The present disclosure relates to an antenna solution to address the need for a multiband, low-profile antenna for satellite and other wideband (Ku/K/Ka/Q) communications applications by using an innovative dielectric traveling wave surface waveguide array.
Commercially available Ku Band or higher frequency antenna solutions such as dish antennas are bulky and unwieldy causing significant drag. In addition, the Commercial off the Shelf (COTS) solutions require large areas of real estate, which for vehicular applications introduces high installation complexity and cost.
To address this need, we have devised a dielectric traveling wave surface wave structure that can be arranged into various types of arrays to yield a cost-effective wideband/multiband antenna that can handle high power.
The geometry of the structure consists of dielectric waveguides with scattering elements on the waveguide surface to operate in a leaky propagation mode.
In optional configurations, to scan the beam along the waveguide axis, the propagation constant of the waveguides is changed using a reconfigurable layered structure in the waveguide.
Wide bandwidth is achieved by optionally embedding chirped Bragg layered structures adjacent the reconfigurable propagation layer in the waveguide to provide equalization of scan angle over frequency. Existing materials and layer deposition processes are used to create this waveguide structure. The design uses low-loss surface wave modes and low-loss dielectric material which provide optimum gain performance which is key to handling power and maintaining efficiency.
In one implementation, an antenna includes a waveguide having a top surface, a bottom surface, a feed (excitation) end and a load end. One or more scattering features are disposed on the top surface of the waveguide or within the waveguide. The scattering features achieve operation in a leaky propagation mode.
The scattering features may take various forms. They may, for example, be a metal structure such as a rod formed on or in the waveguide. In other embodiments the scattering features may be one or more rectangular slots formed on or in the waveguide. In other embodiments the scattering features may be grooves formed in the top surface of the waveguide. The slot and/or grooves may have various shapes.
The scattering feature that provides leaky mode propagation may also be a continuous wedge. The wedge is preferably formed of a material having a higher dielectric constant than the waveguide.
The waveguide may be a dielectric material such as silicon nitride, silicon dioxide, magnesium fluoride, titanium dioxide or other materials suitable for leaky wave mode propagation at the desired frequency of operation.
The scattering feature dimensions and spacing may vary with their respective position along the waveguide. For example, adjusting the spacing of the scattering features may assist with the leaky mode coupling to waves propagating within the waveguide, allowing the waveguide to leak a portion of power along the its entire length, and improving efficiency or bandwidth.
In other embodiments, selected scattering features may be positioned orthogonally with respect to one another. This permits the antenna to operate at multiple polarizations, such as horizontal/vertical or left/right hand circular.
The scattering features can be located at each element position in an array of scattering features or may be arranged as a set of one-dimensional line arrays with the features of alternating line arrays providing different polarizations.
In still other arrangements, a wavelength correction element adds linear delay to incident energy received or transmitted by the antenna. This permits a resulting beam direction of the apparatus to be independent of the wavelength. This correction element may be formed from a set of discrete features embedded in the waveguide with a periodically modulated spacing; or it may be embodied as a material layer that tapers from a thin section at the collection end to a thick section near the detection end.
The leaky propagation mode of operation may be further enhanced by a coupling layer placed between the waveguide and the correction element. With this arrangement the coupling layer has a dielectric constant that changes from the excitation end to the load end, therefore providing increased coupling between the waveguide and the correction layer as a function of the distance along the main axis of the waveguide. This function may also be provided by a coupling layer decreasing in thickness from end to end. Such a coupling layer may equalize the horizontal and vertical mode propagation velocities in the waveguide.
In still other arrangements, the waveguide may itself be formed of two or more layers. Adjacent layers may be formed of materials with different dielectric constants. Gaps may be formed between the layers with a control element provided to adjust a size of the gaps. The gap spacing control element may be, for example, a piezoelectric, electroactive material or a mechanical position control. Such gaps may further control the beamwidth and direction.
In still other arrangements, a multilayer waveguide may provide frequency selective surfaces to assist with maintaining a constant beam shape over a range of frequencies. The spacing in such an arrangement between the layers may follow a chirp relationship.
