Multimode horn antenna

Abstract

A horn antenna is described including a funnel-shaped coupler with a port at the narrow end of the coupler adapted to excite a symmetrical waveguide mode in the coupler and a plurality of side wall input ports adapted to excite at least two difference waveguide modes. The port at the narrow end of the coupler is coupled by a first step to a smaller diameter cylindrical section capable of propagating the dominant symmetrical mode. A larger diameter cylindrical section is coupled to the larger end of the coupler by a second step. A ring divides the larger diameter cylindrical section into two parts. The first and second steps and the dimensioning and placement of the ring are arranged so that the two difference mode radiation patterns have essentially the same combined beam width or pattern envelope as the symmetrical mode radiation pattern.

Description

BACKGROUND OF THE INVENTION

This invention relates to horn antennas and horn antenna feed systems and more particularly to a horn antenna feed system that when properly excited generates at least one symmetrical mode and two asymmetrical modes where the radiated pattern associated with the asymmetrical modes has two main lobes which essentially have the same pattern envelope as the radiated pattern associated with the symmetrical mode having one main lobe.

It is desirable in many applications, such as steerable radar antennas, aircraft collision prevention equipments, tracking antennas, and radiating elements for steerable arrays that a multiple beam position radiating element be provided. It is of particular interest to construct a multiple beam position radiating element in which the beam can be switched several positions, for example, four or five, electronically with a medium gain (in the range of 8 to 20 db) in the peak direction while the sidelobes of the switched beam in the rear hemisphere remain below -25 db relative to the peak of the beam.

SUMMARY OF THE INVENTION

A horn antenna is provided by a structure including a funnel-shaped coupler with an aperture at the narrow end of the coupler adapted to excite a symmetrical mode therein, a larger diameter aperture at the opposite end, and a plurality of side wall coupling apertures adapted to excite two orthogonal asymmetrical difference modes. The narrow end of the coupler is coupled by a first step to a smaller dimensional waveguide section capable of propagating the dominant symmetrical mode. The larger diameter end of the coupler is coupled by a second step to a larger diameter cylindrical waveguide section. The larger diameter cylindrical waveguide section is divided by a ring into two subwaveguide sections. The first and second steps and the ring are dimensioned and arranged so that the radiation patterns associated with the two asymmetrical modes have substantially the same pattern envelope as the radiation pattern associated with the symmetrical mode.

DETAILED DESCRIPTION OF THE INVENTION

A detailed description follows in conjunction with the attached drawing wherein:

FIG. 1 is a side elevation view of the horn antenna according to a preferred embodiment of the present invention.

FIG. 2 is an end view of the antenna of FIG. 1.

FIG. 3 is a front view of the antenna of FIG. 1.

FIG. 4 is a cross-sectional view of the horn antenna in FIG. 2 taken along lines 4--4.

FIG. 5 illustrates the dominant symmetrical TE.sub.11 mode, the asymmetrical TE.sub.12 mode and the orthogonal asymmetrical hybrid TE.sub.21 + TM.sub.01 mode.

FIG. 6 illustrates the patterns associated with the symmetrical and asymmetrical modes from the antenna of FIG. 1.

FIG. 7 is a schematic diagram of a switched beam position antenna using the horn in FIG. 1.

FIG. 8 illustrates a coverage area of the switched beam antenna of FIG. 7.

FIG. 9 illustrates a combined radiation pattern observed by adding the sum and one set of difference modes.

Referring to FIGS. 1 through 4, there is illustrated a horn antenna 10. A horn antenna 10 includes a funnel-shaped coupler 11, a cylindrical waveguide section 13 coupled to one end 11a of coupler 11, a smaller cylindrical waveguide section 15 coupled to the smaller opposite end 11b, a rectangular waveguide section 17 coupled to smaller cylindrical waveguide section 15 at end 17a, and a choke section 19 coupled to the end 13a of cylindrical waveguide 13. The coupler 11 is a hollow, conically shaped member of a diameter at end 11b sufficient at least to excite the dominant TE.sub.11 mode and to reflect those of the TE.sub.12, TE.sub.21 and TM.sub.01 modes that may be coupled thereto. FIG. 5 illustrates these modes. The large aperture at the opposite end 11a is made large enough to couple the TE.sub.11, TE.sub.12, TE.sub.21 and TM.sub.01 modes.

