Abstract:
A device for exciting a waveguide with circular polarization from a plane antenna, said waveguide (10) being a rectilinear hollow waveguide closed at one of its ends (12), said antenna being excited by at least two coaxial ports (13, 14) fed in phase quadrature by a circuit including a hybrid coupler (15), and being constituted by a radiating plane metal pattern (11) disposed on the surface of an insulating substrate (18) closing the waveguide (10) perpendicularly to its axis of symmetry.

Description:
FIELD OF THE INVENTION 
     The invention relates to a device for exciting a waveguide with circular polarization from a plane antenna, e.g. a printed or plated antenna. 
     BACKGROUND OF THE INVENTION 
     This device is a compact device for exciting a waveguide with wideband circular polarization in both directions and with high purity of polarization. It enables a right and/or left circularly polarized wave to be generated in a waveguide having a section which may be square or circular, for example. 
     Such a device is intended for use in any waveguide radiating element requiring compact excitation in circular polarization from a transverse electromagnetic (TEM) line feed, e.g. a coaxial line, a three-plate line, or a microstrip line. 
     Prior systems for generating a circularly polarized wave in a waveguide from a TEM line are: 
     either systems constituted by a TEM line to waveguide transition together with a polarizer which gives rise to considerable bulk (with a typical length being greater than two wavelengths) for performance equivalent to the performance of a device in accordance with the invention; 
     or else compact systems using a resonator at the end of a waveguide, but providing mediocre quality in terms of bandwidth and polarization purity and therefore incompatible with pure circular polarization as used in telecommunications frequency bands. 
     An article by C. H. Chen, A. Tulintseff, and R. M. Sorbello entitled &#34;Broadband two-layer microstrip antenna&#34; published in IEEE 1984 (A.P.S. 8-1 &#34;Antenna and propagation symposium&#34; 1984) describes a broadband two-layer printed antenna that radiates freely. Such an antenna is characterized by two resonant frequencies. By exciting this antenna with two orthogonal modes at equal amplitude and quadrature phase, circular polarization operation is obtained. 
     In contrast, the object of the invention is to generate a right and/or left circularly polarized wave in a waveguide. 
     SUMMARY OF THE INVENTION 
     To this end, the present invention proposes a device for exciting a waveguide with circular polarization from a plane antenna, said waveguide being a rectilinear hollow waveguide closed at one of its ends, said antenna being excited by at least two coaxial ports fed in phase quadrature by a circuit including a hybrid coupler, the device being characterized in that said antenna is constituted by a radiating plane metal pattern disposed on the surface of an insulating substrate closing the waveguide perpendicularly to its axis of symmetry. 
     Such a device provides excellent matching over a broad frequency band and excellent circular polarization purity over said band. 
     In a particular embodiment, the waveguide has an axis of symmetry, with the coaxial ports being situated in pairs at 90° to one another about said axis of symmetry. The antenna includes at least one metal disk disposed on the surface of a plane substrate and symmetically about the axis of symmetry of the guide. 
     Such a device serves to mitigate the drawbacks of prior art systems. It makes it possible: 
     to reduce bulk; and 
     to increase the frequency band width for given values of matching and ellipticity. 
     The device of the invention has the following characteristics: 
     it is extremely compact, circular polarization is directed generated in this case from a TEM line over a length which is shorter than one wavelength; 
     it is provided with longitudinal rear accesses, thereby enabling these accesses to be coupled without additional coaxial cables to a TEM power distributor for transmission and/or reception parallel to the section of the waveguide, at which location hybrid quadrature-imparting couplers may also be implanted; and 
     it can be used with any circular polarization antenna where there is a problem of compactness or bulk for the polarization device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The characteristics and advantages of the invention also appear from the following description given by way of non-limiting example and with reference to the accompanying figures, in which: 
     FIGS. 1 and 2 are respectively a front view as seen in the direction of arrow I in FIG. 2, and a longitudinal section view through a device in accordance with the invention; 
     FIG. 3 is a longitudinal section view through a first variant of the device in accordance with the invention; and 
     FIGS. 4 and 5 are respectively a front view looking along arrow IV in FIG. 5 and a longitudinal section view through a second variant of the device in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The device of the invention as shown in FIG. 1 is constituted by a waveguide 10, e.g. a cylindrical waveguide, which is excited with circular polarization by an antenna 11, having a single resonator and formed by plating or printing, for example. This antenna thus comprises a plane metal pattern deposited on an insulating substrate. The shape of the antenna varies depending on the performance to be achieved (typically it is square or circular depending on the shape of the waveguide). The end 12 of the waveguide serves as a ground plane for the antenna which is in the form of a disk in this case. The antenna is fed by two matched coaxial ports 13 and 14 situated at 90° relative to each other about the center of the waveguide, with said two ports being isolated from each other by means of a dielectric 18. 
     Each coaxial port is fed in phase quadrature by a 90° bybrid coupler 15 which may be a branching hybrid coupler, for example. An access 16 of said hybrid coupler 15 generates right circular polarization; its other access 17 generates left circular polarization. The hybrid coupler 15 is unbalanced in amplitude so as to compensate for the coupling between probes and so as to generate a field in each polarization having a minimum ellipticity ratio. 
     In a first variant embodiment, as shown in FIG. 3, the antenna which may be plated or printed, is constituted by two resonators 11 and 20, thereby increasing the bandwidth of the device. The two portions 11 and 20 of this two-resonator assembly are, by way of example, in the form of two concentric metal disks and they are spaced apart by means of a dielectric 21. 
     In a second variant embodiment, as shown in FIGS. 4 and 5, the antenna 11 (having two resonators or one resonator) and plated or printed, for example, if fed from four coaxial ports 22, 23, 24, and 25 which are fed in quadrature (0°, ±90°, ±180°, ±270°) by a device 26 comprising a hybrid coupler and two matched Ts. Each hybrid coupler and each &#34;rat-race&#34; or each T is balanced (3 dB coupler) and thus generates pure circular polarization waves in the waveguide. 
     The hybrid coupler produces the phase quadrature required for circular polarization. The &#34;rat-races&#34; or Ts constituting a device for providing symmetry, may alternatively be replaced by other types of &#34;balun&#34; or balancing systems. 
     The device of the invention as shown in FIG. 3 may be used with the following dimensions (where mm=millimeters): 
     
