Patent Publication Number: US-6903703-B2

Title: Multiband radially distributed phased array antenna with a sloping ground plane and associated methods

Description:
FIELD OF THE INVENTION 
   The present invention relates to the field of communications, and more particularly, to a multiband phased array antenna. 
   BACKGROUND OF THE INVENTION 
   Existing microwave antennas include a wide variety of configurations for various applications, such as satellite reception, remote broadcasting, or military communication. The desirable characteristics of low cost, light weight, low profile and mass producibility are provided in general by printed circuit antennas. 
   The simplest forms of printed circuit antennas are microstrip antennas wherein flat conductive elements, such as monopole or dipole antenna elements, are spaced from a single essentially continuous ground plane by a dielectric sheet of uniform thickness. An example of a microstrip antenna is disclosed in U.S. Pat. No. 6,417,813 to Durham, which is assigned to the current assignee of the present invention and is incorporated herein by reference in its entirety. 
   The antennas are designed in an array and may be used for communication systems requiring such characteristics as low cost, light weight and a low profile. The bandwidth of such antennas is about 10-to-1. However, a 10-to-1 bandwidth can be limiting for certain applications. For example, electronic warfare support measures (ESM) and electronic intelligence (ELINT) radar systems require antennas having a bandwidth typically greater than 20-to-1, which offers a higher probability of intercepting signals. 
   One approach for increasing the bandwidth of an array of dipole antenna elements is disclosed in U.S. Pat. No. 6,552,687 to Rawnick et al., which is also assigned to the current assignee of the present invention and is incorporated herein by reference in its entirety. The multiband phased array antenna in the &#39;687 patent includes a first array of dipole antenna elements operating over a first frequency band, and a second array of dipole antenna elements operating over a second frequency band so that the phased array antenna is a multiband antenna. 
   The size of the dipole antenna elements in the first array is different from the size of the dipole antenna elements in the second array. Consequently, the ground plane spacing is different between the first and second arrays. One disadvantage of this configuration is that since the higher frequency dipole antenna elements are surrounded by the lower frequency dipole antenna elements, there is a gap or hole in the aperture distribution of the lower frequency dipole antenna elements. Consequently, the layout of the different size antenna elements in the &#39;687 patent presents difficulties in controlling the antenna pattern since this gap or hole may have undesired effects, such as raising the sidelobe levels of the antenna. In addition, the fact that the physical aperture size does not change over a large bandwidth (approximately 10:1) means that the electrical size of the aperture will vary considerably over the band, making this approach unsuitable as a feed for a reflector. 
   A different type antenna that offers a wide bandwidth (greater than 20-to-1) is a spiral antenna. To cover multiple frequency bands, multiple spirals may be used, i.e., a spiral for each frequency band. However, the multiple spirals are non-concentric about the focal point of the antenna when operating as a feed for a reflector, which results in a loss of efficiency due to scan loss compared to that of a completely concentric aperture. In addition, another disadvantage is that the efficiency of spiral antennas is typically much less than 50% since their performance depends on an absorber-filled back cavity. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing background, it is therefore an object of the present invention to provide a multiband antenna that has high efficiency while achieving a constant beamwidth and pattern control. 
   This and other objects, features, and advantages in accordance with the present invention are provided by a multiband phased array antenna comprising a substrate, and a plurality of dipole element arrays extending outwardly from an imaginary center point on the substrate. Each dipole element array may comprise a plurality of dipole antenna elements arranged in an end-to-end relation and having a dipole size different than a dipole size of dipole antenna elements of at least one other dipole element array. A ground plane is adjacent the plurality of dipole element arrays and may have a different spacing therefrom in an outward direction from the imaginary center point. 
   The plurality of dipole element arrays may be radially distributed from the imaginary center point, with the radial distribution being symmetrical. The radial distribution of the dipole element arrays advantageously provides a constant beamwidth when operating the multiband phased array antenna as a reflector feed since all of the arrays use the same focal point. In addition, the pattern of the multiband phased array antenna can be more easily controlled because the radial distribution of the dipole element arrays provides a choice of the radial feed point location, thereby allowing the electrical size of the aperture to be kept relatively constant. 
   The different spacing between the ground plane and the plurality of dipole element arrays may increase from the imaginary center point towards an edge of the substrate. The slope of the ground plane does not necessarily have to be constant. For example, the slope of the ground plane may be logarithmic or exponential. In this case, position of the dipole element arrays may be adjusted accordingly to provide the preferred spacing between the ground plane and the respective dipole antenna elements based upon their size. A dielectric material may be placed between the ground plane and the respective dipole antenna elements. 
   Each dipole antenna element may comprise a printed conductive layer. The plurality of dipole antenna elements are preferably sized and relatively positioned within each respective dipole element array so that the multiband phased array antenna has a total bandwidth equal to or greater than 20-to-1. 
   The plurality of dipole antenna elements in each dipole element array are preferably arranged in rows and columns, with outer columns of dipole antenna elements being resistively loaded. Feed lines are connected to at least one inner column of dipole antenna elements. 
