Patent Publication Number: US-9899746-B2

Title: Electronically steerable single helix/spiral antenna

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 61/916,184, filed Dec. 14, 2014, titled “Electronically Steerable Single Helix/Spiral Antenna,” the entire contents of which are hereby incorporated by reference herein, for all purposes. 
    
    
     TECHNICAL FIELD 
     The present invention relates to radio frequency antennas and, more particularly, to electronically steerable helical or spiral antennas. 
     BACKGROUND ART 
     An antenna, also known as an aerial, is an electronic device that converts electric power into radio waves and vice versa. Antennas are used to transmit and/or receive radio frequency (RF) signals. An antenna element is an electrically conductive member of an antenna. Various arrangements of antenna elements are known, such as dipole, monopole, Yagi and helix, each arrangement having a characteristic radiation pattern, impedance, etc. For example, helical antennas are widely used for space communication, because helical antennas inherently transmit circularly polarized radio waves and can receive linearly polarized signals, regardless of the linear polarization orientation. This is important, at least in part because orientation of an antenna on a spacecraft changes as the spacecraft orbits or spins, thereby making it difficult or impossible to maintain linear polarization alignment between the spacecraft antenna and a ground-based antenna. 
     A directional antenna is an antenna that radiates greater power in one or more directions than in other directions. A directional antenna is correspondingly more sensitive to signals received from one or more directions than from other directions. A directional antenna may be physically aimed toward a receiving antenna, such as an antenna on a spacecraft, to concentrate transmitted power toward the receiving antenna, rather than directions that do not contribute to reception by the spacecraft or to reduce signal power toward an unintended receiver. Similarly, a directional antenna may be physically aimed to receive desired signals from a particular direction and reduce reception of unwanted interference by signals from other directions. 
     Mechanically reorienting an antenna imposes limitations on speed with which the antenna&#39;s orientation can be changed, accuracy of aiming the antenna, reliability of mechanical devices used to support and orient the antenna, etc. To overcome these and other limitations, some antenna arrays (also known as phased arrays) are steered electronically. Signals with particular phase relationships are fed to antennas of a phased array, such that constructive and destructive interference between radiated signals from the individual antennas of the array yield a radiation pattern that is reinforced in a desired direction and suppressed in undesired directions. The radiation pattern can be reshaped very quickly, enabling phased arrays to be used in radar systems to track multiple moving targets. However, phased arrays are much larger than a single antenna of such an array. 
     Phased arrays of helical antennas have been described in the prior art. For example, U.S. Pat. No. 6,243,052 by M. Larry Goldstein, et al. describes a phased array antenna having a spatially periodic array of helical antenna elements and RF feed circuitry. 
     A steerable beam helical antenna is described in U.S. Pat. No. 5,612,707 by Rodney G. Vaughan. The Vaughan device uses a furled dielectric sheet on which one or more conductors are fixed. By furling and unfurling the dielectric sheet, the antenna beam may be steered. However, furling and unfurling the dielectric sheet is a mechanical process, which suffers from the deficiencies mentioned above, with respect to mechanical antenna aiming systems. 
     Thus, while beam steering is important for many applications, it is often limited by space, overall size of the system and other factors. 
     SUMMARY OF EMBODIMENTS 
     An embodiment of the present invention provides an electronically steerable radio frequency antenna. The antenna includes at least one helical antenna element wound about a single longitudinal axis. The at least one helical antenna element defines a volume. The antenna also includes a straight antenna element disposed within the volume. The straight antenna element is oriented along the single longitudinal axis. The straight antenna element may be a monopole antenna element. The straight antenna element is configured to be fed at a proximal end. The distal end of the straight antenna element terminates without any electrical connection to any other antenna element. 
     The electronically steerable radio frequency antenna may also include a feed circuit electrically coupled to the at least one helical antenna element and to the straight antenna element. 
     The feed circuit may be configured to receive an input signal. The feed circuit may also be configured to provide a first version of the input signal to the at least one helical antenna element. The feed circuit may be configured to provide a second version of the input signal to the straight antenna element. The feed circuit may also be configured to control relative amplitude and relative phase of the first and second versions of the input signal. 
     The feed circuit may include a variable delay circuit configured to control the relative phase of the first and second versions of the input signal. 
