Abstract:
An antenna comprises first and second radiating elements for connecting to a first and second potential levels respectively. The first and second potential levels are substantially different for generating an electrostatic field from the first and the second radiating elements. The antenna further comprises first and second field shaping structures for controlling field propagation in a first and second direction respectively. The first and second field shaping structures are interdisplaced for defining a field pathway while the first and second radiating elements are disposed adjacent to the first and second field shaping structures and along the field pathway for directing the electrostatic field in a propagation direction through a liquid medium. More specifically, the propagation direction is defined by the field pathway and substantially perpendicular to at least one of the first and second directions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional application Ser. No. 60/897,898, filed Jan. 29, 2007 and entitled “Directive Antenna For Underwater Communications” incorporated herein by reference in its entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The invention relates generally to antennas. In particular, it relates to an antenna for underwater communications. 
       BACKGROUND 
       [0003]    Conventionally, underwater communications&#39; are achieved using acoustic method. This is because the conductivity of seawater is exceedingly high for practical implementation of underwater communications using electromagnetic radiation methods. 
         [0004]    Specifically, the high conductivity in seawater causes large attenuation in electromagnetic radiation. This results in the electromagnetic radiation incapable of propagating over long distances. 
         [0005]    Although the acoustic method is suitable for long distance underwater communications, the bandwidth of such communications is undesirably limited. Conventional acoustic modem is capable of communicating at a rate of 40 kbps (Kilobytes per second) for up to a distance of a few hundred meters (m). The rate decreases to approximately 10 kbps for a distance greater than 5 kilometers (Km). Additionally, acoustic underwater communications is affected when it occurs close to shore or when there is noise generated by physical movements from underwater objects that are near the acoustic modem. 
         [0006]    With the advent of Autonomous Unmanned Vehicle (AUV), there is an alternative way of transmitting large amount of the data collected via underwater sensors. Instead of sending the data over long distances, AUV is used to reach the proximity of the underwater sensors (in the order of 10 m) to collect data from the sensors. A modem with data transfer rate that is much higher than the 40 kbps achieved by the conventional acoustic modem is desirable. Hence, there is a need for an alternative modem that is capable of delivering high bit rate over a short range in an underwater environment. 
         [0007]    Previous attempts have been made to study underwater communications by means of electromagnetic radiation. Theoretical and experimental studies of dipole antennas immersed in seawater have been proposed by M. Siegel and R. W. P. King in “Electromagnetic Propagation Between Antennas Submerged in the Ocean,” IEEE Trans. Antennas Propagat., vol. 21, pp. 507-513, July 1973. However, the received signal level is undesirably low for existing communication systems. 
         [0008]    This is especially so for existing narrowband systems as the bit rates that the systems are capable of supporting are unclear. A similar concept using a loop antenna is proposed by A. I. Al-Shamma&#39;a, A. Shaw, and S. Saman in “Propagation of Electromagnetic Waves at MHz Frequencies Through Seawater,” IEEE Trans. Antennas Propagat., vol. 52, pp. 2843-2849, November 2004. The authors have proposed that the attenuation in far field propagation is much smaller due to the existence of displacement current. However, this is not verifiable, as suggested by R. Somaraju and J. Trumpf in “Electromagnetic Wave Propagation and the Permittivity of Seawater”. 
         [0009]    A method for underwater communication using electric current has been proposed by H. Momma and T. Tsuchiya in “Underwater Communication by Electric Current” IEEE OCEANS&#39;76, pp. 24C1-24C6. This method is an alternative to the acoustic method for short-range underwater communications. The method is not affected by acoustic noise existing in underwater environment and has been shown to deliver data up to a distance of 150 m. However, the method results in high power consumption. 
         [0010]    There is therefore a need to provide an alternative way for underwater communication that is power efficient and having an improved data transfer rate and communication range. 
       SUMMARY 
       [0011]    Embodiments of the invention are disclosed hereinafter for providing an antenna that is power efficient and having an improved data transfer rate and communication range for underwater communications. 
         [0012]    In accordance with a first embodiment of the invention, there is disclosed an antenna for underwater communications. The antenna comprises a first radiating element for connecting to a first potential level and a second radiating element for connecting to a second potential level, the first and second potential levels being substantially different for generating an electrostatic field from the first radiating element and the second radiating element. The antenna further comprises a first field shaping structure for controlling field propagation in a first direction, and a second field shaping structure for controlling field propagation in a second direction. The first and second field shaping structures are interdisplaced for defining a field pathway while the first and second radiating elements are disposed adjacent to the first and second field shaping structures and along the field pathway for directing the electrostatic field in a propagation direction through a liquid medium. More specifically, the propagation direction is defined by the field pathway and substantially perpendicular to at least one of the first and second directions. 
