Patent Grant
8031128
The application generally relates to electrically small antennas (ESAs). The application relates more specifically to ESAs including metamaterial resonant structure to reduce antenna size. The ESA may be mounted on an aircraft for the identification and mapping of subsurface facilities or features.
One object of gathering intelligence data is the identification, mapping, and location of deeply buried underground facilities. The scientific community is interested in methods for locating and mapping underground facilities in non-accessible territory to determine, for example, whether underground nuclear facilities are situated in underground bunkers. A key factor that makes it difficult to detect, locate or map such underground facilities is that conventional radar does not penetrate the Earth's surface. When using conventional radar the electromagnetic waves are reflected and attenuated by the soil, due to the finite conductivity and dielectric loss of the soil.
Typical ground penetrating radar (GPR) may operate in the frequency range of 100-400 MHz, but in that frequency range, the radar can penetrate the Earth's surface to a depth of only about one meter. In order for radar waves to penetrate deeper into the ground, a radar signal with a lower frequency, e.g., in the range of 10-150 kHz, is required. At frequencies as low as 10-150 kHz, the electromagnetic radar wave can penetrate the Earth to a depth as great as 100 meters or more, depending on the soil characteristics. However, since radar antennas are geometrically proportional to the wavelength, operating a radar system at frequencies as low as 10-150 kHz normally requires an enormous antenna. The corresponding wavelengths of 10-150 kHz radiowaves range from 30 km to 3 km. Such an antenna cannot be carried efficiently by an airplane, and in any event may not radiate sufficient power to generate a ground-penetrating radar wave. Further, the resolution of such a low frequency radar system would have limited diffraction properties. Such a radar system would be diffraction limited and able to resolve only those objects or features of sizes comparable to the wavelength. Such relatively large objects or features are much larger than most of the features that are being sought.
These existing GPRs are based on transmitting a very short pulse which includes all of the long wavelength Fourier components and can thus penetrate the ground to some extent. However, such GPRs at best penetrate the ground within about a meter of the Earth's surface. Such GPRs are typically used to locate wires, pipes etc. under the ground within about a meter of the top surface. None of the short pulse GPRs can penetrate to a subsurface depth of about 100 meters, which is the range of depth illumination that is required for detecting strategic underground facilities.
Existing methods for identification and mapping of underground facilities include satellite imagery that can indicate construction or excavation activities on the Earth's surface. Satellite imagery provides an approximate or general location of such a facility. However, many underground facilities are accessible by a rather long tunnel that leads from the excavation point to the final underground destination point, meaning that identifying the entrance point at the surface may provide an inaccurate indication of the location of the underground facility. Depending on the length of the access tunnel, the area to be mapped underground could cover a rather large physical area, on the order of many square kilometers.
Other suggested methods to identify underground facilities require placement of acoustic sensors in the ground to detect activity associated with such underground facilities. Small sensors placed in the vicinity of such a structure may pick up acoustic signatures for identifying the exact location of the facility. However, it is not always possible to place sensors, conceal them from discovery, and then periodically interrogate such sensors in the vicinity of such an underground facility. The underground facilities of interest are often located in restricted areas, e.g., facilities located on foreign territory. Furthermore, it would be necessary to have determined, in advance, at least a general location of such an underground facility. Unless the ground sensors are placed in the exact location where detection of signals is likely, it would be easy to miss detection of the target. Finally, the logistics and cost of placing a large number of sensors make placing acoustic sensors an impractical and unattractive solution.
Electrically small antennas (ESA) are known, such as an electrically small, low “Q” radiator as disclosed in U.S. Pat. No. 6,437,750. However, these ESAs have not been configured to illuminate subterranean images.
The foregoing examples and limitations associated therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon reading of the specifications and study of the drawings. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs.
