Patent Publication Number: US-8970447-B2

Title: Deployable helical antenna for nano-satellites

Description:
BACKGROUND 
     1. Field 
     This invention relates generally to a helical antenna and, more particularly, to a helical antenna that can be folded both axially and radially into a compact configuration suitable to be stowed on and deployed from a nano-satellite. 
     2. Discussion 
     Satellites orbiting the Earth, and other spacecraft, have many purposes, and come in a variety shapes and sizes. One known satellite type is referred to as a cubed nano-satellite (cubesat) that is typically used solely for communications purposes. Cubesats are modular structures where each module (1U) has a dimension of 10 cm×10 cm×10 cm, and where two or more of the modules can be attached together to provide satellites for different uses. 
     Satellites typically employ various types of structures, such as reflectors, antenna arrays, ground planes, sensors, etc., that are confined within a stowed orientation into the satellite envelope or fairing during launch, and then unfolded or deployed into the useable position once the satellite is in orbit. For example, satellites may require one or more antennas that have a size and configuration suitable for the frequency band used by the satellite. Cubesats typically operate in the VHF or UHF bands. Because cubesats are limited in size, their antennas are required to also be of a small size, especially when in the stowed position for launch. Cubesats have typically been limited to using dipole antennas having the appropriate size for the particular frequency band being used. However, other types of antennas, such as helical antennas, have a larger size, and as thus offer greater signal gain, which requires less signal power for use. 
     It is known in the art to deploy helical antennas on various types of satellites other than cubesats. Known satellites that employ helical antennas typically have been of a large enough size where the antenna can readily be stowed in a reduced area for launch. However, these helical antennas have typically been confined only in an axial direction, i.e., in a lengthwise direction, for subsequent deployment. For a cubesat, this level of confinement and reduced size for stowing of a helical antenna is unsatisfactory. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a helical antenna mounted to a cubesat and showing a stowage compartment for the antenna; 
         FIG. 2  is a perspective view of the helical antenna separated from the cubesat and being in a partially stowed configuration; 
         FIG. 3  is a side perspective view of the helical antenna separated from cubesat and being in a fully stowed configuration; and 
         FIG. 4  is an end perspective view of the helical antenna separated from the cubesat and being in a fully stowed configuration. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a helical antenna capable of being folded in both an axial and radial direction for stowing and launch on a rocket is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the helical antenna described herein has particular application for a cubesat. However, as will be appreciated by those skilled in the art, the helical antenna may have other applications. 
       FIG. 1  is a perspective view of a cubesat  10  including a single modular satellite body  12 . In this non-limiting embodiment, the body  12  is a cube having the dimensions of 10 cm×10 cm×10 cm and is of the type where other cubesat bodies can be mounted to the body  12 . An antenna deployment box  14  having a cover  18  is mounted to the satellite body  12  in the same manner that other modular bodies would be mounted to the body  12 . In this non-limiting embodiment, the deployment box  14  has dimensions of 10 cm×10 cm×5 cm, which is half of the volume of the body  12 . A helical antenna  16  is shown extending from the deployment box  14  in its fully deployed position as would occur when the cubesat  10  is operational in space. In this non-limiting embodiment, the cover  18  includes four sides of the deployment box  14 . However, other types of deployment boxes having other types of covers will be applicable for stowing the antenna  16 . The antenna  16  is attached to an inside surface of a wall  36  of the deployment box  14  that is attached to the body  12  by any suitable mounting structure  20 . 
     As will be discussed in detail below, in order for the helical antenna  16  to be of the size discussed herein to provide the desired antenna performance, and to allow the antenna  16  to be confined and stowed within the deployment box  14  for launch also of the size discussed herein, and for the antenna  16  to properly deploy to the shape shown in  FIG. 1 , the antenna  16  is configured of certain elements, and is folded in both an axial and radial (cross-section) direction for stowing. 
     When the antenna  16  is collapsed and confined within the deployment box  14  it has some amount of strain energy so that when the antenna  16  becomes “free” it will “open” using its own stored energy to its deployed orientation as shown in  FIG. 1 . Various techniques are known in the art to deploy such an antenna from within a deployment box of the type discussed herein, such as using a fuse-type element that when heated, breaks and allows the cover  18  of the deployment box  14  to flip open under a spring force, or some other actuatable mechanism that allows the cover  18  of the deployment box  14  to open causing the antenna  16  to “spring” out using its stored strain energy. 
