EP3158607B1

EP3158607B1 - Dual-polarization, circularly-polarized, surface-wave-waveguide, artificial-impedance-surface antenna

Info

Publication number: EP3158607B1
Application number: EP15810252.5A
Authority: EP
Inventors: Daniel J. Gregoire
Original assignee: HRL Laboratories LLC
Current assignee: HRL Laboratories LLC
Priority date: 2014-06-20
Filing date: 2015-06-16
Publication date: 2020-10-07
Anticipated expiration: 2035-06-16
Also published as: EP3158607A4; EP3158607A1

Description

Technical Field

This invention provides an antenna capable of dual-polarization, circularly-polarized simultaneous Right Hand Circular Polarization (RHCP) and Left Hand Circular Polarization (LHCP) operation.

Background

Linearly-polarized AIS Antennas

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 the prior art, an artificial impedance surface antenna (AISA) is formed from modulated artificial impedance surfaces (AIS). The prior art, in this regard, includes: EP 2822096 constitutes prior art under Article 54(3) EPC and discloses an apparatus (100) comprising a plurality of radiating elements (122,123) and a plurality of surface wave feeds (130). Each radiating element in the plurality of radiating elements comprises a number of surface wave channels (125) in which each of the number of surface wave channels is configured to constrain a path of a surface wave. A surface wave feed in the plurality of surface wave feeds is configured to couple a surface wave channel in the number of surface wave channels of a radiating element in the plurality of radiating elements to a transmission line (156) configured to carry a radio frequency signal.
The document entitled "Substrate Integrated Composite Right-/Left-Handed Leaky- Wave Structure for Polarization-Flexible Antenna Application", by Yuandan Dong and Tatsuo Itoh (IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO.2, FEBRUARY 2012), pages 760-771, discloses an effective development of a composite right-/left-handed (CRLH) leaky-wave (LW) structure for polarization-flexible antenna applications is presented. The proposed leaky transmission line (TL) is a planar passive circuit built using the substrate integrated waveguide technology. It consists of two symmetrical waveguide lines loaded with series interdigital capacitors which radiate orthogonal 45° linearly polarized waves. Its dispersion, Bloch impedance and radiation characteristics are extracted by applying a comprehensive analysis on the unit cell. Its backfire-to-endfire beam-steering capability through frequency scanning due to the CRLH nature is demonstrated and discussed. It is able to generate arbitrary different polarization states by changing the way of excitation, including linear polarization (LP) and circular polarization (CP). This leaky TL is fabricated by the standard printed-circuit board process. Two broadband couplers are also designed and fabricated for the specified excitation purpose. Six different polarization states, including four LP cases and two CP ones, are experimentally verified. The propagation and radiation parameters, including the S-parameters, radiation patterns, gain, and axial ratio (for CP states) are presented for these modes. Measured results are consistent with the simulation. The proposed LW structure shows some desirable merits, such as the simplicity in design, low-cost fabrication, and beam-steering and polarization-flexible capabilities, providing a high degree of flexibility for the real application.

(1) Patel (see, for example, Patel, A.M.; Grbic, A., "A Printed Leaky-Wave Antenna Based on a Sinusoidally-Modulated Reactance Surface", IEEE Transactions on Antennas and Propagation, vol. 59, no. 6, pp. 2087-2096, June 2011) demonstrated a scalar AISA using an endfire-flare-fed one-dimensional, spatially-modulated AIS consisting of a linear array of metallic strips on a grounded dielectric.
(2) Sievenpiper, Colburn and Fong (see, for example, D. Sievenpiper et al, "Holographic AISs for conformal antennas", 29th Antennas Applications Symposium, 2005 & 2005 IEEE Antennas and Prop. Symp. Digest, vol. 1B, pp. 256-259, 2005; and B. Fong et al; , "Scalar and Tensor Holographic Artificial Impedance Surfaces", IEEE TAP., 58, 2010) have demonstrated scalar and tensor AISAs on both flat and curved surfaces using waveguide-fed or dipole-fed, two-dimensional, spatially-modulated AIS consisting of a grounded dielectric topped with a grid of metallic patches.
(3) Gregoire (see, for example, D.J. Gregoire and J.S. Colburn, "Artificial impedance surface antennas", Proc. Antennas Appl. Symposium 2011, pp. 460-475; D.J. Gregoire and J.S. Colburn, "Artificial impedance surface antenna design and simulation", Proc. Antennas Appl. Symposium 2010, pp. 288-303) has examined the dependence of AISA operation on its design properties.