Four side wall slot apertures 21, 23, 25 and 27 are located in the side wall of the coupler. See FIGS. 2 thru 4. The center of apertures 21 and 25 lie in plane 26 and the center of apertures 23 and 27 lie in plane 29. The planes 26 and 29 are at an angle of 45.degree. with respect to the vertical axis 28. The orientation of these slots in the side wall of the coupler determines the polarization of the asymmetrical mode. For vertical polarization shown in FIGS. 2 and 3, the slots are rotated 45.degree. around their centers and relative to the plane containing the centers of the four slots. A rectangular coupling cavity is mounted above each of the side wall apertures. A rectangular cavity 31 is mounted above coupling aperture 21, a rectangular coupling cavity 33 is mounted above coupling aperture 23; a rectangular coupling cavity 35 is mounted above coupling aperture 25; and a rectangular coupling cavity 37 is mounted above coupling aperture 27. The width w and length l.sub.3, as shown in FIGS. 2 and 4, of each of the cavities 31, 33, 35 and 37 is made sufficient to support the TE.sub.10 and the TE.sub.01 rectangular waveguide modes. Coupling probes 41, 43, 45 and 47 act to excite the TE.sub.10 mode in the cavities from some external source to be discussed later. The diameter D.sub.s of the coupler 11 taken as shown in FIG. 4 at the center of the slot apertures 21, 23, 25 and 27 is made so as to support at least the TE.sub.12, TE.sub.21 and TM.sub.01 modes in the coupler. From the above set of modes the TE.sub.12 mode has the lowest ratio of .lambda..sub.c /D.sub.s = 0.5892, and thus D.sub.s .gtoreq..lambda./0.5892, where .lambda. is the operational wavelength. As discussed later, a hybrid mode made up of the TE.sub.21 + TM.sub.01 modes is also excited at the apertures 21, 23, 25 and 27. The dimension of the coupler at the end 11b and the axial distance from the center of apertures 21, 23, 25 and 27 to the end 11b are determined so that the hybrid difference mode (TE.sub.21 + TM.sub.01) is reflected back toward the larger aperture 11a of the coupler and combines in phase with the waves directly travelling from the slot apertures toward the wider end 11a of the coupler.

The symmetrical TE.sub.11 circular waveguide mode and the other symmetrical modes are excited at the end 11b by means of the rectangular waveguide section 17, the circular waveguide section 15 and the step 20 between the waveguide section 15 and the end 11b of the coupler 11. The TE.sub.10 rectangular waveguide mode is excited in rectangular waveguide 17 by means of coupling probe 51. Each of the coupling probes as used herein are coaxial probes having the outer conductor connected to the waveguiding section and the insulated inner conductor extending into the waveguiding section through an aperture therein. This TE.sub.10 mode is coupled through an aperture in the top wall 17a of waveguide 17 into the larger diameter circular waveguide section 15 wherein the TE.sub.11 circular waveguide mode is excited in cylindrical waveguide section 15. The cylindrical waveguide section 15 terminates in the step 20 at the narrow end 11b of the coupler 11 as mentioned previously. This step 20 acts as a controlling device for generating a set of symmetrical modes to produce a Gaussian shape resultant amplitude distribution in the large aperture at end 12 of the device 10 with the Gaussian shaped pattern in both the E and H planes. The TE.sub.11 mode signals together with the other higher order symmetrical modes (for example TE.sub.30 mode) generated by the step 20 are radiated out of end 11a of the coupler 11.

