Abstract:
An antenna configuration includes two closely spaced antennas each positioned so as to be orthogonally polarized with respect to the other. The antenna configuration increases antenna isolation and reduces electromagnetic coupling between donor side antenna and repeat side antenna. The antennas include printed dipoles connected to respective transceivers through respective baluns to balance the non-symmetrical portions of the antenna feed paths to reduce unwanted radiation therein. Printed features such as chokes and non-symmetrical and non-parallel structures are preferably included in the ground plane of a multi-layer circuit board to reduce or eliminate circulating ground currents.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present invention is related to and claims priority from U.S. Provisional Application No. 60/681,948, entitled “INTEGRATED, CLOSELY SPACED, HIGH ISOLATION, PRINTED DIPOLES,” filed May 18, 2005, the contents of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to wireless communications and more specifically to closely spaced antennas utilizing orthogonal polarization to reduce electromagnetic coupling. 
   BACKGROUND OF THE INVENTION 
   In certain circumstances, it becomes necessary to closely position multiple omni directional antennas, such as those used in repeaters, where the antennas for both the donor and subscriber sides of the repeater are placed in close proximity. For example, such closely spaced antennas can be embedded onto low cost printed circuit boards for use in various communications products and systems, such as in the WiDeFi™ TDD based repeater system. It is further desirable for such closely spaced antennas to maintain minimal antenna-to-antenna interaction while maintaining good gain characteristics, to be easily producible in high volume manufacturing using low cost packaging, and to be easy for a user to operate. Further, when the antenna is placed near a reflecting surface, such as a wall, that would otherwise change the free space isolation of the antennas, a mechanism is required to reduce or cancel the effect of the interaction. 
   Three key problems present themselves when attempting to achieve high isolation between multiple, closely-spaced antennas that are printed on a small PCB board with near omni-directional antenna patterns and that must work in close proximity to unknown structures such as walls, furniture, and the like. The problems are coupling of radiated energy, common mode coupling and multi-path or random coupling of in-band signal energy. 
   In dealing with the first problem of coupling of radiated energy from one antenna into the receiver section of another, the radiated fields emanating from the antenna structure must be cancelled somehow to increase isolation. The closer the antennas are in physical proximity, the more they will tend to couple energy, which coupling reduces isolation between the antennas. Additional problems can arise when attempting to maintain an omni or semi-omni directional antenna pattern. 
   Dealing with the second problem of common mode coupling involves a coupling mechanism that is difficult to cancel. Common mode coupling occurs due to a shared ground on a printed circuit board. Voltage perturbations on the ground plane associated with generating and transmitting a signal from one antenna circuit couple into an adjacent antenna circuit either electrically into input circuits through the ground plane or indirectly from energy induced into the ground plane or input circuits by the transmitted signal. The problem of common mode coupling is especially difficult when multiple antennas are integrated together on a very small ground plane. 
   The third problem of random coupling is often the most difficult coupling mechanism to address. With random coupling, energy from indeterminate reflections or interactions with objects that change the radiation patterns or sources of localized coupling are primarily the result of antenna placement. However, attempting to determine an exact antenna placement that reduces or removes the unwanted components while preserving the desired components and the directionality is not generally successful. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes the above noted and other problems by providing an antenna configuration for a repeater in which two closely spaced antennas are orthogonally polarized to increase antenna isolation and reduce electromagnetic coupling. The two antennas may be fed in a balanced configuration to reduce common mode currents. The configuration is provided with a ground structure having various non-parallel and non-symmetrical shapes to reduce circulating currents and ground “hot spots” that can act as additional radiators thereby tending to increase coupling. 
   Alternatively, or in addition, to reducing shape symmetry and parallelism of the ground structure, an exemplary ground structure is provided with various printed structures that “choke” circulating ground currents by inducing opposite polarity currents that will generate electromagnetic (EM) fields with opposite, and thus canceling, polarities. The configuration may also be rotatable and capable of transmitting a sounding signal. By receiving the sounding signal during antenna rotation, the configuration is provided with feedback, which can be output to a user in the form of, for example, a sounding signal strength indicator or the like, providing information regarding antenna signal reflections to enable the user to directionally or spatially reposition the antenna configuration to maximize antenna operation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram illustrating a horizontally and a vertically polarized dipole antenna with resultant signals having respective horizontal and vertical polarization. 
