Patent Description:
Prior Art holographic antennas have an operational bandwidth of less than <NUM>%, limited by the bandwidth of the radiating element, and the instantaneous bandwidth is generally less than <NUM>%, 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 5x 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 <NUM>-<NUM> times. The small thickness of a holographic array is generally on the order of <NUM> 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 <NUM>-<NUM> 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 <NUM> with slotted waveguides, as described in reference [<NUM>] below, and a precursor to these antennas was patented in <NUM>, as described in references [<NUM>,<NUM>] below. 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 [<NUM>] below. 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 (<NUM>):
<MAT>
where β is the wavenumber of the wave propagating along the antenna, k<NUM> is the wavenumber in free space, and θ 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 [<NUM>] below. 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 (<NUM>):.

where m is an integer which represents the spatial mode number and kp is the wavenumber of the modulation. The m=-<NUM> mode is generally the most accessible modulation and other spatial modes predominantly have very minimal coupling or complex radiation angles when the m=-<NUM> 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 [<NUM>]-[<NUM>] below. 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 [<NUM>]-[<NUM>] below. AISAs can be electronically scanned by loading the structure with tunable elements such as varactors, as described by references [<NUM>]-[<NUM>] below. Other holographic structures have also been demonstrated as well, as described in references [<NUM>]-[<NUM>] below.

Prior art reconfigurable slot antennas are described by <NPL>, and by <NPL>. These references are two examples of many that show reconfigurable slot architectures. These elements cannot be used as radiators for a holographic antenna without (<NUM>) being coupled to a traveling wave mode, (<NUM>) fitting into the subwavelength spacing needed for holographic antennas (~λ/<NUM> at the highest frequency), (<NUM>) radiating at the appropriate rate to allow illumination over an electrically long traveling wave antenna, and (<NUM>) 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.

<CIT>
discloses surface scattering antennas with lumped elements that provide adjustable radiation fields by adjustably coupling scattering elements along a wave-propagating structure. In some approaches, the surface scattering antenna is a multi-layer printed circuit board assembly, and the lumped elements are surface-mount components placed on an upper surface of the printed circuit board assembly. In some approaches, the scattering elements are adjusted by adjusting bias voltages for the lumped elements. In some approaches, the lumped elements include diodes or transistors. <CIT> discloses surface scattering antennas with lumped elements that provide adjustable radiation fields by adjustably coupling scattering elements along a waveguide. In some approaches, the scattering elements include slots in an upper surface of the waveguide, and the lumped elements are configured to span the slots provide adjustable loading. In some approaches, the scattering elements are adjusted by adjusting bias voltages for the lumped elements. In some approaches, the lumped elements include diodes or transistors. <CIT> discloses an information terminal unit (<NUM>) that comprises a transceiver (<NUM>) and a processor (<NUM>), while a variable directional antenna (<NUM>) comprises a main antenna element (<NUM>) to which the transceiver directly supplies a radio frequency signal, and a plurality of sub antenna elements (<NUM> to <NUM>). The sub antenna elements are connected with variable phase shifter circuits (<NUM> to <NUM>) for determining a phase shift amount of a reflecting wave, respectively. The control circuit (<NUM>) receives the directivity data from the CPU (<NUM>) of the information terminal unit (<NUM>) and analyzes the received data to control the phase shift amount of each variable phase shifter circuit (<NUM> to <NUM>). Thereby the phase shift amounts of the respective sub antenna elements are adjusted so that the wave fronts of the waves radiated or secondarily radiated from the main antenna element (<NUM>) and sub antenna elements (<NUM> to <NUM>) are aligned in a certain direction, and then the variable directional antenna (<NUM>) is controlled so as to have a directivity in that direction. <CIT> discloses an antenna having radio-frequency (RF) resonators and methods for fabricating the same are described. In one embodiment, the antenna comprises a physical antenna aperture having an array of antenna elements, where the array of antenna elements includes a plurality of radio-frequency (RF) resonators, with each RF resonator of the plurality of RF resonators having an RF radiating element with a microelectromchanical systems (MEMS) device.

