Nested horn radiator assembly

Abstract: A horn radiator assembly (10) includes two horn radiators (12, 14) each of which is formed as a conical horn and a waveguide of constant cross section connected to the small end of the horn as a feed and providing a signal port. The first of the horn radiators (12) is of relatively large cross section and serves to radiate electromagnetic waves at a relatively low frequency. The second of the horn radiators (14) is of relatively small cross section and serves to radiate electromagnetic waves at a relatively high frequency. The second radiator (14) is nested within the first radiator (12), and is positioned with its radiating aperture (28) coplanar with the radiating aperture (20) of the first radiator (12). In the second radiator (14), the waveguide feed (26) is provided with a bend (36) allowing the waveguide feed to pass through a wall (32) of the first radiator (12). A strut (34) may be affixed to the bend (36) to provide a symmetrical transverse support within the first radiator for the second radiator. A doubly-tapered electrically conductive sheet (48) extends from an apex within the throat (18) of the first radiator horn (12) to the transverse support, and from there tapers back to contact the horn of the second radiator (14). The tapered sheet (48) guides low-frequency radiation past the strut (34) and the bend (36) to minimize standing wave ratio.

Description: This invention relates to a horn radiator assembly according to the preamble of claim 1. In communication systems, such as in systems employing a satellite for the communication of signals to various parts of the earth, it is common practice to employ an antenna system comprising a reflector and an array of radiators positioned for illuminating the reflector. Such an antenna system is well suited for the generation of a fan beam which can be directed to a geographical section of the earth. As an example of the construction of such an antenna system, it is common practice to construct the radiators in the form of horn radiators. Signals transmitted by the antenna system may be in one frequency band, while signals received by the antenna system may be in a different frequency band. A situation of particular interest involves the generation of a spot beam in a high frequency band concurrently with the generation of a broad beam at a low frequency band. While it has been the practice, in many situations, to use a separate set of radiators and separate reflectors for generation of beams at high and at low frequency bands, in the present situation of interest, it is desired to locate the radiator of the spot beam concentric with a radiator of the low frequency band, and to use the same reflector for both the beams of the high and the low frequency radiations. A horn radiator assembly according to the preamble of claim 1 is known from US-A- 2, 425, 488 . This known horn radiator assembly comprises two rectangular horn radiators, wherein one of these rectangular horn radiators is smaller in cross-section than the other one so that it can be located therein. Each of these horn radiators comprises an enclosing wall structure of rectangular cross-section which defines a passage of the propagation for a respective radiation, which radiation is coupled in via a respective signal port. A radiation which propagates through these wall structures is leaving the respective wall structure via a radiating aperture at the opposite end of the signal port. Futhermore, there are provided two partitions which operate to separate the smaller horn from the other horn. These partitions are bent towards each other, to provide a mouth flare so that each duct within horn actually operates as a separate smaller horn, the two horns operating in parallel. Due to these partitions the outer (lower frequency) horn operates essentially as two horns in parallel. A problem arises from this known structures in that the outer horn (which consists of the a.m. two parallel horns) may block the inner horn or may otherwise interfere with the radiation characteristics of the inner horn. It therefore is the object of the present invention to improve a horn radiator assembly according to the preamble of claim 1 in such a way that any interference between the two horns thereof can be avoided. This object, according to the present invention, is solved by the advantageous measures indicated in the characterizing part of claim 1. Hence, according to the present invention, the second wall structure is bent to pass through one of the walls of the first wall structure thereby allowing the second signal port to extend ourside the one wall; furtheron, a sheet means configured with tapered surfaces is provided which at least partially covers the second wall structure such that a