Also radiated out of the aperture at end 11a of coupler 11 are waves in the TE.sub.12 circular waveguide mode and in the hybrid mode made up of the TE.sub.21 + TM.sub.01 circular waveguide modes. The signals at the coupler end 11a are coupled to the cylindrical waveguide section 13 which has a larger diameter than that of end 11a of coupler 11 and thus forms a step 30. The second step 30 provides refinement in the control of the symmetrical modes and provides proper phasing of the asymmetrical difference modes TE.sub.12 and hybrid TE.sub.21 + TM.sub.01 modes to maintain their orthogonal relationships. Essentially, the TE.sub.13 mode is compensated for the difference in phase velocity relative to the signals in the hybrid TE.sub.13 + TM.sub.01 modes. Additional compensation for the phasing between the TE.sub.12 modes and the hybrid TE.sub.21 + TM.sub.01 modes is achieved by the ring 13b in the cylindrical waveguide section 13 and by the distance l.sub.7, FIG. 4, the ring 13b is from the step 30. Further, the ring 13b contributes to matching of the different modes to free space. A choke member 19 is located between the output aperture at end 12 and is a continuation of the section 13 from the end 13a of the cylindrical section 13. The choke 19 includes a section in which the inside diameter is reduced and the outside surface diameter of the horn antenna is reduced approximately a quarter wavelength for an axial length of approximately a quarter wavelength, where a wavelength is taken at an operating frequency of the antenna. This quarter wave choke 19 helps to eliminate the flow of surface currents on the outside of the horn antenna.

In an embodiment designed to operate at a frequency of approximately 1825 MHz, the horn antenna described above and shown in FIGS. 1 thru 4 has the following dimensions:

Overall length l.sub.1 of coupler 11 = 7.37 inches

Length l.sub.2 between end 11b and center line of slots 21-25, 23-27 = 5 inches

Length l.sub.3 of cavities 33 and 37 = 6.25 inches

Diameter (D.sub.1) of coupler 11 at end 11b = 7.30 inches

Diameter (D.sub.2) of coupler 11 at end 11a = 10.20 in.

Diameter (D.sub.3) of circular waveguide section 13 between 13a and 11a = 13.23 inches

Diameter of aperture (D.sub.4) in ring 13b, diameter of aperture (D.sub.5) at end 13a of section 13, and diameter (D.sub.7) of aperture at end 12 are each = 10.12 inches

Diameter of coupler 11 at center of slots D.sub.s = 9.25 inches

Diameter (D.sub.6) of circular waveguide section 15 = 5.36 inches

Height H.sub.1 of rectangular waveguide section 17 = 2.880 inches

Axial length (l.sub.5) and depth (D.sub.8) of choke 19 = at 1.5 inches

Length (l.sub.6) of circular waveguide section 13 = 5 inches

The distance (l.sub.7) ring 13b is from step 30 = 3.39 inches

Width (w) of coupling cavity 31, 33, 35 and 37 = 4.4 inches.

By the arrangement described above, the symmetrical mode generated at the step 20 and processed through the antenna has an aperture distribution as shown by curve 54 in FIG. 6. This response approximates a Gaussian type response. By the arrangement of the step 30, the placement of the ring and the dimension of the ring 13b, the asymmetrical difference modes generated in (both the azimuth and elevation plane or in the TE.sub.12 mode and the hybrid TE.sub.21 + TM.sub.01 modes) has an aperture distribution similar to that shown by the dashed line 56 in FIG. 6. As shown in FIG. 6, at about the -3 db point from the center of the main beam (symmetry axis) the symmetrical mode beam width is about .+-.18.degree.. The difference modes fall within the envelope of the pattern defined by the sum mode with the null of the difference mode pattern in the center (at the symmetry axis) and the two peaks of the difference mode at the -3 db point at the .+-.18.degree. points on either side of the null. This difference mode characteristic is achieved by the step 30 and the ring 13b.

A switched beam position antenna or scanning antenna system is provided by coupling the horn antenna 10 shown in FIGS. 1 thru 4 to a circuitry including a monopulse comparator 55 and a switchable power divider network 85 as shown in FIG. 7. The TE.sub.11 mode coupling port 51, FIGS. 1 and 4, is represented by terminal 51a in FIG. 7, and the sidewall apertures 21, 23, 25 and 27 are represented by probes 21a, 23a, and 27a. The coupling cavities 31, 33, 35 and 37 are represented by boxes 31a, 33a, 35a and 37a in FIG. 7. The sidewall coupling members 21a, 23a, 25a and 27a are coupled to a monopulse comparator network 55 via the coupling cavities 31a, 33a and 37a.

