Patent Publication Number: US-7589694-B2

Title: Small, narrow profile multiband antenna

Description:
TECHNICAL FIELD 
   The present invention is generally directed to multiband antennas. In particular, the present invention is directed to a multiband antenna which is small and provides a narrow profile. Specifically, the present invention is directed to a multiband antenna which provides good isolation properties and is small enough for use with an unmanned ground sensor. 
   BACKGROUND ART 
   Currently, unmanned ground sensors are positioned in remote locations. The sensors collect information and data related to weather and ground conditions. And, these ground sensors can detect physical activity such as movement of vehicles, individuals and animals in a defined area. With appropriate antenna systems, these sensors are also capable of transmitting the collected information to a centralized location by transmitting signals to a nearby ground station, over-flying aircraft, or even satellites. The antenna system functions as part of the overall sensor to detect the aforementioned movements and nearby electrical communications. It will also be appreciated that these unmanned sensors can be re-programmed to monitor other characteristics as needed, or the sensors can be turned off remotely if their use is no longer required. 
   The most effective sensors are believed to be those that are not easily detected. For example, sensors used for military purposes must be inconspicuous and not easily detected. In other words, it is desirable for the sensor and antenna to be small and adaptable to various environmental settings. It is also important for such a sensor and associated antenna to be electrically quiet so as to avoid detection by other sensors. 
   Skilled artisans will appreciate that these devices must be able to operate in selected frequency ranges and also operate in multiple frequency ranges. Unfortunately, such antennas known in the art can be quite sizeable and easily detected, thereby defeating their purpose. Therefore, there is a need in the art for a multiband antenna to be small, compact, easily concealed, and capable of operating on multiple and select frequency ranges. And there is a need for the sensor and associated antenna to operate in an electrically quiet manner. 
   SUMMARY OF THE INVENTION 
   In light of the foregoing, it is a first aspect of the present invention to provide a small, narrow profile multiband antenna. 
   It is another aspect of the present invention to provide a multiband antenna system comprising a helical antenna having a first leg and a second leg, wherein the first leg comprises a coaxial conductor, and at least one antenna sub-system coupled to the helical antenna, wherein the coaxial conductor feeds the at least one antenna sub-system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a complete understanding of the objects, techniques and structure of the invention, reference should be made to the following detailed description and accompanying drawings, wherein: 
       FIG. 1  is a perspective view of a multiband antenna according to the concepts of the present invention; 
       FIG. 2  is an electrical schematic diagram of the multiband antenna according to the concepts of the present invention; 
       FIG. 3  is a partial perspective view of selected components of the antenna according to the present invention; 
       FIG. 4  is another partial perspective view of selected components of the antenna; 
       FIG. 5  is a top perspective view of a printed circuit board used in the multiband antenna according to the concepts of the present invention; 
       FIG. 6  is a partial top perspective view of the antenna showing the printed circuit board with a coaxial cable connected thereto; 
       FIG. 7  is a partial bottom perspective view of the printed circuit board and the coaxial cable shown in  FIG. 6 ; 
       FIG. 8  is a plot of voltage standing wave ratio (VSWR) versus frequency in MHz for a helical antenna which is part of the multiband antenna according to the present invention; 
       FIG. 9  is a plot of voltage standing wave ratio (VSWR) versus frequency in GHz for an antenna sub-system coupled to the helical antenna according to the concepts of the present invention; 
       FIG. 10  is a low band gain versus elevation plot for the helical antenna according to the concepts of the present invention; and 
       FIG. 11  is an antenna gain elevation pattern for the antenna sub-system according to the concepts of the present invention. 
