Patent Publication Number: US-9843105-B2

Title: Integrated stripline feed network for linear antenna array

Description:
BACKGROUND 
     In known systems, such as ground reference antennas used in Local Area Augmentation Systems (LAAS) and Ground Based Augmentation Systems (GBAS), generally the feed network board is kept outside of the antenna in its own independent box. The feed network then connects to each antenna element through RF cables of a specific length to maintain the same phase delay to each antenna element. 
     Some current implementations of LAAS/GBAS antenna arrays include several parasitic elements. This increases the cost and complexity of such designs. Feed networks for such antenna arrays are difficult to produce and most feed networks require complex driving boards and numerous phase stable cables to maintain acceptable phase stability. Some current feed networks use microstrip lines and striplines, but issues common to both approaches persist. These issues include the need for enough space in the feed networks to isolate strong and weak signals; coupling the feed network to actual feed lines; and the need for complex assembly processes. 
     SUMMARY 
     An embodiment of an integrated stripline feed network for a linear antenna array comprises a power distribution network coupled to the linear antenna array; a feed signal input/output component coupled to the power distribution network; wherein the input/output component receives a feed signal and splits the feed signal for distributing to a plurality of antenna elements of the linear antenna array through the power distribution network. The integrated stripline feed network is configured to be integrated into a support body of the linear antenna array, wherein, the support body structurally supports the linear antenna array. 
    
    
     
       DRAWINGS 
       Understanding that the drawings depict only exemplary embodiments and do not limit the scope of the invention, the exemplary embodiments will be described with additional specificity and detail through the use of the accompanying drawings, in which: 
         FIG. 1A  is a high-level functional block diagram of a feed network and an antenna array according to one embodiment; 
         FIG. 1B  is a schematic diagram of a feed network according to one embodiment; 
         FIG. 2A  is a diagram illustrating a 3-bay model with circular radiating elements according to one embodiment; 
         FIG. 2B  is a diagram illustrating a perspective view of the 3-bay model with circular radiating elements with an integrated stripline according to one embodiment; 
         FIG. 3  is an exemplary flow chart illustrating an exemplary method of feeding a signal through an integrated stripline feed network to a linear antenna array. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the exemplary embodiments. 
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments. However, it is to be understood that other embodiments may be utilized and that logical, mechanical, and electrical changes may be made. Furthermore, the method presented in the drawing figures and the specification is not to be construed as limiting the order in which the individual steps may be performed. The following detailed description is, therefore, not to be taken in a limiting sense. 
     The embodiments described herein relate to apparatus and methodology for feeding a linear antenna array with an integrated stripline feed network. Integrated, in this context, means configured to integrate inside the antenna structure. The integrated stripline feed network provides a stable feed phase while integrated into the antenna structure through electrical and mechanical connections. Integrating the stripline feed network allows the feed network to couple to the linear antenna array without the need for matched length coaxial cables. This significantly decreases the size requirements of a feed network implementation, allowing the feed network to be integrated into the linear antenna array itself. In some embodiments, electrical connections can be made with shorter lengths of coaxial cable from the feed network to the antenna element. The claimed subject matter is described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. 
       FIG. 1A  illustrates a high-level functional block diagram of a linear antenna array and integrated stripline feed network system  100  according to one embodiment. The system  100  includes an integrated stripline feed network  110  that feeds an antenna array  200 . The feed network  110  includes a feed input/output component  150  that receives the feed signal and initially splits the signal through power distribution units, such as a standard 2-way power divider like the Wilkinson Power Divider, into three output channels. One of the three channels in this example is directly connected to output channel  155 - 6  of the feed network  110 , which provides the most powerful feed signal from the feed input/output component  150 . This output channel directly feeds the center antenna element  135  of antenna array  200  in this example. The remaining two output channels feed the left and right side of the antenna array through a power distribution network  160 . 
       FIG. 1B  illustrates the circuitry of one embodiment of a feed network  110 . The feed network includes a feed input/output component  150 , and a power distribution network  160 . 
     With standard directional couplers, the coupled port has a 90 degree phase difference when compared to the through port. A standard directional coupler can be implemented in stripline using coupled quarter wave striplines. The input signal does not undergo a phase change at the through port directly connected to the input port. The coupled port provides a signal that has a 90 degree advanced phase from the through port. The unused port is an isolated port. Standard directional couplers are used for power distribution that is unbalanced (e.g. less than −10 dB for the weaker channel). 
