Patent Publication Number: US-9837709-B2

Title: Broadband helical antenna with cutoff pattern

Description:
BACKGROUND OF THE INVENTION 
     Global navigation satellite systems (GNSS) are widely used for high-precision positioning, such as the US Global Positioning System (GPS) and Russian global navigation system GLONASS, as well as some others. A GNSS antenna has to provide signal reception in the whole GNSS range, namely, a low-frequency band 1164-1300 MHz and high-frequency band 1525-1610 MHz. 
     One of the most important positioning errors in GNSS systems is a so-called multipath error, when a signal reflected from the underlying ground surface appears at the input of the receiving antenna along with the line-of-sight signal. 
     The value of the multipath error is proportional to the ratio 
     
       
         
           
             
               DU 
               ⁡ 
               
                 ( 
                 θ 
                 ) 
               
             
             = 
             
               
                 F 
                 ⁡ 
                 
                   ( 
                   
                     - 
                     θ 
                   
                   ) 
                 
               
               
                 F 
                 ⁡ 
                 
                   ( 
                   θ 
                   ) 
                 
               
             
           
         
       
     
     This ratio is normally called the Down/Up ratio. In this ratio, θ is the elevation angle over the horizon, and F(+/−θ) is the antenna pattern (AP) at angle θ above and under the local horizon (θ=0°) correspondingly. A spatial region where θ&gt;0 is the upper or front hemisphere, otherwise, a spatial region at θ&lt;0 is called the lower or backward hemisphere. 
     To provide a stable and reliable operation of positioning systems, quality signal reception from all satellites over the local horizon is required. The value F(θ) in the upper hemisphere is not to highly vary. At the same time, the value F(θ) in the lower hemisphere should be as small as possible. So the value F(θ) should have a sharp drop in the vicinity of the local horizon (i.e., near θ=0°). 
     Receiving antennas thus need to provide such an AP whose level is negligibly varied in the upper hemisphere, sharply drops in crossing the direction to the local horizon, and is small in the lower hemisphere. Also, such an antenna pattern needs to be provided over whole operational frequency range. 
     SUMMARY OF THE INVENTION 
     The objective of the invention is an antenna with an antenna pattern whose level varies slightly in the upper hemisphere, drops in the direction of the local horizon, and is small in the lower hemisphere, over the entire desired frequency range. 
     To implement this objective, a circularly-polarized antenna is utilized in the backfire operation mode, the antenna comprising a set of elements each representing a quadruple cylindrical spiral. The spiral winding angle for neighboring elements is different. An excitation circuit is arranged above the antenna. 
     In another embodiment, an antenna for receiving circularly polarized signals includes a hollow dielectric cylinder (used as mechanical support for the conductors) oriented along a vertical axis; four spiral conducting elements wrapped around the cylinder; the four spiral conducting elements are divided into a plurality of longitudinal sections. The conducting elements in each section have a constant winding angle around the cylinder. The winding angle of all of the conducting elements in the same longitudinal section is the same. Neighboring longitudinal sections have different winding angles relative to each other. An excitation circuit is connected to the conducting elements. 
     Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE ATTACHED FIGURES 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
       In the drawings: 
         FIG. 1  shows an appearance of a quadruple cylindrical spiral antenna; 
         FIGS. 2A, 2B  show quadruple cylindrical spiral elements; 
         FIGS. 3A, 3B, 3C  present embodiments of the design of a quadruple cylindrical spiral antenna; 
         FIG. 4  shows parameters for design embodiments of a quadruple cylindrical spiral antenna shown in  FIG. 3A, 3B, 3C ; 
         FIGS. 5A, 5B  show one of embodiments for a quadruple cylindrical spiral antenna; 
         FIG. 6A  depicts graphs of the antenna pattern for the design shown in  FIG. 3A ; 
         FIG. 6B  presents graphs of the antenna pattern for the design shown in  FIG. 3B ; 
         FIG. 6C  shows graphs of the antenna pattern for the design shown in  FIG. 3C ; and 
