Patent Publication Number: US-11038269-B2

Title: Electronically steerable holographic antenna with reconfigurable radiators for wideband frequency tuning

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to and claims the benefit of U.S. Provisional Patent Application No. 62/729,341 filed on Sep. 10, 2018, which is incorporated herein by reference as though set forth in full. 
    
    
     STATEMENT REGARDING FEDERAL FUNDING 
     None 
     TECHNICAL FIELD 
     This disclosure relates to antennas and in particular, to holographic antennas and electronically scanned phased array antennas. 
     BACKGROUND 
     Prior Art holographic antennas have an operational bandwidth of less than 30%, limited by the bandwidth of the radiating element, and the instantaneous bandwidth is generally less than 3%, depending on the size of the antenna. 
     Electronically scanned phased array antennas or beamforming array antennas in the prior art can achieve a wide bandwidth by using a broadband antenna element. However, in order to use this element in an array, the element must have a length of less than half the wavelength on each side. Therefore, in order to achieve wideband operation, the antenna elements must be larger vertically, which has drawbacks in cost, array fabrication, and weight. Wideband phased arrays may be as much as 5× taller than holographic arrays and have more complicated fabrication and electronics, both of which increase cost. 
     In comparison, holographic antenna architectures have shown cost savings on the order of 3-5 times. The small thickness of a holographic array is generally on the order of 2 millimeters, which provides the potential for subarray panels to be folded and later deployed, such as by an operator. Further, holographic arrays have the potential to use significantly less power in receive mode because they have many fewer antenna elements. Phased arrays use significantly more power in receive mode because they have 15-20 times more receive modules than do holographic arrays. 
     Prior art holographic antenna designs may be both fixed-beam and electronically steerable. Leaky wave antennas (LWA) have been studied from as early as 1940 with slotted waveguides, as described in reference [1] below, which is incorporated herein by reference, and a precursor to these antennas was patented in 1921, as described in references [2, 3] below, which are incorporated herein by reference. LWAs are non-resonant antennas in which a wave propagates along the structure and radiates due to the characteristics of the mode supported by the antenna. LWAs can be split into two categories, namely uniform and periodic, as described in reference [4] below, which is incorporated herein by reference. Uniform antennas support a fast-wave mode in which the phase velocity of the antenna is greater than the speed of light. For this condition, the wave radiates based on the wavenumber of the mode along the antenna according to Equation (1):
 
β= k   0  sin θ,  (1)
 
     where β is the wavenumber of the wave propagating along the antenna, k 0  is the wavenumber in free space, and e is the radiation angle with respect to the surface normal of the antenna. Quasi-uniform antennas operate similarly to uniform antennas but have subwavelength periodic loadings in order to improve the antenna characteristics. Composite Right-/Left-Hand (CRLH) transmission line antennas use capacitive and inductive loadings to allow improved beam scanning as describe in reference [5] below, which is incorporated herein by reference. However, these structures generally obtain beam scanning by changing their operating frequency, and this method is not compatible with multiple applications such as mobile satellite communication where a fixed operating frequency is necessary. Periodic LWAs use a slow wave guiding structure which has its wavenumber modulated. Under this condition, the antenna radiates an infinite number of spatial harmonics defined by Equation (2):
 
β= k   0  sin θ+ mk   p ,  (2)
 
     where m is an integer which represents the spatial mode number and k p  is the wavenumber of the modulation. The m=−1 mode is generally the most accessible modulation and other spatial modes predominantly have very minimal coupling or complex radiation angles when the m=−1 mode is excited. In this document, the terms “periodic LWA” and “holographic antenna” are used interchangeably. One early method used to create holographic antennas was artificial impedance surface antennas (AISAs), as described by references [6]-[8] below, which are incorporated herein by reference. These passive structures demonstrated high-gain beams and also polarization control. Surface-wave waveguides were used as a method to confine the travelling wave mode and allow easier biasing as described in references [9]-[11] below, which are incorporated herein by reference. AISAs can be electronically scanned by loading the structure with tunable elements such as varactors, as described by references [12]-[21] below, which are incorporated herein by reference. Other holographic structures have also been demonstrated as well, as described in references [22]-[26] below, which are incorporated herein by reference. 