   

   BEST MODE FOR CARRYING OUT THE INVENTION 
   Referring now to the drawings, and particularly to  FIGS. 1 and 2 , it can be seen that a multiband antenna system according to the concepts of the present invention is designated generally by the numeral  10 . The antenna system may be used with ground sensors, other types of sensors, or any other device that requires the transmission and the reception of wireless signals. As will become apparent as the description proceeds, the antenna system  10  is constructed in such a manner so as to provide a relatively small and narrow profile so that the antenna system is not easily detected. Indeed, the antenna system can be constructed in such a manner to blend with the terrain in which the associated sensor is to be located. For example, the antenna system  10  can be provided with brown/green coloring so as to allow it to be placed in fields, or the antenna system  10  can be constructed to match rock textures in a selected region. To allow a rapid change of the type of camouflage, a radio frequency transparent sock  11  can be supplied with the antenna to allow the user to choose what is most appropriate for the mission. The sock  11  can simply be slipped over the installed antenna during deployment. 
   The antenna system  10  includes a base  12  from which may extend a strain relief  14  that is carried by the base. The base  12  is secured to the sensor or other device in a manner understood in the art. A radome  16  may be retained within the strain relief, if provided, and connected to and carried by the base  12 . As is well understood, the base  12  and radome  16  provide an outer covering for the antenna system components. The radome  16  may be appropriately camouflaged, or the appropriate aforementioned sock could be used. Indeed, the sock  11  form fits over the radome  16  and is secured to the base and/or strain relief in any conventional manner such as with an elastic band, hook and loop fasteners, and the like. And the sock can be appropriately colored. For example, a white sock could be used for snowy areas, tan for desert areas, and so on. The sock  11  could also incorporate camouflage patterns as shown in  FIG. 1 . The sock  11  may be made from nylon or any other form-fitting material. Moreover, the sock could be reversible so as to provide for an easy change to the type of camouflage. 
   The antenna system  10  includes a helical antenna which operates in a first range of frequencies. As will be described in detail later, the helical antenna  18  includes a first leg  20  and a second leg  22 . Coupled to and associated with the helical antenna  18  is an antenna sub-system  24  which operates in a different frequency range. In the present embodiment the frequency range of sub-system  24  is C-band. However, the skilled artisan will appreciate that any frequency band antenna could be associated with the helical antenna  18  in view of its attributes to be described. 
   The base  12  provides a connector  26  which is mountable to the appropriate transmitting/receiving equipment associated with the aforementioned sensor. Helical antenna  18  is connected to the connector  26  as best seen in  FIG. 2  and as specifically shown in  FIGS. 3 and 4 . 
   The first leg  20  comprises a coaxial cable designated generally by the numeral  30 . The cable  30  includes a center conductor which is surrounded by an insulation material and an outer shield conductor. In the present embodiment, the coaxial cable  30  is in the form of a conformable coaxial cable. In other words, the outer shield conductor is flexible, but somewhat rigid so that it can be deformed into a desired shape and retain that shape in most any circumstance. A core made of plastic and electrically transparent to RF (Radio Frequency energy), with helical grooves and radius appropriate to the desired design can be used to form this conformable coaxial cable. As will be discussed, the core also carries the second leg  22  in an appropriate manner. 
   An isolation network  31  is associated with the first leg. The network  31  is configured such that the coaxial cable  30  is inserted through and wrapped around a toroid  32 . Specifically, the toroid  32  is made of a ferrite material and provides an opening  34  therethrough. The coaxial cable has an entry end  36  which is received within the opening  34  and forms loops, designated generally by the numeral  38 , around the toroid  32  any number of times, which in the present embodiment is four times, and exits the opening at a coax end  40 . The number of loops  38  for the coaxial cable to be wrapped around the toroid can be of any number to provide the necessary electrical isolation. In conjunction with the number of loops of the cable around the toroid, a resistor  42  is connected across the entry coax end  36  and the exit coax end  40 . The combination of the resistor and the toroid effectively function to terminate the first leg in the network  31  that appears to this portion of the antenna to be resistive. The value of this resistance is chosen to complement the terminal impedance over band-width to allow a low voltage standing wave ratio (VSWR) for the helical antenna. 
