Patent Publication Number: US-6987493-B2

Title: Electronically steerable passive array antenna

Description:
CLAIMING BENEFIT OF PRIOR FILED PROVISIONAL APPLICATION 
   This application claims the benefit of U.S. Provisional Application Ser. No. 60/372,742 filed on Apr. 15, 2002 and entitled “Electronically Steerable Passive Array antenna with 360 Degree Beam and Null Steering Capability” which is incorporated by reference herein. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an array antenna, and more particularly to an electronically 360 degree steerable passive array antenna capable of steering the radiation beams and nulls of a radio signal. 
   2. Description of Related Art 
   An antenna is used wherever there is wireless communication. The antenna is the last device through which a radio signal leaves a transceiver and the first device to receive a radio signal at a transceiver. Most antennas are designed to radiate energy into a “sector” which can be regarded as a “waste” of power since most of the energy is radiated in directions other than towards the intended transceiver. In addition, other transceivers experience the energy radiated in other directions as interference. As, such a great detail of effort has been made to design an antenna that can maximize the radiated energy towards the intended transceiver and minimize the radiation of energy elsewhere. 
   A scanning beam antenna is one type of antenna known in the art that can change its beam direction, usually for the purpose of maintaining a radio link between a tower and a mobile terminal. Early scanning beam antennas were mechanically controlled. The mechanical control of scanning beam antennas have a number of disadvantages including a limited beam scanning speed as well as a limited lifetime, reliability and maintainability of the mechanical components such as motors and gears. Thus, electronically controlled scanning beam antennas were developed and are becoming more important in the industry as the need for higher speed data, voice and video communications increases in wireless communication systems. 
   Referring to  FIG. 1 , there is illustrated a traditional electronically controlled scanning beam antenna  100  known in the art as a phased array antenna  100 . The phased array antenna  100  has an RF signal input  102  connected to a network of power dividers  104 . The power dividers  104  are connected to a series of phase shifters  106  (eight shown). The phase shifters  106  are used to control the phase of a radio signal delivered to an array of radiating elements  108  (eight shown). The phased array antenna  100  produces a radiation beam  110  that can be scanned in the direction indicated by arrow  112 . As can be seen, the phased array antenna  100  has a complex configuration and as such is costly to manufacture. These drawbacks become even more apparent when the number of radiating elements  108  become larger. 
   Referring to  FIG. 2 , there is illustrated another traditional electronically controlled scanning beam antenna  200  that was described in U.S. Pat. No. 6,407,719 the contents of which are hereby incorporated by reference herein. The array antenna  200  includes a radiating element  202  capable of transmitting and receiving radio signals and one or more parasitic elements  204  that are incapable of transmitting or receiving radio signals. Each parasitic element  204  (six shown) is located on a circumference of a predetermined circle around the radiating element  202 . Each parasitic element  204  is connected to a variable-reactance element  206  (six shown). A controller  208  changes the directivity of the array antenna  200  by changing the reactance X n  of each of the variable-reactance elements  206 . In the preferred embodiment, the variable-reactance element  206  is a varactor diode and the controller  208  changes the backward bias voltage Vb applied to the varactor diode  206  in order to change the capacitance of the varactor diode  206  and thus change the directivity of the array antenna  200 . This array antenna  200  which incorporates varactor diodes  206  has several drawbacks when it operates as a high frequency transmit antenna. These drawbacks include low RF power handling, high linearity distortion and high loss of the RF energy. Accordingly, there is a need to address the aforementioned shortcomings and other shortcomings associated with the traditional electronically controlled scanning beam antennas. These needs and other needs are satisfied by the electronically steerable passive array antenna and method of the present invention. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention is an electronically steerable passive array antenna and method for using the array antenna to steer the radiation beams and nulls of a radio signal. The array antenna includes a radiating antenna element capable of transmitting and receiving radio signals and one or more parasitic antenna elements that are incapable of transmitting or receiving radio signals. Each parasitic antenna element is located on a circumference of a predetermined circle around the radiating antenna element. A voltage-tunable capacitor is connected to each parasitic antenna element. A controller is used to apply a predetermined DC voltage to each one of the voltage-tunable capacitors in order to change the capacitance of each voltage-tunable capacitor and thus enable one to control the directions of the maximum radiation beams and the minimum radiation beams (nulls) of a radio signal emitted from the array antenna. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein: 