What is needed is an electronically steerable holographic antenna with wideband frequency tuning. The embodiments of the present disclosure answer these and other needs.

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 accordance with claim <NUM>.

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.

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 <NUM>:<NUM> 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 <NUM>, radiating elements <NUM>, tuning devices <NUM> in the radiating elements, and bias lines <NUM> that provide individually-controllable voltages to the tuning devices. Note that in <FIG> the bias lines <NUM> appear to be shorted together; however, this is due to the perspective of the figure and in fact the bias lines <NUM> in <FIG> are not shorted together. <FIG> makes it clear that the bias lines <NUM> are independently addressable.

The transmission line structure <NUM> supports a traveling wave mode. Radiating elements <NUM> containing the tuning devices <NUM> 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 (<NUM>). The second purpose is to reconfigure the length of the radiating element in order to change the frequency of operation.

<FIG> show an antenna <NUM> that has a transmission line <NUM>, radiating elements <NUM> along the transmission line <NUM>, and tuning devices <NUM> along the radiating element <NUM>, which in the embodiment shown are radiating slots <NUM>. Bias lines are not shown in <FIG>, but may be located at the edges <NUM> of the transmission line <NUM>. The antenna <NUM> 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> shows a top view of a portion of the antenna <NUM>, showing a slot <NUM> with tuning devices <NUM> controlled by bias lines <NUM>. The waveguide <NUM> may be constructed with metal sheets in the horizontal plane creating top and bottom walls, and vertical vias <NUM> creating the side walls to form a substantially rectangular waveguide <NUM>. Bias lines <NUM> are connected to the tuning devices <NUM> and to metal layers beneath the antenna <NUM> using vias <NUM>.

The red rectangle <NUM> in <FIG> 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 <NUM> 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> the tuning devices <NUM> are connected across the slot <NUM>. The tuning devices <NUM> may be switches <NUM> that are connected across the slot <NUM> at different positions along the slot. Each switch <NUM> may have one electrode touching one side <NUM> of the slot and another electrode touching another side <NUM> of the slot <NUM>. The switch <NUM> is controlled by a bias line <NUM>, which controls the state of the switch <NUM> by applying a voltage or current. In the "short" state, the switch provides a zero impedance or low impedance, which may be less than <NUM> ohms, between the first side <NUM> and the second side <NUM> of the slot <NUM>. In the "open" state, the switch provides a high impedance, which may be greater than <NUM> ohms, between the between the first side <NUM> and the second side <NUM> of the slot <NUM>.

In general, slot antennas radiate power at a given frequency if they are sized appropriately. The tuning devices or switches <NUM> can change the effective length of the slot <NUM>. So, for example, if the appropriate slot length for radiating at a frequency f is L, and if with a length of L/<NUM> radiation is prevented, then by placing a switch in the middle of the slot <NUM>, 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/<NUM> slots and neither of them will radiate at frequency f. <FIG> shows a switch <NUM> implemented with a field effect transistor (FET) <NUM> that has a source electrode <NUM> connected to the first side <NUM> of the slot <NUM> and a drain <NUM> electrode connected to the second side <NUM> of the slot <NUM>. The first side <NUM> and the second side <NUM> of the slot <NUM> are continuous with the waveguide <NUM>. By controlling the gate of the FET with bias line <NUM> the FET switch <NUM> may be controlled to be in the "short" or the "open" state.