The basic principle of AISA operation is to use the grid momentum of the modulated AIS to match the wavevector of an excited surface-wave front to a desired plane wave. In the one-dimensional case, this can be expressed as $k_{sw} = k_{o} {sinθ}_{o} - k_{p},$
where k_o is the radiation's free-space wavenumber at the design frequency, θ _o is the angle of the desired radiation with respect to the AIS normal, k_p =2π/p is the AIS grid momentum where p is the AIS modulation period, and k_sw =n_ok_o is the surface wave's wavenumber, where n_o is the surface wave's refractive index averaged over the AIS modulation. The Surface Wave (SW) impedance is typically chosen to have a pattern that modulates the SW impedance sinusoidally along the Surface Wave Guide (SWG) according to the following equation: $Z (x) = X + M \cos (2 π x / p)$
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 $θ = \sin^{- 1} (n_{0} - λ_{0} / p)$
where n₀ is the mean SW index and λ ₀ is the free-space wavelength of radiation. n₀ is related to Z(x) by $n_{0} = \frac{1}{p} \int_{0}^{p} \sqrt{1 + Z {(x)}^{2}} dx \approx \sqrt{1 + X^{2}} .$
The AISA impedance modulation of Eqn. 2 can be generalized for an AISA of any shape as $Z (\vec{r}) = X + M \cos (k_{o} n_{o} r - {\vec{k}}_{o} \cdot \vec{r})$
where k _o is the desired radiation wave vector, r is the three-dimensional position vector of the AIS, and r is the distance along the AIS from the surface-wave source to 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 $r = \sqrt{x^{2} + y^{2}} .$
For a flat AISA designed to radiate into the wavevector at k _o = k_o (sinθ _o x̂ + cosθ _o ẑ), with the surface-wave source located at x=y=0, the modulation function is $\begin{array}{l} Z (x, y) = X + M \cos γ \\ where γ \equiv k_{0} (n_{0} r - x {sinθ}_{0}) . \end{array}$
The cos function in Eqn. 2 and Eqn. 3 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 disposed on a grounded dielectric that produces the desired index modulation 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 and Fong (see references cited above) 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 (see, for example, O. Luukkonen et al, "Simple and accurate analytical model of planar grids and high-impedance surfaces comprising metal strips or patches", IEEE Trans. Antennas Prop., vol. 56, 1624, 2008) 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°.

Circularly-polarized AIS Antennas

An AIS antenna can be made to operate with circularly-polarized (CP) radiation by using an impedance surface whose impedance properties are anisotropic. Mathematically, the impedance is described at every point on the AIS by a tensor. In a generalization of the modulation function of equation (3) for the linear-polarized AISA [4], the impedance tensor of the CP AISA may have a form like $Z = [\begin{matrix} X - M \cos ϕ \cos γ & \frac{1}{2} M \sin (γ - ϕ) \\ \frac{1}{2} M \sin (γ - ϕ) & X + M \sin ϕ \sin γ \end{matrix}];$
$where \tan ϕ \equiv \frac{y}{x} .$
In the article by B. Fong et al. identified above, the tensor impedance is realized with anisotropic metallic patches on a grounded dielectric substrate. The patches are squares of various sizes with a slice through the center of them. By varying the size of the patches and the angle of the slice through them, the desired tensor impedance of equation Eqn. 5 can be created across the entire AIS. Other types of tensor impedance elements besides the "sliced patch" can be used to create the tensor AIS.

