Abstract:
A small volume antenna ( 100 ) has the form of a polygonal (e.g., square) board with multiple antenna elements ( 104, 110 ) located at vertices ( 114, 116 ) (e.g., opposite vertices). The antenna elements ( 104, 110 ) include two segments ( 118, 120, 124, 126 ) that meet at corners ( 122, 128 ) that are located at the vertices ( 114, 116 ). Peripheral portions ( 134, 136, 138, 140 ) of a ground plane ( 132 ) that underlie the segments ( 118, 120, 124, 126 ) of the antenna elements are deleted, and slots ( 154, 162 ) that have two joined segments ( 156, 158, 164, 166 ) that parallel the segments ( 118, 120, 124, 126 ) of the antenna elements ( 104, 110 ) are formed in the antenna elements. The antenna elements ( 104, 110 ) are selectively loaded by switched impedance (e.g., capacitance) networks ( 172, 176, 178, 180, 182, 186, 190, 192 ). The antenna ( 100 ) is able to support operation in at least two broad operating bands.

Description:
RELATED ART  
       [0001]     This application is related to U.S. patent application Ser. No. 10/945,234, filed on Sep. 20, 2004, entitled “Multi-Frequency Conductive Strip Antenna System”, assigned to the assignee hereof. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to wireless communication devices. More particularly the present invention relates to antennas for wireless communication devices.  
       BACKGROUND  
       [0003]     The deployment of cellular networks, satellite networks and other wireless networks, has greatly expanded the use of mobile wireless communication devices. Whether a wireless communication device is a handheld device or a vehicle mounted device, there is an abiding interest in making the devices small so that they can be conveniently carried or accommodated in a small allocated space.  
         [0004]     Advances, by many orders of magnitude, in the degree of integration and miniaturization of electronics over the past few decades have facilitated extreme miniaturization of transceiver electronic circuits. However, the methods and means used to miniaturize electronic circuits, cannot be applied to miniaturize antennas, because antennas operate under the principles of Maxwell&#39;s equations, which, roughly speaking, indicate that if antenna efficiency is to be preserved, the size of the antenna must be scaled according to the wavelength of the carrier frequency of the wireless signals that are to be received and/or transmitted.  
         [0005]     Compounding the challenge of reducing antennas size, is the fact, that for many wireless communication devices, the antenna system needs to support operation at multiple frequencies, e.g., in multiple relatively wide frequency bands. The obvious expedient of using separate antennas to support separate operating frequencies, is contrary to the desire to reducing the space occupied by the antenna system.  
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0006]     The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.  
         [0007]      FIG. 1  is a top view of an antenna according to an embodiment of the invention;  
         [0008]      FIG. 2  is a bottom view of the antenna shown in  FIG. 1  according to an embodiment of the invention;  
         [0009]      FIG. 3  is a plan view of a plan view of an antenna element of the antenna shown in  FIG. 1  and  2  with a superposed current distribution;  
         [0010]      FIG. 4  is a first graph including S-parameter plots for a prototype of the antenna shown in  FIG. 1  in a first tuning state;  
         [0011]      FIG. 5  is a second graph including S-parameter plots for the prototype of the antenna shown in  FIG. 1  in a second tuning state;  
         [0012]      FIG. 6  is a three-dimensional radiation pattern plot for the antenna shown in  FIG. 1 ;  
         [0013]      FIG. 7  is a block diagram of a radio using the antenna shown in  FIG. 1  according to an embodiment of the invention;  
         [0014]      FIG. 8  is a schematic of an antenna according to another embodiment of the invention;  
         [0015]      FIG. 9 ; is a schematic diagram of an antenna according to yet another embodiment of the invention; and  
         [0016]      FIG. 10  is a third graph including S-parameter plots for the prototype of the antenna of the type shown in  FIG. 1  in five tuning states. 
     
    
       [0017]     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.  
       DETAILED DESCRIPTION  
       [0018]     Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of apparatus components related to antennas. Accordingly, the apparatus components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.  
         [0019]     In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.  
         [0020]     It will be appreciated that embodiments of the invention described herein may comprise one or more conventional processors and unique stored program instructions that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of communication described herein. The non-processor circuits may include, but are not limited to, a radio receiver, a radio transmitter, signal drivers, clock circuits, power source circuits, and user input devices. As such, these functions may be interpreted as steps of a method to perform communication. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. Thus, methods and means for these functions have been described herein. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation.  
