Abstract:
A coaxial helical antenna for transmitting or receiving information through electromagnetic waves includes a first helical antenna comprising a first helix comprising a first diameter and a center cavity; a second helical antenna comprising a second helix comprising a second diameter, wherein the second diameter is smaller than the first diameter, and wherein the second helical antenna is seated within the center cavity of the first helical antenna; a shaped ground plate coupled to the first helical antenna and the second helical antenna; and two microstrip impedance transformers coupled to the first helical antenna, the second helical antenna, and the shaped ground plate.

Description:
GOVERNMENT INTEREST 
       [0001]    The embodiments herein may be manufactured, used, and/or licensed by or for the United States Government without the payment of royalties thereon. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The embodiments herein generally relate to communications systems, and, more particularly, to a communication system for transmitting and receiving information in which information is transmitted on an information-modulated electromagnetic wave that has a carrier frequency, f, and an electric field corresponding to a rotation vector tracing a periodic path at a second frequency that is less than the carrier frequency of the wave. 
         [0004]    2. Description of the Related Art 
         [0005]    Circular polarization (CP) of electromagnetic radiation is a polarization such that the tip of the electric field vector, at a fixed point in space, describes a circle as time progresses with angular velocity ω=2πf. Thus the electric vector, as a function of time, describes a helix along the direction of wave propagation. The magnitude of the electric field vector is constant as it rotates. In conventional systems, when CP is required, the antenna designer has many choices, but for broadband applications a spiral or helical antenna structure often provides the best performance. The principal characteristics of a spiral antenna are broad bandwidth and wide beamwidth. With a spiral antenna, however, designers often have to sacrifice gain to achieve a wide beamwidth. 
       SUMMARY 
       [0006]    In view of the foregoing, an embodiment herein provides an apparatus for sending and receiving information from an electromagnetic wave, the apparatus comprising a first helical antenna comprising a first helix comprising a first diameter and a center cavity; a second helical antenna comprising a second helix comprising a second diameter, wherein the second diameter is smaller than the first diameter, and wherein the second helical antenna is seated within the center cavity of the first helical antenna; a shaped ground plate coupled to the first helical antenna and the second helical antenna; and a microstrip impedance transformer coupled to the first helical antenna, the second helical antenna, and the shaped ground plate. 
         [0007]    Such an apparatus may further comprise a fiberglass shell encasing the first helical antenna and the second helical antenna. Furthermore, the first helical antenna may comprise a first axial length, wherein the second helical antenna may comprise a second axial length, and wherein the first axial length and the second axial length may be equal to each other. In addition, the shaped ground plate may comprise a concave shape. 
         [0008]    Furthermore, such an apparatus may further comprise a splitter comprising a first end coupled to the microstrip impedance transformer and a second end coupled to the first helical antenna and the second helical antenna. Moreover, the first helix may comprise turn-spacing between each turn of the first helix; and a pitch angle for each turn of the first helix. Additionally, the pitch angle may be tan −1 (L/NπD), where L is an axial length of the first helix, N is the number of turns of the first helix and D is the first diameter. In addition, the second helix may comprise turn-spacing between each turn of the second helix; and a pitch angle for each turn of the second helix. Moreover, the pitch angle may be tan −1 (L/NπD), where L is an axial length of the second helix, N is the number of turns of the second helix and D is the second diameter. 
         [0009]    Another embodiment herein provides a system for sending or receiving information from an electromagnetic wave, the system comprising a first helical antenna comprising a first helical element formed as a helix comprising a first diameter and a center cavity; a second helical antenna comprising a second element formed as a helix comprising a second diameter, wherein the second diameter is less than the first diameter and the second helical antenna is seated within the center cavity of the first helical antenna; a shaped ground plate coupled to the first helical antenna and the second helical antenna; a microstrip impedance transformer coupled to the shaped ground plate; and a splitter comprising a first end coupled to the microstrip impedance transformer and a second end coupled to the first helical element and the second helical element. 
         [0010]    In such a system, the splitter may comprise a broadband splitter. Moreover, the splitter may comprise a passive splitter. Furthermore, the splitter may comprise a voltage standing wave ratio approximately equal to two. In addition, the first helical element may comprise first copper tubing and the second helical element comprises second copper tubing. 