Surface-wave waveguide AIS antennas

A variation on the AIS antennas utilizes surface-wave waveguides to confine the surface waves along narrow paths that form one-dimensional ES AISAs. Surface-wave waveguides (SWG) are surface structures that constrain surface-waves (SW) to propagate along a confined path (see, for example, D. J. Gregoire and A. V. Kabakian, "Surface-Wave Waveguides," Antennas and Wireless Propagation Letters, IEEE, 10, 2011, pp. 1512-1515). In the simplest SWG, the structure interacts with surface waves in the same way that a fiber-optic transmission line interacts with light. The physical principle is the same: the wave preferentially propagates in a region of high refractive index surrounded by a region of low refractive index. In the case of the fiber optic, or any dielectric waveguide, the high- and low-index regions are realized with high and low-permittivity materials. In the case of the SWG, the high- and low-index regions can be realized with metallic patches of varying size and/or shape on a dielectric substrate.
The surface-wave fields across the width of the SWG are fairly uniform when the width of the SWG is less than approximately ¾ surface-wave wavelength. So, this is a good rule of thumb for the SWG.
In a linearly-polarized SWG AISA, the impedance of the SWG varies according to equation Eqn. 2. The impedance elements can be square patches of metal on the substrate or they can be strips that span the width of the SWG. The desired impedance modulation is created by varying the size of the impedance element dimensions with position.
In a circularly-polarized SWG, the tensor impedance varies according to equation Eqn. 5 with φ = 0. The impedance elements can be the sliced patches as described by B. Fong et al. (see the B. Fong et al. article referenced above). The impedance element dimensions are varied with position to achieve the desired impedance variation.

Brief description of the Invention

In one aspect the present invention provides a dual-polarization, circularly-polarized artificial-impedance-surface antenna comprising: (1) two adjacent tensor surface-wave waveguides (SWGs); (2) a waveguide feed coupled to each of the two SWGs; (3) a hybrid coupler (which is preferably a 90° coupler) having output ports, each output port of the hybrid coupler being connected to the waveguide feeds coupled to the two SWGs, the hybrid coupler being configured to combine the signals from first and second input ports of the hybrid coupler with phase shifts at its output ports, such that a first signal connected to the first input port is transmitted or received with RHCP polarization while a second signal connected to the second input port simultaneously is transmitted or received with LHCP polarization and front-end electronics arranged for transmitting or receiving said first and second signals; the first signal being independent from the second signal, characterized in that the first and second tensor surface-wave waveguides include a dielectric sheet substrate having a ground plane on a bottom surface thereof and the first and second tensor surface-wave waveguides further include tensor impedance elements which comprise metallic strips or patches disposed in an elongated array on a top surface of the dielectric sheet substrate.
In another aspect the present invention provides a method of simultaneously transmitting two oppositely handed circularly polarized RF signals comprising the steps of: (i) providing a dielectric surface with a ground plane on one side there of and with a pair of elongate artificial impedance surface antennas, each of said artificial impedance surface antennas including a pattern of metallic geometric stripes or shapes disposed on said dielectric surface, the metallic geometric stripes or shapes having varying sizes which form a repeating moire pattern, the moire patterns of the each of said pair of elongate artificial impedance surface antennas having a angular relationship with reference to a major axis of said pair of elongate artificial impedance surface antennas, a first one of said pair of elongate artificial impedance surface antennas having a positive angular relationship to said major axis and second one of said pair of elongate artificial impedance surface antennas having a negative angular relationship to said major axis; and (ii)applying RF energy to said pair of elongate artificial impedance surface antennas, said RF energy applied to said pair of elongate artificial impedance surface antennas having different relative phases selected such that RF signals transmitted by said pair of elongate artificial impedance surface antennas is circularly polarized.
In yet another aspect the present invention provides a method of simultaneously receiving two oppositely handed circularly polarized RF signals comprising the steps of: (i) sending the signals received by two SWGs into two input ports of a 3dB 90 degree hybrid coupler, the coupler also having two output ports; and (ii) extracting LHCP and RHCP signals from the output two ports of the hybrid coupler.

Brief Description of the Drawings

Fig. 1a is top view of one embodiment of the present invention disposed on a printed circuit broad while Fig. 1b is a side elevational view thereof.
Fig. 2 is a schematic view of another embodiment of a SWG which may be used with the present invention.
Fig. 3 is a schematic view of yet another embodiment of a SWG which may be used with the present invention.

