Wide bandwidth hybrid mode feeds
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a division of application Ser. No. 315,670 filed Oct. 28, 1981. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to wide bandwidth hybrid mode feeds and, more particularly, to hybrid mode feeds which are capable of handling very wide bandwidths and include an arrangement which converts a dominant TE 11  mode at the input to the feed into the HE 11  hybrid mode, which hybrid mode is then propagated further or launched into free space. 
     2. Description of the Prior Art 
     An important consideration in designing antennas for terrestrial radio relay and satellite communication is excellent radiation characteristics and very low return loss. In this regard the horn reflector is an excellent antenna, but its metal walls are generally uncorrugated. The horn antenna could be improved with corrugations but generally corrugated structures, especially in the size of the horn reflector, are very difficult and expensive to produce. Additionally, the -40DB return loss over a very wide range of frequencies as found with the present uncorrugated horn reflectors is generally not obtainable with the present corrugated feeds. 
     U.S. Pat. No. 4,040,061 issued to C. G. Roberts et al on Aug. 2, 1977 describes a corrugated horn antenna allegedly having a useful operating bandwidth of at least 2.25:1. There, the antenna is fed with a waveguide in which a TM 11  mode suppressor is disposed in a circular waveguide section before the input wavefront encounters a flared corrugated horn. The mode suppressor functions to prevent the excitation of hybrid modes in the horn at the upper end of a wide band of frequencies which would cause an unacceptable deterioration in the radiation pattern. 
     U.S. Pat. No. 4,021,814 issued to J. L. Kerr on May 3, 1977 relates to a broad-band corrugated horn antenna with a double-ridged circular waveguide feed allegedly having a bandwidth handling capability greater than 2:1 without the introduction of lossy materials or resistive type mode suppressors. There, a plurality of ridges, each having a predetermined width, and a plurality of gaps between the ridges, with each gap having a predetermined width, are provided wherein the width of the gaps is greater than the width of the ridges. 
     It has been found that for a waveguide with finite surface impedances, the fundamental HE 11  mode approaches, under certain conditions the behavior that the field essentially vanishes at the boundary and the field is essentially polarized in one direction. Because of these properties, such a mode is useful for long distance communication since it is little affected by wall imperfections or wall losses and provides an ideal illumination for a feed for reflector antennas. In general, it is difficult to excite the HE 11  mode in a corrugated feed since, at the input, the feed is usually excited by the TE 11  mode of a circular waveguide with smooth metal walls. For the TE 11  mode, the transverse wavenumber, σ, is related to the waveguide radius by σa=1.84184. At the feed aperture, however, for the desired HE 11  mode, σa≃2.4048. Thus the mode parameter u=σa must increase from 1.84184 to about 2.404 as the mode propagates from the input of the feed to the aperture. 
     In a corrugated waveguide, u is known to be a decreasing function of the corrugations depth d. Therefore, in order for u to increase, d must decrease in the direction of propagation. To satisfy this requirement, corrugated feeds are usually designed as shown in FIGS. 1 and 2a of U.S. Pat. No. 3,618,106 issued to G. H. Bryant on Nov. 2, 1977. In this regard, see also the articles &#34;Reflection, Transmission and Mode Conversion in a Corrugated Feed&#34; by C. Dragone in BSTJ, Vol. 56, No. 6, July-August 1977 at pp. 835-867 and &#34;Characteristics of a Broadband Microwave Corrugated Feed: A Comparison Between Theory and Experiment&#34; by C. Dragone in BSTJ, Vol. 56, No. 6, July-August 1977, at pp. 869-888. In such arrangement, the input discontinuity of d causes a reflection which vanishes at the frequency satisfying λ r  ≃2d, where λ r  is the wavelength in the radial lines of the input corrugations. The feed can thus be used effectively only in the vicinity of this frequency and, as a consequence, bandwidths in excess of 100 percent are difficult to obtain. 
     Other arrangements for transforming the TE 11  mode into the HE 11  mode, for subsequent launch from a feed, using helically wound wire structures bonded to the interior surface of a waveguide are disclosed in U.S. Pat. Nos. 4,231,042 issued to R. H. Turrin on Oct. 28, 1980 and 4,246,584 issued to A. R. Noerpel on Jan. 20, 1981. 
     The problem remaining in the prior art is to provide wide bandwidth hybrid mode feeds which are simpler to fabricate than prior art type feeds with wide bandwidth and also provide negligible reflection and generation of unwanted modes over bandwidths in excess of two octaves. 
     SUMMARY OF THE INVENTION 
     The foregoing problem in the prior art has been solved in accordance with the present invention which relates to wide bandwidth hybrid mode feeds and, more particularly, to hybrid mode feeds which are capable of handling very wide bandwidths and include an arrangement which converts a dominant TE 11  mode at the input to the feed into the HE 11  hybrid mode, which hybrid mode is then propagated or launched into free space. 
     It is an aspect of the present invention to provide hybrid mode feeds which are capable of handling very wide bandwidths wherein the dominant TE 11  mode is converted to the HE 11  mode which is then launched. The TE 11  to HE 11  mode conversion is achieved by inserting a circular dielectric rod into a flared end of a smoothwalled cylindrical feedhorn until a small cylindrical section of the dielectric rod engages the inner wall of the unflared portion of the feedhorn. The other end of the dielectric rod is similarly inserted into a flared end of a corrugated cylindrical feedhorn section until a short longitudinal section of the cylindrical portion of the rod engages the corrugations of an unflared cylindrical section of the feedhorn to provide a transition for the HE 11  mode onto the corrugated waveguide for subsequent launch. 
