Abstract:
An antenna. The antenna includes a first dielectric antenna rod having a first dielectric constant. The first dielectric antenna rod is coupled to a first frequency transmission source for propagating first frequency band radiation from the first dielectric antenna rod into a medium having a medium dielectric constant. A second dielectric antenna rod is provided having a second dielectric constant. The second dielectric antenna rod is coupled to a second frequency transmission source for propagating second frequency band radiation from the second dielectric antenna rod into the medium. The first dielectric antenna rod is coaxially mounted within the second dielectric antenna rod. The first dielectric constant is greater than the second dielectric constant. The second dielectric constant is greater than the medium dielectric constant.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of antennas, and more particularly, to antenna structures for covering a diversity of frequency bands. 
     BACKGROUND 
     A large number of different radio frequency systems have come into use for communication, navigation, electronic warfare and radar systems. State of the art automotive and aerospaceborne vehicles which utilize such radio frequency systems could have more than a dozen separate antennas to cover a diversity of frequency bands. However, many mobile platforms have limited space for multiple antennas operating in widely separated frequency bands. 
     Alternatively, a number of wide bandwidth antenna elements have been developed for electronic warfare and signal intelligence systems. Current state-of-art antennas include flared notch elements each with about an octave of bandwidth (2:1). Other antenna elements such as spirals, log periodic elements, biconical dipoles and conical monopoles all have a bandwidth limit of about 2:1 and they tend to have relatively large physical dimensions, and, as such, are not well-suited for mobile platform/vehicular use. 
     One solution to this multi-antenna, multi-aperture problem now faced by land, sea, air and spaceborne vehicles has been multi-function, multi-frequency, phased array antenna apertures with electronic beam forming and scanning/tracking. However, today broadband antenna elements and phased array antennas are limited by the bandwidth and dimensions of the antenna feed elements to a maximum frequency ratio of about one octave (2:1). Broad bandwidth phased array antennas composed of broadband feed elements must address several conflicting design parameters: 
     1) low side lobes require that the phase centers of the feed antennas be closely spaced one half wavelength apart at the highest frequency of operation; 
     2) feed antennas have dimensions approaching one half wavelength at the lowest operating frequency; 
     3) large numbers of broadband amplifiers must be connected to every feed antenna in a 2:1 bandwidth array; and 
     4) often a second set of crossed linear antenna elements and associated electronics are required if the array is to transmit and receive signals in orthogonal linear polarization and in both circular polarizations. 
     Therefore, there exists a need for an effective antenna structure which can cover a diversity of frequency bands, a diversity of polarizations, and can be useful in phased array antenna systems. The present invention provides a unique solution to meet such needs. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an inventive three dimensional, ultra-broad bandwidth, multi-aperture, dielectric antenna is provided which combines features of tapered dielectric rod antennas and coaxial dielectric waveguide transmission lines. The coaxial dielectric rod antenna (CDRA) in accordance with the present invention has multi-frequency collinear apertures which can be optimized for use as individual multi-band antennas or as feed elements in broad bandwidth active aperture phased array antennas. In essence, the CDRA in accordance with the present invention combines into a single structure many separate antennas which cover a diversity of frequency bands. 
     A first embodiment of the invention includes a first dielectric antenna rod having a first dielectric constant. The first dielectric antenna rod is coupled to a first frequency transmission source for propagating first frequency band radiation from the first dielectric antenna rod into a medium having a medium dielectric constant. A second dielectric antenna rod is provided having a second dielectric constant. The second dielectric antenna rod is coupled to a second frequency transmission source for propagating second frequency band radiation from the second dielectric antenna rod into the medium. The first dielectric antenna rod is coaxially mounted within the second dielectric antenna rod. The first dielectric constant is greater than the second dielectric constant. The second dielectric constant is greater than the medium dielectric constant. 
     In accordance with the first embodiment, the second dielectric antenna rod can include an axial cylindrical cavity along the length of the second dielectric antenna rod. The axial cylindrical cavity can be filled with a dielectric powder having the first dielectric constant. The dielectric powder can be secured within the axial cylindrical cavity by end plugs having the first dielectric constant and be located at respective proximal and distal ends of the second dielectric antenna rod. Further, the first frequency transmission source can be axially coupled to the first dielectric antenna rod while the second frequency transmission source can be coupled to the second dielectric antenna by a transmission line axially offset from the second dielectric antenna rod. The second dielectric antenna rod can be made of a thermoplastic resin. The dielectric powder can be barium tetra-titanate or nickel-aluminum titanate. 