   Each dipole antenna element comprises a medial feed portion and a pair of legs extending outwardly therefrom. Adjacent legs of adjacent dipole antenna elements may include respective spaced apart end portions having predetermined shapes and relative positioning to provide increased capacitive coupling between the adjacent dipole antenna elements. Each leg may comprise an elongated body portion, and an enlarged width end portion connected to an end of the elongated body portion. The spaced apart end portions in adjacent legs may comprise interdigitated portions. 
   The multiband phased array antenna may further comprise a respective impedance element electrically connected between the spaced apart end portions of adjacent legs of adjacent dipole antenna elements for further increasing the capacitive coupling therebetween. Alternately, a respective printed impedance element may be adjacent the spaced apart end portions of adjacent legs of adjacent dipole antenna elements for further increasing the capacitive coupling therebetween. 
   Another aspect of the present invention is directed to a method for providing a multiband phased array antenna by providing a substrate, and forming a plurality of dipole element arrays extending outwardly from an imaginary center point on the substrate. Each dipole element array may comprise a plurality of dipole antenna elements arranged in an end-to-end relation and having a dipole size different than a dipole size of dipole antenna elements of at least one other dipole element array. The method further comprises forming a ground plane adjacent the plurality of dipole element arrays. The ground plane may have a different spacing from the plurality of dipole element arrays in an outward direction from the imaginary center point. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram illustrating a multiband phased array antenna mounted on an aircraft in accordance with the present invention. 
       FIG. 2  is a top plan view of the multiband phased array antenna in accordance with the present invention. 
       FIGS. 3 and 4  are cross-sectional views of the multiband phased array antenna as shown in  FIG. 2  respectively taken along radial axes R 1  and R 2 . 
       FIG. 5  is an enlarged schematic view of a center column of one of the dipole element arrays as shown in FIG.  2 . 
       FIG. 6  is a plot of computed VSWR versus frequency for the low-frequency band arrays in the multiband phased array antenna as shown in FIG.  2 . 
       FIGS. 7A and 7B  are enlarged schematic views of the spaced apart end portions of adjacent legs of adjacent dipole antenna elements as may be used in the multiband phased array antenna of FIG.  2 . 
       FIG. 7C  is an enlarged schematic view of an impedance element connected across the spaced apart end portions of adjacent legs of adjacent dipole antenna elements as may be used in the multiband phased array antenna of FIG.  2 . 
       FIG. 7D  is an enlarged schematic view of an impedance element selectively connected across the spaced apart end portions of adjacent legs of adjacent dipole antenna elements as may be used in the multiband phased array antenna of FIG.  2 . 
       FIG. 7E  is an enlarged schematic view of another embodiment of an impedance element connected across the spaced apart end portions of adjacent legs of adjacent dipole antenna elements as may be used in the multiband phased array antenna of FIG.  2 . 
       FIGS. 8A and 8B  are respectively enlarged schematic views of a discrete resistive element and a printed resistive element connected across the medial feed portion of a dipole antenna element as may be used in the multiband phased array antenna of FIG.  2 . 
       FIG. 9  is top plan view of another aspect of the multiband phased array antenna in accordance with the present invention. 
       FIG. 10  is a cross-sectional view of the multiband phased array antenna as shown in  FIG. 9  taken along radial axis R 1 . 
       FIGS. 11A and 11B  are respectively a top plan view and a corresponding side view of another embodiment of the multiband phased array antenna as shown in FIG.  9 . 
       FIG. 12  is a plot of the computed VSWR versus frequency for one of the dipole element arrays having an edge element on a second surface of the substrate as shown in FIG.  11 B. 
       FIG. 13  is top plan view of another aspect of the multiband phased array antenna in accordance with the present invention. 
       FIG. 14  is a cross-sectional view of the multiband phased array antenna as shown in  FIG. 13  taken along radial axis R 1 . 
       FIG. 15  is top plan view of another aspect of the multiband phased array antenna in accordance with the present invention. 
       FIG. 16  is a cross-sectional view of the multiband phased array antenna as shown in  FIG. 15  taken along radial axis R 1 . 
       FIG. 17  is a plot of measured and computed VSWR versus frequency over a frequency range of 2 to 18 GHz for the multiband phased array antenna as shown in FIG.  16 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout, and prime, double prime and triple prime notations are used to indicate similar elements in alternative embodiments. 
   Referring initially to  FIG. 1 , a multiband phased array antenna  50  in accordance with the present invention will now be described. One or more multiband phased array antennas  50  may be mounted on an aircraft  52 , for example. The illustrated multiband phased array antenna  50  is connected to a beam forming network (BFN)  54  which is connected to a plurality of transceivers  56   1 - 56   n . 
   Since the multiband phased array antenna  50  covers multiple frequency bands, each transceiver  56   1 - 56   n  functions over one or more frequency bands. The BFN  54  controls the phase of the multiband phased array antenna  50  to create the desired sum and difference patterns, which forms the desired antenna beams, as readily understood by those skilled in the art. An example BFN  54  is a Butler matrix. 
