Patent Publication Number: US-6989791-B2

Title: Antenna-integrated printed wiring board assembly for a phased array antenna system

Description:
FIELD OF THE INVENTION 
   The present invention relates to phased array antennas, and more particularly to a phased array antenna system incorporating at least one antenna module, and more preferably a plurality of antenna modules, and where each antenna module includes a metal column-like member that significantly improves cross polarization isolation between the RF radiating elements of each antenna module. 
   BACKGROUND OF THE INVENTION 
   The assignee of the present application, The Boeing Company, is a leading innovator in the design of high performance, low cost, compact phased array antenna modules. The Boeing antenna module shown in  FIGS. 1   a - 1   c  have been used in many military and commercial phased array antennas from X-band to Q-band. These modules are described in U.S. Pat. No. 5,886,671 to Riemer et al and U.S. Pat. No. 5,276,455 to Fitzsimmons et al, both being hereby incorporated by reference. 
   The in-line first generation module was used in a brick-style phased-array architecture at K-band and Q-band frequencies. This approach is shown in  FIG. 1   a . This approach requires some complexity for DC power, logic and RF distribution but it provides ample room for electronics. As Boeing phased array antenna module technology has matured, many efforts made in the development of module technology resulted in reduced parts count, reduced complexity and reduced cost of several key components of these antenna modules. Boeing has also enhanced the performance of the phased array antenna with multiple beams, wider instantaneous bandwidths and greater polarization flexibility. 
   The second generation module, shown in  FIG. 1   b , represented a significant improvement over the in-line module of  FIG. 1   a  in terms of performance, complexity and cost. It is sometimes referred to as the “can and spring” design. This design provides dual orthogonal polarization in an even more compact, lower-profile package than the in-line module of  FIG. 1   a . The can-and-spring module forms the basis for several dual simultaneous beam phased arrays used in tile-type antenna architectures from X-band to K-band. The can and spring module was later improved even further through the use of chemical etching, metal forming and injection molding technology. The third generation module developed by the assignee, shown in  FIG. 1   c , provides an even lower-cost production design adapted for use in a dual polarization receive phased array antenna. 
   Each of the phased-array antenna module architectures shown in  FIGS. 1   a - 1   c  require multiple module components and interconnects. In each module, a relatively large plurality of vertical interconnects such as buttons and springs are used to provide DC and RF connectivity between the distribution printed wiring board (PWB), ceramic chip carrier and antenna probes. 
   A further step directed to reduce the parts count and assembly complexity of the antenna module as described above is described in pending U.S. patent application Ser. No. 09/915,836, “Antenna Integrated Ceramic Chip Carrier For A Phased Array Antenna”, hereby incorporated by reference into the present specification This application involves forming an antenna integrated ceramic chip carrier (AICC) module which combines the antenna probe (or probes) of the phased array module with the ceramic chip carrier that contains the module electronics into a single integrated ceramic component. The AICC module eliminates vertical interconnects between the ceramic chip carrier and antenna probes and takes advantage of the fine line accuracy and repeatability of multi-layer, co-fired ceramic technology. This metallization accuracy, multi-layer registration produces a more repeatable, stable design over process variations. The use of mature ceramic technology also provides enhanced flexibility, layout and signal routing through the availability of stacked, blind and buried vias between internal layers, with no fundamental limit to the layer count in the ceramic stack-up of the module. The resulting AICC module has fewer independent components for assembly, improved dimensional precision and increased reliability. 
   In spite of the foregoing improvements in antenna module design, there is still a need to further combine more functions of a phased array antenna into a single component. This would further reduce the parts count, improve alignment and mechanical tolerances during manufacturing and assembly, improve electrical performance, and reduce assembly time and processes to ultimately reduce phased array antenna system costs. More specifically, it would be highly desirable to substantially reduce or eliminate dielectric “pucks” that need to be used in a completed antenna module, as well as to entirely eliminate the use of buttons, button holders, flex members, cans, sleeves, elastomers and springs. If all of these independent parts could be substantially reduced in number or eliminated, then the primary issue bearing on the cost of the antenna assembly would be the material and process cost of manufacturing the antenna assembly. 