         [0011]    Another embodiment herein provides a coaxial helical antenna for capturing an electromagnetic wave comprising a first helical antenna comprising a first helical element formed as a helix comprising a first diameter and a center cavity; a second helical antenna comprising a second element formed as a helix comprising a second diameter, wherein the second diameter is less than the first diameter and the second helical antenna is seated within the center cavity of the first helical antenna; a shaped ground plate coupled to the first helical antenna and the second helical antenna; a first microstrip impedance transformer coupled to the shaped ground plate and the first helical antenna; a second microstrip impedance transformer coupled to the shaped ground plate and the second helical antenna; and a switch comprising a first end coupled to the microstrip impedance transformer and a second end coupled to the first helical element and the second helical element. 
         [0012]    In such a coaxial helical antenna, the switch may allow the first helical antenna and the second helical antenna to be driven independently. Moreover, the first helical element may comprise first copper tubing and the second helical element comprises second copper tubing. In addition, the shaped ground plate may comprise a diameter equal to approximately 0.76λ, where λ is a wavelength of the electromagnetic wave. Furthermore, shaped ground plate may comprise an edge height equal to approximately λ/4, where λ is a wavelength of the electromagnetic wave. Additionally, the shaped ground plate may comprise a concave shape. 
         [0013]    These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The embodiments herein will be better understood from the following detailed description with reference to the drawings, in which: 
           [0015]      FIG. 1  illustrates a schematic diagram of a coaxial helical antenna according to an embodiment herein; 
           [0016]      FIG. 2  illustrates a schematic diagram of a low frequency helical antenna according to an embodiment herein; 
           [0017]      FIG. 3  illustrates a schematic diagram of a high frequency antenna according to an embodiment herein; 
           [0018]      FIG. 4A  illustrates a schematic diagram of a shaped ground plate according to an embodiment herein; 
           [0019]      FIG. 4B  illustrates a schematic diagram of microstrip impedance transformer according to an embodiment herein; 
           [0020]      FIG. 5  illustrates a schematic diagram of a coaxial helical antenna, in a wideband configuration, according to an embodiment herein; and 
           [0021]      FIG. 6  illustrates a schematic diagram of a coaxial helical antenna, in a dual-band configuration, according to an embodiment herein. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein. 
         [0023]    The embodiments herein provide a compact helical radio antenna that is compact in size and capable of both wideband operation and dual-band operation. Referring now to the drawings, and more particularly to  FIGS. 1 through 6 , were similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments. 
         [0024]      FIGS. 1-3  show schematic diagrams of a coaxial helical antenna  1 , and components therein, according an embodiment herein. As shown in  FIG. 1 , coaxial helical antenna  1  includes a low frequency helical (LFH) antenna  10 , a high frequency helical (HFH) antenna  30 , and a shaped ground plate  50 . Coaxial helical antenna  1  also includes a center  3  and optionally includes a fiberglass outer shell  5 . While not shown in  FIG. 1 , different fabrication options are available when fabricating coaxial helical antenna  1 . For example, coaxial helical antenna I may also include a center metal rod support through center  3  and a foam core between the center metal rod support and HFH antenna  30 . Moreover, coaxial helical antenna  1  may also include a hollow core (e.g., without a foam core) and use fiberglass sheets with polyester resin (as described below) to support the structures of coaxial helical antenna  1 . Other options include foam, polyvinyl chloride (PVC) pipe, and a fiberglass tube on which to wind the helix, as discussed in further detail below. In addition, while  FIG. 1  includes a coaxial helical antenna with two antennas covering two separate frequency bands, other configurations are possible. For example, the embodiments herein may include a triaxial helical antenna with three antennas covering three separate frequency bands or configurations with greater than three antennas. 
         [0025]    In  FIG. 2 , with reference to  FIG. 1 , LFH antenna  10  is shown in greater detail. The configuration of LFH antenna  10  includes the circumference, C, of the helical wire coils being chosen near the wavelength, λ c , at the desired center frequency of operation, f a . LFH antenna  10  is designed for a center frequency of operation (e.g., f a =700 MHz) corresponding to a wavelength λ a  (e.g., λ a =16.87-inch). Based on these operating parameters, LFH antenna  10  includes an axial length  12 , a diameter  14 , and an X-turn helix  16  comprising helical element  18 , where each turn of helix  16  has a pitch angle  20  and helix  16  has a turn-spacing  22  between each turn. In addition, diameter  14  forms a helix cavity  24  through the axial length  12  of helix  16 . In addition, LFH antenna  30  may be coupled to a base  26  (e.g., a nylon base, which may be notched). For example, when f a =700 MHz and λ a =16.87-inch, LFH antenna  10  may be a 5-turn helix  16  with pitch angle  20 =15.4° and turn-spacing  22 =4.8-inch. Moreover, helix  16  may have a diameter  14 =5.56-inch and an axial length  12 =2 feet. In addition, while not shown in  FIG. 2 , helical element  18  may comprise hollow copper tubing, with a ¼-inch diameter, embedded in approximately ⅛-inch thick fiberglass (e.g., fiberglass shell  5 , shown in  FIG. 1 ) using polyester resin. 