         [0013]    In accordance with another embodiment of the invention, there is disclosed a method for configuring an antenna for underwater communications. The method involves coupling a first radiating element to a first potential level and a second radiating element to a second potential level, the first and second potential levels being substantially different for generating an electrostatic field from the first radiating element and the second radiating element. The method further involves providing a first field shaping structure for controlling field propagation in a first direction, and a second field shaping structure for controlling field propagation in a second direction. The first and second field shaping structures are interdisplaced for defining a field pathway while the first and second radiating elements are disposed adjacent to the first and second field shaping structures and along the field pathway for directing the electrostatic field in a propagation direction through a liquid medium. More specifically, the propagation direction is defined by the field pathway and substantially perpendicular to at least one of the first and second directions. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]    Embodiments of the invention are described in detail hereinafter with reference to the drawings, in which: 
           [0015]      FIG. 1  is a schematic view of an antenna comprising electrodes for underwater communications according to an embodiment of the invention; 
           [0016]      FIGS. 2 to 4  show schematic views of alternative methods for arranging an array of the electrodes of  FIG. 1 ; 
           [0017]      FIGS. 5 and 6  show a first field shaping structure disposed adjacent to the antennas of  FIGS. 1 and 2  respectively; 
           [0018]      FIGS. 7 and 8  show an additional first field shaping structure disposed adjacent to a pair and an array of the electrodes respectively and opposite to the first field shaping structure; 
           [0019]      FIGS. 9 and 10  show a second field shaping structure disposed adjacent to one and two pairs of the electrodes of  FIG. 1  respectively; 
           [0020]      FIGS. 11 and 12  show an additional second field shaping structure disposed adjacent to one and three pairs of the electrodes of  FIG. 1  respectively and opposite to the second field shaping structure; 
           [0021]      FIGS. 13 and 14  show the electrodes of  FIG. 1  formed adjacent to and directly on the second field shaping structure of  FIG. 9 ; and 
           [0022]      FIG. 15  shows a schematic plan view of the first and second field shaping structures and the additional first and second field shaping structures according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Embodiments of the invention are described hereinafter with reference to the drawings for addressing the need for an antenna that is power efficient and having improved data transfer rate and communication range for underwater communications. 
         [0024]      FIG. 1  shows a schematic view of an antenna  100  for underwater communications according to a first embodiment of the invention.  FIG. 1  also shows a reference coordinate system that consists of an x-axis, a y-axis and a z-axis. The three axes are perpendicular to each other. The antenna  100  has a pair of electrodes  101  comprising a first radiating element  102  and a second radiating element  104 . The first and second radiating elements  102 ,  104  are formed along the x-axis and are preferably coplanar to the x-z plane. 
         [0025]    Each of the first and second radiating elements  102 ,  104  is preferably geometrically shaped as a square. Alternatively, each of the first and second radiating elements  102 ,  104  has a geometric shaped such as a rectangle, square, circle or oval. The first and second radiating elements  102 ,  104  are spaced apart by a separation xl along the x-axis. 
         [0026]    Electric signals are applied to the first and second radiating elements  102 ,  104  for signal transmission through a liquid medium, such as seawater. The first and second radiating elements  102 ,  104  are preferably connected to a first potential level and a second potential level respectively. The first potential level is preferably greater than the second potential level. For example, the first radiating element  102  is connected to a positive voltage while the second radiating element  104  is connected to ground. Alternatively, the electric signals are differential signals. 
         [0027]    Communication range of the antenna  100  is dependable on the separation x 1 . In particular, the communication range is enhanced when the separation x 1  is increased. For a given potential difference between the first and second radiating elements  102 ,  104 , increasing the surface area of the first or second radiating element  102 ,  104  also improves the communication range of the antenna  100 . The increase in the surface area however also increases the power consumption of the antenna  100 . 
         [0028]    Each of the first and second radiating elements  102 ,  104  is preferably made of copper. Conducting materials such as aluminum, gold, silver and alloys are other suitable materials for making the first and second radiating elements  102 ,  104 . 
