Abstract:
A transmission line antenna assembly having a substantially continuous bandwidth from the microwave region of the electromagnetic spectrum to the VHF region of the spectrum. The antenna assembly includes at least one balanced transmission line antenna element of high-temperature superconductor material supported by a substrate, an antenna cavity supporting the substrate and containing a thermally-conductive electromagnetic-energy-absorbing material therein, and a cryogenic cooler for cooling the antenna element to a temperature at which it exhibits superconductivity.

Description:
FIELD OF THE INVENTION 
     The present invention relates to superconducting ultrabroadband antennas generally and, in particular, to a high-temperature superconductor, broadband self-limiting spiral antenna with a controllable signature. While the present invention will be discussed with reference to spiral antennas, it should be understood that the invention is applicable to other forms of transmission line antennas which are not, strictly speaking, spirals, such as log periodic, sinuous, deformed spiral, and ambidextrous antennas. 
     BACKGROUND OF THE INVENTION 
     Broadband antennas are widely used in many contexts, including communications, as radar warning receivers, electronic support measures, and other civil and military applications. In those cases where a broad bandwidth is desired, it is usually necessary to sacrifice some other aspect of antenna performance in order to obtain the desired bandwidth. In some cases, it is necessary to make the antenna larger in order to increase bandwidth. In other cases, increased bandwidth is obtained at the expense of the radiation pattern of the antenna. Neither of these tradeoffs is especially desirable. Where it is desired to mount the antenna on an aircraft or other mobile platform, for example, increased size exacts a penalty in added weight and reduction in space available for other equipment. Degradation of the antenna&#39;s radiation pattern can severely compromise antenna performance. Antenna designers are continually seeking ways to increase antenna bandwidth without sacrificing antenna performance and without incurring increases in size and weight. 
     Much attention has been devoted to improving the bandwidth of transmission line antennas, particularly to extending the bandwidth of spiral transmission line antennas into the VHF/UHF/SHF bands. Prior attempts have primarily focused on improving low-frequency response by increasing the size of the antenna. Increasing the diameter of the spiral, and concomitantly the length of the arms of the spiral, provides frequency scaling that always results in lower gain at the equivalent scaled Lower) frequency. As the length of the spiral arms (sometimes also called the “windings”) is increased, so is their resistance. In many cases, the efficiency of the antenna will become quite low because of losses due to the increased resistance. Thus, a wideband antenna will have lower efficiency at the lower end of the band, since attenuation from loss resistance is greater in that portion of the band. This is because there is a longer distance from the feed points to the “one-wavelength” diameter radiating region of the spiral. The loss per unit length in the windings varies approximately as ƒ ½ , but the length of windings which is effectively used to excite the “one wavelength” diameter radiating region varies as 1/ƒ 2 . As a result, the loss variation due to this effect alone is an inverse function of frequency approximately equal to 1/ƒ {fraction (3/2)} . 
     The inventors have found that reducing conductor resistance by several orders of magnitude will change the conductor loss effect from a 1/ƒ {fraction (3/2)}  relationship (high loss at low frequencies) back toward a ƒ ½  relationship (low loss at low frequencies). The conductor resistance can be decreased dramatically by forming the spiral windings from a high-temperature superconducting (HTSC) material, i.e., materials which exhibit superconductivity at temperatures on the order of 77° K. At 1.0 GHz, HTSC materials are expected to offer surface resistance values as low as 1 μΩ per square, which is several orders of magnitude below cryogenically-cooled copper at 77° K. 
     The inventors are not aware of any previous attempts to improve antenna bandwidth using HTSC or other cryogenic conductors. While A. Septier and N. T. Viet have suggested using low temperature superconductors (which exhibit superconductivity while immersed in liquid helium at about 1.5° K.) to improve antenna Q, it is noted that the result of such an improvement is a very narrowband antenna. See, A. Septier and N. T. Viet, “Microwave applications of superconducting materials,”  J. of Physics E.,  vol. 10, pp. 1193-1207 (1977). U.S. Statutory Invention Registration H653 discloses a superconducting superdirective antenna array using high temperature superconductors, but it discloses only a very narrow bandwidth. 
     There is a need for an ultrabroadband transmission line antenna (i.e., one having a low Q), which covers frequencies from the microwave region down to the VHF region of the spectrum. The present invention fulfills that need. 
     SUMMARY OF THE INVENTION 
     The present invention is broadly directed to a transmission line antenna assembly having a substantially continuous bandwidth from the microwave region of the electromagnetic spectrum to the VHF region of the spectrum. The antenna assembly comprises at least one balanced transmission line antenna element of high-temperature superconductor material (HTSC) supported by a substrate, an antenna cavity supporting the substrate and containing a thermally-conductive electromagnetic-energy-absorbing material therein, and a cryogenic cooler for cooling the antenna element to a temperature at which it exhibits superconductivity. Preferably, the substrate has a crystalline lattice compatible with that of the HTSC and is thermally matched to it. 