   One aspect of the multiband phased array antenna  50  comprises a substrate  60 , and a plurality of dipole element arrays  62 ,  64  extending outwardly from an imaginary center point  66  on the substrate, as illustrated in FIG.  2 . The plurality of dipole element arrays  62 ,  64  may be radially distributed from the imaginary center point  66 , with the radial distribution being symmetrical. The radial distribution of the dipole element arrays  62 ,  64  advantageously provides no scan loss and therefore high efficiency when operating the multiband phased array antenna  50  as a reflector feed since all of the arrays use the same focal point, i.e., the imaginary center point  66 . In addition, the pattern of the multiband phased array antenna  50  can be more easily controlled because the radial distribution of the dipole element arrays  62 ,  64  allows for a choice of one or more feed points. Different feed points correspond to different electrical sizes for the array. By choosing different feed points for different bands of operation, the electrical size may be maintained relatively constant over an extremely broad bandwidth. In addition, yet another benefit of the radial distribution is that it provides the polarization diversity required to obtain sum and difference patterns that are relatively azimuthally constant in amplitude if the proper beam forming network is utilized. 
   Each dipole element array  62 ,  64  comprises a plurality of dipole antenna elements  70   a ,  70   b  arranged in an end-to-end relation and having a dipole size different than a dipole size of dipole antenna elements of at least one other dipole element array. Each dipole element array  62 ,  64  is arranged in rows and columns, such as the 3×5 arrays illustrated in FIG.  2 . The 3×5 arrays are for illustrative purposes, and the actual size of the arrays  62 ,  64  may vary depending on the intended application. 
   As will be discussed in greater detail below, the center column of dipole antenna elements  70   a ,  70   b  are active, whereas the outer columns of dipole antenna elements are passive. The passive elements in the outer columns allow the active elements in the center column to receive sufficient current, which is normally conducted through the dipole antenna elements  70   a ,  70   b  on the substrate  60 . 
   The multiband phased array antenna  50  illustrated in  FIG. 2  includes two sets of dipole element arrays  62 ,  64 . These dipole element arrays  62 ,  64  are separated into high-frequency band arrays and low-frequency band arrays. Dipole element arrays  64  are the low-frequency band arrays, which may cover a frequency range of 4 to 18 GHz, for example. Dipole element arrays  62  are the high-frequency band arrays, which may cover a frequency range of 19 to 28 GHz, for example. In this example, the multiband phased array antenna  50  covers a total bandwidth of 7-to-1. 
   To increase the total bandwidth, additional dipole element arrays may simply be added to the substrate  60  to cover a different frequency range. For example, if the additional dipole element arrays (not shown) cover 1 to 4 GHz, then the total bandwidth is significantly increased to 28-to-1. 
   The size of the dipole antenna elements  70   b  in the low-frequency band arrays  64  is different than the size of the dipole antenna elements  70   a  in the high-frequency band arrays  62 . In particular, the size of the dipole antenna elements  70   a  in the high-frequency band arrays  62  is less than the size of the dipole antenna elements  70   b  in the low-frequency band arrays  64 . 
   The multiband phased array antenna  50  further includes a ground plane  80 .  FIGS. 3 and 4  are cross-sectional views of the multiband phased array antenna  50  as shown in  FIG. 2  respectively taken along radial axes R 1  and R 2 . The spacing X of the ground plane  80  for the dipole antenna elements  70  in the low-frequency band arrays  64  is greater than the spacing Y of the ground plane for the dipole antenna elements in the high-frequency band arrays  62 . The ground plane  80  is preferably spaced from the different size dipole element arrays  62 ,  64  less than about one-half a wavelength of a highest desired frequency within each respective array, as readily appreciated by those skilled in the art. 
   The different spacing between the ground plane  80  and the respective dipole antenna elements  70   a ,  70   b  may be provided by a plateau shaped ground plane. In other words, the ground plane  80  has a stepped shape or thickness between the low-frequency band arrays  64  and the high-frequency band arrays  62 . A dielectric material  81  may be between the ground plane  80  and the respective dipole antenna elements  70 . 
   Referring now to  FIG. 5 , a plurality of feed lines  90  may be connected to the active dipole antenna elements  70   a ,  70   b  in each array  62 ,  64 . As noted above, the center column of each array  62 ,  64  includes active dipole antenna elements  70   a ,  70   b , whereas the outer columns include passive dipole antenna elements. This advantageously reduces the complexity of connecting the feed lines  90  to the dipole antenna elements in the multiband phased array antenna  50 . The active dipole antenna elements  70   b  as shown in  FIG. 5  represent the center column of a low-frequency band array  64 . The feed  72  for each active dipole antenna element  70   b  therein may be referred to as a port. Consequently, the five active dipole antenna elements  70   b  have five ports  72  that may be connected to five separate feed lines  90 . 