     The electronically steerable radio frequency antenna may also include a ground plane disposed adjacent one end of the at least one helical antenna element. The ground plane may be oriented perpendicular to the single longitudinal axis. 
     The at least one helical antenna element may include four helical antenna elements arranged as a quadrifilar helix. 
     The electronically steerable radio frequency antenna may include a feed circuit electrically coupled to the straight antenna element and to each antenna element of the four helical antenna elements. The feed circuit may be configured to receive an input signal. The feed circuit may also be configured to provide a first version of the input signal to the at least one helical antenna element. The feed circuit may be configured to provide a second version of the input signal to the straight antenna element. The feed circuit may also be configured to control relative amplitude and relative phase of the first and second versions of the input signal. 
     An embodiment of the present invention provides a method for electronically steering a radio frequency antenna. The method includes receiving an input signal. A first version of the input signal is provided to at least one helical antenna element. A second version of the input signal is provided to a straight antenna element disposed along an axis of the at least one helical antenna element. The straight antenna element is disposed within a volume defined by the at least one helical antenna element. Relative amplitude and relative phase of the first and second versions of the input signal are controlled. 
     Controlling the relative amplitude and the relative phase of the first and second versions of the input signal may include adjusting the relative amplitude of the second version of the input signal according to a theta direction control signal and adjusting the relative phase of the second version of the input signal according to a phi direction control signal. 
     Adjusting the relative phase of the second version of the input signal may include delaying the second version of the input signal by an amount that depends on the phi direction control signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood by referring to the following Detailed Description of Specific Embodiments in conjunction with the Drawings, of which: 
         FIG. 1  is a perspective schematic view of an electronically steered helical antenna, according to an embodiment of the present invention. 
         FIG. 2  is a perspective schematic view of an electronically steered helical antenna, according to another embodiment of the present invention. 
         FIG. 3  is a schematic block diagram of a signal distribution circuit that feeds the antenna elements of the antenna of  FIG. 2 , according to an embodiment of the present invention. 
         FIG. 4  is a perspective schematic view of an electronically steered spiral antenna, according to another embodiment of the present invention. 
         FIG. 5  is a perspective schematic diagram of an electronically steerable short-circuited, tapered QHA antenna having four meandering antenna elements, according to yet another embodiment of the present invention. 
         FIGS. 6-8  show antenna radiation patterns plotted in the phi direction, with a monopole element being fed with a signal that is phased at various values, relative to a signal fed to a first helical antenna element, according to an embodiment of the present invention. 
         FIGS. 9-11  show beam patterns shifted in the theta direction by changing amplitude of the signal fed to the monopole antenna element, according to an embodiment of the present invention. 
         FIGS. 12-14  show the beam direction change with different phasing being applied to the signal fed to the monopole antenna element, according to an embodiment of the present invention. 
         FIGS. 15-17  show the beam direction change with the phase of the monopole held constant, but with the amplitude of the signal delivered to the monopole adjusted, according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     In accordance with embodiments of the present invention, methods and apparatus are disclosed for electronically steering a helical or spiral antenna. The antenna can be steered, i.e., its radiation pattern can be altered, such that the antenna radiates in a desired direction, without mechanically changing a direction in which the antenna is aimed and without mechanically changing orientation of any of the antenna&#39;s elements. The antenna&#39;s beam can be steered 360° in phi and 90° in the theta direction, while maintaining good gain. Advantages of the disclosed antenna include reduced size and complexity of elements over conventional antennas, leading to simpler designs. 
     As noted, there are many applications that require the beam of an antenna to be steered in different directions, without sacrificing gain or directivity. One method to change the direction of the antenna beam pattern is to physically move the antenna structure to point in the desired direction using a mechanical positioner. However, mechanical positioners tended to be large, slow, and require substantial DC power to move an antenna. 
     Another method to steer an antenna&#39;s beam is to use a phased array. Traditionally, phased arrays consist of multiple antenna elements separated by half a wavelength, which can occupy a considerable amount of space. To steer a beam using a phased array, for a full planner 2D pattern, a minimum array size of 4 elements is needed (2×2), in which horizontally and vertically adjacent elements in the array are separated by ½ wavelength from each other. Each element of the phased array antenna system is excited by various phases and amplitudes to radiate the antenna beam in a desired direction. In order to achieve the proper phasing and amplitude, extra circuitry is required for phase shifting, splitting or combining power and attenuating or amplifying power. This can make the antenna design complex and increase the overall mechanical footprint of the antenna. 
