Abstract:
An X, K u , and K-band omni-directional antenna with dielectric loading is disclosed. It comprises a conductor with a loading dielectric resonator and a ground plane. Broad frequency coverage from 7.5 to 26 GHz includes uniform azimuthal coverage from +10 to +70 degrees. The antenna can be used generally in microwave communications including Digital Radio Frequency Tags (DRaFTs) communicating with airborne and satellite platforms.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention was made with United States Government support under Contract No. W15P7T-05-C-P627 awarded by the U.S. Army. The United States Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to microwave antennas and, more particularly, to the utilization of an X, K u , and K band omnidirectional antenna with dielectric loading. 
     BACKGROUND OF THE INVENTION 
     Broadband microwave communications provide the opportunity for miniaturized systems generally unobtainable at lower frequencies. Components, including antennas, can make these systems very expensive, however. 
     Radio frequency communication with air and space platforms provides the opportunity to remotely track objects over large distances. Military operations especially have a need for tracking technology for air-to-ground Combat Identification (CID). This generally includes microwave communications. 
     As an example, a Digital Radio Frequency Tag (DRaFT) can provide flexible technology to allow radars such as Moving Target Indicator (MTI) and Synthetic Aperture Radar (SAR) to receive data from ground devices. At the frequencies used by these systems, small, lightweight and affordable RF Tags can provide for data extraction from unattended ground sensors and communication with vehicles and personnel throughout an area. This is particularly useful for the identification and location of combined units. Other advanced tag functions include additional communications capabilities for enhanced interoperability with identification and communications systems. 
     Ultra-wideband (UWB) systems provide the benefit of radio transmissions that use a very large bandwidth. This can convey more signal information including data or radar resolution. Although no set bandwidth defines a signal as UWB, systems using bandwidths greater than about ten percent are typically called UWB systems. A typical UWB system may use a bandwidth of one-third to one-half of the center frequency. 
     Broadband operation in the X, K u , and K bands is desirable, but applicable biconical antennas are cost prohibitive and too large for applications. They can cost thousands of dollars and occupy a volume as large as a tennis ball. Currently, multiple antennas are required to cover this bandwidth, especially both above and below the horizontal plane. 
       FIG. 1  is a diagram of a prior-art microwave biconical antenna  100 . It is costly and can be difficult to integrate into a microwave system. 
       FIG. 2  is a plot  200  of the  FIG. 1  prior-art biconical antenna H-plane pattern. It has been normalized based on the average signal from −135 degrees to +135 degrees. 
     Current microwave broadband antennas are expensive, difficult to integrate into systems, and can have relatively narrow operating frequencies. 
     SUMMARY OF THE INVENTION 
     The above problems of biconical and similar antennas are solved by providing an X, K u , and K-band omni-directional antenna with dielectric loading. Advantages of the new antenna are that it is small, very inexpensive, omni-directional, simply constructed, and easily reproducible. It includes the microwave frequency bands of 8 to 12 GHz (X), 12 to 18 GHz (K u ), and 18 to 27 GHz (K). This is approximately twice the bandwidth of prior antennas. Scaling dimensions larger results in performance at lower frequencies. Applications include car-top deployment. 
     Embodiments include a dielectrically loaded omnidirectional broadband antenna comprising a ground plane; a conductor; and a dielectric resonator whereby the antenna is loaded. In embodiments, the radiation is in the X, Ku, and K-bands and the resonant frequency is about approximately between 7.5 GHz and 26 GHz. In other embodiments the dielectric resonator is proximate the ground plane or in contact with the ground plane. For embodiments, the dielectric resonator is a toroid with rectangular cross section of about approximately 99.5 percent pure alumina and the relative dielectric constant ∈ r  of the dielectric resonator is about approximately 9.7. In yet other embodiments, the length of the conductor is about approximately 0.387 inch, the ground plane comprises a copper disk, and the ground plane diameter is about approximately six inches. For embodiments, the radiation polarization is about approximately vertical, the radiation pattern provides transmit and receive reciprocity, and the radiation pattern is substantially omnidirectional in the plane of the ground plane. In antenna embodiments, the radiation pattern azimuth coverage is uniform between about approximately plus ten and about approximately plus seventy degrees. 
     Other embodiments include a dielectrically loaded omnidirectional microwave antenna comprising a ground plane; a conductor having a length of 0.387 inch and a diameter of about approximately 0.050 inch; and a dielectric resonator having an outer diameter of about approximately 0.290 inch, an inner diameter of about approximately 0.102 inch and height of about approximately 0.151 inch; whereby the antenna is loaded. 
