Reconfigurable antenna for multiple band, beam-switching operation

[0002] This invention relates to a reconfigurable antenna array system, and includes an array of dipole antenna elements disposed on a multiple band high impedance surface. The antenna array is configured by changing the resonant frequency of the individual dipoles that constitute the array. At a given frequency band, small changes in the dipoles resonant frequencies allow for the antenna array to be configured so that the reflected radiation forms a beam in the far-field, and can be pointed to selected directions. Larger changes in the dipoles resonant frequencies allow for shifting from one operating frequency band to a different band. This invention has particular applications in satellite radar and airborne communication node (ACN) systems where a wide bandwidth is important and the aperture must be continually reconfigured for various functions. Additionally, this invention has applications in the field of terrestrial high frequency wireless systems.

[0003] The prior art includes U.S. Pat. No. 4,905,014 to Daniel G. Gonzalez, Gerald E. Pollen, and Joel F. Walker, “Microwave passing structure for electromagnetically emulating reflective surfaces and focusing elements of selected geometry”. This patent describes placing antenna elements above a planar metallic reflector for phasing a reflected wave into a desired beam shape and location. It is a flat array that emulates differently shaped reflective surfaces (such as a dish antenna). However it does not disclose a system that is reconfigurable and can operate at multiple frequency bands.

[0004] The prior art includes U.S. Pat. No. 5,541,614 to Juan F. Lam, Gregory L. Tangonan, and Richard L. Abrams, “Smart antenna system using microelectromechanically tunable dipole antennas and photonic bandgap materials”. This patent shows how to use RF MEMS (Micro Electro-Mechanical Switches) and bandgap photonic surfaces for reconfigurable dipoles. Although this invention lists a number of reconfigurable dipole antenna architectures, it does not disclose the dipole reflector antenna, and it does not show how to use multiple band, high impedance surfaces (a sub-class of photonic bandgap material). Furthermore, in the present invention the dipole array is fed from free space rather than a transmission line.

[0005] The present invention also relates to U.S. patent application Ser. No. 09/537,923 entitled “A tunable impedance surface” filed on Mar. 29, 2000 and to U.S. patent application Ser. No. 09/537,922 entitled “An electronically tunable reflector” filed on Mar. 29, 2000, and to U.S. patent application Ser. No. 09/537,921 entitled “An end-fire antenna or array on surface with tunable impedance” filed on Mar. 29, 2000, the disclosures of which are hereby incorporated herein by this reference. The present invention improves upon the high impedance surface of U.S. patent application Ser. No. 09/537,923 entitled “A tunable impedance surface”, and provides a method of broadening the surface operating bandwidth.

[0006] As an aid in understanding the principle of operation of this invention, the prior art is instructive. Turning to FIG. 1a,a dipole element 1, located λ/4 away from a metallic ground plane 2, is shown. An incident plane wave 3 is reflected from the ground plane 2 and also scattered from the dipole element 1. When the dipole element is at its resonant length, (i.e., its length 1_dis appoximately equal to half of the effective signal wavelength, 1_d≈½λ_eff), scattering from the dipole is very strong and the effect from the ground plane is negligible. Thus, the total field has a reflection phase of approximately 180° (at the plane of the dipole). If the dipole is far from its resonant length, then scattering from the dipole is weak and the reflection phase, due primarily to the ground plane, is approximately 0° (at the plane of the dipole). Therefore, the phase of the reflected field from the dipole element can be adjusted by making small changes in the length of the dipole.