   The second leg  22  is formed from a single conductor and is connected to connector  54  that is carried by the base  12 . In this embodiment, the wire  56  is a solid wire and is connected to a transmission line transformer designated generally by the numeral  58 . Although any type of transmission line transformer can be utilized, in the present embodiment it is a 4:1 transmission line transformer. The transformer is of the “Transmission-Line” type of which is even further defined as being of the Guanella type per Transmission Line Transformers . This is not the typical flux coupled transformer but of the type of which windings are actually transmission line whose impedance is the geometric mean of the input and output impedances. Transmission line transformer  58  comprises a toroid  60  which provides a winding form  62 . The wire  56  has an entry end  64  and a loop configuration  66 , best seen in  FIG. 2 , which exits at a wire exit end  68 . The wire is wrapped or wound in a helical configuration per winding schematic and core outline in  FIG. 1 . Prior to the helical windings of the first leg  20  and the second leg  22 , it can be seen that a gas discharge tube  70  is connected across the wire  56  and the outer shield of the coaxial cable  30 . A capacitor  72  is also connected across the wire and the coaxial cable in a similar manner so as to electrically isolate the gas discharge tube. The gas discharge tube is utilized to protect the transceiver equipment from lightening strikes or other large electro-magnetic events. 
   After the connections to the gas discharge tube and the capacitor, the wire  56  and the coaxial cable  30  are formed into corresponding helixes and the wire is formed into a wire helix  74  which is substantially opposite a helix  46  formed by the coaxial cable. The wire and the coax helixes are positioned so as to be on opposite sides of the radome. A plastic core, designated generally by the numeral  76 , may be provided to maintain the desired spacing between the wire helix and the coax helix. Indeed, the core  76  provides an appropriate diameter, pitch and material to properly separate the coaxial cable from the wire and provides the spacing necessary to obtain the desired operational bandwidth of the helical antenna  18 . The core  76  is constructed of a polymeric material such as polycarbonate or other low loss and RF transparent material. At the end of the helical antenna  18 , distal the connectors  26  and  54 , the wire  56  is electrically and mechanically connected to the coaxial cable  30  and in particular to the coax outer shield  78  at a juncture  52 . In the present embodiment, by selectively configuring the components of the helical antenna, it can operate over a frequency band of about 225 to about 450 MHz. 
   The helical antenna  18  is arranged such that it is an electrically shorted, normal mode bi-filar antenna. In this configuration, the first leg  20  is a small coaxial cable that acts as a signal path for another sub-antenna, which is the antenna sub-system  24 , that operates on another frequency range or band. As will be appreciated as the description proceeds, this allows for multiple tactical communications and control links to be established with the unmanned ground sensor. Use of a helical antenna allows for operation of a wide band frequency while providing a small package. Moreover, it is believed the helical antenna differs from known prior art assemblies in that it is electrically shorted on one end, and one leg is terminated in a resistor network that allows for wide band operation and transmitter matching. 
   As best seen in FIGS.  2  and  5 - 7 , it can be seen that the antenna sub-system is designated generally by the numeral  24 . Although any type of antenna configuration could be used, it will be appreciated that the present embodiment provides a double-dipole antenna with an integral choke. The sub-system comprises a circuit board  90  which is isolated from the helical antenna by a choke isolation system  80 . In this embodiment, the system  80  comprises at least one, and if needed a plurality of, ferrite beads  81 . In other words, after the juncture  52 , the coaxial cable is directed through openings in the ferrite beads  81  which function to electrically isolate the helical antenna from the antenna sub-system  24 . Of course, the system  80  may comprise other types of isolation devices. 
   The antenna sub-system  24  includes a circuit board  90  which allows for connections to the coaxial cable  30  and, in particular, an outer shield conductor  78  and a center coaxial conductor  91 . The circuit board  90  provides a substrate  92  which supports both a “grounded coaxial-shield” side  92  and a “hot coaxial center-conductor” side  94  of the overall antenna artwork. The grounded-side  92  of the printed circuit board provides a shield connection point for the feed line  96 . The center conductor of the coaxial cable is connected to the feed line  96  as best seen in  FIG. 6 . The center conductor acts as an excitation probe to energize the hot and grounded sides of the artwork which form a planar-transmission line. This planar feed line  96  is connected to a first dipole  98  which provides dipole arms  100  which are substantially perpendicular to the feed line  96 . Arm extensions  102  extend substantially perpendicularly from the arms  100  and are substantially parallel with the feed line  96 . These extensions  102  extend back toward the helical antenna. The feed line  96  continues beyond the first dipole  98  and a tap point  104  is provided above the dipole arms  100  and extends all the way through the circuit board  90 . The coaxial outer shield  78  is mechanically and electrically connected to the feed line  96 . The center conductor  91  is directed through the tap point  104  and is connected to components on the conductor side  94  of the printed circuit board. 