       FIG. 1  (PRIOR ART) is a diagram that illustrates the basic components of a traditional electronically controlled scanning beam antenna; 
       FIG. 2  (PRIOR ART) is a perspective view that illustrates the basic components of another traditional electronically controlled scanning beam antenna; 
       FIG. 3  is a block diagram of a wireless communications network capable of incorporating an array antenna of the present invention; 
       FIG. 4  is a perspective view that illustrates the basic components of a first embodiment of the array antenna shown in  FIG. 3 ; 
       FIG. 5  is a side view of a RF feed antenna element located in the array antenna shown in  FIG. 4 ; 
       FIG. 6  is a side view of a parasitic antenna element and a voltage-tunable capacitor located in the array antenna shown in  FIG. 4 ; 
       FIGS. 7A and 7B  respectively show a top view and a cross-sectional side view of the voltage-tunable capacitor shown in  FIG. 6 ; 
       FIGS. 8A and 8B  respectively show simulation patterns in a horizontal plane and in a vertical plane that were obtained to indicate the performance of an exemplary array antenna configured like the array antenna shown in  FIG. 4 ; 
       FIG. 9  is a perspective view that illustrates the basic components of a second embodiment of the array antenna shown in  FIG. 3 ; and 
       FIG. 10  is a perspective view that illustrates the basic components of a third embodiment of the array antenna shown in  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE DRAWINGS 
   Referring to the drawings,  FIG. 3  is a block diagram of a wireless communications network  300  that can incorporate an array antenna  302  in accordance with the present invention. Although the array antenna  302  is described below as being incorporated within a hub type wireless communication network  300 , it should be understood that many other types of networks can incorporate the array antenna  302 . For instance, the array antenna  302  can be incorporated within a mesh type wireless communication network, a 24–42 GHz point-to-point microwave network, 24–42 GHz point-to-multipoint microwave network or a 2.1–2.7 GHz multipoint distribution system. Accordingly, the array antenna  302  of the present invention should not be construed in a limited manner. 
   Referring to  FIG. 3 , there is a block diagram of a hub type wireless communications network  300  that utilizes the array antenna  302  of the present invention. The hub type wireless communications network  300  includes a hub node  304  and one or more remote nodes  306  (four shown). The remote nodes  306  may represent any one of a variety of devices. One example is for fixed site users, e.g. in a building, where the remote node  306  (e.g., customer premises equipment, laptop computer) is used to enable a wireless broadband connection to the hub node  304  (e.g., base station). Another example is for mobile site users, where the remote note  306  (wireless phone, personal digital assistant, laptop computer) is used to enable a wireless broadband connection to the hub node  304  (e.g., base station). 
   The hub node  304  incorporates the electronically steerable passive array antenna  302  that produces one or more steerable radiation beams  310  and  312  which are used to establish communications links with particular remote nodes  306 . A network controller  314  directs the hub node  304  and in particular the array antenna  302  to establish a communications link with a desired remote node  306  by outputting a steerable beam having a maximum radiation beam pointed in the direction of the desired remote node  306  and a minimum radiation beam (null) pointed away from that remote node  306 . The network controller  314  may obtain its adaptive beam steering commands from a variety of sources like the combined use of an initial calibration algorithm and a wide beam which is used to detect new remote nodes  306  and moving remote nodes  306 . The wide beam enables all new or moved remote nodes  308  to be updated in its algorithm. The algorithm then can determine the positions of the remote nodes  308  and calculate the appropriate DC voltage for each of the voltage-tunable capacitors  406  (described below) in the array antenna  302 . A more detailed discussion about one way the network controller  314  can keep up-to-date with its current communication links is provided in a co-owned U.S. patent application Ser. No. 09/620,776 entitled “Dynamically Reconfigurable Wireless Networks (DRWiN) and Methods for Operating such Networks”. The contents of this patent application are incorporated by reference herein. 
