Abstract:
A broadband corrugated horn radiator including a hybrid mode exciting member whose inner cross section is constant over its entrie length and a transition member from the cross section of the hybrid mode exciting member to the cross section of the horn aperture. The length and the inner cross section of the hybrid mode exciting member are dimensioned such that the waveguide mode entering this excitation member is converted completely only to the desired hybrid mode type, the pitch of the wall of the transition member is such that no interference waves are able to exist therein, and the corrugation grooves in all members of the horn radiator are matched, by appropriate dimensioning, to the same transmission bandwith.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a broadband corrugated horn radiator including a hybrid mode exciting member whose inner cross section is constant over its entire length and a transition member from the cross section of the hybrid mode exciting member to the cross section of the horn aperture. 
     Corrugated horn radiators are used as primary radiators in reflector antennas. Such radiators are distinguished by a low cross-polarization level, freedom from reflections, good sidelobe suppression and a rotationally symmetrical radiation lobe (E-H matching of the lobe cross sections). A corrugated horn radiator should have these good properties over the broadest possible frequency range. It has, in the past, been attempted, for example in German Auslegeschrift No. 2,152,817, to enlarge the bandwidth by changing the frequency dependency of the impedance at the inner walls of the horn radiator. This is accomplished, according to German Auslegeschrift No. 2,152,817, by special design and dimensioning of the corrugated groove structure in the hybrid mode excitation member and in the transition member to the horn aperture which follows. 
     However, the transmission bandwidth of the horn radiator cannot be enlarged very much merely by modifying the impedance curve within such a horn radiator. 
     SUMMARY OF THE INVENTION 
     It is therefore the object of the present invention to provide a corrugated horn radiator of the above-mentioned type, i.e., a horn radiator including a hybrid mode exciting member and transition member from the constant cross section of the hybrid mode member to the cross section of the horn aperture, whose transmission bandwidth is enlarged without utilization of complicated groove structures. 
     The above object is accomplished according to the present invention in that in a corrugated horn radiator of the above-mentioned type, the length and the inner cross section of the hybrid mode excitation member are dimensioned such that the waveguide mode entering into this excitation member is converted completely only into the hybrid mode to be utilized, the pitch of the inner wall of the transition member is selected such that no interferring higher order modes are able to exist therein, and the corrugation grooves are matched, in all sections of the horn radiator, to the same transmission bandwidth. 
     The present invention can be used for corrugated horn radiators having the most diverse cross-sectional shapes, e.g. circular, elliptical, rectangular, etc. 
    
    
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1a is a graph showing the change of one of the cross-sectional axes, a(z), along the longitudinal axis (z) of the horn radiator of FIG. 1. 
     FIG. 1b is a graph showing the change of the other of the cross-sectional axes, b(z) along the longitudinal axis (z) of the horn radiator of FIG. 1. 
     FIG. 1c is a graph showing a modification of the change of the other of the cross-sectional axes b(z) along the longitudinal axis (z) of the horn radiator of FIG. 1. 
     FIG. 2 is a longitudinal sectional view of a horn radiator of which the groove structure is provided with a capacitive load. 
     FIG. 3 is a longitudinal sectional view of a horn radiator provided with a groove structure according to the invention. 
     FIG. 4 is a graph showing the changes of the groove depth, of the groove width and of the spacing between adjacent grooves. 
     FIG. 5 is a graph showing the different groove depths in the two cross-sectional axes a and b. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, there is shown a horn radiator according to the invention in which, for reasons of clarity, the corrugated groove structure is not shown. The horn radiator begins with a section 1 which converts the incoming waveguide mode to the associated hybrid mode. For example, with a circular cross section as shown, the section 1 converts the incoming waveguide mode to the HE 11  mode. In order for the radiation properties of the corrugated horn radiator not to become worse, care must be taken that in addition to the desired hybrid mode, no other interferring higher order hybrid modes are excited. For that reason, the inner cross section of the hybrid mode exciting section or member 1 is selected to be large enough so that the useful or desired hybrid mode is just excited and no interferring higher order hybrid mode will be able to exist at the highest transmission frequency. Along its axis, the hybrid mode excitation member 1 has a constant inner cross section (circular as shown) and is of such a length that the entire incoming waveguide mode is completely converted to the associated hybrid mode. 
