Abstract:
An antenna array includes two pairs of linear polarized antennas mounted to a perimeter portion of an airframe with one pair having a polarization normal to the airframe and the other pair having a polarization tangential to the airframe. The antenna array eliminates the cross-polarization problem of an electromagnetic wave incident upon the array when using linear polarized perimeter mounted antennas.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to antenna arrays and particularly to antenna arrays for determining an angle of arrival of an incident electromagnetic wave. Still more particularly, this invention relates to an antenna array that is configured on a perimeter portion of an airframe 
     2. Description of the Prior Art 
     Previous interferometric direction finding antennas typically are either externally mounted on an airframe or are center configured. Externally mounted antennas are bulky and introduce aerodynamic drag that diminishes performance. Center configured antennas are difficult to use with other sensors. Center configured antennas can detect only one circular polarization (left or right), which limits their utility in that the opposite sense circular polarization (right or left) cannot be detected. 
     Suitable surface mounted antennas are linearly polarized, which creates a problem with tracking sources of different polarizations. When the airframe rolls, a linearly polarized antenna becomes cross-polarized relative to the incident wave, which causes the antenna to produce no signal or a noisy low power signal in response to the cross-polarized wave. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes problems associated with prior art direction finding antenna arrays and solves the cross-polarization problem by using pairs of antennas having different linear polarizations such that no incident polarization can be cross-polarized. The preferred arrangement of an antenna array according to the present invention includes two pairs of linear polarized antennas with one pair having a polarization normal to the airframe and the other pair having a polarization tangential to the airframe. 
     An object of the invention is to provide a perimeter configured direction finding (DF) antenna array that is capable of providing orthogonal plane direction finding and polarization diversity over a wide bandwidth. 
     A direction finding antenna array according to the invention comprises a first pair of linearly polarized antennas mounted on opposite sides of a perimeter portion of an airframe such that their polarizations are tangentially directed and are perpendicular to a longitudinal axis of the airframe. A second pair of linearly polarized antennas is mounted to the airframe such their polarizations are normally directed and such that each of the second pair of antennas is equidistant from the first pair of antennas. 
     The first and second antennas may be formed as log periodic folded slot antennas. 
     The third and fourth antennas may be formed as flared notch antennas. Alternatively, the third and fourth antennas may be formed as log periodic folded dipole antennas or other log periodic type antenna. 
     A method according to the invention for determining an angle of arrival θ of an electromagnetic wave having a wave polarization that is incident upon an antenna array mounted to a perimeter portion of an airframe comprises the steps of mounting a first pair of linearly polarized antennas on opposite sides of a perimeter portion of an airframe such that their polarizations are tangentially directed and are perpendicular to a longitudinal axis of the airframe, and mounting a second pair of linearly polarized antennas to the airframe such their polarizations are normally directed and such that each of the second pair of antennas is equidistant from the first pair of antennas. The method of the invention further comprises the step of determining the angle of arrival θ measuring phase differences ΔΨ=Ψ 2 −Ψ 1 =(2π/λd 2  sin θ+α 2 )−(2π/λd 1  sin θ+α 1 ), α 1  and α 2  being phase angles of normalized complex voltages v 1 , and v 2  given by: 
     
       
         ν 1   =e   w   ·e   a1   *=e   jα     1     (1) 
       
     
     and 
     
       
         ν 2   =e   w   ·e   a2   *=e   jα     2     (2) 
       