   It should be appreciated that the hub node  304  can also be connected to a backbone communications system  308  (e.g., Internet, private networks, public switched telephone network, wide area network). It should also be appreciated that the remote nodes  308  can incorporate an electronically steerable passive array antenna  302 . 
   Referring to  FIG. 4 , there is a perspective view that illustrates the basic components of a first embodiment of the array antenna  302   a . The array antenna  302   a  includes a radiating antenna element  402  capable of transmitting and receiving radio signals and one or more parasitic antenna elements  404  that are incapable of transmitting or receiving radio signals. Each parasitic antenna element  404  (six shown) is located a predetermined distance away from the radiating antenna element  402 . A voltage-tunable capacitor  406  (six shown) is connected to each parasitic antenna element  404 . A controller  408  is used to apply a predetermined DC voltage to each one of the voltage-tunable capacitors  406  in order to change the capacitance of each voltage-tunable capacitor  406  and thus enable one to control the directions of the maximum radiation beams and the minimum radiation beams (nulls) of a radio signal emitted from the array antenna  302 . The controller  408  may be part of or interface with the network controller  314  (see  FIG. 3 ). 
   In the particular embodiment shown in  FIG. 4 , the array antenna  302   a  includes one radiating antenna element  402  and six parasitic antenna elements  404  all of which are configured as monopole elements. The antenna elements  402  and  404  are electrically insulated from a grounding plate  410 . The grounding plate  410  has an area large enough to accommodate all of the antenna elements  402  and  404 . In the preferred embodiment, each parasitic antenna element  404  is arranged on a circumference of a predetermined circle around the radiating antenna element  402 . For example, the radiating antenna element  402  and the parasitic antenna elements  404  can be separated from one another by about 0.2λ 0 –0.5λ 0  where λ 0  is the working free space wavelength of the radio signal. 
   Referring to  FIG. 5 , there is a side view of the RF feed antenna element  402 . In this embodiment, the feeding antenna element  402  comprises a cylindrical element that is electrically insulated from the grounding plate  410 . The feeding antenna element  402  typically has a length of 0.2λ 0 –0.3λ 0  where λ 0  is the working free space wavelength of the radio signal. As shown, a central conductor  502  of a coaxial cable  504  that transmits a radio signal fed from a radio apparatus (not shown) is connected to one end of the radiating antenna element  402 . And, an outer conductor  506  of the coaxial cable  504  is connected to the grounding plate  410 . The elements  502 ,  504  and  506  collectively are referred to as an RF input  508  (see  FIG. 4 ). Thus, the radio apparatus (not shown) feeds a radio signal to the feeding antenna element  402  through the coaxial cable  504 , and then, the radio signal is radiated by the feeding antenna element  402 . 
   Referring to  FIG. 6 , there is a side view of one parasitic antenna element  404  and one voltage-tunable capacitor  406 . In this embodiment, each parasitic antenna element  404  has a similar structure comprising a cylindrical element that is electrically insulated from the grounding plate  410 . The parasitic antenna elements  404  typically have the same length as the radiating antenna element  402 . The voltage-tunable capacitor  406  is supplied a DC voltage as shown in  FIG. 4  which causes a change in the capacitance of the voltage-tunable capacitor  406  and thus enables one to the control of the directions of the maximum radiation beams and the minimum radiation beams (nulls) of a radio signal emitted from the array antenna  302 . A more detailed discussion about the components and advantages of the voltage-tunable capacitor  406  are provided below with respect to  FIGS. 7A and 7B . 
   Referring to  FIGS. 7A and 7B , there are respectively shown a top view and a cross-sectional side view of an exemplary voltage-tunable capacitor  406 . The voltage-tunable capacitor  406  includes a tunable ferroelectric layer  702  and a pair of metal electrodes  704  and  706  positioned on top of the ferroelectric layer  702 . As shown in  FIG. 6 , one metal electrode  704  is attached to one end of the parasitic antenna element  404 . And, the other metal electrode  704  is attached to the grounding plate  410 . The controller  408  applies the DC voltage to both of the metal electrodes  704  and  706  (see  FIG. 4 ). A substrate (not shown) may be positioned on the bottom of the ferroelectric layer  702 . The substrate may be any type of material that has a relatively low permittivity (e.g., less than about 30) such as MgO, Alumina, LaAlO 3 , Sapphire, or ceramic. 