[0007] As an example, simulation that shows the behavior of the reflected phase versus dipole length is represented in FIG. 2. The simulation assumes that the dipole element is part of an infinite array, and is located in free space, λ/4 away from the ground plane. It further assumes a operating frequency of 11.8 GHz and that the dipole strip is 0.1 inch (CGS) in width. The dipole length varies from 0.1 to 0.8 inch. As can be seen in FIG. 2, the reflection phase of the dipole element can be tuned over a wide range, about 85°, for a length change of only 0.05 inch

[0008] FIG. 2ademonstrates a technique of varying the length of a dipole element using RF MEMS technology. The dipole element 20 is segmented into a main segment 22 and a plurality of smaller segments 21. Each segment is interconnected to the adjacent one by an RF MEMS switch 23. By opening or closing the RF MEMS switches 23, the dipole length can be changed in steps equal to small segment length plus switch length. In this example, the small segments are approximately 200 μm in length, and the switches are about 100 μm long. Consequently, when a switch is opened, the dipole length is increased by 300 μm. This corresponds to approximately a 10° change in the reflected phase. By making the segments and/or switches smaller, a finer phase tunability can be achieved.

[0009] These length-changeable dipole elements can be incorporated into an array, disposed above a ground plane, and tuned to create a reflection phase gradient across the array. In this configuration, the total reflected wave forms a beam, which can be steered to incremental angular directions, by creating uniform phase gratings across the array. FIGS. 3aand 3billustrate this concept for a linear array and a planar array respectively. This type of array can then serve as a stand-alone antenna or as a subreflector to another primary reflecting surface, such as a Cassegrain antenna.

[0010] However, the approach described in the immediately preceding paragraph has bandwidth limitations, as this will now be explained. Each dipole element of the array is modeled as a series resonance circuit 40, located λ/4 from a short circuit 41, as illustrated by FIG. 4. An infinite array approximation is assumed. The values of the inductance and capacitance are functions of the dipole length, width, and unit cell size. When the short circuit is located λ/4 from this susceptance (LC circuit), it appears as an open circuit across the susceptance and the reflection coefficient of the element can be tuned such that the reflection phase takes values over a full range of angles as shown in FIG. 2. However, at a frequency where the distance between the dipole and the ground plane is λ/2, the ground plane effectively shorts out the dipole and the reflected phase is locked at 180°, regardless of dipole length (no tuning is possible). Thus, as the array operates over a range of frequencies, inducing the distance between the ground plane and the dipole to vary between λ/4 and λ/2, the tuning range of the reflected phase becomes more and more limited. The present invention overcomes this limitation by placing the dipole array over a high impedance surface.

[0011] A high impedance surface is a filter structure which has the capability of reflecting an incident plane wave with a 0° phase shift. The basic structure of a high impedance surface is shown in FIG. 5a,and can be fabricated using multi-layer printed circuit board technology. Preferably hexagonal or square metal patches 50 are disposed on the top surface and connected to a lower metal sheet 51, by plated metal posts 52. The high impedance surface 54 acts as a filter to prevent the propagation of electric currents along the surface, over the frequency stopband. Therefore, unlike conventional conductors, propagating surface waves are not supported within the frequency stopband. Furthermore, incident plane waves are reflected without the phase reversal that occurs on an ordinary metal surface. FIG. 5bshows the reflection phase of the high impedance surface 54. The bandwidth over which the reflected phase lies between −90° and 90°, is given by: $\mathrm{Equation}1\text{:}\frac{\Delta f}{f_o}=\frac{\sqrt{\frac{L}{C}}}{\sqrt{\frac{\mu }{ϵ}}}$

[0012] where L and C are related to the equivalent circuit model (see FIG. 5c) of the high impedance surface (and not to be confused with the dipole model of FIG. 4). As shown in FIG. 5c,the capacitance C is due to the proximity of the top metal patches 50, and the inductance L originates from the current loops within the structure. f₀is the frequency for which the reflected wave has a 0° phase shift, μ and ε are the material permeability and permettivity respectively.

[0013] In accordance with this invention, an array of reconfigurable dipole antennas is disposed above a high impedance surface. In this manner, the dipole elements do not have to be placed λ/4 away from the ground plane as required by the prior art. This has the effect of making the system geometry independent of the frequency of operation. Thus, the operating frequency can be changed without having to alter the relative geometry of the array and the back plane, for the purpose of maintaining a λ/4 distance between them. This allows the array to maintain tunability over the full bandwidth of the high impedance surface.

