Abstract:
Antenna systems employing multiple linear polarization antennas that have capacitively loaded magnetic dipoles and that are magnetically coupled to generate a circular polarization. In a first embodiment of the present invention, two intersecting linearly polarized antennas elements are arranged to obtain a circular polarization. In a second embodiment, a first linearly polarization antenna is placed orthogonally to a second linearly polarization antenna where a single active feed excites the first linearly polarization antenna.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application relates to concurrently filed, co-pending application U.S. patent application Ser. No. 09/781,720, filed on Feb. 12, 2001, entitled “Magnetic Dipole Antenna Structure and Method” by Eli Yablonovitch et al., owned by the assignee of this application and incorporated herein by reference. 
    
    
     BACKGROUND INFORMATION 
     1. Field of the Invention 
     The present invention relates generally to the field of wireless communication, and particularly to the design of an antenna. 
     2. Description of Related Art 
     Small antennas are attractive in portable wireless communication devices. One type of compact antennas uses a circular polarization. A circularly polarized antenna may improve the performance of a mobile system. To produce a resonant antenna structure at a certain radio frequency and within a certain bandwidth with a circular polarization, classical antenna structures need to have a certain volume. This volume is fairly large as the bandwidth required is large, especially as the antenna needs to be symmetrical to meet the circular polarization constraint. 
     Accordingly, the present invention addresses the needs of small compact circularly polarized antennas with possibly a wide bandwidth that could be integrated in a mobile device. 
     SUMMARY OF THE INVENTION 
     The present invention provides an antenna system using capacitively loaded magnetic dipoles, magnetically coupled in order to obtain a circular polarization. In a first embodiment of the present invention, two intersecting linearly polarized antenna elements are arranged to obtain a circular polarization. In a second embodiment, a first linearly polarization antenna is placed orthogonally to a second linearly polarization antenna where a single active feed excites the first linearly polarized element. In terms of lengths, the first linear polarization antenna can have a length that is greater or less than the length of the second linear polarization antenna. On the vertical or z-axis, the first linear polarization antenna can be positioned above, below, or at the same level as the second linear polarization antenna. One or more elements, such as an electronic chip, can be inserted between the first linear polarization antenna and the second linear polarization antenna without disturbing the circular polarization generated from the magnetically coupled first and second linearly polarized antennas. In a third embodiment, a circular polarization antenna structure is constructed with two curved linear polarization antennas. In a fourth embodiment, an antenna system that is able to alternate between right hand circular polarization (RHCP) and left hand circular polarization (LHCP). In a fifth embodiment, an antenna system is configured to tune to a wider frequency band. 
     Advantageously, the antenna system in the present invention allows for circularly polarized waves while occupying a small volume. The present invention further advantageously provides high isolation and strong frequency selectivity through the use of capacitively loaded magnetic dipoles. 
     Other structures and methods are disclosed in the detailed description below. This summary does not purport to define the invention. The invention is defined by the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a structural diagram of a first embodiment of a circular polarization antenna employing two crossing antennas feed by a 90-degree phase shifter in accordance with the present invention. 
     FIG. 2 illustrates a structural diagram of a second embodiment of a circular polarization antenna employing two orthogonal antennas with a single feeding point coupled to a coaxial cable in accordance with the present invention. 
     FIG. 3 illustrates a Smith Chart displaying a mode when an antenna is not in an optimized configuration. 
     FIG. 4 shows a Smith Chart that of an optimized antenna showing efficient circularly polarized mode in accordance with the present invention. 
     FIG. 5 is a structural diagram illustrating a linearly polarized antenna element connected to a coaxial waveguide that is applicable to the first and second embodiments in accordance with the present invention. 
     FIG. 6 is a structural diagram illustrating a linearly polarized antenna with a coplanar feeding structure that is applicable to the first and second embodiments in accordance with the present invention. 
     FIG. 7 is a structural diagram illustrating a circular polarization antenna employing two antennas having different lengths applicable to the second embodiment in accordance with the present invention. 
     FIG. 8 is a structural diagram illustrating a circular polarization antenna employing two antennas that are placed at different vertical location on z-axis applicable to the second embodiment in accordance with the present invention. 
     FIG. 9 is a structural diagram illustrating a circular polarization antenna showing inclusion of one or more elements within the circular polarization antenna structure in accordance with the present invention. 
     FIG. 10 is a structural diagram illustrating a top view of a third embodiment in a circular polarization antenna employing two antennas with circular shape in accordance with the present invention. 