   The tunable ferroelectric layer  702  is a material that has a permittivity in a range from about 20 to about 2000, and has a tunability in the range from about 10% to about 80% at a bias voltage of about 10 V/μm. In the preferred embodiment this layer is preferably comprised of Barium-Strontium Titanate, Ba x Sr 1-x TiO 3  (BSTO), where x can range from zero to one, or BSTO-composite ceramics. Examples of such BSTO composites include, but are not limited to: BST—MgO, BSTO—MgAl 2 O 4 , BSTO—CaTiO 3 , BSTO—MgTiO 3 , BSTO—MgSrZrTiO 6 , and combinations thereof. The tunable ferroelectric layer  702  in one preferred embodiment has a dielectric permittivity greater than 100 when subjected to typical DC bias voltages, for example, voltages ranging from about 5 volts to about 300 volts. And, the thickness of the ferroelectric layer can range from about 0.1 μm to about 20 μm. Following is a list of some of the patents which discuss different aspects and capabilities of the tunable ferroelectric layer  702  all of which are incorporated herein by reference: U.S. Pat. Nos. 5,312,790; 5,427,988; 5,486,491; 5,635,434; 5,830,591; 5,846,893; 5,766,697; 5,693,429 and 5,635,433. 
   The voltage-tunable capacitor  406  has a gap  708  formed between the electrodes  704  and  706 . The width of the gap  708  is optimized to increase ratio of the maximum capacitance C max  to the minimum capacitance C min  (C max /C min ) and to increase the quality factor (Q) of the device. The width of the gap  708  has a strong influence on the C max /C min  parameters of the voltage-tunable capacitor  406 . The optimal width, g, is typically the width at which the voltage-tunable capacitor  406  has a maximum C max /C min  and minimal loss tangent. In some applications, the voltage-tunable capacitor  406  may have a gap  708  in the range of 5–50 μm. 
   The thickness of the tunable ferroelectric layer  702  also has a strong influence on the C max /C min  parameters of the voltage-tunable capacitor  406 . The desired thickness of the ferroelectric layer  702  is typically the thickness at which the voltage-tunable capacitor  406  has a maximum C max /C min  and minimal loss tangent. For example, an antenna array  302   a  operating at frequencies ranging from about 1.0 GHz to about 10 GHz, the loss tangent would range from about 0.0001 to about 0.001. For an antenna array  302   a  operating at frequencies ranging from about 10 GHz to about 20 GHz, the loss tangent would range from about 0.001 to about 0.01. And, for an antenna array  302   a  operating frequencies ranging from about 20 GHz to about 30 GHz, the loss tangent would range from about 0.005 to about 0.02. 
   The length of the gap  708  is another dimension that strongly influences the design and functionality of the voltage-tunable capacitor  406 . In other words, variations in the length of the gap  708  have a strong effect on the capacitance of the voltage-tunable capacitor  406 . For a desired capacitance, the length can be determined experimentally, or through computer simulation. 
   The electrodes  704  and  706  may be fabricated in any geometry or shape containing a gap  708  of predetermined width and length. In the preferred embodiment, the electrode material is gold which is resistant to corrosion. However, other conductors such as copper, silver or aluminum, may also be used. Copper provides high conductivity, and would typically be coated with gold for bonding or nickel for soldering. 
   Referring to  FIGS. 8A and 8B , there are respectively shown two simulation patterns one in a horizontal plane and the other in a vertical plane that where obtained to indicate the performance of an exemplary array antenna  302 . The exemplary array antenna  302  has a configuration similar to the array antenna  302   a  shown in  FIG. 4  where each parasitic antenna element  404  is arranged on a circumference of a predetermined circle around the radiating antenna element  402 . In this simulation, the radiating antenna element  402  and the parasitic antenna elements  404  were separated from one another by 0.25λ 0 . 
