Abstract:
A directional antenna has a body made of a stack of layers of dielectric panels. A radiating plate is recessed in the top panel of the stack. A grounding plate is attached to the bottom panel of the stack. A feed wire attaches to the radiating plate to feed a signal to the radiating plate. A grounding conductor attaches to the grounding plate for ground. In at least one embodiment the internal feed wire of a coaxial connector provides the feed wire and the external chassis of the coaxial connector provides the grounding conductor.

Description:
[0001]    This application claims priority from U.S. Provisional Application No. 61/495,519, filed on Jun. 10, 2011. The entire disclosure contained in U.S. Provisional Application 61/495,519, including the attachments thereto, is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This application relates to the field of transmitter and receiver antennas in the ultra-high frequency (UHF) band. More specifically, this application relates to antennas for directional transmission over long distances. 
       BACKGROUND OF THE INVENTION 
       [0003]    Wireless communication systems are ubiquitous and have demanding requirements for their signal transmission components. Components that transmit and receive the signal must be small and easily packaged within the wireless system as well as economically manufactured. A key transmission component is the transmission antenna. Although the antenna must be small, it must also provide high gain and have the capability to produce a directional transmission. The effectiveness of the directional aspect of the antenna may be measured by the ratio of signal strength in the desired direction to signal strength in the opposite direction. This ratio may be called the front-to-back ratio. Current antennas in wireless systems are inconveniently large, and expensive to manufacture. Therefore, there is a need for improvement in transmission antennas, including in their size, effectiveness, and cost, particularly for those used in wireless communication systems. 
       SUMMARY OF THE INVENTION 
       [0004]    To overcome the several weaknesses of current directional antennas, such as excessive size, high cost, difficult application integration, and lack of portability, the many possible embodiments of the directional antenna of the present invention utilize a radiating element and multiple layers of dielectric panel. The multiple layer dielectric panel portion of the directional antenna design comprises a novel, stacked structure that can use conventional printed circuit board (PCB) material as the dielectric panel material to accomplish reduced size and lower cost. By passing the signal through a stack of PCB type layers with different dielectric coefficients and different thicknesses, the signal field can be shaped and directed. While realizing substantial cost savings from a low cost manufacturing process, this antenna provides excellent directional performance. One embodiment of the directional antenna of the present invention provides a high gain of approximately 6 dBi and a high front-to-back ratio of up to 18-20 dB. 
         [0005]    The invention in the present application capitalizes on the phenomenon that an electromagnetic wave travels through dielectric material much slower than it travels through air. Embodiments of the directional antenna stack multiple layers of dielectric material with same or different dielectric constants and same or different thicknesses between the top transmission layer, which contains the radiating element, and the bottom ground layer. This achieves a much smaller antenna ground area, thus decreasing the dimensions of overall antenna. Modeling and experimentation indicate that the higher the dielectric constant of the middle layers and the thicker those middle layers, the smaller the size of the antenna. The antenna can be square, rectangular, or round in shape, and, optimally, the minimum dimension across the ground layer should be no less than ¼ of the wavelength of the operating frequency of the antenna. The transmission layer at the top can be smaller in area than the ground layer. The antenna can incorporate a standard connector such as a 50 ohm or 75 ohm coaxial connector for easy connection to a transmitter circuit. The chassis of the connector is soldered to the antenna&#39;s ground layer, and the central feed wire of the connector is soldered to the radiating element in the transmission layer. According to the specific impedance of the particular antenna design, the feedback point can be positioned at different locations on the transmission layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a perspective view of an embodiment of an antenna of the present invention. 
           [0007]      FIG. 2  is a top view of the embodiment shown in  FIG. 1 . 
           [0008]      FIG. 3  is a side section view of an embodiment of a directional antenna of the present invention. 
           [0009]      FIG. 4  is a bottom view of the embodiment shown in  FIG. 1 . 
           [0010]      FIG. 5  is a 2 dimensional map of the field generated by the embodiment shown in  FIG. 1 . 
           [0011]      FIG. 6  is the instrument testing diagram of an embodiment of an antenna of the present invention—log plot. 
           [0012]      FIG. 7  is the instrument testing diagram of an embodiment of an antenna of the present invention—Smith plot. 
           [0013]      FIG. 8  is the instrument testing diagram of an embodiment of an antenna of the present invention—Phase plot. 
           [0014]      FIG. 9  is the instrument testing diagram of an embodiment of an antenna of the present invention—Delay plot. 
           [0015]      FIG. 10  is the instrument testing diagram of an embodiment of an antenna of the present invention—Polar plot. 
           [0016]      FIG. 11  is the instrument testing diagram of an embodiment of an antenna of the present invention—SWR plot. 
           [0017]      FIG. 12  is a perspective view of a second embodiment of a directional antenna. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0018]      FIG. 1  is a perspective view of an embodiment of an antenna  10  of the present invention. Antenna  10  has a radiating element  20  embedded within a shaping body  30 . Shaping body  30  is comprised of multiple layers  32  of a dielectric material, such as printed circuit board (PCB) material. Each layer  32  is in full contact with neighboring layers to form a contiguous stack with negligible voids in shaping body  30 . Connector  40  at the bottom of shaping body  30  provides a site for connecting to an external connection. Connector  40  conducts a signal to radiating element  20  within shaping body  30 . Ground plate  50  at the bottom of the stack of layers  32  provides the ground structure for directional antenna  10  as well as serving to reflect upward the signal from radiating element  20 . 