       FIG. 2  is a diagram illustrating an exemplary dipole having undesirable circulating currents causing unwanted secondary radiation. 
       FIG. 3  is a diagram illustrating the exemplary dipole of  FIG. 2 , having a Balun for eliminating undesirable circulating currents and associated radiation. 
       FIG. 4  is a diagram illustrating a top layer of a multi-layer printed circuit board having an orthogonally polarized antenna configuration. 
       FIG. 5  is a diagram illustrating a second layer of a multi-layer printed circuit board having an orthogonally polarized antenna configuration. 
       FIG. 6  is a diagram illustrating a third layer of a multi-layer printed circuit board having an orthogonally polarized antenna configuration. 
       FIG. 7  is a diagram illustrating a fourth layer of a multi-layer printed circuit board having an orthogonally polarized antenna configuration. 
       FIG. 8  is a diagram illustrating a fifth layer of a multi-layer printed circuit board having an orthogonally polarized antenna configuration. 
       FIG. 9A  and  FIG. 9B  are diagrams illustrating a pair of perspective views of an exemplary embodiment of a packaged antenna configuration of the present invention that is adjustable/rotatable. 
       FIG. 10  is a diagram illustrating signals incident on an exemplary embodiment of an orthogonally polarized antenna configuration of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings in which like numerals reference like parts, several exemplary embodiments in accordance with the present invention will now be described. To address the above noted problems and other problems, an exemplary antenna configuration is provided where printed dipoles, or dipole elements, are positioned so as to be orthogonally polarized. The interference cause by a signal emanating from one radiating antenna into the adjacent antenna can be cancelled by establishing a polarity or orientation of the adjacent antenna having a natural tendency to cancel the signal energy which is produced with an electromagnetically opposite polarity or orientation from the radiating antenna. 
   It will be appreciated that the polarization of an antenna relates to the orientation of an electric field of a propagating signal radiated from the antenna and can be determined by the physical structure of the antenna and by its orientation. In contrast, the directionality of the antenna relates to the radiation pattern and is somewhat different from orientation. Polarization is typically referred to in terms of horizontal polarization, vertical polarization, circular polarization, and the like. 
   An example of polarization can be seen in  FIG. 1 , where a configuration  100  is shown having a dipole element, or dipole,  101  having a vertical polarization and a dipole element, or dipole,  102  having a horizontal polarization. The dipole  101  and the dipole  102  are separated by a phase angle  120 , which will determine the phase difference between a reference signal propagated from each of the dipole  101  and the dipole  102  in a propagation direction  110 . It will be appreciated that an exemplary signal E y    103  transmitted, for example, from the dipole  101 , will be vertically polarized; that is, it will have an E field component propagating in a plane that is vertically oriented. Similarly, an exemplary signal E x    104  transmitted, for example, from the dipole  102 , will be horizontally polarized; that is, it will have an E field component propagating in a plane that is horizontally oriented. It will be appreciated that due to the orthogonal relationship between the polarization directions of the dipole  101  and the dipole  102 , the likelihood of interference between signals radiated from one of the antennas into the other is low. It will also be appreciated that a signal incident on one of the antennas having a polarization opposite to that of the antenna will not couple well into that antenna. As noted above, some problems arise due to signal reflection, which can change signal polarization. However, by establishing an orthogonal relationship between the polarization of each dipole, maximum cancellation can be achieved even for reflected signals since the polarization can be calculated as the sum of the E field orientations over time relative to an imaginary plane perpendicular to the propagation direction the signal. It should be noted that while the dipole  101  and the dipole  102  are orthogonal, they are separated by a phase  120 . In accordance with various exemplary embodiments, the dipole  101  and the dipole  102  are positioned in an orthogonal relationship on the surface of, for example, a printed circuit board, printed wiring board, or the like as will be described in greater detail hereinafter. 