   For each of the dual polarization antenna modules/systems described above, there are several characteristics used to gauge the effectiveness (i.e., electrical performance) of the design. These characteristics include return loss bandwidth, radiator-to-radiator isolation, insertion loss bandwidth, higher order mode suppression and cross-polarization levels. All of these characteristics affect the overall electrical performance of the antenna module/system. Therefore, it would be highly desirable if these characteristics could be favorably influenced through a new antenna module design which does not involve the use of numerous and/or costly additional components parts, and which further does not significantly complicate the construction of the various antenna module/system designs described above. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a phased array antenna system which incorporates an antenna integrated printed wiring board (AIPWB) assembly. The AIPWB assembly includes circuitry for DC/logic and RF power distribution as well as the antenna probes. The metal honeycomb waveguide plate used with previous designs of phased array antenna modules is eliminated in favor of a multi-layer printed wiring board which includes vias which form circular waveguides and a plurality of layers (stack-up) for providing a honeycomb waveguide structure and wide angle impedance matching network (WAIM). Thus, the antenna system of the present invention completely eliminates the need for dielectric pucks, which previous designs of phased array antenna modules have heretofore required. The entire phased array antenna system is thus formed from at least one multi-layer printed wiring board, or alternatively from two or more multi-layer printed wiring boards placed adjacent to one another. This construction significantly reduces the independent number of component parts required to produce a phased array antenna system. Each of the two printed wiring boards are produced using an inexpensive, photolithographic process. Forming the entire antenna system essentially into one or two, or possibly more, printed wiring boards significantly eases the assembly of the phased array antenna system, as well as significantly reducing its manufacturing cost. 
   In an alternative preferred embodiment of the present invention the antenna system incorporates a metal, column-like element adjacent each pair of antenna probes. The metal, column-like elements are formed in the AIPWB assembly during manufacture. In one preferred manufacturing implementation a plurality of small diameter bores are formed in the AIPWB, with each bore being adjacent, and more preferably in between, each pair of RF probes. Metal is then deposited in each of the bores to form a corresponding plurality of metal, column-like elements. The metal-column like elements effectively form metal “pins”, with each metal pin being associated with a particular pair of probes. antenna probes. 
   The metal, column-like elements significantly improve the overall electrical performance of the probes, and thus the antenna system, by favorably influencing the return loss bandwidth, probe-to-probe cross polarization isolation, insertion loss bandwidth, and the higher order mode suppression of the antenna system. This results in an improved operating bandwidth for a given antenna system. If increased bandwidth is not needed for a given application, these improvements then allow component tolerances to be relaxed, thus increasing the manufacturing yield for such an antenna system. The electrical variations in an array environment, over a range of scan angles, are also reduced by the improvement in operating bandwidth. Importantly, the inclusion of the metal, column-like elements does not significantly complicate the manufacturing process nor does it significantly increase the overall cost of the antenna system. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIGS. 1   a - 1   c  represent prior art module designs of the assignee of the present invention; 
       FIG. 2  is an exploded perspective view of the two major components forming a  64  element phased array antenna system in accordance with a preferred embodiment of the present invention; 
       FIG. 3  is a cross sectional side view through one antenna site taken in accordance with section line  3 — 3  in  FIG. 2 ; 
       FIG. 4  is a cross sectional side view taken in accordance with section line  4 — 4  through the upper printed wiring board shown in  FIG. 2  illustrating the vias used for forming a circular waveguide, honeycomb support structure, and the stack-up for the wide angle impedance matching network (WAIM); 
       FIG. 5  is a detailed, side cross sectional view of portion  5  of the probe-integrated printing wiring board of  FIG. 3  illustrating in greater detail the electrical interconnections formed within the layers of this printed wiring board assembly; 
       FIG. 6  is a plan view of a portion of the probe-integrated wiring board showing the vias that form the can for each pair of RF radiating elements; 