In yet another arrangement, a layer disposed adjacent the waveguide may provide quadratic phase weighting along a primary waveguide axis. This may further assist in maintaining a constant beamwidth. The quadratic phase weight may be imposed by a layer having a thickness that tapers from end to end, or may be provided in other ways such as by subsurface elements formed within the waveguide that vary in length, spacing and/or depth from the surface.
The arrays may be combined to provide beam steering, or a single beam for multiple frequency bands, or multiple beams for a single frequency band.
In still other arrangements, the surface features may themselves be radiating elements, such as an array of patch antennas. The patch antennas may be fed through slots in a ground plane. Rows of these patch antennas may be orthogonally positioned.
The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.
A description of example embodiments follows.
Transceiver System Diagram
In a preferred embodiment herein as shown in
Traveling Leaky Wave Array
In preferred embodiments herein, much improved efficiency is provided by a waveguide structure having surface scattering features arranged in one or more subarrays.
Single line source leaky wave antennas can be used to synthesize frequency scanning beams. The array elements are excited by a traveling wave progressing along the array line. Assuming constant phase progression and constant excitation amplitude, the direction of the beam is that of Equation (1).
Cos θ=β(line)/β−(λm)/s (1)
where s is the spacing between elements, m is the order of the beam, β (line) is the leaky mode propagation constant, and β is the free space propagation constant, and λ is the wavelength. Note the frequency dependence of the direction of the beam.
The antenna uses one or more dielectric surface waveguides with one or more arrays of one-dimensional, sub-array feature (also called “rods” herein). Alternately, one large panel or “slab” of dielectric substrate can house multiple line or subarrays as will be described below.
Treating each of the subarrays as a transmitting case, the rods are excited at one end and the energy travels along the waveguide. The surface elements absorb and radiate a small amount of the energy until at the end of the rod whatever power is left is absorbed by one or more resistive loads at the load end. Operation in the receive mode is the inverse.
Scattering elements 400 disposed along each of the rods 100 can be provided by conductive strips formed on, grooves cut in the surface of, or grooves entirely embedded into, the dielectric. The cross section of the rods may be square or circular and the scattering elements may take many different forms as will be described in more detail below.
The surface wave mode of choice is HE11 which has an exponentially decreasing field outside the waveguide and has low loss. The direction of the resulting beam is stated in Equation 2:
Cos(b)=C/V−wavelength/S (2)
The dispersion of the dielectric waveguide is shown in
Line Array Implementations
As generally shown in
Individual scattering element 400 design is dependent on the choice of construction and will be described in more detail below. It suffices here to say that the scattering elements and can be provided in a number of ways, such as conducting strips or non-conducting grooves embedded into the dielectric waveguide.
Collocated elliptically polarized elements provide polarization diversity to maximize the energy captured when it is randomly polarized. In one embodiment, that shown in
The propagation constant in this “leaky wedge with waveguide” implementation of
Slab Configuration
As mentioned briefly above, groups of sub arrays can be disposed on a substrate formed as a two-dimensional panel or slab 300 as shown in
The waveguide in these slab configurations operates in a TM and TE mode in the vertical and horizontal.
The
A cladding layer (not shown) may be disposed between the main waveguide section and/or tapered core section(s). The cladding layer may be used instead of a ground plane to minimize losses at higher pressure.
This slab implementation can provide ease of manufacture and better performance by eliminating edge effects.
Also significantly, the feed end 260 of the slab 300 can take various forms shown in
The
The
These approaches have similar performance to that of other phased arrays, but with either an order of magnitude less complexity or if our adaptable-delay power divider is used, no modules.
For high power applications the multiple feed is likely preferable, while for SATCOM applications the single feed case may be more cost effective. Both approaches reduce the cost of the system when compared to a typical phased array of the same performance.