     FIG. 11 shows a structural diagram illustrating a fourth embodiment enabling a diversity structure using switching elements in accordance with the present invention. 
     FIG. 12A is a structural diagram illustrating a fifth embodiment showing an antenna configuration for a multi-frequency solution in accordance with the present invention. 
     FIG. 12B is a structural diagram exhibiting a top view of the fifth embodiment of a multi-mode solution in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     The antenna system provided according to the principles of this invention comprises a plurality of antenna elements arranged orthogonally comprising of capacitively loaded magnetic dipoles. The advantages of using capacitively loaded magnetic dipoles are that they offer a high isolation and a strong selectivity for a high K factor, with K as defined using the Wheeler&#39;s law: 
      Δ f/f=K.V/λ   3    
     This law relates the relative bandwidth Δf/f that represents the frequency bandwidth over the frequency. λ is the wavelength. The term V represents the antenna mode volume which is enclosed by the antenna. This volume so far as been a metric and no discussion has been made on the real definition of this volume and the relation to the K factor. 
     FIG. 1 is a structural diagram illustrating the first embodiment of a circular polarized antenna  10  that is comprised of two linearly polarized antennas. A first linearly polarized antenna  11  is connected to a wave-guide as well as a second linearly polarized antenna  12  is connected to another wave-guide. The first linearly polarized antenna  11  is positioned orthogonal relative to the second linearly polarized antenna, where the midpoint of the first antenna  11  crosses with the midpoint of the second antenna  12 . Although FIG. 1 illustrates the midpoint of the first linearly polarized antenna  11  crossing the midpoint of the second linearly polarized antenna  12 , one of ordinary skill in the art should recognize that the first linearly polarized antenna  11  can cross the second linearly polarized antenna  12  at a location other than the midpoint. The feeding system provides a 90-degree phase shift between the first element  11  and the second element  12 . A splitter  13  is coupled to the first linearly polarized antenna  11  and the second linearly polarized antenna  12 . A single feed  14  is coupled to the splitter. The splitter occupies additional space and may reduce the efficiency of the antenna. The first linearly polarized antenna  11  and the second linearly polarized antenna  12  placed orthogonally relative to one another, where a signal line can excite either the first linearly polarized antenna  11  or the second linearly polarized antenna  12  with a quarter wavelength difference in length. Preferably, only one feeding point is necessary, either a signal feeding into the first linearly polarized antenna  11  or feeding into the second linearly polarized antenna  12  without the use of the splitter  13 . 
     FIG. 2 is a structural diagram illustrating a second embodiment of a circular polarized antenna  20  comprising of a first linearly polarized antenna  21  coupled to a single feed  23  coupled to a coaxial cable and a plurality of optimized distances to get a minimized axial ratio. The feed  23  is coupled to an inner conductor of a coaxial cable  24 . A second antenna element  22  is positioned perpendicular to the first antenna element  21 , where both the second antenna element  22  and the first antenna element  21  are on the same z-plane. In this example, the bottom of the first antenna element  21  sits at z=0, while the second antenna element  22  also sits at z=0. It is apparent to one of ordinary skill in the art that the first antenna element  21  and the second antenna element  22  can also be positioned on positive z-axis or on negative z-axis. The first antenna element  21  does not cross with the second antenna element  22  symmetrically in order to avoid the cancellation of magnetic coupling. The first antenna element  21  excites the parasitic second element  22  producing a circularly polarized mode as well as a linearly polarized mode. 
     The merger of the linearly polarized and circularly polarized mode can be accomplished by reducing the interaction between the two elements by increasing the distance between the first element  21  and the second element  22 , and moving the passive element  22  farther away from the feeding point  23 . 
     FIG. 3 is a Smith Chart showing the results of an antenna that has not been optimized. In that case, the loop in the middle of the Smith chart is quite large. This shows that the two modes have different frequencies. By reducing the coupling between both elements, it is possible to reduce the diameter of the loop and then to gather both modes at the same frequency, so that the circular polarized mode has a high efficiency. 
     FIG. 4 shows a Smith Chart  40  that illustrates a reduction in magnetic coupling between a first antenna element and a second antenna element. The smaller loop indicates a reduction in coupling. 
     FIG. 5 is a structural diagram illustrating an embodiment of an antenna feed system  50  having an antenna  51  and a coaxial cable  57 . The antenna  51  has a top plate  52 , a middle plate  53 , and a bottom plate  54 . The antenna  51  is coupled to a first point  55  and a second point  58 , where the first point  55  is coupled to a center conductor  56  of a coaxial waveguide  57 , and a second feed  58  is coupled to an outer conductor  59  of a coaxial waveguide  57 . 