       FIG. 7  is a view of an alternative preferred embodiment of the present invention wherein the probe-integrated printed wiring board and the waveguide printed wiring board are formed as a single, integrated, multi-layer printed wiring board; 
       FIG. 8  is a view of an antenna system in accordance with an alternative preferred embodiment of the present invention, in which the probe-integrated wiring board of  FIG. 2  has been modified to include metal, column-like elements adjacent each pair of RF probes; 
       FIG. 9  is a cross-sectional side view of the probe-integrated wiring board of  FIG. 8  illustrating one of the metal, column-like elements disposed adjacent one of the RF probes; 
       FIG. 10  is a graph illustrating the improvement in return loss bandwidth provided by the metal, column-like elements of the antenna system of  FIG. 8 ; 
       FIG. 11  is a graph illustrating the improvement in probe-to-probe isolation provided by the antenna system of  FIG. 8 ; 
       FIG. 12  is a graph illustrating the improvement in insertion loss for the antenna system of  FIG. 8 ; 
       FIG. 13  is a graph illustrating the improvement in higher order mode suppression for the antenna system of  FIG. 8 ; 
       FIG. 14  is a graph illustrating the improvement in cross-polarization isolation bandwidth for the antenna system of  FIG. 8 ; and 
       FIG. 15  is a view of another preferred implementation of the metal, column-like elements, illustrating one such element disposed within an injection molded antenna module. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
   Referring to  FIG. 2 , there is illustrated a pre-assembled view of a  64  element phased array antenna system  10  in accordance with a preferred embodiment of the present invention. It will be appreciated immediately, however, that the present invention is not limited to a 64 element phased array antenna system, but that the principles and teachings set forth herein could be used to produce phased array antenna systems having a greater or lesser plurality of antenna elements. The phased array antenna system  10  incorporates a multi-layer probe-integrated printed wiring board  12  and a multi-layer waveguide printed wiring board  14  which are adapted to be disposed adjacent one another in abutting relationship when fully assembled. Conventional threaded or non-threaded fasteners (not shown) can be used to secure the two wiring boards  12  and  14  in close, secure abutting contact. The probe-integrated printed wiring board  12  includes a plurality of antenna elements or modules  16  arranged in an 8×8 grid. Each antenna element  16  includes a pair of radio frequency (RF) probes  18 , but it will be appreciated again that merely a single probe could be incorporated, if desired, and that greater than two probes could be included just as well to meet the needs of a specific application. 
   The multi-layer waveguide printed wiring board  14  includes a plurality of integrally formed circular waveguides  20  formed to overlay each of the antenna elements  16 . It will be appreciated that these circular waveguides  20  are integrally formed areas or portions of the waveguide printed wiring board  14  and not independent dielectric pucks. It will also be appreciated that as the operating frequency of the antenna system  10  increases, the thickness of the wiring board  14  will decrease. Conversely, as the operating frequency decreases, the thickness of the board  14  will increase. 
   Referring to  FIG. 3 , the probe-integrated printed wiring board  12  can be seen to include a plurality of 15 independent layers  12   a - 12   o  sandwiched together. Again, it will be appreciated that a greater or lesser plurality of layers could be provided to meet the needs of a specific application. RF vias  22   a  and  22   b  are used to form the probes  18  while vias  24  are arranged circumferentially around the vias  22   a  and  22   b  to effectively form a “cage” or “can”  26  for the antenna element  16 . This is illustrated in greater detail in FIG.  6 . It will be appreciated that the illustration of  20  vias to form the can  26  in  FIG. 6  is presented for illustrative purposes only, and that a greater or lesser plurality of vias  24  could be employed. Also, it will be appreciated that the spacing of the vias  24  does affect how closely the cage  26  approximates a physical can, in an electromagnetic sense. 
   Referring now to  FIG. 4 , the waveguide printed wiring board can be seen to also include a plurality of independent layers  14   a - 14   q  which form a wide angle impedance matching network (WAIM). Vias  28  extending through layers  14   c - 14   q , form the waveguide portion of the wiring board  14 . Again, it will be appreciated that vias  28  are arranged in circular orientations such as shown in FIG.  6 . Layers  14   a  and  14   b  form impedance matching layers. 
