Abstract:
Provided is a low-cost robust antenna that may be discreetly integrated into an article of clothing so that it may be suitable for comfortable use with that article of clothing. The antenna is fabricated from off-the-shelf materials, some of which are commonly available at retail outlets. It may provide the same wear as a good quality piece of clothing. It also reduces electromagnetic coupling with the wearer&#39;s body, reduces apparel weight and bulk, and increases performance when compared to comparable existing systems. The frequency response, gain, or size and shape of the antenna are adjustable, within reason, to user requirements.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The invention described herein may be manufactured and used by or for the government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains to an antenna. In particular, a preferred embodiment is an antenna that is integrated into an object while readily and inconspicuously conforming to irregular soft shapes, such as the human body or an inflatable craft. 
     BACKGROUND 
     Antennas, even so-called conformable antennas, are conventionally fabricated using a relatively inflexible base material. That is to say, the base material may be somewhat flexible in one direction or in one plane, but not easily conformable in all three of its dimensions simultaneously and instantaneously. In other words, these antennas would not be suitable for comfortable active wear as an undergarment, for example. Further, should an antenna be desired for incorporation in a relatively soft object such as an inflatable lifeboat or a weather balloon, the rigid nature of conventional antennas could cause unintended damage when the lifeboat or balloon is subject to buffeting, sudden compression, or abrasion. 
     Another disadvantage of “hard” portable antennas is that when placed in proximity to the human body, electromagnetic coupling with the body degrades antenna performance. Should one desire to reduce or eliminate this coupling, a relatively large ground plane inserted between the antenna and the body is desirable. Using conventional designs, this ground plane would add unnecessary bulk, weight, and cost while reducing mobility of the human or performance of soft objects such as lifeboats and weather balloons. 
     There are available specialized fabrics capable of conducting electromagnetic energy. For example, U.S. Pat. No. 4,722,860, Carbon Film Coated Refractory Fiber Cloth, issued to Doljack et al, Feb. 2, 1988 provides an electrically conductive fabric suitable for use at high temperatures such as might be experienced in electric blankets or as screens for microwave ovens. Processes for coating fabric to change its electrical conductivity while retaining flexibility are evidenced in U.S. Pat. No. 5,723,186, Conductive Fabric and Process for Making Same, issued to Fraser, Mar. 3, 1998, and U.S. Pat. No. 5,804,291, same title and inventor as the &#39;186 patent, Sep. 8, 1998. Further, U.S. Pat. No. 5,906,004, Textile Fabric with Integrated Electrically Conductive Fibers and Clothing Fabricated Thereof, issued to Lebby et al, May 25, 1999, provides electrically conductive clothing. 
     Additionally, there is specialized clothing incorporating readily apparent (to the naked eye) electrical components such as antennas and radios. See U.S. Pat. No. 4,584,707, Cordless Communication System, issued to Goldberg et al, Apr. 22, 1986, and U.S. Patent 5,884,198, Body Conformable Portable Radio and Method of Constructing the Same, issued to Kese et al, Mar. 16, 1999. Although each of these inventions are at least partially incorporated in clothing, their presence is readily observable due to bulk, configuration, or both. That is, they do not conform to the plane of the cloth in which they are integrated. 
     Antennas of recent design intended for portability still incorporate inflexible components. For example, see U.S. Pat. No. 5,874,919, Stub-Tuned Proximity-Fed, Stacked Patch Antenna, issued to Rawnick et al, Feb. 23, 1999, and U.S. Pat. No. 5,949,384, Antenna Apparatus, issued to Ikushima, Sep. 7, 1999. The &#39;919 patent, describes an unconventional design offering performance improvements over existing patch antennas but still incorporating a majority of inflexible components. The &#39;384 patent provides a flexible portable antenna that has no provision for incorporation in an article of clothing or even in layers of cloth. The &#39;384 patent provides for a relatively large antenna that can be folded for transportation, but otherwise is not suitable for continuous wear, for example. Both the &#39;919 and the &#39;384 patents describe very recent antenna technology for use in small spaces or as a portable device but not entirely suitable for discreet insertion in a piece of everyday apparel. 
     Consequently, what is needed is a truly flexible antenna, suited to wear as an article of clothing or incorporation in the fabric of an inflatable such as a lifeboat or a weather balloon. The antenna should be low-cost, robust, and provide a sufficiently large ground plane to prevent unnecessary coupling to human or animal bodies. Further, the antenna should be able to be cleaned, using common methods, if provided as an article of clothing. The antenna can be provided with a suitable connector to enable use with a variety of communications, navigation, medical, emergency, security, and rescue systems, as defined by the user. A preferred embodiment of the present invention addresses these needs. 