     Prior art reconfigurable slot antennas are described by H. Li, J. Xiong, Y. Yu and S. He in “A Simple Compact Reconfigurable Slot Antenna With a Very Wide Tuning Range,” IEEE Transactions on Antennas and Propagation, vol. 58, no. 11, pp. 3725-3728, November 2010, and by Symeon Nikolaou et al., in “Pattern and frequency reconfigurable annular slot antenna using PIN diodes,” IEEE Transactions on Antennas and Propagation, vol. 54, no. 2, pp. 439-448, February 2006. These references are two examples of many that show reconfigurable slot architectures. These elements cannot be used as radiators for a holographic antenna without (1) being coupled to a traveling wave mode, (2) fitting into the subwavelength spacing needed for holographic antennas (˜λ/10 at the highest frequency), (3) radiating at the appropriate rate to allow illumination over an electrically long traveling wave antenna, and (4) providing appropriate impedance to allow wave propagation. For a slot antenna element (or any other small antenna element) designed independently of application to holographic antennas it is almost certain that the element will not operate as desired within a holographic antenna. Further, the innovation of using a reconfigurable radiating element within a holographic antenna is not obvious and has not been previously published. 
     The following references are incorporated herein as though set forth in full. 
     REFERENCES 
     
         
         [1] W. W. Hansen, Radiating electromagnetic waveguide, U.S. Pat. No. 2,402,622, 1940. 
         [2] H. H. Beverage, Radio receiving system, U.S. Pat. No. 1,381,089, 1921. 
         [3] Beverage, Harold H.; Rice, Chester W.; Kellogg, Edward W., “The Wave Antenna A New Type of Highly Directive Antenna,” in American Institute of Electrical Engineers, Transactions of the, vol. XLII, no., pp. 215-266, January 1923. 
         [4] Jackson, D. R.; Caloz, C.; Itoh, T., “Leaky-Wave Antennas,” in Proceedings of the IEEE, vol. 100, no. 7, pp. 2194-2206, July 2012. 
         [5] Caloz, C.; Itoh, T.; Rennings, A., “CRLH metamaterial leaky-wave and resonant antennas,” in Antennas and Propagation Magazine, IEEE, vol. 50, no. 5, pp. 25-39, October 2008. 
         [6] D. Sievenpiper et al, “Holographic AISs for conformal antennas”, 29th Antennas Applications Symposium, 2005. 
         [7] D. Sievenpiper, J. Colburn, B. Fong, J. Ottusch and J. Visher., 2005 IEEE Antennas and Prop. Symp. Digest, vol. 1B, pp. 256-259, 2005. 
         [8] B. Fong et al, “Scalar and Tensor Holographic Artificial Impedance Surfaces,” IEEE TAP., 58, 2010. 
         [9] R. Quarfoth and D. Sievenpiper, “Artificial Tensor Impedance Surface Waveguides,” in IEEE Transactions on Antennas and Propagation, vol. 61, no. 7, pp. 3597-3606, July 2013. 
         [10] R. G. Quarfoth and D. F. Sievenpiper, “Nonscattering Waveguides Based on Tensor Impedance Surfaces,” in IEEE Transactions on Antennas and Propagation, vol. 63, no. 4, pp. 1746-1755, April 2015. 
         [11] A. M. Patel and A. Grbic, “A Printed Leaky-Wave Antenna Based on a Sinusoidally-Modulated Reactance Surface,” in IEEE Transactions on Antennas and Propagation, vol. 59, no. 6, pp. 2087-2096, June 2011. 
         [12] Sievenpiper, D.; Schaffner, J.; Lee, J. J.; Livingston, S.; “A steerable leaky-wave antenna using a tunable impedance ground plane,” Antennas and Wireless Propagation Letters, IEEE, vol. 1, no. 1, pp. 179-182, 2002. 
         [13] Colburn, J. S.; Lai, A.; Sievenpiper, D. F.; Bekaryan, A.; Fong, B. H.; Ottusch, J. J.; Tulythan, P.; “Adaptive artificial impedance surface conformal antennas,” Antennas and Propagation Society International Symposium, 2009. APSURSI &#39;09. IEEE, vol., no., pp. 1-4, 1-5 Jun. 2009. 
