Abstract:
An antenna array for use with a mobile subscriber unit in a wireless network communications system. The invention utilizes a multiplicity of resonant strips provided within the ground plane. These strips couple to an equal multiplicity of monopole array elements located on top of the ground plane. This approach increases antenna gain by more efficiently utilizing the available ground plane area. Additionally, since the active element is on top of the ground plane, the antenna array sensitivity is decreased because the direct coupling between the antenna and external environmental factors is minimized. The multiplicity of antenna elements are electrically isolated from the ground plane. Each antenna element has a bottom end located proximal to the ground plane, and is aligned along a respective antenna axis that is substantially perpendicular to the top side. Each resonant strip has a top end electrically connected to the ground plane and a bottom end spaced apart from a bottom side of the ground plane, and is aligned along the antenna axis of a corresponding antenna element. The multiplicity of antenna elements and the multiplicity of resonant strips are equally spaced about the perimeter of the ground plane, and the combination of each antenna element with a respective resonant strip provides a unbalanced dipole antenna element so that the multiplicity of dipole antenna elements form a composite beam which may be positionally directed along a horizon that is substantially parallel to the ground plane.

Description:
BACKGROUND OF THE INVENTION 
     Code Division Multiple Access (CDMA) communication systems may be used to provide wireless communication between a base station and one or more subscriber units. The base station is typically a computer controlled set of switching transceivers that are interconnected to a land-based public switched telephone network (PSTN). The base station includes an antenna apparatus for sending forward link radio frequency signals to the mobile subscriber units. The base station antenna is also responsible for receiving reverse link radio frequency signals transmitted from each mobile unit. Each mobile subscriber unit also contains an antenna apparatus for the reception of the forward link signals and for transmission of the reverse link signals. A typical mobile subscriber unit is a digital cellular telephone handset or a personal computer coupled to a wireless cellular modem. 
     The most common type of antenna used to transmit and receive signals at a mobile subscriber unit is a omni-directional monopole antenna. This type of antenna consists of a single wire or antenna element that is coupled to a transceiver within the subscriber unit. The transceiver receives reverse link signals to be transmitted from circuitry within the subscriber unit and modulates the signals onto the antenna element at a specified frequency assigned to that subscriber unit. Forward link signals received by the antenna element at a specified frequency are demodulated by the transceiver and supplied to processing circuitry within the subscriber unit. In CDMA cellular systems, multiple mobile subscriber units may transmit and receive signals on the same frequency and use coding algorithms to detect signaling information intended for individual subscriber units on a per unit basis. 
     The transmitted signal sent from a monopole antenna is omnidirectional in nature. That is, the signal is sent with the same signal strength in all directions in a generally horizontal plane. Reception of signals with a monopole antenna element is likewise omnidirectional. A monopole antenna does not differentiate in its ability to detect a signal on one direction versus detection of the same or a different signal coming from another direction. 
     SUMMARY OF THE INVENTION 
     Various problems are inherent in prior art antennas used on mobile subscriber units in wireless communications systems. Typically, an antenna array with scanning capabilities consists of a number of antenna elements located on top of a ground plane. For the subscriber unit to satisfy portability requirements, the ground plane must be physically small. For example, in cellular communication applications, the ground plane is typically smaller than the wavelength of the transmitted and received signals. Because of the interaction between the small ground plane and the antenna elements, which are typically monopole elements, the peak strength of the beam formed by the array is elevated above the horizon, for example, by about 30°, even though the beam itself is directed along the horizon. Correspondingly the strength of the beam along the horizon is about 3 db less than the peak strength. Generally, the subscriber units are located at large distances from the base stations such that the angle of incidence between the subscriber unit and the base station is approximately zero. The ground plane would have to be significantly larger than the wavelength of the transmitted/received signals to be able to bring the peak beam down towards the horizon. For example, in an 800 Mhz system, the ground plane would have to be significantly larger than 14 inches in diameter, and in a PCS system operating at about 1900 Mhz, the ground plane would have to be significantly larger than about 6.5 inches in diameter. Ground planes with such large sizes would prohibit using the subscriber unit as a portable device. 