tapering of the sheet means produces an apex facing the first signal port and the sheet means terminates at or is spaced from the second radiating aperture the second wall structure. It has been found that the a.m. structure ensures a complete decoupling of the two horns so that there will be no interference between the horns. The present invention, consequently, provides a horn radiator assembly which yields great advantages when compared to the known assembly. It is recognized that the insertion of the second radiator within the first radiator introduces a physical structure which may serve as a reflector of energy at the lower frequency. Resulting reflections would increase the standing wave ratio within the first radiator and lower the efficiency of signal transmission. Such sources of reflection include supporting structure employed for holding the second radiator at a designated location within the first radiator, as well as the presence of a feed section of waveguide which conveys microwave energy from a transmitter to the second radiator. In accordance with the present invention, the foregoing structural components which can serve as reflectors are enclosed within a tapered electrically-conductive sheet such as a metallic pyramid. A configuration of tapered sheet is employed on both sides of the reflectors to guide traveling waves, in either a transmission direction or in a reception direction, past the reflectors without interaction with the reflectors. Tapering allows for a smooth transmission within the first horn radiator so as to preserve a low standing wave ratio, and thereby retain the radiation characteristics of the first horn radiator, even though the second horn radiator is nested therein. Thereby, the horn radiator assembly of the invention enables two horns, operating in different frequency bands and having different sizes to be colocated for illumination of a common reflector. The horn radiator assembly of the invention is reciprocal in operation so as to provide the foregoing benefit both in the case of a transmitted beam and a received beam of electromagnetic power. The input port for signal transmission becomes an output port during reception of a signal. The foregoing aspects and other features of the present invention are explained in the following description, taken in connection with the accompanying drawing wherein: Fig. 1 is a perspective view of the horn assembly of the invention showing both a large outer radiator and a smaller inner radiator, a horn of the large radiator being partially cutaway to show a sheet structure for guiding radiation past a feed waveguide for the smaller radiator; Fig. 2 shows the structure of Fig. 1, with the sheet structure being partially cutaway to show a bent waveguide feed of the smaller radiator; Fig. 3 shows an antenna system incorporating the horn assembly of the invention in an array of radiators; and Fig. 4 shows the structure, generally, of Fig. 1 with a simplified sheet which is depicted partially cutaway to show the bent waveguide feed of the smaller radiator. DETAILED DESCRIPTION With reference to Figs. 1 and 2, there is shown a horn assembly 10 which is constructed in accordance with the invention and include a relatively large low-frequency horn radiator 12 and a relatively small high-frequency horn radiator 14 disposed within the large radiator 12. By way of example in the construction of a preferred embodiment of the invention, the large radiator 12 operates at C-band microwave frequencies, 4 - 6 GHz (gigahertz), and the small radiator 14 operates at Ku band, 12 - 18.5 GHz. The dimensions of the components of the horn assembly 10, as disclosed herein, are intended for use in a frequency range of 3.7 - 4.2 GHz and 12.25 - 14.75 GHz for the radiators 12 and 14, respectively. The principles of the invention are applicable to radiators constructed for operation for frequencies other than the foregoing frequencies. The large radiator 12 includes a diverging portion, to be referred to as a horn 16 and a section of waveguide of constant cross-sectional dimensions to be referred to as a throat 18. The throat 18 extends from the end of the horn 16 having a relatively small cross section while the opposite end of the horn 16 having a relatively large cross section serves as a radiating aperture 20 of the large radiator 12. In the construction of the large radiator 12, the throat 18 and the horn 16 may be formed as a unitary structure, as by braising the waveguide of the throat 18 to the small end of the horn 16. Alternatively, as shown in Figs. 1 and 2, the throat 18 may be secured to the horn 16 by means of a mounting flange 22. The construction of the small radiator 14 is substantially the same as that of the large radiator 12, the small radiator 14 having a horn 24 