         [14] Gregoire, Daniel J., and Joseph S. Colburn. “Low cost, 2D, electronically-steerable, artificial-impedance-surface antenna.” U.S. Pat. No. 9,466,887. 11 Oct. 2016. 
         [15] Gregoire, Daniel J. “Two-dimensionally electronically-steerable artificial impedance surface antenna.” U.S. Pat. No. 9,455,495. 27 Sep. 2016. 
         [16] Gregoire, Daniel J., Amit M. Patel, and Michael de la Chapelle. “Two-dimensionally electronically-steerable artificial impedance surface antenna.” U.S. Pat. No. 9,698,479. 4 Jul. 2017. 
         [17] Patel, Amit M., and Ryan G. Quarfoth. “Two-dimensionally electronically-steerable artificial impedance surface antenna.” U.S. Pat. No. 9,871,293. 16 Jan. 2018. 
         [18] Gregoire, D. J.; Colburn, J. S.; Patel, A. M.; Quarfoth, R.; Sievenpiper, D., “An electronically-steerable artificial-impedance-surface antenna,” in Antennas and Propagation Society International Symposium (APSURSI), 2014 IEEE, vol., no., pp. 551-552, 6-11 Jul. 2014. 
         [19] D. J. Gregoire, J. S. Colburn, A. M. Patel, R. Quarfoth and D. Sievenpiper, “An electronically-steerable artificial-impedance-surface antenna,” 2014 IEEE Antennas and Propagation Society International Symposium (APSURSI), Memphis, Tenn., 2014, pp. 551-552. 
         [20] Gregoire, D. J.; Patel, A.; Quarfoth, R., “A design for an electronically-steerable holographic antenna with polarization control,” in Antennas and Propagation &amp; USNC/URSI National Radio Science Meeting, 2015 IEEE International Symposium on, vol., no., pp. 2203-2204, 19-24 Jul. 2015. 
         [21] R. G. Quarfoth, A. M. Patel and D. J. Gregoire, “Ka-band electronically scanned artificial impedance surface antenna,” 2016 IEEE International Symposium on Antennas and Propagation (APSURSI), Fajardo, 2016, pp. 651-652. 
         [22] Avakian, Aramais, et al. “Reconfigurable dielectric waveguide antenna.” U.S. Pat. No. 7,151,499. 19 Dec. 2006. 
         [23] V. A. Manasson et al., “Electronically reconfigurable aperture (ERA): A new approach for beam-steering technology,” 2010 IEEE International Symposium on Phased Array Systems and Technology, Waltham, Mass., 2010, pp. 673-679. 
         [24] Bily, Adam, et al. “Surface scattering antenna improvements.” U.S. Pat. No. 9,385,435. 5 Jul. 2016. 
         [25] Bily, Adam, et al. “Surface scattering antennas.” U.S. Pat. No. 9,450,310. 20 Sep. 2016. 
         [26] Smith, David R., Okan Yurduseven, Laura Pulido Mancera, Patrick Bowen, and Nathan B. Kundtz. “Analysis of a waveguide-fed metasurface antenna.” Physical Review Applied 8, no. 5 (2017): 054048. 
         [27] Balanis, Constantine A. “Antenna Theory: Analysis and Design.” 3 rd  edition, Wiley Interscience (2005), see Chapter 6. 
       
    
     What is needed is an electronically steerable holographic antenna with wideband frequency tuning. The embodiments of the present disclosure answer these and other needs. 
     SUMMARY 
     In a first embodiment disclosed herein, a holographic antenna comprises a transmission line structure having a traveling wave mode along a length of the transmission line structure, and a plurality of reconfigurable radiating elements located along the length of the transmission line structure. 
     In another embodiment disclosed herein, a holographic antenna comprises a rectangular waveguide, a plurality of radiating elements located along a length of the rectangular waveguide, a plurality of tuning devices, a respective set of the plurality of tuning devices coupled to each respective radiating element of the plurality of radiating elements, wherein each respective set of the plurality of tuning devices has a uniform or non-uniform spacing across a width of the respective radiating element. 