     The helical antenna  16  includes a number of elements that are secured together to provide the working antenna element and the structure necessary to support the antenna  16 . Particularly, the antenna  16  includes two helical elements  22  and  24  that are wound and intertwined relative to each other to form an antenna column  26 , where the helical element  22  is wound in a clockwise direction and the helical element  24  is wound in a counter-clockwise direction. In this non-limiting design, only the helical element  22  is an antenna element that receives and transmits the communications signal, where the helical element  24  is a support element. To provide the necessary electrical conductivity, the helical antenna element  22  is covered with or enclosed within an electrically conductive material, such as a copper tape  34  to provide the conductivity to propagate the signals. In other embodiments, the helical element  22  can be made conductive in other suitable ways. Also, in an alternate embodiment, both of the helical elements  22  and  24  can be antenna elements. 
     The column  26  formed by the helical elements  22  and  24  is reinforced by a series of vertical stiffeners  28 , eight in this non-limiting example, circumferentially disposed around the column  26  and being equally spaced apart to provide axial stiffness. In this non-limiting embodiment, the helical elements  22  and  24  are wound outside of the stiffeners  28 . At each location where one of the helical elements  22  or  24  crosses one of the vertical stiffeners  28 , those elements are attached to each other so that they retain their desired shape and configuration. Likewise, at those locations where each of the helical elements  22  and  24  cross each other they are attached together. The stiffeners  28  and the elements  22  and  24  can be secured together in any suitable manner, such as by a suitable adhesive or by using heat to bond or weld the stiffeners  28  and the elements  22  and  24 . The vertical stiffeners  28  and the helical elements  22  and  24  are configured and mounted together so that a mounting end  30  of the antenna  16  at the deployment box  14  has the same diameter as the column  26  and an opposite deployed end  32  of the antenna  16  has a rounded and tapered configuration. 
     In one non-limiting embodiment, the length of the vertical stiffeners  28  and the helical elements  22  and  24  is selected and the helical elements  22  and  24  are wound to have about five coils and a 12° pitch so that the length of the column  28  is about 138 cm to provide the desired antenna performance. In one embodiment, all of the helical elements  22  and  24  and the vertical stiffeners  28  are formed of a fiberglass, such as S-2, that is impregnated with a thermoplastic, such as PEEK, that is pultruded to form a material having a thickness of about 5 mils. These materials give the desired flexibility and rigidity necessary to collapse the antenna  16  as discussed herein, and give the collapsed antenna  16  the necessary spring energy to return to the desired deployed shape. However, as will be appreciated by those skilled in the art, other materials may also be applicable to provide these features. Further, in this non-limiting embodiment, the width of the helical elements  22  and  24  is about ¼ of an inch and the width of the vertical stiffeners  28  is about ⅝ of an inch. Also, the copper tape  34  has a thickness of about 3.5 mils. 
       FIG. 2  is a perspective view of the antenna  16  separated from the satellite  10  shown in a partially folded or stowed position in a radial direction. Particularly, the technician that places the antenna  16  in the stowed position in the deployment box  14  will begin by lining up all of the vertical stiffeners  28  so that they are oriented on top of each other and in contact with each other along the length of the column  26 . Any suitable tool, fixture or other device can be used to assist the technician in performing this operation. In  FIG. 2 , the vertical stiffeners  28  are shown being held together by a series of clips  40 . The clips  40  would not be part of the structure stowed within the deployment box  14 . When the vertical stiffeners  28  are provided in this orientation, the helical elements  22  and  24  are drawn together and extend away from the confined vertical stiffeners  28  in a “rats nest” type orientation. 
     Once the antenna  16  is held in the radially folded position as shown in  FIG. 2 , the technician will then roll the flattened and folded antenna element  16  to form a “ball” shape of the antenna  16  as shown in  FIGS. 3 and 4  that is the final orientation of the antenna  16  that is then placed in the deployment box  14 . The technician can use any suitable tool, fixture or other device to roll the folded antenna  16  to form the antenna ball. For example, the technician can place a cylindrical mandrel (not shown) at an end of the folded column  26  shown in  FIG. 2  and roll the antenna  16  lengthwise around the cylindrical mandrel to form the ball shape. In this design, the technician would begin at the rounded end  32  and roll the antenna  16  towards the mounting end  30 . Once the antenna  16  is formed into the ball shape, the cylindrical mandrel can be slid out of the confined antenna  16 . 
       FIG. 3  shows the vertical stiffeners  28  being configured on top of each other and being wrapped around the helical elements  22  and  24  so that the helical elements  22  and  24  extend outward, as shown. As the antenna  16  is being folded into the flattened configuration and then rolled into the ball configuration, the helical elements  22  and  24  will collapse onto each other into a relatively tight configuration where they will be extending in various directions. Once the antenna  16  is confined within the deployment box  14 , it is under strain, and will quickly deploy to the shape shown in  FIG. 1  when the cover  18  of the deployment box  14  is opened. It is noted that the antenna  16  will collapse on itself when under gravity on earth, but in zero gravity of space, the antenna  16  will maintain its desired shape. 
     The foregoing discussion disclosed and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.