     SUMMARY OF THE INVENTION 
     A preferred embodiment of the present invention comprises a flexible patch antenna in a bonded three-layer fabric configuration. A top layer comprises the antenna, a middle layer comprises a dielectric used as an electrical insulator between the top and bottom layers, and a bottom layer comprises a ground plane for the antenna. The three layers are bonded together using a commercial off-the-shelf (COTS) cloth adhesive. In addition, a COTS connector is affixed to the top (non-ground plane) layer for providing electrical communication to a suitable electronic device such as a communications radio or an emergency beacon. Alternatively, the antenna is fed by a miniature waveguide or similar device. The device that will use the antenna is grounded to the ground plane of the antenna, most often through an element of the connector, such as a connection to coax cable shielding. 
     In a preferred embodiment of the present invention the top and bottom layers are fabricated from a COTS ripstop nylon impregnated with conducting metal fibers, the middle layer is a COTS polyester felt, and the COTS connector is mounted on COTS conducting fabric such as the ripstop nylon of the top layer, and sewn or glued into the top layer. The antenna and ground plane sections are sized to optimize reception/transmission of the operating frequencies of an attached electronic device. 
     Advantages of preferred embodiments of the present invention, include: 
     available for use at a variety of operating frequencies; 
     instantaneously flexible in three dimensions; 
     greatly reduced coupling to human or animal bodies; 
     enhanced performance over conventional designs; 
     increased operational flexibility; 
     modular and adaptable to a variety of missions; 
     robust; 
     cleanable using commonly available methods; 
     simplified design of alternate configurations using COTS parts; 
     uncomplicated fabrication; 
     reduced system bulk, complexity and weight; 
     reduced capital costs via use of COTS parts; 
     low maintenance costs; 
     high reliability; and 
     ready upgradability. 
     Embodiments of the present invention can be employed in jackets, shirts, pants, overalls, and other garments. The applications also include sails for sailboats, coverings for non-metallic boats such as wood, fiberglass, or those using a fabric as part of their structure, such as inflatable lifeboats or other small marine craft. It also can be used with hot air balloons, parachutes, weather balloons, flags, wristbands, hats, helmets, integrated security tags, unmanned aerial vehicles (UAVs), combat aerial vehicles (CAVs), hang gliders, and remotely powered vehicles (RPVs). The electronic systems accommodated by a preferred embodiment of the present invention include: emergency locator systems (ELS), communication (COMM) systems, direction finding (DF) systems, global positioning systems (GPS), navigation (NAV) systems, medical identification and monitoring systems (MIMS), and radio location systems (RLS). This configuration saves capital equipment, as well as replacement and maintenance, costs. Further, a preferred embodiment of the present invention may be used in a variety of military, police, rescue, medical, environmental, scientific, security, and commercial roles not enumerated above. 
     Preferred embodiments are fully disclosed below, albeit without placing limitations thereon. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1A shows a top view of the physical configuration of a preferred embodiment the present invention. 
     FIG. 1B shows an edge view of the physical configuration of a preferred embodiment of the present invention, including a grounding connection. 
     FIG. 2 shows the three-layer configuration of flexible materials comprising a preferred embodiment of the present invention. 
     FIG. 3 represents a preferred embodiment of the present invention used as an article of clothing. 
     FIG. 4 shows a representative connector, and its installation, for a preferred embodiment of the present invention. 
     FIG. 5A shows the antenna response for a horizontally polarized half-fab configuration of a preferred embodiment of the present invention. 
     FIG. 5B shows the antenna response for a vertically polarized half-fab configuration of a preferred embodiment of the present invention. 
     FIG. 6A shows the antenna response for a horizontally polarized full-fab configuration of a preferred embodiment of the present invention. 