     FIG. 6  is a plot of VSWR versus frequency for the low-frequency band arrays  64  with respect to each of the five ports  72 . Port  1  is represented by line  100 , port  2  is represented by line  102 , port  3  is represented by line  104 , port  4  is represented by line  106  and port  5  is represented by line  108 . Lines  106  and  108  overlap one another so that it appears that only one line represents both ports  4  and  5 . Between 4 and 18 GHz, the VSWR for all five ports  72  is substantially the same when operating the multiband phased array  50  as a feed for a reflector. This results in a substantially constant beamwidth over the entire operating bandwidth of the array. 
   Between 2 and 4 GHz, however, the VSWR significantly increases for the outer ports (ports  4  and  5 ), whereas for the inner ports (ports  1 ,  2  and  3 ), the VSWR slightly increases. Each port  72  is a different radial distance from the phase center of the multiband phased array antenna—which is the imaginary center point  66  on the substrate  60 . 
   Since the wavelength changes as the frequency changes, it is preferred that the multiband phased array antenna  50  remains electrically the same for the different size dipole antenna elements  70   a ,  70   b . The radial distance of each port  72  from the phase center  66  determines the beamwidth. Consequently, a corresponding transceiver  56   1 - 56   n  may be connected to any one of the five ports  72  and receive substantially the same antenna performance. This is because the electrical size of the various feeds  90  remains substantially the same as the frequency varies across the multiband phased array antenna by choosing the correct port  50 . 
   Nonetheless, the transceivers  56   1 - 56   n  may be selectively connected to a particular port  72  within the radial distribution of dipole antenna elements  70   a ,  70   b  to achieve constant beamwidth and pattern control. Similarly, the dipole antenna elements  70  for the different frequency bands may be weighted (e.g., amplitude weighted) to also achieve constant beamwidth and pattern control, as readily appreciated by those skilled in the art. 
   A single transceiver may be connected to one or more of the five ports  72  on the low-frequency band arrays  64 , or multiple transceivers may connected. For example, a first transceiver  56   1  operating over the frequency range of 4-to-8 GHz may be connected to port  1 , a second transceiver  56   2  operating over the frequency range of 8-to-12 GHz may be connected to port  2 , and a third transceiver  56   3  operating over the frequency range of 12-to-18 GHz may be connected to port  3 . Different transceivers  56   4 - 56   n  may likewise be connected to the different ports on the high-frequency band arrays  62 . 
   Since the high and low frequency band arrays  62 ,  64  operate over different frequency bands, the respective transceivers  56   1 - 56   n  can operate simultaneously. Even though the illustrated low and high frequency bands are continuous (4-to-18 GHz and 18-to-28 GHz), the multiband phased array antenna  50  may be designed to operate over non-continuous frequency bands, as readily appreciated by those skilled in the art. For example, the low-frequency band arrays  64  may still cover 4 to 18 GHz, but the high-frequency band arrays  62  may cover a different frequency band, such as 30 to 33 GHz instead of 18 to 28 GHz, for example. 
   Referring to  FIGS. 7A-7E , and also to  FIG. 5 , the dipole antenna elements  70   a ,  70   b  as used in the multiband phased array antenna  50  will now be described in greater detail. The dipole antenna elements  70   a ,  70   b  are on a substrate  60 , which is a printed conductive layer. Each dipole antenna element  70   a ,  70   b  comprises a medial feed portion (or port)  72  and a pair of legs  74  extending outwardly therefrom. Respective feed lines  90  would be connected to each feed portion  72  from the opposite side of the substrate  60 . 
   Adjacent legs  74  of adjacent dipole antenna elements  76  have respective spaced apart end portions  78  to provide increased capacitive coupling between the adjacent dipole antenna elements, as shown in FIG.  7 A. Increasing the capacitive coupling counters the inherent inductance of the dipole antenna elements when they are closely spaced, and this is done in such a manner that as the frequency varies a wide bandwidth may be maintained. 
   The adjacent dipole antenna elements  76  have predetermined shapes and relative positioning to provide the increased capacitive coupling. For example, the capacitance between adjacent dipole antenna elements  76  is between about 0.016 and 0.636 picofarads (pF), and preferably between 0.159 and 0.239 pF. Of course, these values will vary as required depending on the actual application to achieve the same desired bandwidth, as readily understood by one skilled in the art. 
   As shown in  FIG. 7A , the spaced apart end portions  78  in adjacent legs  74  may have overlapping or interdigitated portions  80 , and each leg  74  comprises an elongated body portion  82 , an enlarged width end portion  84  connected to an end of the elongated body portion, and a plurality of fingers, e.g., four, extending outwardly from the enlarged width end portion. 
   Each dipole antenna element array  62 ,  64  has a desired frequency range (4 to 18 GHz or 18 to 28 GHz, for example) and the spacing between the end portions  78  of adjacent legs  74  is less than about one-half a wavelength of a highest desired frequency. 
   Alternatively, as shown in  FIG. 7B , adjacent legs  74 ′ of adjacent dipole antenna elements  76  may have respective spaced apart end portions  78 ′ to provide increased capacitive coupling between the adjacent dipole antenna elements. In this embodiment, the spaced apart end portions  78 ′ in adjacent legs  74 ′ comprise enlarged width end portions  84 ′ connected to an end of the elongated body portion  82 ′ to provide the increased capacitive coupling between adjacent dipole antenna elements  76 . 