   Each of the printed wiring boards  12  and  14  are formed through an inexpensive, photolithographic process such that each wiring board  12  and  14  is formed as a multi-layer part. The probe-integrated printed wiring board  12  includes the antenna probes  18  and DC/logic and RF distribution circuitry. On probe-integrated printed wiring board  12 , the discrete electronic components (i.e., MMICs, ASICs, capacitors, resistors, etc) can be placed and enclosed by a suitable lid or cover (not shown) on a bottom surface of layer  12   o . Accordingly, the multiple electrical and mechanical functions of radiation, RF distribution, DC power and logic are all taken care of by the probe-integrated printed wiring board  12 . 
   Referring now to  FIG. 5 , the probe-integrated printed wiring board  12  is shown in further detail. Layer  12   a  comprises a ground pad  30  on an outer surface thereof. Ground pad  30  is electrically coupled to a ground pad  32  on an outer surface of layer  120  by a conductive via  34  extending through each of the layers  12   a - 12   o . Via  34  is also electrically coupled to an RF ground circuit trace  36 . Layers  12   a - 12   i  are separated by ground layers  38 . The ground layers help to reduce the inductance of the vias formed in the board  12 . 
   With further reference to  FIG. 5 , via  39  and pads  39   a  and  39   b  provide electrical coupling to layer  12   o , which forms a stripline for distributing RF energy between the RF probes  18  and the vias  39 . It will be appreciated that for a 64 element phased array antenna, there will be 64 of the vias  39 , with each via  39  associated with one of the 64 antenna elements. 
   Referring further to  FIG. 5 , pad  40  on layer  12   a  and pad  42  on layer  12   o  are electrically coupled by a conductive via  44 . Pad  46  on layer  12   a  and pad  48  on layer  12   o  are electrically coupled by conductive via  50 . Pad  52  on layer  12   a  and pad  54  on layer  12   o  are electrically coupled by conductive via  56 , while pad  58  on layer  12   a  and pad  60  on layer  120  are electrically coupled by conductive via  62 . Via  44  extends completely through all of the layers  12   a - 12   o  and is also electrically coupled to a clock circuit trace  64 . Via  50  extends through all of the layers  12   a - 12   o  and is electrically coupled to a data circuit trace  66 . Via  56  extends through all of layers  12   a - 12   o  and is electrically coupled to a DC source (−5V) circuit trace  68 . Via  62  likewise extends through all of layers  12   a - 12   o  and is electrically coupled to another DC power (+5V) circuit trace  70 . 
   One via  24  is shown which helps to form the can  26  (FIG.  6 ). Via  24  is essentially a conductive column of material that extends through each of layers  12   a - 12   o . Finally, one of the RF vias  18  is illustrated. Via  18  extends through each of layers  12   a - 12   o  and includes a perpendicularly extending leg  74  formed on an outer surface of layer  12   a . Leg  74  defines a surface plane, and the vias  28  ( FIG. 4 ) of the printed wiring board extend beyond the surface plane to perform a waveguide function. 
   Again, however, it will be appreciated that the drawing of  FIG. 5  represents only a very small cross sectional portion of the probe-integrated printed wiring board  12 . In practice, a large plurality of RF probe vias  18 , and a large plurality of vias  24  for forming the can  26 , will be implemented. For the phased array antenna system  10  shown in  FIG. 2 , 128 RF probe vias  18  are formed in the probe-integrated printed wiring board  12 , together with a much larger plurality of vias  24 . Also, it will be appreciated that the various electronic components used with the antenna system  10 , although not shown, will be secured adjacent layer  12 P in FIG.  5 . 
   It will also be appreciated that the probe-integrated printed wiring board  12  and the waveguide printed wiring board  14  could just as easily be formed as one integrally formed, multi-layer printed wiring board to form an antenna system  10  in accordance with an alternative preferred embodiment of the present invention. Such an implementation is illustrated in the cross sectional drawing of  FIG. 7 , wherein reference numeral  78  denotes the single multi-layer printed wiring board which includes a probe-integrated printed wiring board portion  80  and a waveguide printed wiring board portion  82 . RF vias  84  extend through both boards  80 , while a plurality of vias  86  forming the can extend through both boards  80  and  82 . 