     FIG. 6B shows the antenna response for a vertically polarized full-fab configuration of a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The theory behind antenna design is straightforward. Using a patch antenna as an example, the design is iterative, using the following procedure:                  first   estimate:                   W     =     c     2      f          ɛ   r                   (   1   )                                
     where: 
     W=width of rectangular antenna, meters (m) 
     c=speed of light, m/sec 
     f=frequency of operation, Hertz (Hz) 
     ε r =permittivity (a constant)                ɛ   eff     =           ɛ   r     +   1     2     +           ɛ   r     -   1     2            (     1   +       10      H     W       )       1   2                   (   2   )                                
     where: 
     ε eff =effective permitivity 
     H=height of antenna, m                Δ   H     =     0.412                       ɛ   eff     +     0.300                   W   H       +   0.262         ɛ   eff     -     0.258                   W   H       +   0.813                 (   3   )                                
     where: 
     Δ=overlap of dielectric for effective length of antenna, m              L   =       c     2      f          ɛ   eff           -     2                 Δ               (   4   )                                
     where: 
     L=electrical length of rectangular antenna plane, m              G   =         π                 W       η                   λ   0              [     1   -         (   kH   )     2     24       ]               (   5   )                                
     where: 
     G=conductance, (ohms) −1  or mhos 
     η=antenna efficiency,            μ   0       ɛ   r                              
     μ 0 =permeability at λ 0    
     λ 0 =wavelength at center frequency, m 
     k=radiation efficiency factor,          2                 π       λ   0                                        R   =     1     2      G               (   6   )                                 
     where: 
     R=resistance of antenna, ohms (Ω)              B   =     0.01668                   Δ   H          (     W     λ   0       )          ɛ   eff               (   7   )                                
     where: 
     B=capacitive susceptance                R   i     =     R                     sin   2          (       π                 X     L     )                 (   8   )                                
     where: 
     R i =antenna feed point 
     or              X   =       L   π                     sin     -   1                R   i     R                 (   9   )                                
     where: 
     X=arbitrary distance from center line of the width of antenna, m 
     Referring to FIG. 1A, the conductive patch  101  for the antenna  100  is sized using the above equations in an iterative process. The patch  101  lies in a plane having a width, W  102 , and a length, L  103 . To establish these dimensions for an antenna polarized along the plane of the conductive patch  101  in the direction of W  102 , as shown via arrows  104 , values for Δ  105  and X  106  are tried, given a desired effective electrical length, L eff    107 , in order to determine the feed point  108  for the antenna  100 . 
     The position of the feed point  108  relative to the edge of the antenna patch  101  determines the input impedance of the antenna  100 . For a typical design, L  103 =λ/2, where λ is the wavelength corresponding to the operating frequency of the antenna  100 . This design forms an open circuit resonator. Given a sufficient value of W  102 , the antenna patch  101  edges at x=L/2 and −L/2 form slot apertures that radiate in phase to form a broadside radiation pattern. 
     In a preferred embodiment of the present invention, the conductive patch  101  is a piece of metallized ripstop fabric manufactured by Swift Textile Metalizing Corporation, 23 Britton Dr., Bloomfield, Conn.  06002-3616 . Referring to FIG. 1B, the conductive ground plane  109 , shown as the bottom layer of a 3-layer sandwich, is also a piece of metallized ripstop fabric manufactured by Swift Textile Metal Corporation. The dielectric layer  110  is a piece of polyester felt commercially available at any retail fabric outlet. The center feed connection  108  is the point of attachment on the conductive patch  101  of a connector (not completely shown) for an external device (not shown). In FIG. 1B, the ground connection is shown from a conductive outer casing  113  of a coaxial cable to a point  111  on the ground plane  109 , the interior of which carries the feed  112  surrounded by a dielectric covering  114  for the antenna  100 . Although shown as a single layer of material in FIG. 1B, the dielectric layer  110  may be comprised of multiple layers of fabric as necessary to electromagnetically insulate the two conductive layers  101  and  109  one from the other. In prototypes developed for a preferred embodiment of the present invention, as many as 8 layers of polyester fabric were incorporated in the dielectric layer  110 . 
     FIG. 2 depicts the typical 3-element plane  200  that comprises a preferred embodiment of the present invention. The active antenna plane  201  may consist of one or more layers of “metalized cloth” as can the ground plane  203 . Each layer of metalized cloth does not necessarily have to comprise the same type of material and neither the active antenna plane  201  nor the ground plane  203  need comprise the same materials or number of layers of metalized fabric. The non-conducting plane  202  is comprised of one or more layers of non-electrically conductive (dielectric) material such as a common felt. The feed location  204  shown in the ground plane  203  provides a location for connecting an external device (not separately shown) to a preferred embodiment of the present invention. 
     FIG. 3 depicts an operational antenna  300  as a piece of clothing  301  having the antenna  302  integrally incorporated therein, perhaps in an inconspicuous fashion by matching external fabric colors and texture. 
     In a preferred embodiment of the present invention, the antenna  302  is integrated into the fabric by stitching the antenna  302  into the garment or piece of clothing  301  to create a camouflage effect for the operational antenna  300 . In a more preferred embodiment, this camouflage effect is further accomplished by using the same fabric or cloth for the antenna  302  and the garment or piece of clothing  301 . In addition, by stitching the antenna  302  into the fabric or cloth of the garment or piece of clothing  301 , the antenna  302  becomes a swatch of the fabric or cloth, rather than a separate unit affixed to the garment or piece of clothing  301 . Similarly, the antenna  302  may be integrated into other fabric or cloth articles such as sails, parachutes, weather balloons, flags, wristbands, hats and any other item that can be constructed of cloth. Possible fabrics may include but are not limited to cotton, wool, polyester, felt, gabardine, POLARTEC®, GORE-TEX®, chin, broadcloth, canvas, and any combination thereof. 