     Another disadvantage of existing prior art antennas utilizing flat ground planes is that as the ground plane dimensions are reduced in size, the array input impedance becomes highly sensitive to the environment, for example, when the array is placed on a metal surface or table, because the external environment directly couples with the antenna. That is, the external environment becomes part of the antenna. If the dimensions of the ground plane are increased to a sufficient size, this coupling problem is minimized. However, the large size of these ground plans may be undesirable in many applications. Shaped ground planes have been used to pull the beam of monopole arrays down towards the horizon. These shaped ground planes have large three dimensional features. Thus, it is desirable to force the beam down towards the horizon with an antenna structure that is not too large and unwieldy. 
     The present invention greatly reduces problems encountered by the aforementioned prior art antenna systems. The present invention provides an inexpensive antenna array for use with a mobile subscriber unit in a wireless same frequency network communications system, such as CDMA cellular communication networks. The invention utilizes a multiplicity of resonant strips provided within the ground plane. These strips couple to an equal multiplicity of monopole array elements located on top of the ground plane. This approach increases antenna gain by more efficiently utilizing the available ground plane area. Additionally, since the active element is on top of the ground plane, the antenna array sensitivity is decreased because the direct coupling between the antenna and external environmental factors is minimized. 
     The multiplicity of antenna elements are electrically isolated from the ground plane. Each antenna element has a bottom end located proximal to the ground plane, and is aligned along a respective antenna axis that is substantially perpendicular to the top side. Each resonant strip has a top end electrically connected to the ground plane and a bottom end spaced apart from a bottom side of the ground plane, and is aligned along the antenna axis of a corresponding antenna element. The multiplicity of antenna elements and the multiplicity of resonant strips are equally spaced about the perimeter of the ground plane, and the combination of each antenna element with a respective resonant strip provides a unbalanced dipole antenna element so that the multiplicity of dipole antenna elements form a composite beam which may be positionally directed along a horizon that is substantially parallel to the ground plane. Typically, at least one antenna element is connected to a transmission feed line for receiving signals from and transmitting signals to the antenna element. 
     Embodiments of this aspect can include one or more of the following features. The ground plane can be cylindrical such that the top side of the ground plane is a planar end of the cylinder, and the bottom side of the ground plane is an opposite planar end of the cylinder. In this arrangement, each resonant strip is disposed within a respective slot of the ground plane. The walls of each slot are spaced apart from the surface of the resonant strip, and the space between the walls and the surface is filled with nonmetallic material to electrically isolate a non-top end portion of the resonant strip from the ground plane. The ground plane, the antenna elements, and the resonant strips are made of copper, and the nonmetallic material is typically made of PCB materials such as polystyrene or Teflon. 
     The ground plane can be made of a multiplicity of plates equal in number to the multiplicity of resonant strips. Each plate has an outer edge and an inner edge. The resonant strips are aligned along the outer edge of a respective plate, and the inner edges of the plates are joined together at the center of the ground plane forming a central joint with an axis that is substantially parallel to the axes of the resonant strips. The central joint is a hinge which facilitates collapsing the antenna apparatus into a flat compact unit. Each plate includes a first nonmetallic substrate and a first conductive material layered over one side of the substrate. The conductive portion of the ground plane and the resonant strips are made of that conductive material. The nonmetallic substrate can be made from PCB materials such as polystyrene or Teflon, and the conductive material can be copper. Each plate can include a second nonmetallic substrate, a second conductive material sandwiched between the first substrate layer and the second substrate layer, and a third conductive material layered on an opposite side of the second nonmetallic substrate. The conductive portion of the ground plane and the resonant strips can be made of the first conductive material and the third conductive material. 
     In either of the above configurations, at least one antenna element can be connected to a phase shifter. The phase shifters are independently adjustable to affect the phase of respective signals transmitted from the antenna apparatus. Alternatively, at least one antenna element is connected to a delay line. The antenna element can be connected to a lumped or variable impedance element and/or a switch. Ideally, the peak strength of the directed beam rises no more than about 10° above the horizon. In some arrangements, the directed beam rises no more than about 5°, or even less, for example, 0°. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1A is an isometric view of antenna array used by a mobile subscriber unit in a cellular system according to this invention. 
     FIG. 1B is a close-up cutaway view of an antenna element of the antenna array of FIG.  1 A. 
     FIG. 2A is a system level diagram for the electronics used to control the antenna array. 
     FIG. 2B is a system level diagram for an alternative arrangement of the electronics used to control the antenna array. 
     FIG. 3 illustrates a beam directed ten degrees above the horizon by an antenna element configured according to the invention. 
     FIG. 4A is an isometric view of an alternative embodiment of an antenna array according to this invention. 