   Referring again to  FIG. 4 , the antenna array  302   a  operates by exciting the radiating antenna element  402  with the radio frequency energy of a radio signal. Thereafter, the radio frequency energy of the radio signal emitted from the radiating antenna element  402  is received by the parasitic antenna elements  404  which then re-radiate the radio frequency energy after it has been reflected and phase changed by the voltage-tunable capacitors  406 . The controller  408  changes the phase of the radio frequency energy at each parasitic antenna element  404  by applying a predetermined DC voltage to each voltage-tunable capacitor  406  which changes the capacitance of each voltage-tunable capacitor  406 . This mutual coupling between the radiating antenna element  402  and the parasitic antenna elements  404  enables one to steer the radiation beams and nulls of the radio signal that is emitted from the antenna array  302   a.    
   Referring to  FIG. 9 , there is a perspective view that illustrates the basic components of a second embodiment of the array antenna  302   b . The array antenna  302   b  has a similar structure and functionality to array antenna  302   a  except that the antenna elements  902  and  904  are configured as dipole elements instead of a monopole elements as shown in  FIG. 4 . The array antenna  302   b  includes a radiating antenna element  902  capable of transmitting and receiving radio signals and one or more parasitic antenna elements  904  that are incapable of transmitting or receiving radio signals. Each parasitic antenna element  904  (six shown) is located a predetermined distance away from the radiating antenna element  902 . A voltage-tunable capacitor  906  (six shown) is connected to each parasitic element  904 . A controller  908  is used to apply a predetermined DC voltage to each one of the voltage-tunable capacitors  906  in order to change the capacitance of each voltage-tunable capacitor  906  and thus enable one to control the directions of the maximum radiation beams and the minimum radiation beams (nulls) of a radio signal emitted from the array antenna  302   b . The controller  908  may be part of or interface with the network controller  314  (see  FIG. 3 ). 
   In the particular embodiment shown in  FIG. 9 , the array antenna  302   b  includes one radiating antenna element  902  and six parasitic antenna elements  904  all of which are configured as dipole elements. The antenna elements  902  and  904  are electrically insulated from a grounding plate  910 . The grounding plate  910  has an area large enough to accommodate all of the antenna elements  902  and  904 . In the preferred embodiment, each parasitic antenna element  904  is located on a circumference of a predetermined circle around the radiating antenna element  902 . For example, the radiating antenna element  902  and the parasitic antenna elements  904  can be separated from one another by about 0.2λ 0 –0.5λ 0  where λ 0  is the working free space wavelength of the radio signal. 
   Referring to  FIG. 10 , there is a perspective view that illustrates the basic components of a third embodiment of the array antenna  302   c . The array antenna  302   c  includes a radiating antenna element  1002  capable of transmitting and receiving dual band radio signals. The array antenna  302   c  also includes one or more low frequency parasitic antenna elements  1004   a  (six shown) and one or more high frequency parasitic antenna elements  1004   b  (six shown). The parasitic antenna elements  1004   a  and  1004   b  are incapable of transmitting or receiving radio signals. Each of the parasitic antenna elements  1004   a  and  1004   b  are locate a predetermined distance away from the radiating antenna element  1002 . As shown, the low frequency parasitic antenna elements  1004   a  are located on a circumference of a “large” circle around both the radiating antenna element  1002  and the high frequency parasitic antenna elements  1004   b . And, the high frequency parasitic antenna elements  1004   b  are located on a circumference of a “small” circle around the radiating antenna element  1002 . In this embodiment, the low frequency parasitic antenna elements  1004   a  are the same height as the radiating antenna element  1002 . And, the high frequency parasitic antenna elements  1004   b  are shorter than the low frequency parasitic antenna elements  1004   a  and the radiating antenna element  1002 . 
   The array antenna  302   c  also includes one or more low frequency voltage-tunable capacitors  1006   a  (six shown) which are connected to each of the low frequency parasitic elements  1004   a . In addition, the array antenna  302   c  includes one or more high frequency voltage-tunable capacitors  1006   b  (six shown) which are connected to each of the high frequency parasitic elements  1004   b . A controller  1008  is used to apply a predetermined DC voltage to each one of the voltage-tunable capacitors  1006   a  and  1006   b  in order to change the capacitance of each voltage-tunable capacitor  1006   a  and  1006   b  and thus enable one to control the directions of the maximum radiation beams and the minimum radiation beams (nulls) of a dual band radio signal that is emitted from the array antenna  302   c . The controller  1008  may be part of or interface with the network controller  314  (see  FIG. 3 ). 