   To further increase the capacitive coupling between adjacent dipole antenna elements  76 , a respective discrete or bulk impedance element  110 ″ is electrically connected across the spaced apart end portions  78 ″ of adjacent legs  74 ′ of adjacent dipole antenna elements, as illustrated in FIG.  7 C. 
   In the illustrated embodiment, the spaced apart end portions  78 ″ have the same width as the elongated body portions  82 ″. The discrete impedance elements  110 ″ are preferably soldered in place after the dipole antenna elements  70   a ,  70   b  have been formed so that they overlay the respective adjacent legs  74 ″ of adjacent dipole antenna elements  76 . This advantageously allows the same capacitance to be provided in a smaller area, which helps to lower the operating frequency of the respective dipole antenna element arrays  62 ,  64 . 
   The illustrated discrete impedance element  70 ″ includes a capacitor  112 ″ and an inductor  114 ″ connected together in series. However, other configurations of the capacitor  112 ″ and inductor  114 ″ are possible, as would be readily appreciated by those skilled in the art. For example, the capacitor  112 ″ and inductor  114 ″ may be connected together in parallel, or the discrete impedance element  110 ″ may include the capacitor without the inductor or the inductor without the capacitor. Depending on the intended application, the discrete impedance element  110 ″ may even include a resistor. 
   The discrete impedance element  110 ″ may also be connected between the adjacent legs  74  with the overlapping or interdigitated portions  80  illustrated in FIG.  7 A. In this configuration, the discrete impedance element  110 ″ advantageously provides a lower cross polarization in the antenna patterns by eliminating asymmetric currents which flow in the interdigitated capacitor portions  80 . Likewise, the discrete impedance element  110 ″ may also be connected between the adjacent legs  74 ′ with the enlarged width end portions  84 ′ illustrated in FIG.  7 B. 
   Another advantage of the respective discrete impedance elements  110 ″ is that they may have different impedance values so that the bandwidth of the respective dipole antenna element arrays  62 ,  64  can be tuned for different applications, as would be readily appreciated by those skilled in the art. In addition, the impedance is not dependent on the impedance properties of the adjacent dielectric layer  81 . Since the discrete impedance elements  110 ″ are not effected by the dielectric layer  81 , this approach advantageously allows the impedance between the dielectric layer  81  and the impedance of the discrete impedance element  110 ″ to be decoupled from one another. 
   Yet another aspect of the present invention is directed to selectively coupling a discrete impedance element  110   a ″- 110   n ″ between a respective pair of adjacent legs  74 ″ of adjacent dipole antenna elements, as illustrated in FIG.  7 D. Each dipole antenna element  70   a ,  70   b  has associated therewith a plurality of selectable impedance elements  110   a ″- 110   n ″ and a corresponding switch  75 ″. The illustrated switch  751  is a single pole multiple throw (SPMT) switch. Alternately, more than one impedance element  110   a ″- 110   n ″ may be connected at one time to achieve the desired impedance coupling values. In this case, a multiple pole multiple throw (MPMT) switch would be required. 
   A switch controller  77 ″ is connected to all of the switches  75 ″ in the multiband phased array antenna  50 . The switch controller  77 ″ may operate so that the respective impedance elements  110   a ″- 110   n ″ associated with all of the dipole antenna elements  70   a ,  70   b  are synchronously switched. Alternately, the respective impedance elements  110   a ″- 110   n ″ for each dipole antenna element  70   a ,  70   b  may be asynchronously switched with respect to the other dipole antenna elements. 
   The switches  75 ″ and corresponding impedance elements  110   a ″- 110   n ″ advantageously allow the multiband phased array antenna  50  to be retuned. For example, the frequency band of the phased array antenna may be adjusted, i.e., lower or higher. This adjustment may be as much as 10 to 20 percent of the frequency band depending on the range of the impedance values associated with the impedance elements  110   a ″- 110   n ″. In addition, better performance may be achieved at specific frequencies, particularly where the antenna can be better matched, i.e., to operate with a lower VSWR. The active switching may also be combined with the variable height ground plane  80 , as readily appreciated by those skilled in the art. 
   Yet another approach to further increase the capacitive coupling between adjacent dipole antenna elements  76  includes placing a respective printed impedance element  110 ′″ adjacent the spaced apart end portions  78 ′″ of adjacent legs  74 ′″ of adjacent dipole antenna elements  76 , as illustrated in FIG.  7 E. 
   The respective printed impedance elements  110 ′″ are separated from the adjacent legs  74 ′″ by a dielectric layer, and are preferably formed before the dipole antenna layer is formed so that they underlie the adjacent legs  74 ′″ of the adjacent dipole antenna elements  76 . Alternatively, the respective printed impedance elements  110 ′″ may be formed after the dipole antenna layer has been formed. For a more detailed explanation of the printed impedance elements, reference is directed to U.S. patent application Ser. No. 10/308,424 which is assigned to the current assignee of the present invention, and which is incorporated herein by reference. 