     More particularly, the present invention is directed to an antenna assembly having a plurality of antenna elements of high-temperature superconductor material supported by a substrate in which each element forms a spiral. Each element has a first end proximate the first end of each other element. The elements are interwound and define a concentric multi-arm spiral. The antenna assembly also comprises an antenna cavity supporting the substrate and containing a thermally-conductive electromagnetic-energy-absorbing material therein, and a cryogenic cooler for cooling the antenna elements to a temperature at which they exhibit superconductivity. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. 
     FIG. 1 is a simplified schematic illustration of a system incorporating an antenna according to the invention, illustrating one form of mechanical support for the antenna and one way of cryogenically cooling the antenna. 
     FIG. 2 is a top plan view, in simplified form, of a spiral transmission line antenna. 
     FIG. 3 is a top plan view, in simplified form, of a zig-zag spiral transmission line antenna. 
     FIG. 4 is a top plan view, in simplified form, of a wavy spiral transmission line antenna. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, wherein like numerals indicate like elements, there is shown a typical spiral transmission line antenna device  100 . Device  100  comprises a housing  102 , which includes a radome  104  and a thermal shield  106 . Housing  102  is substantially cylindrical in shape. Device  100  further comprises a transmission line antenna  108 , which is supported by a substantially cylindrical body  124 . Body  124  defines channels through which pass video feed lines  110  or a balun  101  to antenna  108 , depending upon which feed method is used. Body  124  is also substantially hollow, and defines a cavity  112  therein which is optionally, and not necessarily, filled with radiation absorbing material  114  as is known in the art, such as treated silicon carbide. Any suitable radiation absorbing material may be used, provided that the material has both good radiation absorption ability and high thermal conductivity, since the antenna device  100  must be cryogenically cooled to approximately 77° K. 
     Body  124  extends below cavity  112  to form a base plate  116 . Base plate  116  is mounted in intimate thermal contact with cryostat  118 . Cryostat  118  is cooled in known manner by a compressor/heat exchanger  120  via gas line  122 . 
     Transmission line antenna  108 , illustrated as a two-arm spiral antenna, is formed on substrate or plate  134 , which is made of magnesium oxide (MgO), lanthanum aluminate (LaAlO 3 ), sapphire, zirconia (YSZ) or any material sufficient to support high temperature superconductors. These materials have a high dielectric constant (∈&gt;10). The plate  134  has a circular periphery  136  and closes the top of cavity  112  within body  124  and is secured thereto to seal the cavity  112 . 
     The top flat surface of the plate  134  supports a pair of antenna elements  139 , respectively comprising spirally interwound conductive windings  140 ,  142  with respective proximate ends  144 ,  146  at the center of the plate  134  and respective distal ends  148 ,  149  close to the periphery  136  of plate  134 . The windings  140  and  142  are made of high-temperature superconducting (HTSC) material such as yttrium barium copper oxide (YBCO), or thallium barium calcium copper oxide TBCCO), HTSC mercury compounds or the like. Of course, any suitable high temperature superconductor can be used without departing from the invention. The proximal ends  144 ,  146  are angularly disposed from each other by about 180° relative to the center of the plate  134 , as are the distal ends  148 ,  149  at the periphery  136  of plate  134 . 
     In practice, it is not vital that the antenna elements  139  be formed of HTSC material on a dielectric substrate. As those skilled in the art will appreciate, the antenna elements could just as easily comprise spiral slots formed in an HTSC plate. Such a spiral slot equivalent antenna will function in the same manner as the antenna described immediately above. In addition, it is possible to fabricate the antenna by forming spiral slots in a metal plate and filling the slots with HTSC material. The resulting metal plate and HTSC element substructure is then mounted to plate  134 . 
     Plate  134  is hermetically sealed to wall  126  of body  124 , and the entire structure is then placed in housing  102  and covered by radome  104 . 
     Because the material from which plate  134  is preferably fabricated has a high dielectric constant, it should be thin in order to minimize reflections from the boundary of the absorbing material  114 . For structural support and heat spreading, a layer  135  of lower dielectric constant is provided between plate  134  and the absorbing material  114 . 
     In order for the electric currents (not radiation) in the spiral windings  140  and  142  to propagate freely in the spiral circuit, the regions immediately above and below plate  134  need to be low-loss, i.e., have a low dielectric constant. The electric field lines of the electric currents in the spiral windings, called the “bound currents,” project a small distance above and below plate  134 . The distance the field lines of these bound currents project is determined, in part, by the dielectric constant and the thickness of plate  134 . The higher the dielectric constant, the shorter the distance the field lines project. Hence, for HTSC spirals using substrates having high dielectric constants, such as LaAlO 3 , the distance is quite small. This permits layer  135  of low dielectric constant material to be thin. 
     In practice, it is not necessary that layer  135  be a physically discrete layer of material. Although it can be, layer  135  can also be a region or portion of the cavity-filling absorbing material  114  that is not treated with an absorbing/resistive material. In other words, the absorbing material  114  in cavity  112  can have a resistive gradation, such that there is no resistive material (and hence a low loss region) near plate  134 , with the resistance (and absorbing properties) increasing gradually toward the cavity bottom. 