     Another embodiment of the present invention includes a first dielectric antenna rod having a first dielectric constant. The first dielectric antenna rod is coupled to a first frequency transmission source for propagating first frequency band radiation from the first dielectric antenna rod into a medium having a medium dielectric constant. A second dielectric antenna rod is provided having a second dielectric constant. The second dielectric antenna rod is coupled to a second frequency transmission source for propagating second frequency band radiation from the second dielectric antenna rod into the medium. The first dielectric antenna rod is coaxially mounted within the second dielectric antenna rod. A third dielectric antenna rod having a third dielectric constant is also provided. The third dielectric antenna rod is coupled to a third frequency transmission source for propagating third frequency band radiation from the third dielectric antenna rod into the medium. The second dielectric antenna rod is coaxially mounted within the third dielectric antenna rod. The first dielectric constant is greater than the second dielectric constant. The second dielectric constant is greater than the third dielectric constant. The third dielectric constant is greater than the medium dielectric constant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows in schematic form a prior art polyrod tapered dielectric antenna. 
     FIG. 2 shows in schematic form an embodiment of the present invention. 
     FIG. 3 shows a partially exploded perspective view of an embodiment of the present invention. 
     FIGS. 4 a-   4   c  show plan and section views of an embodiment of the present invention. 
     FIG. 5 shows FIG. 2 shows in schematic form another embodiment of the present invention. 
     FIGS. 6 a-   6   c  show alternative embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION 
     A uniform rod of dielectric material is a well-known type transmission line for electromagnetic waves ranging in wavelength from radio to optical frequencies. Various microwave and milli-meter wave dielectric transmission lines have been demonstrated, including single dielectric fibers, as described in U.S. Pat. No. 4,293,833 issued to Popa, and coaxial fibers of multiple dielectrics as described in U.S. Pat. No. 4,800,350 issued to Bridges et al. A microwave transition using dielectric waveguide is described in U.S. Pat. No. 5,684,495 issued to Dyott et al. in which a dielectric rod antenna couples a standard metallic waveguide to a dielectric rod transmission line. 
     Similarly, narrowband polyrod dielectric antennas and antenna arrays are well-known. Such antennas include those developed at the Bell Telephone Laboratories during World War II for radar antenna array elements, as described in the Bell System Technical Journal, Vol. XXVI, 1947, pages 837-851. Also, an embedded dielectric rod antenna has been described in U.S. Pat. No. 4,274,097 issued to Krall et al. that embeds a dielectric rod antenna with a relative dielectric constant of 84 in a dielectric cylinder of relative dielectric constant 81. High dielectric constant material is used to form a compact narrow beam antenna. 
     Further, dual frequency antennas have been developed involving a dielectric transmission line. A dual frequency feed satellite antenna horn is described in U.S. Pat. No. 4,785,306 issued to Adams in which a Ku band dielectric transmission line passes along the center of a conventional metallic C-band waveguide and then exits through an end wall. 
     In dielectric transmission lines of this type a portion of the energy travels along the inside of the dielectric rod and a portion travels along in the space outside of the rod. Electromagnetic energy can propagate along the dielectric fiber in a series of modes with the lowest order HE11 mode being the mode of primary interest. The useful bandwidth of the dielectric waveguide extends from the lowest frequency at which the HE11 mode is reasonably well contained up to the lowest frequency where the next lowest order modes, the TM01 and TE01, can propagate. 
     When internal or external discontinuities are encountered along the dielectric rod, radiation takes place. This tendency was used to advantage at the Bell Telephone Laboratories in the 1940s to form the microwave “polyrod” antennas. A representative polyrod tapered dielectric antenna  10  is schematically depicted in FIG.  1  and is discussed in more detail in Chapter 16 of the Antenna Engineering Handbook, published by McGraw-Hill, 1961. Dielectric antenna  10  is coupled to metal waveguide  12  and typically has a feed taper  14 , a body taper  16 , a straight section  18  and a terminal taper section  20 . In the dielectric rod antenna radiation is encouraged from all parts of the rod by gradually tapering the diameter of the rod and then abruptly terminating it at a point where the radiation has been essentially completed. By well-know proper design techniques this radiating structure forms a directional endfire antenna with the gain determined primarily by the length of the taper. 
     The dielectric rod transmission line can be evolved into a coaxial dielectric transmission line by surrounding the core rod with a second dielectric cylinder of slightly lower dielectric constant. This outer sheath confines the electric fields less tightly inside the dielectric material than does air with its relative dielectric constant ∈ of 1, but serves to protect these fields from outside influence. This is the concept used in optical fiber transmission lines. 