   In placing exemplary dipoles on the surface of a printed circuit or wiring board, some problems may arise as shown in exemplary configuration  200  in  FIG. 2 . A dipole antenna  201  is shown, for example, constructed of a coaxial cable with dipole elements  202  and  203 . In some instances unbalanced circulating currents in the dipole  201  from impedance mismatches or the like, can cause unwanted radiation  204  to emanate from portions of the dipole other than the radiating dipole elements  202  and  203 . The effect is greatest when a balanced configuration such as the symmetrical configuration of the dipole element  202  and the dipole element  203  meet the non-symmetrical or unbalanced portion of the dipole antenna  201 . In a circuit board environment, such radiation can cause interference by coupling into input stages of amplifiers, coupling into ground planes, or by coupling into other antenna present on the circuit board. To address the problem, as shown in exemplary configuration  300  in  FIG. 3 , a balun  310 , sometimes referred to as a baluns, or a Marchand Balun, named after Nathan Marchand who described such a configuration in the 1940s for coaxial transmission lines, can be positioned near the dipole elements  202  and  203  of the dipole antenna  201 . It will be appreciated that the balun  301  preferably has a precise 180° phase shift, with minimum loss and equal balanced impedances. The balun  301  provides isolation from ground to eliminate parasitic oscillations. 
   The basic construction/design of the balun  310  consists of two 90° phasing lines that provide the required 180° split. This involves the use of wavelengths in the order of λ/4 and λ/2. It will be appreciated that in a general coaxial example, a wire-wound transformer provides a suitable balun. Miniature wirewound transformers are commercially available covering frequencies from low kHz to beyond 2 GHz. Such balun transformers are often configured with a center-tapped secondary winding. When the center tap is grounded, a short circuit is presented to even-mode, or common-mode signals providing isolation and rejection. Differential or odd-mode signals are passed without effect. 
   As will be described in greater detail hereinafter, wire-wound transformers are expensive and are comparatively unsuitable in form factor for the printed dipoles of the present invention. Thus, the printed or lumped element balun is preferable in practical application. It should be noted the lumped element or printed balun is preferably provided with a center-tapped ground to reject common mode or even mode signals. The Marchand Balun can be adapted for use in a printed circuit configuration to increase isolation and increase noise rejection in the printed dipoles of the present invention, to be described in greater detail hereinafter. 
   With reference to the previously noted first problem, the interaction of EM fields can be canceled by orienting the printed dipole antennas of the present invention such that the respective polarization of the EM fields of each of the antennas are orthogonal to each other, thereby reducing or canceling any coupling therebetween. To reduce other possible points of radiation from the PCB itself such as radiation which would likely emanate from the ground structure, the shape of physical areas of the printed ground structure in close proximity to the antennas can be adjusted such that the ground structure ordinarily situated in parallel relation to the antenna has perpendicular rectangular structures added such that re-radiation points such as corners are shifted away from antenna structures. 
   With reference to the previously noted second problem, generalized coupling through the board substrate can be reduced by driving each of the printed dipole antennas of the present invention in a balanced fashion ensuring better isolation. For example, if any portion one signal couples into the other antenna feed structure, it does so as a common mode signal to both traces of the balanced feed structure and is hence canceled. Further, current choke slots can be printed onto the outer edges of the ground layers to reduce any currents that would tend to circulate around the outside of the ground plane between the two antennas. The choke structures cause the circulating currents to flow in opposite directions thereby generating EM fields with in-turn induce counter currents tending to choke off and cancel the original currents. 
   With reference to the previously noted third problem, several methods including trial and error are possible. However, a preferable approach to dealing with antenna placement is by transmitting a sounding signal from one antenna and receiving or “listening” to the reflections as they propagate back into the other antenna. Based on the arrangement of structures surrounding the antennas, the strength of the signal reflections back into the receiving antenna will be either higher than desired or will be sufficiently low to allow proper system operation. An indication can be provided to a user, either through a visual indicator such as a lamp or an LED, or through a series of LEDs, an external monitoring device, or the like. If the strength of the reflections as indicated by the LEDs is higher than desirable, a user can be directed to move or reposition the antenna until the strength of the reflections are minimized to levels considered to be acceptable. As noted, the feedback to the user could take many forms and the readjustment of the antenna could be in any different direction and any distance. 