and a throat 26 (Fig. 2) connected to the small end of the horn 24. The large end of the horn 24, opposite the throat 26, serves as a radiating aperture 28 of the small radiator 14. The throat 26 and the horn 24 are formed as a unitary structure by braising the throat 26 to the horn 24. The radiators 12 and 14 are formed of a metal, such as brass or aluminum. In the construction of the invention, it is noted that the horns 16 and 24 have rectangular cross section, as do the throats 18 and 26. However, the principles of the invention apply to horn radiators of other cross section, such as circular cross section. Furthermore, while the horns 16 and 24 are disclosed as being tapered structures, it is noted that the principles of the invention also apply to a non-tapered horn such as an open-ended waveguide of constant cross section. In a preferred embodiment of the horn assembly 10, as depicted in Figs. 1 and 2, the radiating apertures 20 and 28 are coplanar. However, if desired, the horn 24 of the small radiator 14 can be positioned such that its radiating aperture 28 is located forward of the radiating aperture 20 (outside the horn 16), or behind the radiating aperture 20 (inside the horn 16). In the construction of the horn assembly 10, the radiation patterns of the large radiator 12 are predicted by numerically integrating the modal fields existing over its aperture, and assuming the electric and magnetic fields to be of zero amplitude over the region of the horn 16 which is blocked by the horn 24. This enables optimization of the position of the horn 24 of the small radiator 14, and also enables accurate prediction of the gain, as well as the co-polar and cross-polar radiation patterns of the large radiator 12. It is advantageous to construct the horn assembly 10 with symmetry in the mounting of the horn 24 within the horn 16. This is accomplished by bending the throat 26 of the small radiator 14 so that a distal end 30 thereof protrudes through a wall section 32 of the large horn 16 so as to provide physical contact with the wall section 32 for supporting the small radiator 14 within the horn 16. Protrusion of the distal end 30 of the throat 26 through the wall section 32 also provides a signal port for access to the small radiator 14 for applying electromagnetic signals to be radiated from the horn 24. A strut 34, which may be fabricated as a section of dummy waveguide is secured to the throat 26 at a bend 36 of the throat 26, and extends parallel to a distal leg 38 of the throat 26 and perpendicular to a proximal leg 40 of the throat 26. Center lines of the strut 34 and of the legs 38 and 40 are coplanar. The strut 34 and the distal leg 38 form a brace which extends transversely of both the horns 16 and 24, and contacts opposed wall sections 32 of the horn 16 to provide for a symmetrical mounting of the horn 24 within the horn 16, By way of example in the fabrication of the foregoing brace, the strut 34 may be brazed to the bend 36 of the throat 26. A mounting flange 42 is brazed to the distal end 30 of the throat 26 to facilitate a connection of microwave circuitry to the small radiator 14 so as to provide a microwave signal to be transmitted by the small radiator 14, or for receiving incoming microwave signals incident upon the radiating aperture 28 of the small radiator 14. In similar fashion, a flange (not shown) may be secured to a distal end of the throat 18 for connection of microwave circuitry to the large radiator 12. The strut 34 may be secured to a wall section 32A by passing an end of the strut 34 through an aperture 44 in the wall section 32A, and then brazing the end of the strut 34 to the wall section 32A. Similarly, the distal leg 38 may be secured to the wall section 32 at an aperture 46 in the wall section 32. The horn 24 is positioned symmetrically within the horn 16, center lines of the two horns coinciding. During manufacture of the assembly 10, the wall sections 32 and 32A may be bowed outward slightly to clear ends of the strut 34 and the throat 26 to allow insertion within the horn 16 and emplacement in the apertures 44 and 46. It is recognized that the strut 34 and the throat 26 constitute a physical structure which can readily reflect waves of radiation propagating through the large radiator 12. Reflections of the radiation are undesirable because they decrease the effectiveness of transmission of microwave power through the large radiator 12 as is indicated by an increased value of standing wave ratio produced by such reflection. In order to operate the horn assembly 10 effectively, it is desirable that the respective radiant signals in both of the radiators 12 and 14 be allowed to propagate without interference from the microwave structures which guide the radiant signals. Accordingly, in accordance