     Phase delay units are used in some channels to counteract a phase advance caused by a short feed line compared to the other channels. Phase delay units should be able to be used repeatedly with low insertion loss and a low VSWR. 
     In this embodiment, the feed input/output component  150  includes two 2-way power dividers  101  and  102  to create three output channels. With the 2-way power dividers  101  and  102 , the output of both ports of the respective power divider typically have approximately the same phase. In a Wilkinson power divider, the input is coupled to two parallel uncoupled quarter wave transmission lines. The output of each quarter wave line is terminated with a load equal to two times the system impedance. The input and output impedances are equal. The line impedance of the system is equal to the system impedance times the square root of two (√2Z 0 ). Power dividers are used for power distribution that is balanced or only slightly unbalanced (e.g. 0 dB to −10 dB for the weaker channel). 
     Power divider  101  splits an input signal into two output channels. One output from power divider  101  is coupled to the second power divider  102  and the other output is coupled directly to the center antenna element  135 , such that the signal to antenna element  135  has the strongest energy distribution. The output channel coupled to the center antenna element  135  has a line length “L” that is pre-selected so that a feed phase that is consistent with the other feed channels is maintained. Power divider  102  further divides the output received from the power divider  101  into two more signal channels, one for a left side power distribution network, defined by the network providing a signal for the antenna elements to the left of the center antenna element  135 , and one for a right side power distribution network, defined by the network providing a signal to the antenna elements to the right of the center antenna element  135 . The output channel for the left side power distribution network is coupled to a power divider  103 . The two outputs from power divider  103  are coupled to a directional coupler  111  and phase delay unit  121 . Phase delay units, such as phase delay unit  121 , are used in some channels to counteract a phase advance caused by a short feed line compared to the other channels. Phase delay units should be able to be used repeatedly with low insertion loss and a low VSWR. 
     Directional coupler  111  can be implemented with a conventional directional coupler. Conventional directional couplers include a coupled port and a through port. With directional couplers, the coupled port has a 90 degree phase difference when compared to the through port. A standard directional coupler can be implemented in stripline using coupled quarter wave striplines. The input signal does not undergo a phase change at the through port directly connected to the input port. The coupled port provides a signal that has a 90 degree advanced phase from the through port. The unused port is an isolated port. Standard directional couplers are typically used for power distribution that is unbalanced (e.g. less than −10 dB for the weaker channel). 
     The through port of directional coupler  111  is connected to power divider  107  and the coupled output is connected to phase delay unit  123 . The outputs of power divider  107  feed antenna elements  130  and  131 . The signal from the coupled port of directional coupler  111  is connected to phase delay unit  123  which adjusts the phase so that it has a phase difference of +90 degrees relative to the signal at antenna elements  130  and  131 . In this embodiment, the phase delay unit  123  adjusts the phase for variations in line length of the signal path to antenna elements  130  and  131 , and antenna element  132 . To adjust for the +90 degree phase advance of antenna element  132 , antenna element  132  is spatially rotated counterclockwise, in relation to the direction of signal propagation, by 90 degrees. Phase delay unit  121  is used to adjust the phase of the signal going to antenna elements  133  and  134  so that they are in phase with the feed signal at antenna elements  130 ,  131 , and  132 . Then the signal is split by power divider  105 , which then feeds the signal to antenna elements  133  and  134 . The length of lines L 1  and L 2  from the outputs of power divider  107  are approximately equal in this example to aid in maintaining the signals to antenna elements  133  and  134  in phase, i.e. L 1 =L 2 . The length of lines L 3  and L 4  from the outputs of power divider  105  are also approximately equal to each other in order to aid in maintaining the signals output from power divider  105  in phase with each other, i.e. L 3 =L 4 . 
     The circuit described above is mirrored for the right side power distribution network. The output channel of power divider  102  for the right side power distribution network is coupled to power divider  104 . One of the two outputs from power divider  104  is coupled to a directional coupler  112  and the other output is coupled to phase delay unit  122 . The through port of directional coupler  112  is connected to power divider  108  and the output of the coupled port is connected to phase delay unit  124 . The outputs of power divider  108  feed antenna elements  139  and  140 , respectively. The signal from the coupled port of directional coupler  112  is connected to phase delay unit  124  which adjusts the phase so that it has a phase difference of +90 degrees relative to the signal at antenna elements  139  and  140 . To adjust for this phase advance of 90 degrees antenna element  138  is spatially rotated counterclockwise, in relation to the direction of signal propagation, by 90 degrees. 