<FIG> shows an illustration of a front elevation view of the antenna structure <NUM>. The top layer <NUM> of the transmission line <NUM> may be on the top layer of a printed circuit board (PCB) or dielectric <NUM> and the bottom layer <NUM> of the transmission line <NUM> may be on an internal layer of the PCB to provide space for biasing lines <NUM> beneath the antenna <NUM>. Bias lines <NUM> come up from the lower bottom layer <NUM> to the tuning devices <NUM>. Using the bottom layer <NUM>, or any number of additional layers below the antenna <NUM>, the bias lines <NUM> 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> shows a side view of the unit cell. The horizontal extent is the horizontal direction of <FIG> and this is also the unit cell width <NUM> shown in <FIG> 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> shows an illustration of a 2D array with <NUM> holographic antenna elements <NUM>, each of which may be the same as antenna <NUM> shown in <FIG>. Each holographic antenna element <NUM> may be fed from a feed network <NUM> by conventional means and with input phase controlled by a phase shifter <NUM>. This architecture allows 2D beam steering enabled by the hologram antenna element <NUM> in one dimension and the phase shifters in the second dimension, as described in references [<NUM>]-[<NUM>] above.

In a preferred embodiment each unit cell, as shown in <FIG>, <FIG> and <FIG>, of each holographic antenna element <NUM> may have the following parameters which were determined by simulation: a <NUM> unit cell length <NUM>; a <NUM> unit cell width <NUM>; an <NUM> waveguide width <NUM>; a 150mil waveguide height <NUM>; a 162mil total unit cell height <NUM>; <NUM> slot width <NUM>; <NUM> slot length <NUM>; a dielectric constant of <NUM> for the dielectric; and copper for the metal in the waveguide <NUM>, vias <NUM> and <NUM>, and bias lines <NUM>. 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 <NUM>. The transmission line <NUM> is preferred to be electrically long, meaning multiple wavelengths long. A preferred embodiment of the transmission line <NUM> may have the following characteristics: operates over a <NUM>:<NUM> frequency range (<NUM>-<NUM>), is filled with a dielectric with a dielectric constant of <NUM>, is a rectangular waveguide, has a length that is <NUM> wavelengths long at the center of the operational frequency band, or <NUM> long at <NUM>, and that is sized to have a frequency cutoff just below the bottom of the operating frequency range.

Radiating elements <NUM> are loaded periodically along the transmission line <NUM> structure and one or more tuning devices <NUM> is coupled to each radiating element <NUM>. A preferred embodiment of a radiating element is a slot <NUM> with four tuning devices <NUM>. Each tuning device <NUM> may be a single FET transistor. Any number of tuning devices <NUM> greater than one coupled to a radiating element <NUM> can provide frequency of operation reconfigurability. Increasing the number of tuning devices increases the number of tuning states that the radiating element <NUM> can achieve. An example showing four tuning devices is shown in <FIG>. Using full wave simulation, it has been found that an optimal slot length for <NUM> is <NUM> which is represented between positions A and F in <FIG>.

The effective length of the slot radiator <NUM> can be changed by switching the appropriate tuning devices <NUM> 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 <NUM> wide which resonates at <NUM>.

As seen in <FIG>, different combinations of switches create center frequencies ranging from <NUM>-<NUM>. Note that the operational bandwidth for each effective slot width is approximately +-<NUM>% of the center frequency. So, for the embodiment of <FIG>, there are frequency ranges where the antenna cannot operate efficiently. In <FIG> each slot <NUM> has four tuning devices <NUM> that are uniformly spaced across the width of the slot <NUM>. The four tuning devices <NUM> from one edge of the <NUM> wide slot are at locations <NUM>, <NUM>, <NUM>, and <NUM>.

By spacing the tuning devices non-uniformly, many more slot lengths can be achieved and thus more center frequencies can be achieved. <FIG> show that by adjusting the tuning device positions, continuous frequency of operation between <NUM>-<NUM> is provided. Again, it is noted that the operational bandwidth of a specific slot length is approximately <NUM>% of the center frequency. Therefore, <FIG> provides a preferred embodiment. In <FIG> each slot <NUM> has four tuning devices <NUM> that are non-uniformly spaced across the width of the slot <NUM>. In <FIG>, the four tuning devices <NUM> from one edge of the <NUM> wide slot are at locations <NUM>, <NUM>, <NUM>, and <NUM>.