   To better appreciate the printed circuit configuration of the closely spaced dipoles, a top layer  400  of an exemplary multi-layer circuit board is shown in  FIG. 4 . A first printed wiring board layer  401  being a top layer of a multi-layer printed orthogonally polarized antenna configuration includes a ground plane  402  occupying a portion of the first printed wiring board layer  401 . A horizontally positioned strip  410  and a vertically positioned strip  411  are portions of the orthogonally positioned printed dipoles. The area of the ground plane with a portion removed shown in a T configuration is a choke  420 , which can be used to reduce circulating currents in the ground plane as described above. Further, a rectangular area  403  can be added to the ground plane  402  in order to disrupt circulating current which could radiate and couple energy into dipole feed sections and other sensitive circuits such as amplifier inputs and the like. 
   A second layer  500  of a multi-layer printed orthogonally polarized antenna configuration is shown in  FIG. 5 . A second printed wiring board layer  501  being a second layer of a multi layer printed orthogonally polarized antenna configuration includes a ground plane  502  occupying at least a portion of the second printed wiring board layer  501 . A horizontally positioned strip  510  and a vertically positioned strip  511  are portions of the orthogonally positioned printed dipoles. It will be appreciated that the dipole strips  510  and  511  are preferably connected through vias (not shown) to the dipole strips  410  and  411  shown in  FIG. 4 . A rectangular area  503  can be added to the ground plane  502  in order to disrupt circulating current which could radiate and couple energy into dipole feed sections and other sensitive circuits such as amplifier inputs and the like. It will be appreciated that ground plane  502  further contains a feed channel  512  and a feed channel  513  for providing clear areas for reducing inductance from the ground planes into signal traces in adjacent layers associated with the feed paths that will couple to dipole sections such as the dipole strips  410 ,  411 ,  510  and  511 . In addition, achoke  520  can be provided corresponding to the choke  420  in the adjacent layer. 
   A third layer  600  of a multi-layer printed orthogonally polarized antenna configuration is shown in  FIG. 6 . A third printed wiring board layer  601  being a third layer of a multi layer printed orthogonally polarized antenna configuration includes a ground plane  602  occupying at least a portion of the third printed wiring board layer  601 . It will be appreciated that the dipole strips  610  and  611  are preferably connected through vias (not shown) to the dipole strips  410  and  411  shown in  FIG. 4  and to the dipole strips  510  and  511  shown in  FIG. 5 . A rectangular area  603  can be added to the ground plane  602  in order to disrupt circulating current which could radiate and couple energy into dipole feed sections and other sensitive circuits such as amplifier inputs and the like. As previously noted a first printed dipole antenna, configured with dipole strips  410 ,  510  and  610  and a second orthogonally positioned printed dipole antenna, configured with dipole strips  411 ,  511  and  611  are fed, at least in part, through traces  612  and  613  respectively. It can be seen that only one portion of the dipole strips  410 ,  510  and  610  and  411 ,  511 ,  611  are fed by the traces  612  and  613 . The other portions are connected to ground as will be described. Signals received and transmitted on first and second printed dipole antennas can be coupled to transceiver input or output circuits (not shown) as appropriate. A connector section  620  is also shown where various connections can be made from traces on the printed wiring board to pins associated with an external connector (not shown) that can be mounted in the area of connector section  620 . 
   A fourth layer  700  of a multi-layer printed orthogonally polarized antenna configuration is shown in  FIG. 7 . A fourth printed wiring board layer  701  being a fourth layer of a multi layer printed orthogonally polarized antenna configuration includes a ground plane  702  occupying at least a portion of the third printed wiring board layer  701 . It will be appreciated that the dipole strips  710  and  711  are preferably connected through vias (not shown) to the dipole strips  410  and  411  shown in  FIG. 4 , to the dipole strips  510  and  511  shown in  FIG. 5 , and to the dipole strips  610  and  611  shown in  FIG. 6 . A rectangular area  703  can be added to the ground plane  702  in order to disrupt circulating current which could radiate and couple energy into dipole feed sections and other sensitive circuits such as amplifier inputs and the like. In a manner similar to the signal portion of the first and second dipoles, for example as described above, a ground portion of the first printed dipole antenna, configured with dipole strips  410 ,  510 ,  610  and  710  and the second orthogonally positioned printed dipole antenna, configured with dipole strips  411 ,  511 ,  611  and  711  are coupled to ground through traces  712  and  713  respectively. A connector section  720  is also shown where various connections can be made from traces on the printed wiring board to pins associated with an external connector (not shown) that can be mounted in the area of connector section  720 . It will also be appreciated that a printed circuit trace for connection to the transceiver through a Marchand Balun can be provided for example, at traces  714  and  715 . 
