Abstract:
The present invention is an apparatus for shifting the phase of an radiofrequency signal. The device has an input line and an output line. An input switch is connected to the input line. The input switch is has several input throws. An output switch is connected to the output line. The output switch has several output throws which correspond to the input throws. The apparatus also has several phase shift lines. Each phase shift line has a true path length that is different from the true path lengths of the other phase shift lines.

Description:
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     The United States Government has ownership rights in this invention. Licensing inquiries may be directed to Office of Research and Technical Applications, Space and Naval Warfare Systems Center, Pacific, Code 72120, San Diego, Calif., 92152; telephone (619) 553-5118; email: ssc_pac_t2@navy.mil. Reference Navy Case No. 101777. 
    
    
     BACKGROUND 
     1. Field 
     This invention relates to the field of antenna arrays, and more specifically, to a high-gain antenna array in which the beam direction can be steered without the use of classical phase shifters and beamformer circuitry. 
     2. Background 
     An antenna is a device that indiscriminately broadcasts a signal in every direction and in a pattern referred as a “radiating signal pattern.” The direction of a signal sent by a single antenna cannot be controlled. Antenna arrays are groupings of antennas that control the direction of a signal by enhancing the signal in a desired direction while diminishing the signal in non-desired directions. Signals transmitted along such a directional path are referred to as “beams”. 
     Beam control is important for coordinating communications. Antennas may be used to communicate between one or more moving stations, such as ships, aircraft, satellites and ground stations. Communication with moving objects requires that the beam path be continuously and precisely adjusted, or the object will lose communication. Precise beam control may also be necessary to prevent a signal from being intercepted. 
     Antennas in arrays are passive devices through which a beam can be mechanically or electronically steered. Antennas are mechanically steered by strategic positioning or by geometric alterations. Antennas are electronically steered by altering the transmission signal fed into them. Electronic steering varies the phase and amplitude of the electronic signal fed into each antenna of the array. This type of array is referred to as a “phased antenna array”. 
     A major component of the phased antenna array is the feed assembly component. The feed assembly receives the incoming radiofrequency (RF) signal for the entire array and splits it between multiple signal-altering components. Signal-altering components include phase shifters, amplifiers and attenuators. Amplifiers increase signal strength and attenuators reduce it. The beam direction for each composite phased antenna array is a result of the output signals emitted by each antenna in the array. 
     Phase shifter components direct the signal down multiple circuit paths of different lengths within the phased antenna array. A switching controller determines which path the signal goes down by opening and closing multiple switches for each path. The different paths delay the signal by varying amounts, altering the phase. The greater the number of paths at varying lengths, the more precise the switching capability. 
     One problem with conventional phase shifters known in the art is a limitation on the number and length of lines it is practical to put into a phase shifter. Increasing the number of lines requires complex circuitry and processing capability, adding to cost and energy requirements. Increasing the number of switches between the input and output lines increases the likelihood of component failure. 
     Another problem known in the art is referred to as “beam squint.” Beam squint refers to the problem of maintaining a consistent beam position over a wide range of frequencies without introducing error inherent in digital phase shifting. 
     A significant cause of beam squint is that classical phase shifters must approximate the path for steering a beam in modulo 2π phase mode. The modulo 2π approximation range is a limitation on accurate approximation. This error is introduced at high and low frequencies. With conventional phase shifters, errors may be produced during phase shift of up to 360/2 n  degrees, where n is the number of path lengths within the phase shifter. A conventional phase shifter with four paths may introduce a phase angle error of up to 22.5 degrees. Phase errors can increase the amount of power wasted, and angle errors can lead to unacceptably poor broadband performance. 
     There is an unmet need for a phased antenna array that provides more precise beam steering capability, that has failure-resistant components, and that is less prone to beam squint error. 