   Referring now to  FIGS. 8 and 9 , an antenna system  100  in accordance with an alternative preferred embodiment of the present invention is shown. With specific reference to  FIG. 8 , antenna system  100  includes a multi-layer, probe-integrated printed wiring board  102  and a multi-layer, waveguide integrated printed wiring board  104 . The waveguide integrated printed wiring board  104  is identical in construction to the waveguide integrated printed wiring board  14 . The probe-integrated printed wiring board  102  is identical to the probe-integrated printed wiring board  12  with the exception that each antenna module  106  thereof includes a metal, column-like element  108  formed adjacent to its associated pair of RF probes  110 . Preferably, each metal, column-like element  108  is disposed centrally in between its associated pair of probes  110 . This placement could be modified as needed to meet the specific design requirements of a given antenna. However, placing each metal, column-like element  108  symmetrically relative to its associated probes will help to reduce cross-talk and suppress higher order modes. The metal column-like elements  108  significantly further improve the overall performance of the antenna system  100  over dual probe antenna systems that do not incorporate such elements. Specific areas of improvement in antenna performance will be discussed further in the following paragraphs. 
   Referring to  FIG. 9 , one of the metal, column-like elements  108  is shown in greater detail. The other various components of the printed wiring board  102  in common with printed wiring board  12  are designated by the same reference numerals used in connection with the description of printed wiring board  12 , but are designated with a prime symbol. The metal, column-like element  108  is formed by first drilling a hole (i.e., bore)  112  through the various layers of the probe-integrated printed wiring board  102  and filling the hole  112  with a metal as part of a plating process so that an electrical connection is formed with the ground pad  32 ′. The finished metal, column-like element  108  essentially forms a “pin” having a generally circular shape when viewed in cross section. Its diameter may vary considerably depending on the specific application and the overall construction of the antenna system  100 , but in one preferred form is preferably between about 0.020 inch (0.508 mm) and about 0.1 inch (2.54 mm), and more preferably between about 0.040 inch (1.016 mm) and about 0.080 inch (2.032 mm). It will be appreciated that the metal, column-like element  108  could readily comprise other cross-sectional shapes as well. The cross sectional shape will obviously be dictated by the shape of the hole  112  that is formed in the probe-integrated printed wiring board  102 . 
   It will be appreciated that for a dual polarized radiator, there are several characteristic used to gauge the effectiveness of the design. These characteristics include return loss bandwidth, probe-to-probe isolation, insertion loss bandwidth, higher order mode suppression and cross polarization levels. Referring to  FIG. 10 , the improvement in return loss bandwidth can be seen by the comparison between a probe design used in a dual polarization receive (DPR) antenna that does not incorporate the metal column-like elements  108 , whose return loss bandwidth is illustrated by curve  114 , and probes that do, whose curves are designated by reference numerals  116  and  118 . Curve  116  represents a metal, column-like element  108  having a diameter of 0.040 inch and curve  118  indicates the performance with a 0.080 inch diameter element  108 . Curve  114  illustrates a 15 dB return loss level at 12.8 GHz, which is a typical return loss value. As shown by curve  116 , the metal, column-like elements  108  having a diameter of 0.040 inch increase the return loss bandwidth from about 11.3 GHz to about 13.2 GHz, which is approximately 3 dB less than curve  114  at 12.8 GHz. As shown by curve  116 , metal, column-like elements  108  having a diameter of 0.080 inch increase the return loss bandwidth from about 11.3 GHz to about 13.8 GHz, which is approximately 12 dB less than curve  114  at 12.8 GHz for an improvement of about 20.80% when elements  108  are incorporated. 
   A radiator&#39;s probe-to-probe isolation is another important characteristic that determines the interaction between inputs applied to each of the RF probes of a dual polarization radiator.  FIG. 11  compares the probe-to-probe isolation slope over the operating bandwidth of the antenna. The baseline radiator  120  has 15 dB isolation at 11 GHz with a +2 dB/GHz slope. Curve  122  indicates the performance gain when a 0.040 inch element  108  is incorporated. Curve  124  indicates the isolation provided by a 0.080 inch diameter element  108 , which provides a −0.25 dB/GHz slope. This a given pair of RF probes  110  to continue to be isolated over a larger operating bandwidth. 