       FIG. 1  is a perspective schematic view of an electronically steered helical antenna  100 , according to an embodiment of the present invention. The antenna  100  includes a helical antenna element  102  and a ground plane  104 , as in a conventional helical antenna. However, the antenna  100  also includes a monopole antenna element  106  disposed within a volume defined by the helical antenna element  102 . The monopole antenna element  106  is disposed along the axis  108  about which the helical antenna element  102  is wound. Thus, the monopole antenna element  106  is centered within the helical antenna element  102 . 
     Dimensions of the helical antenna element  102  and the ground plane  104  may be calculated according to well-known formulas for a desired operating frequency or range of frequencies. Examples of these formulas, as well as other construction details for helical and other antennas, are available in many texts, such as “The ARRL Antenna Book,” 21st Edition, ISBN 0-87259-987-6 (see, for example, pp. 19-6 to 19-9) and Joseph J. Carr, “Practical Antenna Handbook,” ISBN 0-07-137435-3 (see, for example, pp. 427-431), the entire contents of all of which are hereby incorporated by reference herein. 
     The monopole antenna element  106  may be sized as a ¼-wavelength monopole antenna, a ½-wavelength monopole antenna or any other suitable length monopole antenna. (See, for example, the above-referenced “The ARRL Antenna Book,” pp. 2-17 to 2-18, for information about monopole antennas.) 
     It should be noted that the end of the monopole antenna element  106  opposite the feed end is open, i.e., it is not connected to a loop antenna element or an additional helical antenna element, such as to provide a dual-band antenna, as described by P. Eratuuli, P. Haapala, and P. Vainikainen, “Dual frequency wire antennas,” Electronics Letters, Vol. 32, No. 12, pp. 1051-1052, Jun. 6, 1996. 
     The antenna  100  may be electronically steered by varying phase and amplitude of a signal fed to the monopole antenna element  106 , relative to a signal fed to the helical antenna element  102 . By changing the relative phase of the signal fed to the monopole antenna element  106 , the beam direction can be manipulated in the phi direction. By changing the relative weights (amplitudes) of the signals fed to the monopole antenna element  106  versus the helical antenna element  102 , the beam direction can be manipulated in the theta direction. For example, directing all of the power into the helical antenna element  102 , the antenna  100  radiates at zenith (0°). Directing all of the power into the monopole antenna element  106  causes the antenna  100  to radiate through sides of the helical antenna element  102  (90°). If both antenna element signals are weighted equally in amplitude, the antenna  100  radiates at 45°. Other radiation angles may be achieved by other power distribution weightings (ratios). 
     Although the antenna  100  is shown with a single helical antenna element  102 , other embodiments of the present invention may include other numbers of helical antenna elements interwound with each other. One such embodiment  200  is shown schematically in perspective in  FIG. 2 . Here, four helical antenna elements  202 ,  204 ,  206  and  208  are configured as an open-ended quadrifilar helical antenna (QHA). The antenna  200  includes a ground plane  210  and a monopole antenna element  212 . As with the embodiment described with reference to  FIG. 1 , directing all of the power into the QHA causes the antenna  200  to radiate at zenith (0°), whereas directing all of the power into the monopole antenna element  212  causes the antenna  200  to radiate through the sides of the QHA (90°). If both antenna signals are weighted equally in amplitude, the antenna  200  radiates at 45°. 
       FIG. 3  is a schematic block diagram of a signal distribution circuit that feeds the antenna elements of the antenna  200  of  FIG. 2 , according to an embodiment of the present invention. A power divider (weighting) circuit  300  divides an input signal  302  between the QHA and the monopole antenna element  212  according to a theta direction control signal  303 . The ratio M:N represents the relative amount of the input signal  302  that is fed to the QHA, versus to the monopole antenna element  212 . 
     The portion (N)  304  of the input signal is fed into a variable phase shifter  306 , which shifts the phase of the portion (N) by an amount specified by a phi direction control signal  308 . The variable phase shifter  306  may be implemented with a variable delay line or other suitable circuit. The output of the variable phase shifter  306  is fed to the monopole antenna element  212 . 
