Abstract:
Systems and devices relating to antennas and antenna systems. A horizontal omnidirectional antenna has two dipoles with each dipole being in a V-configuration such that the arms of the dipole define an angle. The two dipoles are arranged so that the angles defined by each of the dipoles face and open toward each other. The horizontal omnidirectional antenna can be configured to operate with specific frequency bands. By nesting two instances of this antenna, with one configured for high band frequencies and one configured for low band frequencies, a dualband omnidirectional antenna can be obtained. The resulting antenna is physically compact and can be used in small MIMO systems along with vertical omnidirectional antennas.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to antennas. More specifically, the present invention relates to a physically small horizontal omnidirectional antenna which can be configured for high frequency band or low frequency band applications. 
       BACKGROUND 
       [0002]    The telecommunications revolution of the late 20th and early 21st century has led to a need and a demand for access to wireless services. Access to signals ranging from wireless networking signals to mobile telephone signals, there is now a call for wireless services to be available and ubiquitous as possible. While such wireless services are now possible, there is a further demand that equipment providing such services be as unobtrusive as possible. To this end, antennas which provide access to radio signals for such services are, preferably, as small and unobtrusive as possible. 
         [0003]    Since the users accessing the wireless services may be at any angle to the antenna, a MIMO arrangement including at least a vertically polarized and a horizontally polarized omnidirectional antenna is the most logical choice for quite a few applications. Unfortunately, current horizontally polarized omnidirectional antennas are notorious for being large and bulky. Being able to provide physically small horizontally polarized omnidirectional antennas allows for a number of advantages. For one, a smaller antenna would allow for arrays with more elements and, therefore, a higher number of MIMO (multiple in, multiple out) data streams for the same amount of physical array area. 
         [0004]    It should be noted that implementation of antennas in MIMO arrangements, which can increase the data capacity of wireless networks, is usually required for new base station antennas or access points. For indoor MIMO applications when the antenna is mounted on the ceiling, two types of omnidirectional antennas are required—one which includes a horizontal polarized antenna and another which includes a vertical polarized antenna. For clarity, one antenna needs to have its electrical field in the θ direction (usually referred to as a vertical omnidirectional antenna) and the other antenna needs to have its electrical field in the Φ direction (usually referred to as a horizontal omnidirectional). A vertically polarized omnidirectional antenna is easily achievable by using a monopole. However, producing a horizontally polarized omnidirectional pattern based on the duality theorem requires a uniform current loop. A uniform loop of current is only achievable when the dimensions of the loop are very small compared to the signal wavelength. Such an antenna would have a very low efficiency and cannot be used for indoor access points. Conversely, loops with a larger radius cannot provide the required uniform current distribution around the loops since the current will change its direction after a half wavelength of signal. 
         [0005]    To provide the required uniform current distribution, different approaches have been used in literature including an antenna using four dipoles with each being fed with proper phase and amplitude to provide the required current distribution. Another approach is the Alford loop (and its derivatives). Unfortunately, the Alford loop requires special feeding approaches and is only suitable for certain technologies. 
         [0006]    Other current technologies for omnidirectional antennas have the drawback of high circuit complexity required for MIMO feeds. Another drawback is the requirement for a large physical area for the antenna array as each omnidirectional antenna can be physically large.  FIGS. 1A and 1B  illustrate two previous attempts at a horizontally polarized omnidirectional antenna.  FIG. 1A  shows an Alford loop strip antenna from U.S. Pat. No. 5,767,809.  FIG. 1B  shows an omnidirectional planar antenna (see DEVELOPMENT OF A BROADBAND HORIZONTALLY POLARIZED OMNIDIRECTIONAL PLANAR ANTENNA AND ITS ARRAY FOR BASE STATIONS X. L. Quan, R. L. Li*, J. Y. Wang, and Y. H. Cui School of Electronic and Information Engineering, South China University of Technology, Guangzhou 510641, China, Progress In Electromagnetics Research, Vol. 128, 441-456, 2012). 
         [0007]    There is therefore a need for systems and devices which mitigate if not overcome the shortcomings of the prior art. 
       SUMMARY 
       [0008]    The present invention provides systems and devices relating to antennas and antenna systems. A horizontally polarized omnidirectional antenna has two dipoles with each dipole being in a V-configuration such that the arms of the dipole define an angle. The two dipoles are arranged so that the angles defined by each of the dipoles face and open toward each other. The omnidirectional antenna can be configured to operate with specific frequency bands. By nesting two instances of this antenna, with one configured for high band frequencies and one configured for low band frequencies, a dualband omnidirectional antenna can be obtained. 
