Patent Application
20140097996
1. Field of the Invention
The invention is in the field of tunable electromagnetic devices, such as frequency selective surfaces.
2. Description of the Related Art
Antennas are often placed behind radomes. Radomes are structures which protect the antenna, and which allow electromagnetic energy to pass through in both directions. Often the radome is made so that it transmits electromagnetic energy in a narrow band centered around the operating frequency of the antenna. Frequency selective surfaces, with a grid or lattice of metal patterns or holes in a metal sheet, may be used for this purpose and additionally to deflect or reflect jamming signals at other frequencies. However, such frequency selective surfaces may have the disadvantage of being very selective as to the range of frequencies that they will allow to pass through. Also, such surfaces may have the disadvantage of not being able fully to block incoming electromagnetic energy at all frequencies of interest. Such full blocking would be useful when the antenna is not operating, as the antenna may be made in such a case to appear similar to the surrounding environment or objects, for example appearing to radar as a sheet of metal. This may help in hiding the radar from detection by enemy radar or other sensors.
According to an aspect of the invention, a tunable electromagnetic device includes: a first tunable metamaterial layer; and a second tunable metamaterial layer. The metamaterial layers at least partially physically overlap. Additional tunable metamaterial layers may also be included in the stack.
According to another aspect of the invention, a method of shielding a device that receives and/or sends electromagnetic energy, the method including: selectively altering transmission properties of a tunable electromagnetic device that at least partially covers the device that receives and/or sends electromagnetic energy. The tunable electromagnetic device includes: a first tunable metamaterial layer; and a second tunable metamaterial layer. The metamaterial layers at least partially physically overlap. Altering transmission properties includes selectively altering transmission properties of at least one of the metamaterial layers.
To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.
The annexed drawings, which are not necessarily to scale, show various aspects of the invention.
A tunable electromagnetic device includes at least two overlapping metamaterial layers, wherein the metamaterial layers are selectively tunable by an externally applied means. By selectively altering the properties of the metamaterial layers the frequency response of the electromagnetic device can be controlled, to selectively allow electromagnetic energy of certain frequencies pass through, or alternatively to prevent pass-through of substantially all frequencies of electromagnetic energy. In addition the frequencies for which electromagnetic energy passes through may be altered by controlling one or more of the tunable metamaterial layers. The tunable electromagnetic device may be used to selectively shield radar or other types of sensors, for example being used as all or part of the skin of a vehicle or other object.
The metamaterial layers 32 and 36 may include any of a variety of tunable metamaterials. Metamaterials are materials or combinations of materials that have been engineered to have properties that may not be found in nature. One type of metamaterials is tunable metamaterials, a term that is used herein to refer to a metamaterial with a variable response to an incident electromagnetic wave.
The substrate material layers 35 and 39 may include any of a variety of suitable materials. Examples of suitable materials include ferroelectric materials, for example barium titanates (such as barium strontium titanate), lead titanates, lanthanum titanates, lead arsenate, or ferroelectric polyvinylidene fluoride (PVDF) polymer. The substrate material layers 35 and 39 may have a thickness of from 50 to 500 nm, or more narrowly about 200 nm (although other thicknesses are possible).
The patterned conductive structures 34 and 38 are periodic arrays of metallic (or other electrically conductive) elements with specific geometric shapes, or periodic apertures in a metal (or other electrically conductive material) screen. These periodic arrays may be considered arrays of circuit elements, and form subharmonic structures. The transmission and reflection coefficients for the arrays are dependent on the characteristics of operation, such as the frequency and/or of voltages applied to the patterned conductive structures 34 and 38. The patterned conductive structures 34 and 38 may use any of a variety of suitably-shaped repeating elements or apertures, including squares, circles, and crosses of various configurations (such as Jerusalem crosses). The patterned conductive structures 34 and 38 may have a pair of sets of parallel conductive lines, with the conductive lines in one set being perpendicular to the other set of conductive lines, for example. Elements, such as capacitive elements, diodes, varactor diodes, or other circuit elements, may be placed at various locations between adjacent of the conductive lines. Applying controlled voltages from one or more power sources 50 to the conductive lines, varies the characteristics of the metamaterial layers 32 and 36 in terms of what frequencies of electromagnetic energy will pass through the metamaterial layers 32 and 36. Further details regarding the general arrangement of metamaterial layers and associated conductive structures for controlling frequency response, reference may be had to U.S. Pat. No. 7,612,718, the description and figures of which are incorporated herein by reference.