Detailed Description

This invention provides a solution for a dual-polarization, circularly-polarized AISA with simultaneous Right Hand Circular Polarization (RHCP) and Left Hand Circular Polarization (LHCP) operation.
Referring to Figs. 1a and 1b, one possible embodiment of the invention includes a pair of linearly-polarized SWGs 101 and 102 to form the AISA. The polarization of the two SWGs 101, 102 is preferably rotated by 90° with respect to each other. The SWGs 101, 102 are connected to ports C and D of a 3-dB 90° hybrid coupler 103, the operation of which is well understood in the state of the art (see, for example, www.microwaves101.com/encyclopedia/ hybridcouplers.cfm). The signals at ports C and D are the sum of the signals at ports A and B with preferably either a 90° or a -90° phase shift between them, respectively. The combination of the radiation from the two SWGs 101, 102 with the 90° rotation in polarization and the 90° separation in phase results in circularly polarized radiation. It is well known that circularly polarized radiation can be created by combining radiation from two antennas with orthogonal polarization with a 90° phase shift between them. The signal connected to port A is transmitted or received with RHCP polarization while the signal connected to port B simultaneously is transmitted or received with LHCP polarization. Transmit-Receive (TR) switches 104 enable independent operation of each polarization in transmit or receive modes depending on the positions of switches 104. The two channels are processed in receive mode by conventional front-end electronics 105 and the two channels are provided in transmit mode with transmit signals again by conventional front-end electronics 105. The conventional front-end electronics 105 may be embodied in or by a transceiver with dual inputs (R1 and R2) and dual outputs (T1 and T2) or in or by separate transmitters and receivers or in or by a RF transmit/receive module.
Each of the SWGs 101, 102 is a linear array of tensor impedance elements 106 that radiate with a polarization preferably at a ±45° angle to the polarization of the SW electric field (in the x axis labeled in Fig. 1, the x axis also being the major axis or axis of common elongation of the two SWGs 101, 102). The tensor elements 106 are preferably metallic shapes printed or otherwise formed on the top surface of a dielectric substrate 109 which preferably has a ground plane 111 disposed the opposing (underside) surface of the dielectric substrate 109. The metallic shapes can be stripes as shown in Figs. 1a and 2, or they can be slit squares as shown in Fig. 3. Other electrically conductive shapes can alternatively be utilized as the tensor impedance elements 106 if desired. A ground potential associated with front-end electronics 105 is coupled with the ground plane 111 on bottom side of the dielectric substrate 109. The SWGs 101, 102 should preferably be spaced apart a sufficient distance so that the fields adjacent the SWGs do not couple with each other. In practice the separation distance between SWGs 101, 102 is preferably at least ¼λ.
The SWGs 101, 102 include metallic strips or patches disposed in an elongated array on a top surface of a dielectric sheet, the dielectric sheet having a ground plane on a bottom surface thereof. The tensor impedance elements 106 can be provided by metallic stripes disposed on a top side of the dielectric substrate 109 where the tensor impedance elements 106 in one channel are angled preferably at +45° with respect to the x axis, and the tilt angle of the stripes in the other channel is set to -45° with respect to that same axis. This variation in tilt angle produces radiation of different linear polarization, that when combined with a 90° phase shift via the 90° hybrid 103, produces circularly polarized radiation in transmit mode or allow reception of circularly polarized radiation in receive mode. The impedance elements could also be square patches with slices through them as described in B. Fong et al; , "Scalar and Tensor Holographic Artificial Impedance Surfaces ", noted above. Such an embodiment is depicted by Fig. 3.
