Patent Application
20150009070
This disclosure relates to artificial impedance surface antennas (AISAs).
An antenna whose primary gain lobe can be electronically steered in two dimensions is desirable in many applications. In the prior art the two dimensional steering is most commonly provided by phased array antennas. Phased array antennas have complex electronics and are quite costly.
In the prior art, various electronically steered artificial impedance surface antennas (AISAs) have been described that have one dimensional electronic steering, including the AISAs described in U.S. Pat. Nos. 7,245,269, 7,071,888, and U.S. Pat. No. 7,253,780 to Sievenpiper. These antennas are useful for some applications, but are not suitable for all applications that need two dimensional steering. In some applications mechanical steering can be used to provide steering of a 1D electronically steered antenna in a second dimension. However, there are many applications where mechanical steering is very undesirable. The antennas described by Sievenpiper also require vias for providing voltage control to varactors.
A two dimensionally electronically steered AISA has been described in U.S. Pat. No. 8,436,785, issued on May 7, 2013, to Lai and Colburn. The antenna described by Lai and Colburn is relatively costly and is electronically complex, because to steer in two dimensions a complex network of voltage control to a two dimensional array of impedance elements is required so that an arbitrary impedance pattern can be created to produce beam steering in any direction.
Artificial impedance surface antennas (AISAS) are realized by launching a surface wave across an artificial impedance surface (AIS), whose impedance is spatially modulated across the AIS according a function that matches the phase fronts between the surface wave on the AIS and the desired far-field radiation pattern.
In previous references, listed below, references [1]-[6] describe artificial impedance surface antennas (AISA) formed from modulated artificial impedance surfaces (AIS). Patel [1] demonstrated a scalar AISA using an end-fire, flare-fed one-dimensional, spatially-modulated AIS consisting of a linear array of metallic strips on a grounded dielectric. Sievenpiper, Colburn and Fong [2]-[4] have demonstrated scalar and tensor AISAs on both flat and curved surfaces using waveguide- or dipole-fed, two-dimensional, spatially-modulated AIS consisting of a grounded dielectric topped with a grid of metallic patches. Gregoire [5]-[6] has examined the dependence of AISA operation on its design properties.
Referring to
k
sw
=k
o sin θo−kp (1)
where ko is the radiation's free-space wavenumber at the design frequency, θo is the angle of the desired radiation with respect to the AIS normal, kp=2π/p is the AIS grid momentum where p is the AIS modulation period, and ksw=noko is the surface wave's wavenumber, where no is the surface wave's refractive index averaged over the AIS modulation. The SW impedance is typically chosen to have a pattern that modulates the SW impedance sinusoidally along the SWG according to
Z(x)=X+M cos(2πx/p) (2)
where p is the period of the modulation, X is the mean impedance, and M is the modulation amplitude. X, M and p are chosen such that the angle of the radiation θ in the x-z plane w.r.t the z axis is determined by
θ=sin−1(n0−λ0/p) (3)
where n0 is the mean SW index, and λ0 is the free-space wavelength of radiation. n0 is related to Z(x) by
The AISA impedance modulation of Eqn. (2) can be generalized for an AISA of any shape as
Z({right arrow over (r)})=X+M cos(konor−{right arrow over (k)}o□{right arrow over (r)}) (5)
where {right arrow over (k)}o is the desired radiation wave vector, {right arrow over (r)} is the three-dimensional position vector of the AIS, and r is the distance along the AIS from the surface-wave source to {right arrow over (r)} along a geodesic on the AIS surface. This expression can be used to determine the index modulation for an AISA of any geometry, flat, cylindrical, spherical, or any arbitrary shape. In some cases, determining the value of r is geometrically complex.
For a flat AISA, it is simply r=√{square root over (x2+y2)}.
For a flat AISA designed to radiate into the wave vector at {right arrow over (k)}=ko(sin θo{circumflex over (x)}+cos θo{circumflex over (z)}), with the surface-wave source located at x=y=0, the modulation function is
Z(x,y)=X+M cos(ko(nor−x sin θo)) (6)
The cos function in Eqn. (2) can be replaced with any periodic function and the AISA will still operate as designed, but the details of the side lobes, bandwidth and beam squint will be affected.
The AIS can be realized as a grid of metallic patches on a grounded dielectric. The desired index modulation is produced by varying the size of the patches according to a function that correlates the patch size to the surface wave index. The correlation between index and patch size can be determined using simulations, calculation and/or measurement techniques. For example, Colburn [3] and Fong [4] use a combination of HFSS unit-cell eigenvalue simulations and near field measurements of test boards to determine their correlation function. Fast approximate methods presented by Luukkonen [7] can also be used to calculate the correlation. However, empirical correction factors are often applied to these methods. In many regimes, these methods agree very well with HFSS eigenvalue simulations and near-field measurements. They break down when the patch size is large compared to the substrate thickness, or when the surface-wave phase shift per unit cell approaches 180°.