         [0019]      FIG. 2  is a top view of the embodiment of antenna  10  shown in  FIG. 1 . From the top it can be seen that radiating element  20  is generally centered within shaping body  30  about the vertical axis (z axis). Also, radiating element  20  generally has the contours of shaping body  30 , with a border of material surrounding and enclosing the perimeter of radiating element  20 . The corner cuts of the rectangular radiating element  20  are designed to extend directional antenna&#39;s  10  bandwidth. 
         [0020]      FIG. 3  is a side section view of an embodiment of directional antenna  10 . In  FIG. 3 , it can be seen that layers  32  form a contiguous stack without spacing or voids. In the embodiment of directional antenna  10  shown in  FIG. 3 , radiating element  20  is on top of the top layer of layers  32  of shaping body  30  rather than embedded in it as shown in  FIGS. 1 and 2 . Ground plate  50  is more distinguishable in  FIG. 3  at the bottom of shaping body  30 . Radiating element  20  is made of a material suitable for radiating a field or signal, while ground plate  50  is made of a material suitable to ground directional antenna  10  and to reflect signals from radiating element  20 . Both radiating element  20  and ground plate  50  may be made of copper, for example. 
         [0021]    Layers  32  of shaping body  30  are made of dielectric material. Different layers  32  may be made of the same or different material and may have the same or different thickness. Examples of commercially available dielectric materials include FR- 4  glass reinforced epoxy and Teflon. Boundaries  34  occur between adjacent layers  32  of shaping body  30 . 
         [0022]    Connector  40  can be a standard connector such as SMA connector used for coaxial cable to transfer the signal. Chassis  42  of connector  40  is connected to ground plate  50 , for example by soldering. Central feed wire  44  of connector  40  passes through ground plate  50  and layers  32  of shaping body  30  and connects to radiating element  20 . In the embodiment of directional antenna  10  shown in  FIG. 3 , connector  40  is off center. 
         [0023]      FIG. 4  is a bottom view of the embodiment of antenna  10  shown in  FIG. 1 . Connector  40  is somewhat offset in its location in the bottom surface of shaping body  30 . In  FIG. 4  connector  40  is a coaxial connector. 
         [0024]      FIG. 5  is a  2  dimensional map of the field generated by the embodiment shown in  FIG. 1  to illustrate the directional performance of the antenna. In  FIG. 5 , it may be seen that the field is centered about the vertical z axis like antenna  10  and directed upward. The horizontal axis of the graph corresponds to the bottom of the ground plate, and it can be seen that only a minimal amount of field is project from the bottom of the antenna. The higher gain region of the field is in its upper regions. 
         [0025]    The specific physical embodiment shown in  FIGS. 1 ,  2 ,  3 , and  4  is a 915 MHz directional antenna manufactured in layers as discussed above. It dimensions are 120 mm(L)×120 mm(W)×21 mm(H) square shape with multiple layers of PCB panel. From  FIG. 3 , a SMA connector  40  is soldered to antenna ground layer  50  and the connector&#39;s central feed wire is soldered to radiating element  20 . The middle layers are FR4-S0401 prepreg PCB material. Corner cuts  22  of radiating element  20 , best seen in  FIG. 2 , can expand antenna&#39;s  10  bandwidth.  FIG. 5  is a graph of the simulation result of this particular antenna&#39;s effectiveness, and testing results show a 6 dBi gain and 18-20 db front-to-back ratio. Other physical dimensions and constructions may be used for other applications and situations. 
         [0026]      FIGS. 6-11  are graphs of measured characteristics of the particular embodiment described above. The antenna is operated at the stated 915 MHz.  FIG. 6  presents the measured log-plot of the field pattern&#39;s return loss showing the antenna radiates best at 915 MHz.  FIG. 7  presents the embodiment&#39;s measured data in Smith Chart form displaying the antenna impedance. The data shows that the antenna&#39;s impedance is precisely matched at 915 MHz.  FIG. 8  is a measured phase plot of the embodiment showing the phase changes little in the radiated pattern at 915 MHz.  FIG. 9  shows that the antenna emits at a small frequency range centered on 915 MHz resulting in a high antenna Q.  FIG. 10  displays the embodiments measured polar plot, showing the antenna is in nearly perfectly matched at 915 MHz.  FIG. 11  displays the Standing Wave Ratio (SWR) by frequency of the embodiment, clearly showing an SWR of 1.0 at 915 MHz. 
         [0027]      FIG. 12  is a perspective view of a second embodiment of a directional antenna. In  FIG. 12 , middle layers  36  are thicker than those above and below them. Modeling and experimentation indicate that the higher the dielectric constant of the middle layers and the thicker those middle layers, the smaller the size of the antenna. 
         [0028]    Although at least one specific embodiment of a directional antenna is described above, it should be understood that many other embodiments are possible and that the invention of the current application should not be limited to the specific examples described. Additionally, the structure of the directional antenna may be maintained with any of several possible techniques. For example, the several layers may be held together by adhesives between the layers, or held together by screws passing through the several layers, or held together by an external frame clamping the layers together, etc. Other techniques are possible as well.