     SUMMARY OF THE INVENTION 
     The present invention is an apparatus for shifting the phase of a radiofrequency signal. The device has an input line and an output line. An input switch is connected to the input line. The input switch is has several input throws. An output switch is connected to the output line. The output switch has several output throws which correspond to the input throws. The apparatus also has several phase shift lines. Each phase shift line has a true path length that is different from the true path lengths of the other phase shift lines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary true path beam steering system. 
         FIG. 2  illustrates an exemplary embodiment of a phase shifter used in a true path beam steering system. 
         FIG. 3  illustrates a schematic of amplifiers operatively coupled with radiating elements. 
         FIG. 4  illustrates antenna pattern comparisons between an ideal pattern, those generated via more classical phase shifting approaches, and that according to the detailed exemplary invention description herein. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary embodiment of a true path beam steering system  100 . The elements of the true path beam steering system  100  shown in  FIG. 1  includes a radiofrequency (RF) signal input  10 , a binary splitter  15 , a pair of three-way splitters  17  and  19 , six phase shifters  20 ,  30 ,  40 ,  50 ,  60  and  70 , and six amplifiers  81 ,  82 ,  83 ,  84 ,  85 , and  86 . 
     In the exemplary true path beam steering system  100  shown in  FIG. 1 , an RF signal input  10  passes through a binary splitter  15 . The output of binary splitter  15  is in turn passed through a pair of three-way splitters  17  and  19 . The resulting six-way split signal inputs are then presented to six phase shifters  20 ,  30 ,  40 ,  50 ,  60  and  70 . Finally each of the six phase shifters  20 ,  30 ,  40 ,  50 ,  60  and  70  feeds one of a series of amplifiers  81 ,  82 ,  83 ,  84 ,  85 , and  86 . 
     Splitters  15 ,  17  and  19  are devices capable of splitting a single signal into two or more signals. The exemplary embodiment shown in  FIG. 1  utilizes a binary splitter  15  and a pair of three-way splitters  17  and  19  to create a six-way split of RF signal input  10 . In various alternative embodiments, different numbers and configurations of splitters may be used to split RF signal input  10  into a different number of split signals, ranging from two to about five hundred. 
     Phase shifters  20 ,  30 ,  40 ,  50 ,  60  and  70  each comprise a plurality of physical lines through which an RF signal input  10  passes, resulting in a time delay of that signal as compared to a reference signal that does not pass through the physical lines. While the exemplary embodiment of the true path beam steering system  100  shown utilizes six phase shifters  20 ,  30 ,  40 ,  50 ,  60  and  70 , in alternative embodiments a different number of phase shifters may be utilized to accommodate a different number of split signals ranging from two to about five hundred. Additionally, in alternative embodiments up to five phase shifters may be used in serial on the same split signal to create a more complex time delay. 
     The exemplary true path beam steering system  100  shown in  FIG. 1  provides highly accurate beam steering. Unlike classical phase shifting, which is typically limited to 4 or 5 bits (22.5 or 11.25 degrees) of accuracy, the exemplary embodiment provides highly accurate beam steering capability in which “beam squint” error is controlled. 
     The accuracy of true path beam steering system  100  is limited only by transmission line manufacturing tolerances and switch manufacturing tolerances. Phase accuracy on the order of 1 degree (equivalent of 8 or 9 bits) or better may be possible. The high degree of phase accuracy of true path beam steering system  100  is critical for producing low-side lobe antenna patterns, as are needed for applications where Low Probability of Intercept (LPI), Low Probability of Detection (LPD), or Anti-Jamming (AJ) capabilities are important. 
       FIG. 2  illustrates an exemplary phase shifter  20  used in a true path beam steering system  100 . The exemplary embodiment shown includes a transmission line  21 , an input switch  22 , phase shift lines  25   1 ,  25   2 ,  25   3 ,  25   4 ,  25   5 ,  25   6 ,  25   7 , and  25   8 , output switch  28  and output line  29 . 