     The portion (M)  308  of the input signal is fed into a phase shifter  310 . The phase shifter  310  may be implemented with a delay line whose length depends on the wavelength of the input signal  302  or by any other suitable circuit. One output of the phase shifter  310  provides a signal  312  with 0° phase shift, relative to the input signal  302 , whereas another output of the phase shifter  310  provides a signal  314  with a 180° phase shift, relative to the input signal  302 . The signal  312  is fed into a second phase shifter  316 . One output of the second phase shifter  316  provides a signal  318  with a 0° phase shift, relative to the input signal  302 , whereas another output of the phase shifter  316  provides a signal  320  with a 90° phase shift, relative to the input signal  302 . The signals  318  and  320  are fed to two of the QHA elements  202  and  204  ( FIG. 2 ), respectively. 
     Similarly, a third phase shifter  322  receives the signal  314  and provides a signal  324  with 180° phase shift, relative to the input signal  302 , whereas another output of the third phase shifter  322  provides a signal  326  with a 270° phase shift, relative to the input signal  302 . The signals  324  and  326  are fed to the remaining two QHA elements  206  and  208  ( FIG. 2 ), respectively. 
     In some embodiments, the amplitude and phase of the signals fed to the QHA elements  202 - 208  remain unchanged as the antenna  200  is steered. In these embodiments, only the amplitude and phase of the signal fed to the monopole antenna element  212  varies to electronically steer the antenna  200 . In other embodiments, the amplitude and/or phases of the signals fed to the elements  202 - 208  of the QHA may also be varied to steer the antenna  200 . 
     In some embodiments, the helical elements may be conical or otherwise tapered, open-ended or short-circuited and/or self-phased (not shown). The helical antenna elements may be self-supporting or they may be attached to dielectric supports. The supports may be circular, square or have some other cross-sectional shape. Similarly, the windings of the helical antennal elements by be circular, square or have some other geometric or arbitrary shape. Size reductions of quadrifilar or other helical antennas may be achieved through geometric reduction techniques, such as stub loading, sinusoidal, rectangular, meander line or other techniques. Optionally or alternatively, other variations of a helical antenna may be used. 
     In yet other embodiments, the helical antenna element(s) may be replaced by spiral, i.e., planar, antenna element(s), as shown schematically in  FIG. 4 .  FIG. 4  shows an antenna  400  that includes two interwound spiral antenna elements  402  and  404  and a monopole antenna element  406 , as well as an optional ground plane  408 . In other respects, the antenna  400  of  FIG. 4  is operated in a manner similar to the antennas  100  and  200  described above, mutatis mutandis. Furthermore, an array of steerable helical or spiral antennas, all fed so as to direct their respective beams to be parallel or to be aimed at a desired target, may be used to increase gain over a single such antenna. 
       FIG. 5  is a perspective schematic diagram of an electronically steerable antenna  500 , according to yet another embodiment of the present invention. The antenna  500  includes a short-circuited, tapered QHA having four meandering antenna elements  502 ,  504 ,  506  and  508 . The antenna  500  also includes a ground plane  510  and a monopole antenna element  512 . In other respects, the antenna  500  of  FIG. 5  is constructed and operated in a manner similar to the antennas  100 ,  200  and  400  described above, mutatis mutandis. 
     The antenna  500  was modeled using a finite element analysis program, CST Studio Suite. The design was simulated as a 5-port network, with the first 4 ports being the helical antenna elements of the quadrifilar helix, and the monopole antenna element as the fifth port of the model. 
       FIG. 6  shows an antenna pattern plotted in the phi direction, with the monopole element being fed with a signal that is phased at 0°, relative to the signal fed to the first helical antenna element. Theta is at 115°. 
       FIG. 7  shows the antenna pattern in the phi direction with the monopole being fed with a signal that is phased at 90°, relative to the signal fed to the first helical antenna element. Theta is at 115°. Here we observe the main lobe shift to the direction of the monopole&#39;s phase. 