One embodiment relates to an electrically small antenna including a dipole, a metamaterial hemispherical sphere or shell partially surrounding the dipole, and a ground plane disposed proximate the metamaterial hemispherical sphere or shell. The length of the electrically small antenna is in the range of λ/10 to λ/10,000 of the predetermined wavelength λ.
Another embodiment relates to an airborne antenna system including an airframe and an electrically small antenna disposed on the airframe. The electrically small antenna is in the range of λ/10 to λ/10,000 of the predetermined wavelength λ.
Certain advantages of the embodiments of the invention described herein include an electrically small antenna (ESA) having the capability to resolve very small objects compared to the wavelength of an interrogation signal.
Another advantage of the present invention is to provide an ESA that operates at a frequency of about 100 kHz.
Another advantage of the invention is to provide an ESA with the ability to obtain super-resolution on the order of about λ/100.
A yet further advantage of the present invention is to provide an ESA having an operating wavelength on the order of meters and which has an efficient transmit/receive capability compared to a regular dipole.
A yet further advantage of the present invention is to provide an ESA that is lighter and more efficient than a conventional dipole antenna.
Alternative exemplary embodiments relate to other features and combinations of features as may be generally recited in the claims.
Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
The ESA of the current invention is on the order of meters and has an efficient transmit/receive capability compared to a regular dipole. The ESA is constructed using metamaterial concepts. The metamaterial may be single negative (SNG) (i.e. the permittivity ε<0, or the permeability μ<0) or double negative (DNG) (i.e. both the permittivity ε<0 and the permeability μ<0). In an exemplary embodiment, an ESA is disclosed that is 1/10 of the length of the equivalent dipole length, and may be scaled down to 1/1000 or 1/10,000. Such an ESA may include phase sensitive current injection in the metamaterial resonant structures for loss-compensation. In other words, the unit cells of the ESA may be driven by a current source that is in phase with the exciting electromagnetic wave. The ESA may include a magnetic or electric dipole, and the metamaterial resonant structure may be a metamaterial shell or a metamaterial hemispherical structure. In one embodiment, the ESA includes a magnetic dipole surrounded by a metamaterial hemispherical sphere. In another embodiment, the ESA includes an electric dipole surrounded by a metamaterial shell.
Referring to
The ESA 100 can resolve subwavelength features. Subwavelength features are features that are smaller than the illuminating or probing wavelength. Commonly owned U.S. patent application Ser. No. 12/116,540, entitled “Identification and Mapping of Underground Facilities”, filed concurrently with the present patent application, discloses an exemplary application of the ESA including a method and system for the identification and mapping of subsurface facilities, and the same is hereby incorporated by reference in its entirety.
As can be seen in
The hemispherical sphere 120 is formed by stacking semicircle sheets 122 of differing radii. The semicircle sheets 122 are formed by disposing an array of unit cells 124 on the substrate 126 to form a patterned substrate as shown in
An exemplary embodiment of a unit cell 124 configured to operate at 100 kHz with a λ/d=1000 is shown in
In this exemplary embodiment, the conductive path 510 is formed of a copper wire. In other embodiments, the conductive path 510 may be formed of any conductive material. The substrate 126 is formed of a dielectric material. In one embodiment, the substrate 126 is formed of alumina, however, the substrate 126 may be formed of any dielectric material as would be appreciated by one of ordinary skill in the art. For example, the dielectric material may be Rexolite®, a cross linked polystyrene microwave plastic made by C-Lec Plastics, or Rogers 5880, a glass microfiber reinforced PTFE composite made by Rogers Corporation. In this exemplary embodiment, the capacitor 520 is a 1.79 nF lumped element capacitor. In other embodiments, the capacitor 520 may be chosen in accordance with the inductance of the loop of the unit cell 124 to provide a desired resonant frequency.
It should be understood that the application is not limited to the details or methodology set forth in the following description or illustrated in the figures. It should also be understood that the phraseology and terminology employed herein is for the purpose of description only and should not be regarded as limiting.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
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