with a feature of the invention, an electrically conductive sheet 48 is positioned within the large radiator 12 for enclosing the strut 34 and the throat 26 so as to guide the lower frequency radiation within the large radiator 12 past the region of a strut 34 and the throat 26 without reflection from these components. By way of example, the sheet 48 may be constructed of copper foil or aluminum foil, the foil being sufficiently thick to provide for dimensional stability. The sheet 48 is folded so as to provide the configuration of a double taper. One taper directed towards the throat 18 produces a cone or pyramid 50 having an apex 52. Towards the forward end of the horn 16, the sheet 48 tapers in a tapered section 54 between forward edges of the strut 34 and the leg 38 to the four sides of the horn 24. The tapered. section 54 comprises four trapezoidal wall sections. The aforementioned brace is formed as a composite of the strut 34 and the leg 38 and is indicated at 56 (Fig. 1) as an outline in the sheet 48 of the structure of the brace. In the region of the brace 56, the sheet 48 lies flat on the top and the bottom surfaces of the brace 56, with reference to the orientation of the assembly 10 presented in Fig. 1. Both behind the brace 56, and in front of the brace 56, the sheet 48 undergoes the aforementioned tapering at the pyramid 50 and at the tapered section 54, respectively. The tapering of the sheet 48 provides for a gradual transition in the interior dimensions of the large radiator 12 so as to prevent the generation of excessive reflections. The sheet 48 accomplishes its function of allowing the large radiator 12 to function in a normal fashion, in spite of the presence of the small radiator 14. Thereby, the horn assembly 10 can provide for the co-location of the high and the low frequency radiating apertures in a compact physical configuration while retaining the radiation characteristics of the individual radiator 12 and 14. The following dimensions are employed in constructing the preferred embodiment of the horn assembly 10. With reference to Fig. 1, in the horn 16, the sides 58A and 58B of the radiating aperture 20 each measure 6.0 inches. In the horn 24, the sides 60A and 60B of the radiating aperture 28 measure, respectively, 1.8 inches and 2.2 inches. In the throat 18 of the large radiator 12, the widths of the sides 62A and 62B measure, respectively, 1.145 inches and 2.29 inches. In the waveguide of the throat 26 (Fig. 2) of the small radiator 14, the sides 64A and 64B measure, respectively, 0.375 inches and 0.75 inches. The length of the horn 16, as measured along its center line from the radiating aperture 20 to the flange 22, is 10.0 inches. The length of the horn 24, as measured along its center line from the radiating aperture 28 to the junction with the throat 26, is 4.0 inches. The angles of taper in the construction of the sheet 48, as measured with respect to a center line of the horn assembly 10, are preferably in the range of 15 - 20 degrees, though other angles of taper may be employed, if desired, in accordance with accepted practice in the design of microwave transition structures. Fig. 3 shows an antenna system 66 which is useful in demonstrating use of the horn assembly 10. The antenna system 66 comprises a reflector 68, a plurality of radiators 70 arranged in an array which includes the horn assembly 10, a feed unit 72 such as a power splitter or Butler matrix, a C-band transceiver 74 coupled to the feed unit 72, and a Ku-band transceiver 76 connected by the flange 42 to the small radiator 14 of the horn assembly 10. The feed unit 72 applies C-band microwave power to each of the radiators 70 and also via the throat 18 to the large radiator 12 of the horn assembly 10. Each of the radiators 70 and the large radiator 12 of the horn assembly 10 direct microwave power to the reflector 68 for forming a C-band beam 78 which is transmitted to a distant site. The antenna system operates in reciprocal fashion so that an incoming beam 78 of radiation provides microwave signals which are received by the transceiver 74. Similarly, the small radiator 14 of the horn assembly 10 directs microwave signals from the transceiver 76 towards the reflector 78 for forming a Ku band beam 80. Since the antenna system 66 operates in reciprocal fashion, an incoming band 80 of Ku-band microwave signals is directed by the small radiator 14 of the horn assembly 10 to the transceiver 76. The beams 78 and 80 are concentric by virtue of the use of a common reflector 68 for both the C-band and the Ku-band radiation, and due to the fact that a center one of the radiators of the system 66 employs the invention in the form of the horn assembly 10. In Fig, 3, the radiators 70 are depicted as being horn radiators having the same