     Phase delay unit  122  is used to adjust the phase of the signal going to antenna elements  136  and  137  so that they are in phase with the feed signal at antenna elements  138 ,  139 , and  140 . Then the signal output by phase delay unit  122  is split by power divider  106 , which then feeds the signal to antenna elements  136  and  137 . The length of lines L 1  and L 2  from the outputs of power divider  108  are equal in this embodiment to aid in maintaining the signals from power divider  108  in phase, i.e. L 1 =L 2 . The length of lines L 3  and L 4  from the outputs of power divider  106  are also equal so that the signals from the output power divider  106  are in phase with each other, i.e. L 3 =L 4 . A person having ordinary skill in the art will appreciate that the signals are considered in phase if the difference between the relative phases of the signals is within a predetermined tolerance level depending on the application. 
     This feed network can be implemented in approximately 2-3 layers of stripline in a multilayered printed circuit board (PCB). The strong and weak signals can be isolated from each other by separating the output channels to the antenna elements in different layers. In one embodiment, the output channel associated with the center antenna element is placed on one layer, while antenna elements  133 ,  134 ,  136 ,  137  with a lower power signal are placed on a different layer of the multilayered PCB. Antenna elements  130 ,  131 ,  132 ,  138 ,  139 , and  140  are placed on another layer of the multilayer PCB. 
     This multilayered stripline feed network can be mechanically supported such that each antenna element can be more easily soldered or connected and assembled within the support body  205  of the linear antenna array. In some embodiments, multilayered stripline feed network is mechanically supported by being soldered to the support body itself. 
       FIG. 2A  illustrates one exemplary embodiment of an antenna array  200  using a 3-bay model. Each of a plurality of circular radiating elements  220  is fed through bays  210 . The feed network is integrated into the support body  205 , from where the feed signal is fed to bays  210 . This allows for a compact, novel, low cost feed system for a linear antenna array. 
       FIG. 2B  illustrates a perspective view of one embodiment of an exemplary antenna array with integrated stripline feed lines  230 . The stripline feed lines  230  go through the center of support body  205 . The feed lines  230  couple to an integrated feed network implemented on a multilayered stripline PCB  235  at each bay  210 , upon which radiating elements  220  are mounted. The PCBs  235  are orthogonal relative to the plane of the stripline feed lines  230 . A person having ordinary skill in the art will appreciate that the feed lines can connect to the PCBs at each bay through a variety of means for electrically coupling such feed signals. One such example is through the use of coaxial cables. The PCBs  235  can be mechanically supported within the antenna structure through a variety of means. In one embodiment, the PCBs  235  can be supported by soldering to the antenna structure itself. 
     In some embodiments, the antenna elements  220  are mounted directly on the multilayered PCBs  235 , perpendicular to the plane of the PCB. This can be accomplished by mounting the antenna elements, which have slots in them, onto tabs on the PCB  235 . Then, the connection can be soldered to create both an electrical and mechanical connection. Other means for mounting the antenna elements to the PCBs  235  can be implemented, such as having a slot in the PCB  235 , as opposed to the antenna element  220 . In yet another embodiment, the antenna elements  220  are mounted and spaced equally on four sides of the support body, all along one axis as provided by the support body. 
       FIG. 3  is an exemplary flow chart illustrating one embodiment of a method of operating a linear antenna array with an integrated stripline feed network  300 . At block  301 , a first signal is received by a feed input/output component and split into a second and third signal. At block  303 , the second signal is sent directly to a central antenna element, such as the central antenna element discussed above. Then, further splitting of the third signal depends on the number of antenna elements needing a feed signal. If the number of antenna elements is odd, then the third signal is split into a fourth and fifth signal, which are sent to a power distribution network. At block  305 , the fourth and fifth signals can be further split into more signals, depending on the how many antenna elements are to be fed a signal. The signals are then output to each of a plurality of output channels. The phase delays introduced to the signals by the varying signal paths are adjusted within the feed network so that the phase delay output at each output channel is approximately matched. At block  307 , the feed signals are sent to the antenna elements. At block  309 , antenna elements that receive a signal with a phase delay or advancement introduced by the various feed network components are spatially rotated to adjust for the phase delay or advancement. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.