         [0009]    In a first aspect, the present invention provides an antenna comprising:
       a first dipole having a first arm extending outwardly from a first center of said first dipole and a second arm extending outwardly from said first center;   a second dipole having a third arm extending outwardly from a second center of said second dipole and a fourth arm extending outwardly for said second center;
 
wherein
   said first arm and said second arm define a first angle with a first opening and with said first center being a first vertex of said first angle;   said third arm and said fourth arm define a second angle with a second opening and with said second center being a second vertex of said second angle;   said first and second dipoles being constructed and arranged such that said first opening and said second opening face each other;   said antenna is a horizontally polarized omnidirectional antenna.       
 
         [0016]    In a second aspect, the present invention provides an antenna comprising:
       two assemblies for use as antenna elements, a first assembly being nested inside a second assembly, said second assembly comprising:
           a first dipole having a first pair of arms, said first pair of arms being in a V-configuration defining a first opening;   a second dipole having a second pair of arms, said second pair of arms being in another V-configuration defining a second opening;
 
wherein
   
           said first opening and said second opening are facing each other;   said first assembly is located between said first opening and said second opening.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The embodiments of the present invention will now be described by reference to the following figures, in which identical reference numerals in different figures indicate identical elements and in which: 
           [0023]      FIGS. 1A and 1B  illustrate examples of horizontally polarized omnidirectional antenna according to the prior art; 
           [0024]      FIG. 2  illustrates one implementation of an omnidirectional antenna according to one aspect of the invention; 
           [0025]      FIG. 3  is a schematic diagram for use in explaining a concept of one aspect of the invention; 
           [0026]      FIG. 4  illustrates one example of co-polarization and cross-polarization patterns for a horizontal omnidirectional antenna; 
           [0027]      FIG. 5  shows current distributions on dipoles according to one aspect of the invention; 
           [0028]      FIG. 6  is an illustration of 2D radiation patterns for a horizontal omnidirectional antenna; 
           [0029]      FIG. 7  shows 3D simulated radiation patterns for one implementation of the present invention; 
           [0030]      FIG. 8  is an illustration of another implementation of one aspect of the present invention; 
           [0031]      FIGS. 9 and 10  schematically illustrate variants of angle configurations which may be used with implementations of the invention; 
           [0032]      FIG. 11  shows 2D radiation patterns for differently angled dipoles used in omnidirectional antennas; 
           [0033]      FIG. 12  are 3D radiation patterns for 60 degree dipoles used in an omnidirectional antenna according to another aspect of the invention; 
           [0034]      FIGS. 13 and 14  are pictures illustrating a dual band omnidirectional antenna which use two instances of an omnidirectional antenna according to another aspect of the invention; 
           [0035]      FIGS. 15, 16, and 17  show measured 2D patterns for the antennas illustrated in  FIGS. 13 and 14  for three different frequency bands; 
           [0036]      FIGS. 18 and 19  show examples of co-polarization and cross-polarization measured 3D patterns for two different frequency bands at 2496 MHz and 912 MHz; 
           [0037]      FIG. 20  illustrates a 4*4 MIMO antenna utilizing one aspect of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    Referring to  FIG. 2 , one implementation of antenna according to one aspect of the invention is illustrated. As can be seen, the antenna  10  includes a first dipole  20  and a second dipole  30 . The first dipole  20  has two arms  40 A,  40 B extending outwardly from a center  50 . Similarly, the second dipole  30  has two arms  60 A,  60 B extending outwardly from a center  70 . The two arms  40 A,  40 B define an angle A between them while arms  60 A,  60 B also define an angle B between them. The two dipoles  20 ,  30  are configured so that angles A and B are facing each other, i.e., each pair of arms open towards the other pair. It should be clear that center  50  acts as the vertex for angle A while center  70  acts as the vertex for angle B. It should be noted that  FIG. 2  includes a splitter  75  used for splitting a signal between the two dipoles. As can be seen, the signal is split between the two dipoles. It should further be noted that the output cables from the splitter to the dipoles are of the same length. The length of these cables can be adjusted or replaced to adjust the resulting patterns. 