Bias lines <NUM> provide independent voltage control for each tuning device <NUM>. The metal surrounding the slot <NUM> is the transmission line structure <NUM>, which may be at ground. The bias lines <NUM> can be brought in from a lower plane of the antenna <NUM> as shown in <FIG>, <FIG>.

A preferred embodiment uses multiple tuning devices <NUM> across the slot <NUM>, with each single one of the multiple tuning devices <NUM> being a single transistor FET switch <NUM>, as shown in <FIG> shows a switch <NUM> implemented with a field effect transistor (FET) <NUM> that has a source electrode <NUM> connected to the first side <NUM> of the slot <NUM> and a drain electrode <NUM> connected to the second side <NUM> of the slot <NUM>. The first side <NUM> and the second side <NUM> of the slot <NUM> are continuous with the waveguide <NUM>. By controlling the gate of the FET with bias line <NUM> the FET switch <NUM> may be controlled to be in the "short" or the "open" state.

At higher frequencies, the width of a slot <NUM> may be narrower and in that case it may be challenging to fit multiple single transistor FET switches <NUM> across the slot <NUM>. In such a case an integrated tuning device <NUM>, as shown in <FIG>, may be used for each slot <NUM>. The integrated tuning device <NUM> 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 <NUM> are shown in <FIG> shows a series of <NUM> transistors <NUM> 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 <NUM> are on the integrated tuning device <NUM> and connected to the transmission line structure <NUM>. The example of <FIG> also has three transistors <NUM> which are controlled by a decoder <NUM>, which decodes either a digital or analog input <NUM> to set the state of each of the transistors <NUM> to be either in a "short" state or in an "open" state across the slot <NUM>. For example, one of the transistors <NUM> may be in a "short" state, while the other two transistors <NUM> are in an "open" state. Three transistors <NUM> are shown within the multi-transistor tuning device examples of <FIG>; however, any number of transistors may be used for various applications. Also, more than one of these integrated tuning devices <NUM> may be used to control the effective width and therefore the operating frequency of a single slot <NUM>. Also, the tuning device <NUM>, shown as transistors in <FIG>, 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>; 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 <FIG>, respectively.

The preferred embodiment of the transmission line is a rectangular waveguide, as shown in <FIG>. However, other transmission line geometries may be used, such as a ridged waveguide, a coaxial waveguide, or a parallel plate, as shown in <FIG>, 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>, which is a zoomed out view of <FIG>, and this structure has been simulated at multiple frequencies. <FIG> shows the simulation results for a <NUM> center frequency. The analytic formulation is an array factor analysis that is calculated by traditional methods for antenna arrays, as described in reference [<NUM>]. <FIG> shows analytic results of a sweep of modulation period showing that the antenna <NUM> is capable of wide-angle beam steering. <FIG> shows a legend showing the different modulation periods, kp, which is the spatial domain representation of the period kp=<NUM>*pi/period.

Claim 1:
A holographic antenna (<NUM>) comprising:
a transmission line structure (<NUM>) having a traveling wave mode along a length of the transmission line structure (<NUM>); and
a plurality of reconfigurable radiating elements (<NUM>) located along the length of the transmission line structure (<NUM>);
a plurality of tuning devices (<NUM>) coupled to and arranged along at least one respective reconfigurable radiating element (<NUM>) of the plurality of reconfigurable radiating elements (<NUM>); and
a plurality of bias lines (<NUM>), wherein a respective bias line (<NUM>) is coupled to a respective tuning device (<NUM>) for controlling the respective tuning device (<NUM>) of the plurality of tuning devices to be shorted to the transmission line structure (<NUM>) or to be not shorted to the transmission line structure (<NUM>) to reconfigure the respective reconfigurable radiating element (<NUM>) to steer a radiation from the antenna in a desired direction and to tune a frequency of operation of the antenna.