     In accordance with the present invention, features of the dielectric rod antenna and coaxial dielectric transmission lines are combined to form a series of concentric collinear apertures, each operating in the fundamental HE11 mode over greater than 2:1 frequency ratios in their respective frequency bands. 
     Referring to FIG. 2 the essence of the present invention is depicted in schematic form. Antenna  20 , which in the embodiment depicted hereinbelow is configured for operation both at 9.4 GHz in “low” frequency X-band and at 94 GHz in “high” frequency W-Band, includes core rod  22  of dielectric constant ∈ 3  which is inserted into rod  24  of dielectric constant ∈ 2 , which in turn is surrounded by medium  26  of dielectric constant ∈ 1  (usually air), forming two concentric dielectric transmission lines, which are respectively coupled to high band waveguide transducer  27  and low band waveguide transducer  28 . Dielectric constant ∈ 3  will be greater than dielectric constant ∈ 2 , which will be greater than dielectric constant ∈ 1 . By tapering this combined structure in a controlled manner, the transmission line formed by dielectric rods ∈ 1  and ∈ 2  will provide radiating of low band radiation  30  along the tapered surface followed collinerally by radiating of high band radiation  32  from the second embedded transmission line formed by dielectric rods ∈ 2  and ∈ 3 . The bandwidth, gain and beamwidth of each of these apertures can be individually adjusted for a specific application or they can be optimized for combined operation as feed antennas as part of a large active aperture phased array antenna system. 
     Referring collectively to FIGS. 3 and 4 a-   4   c  there is depicted a first embodiment of the present invention. Antenna  40  includes support housing  42 , which is made from two symmetrical mirror image aluminum housing blocks  44   a,    44   b,  each having length  43  of 3.5″, width  45  of 2.25″ and combined height  47  of 1.625″. Block  44   a  clamps down on block  44   b  and is secured in place by screws  46   a-   46   d  passing through clearance holes  48   a-   4   d  coupling with threaded holes  50   a-   50   d.  Support rod  52  includes tapered rod  54 , thin tubing  56  and tapered transition  58 . Tapered transition  58  at proximal end  59  of tapered rod  54  has a 45° taper thereat and couples tapered rod  54  with thin tubing  56 . Support rod  52  is made of a relatively loss-less dielectric material having a dielectric constant greater than that of air, e.g., having an ∈ 2 =2.08, such as that provided by thermoplastic resins, and in particular, the commonly known fluorocarbon resin Teflon (trademark). Thin tubing  56  can be formed from standard AWG20 teflon tubing. Tapered rod  54  has a straight section  60  having a diameter  62  of approximately 0.75″ for tapered rod  54  support in cylindrical recess  64  of housing blocks  44   a,    44   b,  and having a support length  66  of 1″. Thin tubing  56  is likewise supported in cylindrical recess  68  of housing blocks  44   a,    44   b,  cylindrical recess  68  being dimensioned to allow a press-fit of AWG20 size tubing . Cylindrical recess  68  is in axial alignment with cylindrical recess  64 . Tapered rod  54  tapers from dimension  62  at the edge of housing blocks  44   a,    44   b  to dimension  70  of 2 mm at tapered rod distal end  72  over taper length  74  of 4.75″. 
     Support rod  52  axially houses therein an axial cylindrical cavity  76  of approximately 1 mm diameter. Cylindrical cavity  76  is filled with powder-like high dielectric material  78  and has proximal end cap  80  and distal end cap  82  terminating each end. Proximal end cap  80  and distal end cap  82  are typically rigid pieces of approximately 1 mm diameter press-fit supported over a suitable length of cylindrical cavity  76 , typically made of the same material as powder-like material  78 , and act as plugs. Proximal end cap  80  has a taper  81  over length  84  of 2 mm and protrudes the same amount from housing blocks  44   a,    44   b.  Distal end cap  82  has a similar taper  83  over length  86  of 2 mm. Distal end cap  82  extends distance  88  of approximately 1.125″ from tapered rod distal end  72 . 
     In the first embodiment, material with a dielectric constant of 30, such as barium tetra-titanate powder or nickel-aluminum titanate powder, as is described in U.S. Pat. No. 4,800,350 entitled “Dielectric Waveguide Using Powdered Material”, was found to be a most effective powder-like material  78 . Those skilled in the art will recognize that the material and the powder consistency can be varied to enable changeable antenna frequencies. 