     
     with e w  and e a  representing complex vectors of the wave and antenna polarizations, respectively. 
     The structure and function of the invention may be best understood by referring to the accompanying drawings, which are not to scale, and to the following detailed description. 
     For a continuously rotating airframe, the phase difference is:              ψ   =         2      π     λ        d                 sin                   θ   source        cos                   φ   roll               (   3   )                                
     where φ roll  is the roll angle. The unambiguous angle of arrival can be determined using the expression:                θ   source     =     arcsin        [       λ     4      π                 d                       ψ     p   -   p         ]               (   4   )                                
     φ source  is determined by peak or zero locations where Ψ p-p  is the peak to peak value of the unwrapped phase difference in co-polarized regions of the two antenna pairs that have been spliced together effectively replacing the cross-polarized regions. Co-polarized regions are regions throughout a roll that are within 3 dB down from perfectly co-polarized points. 
     Alternatively, amplitude comparison may be used to solve ambiguities if sufficient squint is obtained. Side by side antennas may be used to solve the ambiguities. Array processing or monpulse techniques may be used. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a four-element perimeter configured antenna array according to the present invention; 
     FIG. 2A illustrates a first antenna element that may be included in the perimeter configured antenna array of FIG. 1; 
     FIG. 2B illustrates a second antenna element that may be included in the perimeter configured antenna array of FIG. 1; 
     FIG. 3 illustrates antenna element polarizations configured in the array; and 
     FIG. 4 illustrates antenna element locations for a perimeter configured antenna array having eight antenna elements. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a perimeter antenna array  10  includes a first log periodic folded slot (LPFS) antenna  12  formed on a surface  20  adjacent an antenna location A 1 . LPFS antennas are well known in the art. A second LPFS antenna that preferably is substantially identical to LPFS  12  is formed on surface  20  adjacent an antenna location A 2  that is diametrically opposite antenna location A 1 . 
     Surface  20  typically is an exterior portion of an airframe that is illustrated as being cylindrical only for convenience in describing the features of antenna array  10 . Antenna array  10  may configured conformally to an exterior portion of an airframe for a missile or other platform which may for example be an aircraft or it may be enclosed in a radome (not shown). Surface  20  is not limited to a cylindrical geometry, but instead may have any shape that is convenient for forming an airframe of a missile (not shown) or other platform (not shown). 
     A first flared notch antenna  14  is formed on cylindrical surface  20  adjacent an antenna location A 3  that is midway between antenna locations A 1  and A 2 . Flared notch antennas (also called Vivaldi antennas) are also well known in the art. A second flared notch antenna that is preferably substantially identical to flared notch antenna  14  is formed on cylindrical surface  20  at an antenna location A 4  that is diametrically opposite from antenna location A 3 . Therefore, it may be seen that antenna locations A 1 -A 4  are spaced apart by 90° arcs on cylindrical surface  20 . 
     Referring to FIGS. 1 and 2A, folded slot antenna  12  includes a series of spaced apart folded slots  25 ,  27 ,  29  and  31  whose distances from a common origin form a geometric progression. The slots  25 ,  27 ,  29  and  31 , which are radiating slots, are adjacent conducting elements  24 A,  24 B,  24 C and  24 D. The lengths, and hence the resonant frequencies of the slot  25 ,  27 ,  29  and  31  also form the same geometric progression. The folded slots  25 ,  27 ,  29  and  31  are formed in the area adjacent elements  24 A,  24 B,  24 C and  24 D in areas which are etched to expose the dielectric. The slots are etched in pairs as shown in FIG.  1 . and when folded form a mirror image of one another. The specific embodiment illustrated is merely an example of a possible structure for folded slot antenna  12 . 
     There are also phasing slots  16 ,  18 ,  21  and  22  which are dielectric regions inside an electrically conductive layer  24 . Phasing slots  16 ,  18 ,  21  and  22  are filled with a dielectric material. In the elevation view of FIG. 2A, phasing slots  16 ,  18 ,  21  and  22  appear to have trapezoidal shapes. However, because phasing slots  16 ,  18 ,  21 , and  22  conform to the surface of layer  24 , which is shown to be curved in the form of a cylinder, the surface of phasing slots  16 ,  18 ,  21  and  22  are curved. The phasing slots  16 ,  18 ,  21 , and  22  have lengths that form the same geometric progression as their spacings. 
     At this time it should be noted that phasing slots  16 ,  18 ,  21  and  22  are normally present in LPFS antennas, however these phasing slots may be omitted in embodiment illustrated in FIG.  1 . The phasing slots  16 ,  18 ,  21  and  22  are not critical to the operation of the present invention, rather it is the folded slots  25 ,  27 ,  29  and  31  which are required for the operation of antenna array  10  comprising the present invention. 
     It should also be noted that while only four arms are shown on the embodiment depicted in FIG. 1, it is possible to design an antenna array which uses substantially more folded slots than the antenna array depicted in FIG.  1 . 
     Electrically conducting section  24  is surrounded by a dielectric border  26  that separates electrically conducting section  24  from surface  20 , which is also electrically conductive. Electrically conducting section  24  includes a plurality of conducting elements  24 A,  24 B,  24 C and  24 D that surround corresponding phasing slots  16 ,  18 ,  21  and  22 . A conducting strip  24 E extends between conducting elements  24 A and  24 B. Similar conducting strips  24 F and  24 G extend between conducting elements  24 B and  24 C and conducting elements  24 C and  24 D. 
     Flared notch antenna  14  is formed as a dielectric region  28  formed on the surface  20 . Dielectric region  28  has sides  30  and  32  that are separated by a distance that increases from a narrow end  34  to a wide end  36 . 
     FIG. 2A shows the Cartesian coordinated for LPFS antenna  12 . FIG. 2B shows the Cartesian coordinates for flared notch antenna  14 . Antenna polarization may be conveniently defined as the orientation that the electric field vector in an incident electromagnetic wave must have for maximum gain. In both FIGS. 2A and 2B the radiating pattern of the antennas  12  and  14  are Z-directed. The electric field of LPFS antenna  12  has primarily only an x-component Ex whereas the electric field of flared notch antenna has primarily only a y-component Ey. These two polarizations are orthogonal when antennas  12  and  14  are oriented as shown in FIGS. 2A and 2B. 
     When the antenna array  10  has antennas  12  and  14  oriented as shown in FIG.  1  and placed at the antenna locations A 1 -A 4  as described above, the polarizations are aligned as shown by the arrows in FIG.  3 . This perimeter configuration allows for the unambiguous source angle of arrival. 
     The phase difference between a baseline pair of antennas is              ψ   =         2      π                 d     λ        sin                 θ             (   5   )                                
     where λ is the wavelength, θ is the angle of arrival of an incident electromagnetic wave and d is the distance between the antennas. The angle of arrival of an incident wave can be determined unambiguously if the baseline separation (d) is not more than λ/2. In the present invention, the antenna array  10  typically may be located on the perimeter of an airframe so that the distance between antennas may be many wavelengths long. Therefore, if only baseline phase measurements are made, the angle of arrival will be ambiguous. Ambiguities in angle of arrival can be resolved by measuring the change of phase ΔΨ as antenna array  10  is rotated from a first angular orientation to a second angular orientation indicated in subsequent equations by corresponding subscripts  1  and  2 . Antenna array  10  may be rotated by rotating the airframe to which the antenna array  10  is mounted or by mechanical rotation of antenna array  10 . The phase change may be written as: 
     