     Other and further aspects of the present invention will become apparent during the course of the following description and by reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
    
    
     Referring now to the drawings, in which like numerals represent like parts in the several views: 
     FIG. 1 illustrates a cross-sectional view of the TE 11  to HE 11  mode conversion section in accordance with the present invention; 
     FIG. 2 illustrates a cross-sectional view of a feed arrangement in accordance with the present invention which includes the mode conversion section of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a mode conversion arrangement which transforms efficiently, over a wide range of frequencies, the TE 11  mode into the HE 11  mode. Such transformation into the HE 11  mode is desired in order to obtain from a circular feed the radiation characteristics where the field essentially vanishes at the boundary and the field is essentially polarized in one direction. The arrangement of FIG. 1 comprises a circular waveguide 10 which includes an outwardly-flared end section 11, and a rod 12 of dielectric material which has an end section thereof in radial engagement with a longitudinal section 14 of the inner surface 15 of waveguide 10, adjacent the flared end section 11, and extends longitudinally outward from the flared end section 11. 
     Dielectric rod 12 is shown as comprising a conical end 16 for providing a smooth transition interface for the TE 11  mode entering dielectric rod 12 from waveguide 10. It is to be understood that such conical end 16 of dielectric rod 12 is preferred but optional and is for purposes of exposition and not for purposes of limitation since other shaped ends such as, for example, a flat end, which is not preferred due to reflections being directed directly backward, or a tapered end could be used to provide a proper transition boundary. Also shown is an optional helical wire structure 18 surrounding dielectric rod 12 in the area both within and beyond the flared end section 11 of waveguide 10, which can be used to improve the performance by containing any of the field found at the boundary. 
     In operation, the TE 11  mode propagates from a source (not shown) down waveguide 10 and enters the conical end 16 of dielectric rod 12 and propagates therein until it reaches the beginning of flared end 11 of waveguide 10. It has been found that by placing a dielectric rod 12 inside an ordinary waveguide 10 comprising smooth metal walls, the mode parameter, u, is found to decrease as the distance d between the outer surface of dielectric rod 12 and the inside wall 15 of waveguide 10 is gradually increased. As a consequence, to obtain the HE 11  mode, starting from the TE 11  mode, it is sufficient to increase d in the direction of propagation, starting from d=0 as shown in FIG. 1 to the end of flared section 11. Beyond the wide end of flared section 11, the distance d is so large that it can be assumed that the HE 11  mode is guided entirely by dielectric rod 12. Therefore, the metal walls of waveguide 10 and its flared end 11 can be removed especially since, for the HE 11  mode, the field essentially vanishes at the boundary of dielectric rod 12. The HE 11  mode can then be propagated further down dielectric rod 12. Optional helical windings 18 merely aid in containing any of the HE 11  mode at the boundary within rod 12 as stated hereinbefore. 
     Having obtained the HE 11  mode in a dielectric rod 12 as shown in FIG. 1 and described hereinbefore, the ensuing description relates to arrangements which expand the arrangement of FIG. 1 to permit the launching of the HE 11  mode into free space as found with an antenna feed. One such arrangement in accordance with the present invention is shown in FIG. 2. There, the HE 11  mode propagating in dielectric rod 12 enters a corrugated waveguide structure 20 comprising a first flared end 21, a cylindrical section 22 and a second flared end 23. More particularly, the HE 11  mode propagating in dielectric rod 12 enters the first flared end 21 of corrugated waveguide 20 where the distance, d, of the corrugated walls from the dielectric rod 12 is large to prevent reflection or excitation of unwanted modes. In first tapered end 21, the distance d is gradually decreased until the corrugated walls touch the outer periphery of dielectric rod 12. The HE 11  mode will propagate in first tapered end 21 without conversion to other modes provided Y≠∞, where Y=-j(Z/Z 1 ),Z is the wave impedance of the homogeneous medium filling the waveguide and Z 1  is the finite surface impedance in the longitudinal direction of the waveguide. By properly choosing the parameters of the corrugated waveguide, such condition can be satisfied over a very wide frequency range. 
     On reaching cylindrical corrugated waveguide section 22 the dielectric rod 12 can be terminated in cylindrical section 22 by any suitable configuration as, for example, the conical end 24 shown or other tapered configuration. It can be shown that such arrangement does not result in the generation of unwanted modes, assuming the transition is long enough. The HE 11  mode then propagates down waveguide section 22 for any desirable distance and is launched into free space, if desired, by second flared end 23 as is well known in the art for providing a smooth transition between a circular waveguide and free space. It is to be understood that the helical wound wire structure 18 of FIG. 1 could be included in the arrangement of FIG. 2 between cylindrical waveguide 10 and the cylindrical corrugated waveguide section 22, which cylindrical waveguide sections should be of a diameter to support the desired frequency range of interest. 
     It is to be understood that in the arrangement of FIG. 2, dielectric rod 12 may not be manufactured to precisely match the inner diameter of smooth walled waveguide 10 and corrugated waveguide section 22. Therefore, in actual construction, a frame (not shown) can fixedly support both waveguides in position rather than depending on a tight fit of dielectric rod 12. In addition, dielectric rod 12 need not correspond to the inner diameter of the corrugated waveguide section 22 which can be slightly greater than the outer diameter of dielectric rod 12, and in such arrangement dielectric rod 12 can then be supported to the corrugations by dielectric washers or spacers (not shown) or held in position by the frame. In such latter arrangement, the HE 11  mode will still be transferred to corrugated waveguide section 22 provided the tapered end of dielectric rod 12 is sufficiently long.