     In the exemplary embodiment shown in  FIG. 2 , the split signal input enters on transmission line  21  to an input switch  22 , which directs the split signal input to any of eight phase shift lines  25   1 ,  25   2 ,  25   3 ,  25   4 ,  25   5 ,  25   6 ,  25   7 , or  25   8 . Note that the various phase shift lines  25   i  have different physical lengths relative to each other. The various phase shift lines  25   i  carry the split signal input to collecting terminals in an output switch  28 , which selects the particular output line that is carrying the signal (as directed by input switch  22 ), and connects that particular output line to an output line  29 . In this embodiment, eight beam positions can be created by the eight phase shift lines  25   i . In alternate contemplated embodiments, phase shift lines  25   i  may number from four to one hundred. 
     Classical phase shifters operate in modulo 2π phase mode. These phase shifters approximate beam steering phases by trying to equal the phase in a modulo 2π framework. The modulo 2π approximation may deviate significantly in a true path or total phase sense. This disparity leads to unacceptably poor broadband performance. 
     In the exemplary embodiment shown in  FIG. 2 , the net effect of the split signal input having traversed the phase shifter  20  is an increase in the true time delay that when translated into phase can be much larger than 2π. 
     Phase shifter  20  uses delays that are physical shift-line-paths (herein also identified as “true paths”) and not digital approximations. In the exemplary embodiment shown, the lengths of the true paths are capable of being adjusted to take into account factors that include, but are not limited to, actual size of the phased antenna array, the number of radiating antennas in the phased antenna array, antenna spacing, variations in antenna spacing, and frequency ranges. 
     Unlike phase shifters which rely on digital approximations and are limited to lengths of 2π, phase shifter  20  eliminates errors at high and low frequencies caused by the artificial 2π limitation inherent in methods known in the art. 
     In true path beam steering system  100 , phase errors may be reduced to very low levels. Errors within true path beam steering system  100  are caused by fabrication tolerances in lines and switches. Because phase shifter  20  uses physically true paths, the narrow bandwidth nature of classical phase shifting (i.e. only getting the modulo 2π phase correct) is overcome. 
     In various embodiments, phase shifter  20  may enable beam steering with very large fractional bandwidths, ranging from a factor of about 30% to multiple decades. 
       FIG. 2  illustrates that phase shift lines  25   i  can be realized in a variety of embodiments including, but not limited to, microstrip, stripline, co-planar waveguide and waveguide transmission lines. In the present embodiment, phase shift lines  25   i  are microstrip lines, but may be any other form or material known in the art from which phase shift lines can be fabricated. 
     In alternative embodiments input switch  22  and output switch  28  may be any type of switch known in the art, including a semi-conductor, electro-mechanical, PIN or micro-electronic mechanical systems (MEMS) switch. 
     In the embodiment shown in  FIG. 2 , the input switch  22  and output switch  28  are configured as single-pole, eight-throw (“1 by 8”) switches (“1 by N” known in art) but in various alternate embodiments may be arranged in any configuration that may accommodate individual selection of lines within the phase shifter. The exemplary embodiment utilizes a commercially available embodiment of a surface mount package switch is manufactured by Hittite Microwave (www.hittite.com). The switch model HMC321LP4(E) is a broadband, non-reflective GaAs MESFET SP8T switch in a low-cost leadless-surface mount package. 
       FIG. 3  is a schematic of amplifiers of the exemplary true path beam steering system  100  operatively coupled with radiating elements. The exemplary embodiment in  FIG. 3  includes six amplifiers  81 ,  82 ,  83 ,  84 ,  85 , and  86  and corresponding radiating elements  221 ,  222 ,  223 ,  224 ,  225 , and  226  of an electronically steered antenna array  200 . 
     In various alternate embodiments, radiating elements  221 ,  222 ,  223 ,  224 ,  225 , and  226  may have alternate configurations, known in the art as “steerable RF arrays”. In one alternative embodiment, radiating elements  221 ,  222 ,  223 ,  224 ,  225 , and  226  may be slot-coupled patches. In another alternative embodiment, radiating elements  221 ,  222 ,  223 ,  224 ,  225 , and  226  may be pin-feed patches. Various alternative embodiments may also include more or fewer radiating elements or alternate types of radiating elements. 