Scattering Feature (Element) Designs
There are a multitude of possible scattering element configurations that provide varying degrees of efficiency in the desired leaky mode of operation. Due to metal Ohmic heating losses and manufacturability at these sizes, it is desirable to use a dielectric groove or imprint structure. However, it is also possible to use metalized elements to capture the same effect, albeit with higher losses. The following figures show element shapes that have varying degrees of ellipticity, and/or high efficiency in a single polarization.
With all element cases, there remain two similarities. The element spacing distribution has an effect on the frequency of operation and bandwidth of the array. For each element type and bandwidth desired, the spacing of element to element is optimized. For most element types, there is a width distribution increasing along the long axis of the subarray, as mentioned above. The intention of this increasing width distribution is to couple and scatter a similar amount of energy from each element. To do this, the elements near the excitation end 260 (or feed) are narrower, so they do not scatter as much energy per unit area as the elements further down the long axis. The width distribution is adapted for example, from Rodenbeck, Christopher T., “A novel millimeter-wave beam-steering technique using a dielectric-image-line-fed grating film”, Texas A & M University, 2001, at equation 3. This width relationship is optimized for each element type to maximize array radiation efficiency.
It should be understood that surface features resulting in other types of array polarizations (such as Left/Right Hand Circular Polariation (L/RHCP) can also be utilized.
Correction Wedge
A significant challenge is the instantaneous bandwidth of the array. Equation (1) indicates that there is a shift in the beam direction as the frequency changes. This distortion is caused by the fact that all usable beams are higher order beams.
shows a one-dimensional (1-D) subarray 305 configuration with surface scattering features similar to that of
The approach to correcting frequency distortion introduced by this geometry is to situate a correcting layer 700 on top of the subarray 305. This layer, shown in
The idea behind the correction layer 700 is to linearly add increasing delay to the scattering elements from the resistive load 250 to the excitation end 260. Incident radiation enters along the top surface of the correction layer 700 and is delayed depending upon the location of incidence. When this is done properly, the quiescent delay for each element of the subarray across the top plane of the correction layer 700 is therefore the same, regardless of the position along the subarray at which the energy was received (or transmitted). The effect is that in the far-field, the beams over frequency line up at the same point.
One implementation that has been modeled indicates a TiO2 top wedge layer 700, and a lower dielectric SiO2 waveguide 100. Forming the correction wedge of a higher dielectric permits it to be “shorter” in height”. There are a multitude of materials that can be used to implement the correction wedge 700. The propagation constant of the waveguide should also be constant as a function of frequency, which is achieved by operating in the constant propagation regions of the waveguide as was shown in
Linear delay can be implemented in other ways. For a multiple rod implementation, depositing a set of wedges, such as a wedge 700 for each 1-D array would be tedious. Instead, one can fabricate a molded plastic sheet with a series of wedges. In other implementations, a TiO2 layer with top facing groves can replace the wedge to re-radiate the energy incident on the scattering elements as per
Since the wedge of
Chirped Bragg Layers to Provide Broadband Operation
Chirped Bragg layers situated underneath the waveguide structure can alter the propagation constant of the waveguide as a function of frequency. In this way, it is possible to line up beams in the far-field, making this antenna broadband.
An embodiment of an apparatus using such Frequency Selective Surfaces (FSS) 1011 shown in
Spacing of the Bragg layers 1010, 1012 can be determined as follows. An equation governing the beam angle of a traveling wave fed linear array is:
cos(theta)=beta(waveguide)/beta(air)+lambda/element spacing
where beta (waveguide) is the propagation constant of the guide.
To eliminate the frequency dependency of theta, we solve the equation for beta (waveguide). The required frequency dependency of beta can be fashioned by controlling the effective thickness of the waveguide as a function of frequency derived by using the general dispersion curve of the waveguide itself.
The effective thickness as a function of frequency is then provided by a series of chirped Bragg layers as shown in
Beamwidth Control
To further assist with controlling a beamwidth, quadratic phase weights may be added. This can be done by implementing a quadratic phase weighting along the primary axis of a 1-D array, and can be achieved with either 1) gradually tapering a dielectric layer 1050 (as shown in
The sub-surface elements within the waveguide can be varied in length, spacing, and or depth within the waveguide to obtain the desired quadratic phase weighting. Regardless, the sub surface elements are located deep enough within the waveguide so as to not radiate outside the waveguide. The weighting layer be defined by
φ(x)=eiαx
where x is the distance along the waveguide and a is a weighting constant.