     FIG. 4B is a close-up cutaway view of an antenna element of the antenna array of FIG.  3 A. 
     FIG. 4C is a view of the antenna array of FIG. 4A collapsed into a flat compact unit. 
     FIG. 4D an alternative configuration of the multiple layers of a plate of the antenna array. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description of preferred embodiments of the invention follows. Turning now to the drawings, there is shown in FIG. 1 an antenna apparatus  10  configured according to the present invention. Antenna apparatus  10  serves as the means by which transmission and reception of radio signals is accomplished by a subscriber unit  11 , such as a laptop computer  14  coupled to a wireless cellular modem, with a base station  12 . The subscriber unit provides wireless data and/or voice services and can connect devices such as the laptop computer  14 , or personal digital assistants (PDAs) or the like through the base station  12  to a network which can be a Public Switched Telephone Network (PSTN), a packet switched computer network, or other data network such as the Internet or a private intranet. The base station  12  may communicate with the network over any number of different efficient communication protocols such as primary ISDN, or even TCP/IP if the network is an Ethernet network such as the Internet. The subscriber unit may be mobile in nature and may travel from one location to another while communicating with base station  12 . In the typical scenario, a number of subscriber access units  11  are located within the area surrounding the base station  12  and are serviced by the common base station. However, other arrangements are possible. 
     It is also to be understood by those skilled in the art that FIG. 1 may be a standard cellular type communication system such as CDMA, TDMA, GSM or other systems in which the radio channels are assigned to carry data and/or voice signals between the base station  12  and the subscriber unit  14 . In a preferred embodiment, FIG. 1 is a CDMA-like system, using code division multiplexing principles such as those defined in U.S. Pat. No. 6,151,332. 
     Antenna apparatus  10  includes a cylindrically shaped base or ground plane  20  upon which are mounted six antenna elements  22 . As illustrated, the antenna apparatus  10  is coupled to the laptop computer  14  (not drawn to scale). The antenna apparatus  10  allows the laptop computer  14  to perform wireless communications via forward link signals  30  transmitted from the base station  12  and reverse link signals  32  transmitted to the base station  12 . 
     In a preferred embodiment, each antenna element  22  is disposed on the ground plane  20  in the dispersed manner as illustrated in the figure. That is, a preferred embodiment includes five elements which are equally spaced about the perimeter of the ground plane  20  and a sixth element is positioned at a location corresponding to a center of the ground plane  20 . 
     Turning attention to FIG. 2A, there is shown a block diagram of the electronics which control the subscriber access unit  11 . The subscriber access unit  11  includes the antenna array  10 , antenna Radio Frequency (RF) sub-assembly  40 , and an electronics sub-assembly  42 . Wireless signals arriving from the base station  12  are first received at the antenna array  10  which consists of the antenna elements  22 - 1 ,  22 - 2 , . . . ,  22 -N. The signals arriving at each antenna element are fed to the RF subassembly  40 , including, for example, a phase shifter  56 , delay  58 , and/or switch  59 . There is an associated phase shifter  56 , delay  58 , and/or switch  59  associated with each antenna element  22 . 
     The signals are then fed through a combiner divider network  60  which typically adds the energy in each signal chain providing the summed signal to the electronics sub-assembly  42 . 
     In the transmit direction, radio frequency signals provided by the electronic sub-assembly  42  are fed to the combiner divider network  60 . The signals to be transmitted follow through the signal chain, including the switch  59 , delay  58 , and/or phase shifter  56  to a respective one of the antenna elements  22 , and from there are transmitted back towards the base station. 
     In the receive direction, the electronics sub-assembly  42  receives the radio signal at the duplexer filter  62  which provides the received signals to the receiver  64 . The radio receiver  64  provides a demodulated signal to a decoder circuit  66  that removes the modulation coding. For example, such decoder may operate to remove Code Division Multiple Access (CDMA) type encoding which may involve the use of pseudorandom codes and/or Walsh codes to separate the various signals intended for particular subscriber units, in a manner which is known in the art. The decoded signal is then fed to a data buffering circuit  68  which then feeds the decoded signal to a data interface circuit  70 . The interface circuit  70  may then provide the data signals to a typical computer interface such as may be provided by a Universal Serial Bus (USB), PCMCIA type interface, serial interface or other well-known computer interface that is compatible with the laptop computer  14 . A controller  72  may receive and/or transmit messages from the data interface to and from a message interface circuit  74  to control the operation of the decoder  66 , an encoder  74 , the tuning of the transmitter  76  and receiver  64 . This may also provide the control signals  78  associated with controlling the state of the switches  59 , delays  58 , and/or phase shifters  56 . For example, a first set of control signals  78 - 3  may control the phase shifter states such that each individual phase shifter  56  imparts a particular desired phase shift to one of the signals received from or transmitted by the respective antenna element  22 . This permits the steering of the entire antenna array  10  to a particular desired direction, thereby increasing the overall available data rate that may be accomplished with the equipment. For example, the access unit  11  may receive a control message from the base station commanded to steer its array to a particular direction and/or circuits associated with the receiver  64  and/or decoder  66  may provide signal strength indication to the controller  72 . The controller  72  in turn, periodically sets the values for the phase shifter  56 . 