The dielectric substrate 109 may preferably be made from Printed Circuit Board (PCB) material which has a metallic conductor (such as copper) disposed preferably on both of its major surfaces, the metallic conductor on the top or upper surface being patterned using conventional PCB fabrication techniques to define the aforementioned tensor impedance elements 106 from the metallic conductor originally formed on the upper surface of the PCB. The metallic conductor formed on the lower surface of the PCB would then become the ground plane.
In transmit operation, the front-end electronics 105 sends two independent signals from its transmit channels (T1 and T2) to the transmit connections of the two TR switches 104. The TR switches 104 send the two transmit signals to ports A and B of the 90° hybrid coupler 103. If the voltages at ports A and B are V_A and V_B , then the voltages V_C and V_D at ports C and D are $({iV}_{A} + V_{B}) / \sqrt{2}$
and $(V_{A} + {iV}_{B}) / \sqrt{2},$
respectively where $i = \sqrt{- 1}$
and represents a 90° phase shift.
The signals from ports C and D of the 90° hybrid coupler 103 pass through optional coaxial cables 110 to end launch Printed Circuit Board (PCB) connectors 107 which are connected to surface-wave (SW) feeds 108. The coaxial cables 110 and connectors 107 may be omitted if coupler 103 is connected directly the SW feeds 108, for example. If coaxial cables 110 are utilized, then their respective center conductors are connected to the SW feeds 108 while their shielding conductor are connected to the ground plane 111. Instead of using coaxial cables 110 to connect outputs of the coupler 103 to the feeds 108, a link between the two can alternatively be provided by rectangular waveguides, microstrips, coplanar waveguides (CPWs), etc. The SW feeds 108 preferably have a 50 Ω impedance at the end that connects to coupler 103 via the end-launch connector 107 (if utilized). The SW feed 108 flares from one end, preferably in an exponential curve, until its width matches the width of the SWGs 101, 102. The SW feeds 108 launch surface waves with a uniform field across their wide ends into the SWGs 101, 102. The SW feeds 108 are preferably formed using the same techniques to form the tensor impedance elements 106 (this is, by forming them from them the metallic conductor found on a typical PCB). The widths of the SWGs 101, 102 is preferably between 1/8 to 2 wavelengths of an operational frequency (or frequencies) of the SWGs 102, 102.
The SWGs 101, 102 are preferably composed of a series of metallic tensor impedance elements 106 whose sides are preferably angled at 45° or having angled slices as in the embodiment of Fig. 3 with respect to the SWG axis (the x-axis in Fig. 1) as noted above. The slices are angled at ±45° with respect to the major axis of the SWGs 101, 102 axis so that the impedance tensor's principal axis is aligned with the slice. It should be noted that series of metallic tensor impedance elements 106 with angled slices or sides could be angled at some other angle than ±45° with respect to the SWG axis (the x-axis in Fig. 1), but in that case the hybrid coupler 103 has to have a phase shift that is different from 90 degrees at its outputs. Such a hybrid coupler 103 is not believed to be commercially available, so it would be a custom designed coupler, but such a coupler could designed and made if desired. So the angles of ±45° with respect to the SWG axis (the x-axis in Fig. 1) set for the angles of the metallic tensor impedance elements 106 (or the angles of the slices or sides of the as in the embodiment of Fig. 3) is preferred as those angles are believed to be compatible with commercially available hybrid couplers for element 103.