         [0026]    Optionally, a slightly larger diameter  14  (e.g., D=5.56-inch) may be used, based on the outer diameter of a standard 5-inch PVC pipe (not shown) as a convenient way to support the ¼-inch outside diameter copper tubing. Moreover, the fiberglass thickness is non-uniform owing to the overlapping glass mat but may include an approximately 1/16-⅙-inch thickness when using two or five woven fiberglass mats to encase the ¼-inch diameter hollow copper tubing. In addition, roughly uniform performance over the entire bandwidth may be achieved by using a pitch angle  20  α=tan −1 (L/NπD) for N turns in the helical coil of helix  16 . Although the optimum pitch angle  20  may vary, and tapered windings can be used, the typical choice is a constant pitch angle in the range of approximately 12°-15°. 
         [0027]      FIG. 3 , with reference to  FIGS. 1 and 2 , shows HFH antenna  30  in greater detail. As shown, HFH antenna  30  includes an axial length  32 , a diameter  34 , and an X-turn helix  36  comprising wire helical element  38 , where each turn of helix  36  has a pitch angle  40  and has a turn-spacing  42 . In addition, HFH antenna  30  may be coupled to a base  44  (e.g., a nylon base, which may be notched). Preferable, HFH antenna  30  is configured to operate at a higher frequency than LFH antenna  10 . For example, HFH antenna  30  may operate from 1-1.6 GHz and may have a diameter  34 =2.7-inch so it can fit inside helix cavity  24  (shown in  FIG. 2 ) of LFH antenna  10 . In addition, HFH antenna  30  may include 2-ft axial length  32  that comprises a 10-turn helix  36  with each turn having a pitch angle  40 =15.8° and turn-spacing  42  of 2.4-inch. Although  FIGS. 1-3  illustrate LFH antenna  10  and HFH antenna  30  with equal axial lengths (axial length  12  and axial length  32 , respectively), axial length  12  and axial length  32  may include lengths that are different with respect to each other. In addition, while not shown in  FIG. 3 , helical element  38  may comprise hollow copper tubing, with a ¼-inch diameter, embedded in approximately ⅛-inch thick fiberglass (e.g., fiberglass shell  5 , shown in  FIG. 1 ) using polyester resin. 
         [0028]      FIG. 4A , with reference to  FIGS. 1 through 3 , shows a schematic diagram of shaped ground plate  50 , according to an embodiment herein. As shown, shaped ground plate  50  includes a diameter  52 , with a height  54 . For example, when λ a =16.87-inch, diameter  52  may be 0.76λ a  or 12.75-inch and edge height  54  may be λ a /4=4.22-inch. The size of shaped ground plane  50  may be chosen as small as possible without reducing the gain or pattern purity over the desired bandwidth, although the front-to-back (F/B) ratio decreases with a smaller shaped ground plane  50  size. In addition, the shaped (or cupped) form of shaped ground plane  50  improves the gain ˜1 dB over the entire bandwidth. Additionally, shaped ground plate  50  may also include an outer shell  56  (comprising, e.g., thin fiberglass) attached to shaped ground plate  50  and providing protection to coaxial helical antenna  1 . Shaped ground plate  50  is optionally coupled to at least one microstrip impedance transformer  60 . 
         [0029]      FIG. 4B , with reference to  FIGS. 1 through 4A , shows a schematic diagram of microstrip impedance transformer  60 , according to an embodiment herein. As shown microstrip impedance transformer  60  includes length  62 , a bottom ground plate  64 , and a transmission line  66 . For example, microstrip impedance transformer  60  may be a 50 to 100Ω linear tapered microstrip impedance transformer. Moreover, in one embodiment, microstrip impedance transformer  60  may be approximately three inches along length  62 . In addition, ground plate  64  may include a 1.25-inch wide bottom ground plane, which may be fabricated with two layers of PTFE composites (not shown) using circuit board milling techniques. The material for each layer may have a 125 mil thickness with single sided ½ ounce copper (not shown) and may have a relative dielectric constant, ε r =2.33 and loss tangent, tanδ=0.0012. Two unclad sides are shown in  FIG. 4B  (e.g., side  64   a  and side  64   b ), which may be bonded together with an adhesive film (not shown). As shown in  FIG. 4B , transmission line  66  may have a width that tapers linearly from first width  68   a  (e.g., 669 mil or 17 mm) to a second width  68   b  (e.g., 158 mil or 4 mm) with a wire connection at first width  68   a  (not shown) and a helical element (e.g., helical element  18  or helical element  38 ) directly soldered to the second width  68   b.    