         [0029]      FIGS. 2 to 4  show different antenna arrangements for arranging an array of electrodes  201 . The array of electrodes  201  comprises multiple pairs of electrodes  101  shown in  FIG. 1  arranged spatially on the x-z plane. Each pair of electrodes  101  generates an electrostatic field that propagates along a propagation direction for generating an electric current along the propagation direction. The magnitude of the electric current is proportional to the strength of the electric field generated. Specifically, the propagation direction is along the y-axis. The positive (+) or negative (−) signs on each of the array of electrodes  201  represent positive or negative potential that is applied to the respective electrodes  101 . Correct signal polarity should be applied to each of the array of electrodes  201  in order to focus the electrostatic field along the y-axis. 
         [0030]      FIG. 2  shows three pairs of electrodes  101  arranged in a row. The three pairs of electrodes  101  are arranged substantially in line along the z-axis, with the centre pair  202  formed directly on the x-axis. Each pair of electrodes  101  is separated along the z-axis from an adjacent pair of electrodes  101  by a separation x 2 . Specifically, the separation x 1  is preferably greater than the separation x 2 . For instance, the separation x 1  is 10 centimeters (cm) while the separation x 2  is 2.5 cm. 
         [0031]      FIG. 3  shows two pair of the electrodes  101  arranged in a non-row and off-line arrangement. Specifically, the upper pair  302  is offset along the x-axis to the left of the z-axis while the lower pair  304  is offset along the x-axis to the right of the z-axis. Alternatively, the upper pair  302  is offset along the x-axis to the right of the z-axis while the lower pair  304  is offset along the x-axis to the left of the z-axis. The positive electrode of the upper pair  304  is separated from the negative electrode of the lower pair  304  by a separation x 3 . For optimal performance, the separation x 1  is preferably smaller than the separation x 3 . 
         [0032]      FIG. 4  shows three pairs of the electrodes  101  formed directly along the x-axis in a nesting arrangement. Specifically, a first pair  402  with the smallest separation x 1  is nested in a second pair  404 . The second pair  404  is in turn nested in a third pair  406 . The electrode orientation of the first pair is a mirror of the second and third pairs  404 ,  406 . 
         [0033]    The use of directive antenna enhances communication range in air. This concept is applicable to underwater communications through electric conduction. By using a principle method of in-phase image, radiation generated by the electrodes is enhanced. This is achieved by forming minors or field shaping structures adjacent to the antenna  100  of  FIG. 1 . 
         [0034]    With reference to  FIG. 5 , a first field shaping structure  500  is formed on the y-z plane. The first field shaping structure  500  is preferably made of aluminum. Alternatively, the first field shaping structure  500  is made of other conductive materials such as copper, gold or alloys. The electrostatic field generated by the first and second radiating elements  102 ,  104  of the antenna  100  of  FIG. 1  is thereby focused along the y-axis. 
         [0035]    The first field shaping structure  500  has a length l, thickness w and height (not shown). The length l is preferably several times greater than the length Eh of each of the first and second radiating elements  102 ,  104  for effective focusing of the electrostatic field along the y-axis. The thickness w is preferably a few millimeters (mm), for example 2 mm. The communication range of the antenna  100  is proportional to the height of the first field shaping structure  500 . Exemplary dimensions for the length l and height are 30 cm and 50 cm respectively. 
         [0036]    The first field shaping structure  500  is spatially separated from the first radiating element  102  by a separation s 1  in an arrangement where the first radiating element  102   1− is proximal to the first field shaping structure  500  and the second radiating element  104  is distal thereto. The separation s 1  is preferably as small as possible but the first field shaping structure  500  and the first radiating element  102  should not be contacting each other. For example, the separation s 1  is approximately 5 cm. 
         [0037]      FIG. 6  shows the first field shaping structure  500  formed adjacent to the array of electrodes  201  of  FIG. 2  in an arrangement similar to that of  FIG. 5 , where the first radiating element  102  is proximal to the first field shaping structure  500  and the second radiating element  104  is distal thereto. The array of electrodes  201  enhances the communication range of the antenna  100  along the y-axis. The array of electrodes  201  has an array height Ah. Specifically, the length l of the first field shaping structure  500  is preferably greater than the array height Ah. 