     Referring now to FIG. 2B, there is shown an alternative arrangement for the antenna array  10  of the access unit  11 . In this configuration, a single active antenna element  22 -A is positioned in the middle of the ground plane  20  and is surrounded by a set of passive antenna elements  22 - 1 ,  22 - 2 ,  22 - 3 , . . . ,  22 -N. (For example, in FIG. 1A, the five outer elements are passive antenna elements for this configuration.) Here only the active antenna element  22 -A is connected, directly through the duplexer filter  62 , to the electronics sub-assembly  42 . An associated delay  58 , variable or lumped impedance element  57 , and switch  59  is connected to a respective passive antenna element  22 - 1 ,  22 - 2 ,  22 - 3 , . . . ,  22 -N. 
     In the arrangement shown in FIG. 2B, the transmit/receive signals are communicated between the base station and the active antenna element  22 -A. In turn, the active antenna element  22 -A provides the signals to the electronics sub-assembly  42  or receives signals from the assembly  42 . The passive antenna elements  22 - 1 ,  22 - 2 ,  22 - 3 , . . . ,  22 -N either reflect the signals or direct the signals to the active antenna element  22 -A. The controller  72  may provide control signals  78  to control the state of the delays  58 , impedance elements  57 , and switches  59 . 
     Referring now to FIG. 1B, each antenna element  22  is mounted to the top of the ground plane  20 . A transmission feed line  182  is connected to the antenna element  22  at a bottom feed point  183 , and to the delay line  58  which in turn is connected to the lumped or impedance element  57  and the switch  59 . The antenna element  22 , and the transmission feed line  182  are electrically isolated from the ground plane  20 . The delay line  58 , the lumped or variable impedance element  57 , and the switch  59  are located within the ground plane  20  but are also electrically isolated from the ground plane. Instead of the delay line  58 , the phase shifter  56  (FIG. 2A) can be connected to the antenna element  22 . The phase shifter  56  of each antenna element  22  is independently adjustable to facilitate changing the phase of a signal transmitted from the antenna element  22 . The transmission line  182  provides a path for transmitted signals to and received signals from the antenna element  22 . 
     Beneath each antenna element  22  is a resonant strip  190  positioned in a slot  192  formed in the ground plane  20 . The slot  192  is slightly larger in size than the resonant strip  190  to define a space  194 . A top end  196  of the resonant strip  190  is electrically coupled to the ground plane  20 . However, the space  194  is filled with nonmetallic material, for example, PCB materials such as polystyrene or Teflon, to electrically isolate the non-top end portion  198  of the resonant strip  190  from the ground plane  20 . 
     Both the antenna element  22  and the resonant strip  90  are made, for example, from copper. For applications in the PCS bandwidth (1850 Mhz to 1990 Mhz), the antenna element  22  has a length of about a quarter wavelength of the operating signal and a thickness of about one-tenth a wavelength. Each resonant strip  190  is also about a quarter wavelength long and about one-tenth wavelength in thickness. The bottom of the resonant strip  190  is positioned at a height, “h,” of about a one-eighth wavelength above the bottom of the ground plane  20  (FIG.  1 A), although the bottom of the resonant strip  190  can be nearly touching the bottom of the ground plane  20 . 