     The antenna device may include a diode  150  positioned at the central region of plate  134  between the proximal ends  144  and  146  of the windings  140  and  142 . If used, the diode  150  is electrically connected between proximal ends  144  and  146  by soldering or other suitable method. The distal end  148  of the winding  140  and the distal end  149  of the winding  142  are connected to video lines  152  and  154 , which are connected through the RF front end/bias circuits  155  to connectors  156  and  158  to suitable external bias circuits (not shown). 
     The antenna device  100  may use a balun  101  connected to the central region of plate  134  between the proximal ends  144  and  146  of the windings  140  and  142 . If used, the balun is electrically connected between proximal ends  144  and  146  by soldering or other suitable method. The unbalanced end of balun  101  is connected to the RF front end/bias circuit  155  through connectors  156  and  158  to suitable external bias circuits (not shown). 
     The suitable external bias circuits are not shown since they do not form any part of the invention and are well known to those skilled in the art. 
     With essentially zero ac (RF) losses in the spiral antenna, additional techniques can be applied to slow the wave propagation on the spiral circuit to enhance radiation performance at lower frequencies. Geometric “slow wave” circuits, such as the zig-zag spiral shown in FIG.  3  and the wavy spiral shown in FIG. 4, add significant spiral arm length. However, with the use of HTSC windings, as in the present invention, the added length does not add resistive losses that would reduce spiral gain at low frequencies. 
     Another performance feature made possible by the present invention is the ability to vary the antenna RF signature by varying the temperature (and therefore the conductive state) of the HTSC spiral. By varying the temperature of the spiral about the critical temperature, the spiral can be made to vary from superconducting to exhibiting normal conductivity. This can give the antenna a radiation pattern, or “signature,” that is very difficult to detect. 
     The radiation signature relates to energy which emanates from the antenna, and includes two types of energy: (1) the RF scattered field from the antenna and its mounting host (e.g., an aircraft) caused by an external source (e.g., a search radar), and (2) the direct infrared/electro-optical/ultraviolet radiation (Planck blackbody radiation) that the HTSC antenna itself emits as influenced by its mounting host. Generally, it is desired to not increase the host and HTSC antenna signature above the so-called “slick and clean” condition of the host. In addressing these effects, there are at least two cases to consider: (1) the mounting host is generally metallic, and (2) the mounting host is generally nonmetallic. 
     For RF scattered signature control for a generally metallic mounting host, with the antenna not radiating, the HTSC needs to be in the conducting state so as to “blend in” with the host. That is, the antenna should appear to be metallic. This can be done by fabricating the antenna from a metallic plate and forming spiral slots in the plate. The spiral slots are filled with an HTSC to create windings  140  and  142 . The resulting structure is mounted on plate  134 . At temperatures below the critical temperature, the HTSC is conductive, and the antenna windings  140  and  142  and the metallic plate appear as a continuous piece of metal, thus blending in with the metallic host. At temperatures above the critical temperature, the windings  140  and  142  are nonconductive, whereas the surrounding metallic plate remains conductive, and the normal antenna “on” (radiating) condition occurs. 
     For a generally nonmetallic host, with the antenna not radiating, the antenna needs to be in a nonconductive state in order to blend in with the host. This can be accomplished by keeping the antenna device  100  as already described above the critical temperature. In this case, the windings  140  and  142  and plate  134  all appear to be nonmetallic. Alternatively, the antenna can be fabricated from a sheet of HTSC material with spiral slots formed therein, where the slots behave as the radiating element. Again, by keeping the temperature above the critical temperature, the sheet of HTSC material appears to be nonmetallic. When the temperature is lowered below the critical temperature, the windings become conductive and the antenna functions in the normal radiating mode. 
     The uniqueness of this type of signature control lies in the ability of the HTSC material to vary its conductivity as a function of temperature and to appear as a reasonably good dielectric material when in the “normal” state (above the critical temperature) and as a metallic conductor when in the superconducting state (below the critical temperature). Varying the conductivity of the HTSC is accomplished by changing the temperature of the HTSC well above or well below the critical temperature. 
     RF scattered signature control with the antenna radiating lies partly in conventional methods, which include shaping and physically blending into the host structure so as to minimize discontinuities that end to enhance the RF scattered field. However, HTSC materials offer a further dimension of control in that only portions of the antenna need be turned “on” (i.e., rendered conductive) to achieve improved blending into the host. For example, the central region of the spiral could be cooled so as to be superconducting, while the outer portion of the spiral is heated or otherwise allowed to rise in temperature to the critical temperature or even above. 
     A variation of signature control can be further realized by operating the antenna in a “degraded” mode near the HTSC critical temperature. At these temperatures, the HTSC windings could have higher resistivity than the superconducting state, but still offer useful antenna gain. 
     The quality of the direct IR/EO/UV radiation (Planck blackbody radiation) signature control, whether the antenna is radiating or not radiating, is affected by the coverings or material layers between the HTSC antenna and the “outside world” and their transparency in the IR/EO/UV bands. Assuming that these coverings or materials are IR/EO/UV transparent, then the external signature of the antenna is further affected by the HTSC element temperature and the differential emissivity with respect to the surrounding host and the background temperature at each specific band (IR, EO, and UV), which can be controlled by varying the temperature of the HTSC element. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.