[0014] The present invention provides an apparatus and method for tuning the array by changing the length of the dipole elements using RF MEMS technology, which overcomes the problems posed in the prior art, by the use of photoconductive switches.

[0015] This invention provides a multiple band, reconfigurable electromagnetic reflecting antenna system which can be reconfigured to operate at multiple frequency bands; the user can select the operating frequency band from a range that can be anywhere within the total surface bandwidth. Furthermore, at a given operating frequency band, the antenna system is capable of forming an antenna beam in the far-field and pointing the beam to selected directions.

[0016] In accordance with this invention, an array of dipole antenna elements is fabricated on top of a multiple band, high impedance surface. Reconfigurability is achieved by varying the resonant frequency of each dipole which is a function of dipole length. Thus by changing the dipole length, one can vary the resonant frequency of the dipole. Each dipole antenna element is segmented, and the segments are interconnected with RF MEMS (Micro Electro-Mechanical Switches) which can be opened or closed to change the length of the dipole. Small changes in dipole length allow for beam steering and forming a beam in the far-field, while larger changes allow for changes in the antenna array operating frequency band.

[0017] This invention further provides a method of increasing the bandwidth of the high impedance surface that supports the array of dipoles, by increasing the surface inductance.

[0018] FIG. 1 illustrates the principle of operation of the proposed array. An element of the dipole array placed λ/4 away from a ground plane is shown.

[0019] FIG. 2 shows a simulated model of the reflection phase as a function of dipole length for an infinite array similar to the array of FIG. 1

[0020] FIG. 2adepicts a dipole element whose length can be changed by actuating the RF MEMS switches which connect the different segments that constitute the dipole.

[0021] FIGS. 3aand 3billustrate beam steerability in the case of a linear and planar dipole array, respectively.

[0022] FIG. 4 is a series resonance circuit equivalent of a dipole element of the array shown in FIG. 3.

[0023] FIG. 5adepicts a perspective view of a high impedance surface.

[0024] FIG. 5bshows the measured reflection phase as a function of frequency for the high impedance surface of FIG. 5a.

[0025] FIG. 5cis a circuit equivalent model of two elements of the high impedance surface of FIG. 5a.

[0026] FIG. 6 depicts a dipole element whose length can be changed by small increments and/or large increments by actuation of the RF MEMS switches that connect the dipole segments.

[0027] FIG. 7 depicts an embodiment of the invention where dipole elements as shown in FIG. 6 are fabricated on a high impedance surface, and whose lengths are controlled by an actuation logic circuit.

[0028] FIG. 8 is a perspective view of a high impedance surface illustrating a method of broadening the surface bandwidth by inserting a layer of spiral inductors.

[0029] FIG. 9 is a cross-section view of the an embodiment of the invention showing spiral inductors in the middle layer and MEMS switches on the top.

[0030] In accordance with this invention, and referring to FIG. 7, a reconfigurable array of dipole elements 60 is fabricated above a multiple band, high impedance surface 54, so that the array can be tuned to be resonant at different frequencies, and a beam can be steered at those frequencies. Assuming a high impedance surface that reflects with little phase shift over an octave or more in bandwidth, the dipole lengths can be changed in large increments to change the array operating frequency band. For example, reducing the length of a dipole by half, will move its resonant frequency up an octave, from f to 2f. This concept is illustrated by FIG. 6. For ease of understanding, let us assume that RF MEMS switches 67 are all closed, thereby conductively connecting dipole segment 62 to dipole segments 65 and 61 on one side and to dipole segments 65 and 63 on the other side. Let us further assume that RF MEMS switches 66 are open. In this configuration the length L of the dipole element 60 is equal to the sum of the lengths of segments 61, 62, 63, 65, and RF MEMS switches 67. Finally, let us call f the resonant frequency of this dipole element 60 of length L. In this configuration the user can:

[0031] (1) Steer the beam by closing selected ones of RF MEMS switches 66, thereby increasing the length of the dipole 60 by small amounts. In the particular example of FIG. 6, six small segments 64 can be added to the dipole main body, three on each side. These small changes in length have the effect of modifying the dipole resonant frequency, thereby changing its reflection phase. When such dipoles are disposed in an array, they can be tuned to create a reflection phase gradient across the array, allowing for steering of the reflected beam.