         [0030]    As shown in  FIGS. 5 and 6 , with reference to  FIGS. 1 through 4B , LFH antenna  10  and HFH antenna  30  may be combined in a coaxial arrangement to form coaxial helical antenna  1 . As described in further detail below, LFH antenna  10  and HFH antenna  30  may be connected in parallel (as shown in  FIG. 5 ) or driven individually (as shown in  FIG. 6 ) to yield wideband or dual band operation. For example, coaxial helical antenna  1  may include a splitter  70  (e.g., a broadband splitter) coupled to microstrip impedance transfoiiuer  60  to provide a 50Ω input to LFH antenna  10  and HFH antenna  30 , enabling coaxial helical antenna  1  to operate as a wideband antenna. Splitter  60  may also be embedded within a notch  72  cut into HFH antenna  30 . Thus, as shown in  FIG. 5 , splitter  70  enables coaxial helical antenna  1  to operate as a single feed wideband antenna. 
         [0031]    When connected in parallel, as shown in  FIG. 5 , for example, coaxial helical antenna  1  may include an input impedance near 70Ω and can be driven with 50Ω source impedance. With this arrangement, the input reactance may oscillate approximately at 0±50Ω but the input resistance have may large excursions at the lower frequencies. Above 1 GHz, the reactance may become inductive—increasing to approximately 25Ω at 1.8 GHz. Including the fiberglass structures (not shown in  FIG. 5 , but see fiberglass shell  5  shown in  FIG. 1 ) provides a better match by reducing these low frequency oscillations in the input resistance while the reactance is about the same as without dielectric loading. While not shown in  FIG. 5 , coaxial helix antenna may also comprise increasing diameter  34  of HFH antenna  30  by approximately 20%. 
         [0032]    As noted above, microstrip impedance transformer  60  could also be coupled to a splitter  70  to feed both LFH antenna  10  and HFH antenna  30  with a single input connection (e.g., microstrip impedance transfoirner  60 ). For example, splitter  70  may include a broadband splitter or splitter  70  may include a passive splitter, where splitter  70  may have a voltage standing wave ratio (VSWR) approximately equal to two. When terminated by both LFH antenna  10  and HFH antenna  30 , the return loss oscillates approximately 10 dB by ±5 dB over the entire bandwidth. Moreover, splitter  70  may also have a VSWR; approximately 1.3 for 50Ω loads which increases with the load imbalance and deviation from 50Ω. While not shown in  FIG. 5 , wideband operation may include a single source (e.g., provided via input connector  74 ) to drive both LFH antenna  10  and HFH antenna  30  and is possible through a number of different configurations. For example, a splitter  70  may be replaced with a single transformer situated between an input source and the two helices (e.g., LFH antenna  10  and HFH antenna  30 ) connected together, or a splitter  70  may include a passive splitter situated between an input source and two transformers (not shown in  FIG. 5 ), where the output of each transformer goes to one of LFH antenna  10  and HFH antenna  30 . 
         [0033]    In  FIG. 6 , LFH antenna  10  and HFH antenna  30  are driven individually to yield dual-band operation. As shown, coaxial helical antenna  1  includes a switched input  75  to allow coaxial helical antenna  1  to operate in a dual-band operation and optionally includes a notch  72  in HFH antenna  30 . In addition, coaxial helical antenna  1  may include a microstrip transformer  60  on each helix to provide two 50Ω input connectors, where notch  72  optionally allows a microstrip transformer  60  to provide a 50Ω input connector to HFH antenna  30 . Moreover, during dual-band operation, the non-driven antenna is either left open or terminated. Consequently, dual-band operation is accomplished with switched input  75  coupled to two inputs (e.g., input  76  and input  78 ), possibly from two sources, where only one antenna is driven at a time. In addition, dual-band operation may be configured with a single input (not shown) coupled to input switch  75 , which excites either LFH antenna  10  or HFH antenna  30 . 
         [0034]    The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.