         ______________________________________distance between each of the coaxial ports 13                    about 20.5 mm;and 14 and the center of the circular resonator11:thickness of the dielectric 18:                    about 3 mm;thickness of the resonator 11:                    about 0.5 mm;thickness of the dielectric 21:                    about 7 mm;thickness of the resonator 20:                    about 0.5 mm;diameter of the circular resonator 11:                    about 41 mm;diameter of the circular resonator 20:                    about 28 mm;diameter of the cylindrical waveguide 10:                    about 52 mm.______________________________________ 
    
     The following performance can then be obtained: 
     frequency band: 15% (e.g. 3700 MHz to 4200 MHz); 
     matching: SWR in this band&lt;1.20; and 
     ellipticity&lt;0.6 dB. 
     Naturally the present invention has been described and shown merely by way of preferred example and its component parts could be replaced by equivalent parts without thereby going beyond the scope of the invention. 
     Thus, the device of the invention may comprise one resonator (FIGS. 1, 2), two resonators (FIG. 3), or some large number of resonators: three, four, . . . 
     These resonators are not necessarily circular in shape; they may be of any shape: circular, square, cross-shaped, star-shaped, hexagonal, and they may include asymmetrical features or notches. They may also include holes (non-metallized areas) of arbitrary shape within their outlines. 
     Thus, the dielectric layers (18, 21) supporting these resonators (11, 20) may be replaced in part or completely by other types of support (spacers, standoffs) of any type of material (conducting or insulating) known to the person skilled in the art. 
     Thus, the resonators may be extended out from their places or within their planes by metal pieces which may optically come into electrical contact with the wall of the waveguide. 
     Thus, the waveguides used may be circular or square in shape and also hexagonal, polygonal, elliptical, or other. They may have features such as excess thickness or grooves in the longitudinal, oblique, or transverse directions, or they may have local features such as pegs, irises, or slots. They may also be flared or narrowed locally or globally, or one after the other, e.g. in accordance with some predetermined law. 
     Thus, the excitation system may equally well be situated inside the waveguide. 
     Thus, the device of the invention may be fed by 2, by 4, or by some larger number of accesses, which may be connected to the first resonator (11) but also to the other resonators (20, . . .).