[0032] (2) Reconfigure the operating frequency by opening RF MEMS switch 68 (the particular switch of group 67 that is connected to segment 61) and 69 (the particular switch of group 67 that is connected to segment 63), thereby conductively disconnecting segments 61 and 63 from center segment 62, and reducing the length of the dipole 60 from L to L/2. This has the effect of moving the dipole resonant frequency up an octave, from f to 2f, thereby reconfiguring the dipole operating frequency. In a manner analogous to (1), beam steering can be performed at this new operating frequency, by actuating RF MEMS switches 67, and changing the length of dipole 60 by small amounts.

[0033] Numerous other embodiments than the one shown in FIG. 6 can easily be imagined. For example the dipole could be finely segmented along its entire length, with RF MEMS switches interconnecting the segments, thus achieving a high degree of functionality and a multitude of frequency bands. An array of such dipoles can be fabricated on a single substrate tile, with larger antennas requiring multiple tiles.

[0034] Referring to FIG. 7, an array of RF MEMS switched dipoles 60 is fabricated on top of a thin insulating layer 72, and disposed on a multiple band high impedance surface 54. As previously explained, the operational frequency band of the array is set by switching in or out the larger metallic segments of each dipole. Switching in or out the smaller metallic segments allows to steer the reflected beam in two angular directions. A switch actuation logic control circuit 70 is preferably placed behind the high impedance surface 54, so as to isolate it from the potentially disturbing radiating dipoles. Each switch comprises two DC lines to apply the actuation voltage, and since the lines carry solely DC voltage, they can be placed very close together in a very dense actuation network disposed behind the high impedance surface 54. Furthermore, the cantilever beam that opens and closes the switch has a DC actuation electrode that is set apart from the RF electrode, thereby completely isolating the DC pads from the RF pads within the switch. Thus, without seriously affecting the dipoles, very tiny feed-through via holes 71, can be made to bring the actuation voltage through the high impedance surface 54, from the backside network. The switch actuation lines originate from the logic control circuit 70, which allows a desired mode of operation to be selected by actuating the required switches.

[0035] The high impedance surface bandwidth must be made broad enough to allow the array to operate over the desired frequencies. When this is achieved, the high impedance surface effectively behaves like an open circuit. Thus, when the dipoles are located just a fraction of a wavelength away from this surface, the tuning range of the dipoles can be maintained over their full phase range for the bandwidth of the surface. It can be noted from equation 1, that the surface bandwidth can be broadened by increasing the equivalent inductance of the surface. FIG. 8 illustrates a technique for increasing the surface equivalent inductance. A three layer circuit board is used, with the middle layer consisting of printed circuit spiral inductors 80. The inductances and patch sizes are set to the desired center frequency and bandwidth, and maintain a 0° phase change at reflection. FIG. 9 is a view of the circuit board in cross-section. The spiral inductors 80 are printed in the middle layer, while the patches 50 are printed on the top layer. The dipoles are disposed on top the high impedance surface and the MEMS switches 90 are shown in cross-section. The control lines for the MEMS switches are run through the via holes 71.

[0036] Other methods of increasing the bandwidth of the high impedance surface include decreasing the surface equivalent capacitance, or using complicated resonant structures that have additional frequencies where the reflected phase goes to 0°.

[0037] Having described the invention in conjunction with certain embodiments thereof, modifications and variations will now certainly suggest themselves to those skilled in the art. As such, the invention is not limited to the disclosed embodiments except as required by the appended claims.