configuration as the large radiator 12 of the horn assembly 10. However, if desired, the horn assembly 10 of the invention can be employed with radiators of other physical configuration. Fig. 4 shows a further embodiment of the invention which functions in the same manner as that disclosed in Figs. 1 and 2, but is preferred because of its simpler construction. In Fig. 4, a horn assembly 82 comprises a large radiator 84 and a small radiator 86 nested within the large radiator 84 as was described in Figs. 1 and 2 with reference to the radiators 12 and 14, respectively. The large radiator 84 has the same configuration as the radiator 12. The small radiator 86 comprises the horn 24 and the throat 26 of the radiator 14 but differs in construction from the radiator 14 in that the horn 24 is joined to the throat 26 by a flange 88 rather than by the unitary construction of the radiator 14. The strut 34 and the distal leg 38 of the throat 26 are joined together to form the brace 56 which is oriented transversely of the common axis of the horns 16 and 24 for securing the small radiator 14 to the large radiator 12. In accordance with the invention, the horn assembly 82 includes a sheet 90 which encloses the horn 24, the proximal leg 40 of the throat 26, the flange 88, and the central portion of the brace 56. The sheet 90 functions in the same fashion and serves the same purpose as the sheet 48 (Figs. 1 and 2). The sheet 90 has a simpler geometric form than the sheet 48, the sheet 90 being in the form of a simple pyramid which extends from a base at the radiating aperture 28 of the horn 14 to an apex at the flange 22 at the junction of the horn 16 with the throat 18 of the large radiator 12. Due to the simpler configuration of the sheet 90, the outer ends of the brace 56 extend through the sheet 90 to be exposed to the lower frequency radiation propagating within the large radiator 12. However, the resulting reflections of the electric field, E, of the lower frequency radiation may be regarded as being negligible because of the very small reflection of the electric field from the outer ends of the brace 56. The small amount of reflection is due to the presentation of the narrow wall of the distal leg to the radiation with the direction of the electric field, E, being perpendicular to the brace 56.

Claims: A horn radiator assembly, comprising: [a] a first rectangular horn radiator (12; 84) comprising a first enclosing wall structure (16) of rectangular cross-section having two pairs of opposed walls (58A, 58B) and defining a first passage for propagation of a first radiation which is coupled in via a first waveguide signal port (18) at one end of said first wall structure (16) and is radiated via a first radiating aperture (20) at the other end thereof; [b] a second rectangular horn radiator (14; 86) comprising a second enclosing wall structure (24) being smaller in cross-section than said first wall structure (16) so as to be located within said first wall structure (16) and defining a second passage for propagation of a second radiation which is coupled in via a second waveguide signal port (38) at one end of said second wall structure (24) and is radiated via a second radiating aperture (28) at the other end thereof; characterized in that [c] said second waveguide signal port (38) is bent to pass through one of said walls (58A) of said first wall structure (16) thereby allowing said second waveguide signal port (38) to extend outside said wall (58A); and in that [d1] a sheet means (48; 90) configured with tapered surfaces (50, 54) is provided which at least partially covers said second wall structure (24) such that a tapering of said sheet means (48; 90) produces an apex (52) facing said first signal port (18), [d2] said sheet means (48; 90) terminating at (Fig. 4) or being spaced from (Fig.1, 2) said second radiating aperture (28) of said second wall structure (24). Horn radiator assembly according to claim 1, characterized in that each of said wall structures (16, 24) defines a diverging horn opening at a larger cross-section into a respective radiating aperture (20, 28) and opening at a smaller cross-section into a respective port (18, 38) of constant cross-sectional dimension. Horn radiator assembly according to claim 1 or 2, characterized by a strut (34) for supporting said second wall structure (24) within said first wall structure (16). Horn radiator assembly according to claim 3, characterized in that said sheet means (48; 90) at least partially encloses said strut (34) and said second signal port (38).

Inventors: BAINS PARAMJIT S, GAWLAS GARY J, RAGHAVAN KRISHNAN, BAINS, PARAMJIT S., GAWLAS, GARY J., RAGHAVAN, KRISHNAN
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1990
Main classification: H01Q5
IPC codes: H01Q13, H01Q21, H01Q5