   A fifth or bottom layer  800  of an exemplary multi-layer circuit board is shown in  FIG. 8 . A fifth printed wiring board layer  801  being a bottom layer of a multi-layer printed orthogonally polarized antenna configuration includes a ground plane  802  occupying a portion of the fifth printed wiring board layer  801 . A horizontally positioned strip  810  and a vertically positioned strip  811  are portions of the orthogonally positioned printed dipoles. The area of the ground plane with a portion removed shown in a T configuration is a choke  820 , which can be used to reduce circulating currents in the ground plane as described above. Further, a rectangular area  803  can be added to the ground plane  802  in order to disrupt circulating current which could radiate and couple energy into dipole feed sections and other sensitive circuits such as amplifier inputs and the like. 
   In  FIG. 9A  and  FIG. 9B , perspective views of an exemplary embodiment of a packaged antenna configuration  900  of the present invention are shown. The antenna package  901  is adjustable/rotatable about an axis or hinge which is located in the portion of the package surrounding plug  910  that can be plugged into a standard wall socket  920 . Such a configuration provides for potential positioning of the antenna package  901  for placement that reduces or eliminates interference. As depicted, the antenna package  901 , which could be associated with a WiDeFi™ TDD repeater, has an align LED  911  at the top of the antenna package  901 . Additionally the antenna package  901  can be rotated through an arc  902  such that the top of the antenna package  901  could be rotated down and away from a wall  903 . Such rotation would bring the antenna package  901  from a starting position parallel to the wall  903  to a position where one end of the dipole antennas is closer to the wall  903  and the other end is father away from the wall  903 , thereby providing a high degree of change in any coupling mechanisms that may be present due to the wall  903 . In such a configuration, the LED  911  will flash until the operation of sending and receiving the sounding signal as described above, while repositioning the antenna package  901  results in an acceptable position at which time it will stop, change color, or some other indicia that the interference between the sounding signal transmitter and receiver has been reduced to acceptable levels. When such an indication is provided, the user should stop rotating the antenna package  901 . 
   By placement of the first and second dipoles in orthogonal relation on a printed wiring board as described and illustrated herein, maximum isolation can be achieved.  FIG. 10  shows a configuration  1000  where a first dipole  1001  and a second dipole  1002  are positioned in orthogonal relation, such as a 90° relation  1020 , on the surface of a printed wiring board. The first dipole  1001  can transmit signals  1010  with a corresponding polarization and optimally receive signals  1010  with the same polarization. Signals incident on the second dipole  1002  having the polarization of the first dipole  1001 , such as incident signal  1012 , will not be received, that is, will not effectively couple energy into the second dipole  1002 , since the polarization of the second dipole is directed orthogonally away from the polarization direction of the incident signal  1012 . Such signal rejection is true of incident signals  1012  incident from remote transmitters and from signal components associated with incident signals  1012  generated by the first dipole  1001 . Likewise, the second dipole  1002  can transmit signals  1011  with a corresponding polarization and optimally receive signals  1011  with the same polarization. Signals incident on the first dipole  1001  having the polarization of the second dipole  1002 , such as incident signal  1013 , will not be received, that is, will not effectively couple energy into the first dipole  1001 , since the polarization of the first dipole is directed orthogonally away from the polarization direction of the incident signal  1013 . Such signal rejection is true of incident signals  1013  incident from remote transmitters and from signal components associated with incident signals  1013  generated by the second dipole  1002 . 
   It should be noted that the respective first dipole  1001  and the second dipole  1002  can be coupled to a first transceiver/STA  1020  and a second transceiver/STA  1030  for providing a transmit signal and for receiving a signal received on the respective antenna. It will be appreciated that in various exemplary embodiments, the first transceiver/STA  1020  and a second transceiver/STA  1030  can be configured to operate by sending and receiving signals in various modes such as in a TDD mode using one or more frequency channels, in frequency division duplex (FDD) mode and the like, and can be configured to operated according to various standards under 802.11, 802.16, and the like. 
   The invention is described herein in detail with particular reference to presently preferred embodiments. However, it will be understood that variations and modifications can be effected within the scope and spirit of the invention.