       
         ΔΨ=Ψ 2 −Ψ 1   (6) 
       
     
     where Ψ 1 =2π/λd 1  sin θ+α 1  and Ψ 2 =2π/λ d 2  sin θ+α 2  and α 1  and α 2  are phase angles of normalized complex voltages v 1 , and v 2  given by: 
     
       
         ν 1   =e   w   ·e   a1   *=e   jα     1     (7) 
       
     
     and 
     
       
         ν 2   =e   w   ·e   a2   *=e   jα     2     (8) 
       
     
     with e w  and e a  representing complex vectors of the wave and antenna polarizations, respectively. 
     The symbols e w  and e a  represent the complex vectors of the wave and antenna polarizations, respectively. The angle of arrival θ can be determined unambiguously from the expression for the change of phase ΔΨ. 
     Rotation of the antenna array  10  also provides polarization diversity. The antenna array  10  experiences co-polarization and cross-polarization throughout a roll. Comparing amplitudes of the received signal allows for a determination of when the antennas are co-polarized or sufficiently matched to make good phase measurements. The phase difference between baseline pairs of antennas is ignored near or at cross-polarization. Therefore rotation of antenna array  10  provides for the capability of using the antenna array  10  to direction find on any received polarization (linear, slant, right hand circular, left hand circular and elliptical). 
     For a continuously rotating airframe, the phase difference is:              ψ   =         2      π     λ        d                 sin                   θ   source        cos                   φ   roll               (   9   )                                
     where φ roll  is the roll angle. The unambiguous angle of arrival can be determined using the expression:                θ   source     =     arcsin        [       λ     4      π                 d                       ψ     p   -   p         ]               (   10   )                                
     φ source  is determined by peak or zero locations where Ψ p-p  is the peak to peak value of the unwrapped phase difference in co-polarized regions of the two antenna pairs that have been spliced together effectively replacing the cross-polarized regions. Co-polarized regions are regions throughout a roll that are within 3 dB down from perfectly co-polarized points. 
     Alternatively, amplitude comparison may be used to solve ambiguities if sufficient squint is obtained. Side by side antennas may be used to solve the ambiguities. Array processing or monopulse techniques may be used. 
     Referring to FIG. 4, polarization diversity may be achieved by using another set of four antennas like the array  10  of FIG. 1 at antenna locations A 1 -A 8  spaced apart by 45° if enough space is available around the perimeter of the airframe. Alternatively, a six element array may comprise the antenna array. 
     Antenna array  10  may be configured conformally to the outer surface of a missile or enclosed by a radome. A cylindrical geometry has been described for simplicity; however, antenna array  10  may be configured on irregular shaped airframes. Flared notch antenna  14  may be replaced by a log periodic folded dipole antenna (not shown) by interchanging the conducting and dielectric portions of the LPFS antenna  12 . Either machining or printed circuit board techniques may be used in forming the antenna array  10 . 
     The antenna elements can be either etched copper antenna elements or mechanically constructed antenna elements. The polarization of the antenna elements of antenna array  10  is critical to the operation of antenna array  10 , the type of antenna elements used on antenna array  10  may vary. 
     The structures and methods disclosed herein illustrate the principles of the present invention. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects as exemplary and illustrative rather than restrictive. Therefore, the appended claims rather than the foregoing description define the scope of the invention. All modifications to the embodiments described herein that come within the meaning and range of equivalence of the claims are embraced within the scope of the invention.