This comparator 55 network is like that of comparator network 15 in applicant's copending application, Ser. No. 470,574, filed May 16, 1974. The comparator 55 consists of two magic-tee (0.degree. hybrids) hybrids 65 and 67 and two short-slot hybrids (90.degree. hybrids) 69 and 71 and connections therebetween. One terminal of each of the magictee hybrids is coupled to a load (indicated by resistor). One of the magic-tee hybrids 65 is coupled at one end to terminal 75 of the comparator 55 and at the opposite end to terminal 69b of short-slot hybrid 69 and terminal 71a of short-slot hybrid 71. The other magic-tee hybrid 67 is coupled at one end to the terminal 77 of the comparator 55 and at the opposite end to terminal 69a of short-slot hybrid 69 and terminal 71b of short-slot hybrid 71. The terminals 69c, 69d, 71c and 71d of short-slot hybrids 69 and 71 form the terminals of the monopulse comparator 55. The terminals 69c, 69d, 71d of the comparator 55 are coupled via suitable transmission lines 79, 80, 81 and 82 and the respective coupling cavities 33a, 31a, 37a and 35a to the coupling members 23a, 21a, 27a and 25a. The signal waves at terminal 75 are power divided at hybrid 65 and are coupled with equal phase to terminal 69b of hybrid 69 and terminal 71a of hybrid 71. The wave at terminal 69b is power divided with the output waves coupled to coupling member 23a via terminal 69c undergoing 90.degree. more phase shift than the wave coupled to coupling member 21 a. This additional phase shift is due to the coupling of the waves through the short slot 69e of hybrid 69. The signal waves at terminal 71a are power divided with the output power divided waves coupled through slot 71e to coupling member 25a undergoing 90.degree. more phase shift than the power divided waves coupled to coupling member 27a. With this phase distribution the signals at coupling members 23a and 25a undergo 90.degree. more phase shift than the signals at the coupling members 21a and 27a. If the coupling members 21a, 23a, 25a and 27a are oriented as shown in FIG. 7 and the above phase relationships exist, the linearly polarized TE.sub.12 mode circular waveguide as shown in FIG. 5 is excited with the null in the plane of coupling members 23a and 27a. The waves at terminal 77 are power divided by hybrids 67, 69 and 71. This results in 90.degree. more phase shift to the waves at coupling members 21a and 27a than at coupling members 23a and 25a. When the coupling slots are oriented in the same direction as stated previously (shown in FIG. 2), the signals in the coupler 11 of antenna 10 are excited in the TE.sub.21 + TM.sub.01 so called circular waveguide hybrid mode as shown in FIG. 5. The null plane is in the plane of the coupler determined by coupling members 21a and 25a. The radiation pattern from these two sets of modes will have two out of phase main lobes and their null planes will be orthogonal to each other and will coincide with the plane of the coupling members. The difference mode pattern is illustrated by dashed line 56 in FIG. 6.

It has been found that by controlling the amount of power to terminal 51a of the antenna 10 relative to the power at terminals 75 and 77 of the comparator 55, a switched beam position antenna system is provided. In other words with respect to the above described arrangement, by controlling the power levels of the excited symmetrical mode such as the dominant TE.sub.11 mode, and the asymmetrical modes such as the TE.sub.12 and hybrid TE.sub.21 + TE.sub.01 modes in the antenna coupler 11, a switched beam position antenna system can be produced. This switchable power division is provided by the switchable power divider network 85. The switchable power dividing network 85 includes a power divider 95, three switches 96, 97 and 99 and a 180.degree. phase shifter 101. The terminal 91 of switchable power divider network 85 is coupled to a source of signal waves. The power from that source is power divided at power divider 95 wherein waves with a selected percentage of the input power from the source are coupled via a trimming phase shifter 104 to terminal 51a of antenna 10 and the remaining power in the form of signal waves is coupled to switch 96. The switch 99 in a first position (contacting terminal 99a) couples the applied remaining power via synchronized switches 96 and 97 to terminal 75 of comparator network 55. In a second position of switch 99 (contacting terminal 99b) the applied remaining power is coupled to terminal 77. If switches 96 and 97 are in a first position (contacting terminals 96a and 97a) the remaining power is coupled over a transmission line path 105 therebetween to terminal 75 or 77. If switches 96 and 97 are in their second position (contacting terminals 96b and 97b), the remaining power is coupled over a transmission path 106 including 180.degree. phase shifter 101 to terminal 75 or 77. Thus, the switchable power divider delivers the remaining power to either terminal 75 or 77 in either 0.degree. or 180.degree. phase, providing a total of four conditions.