     In yet another embodiment disclosed herein, a method of providing a holographic antenna comprises providing a printed circuit board having multiple layers, forming a metallic top layer of a transmission line structure on top of the printed circuit board, forming a metallic bottom layer of the transmission line structure on an internal layer of the printed circuit board, forming a plurality of metallic vias coupled between the top layer of the transmission line structure and the bottom layer of the transmission line structure, forming a plurality of radiating elements in the top layer of the transmission line along a length of the transmission line, and providing a plurality of tuning devices, a respective set of the plurality of tuning devices coupled to each respective radiating element of the plurality of radiating elements, wherein each respective set of the plurality of tuning devices has a uniform or non-uniform spacing across a width of the respective reconfigurable radiating element. 
     These and other features and advantages will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features, like numerals referring to like features throughout both the drawings and the description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a perspective view of the antenna and  FIG. 1B  shows slot radiating elements, and tuning devices in accordance with the present disclosure; 
         FIG. 2  shows a more-detailed unit cell top view of the structure in accordance with the present disclosure; 
         FIG. 3  shows a front view of unit cell in accordance with the present disclosure; 
         FIG. 4  shows side view of a unit cell in accordance with the present disclosure; 
         FIG. 5  shows a perspective view of a two-dimensional (2D) array in accordance with the present disclosure; 
         FIG. 6A  shows an example of four tuning devices and  FIG. 6B  shows the positions of the tuning devices in accordance with the present disclosure; 
         FIGS. 7A and 7B  show an adjustment to the tuning device positions of  FIG. 6  that allow continuous operation between 6-18 GHz in accordance with the present disclosure; 
         FIGS. 8A, 8B, 8C, and 8D  show the device topology in relation to the slot and show single- and multi-transistor tuning device architectures in accordance with the present disclosure; 
         FIGS. 9A, 9B, 9C, 9D and 9E  show different slot geometries in accordance with the present disclosure; 
         FIGS. 10A, 10B, 10C and 10D  show different transmission line geometries in accordance with the present disclosure; 
         FIG. 11  shows a geometry used for simulation of the antenna performance in accordance with the present disclosure; 
         FIG. 12  shows simulation results compared to an analytic formulation in accordance with the present disclosure; and 
         FIG. 13  shows analytic results of a sweep of modulation period showing wide-angle beam steering in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to clearly describe various specific embodiments disclosed herein. One skilled in the art, however, will understand that the presently claimed invention may be practiced without all of the specific details discussed below. In other instances, well known features have not been described so as not to obscure the invention. 
     The described invention is for an electronically steerable holographic antenna with reconfigurable radiating elements. The preferred embodiment is a rectangular waveguide with slot radiating elements spaced along the rectangular waveguide at a sub-wavelength of the traveling wave mode of the antenna. The antenna uses traditional holographic beam steering techniques. A periodic pattern of open and shorted slots is applied along the length of the antenna. The beam steering direction is based on the periodicity of open and shorted slots. Switches are used to control whether a slot is open or shorted, and the periodicity can be reconfigured electronically, thus providing electronic beam steering. The present disclosure describes multiple switches that are placed in each radiating element, so that by operating the switches, the effective length of the slot can be changed. Each of the switches in the slot are independently controllable, and this allows the slot to take on a discrete set of lengths based on the number of switches and their positions. The operational frequency of the holographic antenna is based on the length of the slot, so the frequency of the holographic antenna can be reconfigured by shorting out portions of the slot. The preferred embodiment provides a 3:1 tuning range while still allowing wide angle beam steering. Other embodiments could provide wider tuning ranges or steering ranges. 
     Four components are used together to form the electronically steerable holographic antenna with reconfigurable radiators for wideband frequency tuning: a transmission line structure  12 , radiating elements  14 , tuning devices  16  in the radiating elements, and bias lines  20  that provide individually-controllable voltages to the tuning devices. Note that in  FIG. 3  the bias lines  20  appear to be shorted together; however, this is due to the perspective of the figure and in fact the bias lines  20  in  FIG. 3  are not shorted together.  FIG. 4  makes it clear that the bias lines  20  are independently addressable. 
     The transmission line structure  12  supports a traveling wave mode. Radiating elements  14  containing the tuning devices  16  are located periodically along the transmission line structure to provide reconfigurability. The tuning devices have two purposes. The first purpose is to apply an overall holographic pattern to the antenna, so that the antenna radiates a beam in a desired direction as described in equation (2). The second purpose is to reconfigure the length of the radiating element in order to change the frequency of operation. 