     The hybrid mode excitation member 1 is followed by a transition member 2, 3 which changes from the circular cross section of the hybrid mode excitation member 1 in plane Z 1  of the illustrated embodiment to the elliptical cross section of the horn aperture 4 in plane Z 3 . This transition member 2, 3 must be dimensioned such that, as with the hybrid mode exciting member 1, no interferring higher order modes can be excited therein. For this reason, the transition from the cross section of the hybrid mode exciting member 1 to the cross section of the horn aperture takes place in two transition zones or sections 2 and 3. In the first transition zone 2, the cross section of the hybrid mode exciting member 1 is changed to a cross section which corresponds in its shape but not yet in its size to the cross section of the horn aperture 4. The final inner cross section of this zone 2, as shown in plane Z 2 , should differ as little as possible from the cross section of the hybrid mode exciting member 1 so as to exclude any possibility of interferring higher order mode excitation. This is accomplished in the transition zone 2 of the embodiment shown in FIG. 1 in that beginning with the circular cross section of the hybrid mode exciting member 1 in plane Z 1 , only one of the two mutually perpendicular cross-sectional axes, e.g., the axis a(z), widens along the z-axis whereas the other axis, e.g. the axis b(z), remains unchanged. 
     FIGS. 1a and 1b show the changes in the cross-sectional axes a(z) and b(z) along the z-axis of the embodiment of FIG. 1. The cross-sectional axis a(z) ascends from a value of a 1  =a 0  in the Z 1  plane at the end of the hybrid exciting member 1 to the value a 2  in the Z 2  plane at the end of the transition zone 2. The value of the cross-sectional axis b(z) is unchanged from the plane Z 1  to the plane Z 2 , so that the following applies: b 2  =b 1  =b 0 . 
     Due to the increase in the value of the cross-sectional axis a(z) and thus the increase in the inner diameter within the transition zone 2, the cutoff frequencies of the interferring higher order modes able to exist in this zone 2 is reduced, which leads to a reduction of the transmission bandwidth. This drawback can be overcome with the following measure. As shown in FIG. 1c, the value of the cross-sectional axis b(z) is not kept constant but rather is reduced to a lower value b 2  &#39;. In this connection it must be noted that the value of b(z) must not fall below a limit value b c  (shown in FIG. 1c by a dot-dash line), below which the useful or desireable hybrid mode can no longer propagate. 
     In either case the axis ratio a 2  /b 2  or a 2  /b 2  &#39; of the final cross section of the transition zone 2 should correspond to the axis ratio a 3  /b 3  of the elliptical horn aperture 4. In the transition zone 3, the elliptical cross section of the plane Z 2  is then widened to the horn aperture only in the Z 2  plane, with the cross-sectional shape remaining the same. 
     In the corrugated horn radiator shown in FIG. 1, the values of the cross-sectional axes a(z) and b(z) change linearly in the transition region along the z-axis. However, it is also possible for the values of a(z) and b(z) to have nonlinear but steady function curves (shown by dotted lines in FIGS. 1a, 1b). In this way it is possible to avoid undesireable sudden bends in the transition member 2, 3. 
     Turning now to FIG. 2, there is shown an example of a corrugated horn radiator which has been cut open in the longitudinal direction and in which the corrugation grooves 5 have such a shape that they are matched in all sections 1, 2 of the horn radiator to the same transmission bandwidth. Normally a corrugation groove 5 has a smaller bandwidth in a waveguide section having a small cross section than a corrugation groove in the waveguide section having a larger cross section. In order to realize homogeneous bandwidth matching along the length of the horn radiator axis, the capacitive charge is reduced with increasing cross-sectional size. As can be seen in FIG. 2, the width of the corrugation grooves 5 is therefore enlarged in the transition section or zone 2 together with the increases in cross section and the trap 6 at the end of the corrugation grooves is eventually reduced or even avoided. This measure is required only for extremely large bandwidths (about one octave or higher). Generally it is sufficient to adapt the corrugation groove structure in the excitation member 1 and in the transition member 2, 3 to the required transmission bandwidth in a known manner, e.g. according to the manner disclosed in German Pat. No. 2,616,125. Such matching of the corrugation groove structure involves only a variation of the groove width, depth, and/or spacing in dependence on the location of the respective corrugation groove along the longitudinal axis of the horn radiator. In the horn section 1, such a variation of the corrugation groove structure is necessary already to completely transform the waveguide modes to the corresponding hybrid modes, assuming the lowest possible inherent reflection coefficient. 