   In the particular embodiment shown in  FIG. 10 , the array antenna  302   c  includes one radiating antenna element  1002  and twelve parasitic antenna elements  1004   a  and  1004   b  all of which are configured as monopole elements. The antenna elements  1002 ,  1004   a  and  1004   b  are electrically insulated from a grounding plate  1010 . The grounding plate  1010  has an area large enough to accommodate all of the antenna elements  1002 ,  1004   a  and  1004   b . It should be understood that the low frequency parasitic antenna elements  1004   a  do not affect the high frequency parasitic antenna elements  1004   b  and vice versa. 
   The antenna array  302   c  operates by exciting the radiating antenna element  1002  with the high and low radio frequency energy of a dual band radio signal. Thereafter, the low frequency radio energy of the dual band radio signal emitted from the radiating antenna element  1002  is received by the low frequency parasitic antenna elements  1004   a  which then re-radiate the low frequency radio frequency energy after it has been reflected and phase changed by the low frequency voltage-tunable capacitors  1006   a . Likewise, the high frequency radio energy of the dual band radio signal emitted from the radiating antenna element  1002  is received by the high frequency parasitic antenna elements  1004   b  which then re-radiate the high frequency radio frequency energy after it has been reflected and phase changed by the high frequency voltage-tunable capacitors  1006   b . The controller  1008  changes the phase of the radio frequency energy at each parasitic antenna element  1004   a  and  1004   b  by applying a predetermined DC voltage to each voltage-tunable capacitor  1006   a  and  1006   b  which changes the capacitance of each voltage-tunable capacitor  1006   a  and  1006   b . This mutual coupling between the radiating antenna element  1002  and the parasitic antenna elements  1004   a  and  1004   b  enables one to steer the radiation beams and nulls of the dual band radio signal that is emitted from the antenna array  302   c . The array antenna  302   c  configured as described above can be called a dual band, endfire, phased array antenna  302   c.    
   Although the array antennas described above have radiating antenna elements and parasitic antenna elements that are configured as either a monopole element or dipole element, it should be understood that these antenna elements can have different configurations. For instance, these antenna elements can be a planar microstrip antenna, a patch antenna, a ring antenna or a helix antenna. 
   In the above description, it should be understood that the features of the array antennas apply whether it is used for transmitting or receiving. For a passive array antenna the properties are the same for both the receive and transmit modes. Therefore, no confusion should result from a description that is made in terms of one or the other mode of operation and it is well understood by those skilled in the art that the invention is not limited to one or the other mode. 
   Following are some of the different advantages and features of the array antenna  302  of the present invention:
         The array antenna  302  has a simple configuration.   The array antenna  302  is relatively inexpensive.   The array antenna  302  has a high RF power handling parameter of up to 20 W. In contrast, the traditional array antenna  200  has a RF power handling parameter that is less than 1 W.   The array antenna  302  has a low linearity distortion represented by IP3 of upto +65 dBm. In contrast, the traditional array antenna  200  has a linearity distortion represented by IP3 of about +30 dBm.   The array antenna  302  has a low voltage-tunable capacitor loss.   The dual band array antenna  302   c  has two bands each of which works upto 20% of frequency. In particular, there are two center frequency points for the dual band antenna f 0  each of which has a bandwidth of about 10%˜20% [(f 1 +f 2 )/2=f 0 , Bandwidth=(f 2 −f 1 )/f 0 *100%] where f 1  and f 2  are the start and end frequency points for one frequency band. Whereas the single band antenna  302   a  and  302   b  works in the f 1  to f 2  frequency range. The dual band antenna  302   c  works in one f 1  to f 2  frequency range and another f 1  to f 2  frequency range. The two center frequency points are apart from each other, such as more than 10%. For example, 1.6 GHz˜1.7 GHz and 2.4 GHz˜2.5 GHz, etc. The traditional array antenna  200  cannot support a dual band radio signal.       

   While the present invention has been described in terms of its preferred embodiments, it will be apparent to those skilled in the art that various changes can be made to the disclosed embodiments without departing from the scope of the invention as set forth in the following claims.