   Referring now to  FIGS. 8A and 8B , a resistive load may be connected across the medial feed portions  72 ′ of the dipole antenna elements  70   a ′,  70   b ′ in the outer columns of the respective dipole antenna element arrays  62 ,  64 . As discussed above, the passive elements  70   a ′,  70   b ′ in the outer columns allow the active elements in the center column to receive sufficient current, which is normally conducted through the dipole antenna elements on the substrate  60 . 
   The resistive load may include a discrete resistor  120 , as illustrated in  FIG. 8A , or a printed resistive element  122 , as illustrated in FIG.  8 B. Each discrete resistor  120  is soldered in place after the dipole antenna elements  70   a ,  70   b  have been formed. Alternatively, each discrete resistor  120  may be formed by depositing a resistive paste on the medial feed portions  72 , as would be readily appreciated by those skilled in the art. 
   The respective printed resistive elements  122  may be printed before, during or after formation of the dipole antenna elements  70   a ,  70   b , as would also be readily appreciated by those skilled in the art. The resistance of the load is typically selected to match the impedance of a feed line connected to an active dipole antenna element, which is in a range of about 50 to 100 ohms. 
   Other aspects of the present invention will now be discussed. One such aspect is still directed to a multiband phased array antenna  150 , as illustrated in FIG.  9 . The multiband phased array antenna  150  is also a radially distributed phased array antenna covering multiple frequency bands. 
   However, the multiband phased array antenna  150  comprises a substrate  160 , and a plurality of dipole element arrays  161 ,  162 ,  163 ,  164  and  165  extending outwardly from an imaginary center point  166  on the substrate  160 . The imaginary center point  166  is not necessarily the center of the substrate  160 , but may be slightly off center. 
   Each dipole element array  161 - 165  comprises a plurality of dipole antenna elements (generally referred to by reference numeral  170 ) arranged in end-to-end relation and having a dipole size different than a dipole size of dipole antenna elements of at least one other dipole element array. In other words, each dipole element array  161 - 165  is sized to cover a respective frequency band so that collectively, the multiband phased array antenna  150  covers a wide bandwidth. 
   As the dipole element arrays  161 - 165  decrease from a larger size to a smaller size, the frequency inversely changes, as readily understood by those skilled in the art. For example, the five dipole element arrays may cover the following five frequency bands: 0.1 to 1 GHz for dipole element array  161 , 1 to 2 GHz for dipole element array  162 , 2 to 4 GHz for dipole element array  163 , 4 to 8 GHz for dipole element array  164 , and 8 to 16 GHz for dipole element array  165 . 
   Only five dipole element arrays  161 - 165  within a single “pie” section are illustrated in FIG.  9 . Depending on the intended application, the five dipole element arrays  161 - 165  are repeated in other pie sections around the substrate  160 . The distribution of the dipole element arrays  161 - 165  may be symmetrical, although this is not required. The embodiment of five dipole element arrays  161 - 165  is for illustrative purposes only, and the actual number of dipole element arrays may vary, as readily appreciated by those skilled in the art. 
   Each dipole element array  161 - 165  includes an active dipole antenna element (which is the center element), and may include passive dipole antenna elements adjacent to the active element. The passive dipole antenna elements include a resistive load (not shown) connected across the medial feed portions. The resistive load may be a discrete resistor  120 , as illustrated in  FIG. 8A , or a printed resistive element  122 , as illustrated in FIG.  8 B. The passive elements allow the active element in the center to receive sufficient current, which is normally conducted through the dipole antenna elements  170  on the substrate  160 . 
   The actual size of each dipole element array  161 - 165  may vary, as readily appreciated by those skilled in the art. As illustrated in  FIG. 9 , each dipole element array  161 - 165  is a 1 by 3 array. Depending on the intended application, the size of the arrays  161 - 165  may be adjusted accordingly. For example, a 2 by 3 or a 3 by 5 array would be readily applicable. 
   As noted above, a ground plane for a multiband phased array antenna is preferably spaced from the different size dipole element arrays  161 - 165  less than about one-half a wavelength of a highest desired frequency within each respective array. Referring now to  FIG. 10 , a cross-sectional view of the multiband phased array antenna  150  as shown in  FIG. 9  is taken along radial axis R 1 . The ground plane  180  has a different spacing from the plurality of dipole element arrays  161 - 165  in an outward direction from the imaginary center point  166 . 
   In other words, the illustrated ground plane  180  is sloping so that the spacing between the ground plane and the dipole element arrays  161 - 165  increases. Alternately, the dipole element arrays  161 - 165  may be positioned so that the spacing between the ground plane  180  and the dipole element arrays  161 - 165  decreases. When the slope of the ground plane  180  increases, the lower frequency arrays are positioned on the substrate  160  further away from the imaginary center point  166 , whereas the higher frequency arrays are positioned closer to the imaginary center point. Furthermore, the position of each dipole element array  161 - 165  on the substrate  160  may also be radially adjusted for the different frequency bands to achieve a constant beamwidth across the total bandwidth. 