   The feed line  96  further extends from the first dipole  98  to a second dipole designated generally by the numeral  110 . The dipole  110  includes dipole arms  112  which extend substantially perpendicularly from the feed line  96 . And further extending from the dipole arms  112  are arm extensions  114  which are substantially perpendicular to the arms  112  and extend back toward the first dipole and, in particular, the dipole arms  100 . 
   As best seen in  FIG. 7 , the center conductor is directed through the tap point  104  and is connected to a conductor feed line designated generally by the numeral  118 . The first dipole  98  is provided and connected to the conductor feed line  118  by arms  120  which extend substantially perpendicularly therefrom. Extending from the arms  120  are arm extensions  122  which are substantially parallel to the feed line  118  and extend in a direction opposite that of the arm extensions  102  provided on the shield side of the circuit board. 
   The second dipole  110  includes a pair of arms  126  extending substantially perpendicularly from the feed line  118  and which further extend to arm extensions  128  that are substantially parallel with the feed line  118 . Configuration of the arms and arm extensions with respect to the first and second dipoles is such that a double dipole antenna is formed on the circuit board  90 . By selectively configuring and sizing the spacing of the dipoles, the desired electrical transmission and operating frequency can be obtained for the antenna sub-system  24 . 
   Although a double dipole antenna is shown, the antenna sub-system  24  may be an array of elements which serve to provide other antenna radiation functions. In other words, the antenna sub-system may comprise a single antenna or an array of antennas. In the present embodiment, the double di-pole configuration operates over a frequency band of about 5.0 GHz to about 6.0 GHz. However, it will be appreciated that any antenna configuration could be associated in view of the helical antenna providing a coaxial antenna as one of the branches of the helix. Such a configuration provides a method of effectively feeding and realizing multi-band operation with good gain, isolation and efficiencies which are normally difficult to achieve with small antenna apertures. This configuration is advantageous in that the antennas can be co-located and/or coaxially positioned within a single narrow radome to create a narrow profile, low visual detection antenna system for a tactically deployed unmanned ground sensor or an entire network of such sensor devices. The present construction is also advantageous in that it provides a covering, such as the sock  11 , to help conceal the antenna in different types of environments. 
   As configured, the antenna sub-system is a C-band antenna which provides uniform azimuth of about 7.5 dBi+/−0.5 dB. The antenna sub-system provides an elevation beam width (3 db) of about 12 degrees and a dipole array length of about 8 centimeters. In the present configuration, the C-band antenna provides a frequency range of about 5.0 to about 6.0 GHz with a VSWR of less than 2.5. 
   Referring now to  FIGS. 8-11 , it can be seen that plots of calculated measurements for an antenna configuration utilizing the components described herein provide a helical antenna with a VSWR versus frequency (MHz) plot as shown in  FIG. 8 . A similar plot for the antenna sub-system is shown in  FIG. 9  wherein the VSWR is less than 1.5 over the selected range of frequency. A low band gain versus elevation plot is provided for the helical antenna in  FIG. 10  and an antenna gain elevation pattern for the antenna sub-system  24  is provided in  FIG. 11 . It will be appreciated that these plots are approximations based upon software modeling of the electrical characteristics of the antenna presented herein. 
   Thus, it can be seen that the objects of the invention have been satisfied by the structure and its method for use presented above. While in accordance with the Patent Statutes, only the best mode and preferred embodiment has been presented and described in detail, it is to be understood that the invention is not limited thereto or thereby. Accordingly, for an appreciation of the true scope and breadth of the invention, reference should be made to the following claims.