       FIG. 8  shows the antenna pattern in the phi direction, with the monopole being fed with a signal that is phased at 180°, relative to the signal fed to the first helical antenna element. Theta is at 115°. Here we observe the main lobe shift to the direction of the monopole&#39;s phase. 
     In  FIGS. 9-11 , the beam pattern is shifted in the theta direction by changing the amplitude of the signal fed to the monopole antenna element.  FIG. 9  shows an antenna radiation pattern when the amplitude of the signal fed to the monopole antenna element is equal to the amplitude of the signal fed to the helical antenna elements.  FIG. 10  shows the antenna radiation pattern when the amplitude of the signal fed to the monopole antenna element is equal to two times the amplitude of the signal fed to the helical antenna elements.  FIG. 11  shows the antenna radiation pattern when the signal fed to the monopole antenna element is equal to three times the amplitude of the signal fed to the helical antenna elements. 
     A functional embodiment was tested. Measurements were made using a quadrifilar helix antenna as a transmitting antenna.  FIGS. 12-14  show the beam direction change with different phasing being applied to the signal fed to the monopole antenna element. The monopole element receives four times the power of the helix, and the antenna is held constant in the theta angle but swept across the phi angle 0° to 360°. 
       FIG. 12  shows a polar plot when the monopole element is fed with a signal phased at 90°, relative to the signal fed to the helical elements.  FIG. 13  shows a polar plot when the monopole element is fed with a signal phased at 180°, relative to the signal fed to the helical elements.  FIG. 14  shows a polar plot when the monopole element is fed with a signal phased at 0°, relative to the signal fed to the helical elements. 
     In the  FIGS. 15-17 , the phase of the monopole was held constant, but the amplitude of the signal delivered to the monopole was adjusted. The phi angle was held constant and the theta angle was swept 0° to 360°. 
       FIG. 15  shows a polar plot when the monopole element is fed with a signal whose amplitude is four times the amplitude of the signal fed to the helical antenna elements.  FIG. 16  shows a polar plot when the monopole element is fed with a signal whose amplitude is two times the amplitude of the signal fed to the helical antenna elements.  FIG. 17  shows a polar plot when the monopole element is fed with a signal whose amplitude is equal to the amplitude of the signal fed to the helical antenna elements. 
     Since the phi angle is held constant, it is not as obvious to see the elevation of the beam direction change. The change can be seen by looking at the nulls. When the monopole antennal element receives more power than the helical antenna elements, and the beam radiates closer to the horizon, two nulls seen in a typical monopole radiation pattern are evident, even though the second null is slightly shifted due to the helical elements. As progressively more power is fed to the helical elements, the second null eventually converges with the first one, and the antenna radiates at broadside, as a helical antenna naturally radiates. In the previous plots, the back lobe of the radiation pattern shrinks as power is directed away from the monopole element and relatively to the helical elements. 
     While the invention is described through the above-described exemplary embodiments, modifications to, and variations of, the illustrated embodiments may be made without departing from the inventive concepts disclosed herein. Furthermore, disclosed aspects, or portions thereof, may be combined in ways not listed above and/or not explicitly claimed. Accordingly, the invention should not be viewed as being limited to the disclosed embodiments. 
     Although aspects of embodiments may be described with reference to flowcharts and/or block diagrams, functions, operations, decisions, etc. of all or a portion of each block, or a combination of blocks, may be combined, separated into separate operations or performed in other orders. All or a portion of each block, or a combination of blocks, may be implemented as computer program instructions (such as software), hardware (such as combinatorial logic, Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs) or other hardware), firmware or combinations thereof. Embodiments may be implemented by a processor executing, or controlled by, instructions stored in a memory. The memory may be random access memory (RAM), read-only memory (ROM), flash memory or any other memory, or combination thereof, suitable for storing control software or other instructions and data. Instructions defining the functions of the present invention may be delivered to a processor in many forms, including, but not limited to, information permanently stored on tangible non-writable storage media (e.g., read-only memory devices within a computer, such as ROM, or devices readable by a computer I/O attachment, such as CD-ROM or DVD disks), information alterably stored on tangible writable storage media (e.g., floppy disks, removable flash memory and hard drives) or information conveyed to a computer through a communication medium, including wired or wireless computer networks. Moreover, while embodiments may be described in connection with various illustrative data structures, systems may be embodied using a variety of data structures.