When the variable power divider 95 is adjusted so that all of the power is coupled to terminal 51a, a beam is excited having its maximum along the mechanical axis (symmetry axis in FIG. 6) of the radiating antenna 10. If the variable power divider 95 couples one half of the power to terminal 51a and the remaining power to switch 96, four additional beam positions are provided by the four conditions of switchable power divider 85 which have their maximum in the 45.degree., 135.degree., 225.degree. and 315.degree. azimuth directions and at 1/2 .theta..sub.3 angle away from the original mechanical (symmetry) axis of the system. Here .theta..sub.3 is the 3 db beamwidth of the symmetrical mode beam associated with signals in the TE.sub.11 mode coupled to terminal 51a.

The square 120 outlined in FIG. 8 illustrates a given required angular coverage, the dot 123 in the center represents the mechanical axis of the antenna and the small circle 121 represents the angular coverage when all the input power is delivered to the sum port 51a. When one half of the power is coupled either in phase or with 180.degree. phase reversal to either terminals 75 or 77 the center of the radiated pattern is switched in the direction of the arrows to provide the coverage areas A, B, C and D which are quadrants of coverage area 120.

As mentioned previously, the two difference mode patterns have two out of phase main lobes. By the arrangement discussed above a first of these two main lobes associated with each difference mode is in phase with the single main lobe associated with the TE.sub.11 mode. These in phase lobes add and the out of phase lobe subtracts, resulting in a combined beam with maximum signal in a direction toward the peak of the in phase lobe of the difference mode. By adding, the 180.degree. phase shift to the signals coupled to terminals 75 or 77, the sum mode (symmetrical mode) wave associated with terminal 51a is in phase with the second of the two main lobes associated with each difference mode and is out of phase with the first of these modes. Consequently, the maximum beam direction is moved to the directly opposite quadrant (for example from A to D or C to B).

To summarize the variable power divider switch 95 controls the percentage of the power to excite the main symmetrical TE.sub.11 mode. The switch 99 switches the remaining percentage of power to excite either the TE.sub.12 mode via terminal 75 (if 50 percent of the power at that terminal 75 illuminate quadrants A or D in FIG. 8) or excite the hybrid mode of TE.sub.12 + TM.sub.01 modes via terminal 77 (if 50 percent of the power at the terminal 77 illuminate quadrants B or C in FIG. 7). The synchronized switches 96 and 97 control which of the two quadrants A or D are illuminated when the power goes via terminal 75. The synchronized switches 96 and 97 control which of the two quadrants B or C are illuminated when the power goes via terminal 77.

FIG. 9 illustrates a combined beam 131 which is the result of 50 percent of the power being coupled to terminal 51a and 50 percent of the power to either terminals 75 or 77. Because the sum and difference mode component patterns have the same envelope, the resultant scanned pattern is practically sidelobe free. This same envelope characteristic as discussed previously is achieved by the dimensioning of the steps at the junction of the conical coupler section 11 and the cylindrical section, by the length of the cylindrical section and the position of the ring 13b in the cylindrical section.

Claims

What is Claimed is:

1. A horn antenna comprising:

a funnel-shaped coupling member having two orthogonal symmetry planes and having an aperture at the narrow end adapted to excite a symmetrical mode wave therein and a larger diameter aperture at the opposite end,

a first pair of side wall coupling apertures in the side wall of said member at diametrically opposite surfaces of the member in one symmetry plane,

a second pair of side wall coupling apertures in the side wall of said member at diametrically opposite surfaces of said member in the second of said orthogonal symmetry planes, said first and second pair of side wall coupling apertures being adapted to excite two orthogonal asymmetrical modes,

a waveguide section of a diameter large enough to support said symmetrical mode and small enough to reflect said asymmetrical modes and smaller than the diameter of the aperture at the narrow end of said member coupled by a first step to said narrow end of said member,

a cylindrical waveguide of a diameter larger than said larger diameter aperture of said member coupled by a second step to said opposite end of said member, said cylindrical waveguide section having a ring-like member dividing said cylindrical waveguide into two sections, said first and second steps and said ring-like member being dimensioned and arranged so that the two main lobes in the radiation patterns associated with the two asymmetrical modes have substantially the same beamwidth pattern of the single main lobe radiation pattern associated with the symmetrical mode.