         [0039]    To explain the invention, it should be noted that if two currents with opposite directions are separated from each other by a distance d, there will always be a null in the pattern along their normal bisecting plane. This will reduce the cross polarization component in the main planes. The spacing between the currents as shown in  FIG. 3  will determine the location of the maximum peak of the elevation patterns. Referring to  FIG. 3 , when current is travelling in the directions indicated by A, B, C and D in  FIG. 3  emit an electromagnetic field, the electromagnetic pattern produced be a omnidirectional pattern with a null in the middle and an electrical field in Φ direction. This is very similar to the pattern of a vertical monopole with an electrical field in θ direction. The co-polarization and the cross-polarization patterns in 3D are shown in  FIG. 4 . In  FIG. 4 , in the Φ direction, Φ is a dependent unit vector with angle Φ measured from the x-axis while in the θ direction, θ is a dependent vector with angle θ measured from the z-axis. 
         [0040]    One main challenge is in how to produce the current distribution shown in the figures. The approach taken in the present invention only requires two dipoles. Since the spacing between the two dipoles can be small, the resulting antenna can be physically small. As well, the feeding network can also be simple such as one where both dipoles are fed using, in one implementation, a 3 dB splitter (e.g. element  75  in  FIG. 2 ) with two output cables. This approach is schematically illustrated in  FIG. 5 . In this approach, two dipoles, each in a V-configuration, is placed in front of one another with their openings facing each other as in the figure. By judiciously feeding each dipole with a signal from a splitter such that the current is as shown on the right side of  FIG. 5 , the resulting 2-D radiation pattern in  FIG. 6  is achieved. A 3-D radiation pattern for the ideal version of the V-configuration horizontal omnidirectional antenna is illustrated in  FIG. 7 . 
         [0041]    Regarding implementation, the dual dipoles of the antenna can be implemented as illustrated in  FIG. 2 . In  FIG. 2 , the dipoles are implemented as metallic traces on a printed circuit board with each arm of each dipole extending outwardly from each dipole&#39;s respective center. In another embodiment,  FIG. 8  illustrates a metallic rod or wire implementation of the present invention. As can be seen,  FIG. 8  uses similar reference numbers parts similar to those in  FIG. 2 . 
         [0042]    In terms of variants, it should be noted that the angles A and B (as noted in  FIGS. 2 and 2 ) may be varied.  FIGS. 2 and 8  illustrate implementations where the angles A and B between the arms are both at 90 degrees. However, other angles are also possible.  FIG. 9  illustrates a top down schematic view of another implementation of the invention where the angles A and B are set at 60 degrees.  FIG. 10  illustrates another top down schematic view of another implementation, this time where the angles A and B are set at 120 degrees. It should be clear that the angles A and B may be considerably varied and the resulting antenna will still be useful. Experiments have shown that an angle between the arms as low as 50 degrees and as high as 120 degrees will still yield an antenna that is useful.  FIG. 11  illustrates the 2D radiation pattern for various angles while  FIG. 12  illustrates the 3D radiation pattern for a dual dipole antenna according to the invention where the angle between the arms is set to 60 degrees. 
         [0043]    It should be clear that the implementations illustrated in the Figures use symmetrical dipoles as in each dipole is a mirror of the other dipole. However, this is not necessary as antennas where one dipole has a different angle from the other dipole. To clarify, if one uses the terminology used for  FIG. 10 , angles A and B can be different. Such an antenna would produce asymmetrical beams and may be useful for some applications. 
         [0044]    It should also be clear that the implementations illustrated in the Figures use symmetrical dimensions for the arms. This means that the same dimensions for the arms are used for the two dipoles, i.e. dipole arm length is constant for the two dipoles. However, implementations where one dipole has one arm longer than the other are also possible. The other dipole can also have one dipole arm longer than the other, resulting in a rectangular top down outline of the dipole arms. For the symmetrical implementation illustrated in the Figures, the top down outline of the dipole arms is that of a square. 
         [0045]    It should be noted that the resulting dual dipole antenna may be used for different frequency bands. The spacing between the two dipoles would be dependent on the frequencies (and thereby wavelengths) of the signals for which the antenna will be used. Experiments have shown that the dipoles can be separated by a distance of between 0.3 to 0.7 of a signal wavelength. 