   The slope of the ground plane  180  does not necessarily have to be constant. For example, the slope of the ground plane  180  may be logarithmic or exponential. In this case, position of the dipole element arrays  161 - 165  would be adjusted accordingly to provide the preferred spacing between the ground plane  180  and the respective dipole antenna elements  170  based upon their size. A dielectric material  181  is between the ground plane  180  and the respective dipole antenna elements  170 . 
   Depending on the desired overall size of the multiband phased array antenna  150 , crowding of the dipole antenna elements  170  within each pie section on the substrate  160  could be a problem. One approach to alleviating this problem is to turn the outermost passive dipole antenna elements near the edge of the substrate 90 degrees, as illustrated in  FIGS. 11A  (top view) and  11 B (side view). 
   In this embodiment of the multiband phased array antenna  150 ′, the substrate has a first surface  160   a ′, and a second surface  160   b ′ adjacent thereto and defining an edge  169 ′ therebetween. In the illustrated embodiment, the second surface  160   b ′ is orthogonal to the first surface  160   a ′. The substrate  160   a ′,  160   b ′ may be a monolithic flexible substrate, and the second surface is formed by simply bending the substrate so that one of the legs of the edge elements  170   b ′ extends onto the second surface. 
   Dipole element arrays  163 ′,  164 ′ and  165 ′ extend outwardly from the imaginary center point  166 ′ only the first surface  160   a ′ of the substrate  160   a ′, and dipole element arrays  161 ′ and  162 ′ extend outwardly from the imaginary center point  166 ′ on both the first and second surfaces  160   a ′,  160   b ′ of the substrate. The dipole antenna elements on the first surface of the substrate  160   a ′ are indicated by reference  170   a ′, whereas the dipole antenna elements on the second surface of the substrate  160   b ′ (partially or fully thereon) are indicated by reference  170   b′.    
   The dipole antenna elements  170   b ′ on the second surface  160   b ′ of the substrate may also be referred to as “edge elements.” A plot of the computed VSWR versus frequency for the low frequency dipole element array  161 ′ having a dipole antenna element  170   b ′ on the second surface  160   b ′ of the substrate is represented by line  186  in FIG.  12 . 
   Another aspect of the present invention is directed to a multiband phased array antenna  250 , as illustrated in FIG.  13 . The multiband phased array antenna  250  is also a radially distributed phased array antenna covering multiple frequency bands. In particular, the multiband phased array antenna  250  comprises a substrate  260 , and a plurality of dipole element arrays  262  extending outwardly from an imaginary center point  266  on the substrate. The distribution of the dipole element arrays  262  may be symmetrical, although this is not required. 
   Each dipole element array  262  comprises a plurality of dipole antenna elements  270   a - 270   e  arranged in end-to-end relation and having different dipole sizes for dipole antenna elements in a direction extending outwardly from the imaginary center point  266 . In other words, the multiband phased array antenna  250  is “graded” in the sense that the size of the dipole antenna elements  270   a - 270   e  changes from the imaginary center point  266  toward the outer edge of the substrate  260 . 
   Each illustrated dipole element array  262  comprises five active dipole antenna elements  270   a - 270   e . The actual number of elements could vary depending on the intended application. The multiband phased array antenna  250  may cover the following frequency bands: dipole antenna element  270   a  covers 0.1 to 1 GHz, dipole antenna element  270   b  covers 1 to 2 GHz, dipole antenna element  270   c  covers 2 to 4 GHz, dipole antenna element  270   d  covers 4 to 8 GHz and dipole antenna element  270   e  covers 8 to 16 GHz. Of course, the active dipole antenna elements  270   a - 270   e  vary in size to cover different frequency bands, as readily appreciated by those skilled in the art. 
   As noted above, a ground plane for a multiband phased array antenna is preferably spaced from the different size dipole element arrays  262  less than about one-half a wavelength of a highest desired frequency within each respective array. Referring now to  FIG. 14 , a cross-sectional view of the multiband phased array antenna  250  as shown in  FIG. 13  is taken along radial axis R 1 . The ground plane  280  has a different spacing from the different dipole antenna elements  270   a - 270   e  in the plurality of dipole element arrays  262 . 
   The illustrated ground plane  280  is sloping so that the spacing between the ground plane and the dipole antenna elements  270   a - 270   e  increases as you move from the imaginary center point  266  toward the outer edge of the substrate  260 . Consequently, the lower frequency dipole antenna elements  270   d  and  270   e  are positioned on the substrate  260  further away from the imaginary center point  266 , whereas the higher frequency dipole antenna elements  270   a ,  270   b  and  270   c  are positioned closer to the imaginary center point. 
   The transceivers  56   1 - 56   n  may be selectively connected to a particular port within the radial distribution of dipole antenna elements  270   a - 270   e  to achieve constant beamwidth and pattern control. Although not illustrated in  FIG. 13 , passive elements may be connected to the innermost and outermost dipole antenna elements  270   a ,  270   e  to increase bandwidth. In addition, each dipole element array  262  is not limited to a 1×5 matrix of dipole antenna elements, and other size arrays are acceptable, as readily appreciated by those skilled in the art. 