The widths of the individual metallic tensor impedance elements 106 are typically much narrower than the widths of the SWGs 101, 102 which they form. In Fig. 1 the widths of the individual metallic tensor impedance elements 106 averages about 1/7th of the width of the SWGs 101, 102. Typically, the individual metallic tensor impedance elements 106 will be spaced by 1/20 to 1/5 of a wavelength apart from each other along the length of the SWGs 101, 102. The width of the individual metallic tensor impedance elements 106 determines the SW propagation impedance locally along the SWG. The width of the tensor impedance elements 106 varies with distance along the SWG such that the SW impedance is modulated according to equation (Eqn. 2), in order to have the radiation pattern directed at an angle θ determined by equation (Eqn. 3) with respect to the z axis in the x-z plane noted on Fig. 1. This variation in the widths of the tensor impedance elements 106 can be seen in Fig. 1 as a noticeable moire pattern caused by the changing widths of the tensor impedance elements 106. This pattern repeats itself continuously along the length of the SWG, no matter how long the SWG is. The length of the SWG 101, 102 will depend on a number of factors related to the antenna's engineering parameters, such as desired radiation beam width, gain, instantaneous bandwidth, aperture efficiency, etc. Typically the length of the SWGs 101, 102 will fall in the range of 2 to 30 wavelengths at the operational frequency of the SWGs 101, 102.
The relation between the impedance-element geometry (e.g. the strip width) and the SW impedance is well understood. See the papers by Patel, Sievenpiper, Colburn, Fong and Gregoire identified above.
The metallic tensor impedance elements 106 in SWG 101 are angled in a direction opposite to the tensor impedance elements 106 in the other SWG 102. The radiation from the two SWGs will be polarized in the direction across the gaps between the strips. Therefore, the radiation from the two SWGs 101, 102 depicted by Fig. 1 will be orthogonal to each other. When the 90° phase shift difference is applied to the feeds 108 with the hybrid power splitter 103, the net radiation from the combination of the two SWGs 101, 102 is circularly polarized. However, as noted above other angles (then 45°) for the metallic tensor impedance elements 106 relative to the x-axis can be utilized if a custom designed coupler 103 is employed and still the resulting polarization will be polar.
The radiation from each SWG 101, 102 is polarized as it is because the slanted metallic strips are tensor impedance elements 106 whose major principal axis is perpendicular to the long edge of the strips and the minor axis is along them. The local tensor admittance of the SWG in the coordinate frame of the principal axes is $Y_{sw} = [\begin{matrix} Y (x) & 0 \\ 0 & 0 \end{matrix}]$
where Y(x) is determined by the voltage applied to the metallic strips at position x. Then the SW current is $J_{sw} = Y_{sw} E_{sw} = [\begin{matrix} Y (x) & 0 \\ 0 & 0 \end{matrix}] E_{sw} [\begin{matrix} 1 \\ 1 \end{matrix}] / \sqrt{2} = E_{sw} / \sqrt{2} [\begin{matrix} 1 \\ 0 \end{matrix}]$
which is along the major principal axis that is perpendicular to the long edge of the strips forming the tensor impedance elements 106. The radiation is driven by the SW currents according to $E_{rad} \propto [\int [\{\hat{k} \times J_{sw}\} \times \hat{k}] e^{- i k \cdot r'} dx] e^{i k \cdot r}$
and is therefore polarized in the direction across the gaps between the strips.
The preferred embodiment for a 12 GHz version of a radiating element of the invention is shown in Fig. 1. Everything is scaled to a free-space wavelength at 12 GHz is λ₀ =2.5 cm ≅1.0". The SWGs 101 and 102 are preferably ½ λ₀ wide. The exponentially-tapered, surface-wave feeds 108 are preferably 2 λ₀ long. The period of the tensor impedance elements 106 ≅ 1/12 λ₀ .
Fig. 2 illustrates a preferred embodiment where an RF feed assembly 108 is also disposed at the other of the SWGs with RF terminators 201 attached to the end. This prevents the surface-wave from reflecting off the end of the AISA which could lead to unwanted distortion in the radiation pattern.
Broadly, this writing discloses at least the following:
A dual-polarization, circularly-polarized artificial-impedance-surface antenna has two adjacent tensor surface-wave waveguides (SWGs), a waveguide feed coupled to each of the two SWGs and a hybrid coupler having output ports, each output port of the hybrid coupler being connected to the waveguide feeds coupled to the two SWGs, the hybrid coupler, in use, combining the signals from input ports of the 90° hybrid coupler with phase shifts at its output ports.