     FIG. 4 depicts an example complete embodiment  400  of the present invention incorporating a coaxial connector  401  used to connect an external device. To improve comfort for the wearer, the coaxial connector is connected via an elbow  402  to the active antenna plane  201 . This connection is electrically insulated from the ground plane  203  by the dielectric layer  202  and a gasket  404  of dielectric material that insulates the exterior of the coax cable  401  from the active antenna  201 . The outside of the coaxial connector  401  is connected via a ground wire  403  to provide a common ground (shown separately for clarity only) for the external device to the ground plane  203  of the antenna. 
     EXAMPLE 
     For an initial configuration, design parameters were copied from an existing conventional, i.e., relatively inflexible in three dimensions, patch antenna. This flexible patch antenna  100  is fabricated from the ripstop material identified above for the conductive layers  101  and  109  and a relatively thin material known generically as “embroidery stabilizer,” having the trade name CUT-AWAY PLUS™, for the dielectric layer  110 . The antenna was constructed as follows: 
     a. Four thicknesses of CUT-AWAY PLUS™ were fused together with an adhesive to serve as the dielectric layer  110 . It was estimated that four thicknesses were needed to provide the required dielectric strength. Bonding was achieved with an adhesive having the trade name FUSEBOND™. 
     b. A single layer of ripstop fabric was fused to one side of this dielectric layer  110 , serving as the conductive ground patch  109 , and an appropriately sized piece of ripstop fabric was fused to the other side of this dielectric layer to serve as the conductive patch  101 . 
     c. An antenna feed was located on the surface of the conductive patch  101  in keeping with the conventional antenna&#39;s design noted above. Through the ground conductive layer  109  and the dielectric layer  110 , a coaxial cable  113 , having properly grounded shielding  112 , was attached to the antenna  100  as shown in FIG.  1 B and used to feed a signal to the antenna  100 . 
     Characteristics of this antenna  100  were then measured, yielding radiation (Gain, dB as a function of angle, θ, off boresite of the antenna for the polarization at which it was measured) and impedance (resistance, Ω) parameters as well as the critical design parameter of relative permitivity, ε r . Having an estimate of ε r , the conductive patch  101  dimensions, including feed location  108 , are adjusted for optimum performance. In addition, the dielectric strength of the dielectric layer  110  is adjusted by altering the number of layers of CUT-AWAY PLUS™. Having determined that multiple thicknesses of CUT-AWAY PLUS™ are needed in most cases, a thicker polyester felt was selected in order to reduce the number of thicknesses required for the dielectric layer  110 . Performance characteristics of Gain vs. θ for various permutations of the basic design are provided in FIGS. 5-6. 
     EXAMPLE 
     A dimensionally smaller antenna patch is affected by shorting to ground one of the radiated sides of the conventionally designed (designated “full-fab”) λ/2 or the smaller λ/4 antennas. When one side is shorted for either the λ/2 or the λ/4 antenna design these antennas are referenced as “half-fab” antennas, indicating a size one-half the length, L  103 , of a conventional “full-fab” or “whole” antenna. The length, L  103 , is the dimension across which the polarization  104  of the antenna  100  is defined. See  Antenna Engineering Handbook , Johnson &amp; Jasik, pp. 7-2 to 7-3. These “shorted” antenna patches were tested and results compared to tests run on “full size” antenna patches. FIG. 5A provides results for a half-fab horizontally-polarized antenna showing a fairly flat, low amplitude response suitable for use as an omni-directional antenna, for example. FIG. 5B provides results for a half-fab vertically-polarized antenna showing a peaked higher gain response, suitable for use as a high-gain directional antenna. In FIG. 6A, a peaked response for a full-fab horizontally polarized antenna indicates the same use of this horizontally polarized antenna as for the half-fab vertically polarized antenna of FIG.  5 B. Conversely, results for the full-fab vertically polarized antenna shown in FIG. 6B parallel the results for the half-fab horizontally polarized antenna whose results are displayed in FIG.  5 A. 
     This brief exploration of the capabilities of different configurations of cloth antennas demonstrates that, as with conventional antenna designs, these flexible fabric antennas are suitable for adaptation, through design manipulation, to a number of different missions. Depending on specific user requirements, any of the parameters in Eqns. 1-9 are manipulated to achieve a desired performance, size, composition, or other characteristic. 
     The above descriptions should not be construed as limiting the scope of the invention but as mere illustrations of preferred embodiments. For example, although examples discussed clothing and soft objects, the antennas may also serve a function as a covering for an irregularly shaped hard object, such as a radiosonde or sonobuoy. The scope shall be determined by appended claims as interpreted in light of the above specification.