2. The combination in claim 1 including a choke in the outer surface wall of said cylindrical waveguide.

3. A variable beam position antenna comprising,

in combination:

a hollow member having a first opening at one end and an opening at the opposite end;

a plurality of side wall coupling apertures in the side wall of said member;

a first power divider means coupled between said plurality of side wall coupling apertures and a terminal thereof for in response to signal waves coupled to said terminal power dividing said waves and coupling the power divided waves in a given phase relationship to said side wall apertures to excite within said member a first asymmetrical difference mode that provides a radiated pattern with two main out of phase lobes;

means adapted at said one end of the hollow member to couple a symmetrical mode wave and reflect said asymmetrical difference mode waves, and means at said opposite end to couple said symmetrical and asymmetrical modes, and

a controllable power divider means adapted to be coupled to a source of signal waves for coupling signal waves of a selected percentage of the power from said source to said one end, whereby a symmetrical mode radiated pattern with one main lobe is produced, and for coupling signal waves at the remaining power from said source to said terminal of said first power divider means.

4. The combination claimed in claim 3, wherein said member is a funnel-shaped member.

5. The combination claimed in claim 3, wherein said member has two orthogonal symmetry planes.

6. The combination claimed in claim 5, wherein a first pair of said plurality of side wall apertures is in a first of said planes and a second pair of said plurality of side wall apertures is in a second of said planes orthogonal to said first plane.

7. The combination claimed in claim 6, including means coupled to said one end for controlling said symmetrical mode and means coupled to said opposite end for controlling said asymmetrical mode whereby the two out of phase difference lobes in the pattern associated within the asymmetrical mode fall within the main pattern lobe associated with the symmetrical mode.

8. The combination claimed in claim 6 wherein said two asymmetrical difference modes are the TE.sub.12 mode and a hybrid TM.sub.01 + TE.sub.21 mode.

9. The combination claimed in claim 6 wherein said first power divider means includes a second terminal which in response to signal waves coupled thereto power divides said waves and couples these power divided waves in a phase relationship to excite within said member a second asymmetrical difference mode orthogonal to said first asymmetrical difference mode.

10. A variable beam position antenna comprising, in combination:

a funnel-shaped hollow member having two orthogonal symmetry planes and a first opening at one end and a larger opening at the opposite end adapted to be coupled to free space;

a first terminal means coupled to said one end;

first and second apertures in the side wall of said member at diametrically opposite surfaces of the member in one of said symmetry planes;

third and fourth apertures in the side wall of said member at diametrically opposite surfaces of said member in the second of said planes;

second and third terminal means;

means coupled between the first, second, third and fourth apertures and said second and third terminal means for, in response to signal waves at said second terminal means, equally distributing the energy of said signal waves applied thereto to each of said side wall apertures with signal waves at the first and third side wall apertures being advanced 90.degree. relative to signal waves at the remaining second and fourth side wall apertures and for, in response to signal waves at said third terminal means, equally distributing the energy of said signal waves applied thereto to each of said sidewall apertures with the signal waves at the second and fourth apertures being advanced 90.degree. relative to the signal waves at the first and third sidewall apertures;

a controllable power divider means adapted to be coupled to a source of radio frequency signals for power dividing said signals in a selective manner to provide at least one half of the signal energy of said signals at said source to said first terminal means, whereby a symmetrical mode pattern is exited in the member, and the remaining percentage of signal energy of said signals at said source to a selected one of said second and third terminals in either a selective in phase at 180.degree. out of phase relationship with said signals applied to said first terminal.