In the prior art electronically-steerable AIS antennas described in [8] and [9], the AIS is a grid of metallic patches on a dielectric substrate. The surface-wave impedance is locally controlled at each position on the AIS by applying a variable voltage to voltage-variable varactors connected between each of the patches. It is well known that an AIS's SW impedance can be tuned with capacitive loads inserted between impedance elements [8], [9]. Each patch is electrically connected to neighboring patches on all four sides with voltage-variable varactor capacitor. The voltage is applied to the varactors though electrical vias connected to each impedance element patch. Half of the patches are electrically connected to the groundplane with vias that run from the center of each patch down through the dielectric substrate. The rest of the patches are electrically connected to voltage sources that run through the substrates, and through holes in the ground plane to the voltage sources.
Computer control allows any desired impedance pattern to be applied to the AIS within the limits of the varactor tunability and the AIS SW property limitations. One of the limitations of this method is that the vias can severely reduce the operation bandwidth of the AIS because the vias also impart an inductance to the AIS that shifts the SW bandgap to lower frequency. As the varactors are tuned to higher capacitance, the AIS inductance is increased and this further reduces the SW bandgap frequency. The net result of the SW bandgap is that it does not allow the AIS to be used above the bandgap frequency. It also limits the range of SW impedance that the AIS can be tuned to.
What is needed is an electronically steered artificial impedance surface antenna (AISA) that can be steered in two dimensions, while being lower cost. The embodiments of the present disclosure answer these and other needs.
In a first embodiment disclosed herein, a steerable artificial impedance surface antenna steerable in phi and theta angles comprises dielectric substrate, a plurality of metallic strips on a first surface of the dielectric substrate, the metallic strips spaced apart across a length of the dielectric substrate and each metallic strip extending along a width of the dielectric substrate, and surface wave feeds spaced apart along the width of the dielectric substrate near an edge of the dielectric substrate, wherein the dielectric substrate is substantially in an X-Y plane defined by an X axis and a Y axis, wherein the phi angle is an angle in the X-Y plane relative to the X axis, and wherein the theta angle is an angle relative to a Z axis orthogonal to the X-Y plane.
In another embodiment disclosed herein, a steerable artificial impedance surface antenna steerable in phi and theta angles comprises a dielectric substrate, a plurality of metallic strips on a first surface of the dielectric substrate, the metallic strips spaced apart across a length of the dielectric substrate, the metallic strips having equally spaced centers, the metallic strips varying in width with a period of p, and each metallic strip extending along a width of the dielectric substrate, and surface wave feeds spaced apart along a width of the dielectric substrate near an edge of the dielectric substrate, wherein the dielectric substrate is substantially in an X-Y plane defined by an X axis and a Y axis, wherein the phi angle is an angle in the X-Y plane relative to the X axis, and wherein the theta angle is an angle relative to a Z axis orthogonal to the X-Y plane.
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 electronically steered artificial impedance surface antenna (AISA) of
The artificial impedance surface antenna (AISA) 101 in the embodiment of
The voltage control network 102 applies direct current (DC) voltages to the metallic strips 107 on the AISA structure. Control bus 105 provides control for the voltage control network 102. The control bus 105 may be from a microprocessor, central processing unit, or any computer or processor.
Control bus 104 provides control for the 1D RF feed network 103. The control bus 104 may be from a microprocessor, central processing unit, or any computer or processor.
A varactor is a type of diode whose capacitance varies as a function of the voltage applied across its terminals, which makes it useful for tuning applications. When varactors 109 are used between the metallic strips 107, as shown in
The polarities of the varactors 109 are aligned such that all the varactor connections to any one of the metallic strips 107 are connected with the same polarity. One terminal on a varactor may be referred to as an anode, and the other terminal as a cathode. Thus, some of the metallic strips 107 are only connected to anodes of varactors 109, and other metallic strips 107 are only connected to cathodes of varactors 109. Further, as shown in
The spacing of the metallic strips 107 in one dimension of the AISA, which may, for example, be the X axis of
The spacing between varactors 109 connected to the metallic strips 107 in a second dimension of the AISA, which is generally orthogonal to the first dimension of the AISA and which may be the Y axis of
The RF SW feeds 108 may be a phased array corporate feed structure, or may be conformal surface wave feeds, which are integrated into the AISA, such as by using micro-strips. Conformal surface wave feeds that may be used include those described in U.S. patent application Ser. No. 13/242,102 filed Sep. 23, 2011, or those described in “Directional Coupler for Transverse-Electric Surface Waves”, published in IP.com Prior Art Database Disclosure No. IPCOM000183639D, May 29, 2009, which are incorporated herein by reference as though set forth in full.
The spacing between the RF SW feeds 108 in the second dimension of the AISA or the y dimension of
The thickness of the dielectric substrate 106 is determined by its permittivity and the frequency of radiation to be transmitted or received. The higher the permittivity, the thinner the substrate can be.
The capacitance values of the varactors 109 are determined by the range necessary for the desired AISA impedance modulations to obtain the various angles of radiation.