         [0021]      FIG. 1  is a top view of an antenna  100  according to an embodiment of the invention and  FIG. 2  is a bottom view of the antenna  100  shown in  FIG. 1 . The antenna  100  is built on square dielectric substrate  102 . The dielectric substrate  102  is suitably made out of Duroid, FR-4 or other suitable materials. A first driven antenna element  104  is supported by a first dielectric spacer  106  on a top surface  108  of the dielectric substrate  102 . Similarly a second driven antenna element  110  is supported by a second dielectric spacer  112  above the dielectric substrate  102 . The first dielectric spacer  106  and the second dielectric spacer  112  are suitably made out of polytetrafluoroethylene, or other low loss tangent material. The first antenna element  104  and the second antenna element  110  are suitably made out of a highly conductive material such as copper or silver. The first antenna element  104  and the second antenna element  110  can be formed by metal working (e.g., stamping, machining), lift-off deposition, printing, lithography, electroless deposition or other suitable processes. The first antenna element  104  is located at a first vertex  114  of the square dielectric substrate  102  and the second antenna element  110  is located at a second (opposite) vertex  116  of the square dielectric substrate  102 . In as much as a square is a convex polygon, positioning the first antenna element  104  and the second antenna element  110  at vertices, increases the utilizable electrical length of the antenna  100 , for modes that involve strong current components directed radially from the antenna elements  104 ,  110  (e.g., along the diagonal of the square), thereby allowing the antenna  100  to be smaller for a given operating frequency. The design of the antenna  100 , which is further described below, is such that the volume of the antenna  100 , judged in view of the operating wavelengths of the antenna, is relatively small. For example, an embodiment of the antenna capable of supporting efficient operation in two frequency bands centered at 253 MHz and 303 MHz corresponding to free-space wavelengths of 1.18 meters and 0.99 meters has plan view dimensions of 30 centimeters by 30 centimeters and a height of 0.5 centimeters.  
         [0022]     The first antenna element  104  comprises a first linear segment  118  and a second linear segment  120  that join contiguously at a right angle forming a first corner  122 . The first corner  122  is located at the first vertex  114  of the antenna  100 . Similarly, the second antenna element  110  comprises a third linear segment  124  and a fourth linear segment  126  that join contiguously at a right angle forming a second corner  128 . The second corner  128  of the second antenna element  110  is located at the second vertex  116  of the antenna  100 .  
         [0023]     A first signal feed conductor  130  extends from the top surface  108  of the dielectric substrate  102  proximate the first corner  122  to the first linear segment  118 .  
         [0024]     The antenna  100  further comprises a ground plane  132  disposed on the dielectric substrate  102  opposite the dielectric spacers  106 ,  112  and the antenna elements  104 ,  110 . Alternatively, the ground plane  132  is located on the top surface  108  of the dielectric substrate  102  as the aforementioned components, or within a multilayered substrate that is used in lieu of the dielectric substrate  102 . Such a multilayered substrate can take the form of a multilayer circuit board that has one or more ground planes.  
         [0025]     As shown in  FIG. 2  the ground plane  132  has four deleted areas  134 ,  136 ,  138 ,  140 , including a first deleted area  134  and a second deleted area  136  that are disposed under the first segment  118  and the second segment  120  of the first antenna element  104  respectively. Similarly a third deleted area  138  and a fourth deleted area  140  are located under the third segment  124  and the fourth segment  126  of the second antenna element  110  respectively. Accordingly, a perimeter  142  of the ground plane  132  is reentrant (with respect to an otherwise square shape) at the deleted areas  134 ,  136 ,  138 ,  140 . The ground plane  132  can be patterned using various methods such as the methods mentioned above in reference to the antenna elements  104 ,  110 .  
         [0026]     The first linear segment  118  and the second linear segment  120  extend parallel to a first edge  144  and a second edge  146  of the antenna  100  that join at the first vertex  114 . Similarly the third segment  124  and the fourth segment  126  extend parallel to a third edge  148  and a fourth edge  150  of the antenna  100  that join at the second vertex  116 . The antenna elements  104 ,  110  are shaped to guide currents along the edges  144 ,  146 ,  148 ,  150 , thereby bringing the currents over the deleted areas  134 ,  136 ,  138 ,  140 . Although not wishing to be bound to any particular theory of operation, it is believed that the deleted areas  134 ,  136 ,  138 ,  140  create a field configuration that increases the radiative efficiency of the antenna  100 , lowering the Q of the antenna, and thereby increasing the bandwidths of the antenna  100  for modes associated with two antenna elements  104 ,  110 . Furthermore, it is believed that having the segments  118 ,  120 ,  124 ,  126  of the antenna elements  104 ,  110  run along the edges  144 ,  146 ,  148 ,  150  of the antenna  100  enhances the radiation associated with the deleted areas by inducing strong currents, charge densities and fields on the perimeter  142 , where the fields more readily couple to free space (compared to a case where the deleted area is interior to the ground plane  132 . Although the two antenna elements  104 ,  110  share the ground plane  132 , the two elements  104 ,  110  are able to support operation in two different frequency bands without substantial mutual interference.  