     An embodiment is a microwave frequency tag comprising at least one broadband microwave antenna comprising a ground plane; a conductor; a dielectric resonator whereby the antenna is loaded; and circuitry in electrical communication with the antenna whereby the microwave frequency tag communicates with a transceiver. For embodiments, the tag is associated with personnel, the tag is associated with vehicles, and the tag is a digital radio frequency tag (DRaFT). Other embodiments comprise two antennas in close proximity wherein there is less than 1 dB of gain pattern variation in azimuth. 
     The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a prior-art microwave biconical antenna. 
         FIG. 2  is a plot of the prior-art  FIG. 1  biconical antenna H-plane pattern. 
         FIG. 3  is a simplified perspective diagrammatic illustration of an X, K u , K-band omnidirectional antenna with dielectric loading configured in accordance with one embodiment of the present invention. 
         FIG. 4  is a polar plot of the pattern of the antenna represented in  FIG. 3  configured in accordance with one embodiment of the present invention. 
         FIG. 5  is a voltage standing wave ratio (VSWR) plot of the pattern of the antenna represented in  FIG. 3  configured in accordance with one embodiment of the present invention. 
         FIG. 6  is a simplified illustration of the subject antenna deployed in a broadband microwave DRaFT system configured in accordance with one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the antenna are very small (one fortieth 1/40th of a cubic inch), have good azimuth coverage from at least +10 degrees to at least +70 degrees elevation, and have extremely wide bandwidth from approximately 7.5 to approximately 26 GHz. They are very low cost and very simple to connect to a transmit/receive microwave apparatus. Embodiments have vertical polarization. 
       FIG. 3  is a simplified schematic illustration  300  of an embodiment of an X, K u , K-band omnidirectional antenna with dielectric loading. Conductor  305  has a length  310  of 0.387 inch and a diameter of 0.050 inch. Dielectric resonator  315  has an outer diameter  320  of 0.290 inch and inner diameter  325  of 0.102 inch. Its height  330  is 0.151 inch. Dielectric resonator  315  embodiments are made of aluminum oxide Al 2 O 3 , but other dielectrics may be used. Dielectric resonator  315  provides loading to the antenna system. Ground plane  335  can incorporate a backside 50 ohm coaxial feed (not shown). In embodiments, feedpoint is flush with groundplane  335 . Ground plane  335  can be greater than or equal to approximately the wavelength of the antenna&#39;s lowest frequency. Ground plane  335  can be of varied shape. Nonlimiting examples include a circle or rectilinear shape. Size can include an approximate six inch diameter, smaller or larger depending on application requirements. Materials can include copper, brass, and aluminum. Dielectric resonator  315  of the antenna is located on ground plane  335 , with no separation. Embodiments include a four-hole flange subminiature A (SMA) connector and a 99.5 percent alumina dielectric toroid  315  with rectangular cross section and relative dielectric constant ∈ r  of 9.7. Scaling dimensions larger results in performance at lower frequencies. 
       FIG. 4  is a polar plot  400  of the pattern of the antenna represented in  FIG. 3 . The scale ranges from +5 to −25 dBi. Four patterns shown are of 7 GHz  405 , 9 GHz  410 , 15 GHz  415 , and 18 GHz  420 . Elevation patterns show greater than +5 dBi gain from 10 to 25 degrees elevation. They exhibit good azimuth coverage from at least +10 degrees to at least +70 degrees. Performance in airborne communications benefits from this pattern. Maximum gain occurs in the direction of maximum range to an aircraft and is decreased overhead where range to the aircraft is least. This directs energy where it is most beneficial. 
       FIG. 5  is an input VSWR plot  500  of the pattern of the antenna represented in  FIG. 3 . The scale is from zero to five and covers 2 GHz to 28 GHz. It depicts the influence of ground plane size with curves  505  and  510  portraying larger ground planes and curve  515  a smaller ground plane. Each curve presents a VSWR between 1.0 and 2.5 for 6 GHz to 26 GHz. This is a distinguishing feature of this antenna. It is expected that dimensional scaling produces similar results for frequencies in addition to this band. 
       FIG. 6  illustrates a simplified diagram of an embodiment of the subject antenna deployed in a broadband microwave Digital Radio Frequency Tag (DRaFT) system  600 . DRaFT  605  includes broadband antenna  610  and is in communication with remote airborne platform  615 . Circuitry on DRaFT  605  is in electrical communication with microwave antenna  610  and supports two-way communication with a tag communication device that can be other than an airborne platform  615 . Also shown is a second DRaFT  625  also incorporating broadband antenna  620 . DRaFTs  605  and  625  communicate with each other and remote platform  615 . 
     In embodiments, two antennas perform transmit/receive functions. The mutual effects of two antennas in close proximity (approximately two wavelengths apart on a common ground plane) display only slight azimuth pattern perturbation. There is less than 1 dB of “wobble” as azimuth as the pattern is measured over 360 degrees (passive antenna rotated about the active antenna). 
     The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of this disclosure. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.