Abstract:
A circularly polarized multifrequency antenna is described. The antenna includes a reflector having a first side and a second side, a first crossed dipole pair having a first resonant frequency and a second crossed dipole pair having a second resonant frequency. The first and second dipole pair are symmetrically disposed on the first side of the reflector and configured to be fed with equal power in a relative phase rotation of 0°, 90°, 180° and 270°.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to circularly polarized (CP) radio antennas and, more particularly, to an antenna comprising at least two pairs of crossed dipole antennas. 
     BACKGROUND OF THE INVENTION 
     Conventional CP radio antennas in a crossed-dipole or “turnstile” configuration are well known in the art. An exemplary conventional CP radio antenna includes crossed dipole antennas fed by a balanced four-phase transmission line and located above a reflecting screen. Its dipole legs of the crossed dipole antennas incline downward toward the screen in order to increase the CP radiation at lower elevation angles relative to the plane of the screen. Antennas of this type can be constructed using simple wires, rods, or printed conductors for the dipole legs. A CP radio antenna having the above discussed features is depicted in FIG.  28-7  of the 3rd edition of the  Antenna Engineering Handbook , published by McGraw-Hill, relevant portions of which are incorporated herein by reference. 
     In U.S. Pat. No. 5,519,407, a CP dual frequency antenna is described. This CP antenna includes four identical antenna elements each of which includes an inductor-capacitor trap positioned along the length of each antenna element. This configuration permits the disclosed CP antenna to operate at two different frequency bands. 
     Furthermore, in U.S. Pat. No. 5,526,009, a linearly polarized (LP) dual frequency antenna is described. This LP antenna includes an antenna assembly that comprises four antenna elements. Each antenna element includes a coil and an elongated arm. Pairs of the elongate arms form dipoles which are of differing lengths so that each pair of antenna elements resonates at a different frequency. 
     SUMMARY OF THE INVENTION 
     The present invention provides a nested turnstile antenna structure capable of transmitting and/or receiving CP electromagnetic waves in more than one frequency band. The antenna of the present invention also has a capability to achieve desired elevation radiation patterns within each frequency band. 
     The present invention is preferably used in reception of CP signals from Global Positioning System (GPS) satellites, and for transmission and reception of L-band communications satellite CP signals (e.g., signals used in the International Maritime Satellite System (INMARSAT) service), but it is not limited to use with above-discussed systems. For instance, the present invention may also be used for multifrequency communications using CP signals, for which omnidirectional, elevation-tailored radiation patterns are required. 
     In the present invention two or more turnstile antenna structures share a common symmetry axis and common reflector. Various design characteristics (e.g., lengths, positions along its symmetry axis, inclinations to a reflector and like) of radiating elements of crossed dipole pairs are preferably selected to achieve the aforementioned radiation characteristics. 
     In particular the present invention provides a circularly polarized multifrequency antenna. The antenna includes a reflector having a first side and a second side, a first crossed dipole pair having a first resonant frequency and a second crossed dipole pair having a second resonant frequency. The first and second dipole pair are symmetrically disposed on the first side of the reflector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred features of the present invention are disclosed in the accompanying drawings, wherein similar reference characters denote similar elements throughout the several views, and wherein: 
     FIG. 1 is a top view of an antenna according to one embodiment of the present invention; 
     FIG. 2 is an elevation view of the antenna of FIG. 1 illustrating one of the two sets of crossed dipoles; 
     FIG. 3 is a schematic diagram illustrating the relative phase between the dipole elements in the arrangement of FIG. 1; and 
     FIG. 4 is a perspective view of the antenna of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1-3, an antenna of the present invention preferably includes a reflector  10  supporting a pair of circuit boards  40   a  and  40   b . Reflector  10  is preferably planar. It should be noted that reflector  10  is not required to be planar. Therefore, in alternative embodiments, reflector  10  may have curved or cavity surfaces or other shaped surfaces as known in the art. The antenna is enclosed in a radome (not shown) for weather protection. 
     Reflector  10  preferably is in the shape of a circle as illustrated in FIG.  1 . The diameter of the circular shaped reflector is approximately 8 inches. Alternatively, reflector  10  may have any quadranal symmetrical shape such as a square or an octagon. A vertical axis perpendicular to reflector  10  passes through the center thereof. The vertical axis is also the symmetry axis of the antenna. The transmission and reception characteristics of the antenna are of concern primarily in the “half-space” above a plane containing reflector  10 . Reflector  10  also establishes a ground plane below the antenna for electromagnetically isolating circuits and other structures underneath reflector  10  from the antenna. 
     Circuit boards  40   a  and  40   b  include a pair of opposing slots (not shown), cut at least halfway across the center of the two circuit boards, allowing the two boards to be slipped together, resulting in an interlocking structure. Each circuit board is preferably fabricated from high frequency circuit material, 0.031 inch thick, with electro-deposited copper on both sides (e.g., type RO4003 from Rogers Corporation, Chandler, Ariz.). Other circuit board material may be used depending on the electrical characteristics of the material at the desired operating frequencies. Using standard printed circuit technology, circuit boards  40   a  and  40   b  are etched to remove the electro-deposited copper. This leaves copper lines on opposite sides of circuit boards  40   a  and  40   b  which form the radiating elements  20   a-d ,  30   a-d  and feed lines  22   a-d ,  32   a-d . The widths of the copper lines are substantially equal to 0.1 inch. To maintain equal electric field potential between the conductors on opposite sides of the boards, plated through holes  50  are preferably placed every 0.2 inch along the center of the copper lines as shown in FIG. 2 with black dots. Subsequently, the copper lines on circuit boards  40   a  and  40   b  are tin-lead plated for corrosion prevention. The above-mentioned values given for the circuit board thickness, conductor line width, and spacing of the plated through holes may be chosen as a matter of convenience, although they preferably should be no more than 5% of the wavelength at the highest operating frequency of the antenna. 