     FIG. 6 is a structural diagram illustrating an embodiment of an antenna feed system  60 . An antenna element  61  is placed on a micro-strip or a coplanar waveguide  62 . Additional antenna elements can be added for placement on the coplanar waveguide  62 . 
     FIG. 7 shows a structural diagram illustrating an antenna system  70  with antenna elements of unequal dimensions. A first antenna element  74  having a length L 1  is positioned perpendicular to a second antenna element  75  having a length L 2 , where the length of L 2  is greater than the length of L 1 , or represented in mathematical form, L 1 &lt;L 2 . Alternatively, the length L 1  in the first antenna element  74  can be selected to be less than the length L 2 , or L 1 &gt;L 2 . A further variation of the antenna system  70  is to select the length L 1  equal to the length of L 2 , or L 1 =L 2 . In this example, the bottom of the first antenna element  74  sits at z=0 position, while the bottom of the second antenna element  75  sits at z≠0 position, which means that the second antenna element  75  is positioned on positive z-axis or on negative z-axis. 
     FIG. 8 shows a structural diagram illustrating an antenna system configuration  80 . A first element  81  is coupled to a feed  83 . In this example, a second element  82  is positioned at a lower level than the first element  81 . More specifically, the first antenna element  81  sits at z=0 position, while the second antenna element  82  sits at z=z 1  position, where z 1  represents a negative z-axis, or z&lt;0. One of ordinary skill in the art should recognize that z 1  can be selected to be on positive z-axis, or z&gt;0. The level difference is smaller than the height of the element placed in the lower position. This configuration is used when there is a need for a particular volume to be achieved by the antenna. 
     FIG. 9 shows a structural diagram illustrating an antenna system configuration  90 . A first antenna element  91  is arranged orthogonal to a second antenna  92 . The magnetic coupling between element  91  and element  92  is not disturbed when an external element  93 , such a chip or an electronic component, is placed between the first element  91  and the second element  92 . The antenna behavior is not changed. 
     FIG. 10 is a diagram showing the top view of an antenna arrangement  100 . A first antenna element has been changed to a curved form  101 . A second antenna element of a curved form  102  is placed orthogonally in relation to the first curved antenna element  101 . Different shapes of the elementary radiating parts can be employed just as long as they are placed orthogonally from each other. Although curved antennas are shown in this embodiment, other geometric shapes are possible for implementing the present invention, such as circular, square, and s-type curve. Furthermore, the length of the curved antenna  101  can be of the same or different length than the curve antenna  102 . 
     FIG. 11 shows a structural diagram that utilizes three antenna elements  110 . A central antenna element  113  is coupled to a feed  114 . A parasitic antenna element  111  is placed perpendicular to one side of the feed antenna element  114 . A control element  112  is coupled to the passive antenna element  111 . A parasitic antenna element  115  is placed orthogonally with respect to the feed antenna element  113  on the opposite side of the parasitic antenna element  111 . Attached to the elements  111  &amp;  115  are control elements respectively  112  &amp;  116 . The control element is an active component that switches from an open circuit to a short circuit; the first element is alternate of the second element. Depending on which side the parasitic element is not short-circuited, it is possible to control between left hand circular polarization (LHCP) and right hand circular polarization (RHCP). 
     FIG. 12A illustrates an antenna configuration for a multi-mode solution having multiple set of circular polarized antennas, and FIG. 12B illustrates a top view of the multi-frequency arrangement as described in FIG. 12A. A first set of antenna  121  has a first antenna  122  positioned orthogonal to a second antenna  123  in producing a first circular polarization at a first frequency f 1    131 . A second set of antenna  124  has a third antenna  125  positioned orthogonal to a fourth antenna  126  in producing a second circular polarization at a second frequency f 2    132 . A third set of antenna  127  has a fifth antenna  128  positioned orthogonal to a sixth antenna  129  in producing a third circular polarization at a third frequency f 3    133 . Arranging the antenna elements with nearby frequencies in such a fashion increases the bandwidth that can be tuned by the circularly polarized antenna. 
     The foregoing descriptions of specific embodiments of the invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to explain the principles and the application of the invention, thereby enabling others skilled in the art to utilize the invention in its various embodiments and modifications according to the particular purpose contemplated. The scope of the invention is intended to be defined by the claims appended hereto and their equivalents.