     As referred to above, the low frequency antenna of the present embodiment is designed to operate at 9.4 GHz while the high frequency antenna operates at 94 GHz. There are, accordingly, two corresponding waveguide ports for the respective frequency inputs, namely, low frequency port  90  and high frequency port  92 . Low frequency port  90  is a standard WR90 waveguide port, having a 0.9″ by 0.4″ waveguide mouth. High frequency port  92  is a standard WR8 waveguide port having a 0.08″ by 0.04″ waveguide mouth. Standard mounting holes are provided to enable corresponding WR90 and WR8 feed transmission lines (not shown) to be coupled to support housing  42 . In the first embodiment low frequency port  90  is physically located at 90° to high frequency port  92 . High frequency port  92  is axially in line with the dielectric rods of the antenna. Low frequency port  90  tapers over 90° bend  94  to interface with end  96  of housing cylindrical recess  64 . As such, low frequency port  90  tapers to end  96  having guide dimensions  98 ,  100  of 0.9″ by 0.9″ respectively. 
     Support rod  52  can be press fit into housing cylindrical recess  64 . However, support rod  52  can be allowed to be axially moveable to allow frequency tuning of the antenna if desired. 
     Those skilled in the art will appreciate that it is possible to extend this invention to operation in three frequency bands by triaxially embedding dielectric rods of increasing large dielectric constant. This is schematically depicted in FIG.  5 . Core rod  122  of dielectric constant ∈ 4  is inserted into rod  124  of dielectric constant ∈ 3 , which in turn is inserted into rod  125  of dielectric constant ∈ 2 . The non-imbedded portions of the respective rods are surrounded by medium  126  of dielectric constant ∈ 1  (usually air), forming three concentric dielectric transmission lines, which are respectively coupled to high band waveguide transducer  127 , mid-band waveguide transducer  128  and low band waveguide transducer  130 . Dielectric constant ∈ 4  will be greater than dielectric constant ∈ 3 , which will be greater than dielectric constant ∈ 2 , which will be greater than dielectric constant ∈ 1 . By tapering this combined structure in a controlled manner, the transmission line formed by dielectric rods ∈ 1  and ∈ 2  will provide low band radiation  132 , followed collinerally by radiating mid-band radiation  134  from the second embedded transmission line formed by dielectric rods ∈ 2  and ∈ 3 , followed collinerally by radiating high band radiation  136  from the second embedded transmission line formed by dielectric rods ∈ 3  and ∈ 4 . 
     Those skilled in the art can also appreciate that it is possible to extend this invention to operation in four or more frequency bands by increasing the multiple embedding dielectric rods of increasingly large dielectric constant. 
     Further, dielectric rod antennas with periodic perturbations excited by dielectric rod transmission lines have been developed for use over smaller bandwidths (a few percent) to shape the radiation patterns for omndirectional coverage and are described in the literature. These configurations, examples of which are depicted in FIGS. 6 a,    6   b,  and  6   c,  could also be incorporated by those skilled in the art. 
     As has been described hereinabove a coaxial dielectric rod antenna (CDRA) has been provided with multi-frequency collinear apertures that combines thin (relative to a half wavelength in air) dielectric rod antenna elements embedded with a series of one or more coaxial dielectric waveguides with collinear tapered radiating apertures of increasing dielectric constant, forming an array of two or more radiating apertures. Each of the radiating apertures on the CDRA can operate over a broad bandwidth in different frequency bands. All of the elements in the CDRA support both linear and circular polarizations and each of the collinear apertures can be coupled to separate electronics modules each of which are optimized for use in the specific frequency band of operation. 
     When combined into a phased array antenna the CDRA antenna elements can provide several novel features: 
     1) Each radiating aperture on the coaxial rod has an operating bandwidth ratio of at least 2:1. Thus, a two aperture antenna would provide an operating bandwidth of 4:1 and a three aperture antenna would operate over an 8:1 frequency range. 
     2) A multi-aperture CDRA could operate in widely separated frequency bands such as X-Band and W-Band. 
     3) The diameter of the CDRA dielectric waveguides can be very small at the lowest operating frequencies, enabling dense spacing to support operation at the highest operating frequencies. 
     4) The CDRA feed elements reduce the number and complexity of the electronics in the feed manifold by enabling separate, optimized electronics transmitter/receiver (T/R) circuits to be packaged in separate planes located behind the antenna surface. 
     5) The endfire nature of the CDRA eliminates the need for a metallic ground plane at the base of the feed antennas which is required for most currently used broadband antenna feed elements. This will reduce the weight of phased array antennas and enable mounting antennas of this type on plastic and composite surfaces now in common use in aircraft, spacecraft and automotive structures.