         FIG. 7  shows graphs of the DU ratio for elevation θ=10° for embodiments shown in  FIG. 3A, 3B, 3C . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     A wideband circularly-polarized antenna is proposed to receive GNSS signals. According to  FIG. 1 , the antenna comprises a set of quadruple spiral elements  101 , an excitation circuit  102 , and a power cable  103 . The antenna design is elongated along the vertical axis (z). Positive direction of axis z corresponds to θ=90°. 
     The excitation circuit  102  is located above, and, thereby, the backfire operation mode is implemented. The power cable  103  is in the center of the antenna. The upper end of the power cable  103  is connected to the excitation circuit  102 . The lower end of the power cable  103  is connected to the input of a low-noise amplifier (the LNA is not shown). 
     The excitation circuit is well-known and is an equal-amplitude power splitter with one input and four outputs. The phase difference between neighboring outputs is 90 degrees. Each output of the excitation circuit is connected to a corresponding conductor of the first (upper) quadruple spiral element, thereby providing excitation of a right hand circular polarization (RHCP) wave in the positive direction of the vertical antenna axis z. The antenna pattern has maximum in this direction. 
     Each of quadruple spiral elements consists of four conductors wound at the same angle and forming a quadruple spiral whose axis is aligned with the z axis. Each conductor is one spiral turn of the quadruple spiral. The winding angle for the conductors is the same for the entire quadruple spiral element. 
       FIG. 2A  shows quadruple spiral elements  201 ,  202 ,  203 ,  204  and corresponding forming conductors:  2011 ,  2012 ,  2013 ,  2014 ;  2021 ,  2022 ,  2023 ,  2024 ,  2031 ,  2032 ,  2033 ,  2034 . The conductors are applied to a dielectric substrate (not shown) that is further bent to form a hollow cylinder. 
     Each conductor has a first (top) and second (bottom) ends. From  FIG. 2B , the first and second conductor ends (for example,  2024  and  2034 ) of neighboring spiral elements (for example,  202  and  203 ) geometrically match. 
     The exception of this rule is conductors of the first (top) and the last (bottom) elements. First (top) conductor ends of the first quadruple spiral element are connected to the excitation circuit, and second (bottom) conductor ends of the last quadruple spiral element are open. 
     Thus, the antenna includes a set of two or more quadruple spiral elements. A feature of the design is the same winding angle for the conductors of the same spiral elements, while the conductors of the neighboring spiral elements have different winding angles. 
       FIGS. 3A, 3B, 3C  show possible embodiments of the spiral antenna.  FIG. 3A  presents a design of the spiral antenna with seven spiral elements,  FIG. 3B  shows a design with nine spiral elements, and the embodiment of  FIG. 3C  includes eleven spiral elements. In Table of  FIG. 4  there are parameters of the embodiments shown. Note that although the described embodiments use 4 spiral conductors, more (e.g., 6 or 8) or fewer (e.g., 3) can also be used. 
     First and second conductor ends of the neighboring spiral elements can mismatch. 
       FIG. 5A, 5B  show an embodiment with mismatching first and second conductor ends of the neighboring elements. In this case, the conductors of the neighboring spiral elements are connected to each other by conductors  51 ,  52 ,  53 ,  54  which are circle segments. 
       FIG. 6A ,  FIG. 6B , and  FIG. 6C  show graphs of antenna patterns normalized to the zenith (θ=90°) for different design embodiments. Parameters of these embodiments are given in  FIG. 4 . It can be seen that the antenna provides an AP with a nearly stable level in the upper hemisphere, a drop in the level close to the horizon, and a small level in the lower hemisphere. 
       FIG. 7  presents frequency graphs for DU ratio at θ=10°, that is 
               DU   ⁡     (   θ   )       =       F   ⁡     (       -   10     ⁢   °     )         F   ⁡     (     10   ⁢   °     )               
for different embodiments. Embodiments 2 and 3 are seen to provide a DU (θ=10°) ratio at least −15 dB in the whole frequency range from 1164-1610 MHz. Embodiment 1 produces the worst ratio DU (θ=10°) in the high-frequency part of the range, but the actual antenna has the smallest dimensions, of the three embodiments discussed herein.
 
     Having thus described a preferred embodiment, it should be apparent to those skilled in the art that certain advantages of the described method and apparatus have been achieved. 
     It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims.