outer loop  8  has a larger diameter than the middle loop  16 . The middle loop  16  is arranged proximate to the inner loop  12  such that the two are electrically coupled together to form a composite electrical structure  19 . The effect of this is that the parasitic strips  15  of the middle loop  16  tune the modes of the inner loop  12 . 
     The curved strips  11 ,  15  of the composite electrical structure  19  are dimensioned such that when excited by a microwave signal from the second port  14  the composite electrical structure  19  excites horizontally polarised TE modes in an upper frequency band, the upper frequency band containing 5.2 GHz and 5.8 GHz. 
     Shown in  FIG.  3    is the tri-frequency multi-polarisation omnidirectional antenna  1  of  FIG.  1    in vertical cross section. Arranged on the first face  3  is a cylindrical dielectric resonator  20 . The dielectric resonator  20  is typically a glass. The dielectric resonator  20  comprises first and second faces  21 ,  22  and a side wall  23  extending therebetween. The dielectric resonator  20  has a dielectric axis  24  which extends from the middle of the first face  21  to the middle of the second face  22 . The first face  21  of the dielectric resonator  20  is arranged on the first face  3  of the substrate  2  with the dielectric axis  24  coaxial with the symmetry axis  18  as shown. The dielectric resonator  20  has a step change in diameter part way along its length. Arranged in the first face  21  of the dielectric resonator  20  is a small recess  25  which reduces the cross polarised fields generated by the power dividers  9 ,  13 . 
     An aperture  26  extends through the dielectric resonator  20  along the dielectric axis  24 . Extending along the dielectric axis  24  is an electrically conductive probe  27  which is connected to a probe port  28  extending through the substrate  2  proximate to the dielectric axis  24 . The dielectric resonator  20  is dimensioned such that when excited by a microwave signal provided by the probe  27  it excites vertically polarised TM modes in a lower band and an upper band, the lower band containing 2.4 GHz and the upper band containing 5.2 GHz and 5.8 GHz. 
     The probe port  28  is connected to a probe signal source  29 . The probe signal source  29  is configured to provide a microwave signal at at least one of 2.4 GHz, 5.2 GHz and 5.8 GHz. Preferably the probe signal source  29  is configured to provide a microwave signal at all three frequencies. The first port  10  is connected to a first signal source  30 . The first signal source  30  is configured to provide a microwave signal in a lower band at 2.4 GHz. The second port  14  is connected to a second signal source  31 . The second signal source  31  is configured to provide a microwave signal in an upper band containing 5.2 GHz and 5.8 GHz. The antenna  1  further comprises a ring slot  32  etched in an electrically conductive ground plane  4   a  on the second side  4  of the substrate  2  for impedance matching of the probe port  28 . 
     In use the probe signal source  29  provides a microwave signal to the probe  27 . The probe  27  excites vertically polarised TM modes in the dielectric resonator  20  at one or more of 2.4 GHz, 5.2 GHz and 5.8 GHz. Simultaneously, the first signal source  30  provides a microwave signal to the strips  7  of the outer loop  8  which excites horizontally polarised TE modes in the strips  7  of the outer loop  8  at 2.4 GHz. In addition, the second signal source  31  provides a microwave signal to the strips  11  of the inner loop  12  which excites horizontally polarised TE modes in the composite electrical structure  19  of at least one of 5.2 GHz and 5.8 GHz. In use the antenna  1  according to the invention can therefore operate at all of 2.4 GHz, 5.2 GHz and 5.8 GHz in both TE and TM modes. 
       FIGS.  4 ( a ) to  4 ( d )  show the simulated and measured S-parameters, antenna gains, total efficiencies and envelope correlation coefficients for an antenna according to the invention. With reference to  FIG.  4 ( a )  the measured −10 dB impedance passbands for the probe port ( 28 ) are 2.2-2.7 GHz and 5.05-5.86 GHz. As for the first and second ports ( 10 ,  14 ) the measured passband is 2.38-2.5 GHz for the first port ( 10 ) and 4.9-5.27 GHz and 5.6-5.85 GHz for the second port ( 28 ). Overlapped bandwidths are 4.9% (2.38-2.5), 4.26% (5.05-5.27) and 4.36% (5.6-5.85) fully covering the three Wi-Fi bands. Among the three pairs of modes of orthogonal polarisations there are five dielectric resonator modes and one dielectric resonator loaded probe mode. Also, the port isolations are as high as 30 dB, 25 dB and 20 dB of the three bands. As shown in  FIG.  4 ( b )  the measured vertically polarised antenna gains at (φ=0 degrees, θ=sixty degrees) for the probe port ( 28 ) are 1, 0.3 and 1.3 dBi at the 2.4-, 5.2-, and 5.8-GHz bands respectively. The measured horizontally polarised gain at this direction is 3 dBi at 2.4 GHz band at the first port ( 10 ) whereas they are 2 and -0.75 dBi at the 5.2 and 5.8 GHz bands for the second port ( 14 ). The relatively low gain of the 5.8 GHz band is because the peak radiation direction is not at θ=sixty degrees, caused by the increased titled angle of radiation patterns. As can be seen from  FIG.  4 ( c )  the antenna total efficiencies of the probe port ( 28 ) are 93%, 77% and 72% at the three bands which takes impedance matching into account. As for the first and second ports ( 10 ,  14 ) the efficiencies of the three bands are 80%, 73% and 70%. The antenna efficiency of the 5 GHz band is generally lower than that of the 2.4 GHz band. This is expected as the loss caused by the glass increases with frequency. The envelope correlation coefficients (ECCs) of the antenna, which is a performance index for a MIMO antenna, are shown in  FIG.  4 ( d ) . The simulated and measured ECCs were obtained from radiation patterns. Both the simulated and measured ECCs between different polarised ports are lower than −20, −28 and −18 dB for the 2.4-, 5.2- and 5.8 GHz bands respectively. Due to the orthogonal polarisation they are desirably much lower than the criteria of −3 dB. 
       FIGS.  5 ( a ) to  5 ( f )  show simulated and measured normalised radiation patterns at 2.44, 5.2 and 5.8 GHz.  FIGS.  5 ( a ) to  5 ( c )  show the patterns for a signal provided to the probe port ( 28 ).  FIG.  5 ( d )  shows the pattern for a signal provided to the first port ( 10 ).  FIGS.  5 ( e ) and  5 ( f )  show the radiation pattern for a signal provided to the second port ( 14 ). The solid lines show the simulated patterns. The dotted lines show the measured patterns.