       FIGS. 1A and 1B  show an antenna  10  that has a transmission line  12 , radiating elements  14  along the transmission line  12 , and tuning devices  16  along the radiating element  14 , which in the embodiment shown are radiating slots  14 . Bias lines are not shown in  FIGS. 1A and 1B , but may be located at the edges  18  of the transmission line  12 . The antenna  10  may be constructed using a printed circuit board which is a laminate consisting of layers of metal and layers of dielectric. Plated metal vias may be used to provide conductive connections vertically between horizontal metal layers. 
       FIG. 2  shows a top view of a portion of the antenna  10 , showing a slot  14  with tuning devices  16  controlled by bias lines  20 . The waveguide  12  may be constructed with metal sheets in the horizontal plane creating top and bottom walls, and vertical vias  22  creating the side walls to form a substantially rectangular waveguide  12 . Bias lines  20  are connected to the tuning devices  16  and to metal layers beneath the antenna  10  using vias  24 . 
     The red rectangle  31  in  FIG. 3  represents the four walls of the waveguide. The top and bottom walls are solid metal that is located on the PCB. The side walls are created by the vias  22  and they make contact with the top and bottom layers. In order be a “wall” electromagnetically-speaking these vias are spaced closer than than the wavelength. With this small spacing the vias form a “wall” that electromagnetic (EM) waves can not penetrate. Other names for this are via fence, conductive fence, or more generally faraday cage. 
     As shown in  FIG. 2  the tuning devices  16  are connected across the slot  14 . The tuning devices  16  may be switches  16  that are connected across the slot  14  at different positions along the slot. Each switch  16  may have one electrode touching one side  23  of the slot and another electrode touching another side  25  of the slot  16 . The switch  16  is controlled by a bias line  20 , which controls the state of the switch  16  by applying a voltage or current. In the “short” state, the switch provides a zero impedance or low impedance, which may be less than 10 ohms, between the first side  23  and the second side  25  of the slot  16 . In the “open” state, the switch provides a high impedance, which may be greater than 100 ohms, between the between the first side  23  and the second side  25  of the slot  16 . 
     In general, slot antennas radiate power at a given frequency if they are sized appropriately. The tuning devices or switches  16  can change the effective length of the slot  14 . So, for example, if the appropriate slot length for radiating at a frequency f is L, and if with a length of L/2 radiation is prevented, then by placing a switch in the middle of the slot  14 , the slot can be switched from a radiating slot to a non-radiating slot. In the “open” state the effective length is L, and the slot radiates. In the “short” state the slot does not radiate. In the “short” state the slot does not radiate because the slot is changed to two L/2 slots and neither of them will radiate at frequency f.  FIG. 8A  shows a switch  16  implemented with a field effect transistor (FET)  60  that has a source electrode  80  connected to the first side  23  of the slot  14  and a drain  82  electrode connected to the second side  25  of the slot  14 . The first side  23  and the second side  25  of the slot  14  are continuous with the waveguide  12 . By controlling the gate of the FET with bias line  20  the FET switch  60  may be controlled to be in the “short” or the “open” state. 
       FIG. 3  shows an illustration of a front elevation view of the antenna structure  10 . The top layer  30  of the transmission line  12  may be on the top layer of a printed circuit board (PCB) or dielectric  32  and the bottom layer  34  of the transmission line  12  may be on an internal layer of the PCB to provide space for biasing lines  20  beneath the antenna  10 . Bias lines  20  come up from the lower bottom layer  36  to the tuning devices  16 . Using the bottom layer  36 , or any number of additional layers below the antenna  10 , the bias lines  20  can be connected to traditional biasing hardware, such as digital-to-analog converters, digital input control lines, and so on. It is preferred that a horizontal extent of the unit cell be on the order of half the wavelength of the lowest frequency of operation so that holographic antenna elements can be arrayed horizontally to provide two-dimensional beam steering.  FIG. 4  shows a side view of the unit cell. The horizontal extent is the horizontal direction of  FIG. 3  and this is also the unit cell width  46  shown in  FIG. 2  and discussed further below. 