         [0046]    It should be clear that, as noted above, the preferred separation distance is between 0.3 to 0.7 of a signal wavelength. For a certain frequency band, implementations have used a frequency whose wavelength is approximately midway through the frequency band for the distance calculations. As an example, for a desired frequency band of between 1695 MHz-2690 MHz (or 1.695 GHz to 2.690 GHz), a middle frequency of approximately 2.2 GHz can be used. For such a middle frequency, the signal wavelength would be approximately 136 mm. Since the separation is desired to be between 0.3 to 0.7 of a signal wavelength, a separation of 0.5 (or half) of the 136 mm wavelength can be used. This results in a separation distance between the dipoles of 68 mm. With such a separation distance, and taking the extremes of the frequency band of 1.695 GHs to 2.690 GHz (i.e. of a wavelength band of from 178.7 mm to 111.4 mm), the separation distance between the two dipoles therefore ranges from 0.38 of the longest wavelength to 0.61 of the shortest wavelength in the desired frequency band. For clarity, the 68 mm fixed separation distance is equal to 0.38×178.7 mm (the longest wavelength in the desired frequency band) and to 0.61×111.44 mm (the shortest wavelength in the desired frequency band). Care should be taken when determining the separation distance between the dipoles so that, preferably, this distance remains between 0.3 to 0.7 of any wavelength in the desired frequency range. This is preferred to ensure that a proper omnidirectional pattern is produced. 
         [0047]    In another implementation of the invention, an antenna for use with the 698-960 MHz frequency band had a separation distance of 160 mm between the two vertices of the dipoles. In another implementation, an antenna for use with the 1695-2690 MHz frequency band had a spacing of 60 mm between the two vertices of the dipoles. 
         [0048]    For clarity, the distance between the dipoles is, in this case, measured to be the distance between the vertices of the two dipoles. 
         [0049]    Since the antenna may be configured for different frequency bands, a dual band antenna using nested V-configured antennas can be created. A low band antenna configured for low frequencies can be created while a high frequency antenna can be placed in the space between the V-configured dipoles of the low band antenna. Such a two-port dual band antenna is illustrated in  FIGS. 13 and 14 . 
         [0050]    As can be seen in  FIGS. 13 and 14 , a first dual dipole antenna is placed in the space between two dipoles of a second dual dipole antenna. The first antenna is physically smaller than the second antenna and is configured to operate with a frequency band that is different from the frequency band for the second antenna. In one implementation, the first antenna is configured for a high band frequency range (e.g. 1710-2690 MHz) while the second antenna is configured for a low frequency band (e.g. 698-960 MHz). When the first antenna and the second antenna are combined, the resulting dual band omnidirectional antenna (fed by a diplexer) can be used in an antenna panel for use in MIMO applications. 
         [0051]    It should be clear that, following from the example illustrated in  FIG. 2 , two splitters would be used for the dual band omnidirectional antenna. One splitter would be used for high band signals while a second splitter would be used for low band signals. The first splitter would feed the high band dipoles while the second splitter would feed the low band dipoles. 
         [0052]    For the dual-band omnidirectional antennas in  FIGS. 13 and 14 , the measured 2D patterns for these antennas at three different frequency bands are presented in  FIGS. 15, 16, and 17 .  FIG. 15  shows the measured omnidirectional patterns for the 698-960 MHz band.  FIG. 16  shows the measured omnidirectional pattern for the 2.3-2.690 GHz frequency band.  FIG. 17  shows the measured omnidirectional pattern for the 1.850-1995 GHz frequency band. 
         [0053]    Similar to  FIGS. 15-17 ,  FIGS. 18 and 19  show examples of co-polarization and cross-polarization measured 3D patterns for two different frequency bands for the dual-band omnidirectional antennas in  FIGS. 13 and 14  respectively at 2496 MHz for high band and 912 MHz for lowband. 
         [0054]    It should be noted that aspects of the invention may be used in various antenna configurations. Referring to  FIG. 20 , one aspect of the invention is illustrated in a MIMO-antenna. The 4*4 MIMO antenna in  FIG. 20  has two dualband horizontal polarized omnidirectional antennas  100 A,  100 B, each being connected to the two ports of a diplexer  105 A,  105 B to provide two ultra wideband horizontal polarized ports. These horizontal omnidirectional antennas as similar to the antennas illustrated in  FIGS. 13 and 14 . Also present in  FIG. 20  are two ultra wideband monopoles  110 A,  110 B to provide two vertical polarization ports. The omnidirectional antennas  100 A,  100 B, in combination with the monopoles  110 A,  110 B provides an ultra wideband 4*4 MIMO with enough space for a sniffer port  120  in the middle of the assembly. 
         [0055]    A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above all of which are intended to fall within the scope of the invention as defined in the claims that follow.