     FIG. 12  illustrates the improvement in the co-polarization insertion loss bandwidth provided by the metal, column-like elements  108 . The co-polarization insertion loss bandwidth represents the loss in energy at the inputs to the RF probes caused by ohmic and dielectric losses, cross-polarization components and to higher order mode conversion. Choosing a 0.5 dB insertion loss for comparison, represented by curve  126 , shows that a baseline radiator can operate from 11.1 GHz to 13.1 GHz for a variation of 16.5%. The increase in bandwidth when including the 0.0.040 inch metal, column-like element  108  is represented by curve  128 . The increase in bandwidth when incorporating a 0.080 inch element  108  is represented by curve  130 . Curve  130  increases the 0.5 dB insertion loss bandwidth from 11.0 GHz to 14.2 GHz or by 25.4% This increased bandwidth, even if not needed by an application, increases manufacturing yield, reduces component tolerances and reduces electrical variations in an array environment over a range of scan angles. 
     FIG. 13  illustrates the increase in higher order mode suppression provided by the antenna system  100 . Suppressing the generation and propagation of the next higher order allows the operating bandwidth of a radiator to be increased. For a circular waveguide radiator, the TM 01  mode is the next higher order mode which, if generated, will increase the co-polarization insertion loss of the element.  FIG. 13  illustrates a curve  132  representing the next higher order mode suppression of a baseline probe having 32 dB suppression at 10.8 GHz with a +6 dB/GHz slope. Curve  134  illustrates the improvement in suppression provided by the use of metal column-like elements  108  having a diameter of 0.040 inch. Curve  136  illustrates the improvement in suppression provided by an element  108  having a diameter of 0.080 inch. The 0.080 inch diameter element  108  also provides 32 dB suppression at 10.8 GHz but reduces the slope to −1.25 dB/GHz. The shows that the element  108  further serves to suppress the TM 01  mode throughout the operating bandwidth of the radiator. This translates to more predictable element performance and reduced mode conversion losses. 
     FIG. 14  shows how the use of the metal, column-like elements  108  serve to even further improve the cross-polarization isolation. Curve  138  represents a typical prior art dual polarization radiator having a −27 dB cross-polarization level over a 14.1 bandwidth. Curves  140  and  142  illustrate the gain in bandwidth when 0.040 inch and 0.080 diameter metal, column-like elements  108 , respectively, are employed in the antenna system  100 . Curve  142  shows a −27 dB cross-polarization level over a frequency range of 11.4 G Hz to 15 GHz, or a 27.3% bandwidth. Thus, the baseline radiator “nulls” out the cross-polarization near the middle of its operating bandwidth while the antenna system  100  maintains a near-flat −27 dB response. The near-flat response serves to increase the useful operating bandwidth of the element. 
   It will be appreciated that while the use of the metal-column like elements  108  have been described and illustrated in connection with probe-integrated printed wiring board  102 , that the elements  108  could be implemented into virtually any design of dual polarization radiator with only minor manufacturing modifications. For example, referring to  FIG. 15 , an antenna module  200  in accordance with another alternative preferred embodiment  200  of the present invention is shown. Antenna module  200  is injection molded and includes a metal pin  202  molded in between a pair of probes  204  within a plastic body portion  206 . The metal pin  202  has a thickness of preferably between about 0.020 inch and 0.10 inch, and more preferably between about 0.040 inch and 0.080 inch. Thus, it will be appreciated that a metal pin  202  or other form of metal, column-like element could readily be implemented in the antennas disclosed in U.S. Pat. Nos. 5,276,455 and 5,886,671 with relatively minor manufacturing modifications. 
   The preferred embodiments disclosed herein thus provide a means for forming a phased array antenna from a significantly fewer number of component parts, as well as improving the electrical performance of a phased array antenna system. The metal, column-like elements  108  serve to significantly cancel out any higher order modes which were previously generated and suppress the cross-talk over nearly twice the operating bandwidth of an antenna that does not incorporate the elements  108 . 
   Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and following claims.