An AISA operating at about 10 GHz may use for the dielectric substrate 106, a 50-mil thick Rogers Corp 3010 circuit board material with a relative permittivity equal to 11.2. The metallic strips 107 may be spaced 2 millimeters (mm) to 3 mm apart on the dielectric substrate 106. The RF surface wave feeds 108 may be spaced 1.5 centimeters (cm) apart and the varactors 109 may be spaced 2 mm to 3 mm apart. The varactors 109 vary in capacitance from 0.2 to 2.0 pico farads (pF). Designs for different radiation frequencies or designs using different substrates will vary accordingly.
To transmit or receive an RF signal, transmit/receive module 110 is connected to the feed network 103. The feed network 103 can be of any type that is known to those skilled in the state of the art of phased array antennas. For the sake of illustration, the feed network 103 shown in
The antenna main lobe is steered in the phi direction by using the feed network 103 to impose a phase shift between each of the RF SW feeds 108. If the RF SW feeds 108 are spaced uniformly, then the phase shift between adjacent RF SW feeds 108 is constant. The relation between the phi (φ) steering angle, and the phase shift may be calculated using standard phased array methods, according to equation,
φ=sin−1(λΔψ/2πd) (7)
where λ is the radiation wavelength, d is the spacing between SW feeds 108, and Δψ is the phase shift between SW feeds 108. The RF SW feeds 108 may also be spaced non-uniformly, and the phase shifts adjusted accordingly.
The antenna lobe is steered in the theta (θ) direction by applying voltages to the varactors 109 between the metallic strips 107 such that AISA 101 has surface-wave impedance Zsw, that is modulated or varied periodically with the distance (x) away from the SW feeds 108, according to equation,
Z
sw
=X+M cos(2πx/p) (8)
where X and M are the mean impedance and the amplitude of its modulation respectively, and p is the modulation period. The variation of the surface-wave impedance Zsw may be modulated sinusoidally. The theta steering angle θ, is related to the impedance modulation by the equation,
θ=sin−1(nsw−λ/p) (9)
where λ is the wavelength of the radiation, and
n
sw=√{square root over ((X/377)2+1)} (10)
is the mean surface-wave index.
The beam is steered in the theta direction by tuning the varactor voltages such that X, M, and p result in the desired theta θ. The dependence of the surface wave (SW) impedance on the varactor capacitance is calculated using transcendental equations resulting from the transverse resonance method or by using full-wave numerical simulations.
In the embodiment of
An advantage of grounding half of the metallic strips 107 is that only half as many voltage control lines 116 are required as there are metallic strips 107. A disadvantage is that the spatial resolution of the voltage control and hence the impedance modulation is limited to twice the spacing between metallic strips.
The antenna main lobe is steered in the phi direction by using the feed network 203 to impose a phase shift between each of the RF SW feeds 208 in the same manner as described with reference to
One or more varactors diodes 309 may be in each gap between adjacent metallic strips 340 and electrically connected to the metallic strips in the same manner as shown in
The metallic strips may have centers that are equally spaced in the x dimension, with the widths of the metallic strips 340 periodically varying with a period p 346. The number of metallic strips in a period 346 can be any number, although 10 to 20 is reasonable for most designs. The width variation is designed to produce surface-wave impedance with a periodic modulation in the X-direction with period p 346, for example, the sinusoidal variation of equation (8) above.
The surface-wave impedance at each point on the AISA is determined by the width of the metallic strips and the voltage applied to the varactors 309. The relation between the surface-wave impedance and these parameters is well understood and documented in the references [1]-[9].
The capacitance of the diode varactors 309 varies with the applied voltage. When the voltage is 0 volts, the diode capacitance is at its maximum value of Cmax. The capacitance decreases as the voltage is increased until it reaches a minimum value of Cmin. As the diode capacitance is varied, the impedance modulation parameters, X and M in Eqn. (8) vary also from minimum values Xmin and Mmin to maximum values of Xmax and Mmax. Likewise, the mean surface-wave index of Eqn. (10) varies from nmin=√{square root over ((Xmin/377)2+1)} to nmax=√{square root over ((Xmax/377)2+1)}.
Then from Eqn. (9), the range that the AISA's radiation angle can be scanned varies from a minimum of
θmin=sin−1(nmin−λ/p) (11)
to a maximum of
θmax=sin−1(nmax−λ/p) (12)
with variation of a single control voltage.
In another embodiment shown in the elevation view of
In a variation on this, shown in the elevation view of
The antenna main lobe is steered in the phi direction by using the feed network 303 to impose a phase shift between each of the RF SW feeds 308 in the same manner as described with reference to
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 . . . . ”
This application is related to the disclosure of U.S. patent application Ser. No. 12/939,040 filed Nov. 3, 2010, and U.S. patent application Ser. No. 13/242,102 filed Sep. 23, 2011, the disclosures of which are hereby incorporated herein by reference.