     Even with complete hybrid mode transformation at the output of member 1, it is sometimes necessary to adapt the corrugation grooves in section 2 as well (e.g. corrugation groove depth, corrugation groove spacing) to the local cross section, since the waveguide wavelengths differ locally. With asymmetrical cross sections (e.g. elliptical) of the transition zones 2 and 3, it may become necessary to make this local adaptation of the corrugation grooves different in the two cross-sectional axes a(z) and b(z). For this purpose, it is preferable to vary the corrugation groove depth, which should decrease in a direction toward the horn aperture 4, because corrugation groove depth variation is easiest from a manufacturing point of view. With the aid of such measures it is possible to manufacture an elliptical corrugated horn radiator which has identical phase shifts in two polarizations. 
     FIG. 3 shows a specific example of an embodiment of a corrugated horn radiator according to the invention. This figure depicts a longitudinal section view (along the axis b) of the horn radiator. The hybrid mode excitation member 1 and its groove structure are dimensioned as follows. Member 1 has the length l 1  ≃3λ o , and a diameter b o  ≃λ o , whereby λ o  characterizes the free-space wavelength. 
     The manner in which the groove dimensions, that is, the depth t, the width c and the spacing h between adjacent grooves, vary along the z-axis is shown in the graph of FIG. 4. 
     The hybrid mode excitation member 1 has the following dimensions at an operating frequency of 15 GHz; 
     
         ______________________________________b.sub.o = b.sub.1 ≃ 20 mml.sub.1 ≃ 60 mmt.sub.o ≃ 7.4 mm                t.sub.1 ≃ 5 mmc.sub.o ≃ 0.4 mm                c.sub.1 ≃ 3.6 mmh.sub.o ≃ 1.8 mm                h.sub.1 ≃ 2.5 mm______________________________________ 
    
     The maximum amount by which the pitch of the waveguide transition section 2 can vary, i.e. the maximum pitch angle, is 10°. In the embodiment shown in FIG. 3, the transition section 2 is only enlarged in its cross-sectional axis a (a 2  &gt;a 1 ). The cross-sectional axis b is unchanged (b 1  =b 2 ). The axial ratio a 2  /b 2  of the elliptical section 2 as well as the axial ratio a 3  /b 3  of the elliptical aperture 4 of the section 3 is equal or less than 2.7. 
     As FIG. 3 shows, the grooves in each cross section of the members 2 and 3 have the same widths and spacings between one another. But the groove depth in each cross-section of the elliptically shaped members 2 and 3 increases from a minimum depth t min  (t min  ≃0.24λ o  is constant along the z-axis) near the large axis a to a maximum depth t max  near the small axis b. FIG. 5 shows the variation of the depth t(x,y) of the groove in one cross section. The increase of the groove depth t(x,y) from the value t min  to the value t max  is given by the formula ##EQU1## wherein a and b represent the respective half-axes as indicated in FIG. 5. 
     At an operating frequency of 15 GHz, the sections 2 and 3 have the following dimensions: 
     l 2  ≃66 mm 
     l 3  ≃240 mm 
     a 3  ≃62.5 mm 
     b 3  ≃24 mm 
     t min  ≃5 mm 
     The above described horn radiator has a round hybrid mode excitation member 1 and elliptical waveguide sections 2 and 3. The shape of member 1 can also be square and the shapes of the sections 2 and 3 can also be rectangular. 
     It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.