   As noted above, the slope of the ground plane  280  does not necessarily have to be constant. For example, the slope of the ground plane  280  may be logarithmic or exponential. In this case, position of the dipole element arrays  262  would be adjusted accordingly to provide the preferred spacing between the ground plane  280  and the respective dipole antenna elements  270   a - 270   c  based upon their size. A dielectric material  281  is between the ground plane  280  and the respective dipole antenna elements  270   a - 270   e.    
   Yet another aspect of the present invention is directed to a multiband phased array antenna  350 , as illustrated in FIG.  16 . In particular, the multiband phased array antenna  350  comprises a substrate  360 , and a plurality of dipole element arrays  361 ,  362 ,  363 ,  364  and  365  extending in concentric polygonal rings about an imaginary center point  366  on the substrate. 
   The plurality of dipole element arrays  361 - 365  are concentric about the imaginary center point  366 . This is in contrast to the dipole element arrays in the multiband phased array antennas  50 ,  150  and  250  as discussed above, which are all radially distributed with respect to an imaginary center point. 
   Each dipole element array  361 - 365  comprises a plurality of dipole antenna elements  370   a - 370   e  arranged in an end-to-end relation and having a dipole size different than a dipole size of dipole antenna elements of at least one other dipole element array. The specific features of the dipole antenna elements as discussed above are also applicable to the multiband phased array antenna  350 , and will not be discussed in any greater detail. 
   In the illustrated embodiment, each concentric polygonal ring (i.e., a dipole element array) includes N individual dipole antenna elements, wherein N=8. The actual number N of individual dipole antenna elements can vary depending on the intended application. For example, the lower limit of N may be 3, and the upper end of N may be determined by the intended application. 
   The ground plane  380  for the multiband phased array antenna  350  is preferably spaced from the different size dipole element arrays  361 - 365  less than about one-half a wavelength of a highest desired frequency within each respective array. Referring now to  FIG. 17 , a cross-sectional view of the multiband phased array antenna  350  as shown in  FIG. 16  is taken along radial axis R 1 . The ground plane  380  has a different spacing from the different dipole antenna elements  370   a - 370   e  in the plurality of dipole element arrays  361 - 365 . 
   The illustrated ground plane  380  is sloping so that the spacing between the ground plane and the dipole antenna elements  370   a - 370   e  increases as you move from the imaginary center point  366  toward the outer edge of the substrate  360 . Consequently, the lower frequency dipole antenna elements  370   a ,  370   b  (i.e., arrays  361 ,  362 ) are positioned on the substrate  360  further away from the imaginary center point  366 , whereas the higher frequency dipole antenna elements  370   a ,  370   b ,  370   c  (i.e.,  363 ,  364 ,  365 ) are positioned closer to the imaginary center point. 
   The transceivers  56   1 - 56   n  may be selectively connected to a particular concentric ring to achieve constant beamwidth and pattern control. As noted above, the slope of the ground plane  380  does not necessarily have to be constant. For example, the slope of the ground plane  380  may be logarithmic, exponential, or stepped. In this case, position of the dipole element arrays would be adjusted accordingly to provide the preferred spacing between the ground plane  380  and the respective dipole antenna elements  370   a - 370   e  based upon their size. A dielectric material  381  is between the ground plane  380  and the respective dipole antenna elements  370   a - 370   e.    
   The concentric rings are illustrated as being circumscribed in a circle, but they may also be circumscribed in any other shape, such as an ellipse. The concentric rings may also be triangular or rectangular, as readily appreciated by those skilled in the art. In addition, the spacing of the concentric rings may be symmetrical, as shown in FIG.  15 . 
   Measured and computed VSWR versus frequency over a frequency band of 2 to 18 GHz for the multiband phased array antenna  350  is provided in FIG.  17 . Line  386  represents the measured VSWR, whereas line  388  represents the computed VSWR. The measured and computed VSWR versus frequency is relatively constant between 8 and 18 GHz. 
   In addition, other features relating to the multiband phased array antennas are disclosed in copending patent applications filed concurrently herewith and assigned to the assignee of the present invention and are entitled PHASED ARRAY ANTENNA WITH SELECTIVE CAPACITIVE COUPLING AND ASSOCIATED METHODS, Ser. No. 10/702,713; MULTIBAND POLYGONALLY DISTRIBUTED PHASED ARRAY ANTENNA AND ASSOCIATED METHODS, Ser. No. 10/703,132; MULTIBAND RADIALLY DISTRIBUTED GRADED PHASED ARRAY ANTENNA AND ASSOCIATED METHODS, Ser. No. 10/702,899; and MULTIBAND RADIALLY DISTRIBUTED PHASED ARRAY ANTENNA WITH A STEPPED GROUND PLANE AND ASSOCIATED METHODS, Ser. No. 10/702,853, the entire disclosures of which are incorporated herein in their entirety by reference. 
   Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claims.