Scanning and Steering
The high gain fan beams of the 1D subarrays can be steered in order to track a desired transmitter or receiver. This steering can be achieved in two ways: mechanical and electrically. The 1D tracking requirement facilitates either mechanical or electrical tracking methodologies.
Mechanical
In this approach, the leaky wave mode antenna is placed on a support that is mechanically positioned utilizing a positioner or some other mechanical means such as MEMs or electro active devices.
Electrical
In this approach, the system electrically scans the main beam by dynamically changing the volume or spacing of gaps 1022 in the dielectric waveguide. It is equivalent to changing the “effective dielectric constant,” causing more or less delay through the waveguide. The fields associated with the HE11 mode (the mode operating in the rod type waveguide) are counter propagating waves traversing across the gaps 1077 as shown in
The fields associated with the HE11 mode are counter propagating waves traversing across the gaps 1077. The propagation constant of the rod is increased by the factor K=sqrt[(1+w)/(1−w)] for small dielectric spacing w, which is equivalent to an increase in the rod's effective dielectric constant. The increase is independent of frequency as long the as the gap spacing, s, is less than ¼ wavelength. The idea is to control the gap size by using piezoelectric or electroactive actuator control elements to effect a change in the propagation constant of the rod.
Electrical scanning can be achieved by controlling the gap size by with piezoelectric, electro active, or any other suitable control element that is fast acting to effect a change in the propagation constant of the waveguide. The wedge configurations of
To achieve wideband propagation constant control, an additional chirped Bragg structure can be provided to adjust the effective rod diameter as a function of frequency.
The FSS 1011 are fixed layers of low dielectric constant material alternated with high dielectric constant material. The spacing of the layers is such that the energy is reflected where the spacing is ¼ wavelength. The higher frequencies are reflected by the layer at position P1 and the lower frequencies by the layer at position P2. The local (or specific) spacing as functions of distance along P1 to P2 is adjusted to affect a wide band equalized propagation constant value. The dispersion curve of
As an added degree of freedom, enhancing the Bragg FSS structure with reconfigurable Chirp dielectric layers 1079 (
The feed end 260 may be arranged with a single feed as per
Beam steering with a single beam in the Y-Axis Field of Regard from 0° to ±90° can be accomplished by arraying the dielectric waveguide antenna line arrays and applying a range of different phase shifts as shown in
It is possible to interleave dielectric traveling wave line arrays having different types of surface features, or of different lengths in order to accomplish two (2) different functions: Single Beams for Multiple Frequency Bands (as per
This technology is therefore not only suited for a single-band, single or multi-beam application for the Ka-band data link, but is also suited for collocated multiple bands. There is a bandwidth vs. radiation efficiency vs. surface area trade that must be heeded. Single-band, multi-aperture side-by-side arrays (such as shown in
Multi-band interleaved apertures (as per
The preferred array layout of the dielectric traveling wave line arrays is important depending upon the overall Conception of Operation (Con-Ops) for the particular system of interest. In some cases a multiple beam solution could be more advantageous than a single beam solution if switching speeds are an issue. Additionally, for single beam solutions, it could be useful to have multiple single beams for differing frequency bands as opposed to a single beam across a single frequency band.
In yet another implementation of the—array as shown in
The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.
While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
This application claims the benefit of U.S. Provisional Application No. 61/441,720, filed on Feb. 11, 2011, U.S. Provisional Application No. 61/502,260 filed on Jun. 28, 2011 and is a continuation-in-part of U.S. application Ser. No. 13/357,448, filed Jan. 24, 2012. The entire teachings of the above application(s) are incorporated herein by reference. This application also claims the benefit of U.S. Provisional Application No. 61/540,730 filed Sep. 29, 2011.
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