       FIG. 4  illustrates antenna pattern comparisons between an ideal pattern, those generated via more classical phase shifting approaches, and that according to the detailed exemplary invention description herein. In this figure, the vertical “y” axis represents normalized antenna gain (dB) and the horizontal “x” axis is azimuth angle from broadside (degrees). Ideal pattern  410  corresponds to a 6-element array that is ideal Taylor weighted resulting in with −45 dB weighting and generating −48 dB side lobes. Patterns  412 ,  414  and  416  are generated with the same ideal Taylor weighting but are created with classical 4-bit, 5-bit, and 6-bit accuracy phase shifters, respectively. Finally, pattern  418  also utilizes the ideal Taylor weighting but couples this with the exemplary phase shifting and beam steering invention approach as described in this detailed description. 
     With respect to “ideal” generated antenna pattern  410 , the Taylor weighted antenna array used has elements near the center of the array that are assigned large signal amplitudes and has elements of progressively decreased amplitudes toward the edges of the array. 
     Ideal pattern  410  reflects phase errors that are essentially zero, and a beam steered to 10 degrees off of normal to avoid computational issues that occur with 0 degree beam pointing. The calculation used for this exemplary embodiment assumes ideal magnitude weights and, as indicated, these weight assumptions were used for all subsequent antenna patterns generated. The assumed antenna element spacing within the array is one-half wavelength. When no phase errors are present, highly suppressed side lobes are made feasible by the use of the Taylor weighting. 
       FIG. 4  further illustrates the effect of using low-bit (i.e. less accurate) classical phase shifters on beam steering. The first output of this approach is identified in the figure as  412  and is one generated with classical assumed 4-bit accuracy phase shifters. Although a −45 dB ideal Taylor weighting is assumed, the side lobes of the 4-bit approach are worse than −25 dB. This indicates that the side lobes utilize 23 dB more power, i.e. 200× more power than in the ideal approach illustrated by pattern  410  of  FIG. 4 . 
     Similarly, graph line  414  of  FIG. 4  is generated with classical assumed 5-bit accuracy phase shifters. Even though a −45 dB Taylor weighting is assumed, the side lobes are worse than −30 dB. This indicates that the side lobes are utilizing 18 dB more power (i.e. 60× more power) than the ideal case illustrated as  410  in the figure. 
     Finally, the output  416  of  FIG. 4  uses the same array configuration as before, but with assumed 6-bit accuracy phase shifters. Notice that even with the −45 dB Taylor weighting, the side lobes are worse than −35 dB. This indicates that the side lobes are seeing 13 dB more power (i.e. 20× more power) than the ideal embodiment illustrated as  410  in the figure. The 6-bit phase shifters represent high performance (and cost) commercially available phase shifters. 
     True path beam steering system  100  achieves the results of the graph line  418  also labeled “9-bit” in  FIG. 4 . The approach uses the same array configuration as the ideal approach of pattern  410 , but with the assumed 9-bit (equivalent) accuracy phase shifters of the present invention. In this exemplary embodiment, the side lobes are suppressed to −45 dB, which is near the 3 dB level of the ideal embodiment of  410 . 
     As illustrated in  FIG. 4 , the performance of true path beam steering system  100  can be improved to achieve a result that is 20 dB more efficient (i.e. 100× more power) than 4-bit phase shifters and 15 dB more efficient (30× more power) than 5-bit phase shifters with regard to side-lobe suppression. 
     In the exemplary embodiment shown in  FIG. 1 , the number of antenna beams is equivalent to the number of possible true path lengths that lead to an antenna element of the antenna array. Also in that embodiment, the number of beam positions is a function of the number of potential switch positions (i.e., “throws”) of either switch used to make up each phase shifter. If “n” beams are required, the phase shifter may be made with single pole, n-throw input and output switches. 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated to explain the nature of the invention, may be made by those skilled in the art within the principal and scope of the invention as expressed in the appended claims.