     The copper lines (i.e., conductors) on circuit boards  40   a  and  40   b  form a first turnstile antenna (i.e., a first pair of crossed dipole antennas) operating within a first frequency band and a second turnstile antenna (i.e., a second pair of crossed dipole antennas) operating within a second frequency band. The first antenna comprises radiating elements  20   a-d , that are connected to feed lines  22   a-d . The second turnstile antenna comprises radiating elements  30   a-d , that are connected to feed lines  32   a-d . In reflector  10 , holes  24   a-d , for the first turnstile antenna, and holes  34   a-d , for the second turnstile antenna, allow connection of the corresponding feed lines to circuits (not shown) located beneath reflector  10 . 
     Radiating elements  20   a-d  of the first turnstile antenna, and corresponding feed lines  22   a-d , and radiating elements  30   a-d  of the second turnstile antenna, and corresponding feed lines  32   a-d , are spaced at 90° intervals about the vertical axis of reflector  10 . This allows each of the first and second turnstile antennas, in combination with the reflector, to exhibit quadranal symmetry about the vertical axis. As a result, when signals of equal magnitude, in the relative phase rotation of 0°, 90°, 180° and 270° as illustrated in FIG. 4, propagate either on feed lines  22   a-d  in combination, or on feed lines  32   a-d  in combination, the corresponding first or second turnstile antenna transmits or receives a CP electromagnetic wave along the vertical axis. 
     There are many well known dividing/phasing circuits which can divide a signal into four equal amplitude signals having relative phase of 0°, 90°, 180° and 270°. Examples of suitable dividing/phasing circuits include, but are not limited to, an 180° hybrid coupler which feeds into two 90° hybrid couplers or a 90° hybrid coupler which feeds into two 180° hybrid couplers; and a four-way in-phase divider which feeds four transmission lines each progressively increasing in length by 90°. 
     Returning back to the discussion of the circuit boards  40   a  and  40   b , the spacings of the centers of the first antenna feed lines  22   a-d  and the second antenna feed lines  32   a-d  from the vertical axis discussed above in connection with reflector  10  are substantially equal to 0.1 inch and 0.3 inch. The lengths of the first and second antenna feed lines  22   a-d ,  32   a-d  are substantially equal to 3.762 and 3.562 inches, and the lengths of the first and second antenna radiating elements  20   a-d ,  30   a-d  are substantially equal to 2.593 and 2.360 inches. Radiating elements  20   a-d  of the first (low band) turnstile antenna are preferably inclined at an angle substantially equal to 12.5° below the horizontal, and radiating elements  30   a-d  of the second (high band) turnstile antenna are preferably inclined at an angle substantially equal to 60° below the horizontal. 
     It should be noted that one skilled in the art will recognize that there is a wide variation of possible dimensions, depending on the operating frequencies and desired performance, which will provide a useful multifrequency CP antenna. The resulting antenna impedances may require additional impedance matching structures. The lengths of the radiating elements will nominally be 0.25λ at the corresponding operating frequencies but may be longer or shorter by substantial amounts depending on the other dimensions and whether or not impedance matching circuits are included. For instance, it can be in the range of 0.20λ-0.35λ. Similarly, the lengths of the feed lines will nominally be 0.5λ but may also vary substantially. For instance, it can be in the range of 0.35λ-0.55λ. The inclination angles of the radiating elements and the spacings of the feed lines from the vertical axis will also influence the performance and be subject to a substantial range of dimensions. 
     Even though the above discussed crossed dipole pairs of the present invention use linear dipole elements, other types of elements in various combinations may also be used such as, but not limited to, segmented linear, arcuate, folded dipole elements, as well as elements with more general two-dimensional shapes. In addition, the invention is not limited to the geometry of the preferred embodiment in which the crossed dipole antennas are rotationally aligned. For example, the crossed dipole antennas may be disposed, relative to each other, at an angle of rotation of 45° about the common symmetry axis (i.e., the vertical axis discussed above in connection with reflector  10 ). Furthermore, a transmission line feed as described herein with quadranal symmetry and comprising four conductors may additionally include, for example, a single shield, grounded to the reflector, which surrounds all feed line conductors, or grounded shields each surrounding a feed line conductor so that each conductor-shield pair constitutes a coaxial transmission line. 
     It should be noted that additional turnstile antennas may be included in embodiments of the present invention, thus providing operational capability at corresponding additional frequencies. Moreover, the crossed dipole pairs and the transmission line feeds may be connected in various combinations which may seem more advantageous when used in combination with particular system components including transmitters, receivers, multiplexers and phasing networks. For example, one set of feed lines may be connected to two sets of radiating elements. 
     The antenna of the present invention is preferably utilized in a system which operates from a terrestrial vehicle, with the antenna mounted atop the vehicle such that the reflector  10  is parallel to the ground when the vehicle is level. Because the vehicle may be oriented in an arbitrary direction, it is desirable that the antenna radiation pattern be substantially omnidirectional (i.e., having little variation in azimuth) and further that there be reasonable pattern coverage from zenith down to low elevation angles for operation from the equator to higher latitudes. 
     The preferred operating frequencies of the antenna of the present invention are: 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Signal 
                 Frequency 
               