         [0027]     A first ground conductor  152  extends from the second linear segment  120  of the first antenna element  104  to the ground plane  132  proximate the first corner  122 . A second ground conductor  202  extends from the third linear segment  124  of the second antenna element  110  to the ground plane  132  proximate the second corner  128 . A second signal feed conductor (not shown) extends from the top surface  108  of the dielectric substrate  102  to the fourth linear segment  126  of the second driven antenna element  110 . Signal lines (not shown) that are suitably formed on the top surface  108  of the dielectric substrate  102  connect the antenna elements  104 ,  110  to transceiver circuits (not shown). Alternatively, the antenna elements  104 ,  110  are coupled to transceiver circuits located on a separate circuit board.  
         [0028]     The proximity of the signal feed conductors  130 , and the ground conductors  152 ,  202  to the corners  122 ,  128  of the antenna elements  104 ,  110  effects input impedances of the antenna  100 . A particular spacing which can be found by experimentation yields a particular desired real impedance e.g., 50 Ohms. The spacing that gives a desired real impedance is also dependent on the spacing of the antenna elements  104 ,  110  from the ground plane  132 . As the spacing of the antenna elements  104 ,  110  from the ground plane increases the input impedance will increase. By way of example for an embodiment of the antenna  100  designed for operation at 300 MHz, that has an overall edge dimension of 30 cm, in which the lengths of the linear segments  118 ,  120 ,  124 ,  126  were about 130 millimeter and the antenna elements  104 ,  110  spaced from the ground plane  132  by 5 mm, the ground conductors  152 ,  202  and the signal feed conductors  130  are suitably spaced from the corners  122 ,  128  by about 4 mm.  
         [0029]     A right angle shaped slot  154  is formed in the first antenna element  104 . The right angle shaped slot  154  includes a fifth linear segment  156  and a sixth linear segment  158  that join at a third corner  160 , that is located proximate the first corner  122  of the first antenna element  104 . The fifth linear  156  segment is arranged parallel to the first linear segment  118 , and the sixth linear segment is arranged parallel to the second linear segment  120 .  
         [0030]     A three legged slot  162  is formed in the second antenna element  110 . The three legged slot  162  includes a seventh linear segment  164  arranged parallel to the third linear segment  124  of the second antenna element  110 , an eighth linear segment  166 , that extends parallel to the fourth linear segment  126  of the second antenna element  110  and intersects the seventh linear segment  164  at an intersection  168 , that is located proximate the second corner  128  of the second antenna element  110 . The three legged slot  162  also includes a ninth linear segment  170  that extends from the intersection  168  toward the second corner  128  of the second antenna element  110 . Although linear segments are discussed above alternatively curved or curvilinear segments are used.  
         [0031]     The right angle slot  154  and the three legged slot  162  are used to control the operating frequencies of the first and second antennas, respectively. In general, increasing the length of the slot legs will reduce the operating frequency of the antenna element.  
         [0032]     A first microstrip  172  connects an inside edge  174  of the second segment  120  of the first antenna element  104  to a first switch  176 . The first microstrip  172  runs up an inward facing side wall (not visible) of the first dielectric spacer  106 . A second microstrip  178  connects the first switch  176  to a first capacitor  180 . Thus, the first switch  176  selectively couples the first antenna element to the first capacitor  180 . Similarly, a third microstrip  182  connects an inside edge  184  of the third segment  124  of the second antenna element  110  to a second switch  186 . The third microstrip  182  runs up an inward facing side wall  188  of the second dielectric spacer  112 . A fourth microstrip  190  connects the second switch  186  to a second capacitor  192 . The first capacitor  180  and the second capacitor  192  are suitably grounded to the ground plane  132  through vias (not shown) that pass through the dielectric substrate  102 . By selectively coupling the capacitors  180 ,  192  to the antenna elements  104 ,  110  the frequency bands of the antenna  100  can be shifted, effectively broadening the bandwidth of the antenna  100 . This broadening effect compounds the bandwidth broadening provided by the deleted areas  134 ,  136 ,  138 ,  140  of the ground plane  132  and the bandwidth broadening provided by the slots  154 ,  162 . The first switch  176  and the second switch  186  can be Micro-Electro Mechanical (MEMS) switches, or a solid state switch.  
         [0033]     The exact positions on the inside edges  174 ,  184  of the antenna elements at which the antenna elements  104 ,  110  are capacitively loaded (i.e., the points at which the first microstrip  172  and the third microstrip  182  connect) are suitably close to an inside corner  194  of the first antenna element  104 , and an inside corner  196  of the second antenna element  196  respectively. If it is only necessary to obtain a limited tuning range, the loading point could be connected at the inside corners  194 ,  196 , but to obtain an increased tuning effect the point of connection is located away from the corner  310 . On the other hand, moving the loading points too far away from the inside corners  194 ,  196  (e.g., beyond the longitudinal midpoints of the linear segments  118 ,  120 ,  124 ,  126 ) leads to degraded antenna performance.  
         [0034]      FIG. 3  is a plan view of a plan view of the second antenna element  110  of the antenna  100  shown in  FIG. 1  and  2  with a superposed current distribution. The position of the second feed conductor is indicated by reference numeral  302  and the position of the second ground conductor  202  is indicated by reference numeral  304 . The position at which the second antenna element  110  is loaded (connected to the third microstrip  182 ) is indicated by reference numeral  306 . In the modeled prototype on which  FIG. 3  is based, the ninth linear segment  170  of the three legged slot  162  is bridged by a conductive bridge  308 . The bridge  308  is used for tuning the input impedance. As shown in  FIG. 3  the current pattern that is established when operation the antenna  100  includes a current flow that flows partly around the three legged slot  162 , before diverging onto the third linear segment  124  and fourth linear segment  126 . Note, also that the current is concentrated in areas overlying the ground plane. Consequently, the deleted areas of the ground plane serve to concentrate the current toward the inside of the antenna element  110 . An effect of having both the slot  162  and the deleted areas  138 ,  140  is force a create a convoluted current path. Although not wishing to be bound to any particular theory of operation, it is believed that this convoluted current path serves to increase the effective electrical size of the antenna  100 , allowing the antenna have a relatively reduced size for a given frequency of operation.  
         [0035]      FIG. 4  is a first graph  400  including S-parameter plots  402 ,  404 ,  406  for a prototype of the antenna shown in  FIG. 1  in a first tuning state and  FIG. 5  is a second graph  500  including S-parameter plots  502 ,  504 ,  506  for the prototype of the antenna shown in  FIG. 1  in a second tuning state. In the prototype tested to obtain the data shown in  FIGS. 4 and 5 , the antenna elements were designed to provide two separate operating bands including a lower band centered at about 253 MHz and an upper band centered at about 303 MHz. Each antenna element plays a primary role in supporting one of the operating bands. The first graph  400  shows the S-parameters with no capacitive loading on either antenna element  104 ,  110  but the second graph  500  shows the S parameters with the antenna element associated with the upper band loaded with a capacitor (e.g.,  180 ,  192 ). In the first graph  400 , a first plot  402  (correspond to port  1 ) shows the return loss for the upper band and a second plot  404  (corresponding to port  2 ) shows the return loss for the lower band. Correspondingly, in the second graph  500 , a third plot  502  (corresponding to port  1 ) shows the return loss for the upper band and a fourth plot  504  (corresponding to port  2 ) shows the return loss for the lower band. Comparing the two graphs  400 ,  500  it is seen that switching in the capacitive loading on the antenna element associated with the upper band, causes the upper band to shift down in frequency by about 6 MHz, thereby effectively increasing the obtainable bandwidth. Note that the lower band is also somewhat sharpened by capacitively loading the antenna element associated with the upper band, however the change in efficiency in the lower band is relatively small. (Note that a port is an abstraction that is physically embodied by the combination of a signal feed conductor, e.g.,  130  and ground conductor e.g.,  152 )  
         [0036]     A fifth plot  406  in the first graph  400  and a sixth plot  506  in the second graph shows the coupling between the ports feeding the two antenna elements  104 ,  110 . Note that the coupling is limited to about 16dB, which corresponds to a high degree of isolation. Thus, the two antenna elements  104 ,  110  are able to achieve operation in two bands while sharing the common ground plane without suffering from excessive mutual interference.  
         [0037]     Frequency tuning can be achieved by varying the lengths of the segments  118 ,  120 ,  124 ,  126  of the antenna elements  104 ,  110  and by varying the lengths of the slot segments  156 ,  158 ,  164 ,  166  that run parallel to the segments  118 ,  120 ,  124 ,  126  of the antenna elements.  
         [0038]      FIG. 6  is a three dimensional radiation pattern plot  600  for the antenna shown in  FIG. 1 . The plot  600  shows a series of level curves on a sphere to indicate the gain in each direction. In the plot Cartesian X, Y and Z axes are indicated. The Z-axis is aligned so as to pass through the first vertex  114  and the second vertex  116  of the antenna and the X-axis is aligned normal to the dielectric substrate  102 .  
         [0039]      FIG. 7  is a block diagram of a radio  700  using the antenna  100  shown in  FIG. 1  according to an embodiment of the invention. The radio  700  includes a transceiver  702  that is coupled to the antenna  100  by a receive signal line  704  and a transmit signal line  706 . The receive signal line  704  is suitably coupled to one of the antenna elements  104 ,  110  and the transmit signal line is suitably couple to another of the antenna elements  104 ,  110 . Alternatively, both antenna elements  104 ,  110  are coupled to both receive signal lines and transmit signal lines. A first control line  708  is coupled to a first switched reactive load network  710  (e.g., made up of first microstrip  172 , first switch  176 , second microstrip  178  and first capacitor  180 ). Similarly, a second control line  712  is coupled to second switched reactive load network  714  (e.g., made up of third microstrip  182 , second switch  186 , fourth microstrip  190  and second capacitor  192 ). The control lines  708 ,  712  are used to apply signals to control the switches (e.g.,  176 ,  186 ), in order to shift the operating bands of the antenna  100 , in coordination with shifting of the frequency of signals transmitted from or received by the transceiver  702 . The transceiver suitably comprises a Frequency Division Multi-Access (FDMA) transceiver, or a Frequency Hopping Spread Spectrum (FHSS) transceiver, or another type of transceiver that works with signals that change frequency.  
         [0040]      FIG. 8  is a schematic of an antenna  800  according to another embodiment of the invention. The antenna  800  includes an antenna element  802  (such as  104 ,  110 ) coupled to a common terminal of a first single pole double throw (SPDT) switch  804 . A MEMS SPDT switch is suitably used. A first throw of the switch  804  is coupled to a first reactive load  806  and a second throw of the switch  804  is coupled to a second reactive load  808 . Alternatively, one of the throw connections is left open. Thus, as in the case of the antenna  100  shown in  FIGS. 1 and 2 , in the antenna  800  two loading conditions can be obtained in the antenna  800 , so that an operating band of the antenna  800  can be shifted.  
         [0041]      FIG. 9  is a schematic diagram of an antenna  900  according to yet another embodiment of the invention. The antenna  900  includes an antenna element  902  (such as  104 , 110 ) coupled to a first SPDT switch  904 . A first throw of the first SPDT switch  904  is coupled to a second SPDT switch  906  and a second throw of the first SPDT switch  904  is coupled to third SPDT switch  908 . The second SPDT switch  906  is coupled to a first reactive load  910  and a second reactive load  912 , and the third SPDT switch  908  is coupled to a third reactive load  914  and a fourth reactive load  916 . Thus, by setting the states of the SPDT switches  904 ,  906 ,  908  the antenna  900  can be selectively coupled to one of the four reactive loads  910 ,  912 ,  914 ,  916 . If the first SPDT switch  904  is a Single Pole Centre Off (SPCO) device, then the antenna element  902  can be decoupled from all of the reactive loads  910 ,  912 ,  914 ,  916 .  
         [0042]      FIG. 10  is a third graph  1000  including S-parameter plots  1002 ,  1004 ,  1006 ,  1008 ,  1010  for the prototype of the antenna of the type shown in  FIG. 1  in five tuning states. A first plot  1002  shows the return loss with no loading on the antenna element e.g.,  104 ,  110 , and the sequence of plots  1004 - 1010  show the return loss with increasing capacitive loading of the antenna element, e.g.,  104 ,  110 .  FIG. 9  illustrates one form of switched capacitance network that can alter the capacitive loading on the antenna element, e.g.,  104 ,  110  in steps in order to shift the return loss plot in steps. By incrementally increasing the capacitive loading on at least one of the antenna elements  104 ,  110  the operating band of the antenna can be shifted so that the antenna  100  is able to support operation over a relatively broad frequency band.  
         [0043]     In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The inventionis defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.