     The antenna may be fabricated using wafer-based fabrication and assembly with tuning devices integrated on-wafer together with the traveling wave structure and the radiators. The traveling wave structure and radiator may also be machined and coupled to a circuit board or a wafer with the tuning devices. 
       FIG. 5  shows an illustration of a 2D array with 6 holographic antenna elements  10 , each of which may be the same as antenna  10  shown in  FIG. 1A . Each holographic antenna element  10  may be fed from a feed network  40  by conventional means and with input phase controlled by a phase shifter  42 . This architecture allows 2D beam steering enabled by the hologram antenna element  10  in one dimension and the phase shifters in the second dimension, as described in references [14]-[17] above, which are incorporated herein by reference. 
     In a preferred embodiment each unit cell, as shown in  FIG. 1B ,  FIG. 2  and  FIG. 3 , of each holographic antenna element  10  may have the following parameters which were determined by simulation: a 2 mm unit cell length  44 ; a 13 mm unit cell width  46 ; an 11 mm waveguide width  48 ; a 150 mil waveguide height  50 ; a 162 mil total unit cell height  52 ; 9.5 mm slot width  54 ; 0.4 mm slot length  56 ; a dielectric constant of 6 for the dielectric; and copper for the metal in the waveguide  12 , vias  22  and  24 , and bias lines  20 . Depending on the frequency of operation or manufacturing method, other lengths, widths, or materials can be used. 
     An electromagnetic wave (EM wave) which travels along the structure through the transmission line  12 . The transmission line  12  is preferred to be electrically long, meaning multiple wavelengths long. A preferred embodiment of the transmission line  12  may have the following characteristics: operates over a 3:1 frequency range (6-18 GHz), is filled with a dielectric with a dielectric constant of 6, is a rectangular waveguide, has a length that is 12.8 wavelengths long at the center of the operational frequency band, or 320 mm long at 12 GHz, and that is sized to have a frequency cutoff just below the bottom of the operating frequency range. 
     Radiating elements  14  are loaded periodically along the transmission line  12  structure and one or more tuning devices  16  is coupled to each radiating element  14 . A preferred embodiment of a radiating element is a slot  14  with four tuning devices  16 . Each tuning device  16  may be a single FET transistor. Any number of tuning devices  16  greater than one coupled to a radiating element  14  can provide frequency of operation reconfigurability. Increasing the number of tuning devices increases the number of tuning states that the radiating element  14  can achieve. An example showing four tuning devices is shown in  FIG. 6A . Using full wave simulation, it has been found that an optimal slot length for 6 GHz is 9.5 mm which is represented between positions A and F in  FIG. 6A . 
     The effective length of the slot radiator  14  can be changed by switching the appropriate tuning devices  16  to a “short” or ON state. For example, the effective slot width is only the distance between A and E if the tuning device at position E is turned ON or is put in an “short” state in every row of the antenna. In this example, only the tuning devices in positions B, C, and D would be in the “open” or OFF state. The result is a slot that is 7.6 mm wide which resonates at 7.6 GHz. 
     As seen in  FIG. 6B , different combinations of switches create center frequencies ranging from 6-15 GHz. Note that the operational bandwidth for each effective slot width is approximately +−20% of the center frequency. So, for the embodiment of  FIG. 6A , there are frequency ranges where the antenna cannot operate efficiently. In  FIG. 6A  each slot  14  has four tuning devices  16  that are uniformly spaced across the width of the slot  16 . The four tuning devices  16  from one edge of the 9.5 mm wide slot are at locations 1.9 mm, 3.8 mm, 5.7 mm, and 7.6 mm. 
     By spacing the tuning devices non-uniformly, many more slot lengths can be achieved and thus more center frequencies can be achieved.  FIGS. 7A and 7B  show that by adjusting the tuning device positions, continuous frequency of operation between 6-18 GHz is provided. Again, it is noted that the operational bandwidth of a specific slot length is approximately 20% of the center frequency. Therefore,  FIG. 7  provides a preferred embodiment. In  FIG. 7A  each slot  14  has four tuning devices  16  that are non-uniformly spaced across the width of the slot  16 . In  FIG. 7A , the four tuning devices  16  from one edge of the 9.5 mm wide slot are at locations 1.9 mm, 3.8 mm, 6.2 mm, and 8.6 mm. 