               
                   
                   
               
             
             
               
                   
                 GPS L2 
                 1227.6 MHz 
               
               
                   
                 L-band Receive 
                 1520-1560 MHz 
               
               
                   
                 GPS L1 
                 1575.42 MHz 
               
               
                   
                 L-band Transmit 
                 1620-1660 MHz 
               
               
                   
                   
               
             
          
         
       
     
     It should be noted that satisfactory performance can be obtained by operating the antenna in two frequency bands, a low band for the GPS L2 signal and a high band encompassing the L-band Receive, GPS L1 and L-band Transmit signals. The first turnstile antenna, comprising radiating elements  20   a-d  preferably operates in the low band, and the second turnstile antenna, comprising radiating elements  30   a-d  preferably operates in the high band. 
     Operation in the high band results in strong signal coupling from the second turnstile antenna to the first turnstile antenna, which may cause severe detuning or loss of signal strength caused by coupling of high band signals to the low band circuits located beneath reflector  10 . These effects are mitigated by using a set of open-circuited transmission-line stubs. Each stub is approximately a quarter wavelength long in the high high band. One stub is connected in shunt to each of the low band circuits beneath reflector  10 , close to each of holes  24   a-d  through which the corresponding low band feed lines  22   a-d  are connected. Each stub presents a very low shunt impedance in the high band, thus decoupling the corresponding low band circuit. Operation in the low band results in negligible signal coupling from the first turnstile antenna to the second turnstile antenna, and therefore corresponding low band decoupling stubs are not required. 
     Although the invention has been described with respect to a preferred embodiment which comprises the best mode contemplated within the present invention, it will be obvious to those skilled in the art that many changes could be made and many apparently different embodiments thus derived without departing from the spirit and scope of the invention. Consequently, it will be appreciated by those skilled in the art that the scope of the invention should not be limited by any of the aforementioned embodiments, but rather that it be interpreted only from the following claims.