         [0038]    Each of  FIGS. 7 and 8  shows an additional first field shaping structure  700  formed opposite and substantially parallel to the first field shaping structure  500  on the y-z plane. The additional first field shaping structure  700  further enhances focusing of the electrostatic field along the y-axis. The additional first field shaping structure  700  is separated from the second radiating element  104  by a separation s 2 . Specifically, the second radiating element  104  is proximal to the additional first field shaping structure  700  and the first radiating element  102  is distal thereto. The separation s 2  is preferably as small as possible but the additional first field shaping structure  700  and the second radiating element  104  should not be contacting each other. For example, the separation s 2  is approximately 5 cm. 
         [0039]    With reference to  FIG. 9 , a second field shaping structure  900  is formed on the x-y plane. The second field shaping structure  900  also enhances the electrostatic field along the y-axis. At the same time, the electrostatic field in the space on the opposite side of the second field shaping structure  900  is reduced. This is because the second field shaping structure  900  impedes propagation of the electrostatic field. 
         [0040]    The second field shaping structure  900  is preferably made of perspex. Alternatively, the second field shaping structure  900  is made of other insulating or non-conductive materials such as fiberglass, plastics or air. 
         [0041]    The second field shaping structure  900  has a length l, thickness w and height (not shown). The length l is preferably several times greater than the width Ew of each of the first and second radiating elements  102 ,  104  for effective focusing of the electrostatic field along the y-axis. The thickness w is preferably a few mm, for example 5 mm. The communication range of the antenna  100  is proportional to the height of the second field shaping structure  900 . Exemplary dimensions for the length land height are 60 cm and 50 cm respectively. 
         [0042]    The second field shaping structure  900  is separated from the first and second radiating elements  102 ,  104  by a separation s 3 . Specifically, the first and second radiating elements  102 ,  104  are equally separated from the second field shaping structure  900  by the separation s 3 . The separation s 3  is preferably as small as possible. For example, the separation s 3  is approximately 10 cm. 
         [0043]      FIG. 10  shows two pairs of electrodes  101  formed adjacent to the second field shaping structure  900 . The two pairs of electrodes  101  enhance the communication range of the antenna  100  along the y-axis. 
         [0044]      FIGS. 11 and 12  show an additional second field shaping structure  1100  formed opposite and substantially parallel to the second field shaping structure  900  on the x-y plane. The additional second field shaping structure  1100  further enhances focusing of the electrostatic field along the y-axis. The additional second field shaping structure  1100  is separated from the first and second radiating elements  102 ,  104  by a separation s 4 . Specifically, the first and second radiating elements  102 ,  104  are equally separated from the additional second field shaping structure  900  by the separation s 4 . The separation s 4  is preferably as small as possible. For example, the separation s 4  is approximately 10 cm. 
         [0045]      FIGS. 13 and 14  show the electrodes  101  formed adjacent to the second field shaping structure  900  of  FIG. 9 . In particular, the direction of propagation is along the positive y-axis when the electrodes  101  are formed directly on the second field shaping structure  900 , as shown in  FIG. 14 . In this case, there are no separation between the electrodes  101  and the second field shaping structure  900 . 
         [0046]      FIG. 15  shows a schematic plan view of the first and second field shaping structures  500 ,  900  as well as the additional first and second field shaping structures  700 ,  1100 , according to another embodiment of the invention. In particular, the first field shaping structure  500  is substantially parallel to the additional first field shaping structure  700  and is disposed along the x-axis. Similarly, the second field shaping structure  900  is substantially parallel to the additional second field shaping structure  1100  and is disposed along the z-axis. 
         [0047]    The pair of electrodes  101  is modeled as an electric current element  1500  represented by a solid arrow. The electric current element  1500  is mirrored about the first field shaping structure  500  and the additional first field shaping structure  700  as well as the second field shaping structure  900  and the additional second field shaping structure  1100 . Dashed arrows represent the mirrored or virtual electric current elements  1502  that are in-phase with the electric current element  1500 , as shown in  FIG. 15 . 
         [0048]    The first field shaping structure  500  and the second field shaping structure  900 , together with the additional first field shaping structure  700  and the additional second field shaping structure  1100  advantageously define a field pathway to provide directivity of the electrostatic field along the y-axis. This allows the communication range of the antenna  100  to be enhanced without increasing power consumption. 
         [0049]    In the foregoing manner, an antenna for providing underwater communications that is power efficient and having improved data transfer rate and communication range is disclosed. Although only a number of embodiments of the invention are disclosed, it becomes apparent to one skilled in the art in view of this disclosure that numerous changes and/or modification can be made without departing from the scope and spirit of the invention.