     In use, signals are transmitted to and received from the antenna element  22  along the transmission feed line  182  to enable the antenna array  10  to communicate with the base station  12 . The curved outer surface  200  of the ground plane  20  brings the beam formed by the antenna array  10  down to the horizon since the surface normal of the curved surface  200  points towards the horizon. Alone, each antenna element  22  acts as a monopole element. However, because of the presence of the resonant strip  190 , the antenna elements  22  couple with the resonant strips  190  to form an unbalanced dipole antenna. As such, the combination of the antenna element  22  and the resonant strip  190  provide further capabilities to direct the array beam along the horizon so that the ground plane  20  maybe reduced in size without sacrificing the beam directing capability of the antenna array  10 . As an array of unbalanced dipole antenna elements, the antenna array  10 , as illustrated in FIG. 3, is capable of forming a beam with a peak beam strength which rises no more than about 10° above the horizon, or even less, for example, no more than 0°. 
     In addition, the coupling of the antenna element  22  with the resonant strip  190  increases the effective area of the antenna and consequently the gain. And, since the antenna elements  22  are mounted on top of the ground plane  20 , the antenna array sensitivity to external environmental factors (such as when the array is placed on a metallic table) is decreased because the direct coupling of the antenna element  22  to these factors is minimized. 
     The present invention is not limited to cylindrical ground planes. For example, referring to FIG. 4A, there is shown an alternative arrangement of an antenna array  201 . The antenna array  201  includes a ground plane  202  made of six plates  204 . Seven antenna elements  206  are mounted on the ground plane  202  in the manner illustrated in the figure. That is, the embodiment includes six passive director/reflector elements which are equally spaced about the perimeter of the ground plane  202  above an outer edge  208  of each plate  204 , and a seventh active element is positioned at a location corresponding to a center of the ground plane  202 . An inner edge  207  of each plate  204  is joined together with the other inner edges  207  at the center of the ground plane  202  to form a hinge  209 . The hinge  209  can be spring loaded so that the plates  204  are collapsible to form a flat compact unit (FIG.  4 C), thereby making the antenna array convenient for transporting. 
     Referring in particular to FIG. 4B, each antenna element  206  is mounted to the top of the ground plane  202 , but is electrically isolated from the ground plane  202 . The antenna element  206  is connected to a transmission feed line  210  at a bottom feed point  212 . Each plate  204  is provided with a delay line  214  connected to a lumped or variable impedance element  215  and a switch  216  which are connected to the antenna element  1206  through the transmission feed line  210 . The transmission feed line  210 , the delay line  214 , the lumped or variable element  214 , and the switch  216  serve the same functions as the transmission feed line  182 , the delay line  58 , the lumped or variable impedance element  57 , and the switch  59  for the embodiment described in reference to FIGS. 1A and 1B. 
     Each plate  204  is also provided with a resonant strip  216  positioned along the outer edge  208  of the plate  204 . A top end  220  of the resonant strip  216  is electrically coupled to the ground plane  202  by a top band  203 . 
     Each plate  204  includes a nonmetallic dielectric substrate  222  made from, for example, PCB materials such as polystyrene or Teflon. For PCS applications, the substrate has a height of about one-third the wavelength of the operating signal, and a width of about one-quarter wavelength and is about 0.03 inch thick. The ground plane  202  and the resonant strip  216  are produced with printed circuit board (PCB) techniques by depositing on one side  218  of the substrate  222  with copper having a thickness of about 0.0015 inch, and then photo-etching the copper into the desired shapes. Thus the ground plane  202 , the top band  203  and the resonant strip  216  form a continuous layer of copper surrounding an inner region  224  of the substrate  222 . In addition, there is a thin region  226  of height, “h 1 , ” separating the bottom of the resonant strip  216  from the bottom of the plate  204 . PCB techniques are also used to print the transmission feed line  210 , the delay line  214 , the lumped or variable impedance element  215 , and the switch  216  on the opposite side of the substrate  222 . The antenna elements  206  are also typically made from copper. The antenna elements  206  and the resonant strips  216  are about one-quarter wavelength long, and are about a one-tenth wavelength wide. 
     Referring to FIG. 4D, there is shown an alternative lay-up for the plate  204 . Here, a conductive material  304 , for example, copper, is sandwiched between two substrates  302 A and  302 B made from a dielectric material. On the outer sides of the substrates  302 A and  302 B, there is a respective layer of conductive material  306 A and  306 B. The inner conductive material  304  is used for transmission line activity for the antenna element  206 , as well as the delay line  214 , the lumped or variable impedance element  215 , and the switch  216  which are typically imbedded in one of the substrates  302 A or  302 B. The two outer layers of conductive material  306 A and  306 B serve as the ground plane  202  and the resonant strip  216 . 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.