U.S. Pat. No. 7,612,718 describes an apparatus and methods for operating a frequency selective surface. Lines of conductors are placed on one or both major surfaces of a metamaterial layer. Circuit elements, such as varactor diodes, are placed between the lines of conductive material. Providing voltage differences across adjacent of the conductor lines, at a given frequency, will alter the inherent capacitance of the system, thereby changing the frequency response of the material.
In the lattice described in U.S. Pat. No. 7,612,718 the distance between adjacent conductor lines may be from 1/15 of the wavelength to ½ of the wavelength. The conductive structures 34 and 38 may have distances between conductor lines that are an order of magnitude less. This results in conductive structures 34 and 38 that have greater concentration, allowing greater control of the properties of the metamaterial layers 32 and 36.
The electromagnetic device 14 may cover a sensor/antenna, such as radiofrequency (RF) feeds 52 and 54. The RF feeds 52 and 54 may be parts of an antenna for use in sending and receiving signals, as part of a radar system. The feeds 52 and 54 may feed through holes in a metal sheet 56. Other sorts of devices that send and/or receive electromagnetic energy may be covered at least in part by the electromagnetic device 14.
The two metamaterial layers 32 and 36 may be independently controlled to achieve any of a variety of effects. The tunable electromagnetic device 14 may be tuned to provide a high degree of isolation (preventing ingress and egress of electromagnetic radiation) by tuning the metamaterial layers 32 and 36 so that their transmissive parts of the frequency spectrum have substantially no frequency overlap. This is illustrated in
The electromagnetic device 14 may be used to selectively allow transmission of electromagnetic energy therethrough at some times, while blocking substantially all transmission at other times. For example, the electromagnetic device 14 may be configured (such as by being selectively tuned) to allowed electromagnetic energy through for operation of a radar system, when the radar system is sending and receiving signals, and to reflect electromagnetic energy when the radar system is not operating. This makes the radar system less visible to enemy radar, since the electromagnetic device 14 appears similar to surrounding electromagnetically-reflecting surfaces, such as the metal skin of an aircraft, other vehicle, or other object.
The tunable electromagnetic device 14 may be used for any of a variety of purposes. Besides shielding radar systems, it may be used for shielding any of a variety of other sensors and devices, such as communications devices, electronic warfare devices for transmitting and/or receiving signals, and radiofrequency (RF) sensors. In addition, the tuning of the metamaterial layers 32 and 36 may be modulated to control the bandwidth of the open frequency range of electromagnetic radiation that passes through the electromagnetic device 14. By partially separating the frequency responses of the metamaterial layers 32 and 36 (by selective tuning of the metamaterial layers 32 and 36), the frequency range of the opening can be tailored to be similar to that of the radar system or other device that sends and/or receives electromagnetic energy passing through the electromagnetic device 14. For example, the frequency range may be set 1-2% wider than the bandwidth of a radar system antenna (or other antenna, sensor, or device).
Alternatively or in addition, the bandwidth of a frequency window may be a function of frequency. As illustrated in
The metamaterial layers 32 and 36 may be made of the same materials, with or without having the same tunability properties, or may be made of different materials, having different properties. To give one example of the size of the electromagnetic device 14, the device 14 may be on the order of 1 lambda (where lambda corresponds to the wavelength of the target operating frequency) in each of the lateral directions, with each of the substrate metamaterial layers 35 and 39 having a thickness of 10 mils, and each of the conductive structures 34 and 38 having 400 circuit elements. However, the electromagnetic device 14 may have a wide variety of other sizes, for example being large enough to cover a surface of a large vehicle, such as a ship.
The tunable electromagnetic device 14 may provide various advantages in use. It reduces or otherwise alters the radar signature of a sensor system, such as a radar antenna. The signature can be reduced by closing the transmission window when the radar or other sensor is not in use, as well as by shaping the transmission window (its bandwidth and peak frequency) by controlling the transmission properties of the metamaterial layers 32 and 36. The tunable electromagnetic device 14 may be used to change the radar signature in other ways, by altering the transmission properties to achieve other effects.
Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.