     Bias lines  20  provide independent voltage control for each tuning device  16 . The metal surrounding the slot  14  is the transmission line structure  12 , which may be at ground. The bias lines  20  can be brought in from a lower plane of the antenna  10  as shown in  FIGS. 2, 3, and 4 . 
     A preferred embodiment uses multiple tuning devices  16  across the slot  14 , with each single one of the multiple tuning devices  16  being a single transistor FET switch  60 , as shown in  FIG. 8A .  FIG. 8A  shows a switch  16  implemented with a field effect transistor (FET)  60  that has a source electrode  80  connected to the first side  23  of the slot  14  and a drain electrode  82  connected to the second side  25  of the slot  14 . The first side  23  and the second side  25  of the slot  14  are continuous with the waveguide  12 . By controlling the gate of the FET with bias line  20  the FET switch  60  may be controlled to be in the “short” or the “open” state. 
     At higher frequencies, the width of a slot  14  may be narrower and in that case it may be challenging to fit multiple single transistor FET switches  60  across the slot  14 . In such a case an integrated tuning device  62 , as shown in  FIG. 8B , may be used for each slot  14 . The integrated tuning device  62  integrates multiple tuning elements into the integrated tuning device, which may be an integrated circuit or a monolithic integrated circuit. Two examples of integrated tuning devices  62  are shown in  FIGS. 8C and 8D .  FIG. 8C  shows a series of 3 transistors  64  that may be fed by a resistive network that controls which devices are ON or in a “short” state based on an analog voltage input. Pads  68  are on the integrated tuning device  62  and connected to the transmission line structure  12 . The example of  FIG. 8D  also has three transistors  64  which are controlled by a decoder  70 , which decodes either a digital or analog input  71  to set the state of each of the transistors  64  to be either in a “short” state or in an “open” state across the slot  14 . For example, one of the transistors  64  may be in a “short” state, while the other two transistors  64  are in an “open” state. Three transistors  64  are shown within the multi-transistor tuning device examples of  FIGS. 8C and 8D ; however, any number of transistors may be used for various applications. Also, more than one of these integrated tuning devices  62  may be used to control the effective width and therefore the operating frequency of a single slot  14 . Also, the tuning device  62 , shown as transistors in  FIGS. 8A, 8C and 8D , may also be implemented using micro-electro-mechanical systems (MEMS) switches, phase change material (PCM) switches, semiconductor switches, other switches, or any two state (ON/OFF) or “short”/“open” device. 
     The preferred embodiment for a slot is a straight slot, as shown in  FIG. 9A ; however, other slot geometries are possible. The slot may be a straight slot, a bent slot, an annular ring, a split ring, or a slot of arbitrary geometry, as shown in  FIGS. 9A, 9B, 9C, 9D and 9E , respectively. 
     The preferred embodiment of the transmission line is a rectangular waveguide, as shown in  FIG. 10A . However, other transmission line geometries may be used, such as a ridged waveguide, a coaxial waveguide, or a parallel plate, as shown in  FIGS. 10B, 10C and 10D , respectively. Each of these other geometries may provide improved bandwidth. 
     A preferred embodiment with a straight slot and a rectangular waveguide has been simulated in a full-wave 3D electromagnetic solver (ANSYS HFSS) in order to determine its performance. The simulation geometry of the structure is shown in  FIG. 11 , which is a zoomed out view of  FIG. 1A , and this structure has been simulated at multiple frequencies.  FIG. 12  shows the simulation results for a 12 GHz center frequency. The analytic formulation is an array factor analysis that is calculated by traditional methods for antenna arrays, as described in reference [27], which is incorporated herein by reference.  FIG. 13  shows analytic results of a sweep of modulation period showing that the antenna  10  is capable of wide-angle beam steering.  FIG. 13  shows a legend showing the different modulation periods, kp, which is the spatial domain representation of the period kp=2*pi/period. 
     Having now described the invention in accordance with the requirements of the patent statutes, those skilled in this art will understand how to make changes and modifications to the present invention to meet their specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention as disclosed herein. 
     The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the Claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the step(s) of . . . .”