Claims

A dual-polarization, circularly-polarized artificial-impedance-surface antenna comprising:
(1) a first tensor surface-wave waveguide (101) adjacent a second tensor surface-wave waveguide (102);

(2) a first waveguide feed (108) coupled to the first surface-wave waveguide (101) and a second waveguide feed (108) coupled to the second surface-wave waveguide (102);

(3) a hybrid coupler (103) having a first (C) output port connected to the first waveguide feed (108) and a second (D) output port connected to the second waveguide feed (108), the hybrid coupler being configured to combine the signals from first (A) and second (B) input ports of the hybrid coupler (103) with phase shifts at its output ports such that a first signal connected to the first input port (A) is transmitted or received with RHCP polarization while a second signal connected to the second input port (B) simultaneously is transmitted or received with LHCP polarization; and
front end electronics (105) arranged for transmitting or receiving said first and second signals; the first signal being independent from the second signal, characterized in that the first and second tensor surface-wave waveguides (101, 102) include a dielectric sheet substrate (109) having a ground plane (111) on a bottom surface thereof and the first and second tensor surface-wave waveguides (101, 102) further include tensor impedance elements which comprise metallic strips (106) or patches (106) disposed in an elongated array on a top surface of the dielectric sheet substrate (109).
The antenna of claim 1 wherein the surface-wave waveguides (101, 102) are disposed on a common substrate (109).
The antenna of claim 1 or 2 wherein tensor impedance elements (106) on adjacently disposed surface-wave waveguides (101, 102) have principal axes of their impedance tensors rotated 90° with respect to each other and wherein the hybrid coupler is a 90° hybrid coupler.
The antenna of any one of the preceding claims:
wherein the first and second surface-wave waveguides (101, 102) are elongated and each have a width which is between 1/8 to 2 wavelengths of an operational frequency of the surface-wave waveguides (101, 102) and have a length which is between 2 and 30 wavelengths of said operational frequency of the surface-wave waveguides (101, 102); or
wherein the first and second surface-wave waveguides (101, 102) include tensor impedance elements (106) that are spaced with a period of 1/20 to 1/5 wavelength apart from each other along the length of the surface-wave waveguides (101, 102).
The antenna of any one of claims 1 through 3 wherein said metallic strips (106) of each of the surface-wave waveguides (101,102) are slanted at an angle with respect a common direction of elongation of the surface-wave waveguides (101, 102).
The antenna of claim 5 wherein said metallic strips (106) are disposed at 45° angle with respect to said common direction of elongation of the surface-wave waveguides (101, 102).
The antenna of claim 6 wherein said metallic strips (106) in one surface-wave waveguide (101, 102) are disposed at 90° angle with respect said metallic strips (106) in the other surface-wave waveguide (102, 101).
The antenna of claim 7 wherein said metallic strips (106) are distributed along a length of each surface-wave waveguide (101, 102).
The antenna of any one of claims 1 through 3 wherein said tensor impedance elements of the surface-wave waveguides (101, 102) are configured by their shape to produce a modulated impedance pattern according to $Z (x) = X + M \cos (2 nx / p)$
where p is the period of the modulation, X is the mean impedance, and M is the modulation amplitude; X, M and p can be tuned such that the angle of the radiation Θ in the x-z plane with respect to the z axis is scanned according to $Θ = \sin^{- 1} (n_{0} - λ_{0} / p)$
where n₀ is the mean SW index, and λo is the free-space wavelength of radiation and no is related to Z(x) by $n_{0} = \frac{1}{p} \int_{0}^{p} \sqrt{1 + Z {(x)}^{2}} dx \approx \sqrt{1 + X^{2}}$
The antenna of any one of claims 1 to 4 and 9, wherein said patches of the surface-wave waveguides (101, 102) are formed with slices through them and wherein said slices are angled at 45° with respect to a major axis of the surface-wave waveguides (101, 102) so as to form an impedance tensor having an impedance tensor principal axis which is aligned with said slices.
The antenna of claim 1, comprising a transmit-receive switch (104) between each of the first (A) and second (B) input ports of the hybrid coupler (103) and the front end electronics (105), the front-end electronics (105) comprising one of: a transceiver with dual inputs, R1 and R2 and dual outputs, T1 and T2; separate transmitters and receivers; and a RF transmit/receive module; said switches (104) being arranged to enable independent operation of each of the RHCP and LHCP polarizations in transmit or receive modes depending on the positions of switches (104).
A method of simultaneously transmitting a first and a second oppositely handed circularly polarized RF signals using the antenna of any one of claims 1 to 10; the method comprising the steps of:
applying the first RF signal to the first input of the hybrid coupler (103) and applying the second RF signal to the second input of the hybrid coupler (103).
A method of simultaneously receiving a first and a second oppositely handed circularly polarized RF signals using the antenna of any one of claims 1 to 10; the method comprising the steps of: sending a first signal received by the first and second surface-wave waveguides (101, 102) into the output ports of the coupler (103), and sending a second signal received by the first and second surface-wave waveguides (101, 102) into the output ports of the coupler (103); and extracting Left Hand Circular Polarization and Right Hand Circular Polarization signals from the first and second input ports of the hybrid coupler (103).