US3605100A

US3605100A - Electrically scanned tracking feed

Info

Publication number: US3605100A
Application number: US853704A
Authority: US
Inventors: Leonard I Parad
Original assignee: Sylvania Electric Products Inc
Current assignee: GTE Sylvania Inc
Priority date: 1969-08-28
Filing date: 1969-08-28
Publication date: 1971-09-14
Anticipated expiration: 1988-09-14
Also published as: DE2042588A1

Abstract

Apparatus for electrically scanning a radio frequency tracking feed beam employing a feed in which the phase center of the beam is located at the aperture. Two lobes with 180* phase reversal are provided in both azimuth and elevation by exciting the feed in the higher order modes, typically the TE20 mode for azimuth and the hybrid TE11+TM11 mode in elevation. Sequentially lobed tracking is obtained by moving the phase center to the left and the right of the focal axis of the feed for azimuth tracking and above and below the focal axis for elevation tracking.

Description

United States Patent Leonard l. Parad Framlngbam, Mass.

Aug. 28, l969 Sept. 14, 1971 Sylvania Electric Products, Inc.

Inventor Appl. No. Filed Patented Assignee ELECTRICALLY SCANNED TRACKING FEED 5 Claims, 25 Drawing Figs.

US. Cl 343/777, 343/854 Int. Cl l-IOlq 13/00 Field of Search 343/777, 778, 779, 786, 853, 854

[56] References Cited UNITED STATES PATENTS 2,994,869 8/l96l Woodyard 343/777 3,35l,944 ll/l967 Dunn et aL 343/786 3,383,688 5/1968 Renaudie 343/786 3,423,756 1/1969 Foldes 343/786 Primary Examiner-Eli Lieberman Attorney-Robert T. Orner ABSTRACT: Apparatus for electrically scanning a radio frequency tracking feed beam employing a feed in which the phase center of the beam is located at the aperture. Two lobes with 180 phase reversal are provided in both azimuth and elevation by exciting the feed in the higher order modes, typically the TE, mode for azimuth and the hybrid TE +TM mode in elevation. Sequentially lobed tracking is obtained by moving the phase center to the left and the right of the focal axis of the feed for azimuth tracking and above and below the focal axis for elevation tracking.

all L I ll] PATENIED SEP I 4 I971 3. 6 O5 1 0O SHEET 2 [IF 4 DIRECTION OF DISTRIBUTION OF ELECTRIC FIELD ELECTRIC FIELD TE) II I I I 4A. b E

I zc IIII I I I7 I I V TE11+ TM" 46 III III IIIIIII IIIIIIIII IIIIIIII TE -I-TE III III I I 10" 2o I II I I M I TE (I-E" +TM") IIII IIIIII IIIIIII 4. I.\'\'III\"I'()I\ LEONARD I. PARAD SHEET 3 BF 4 E of TE E of T5 TO LOGIC CIRCUIT V l\\\n DRIVER E of TE +TE Fig. 5.

e0 FLIP-FLOP FLIP-FLOP OSCILLATOR A D Fig. 6'.

P Lg. 5.

J EXCLUSIVE v WITH COMPLEMENT L G DRIVER PROBE DRIVER DRIVER LEONARD I. PARAD HYQ'DMZJW PATENTEUSEPIMQYI 3.605.100

SHEET Q UF 4 VOLTAGE H I I OSCILLATOR 60 7A.

H FF 62 73 H FF 62 K FF 64 L H a 7 D. DRIvER 74 L FF 64 K a Y E. DRIvER 76 EX. OR 66 G H 8 TH DRIVER 72 L Ex. OR 66 H H a 7G. DRIVER 70 PROBE TABLE 7H. NUMBER CONDITION OF PROBE 34 OPENED sHORTED sHORTED OPENED OPENED 35 sHORTED sHORTED OPENED OPENED SHORTED 3e OPENED OPENED sHORTED sHORTED OPENED 37 sHoRTED OPENED OPENED sHoRTED sHORTED POSITION OF PHAsE OENTER RIGHT DOWN LEFT UP RIGHT to t1 t2 t3 t4 5 I Z II'\'\'I'.'.\"I()I\' LEONARD I. PARAD ATTORNEY EIJEC'I'RICALLY SCANNEI) TRACKING FEED BACKGROUND OF THE INVENTION This invention relates to antenna systems and in particular to a: :nna feed assemblies useful, for example, in radar tracking system employing sequential lobing.

Tracking or direction finding by sequential lobing is performed by sequentially comparing the signals received via antenna patterns at various positions about its focal axis. The most common techniques for sequential lobing employ beam switching or conical scan. When an RF signal appears within each of two beams at an angle, 6, with input to the boresight axis of an antenna, a voltage of amplitude E,(6) is received by the upper beam and a voltage amplitude (6) is received by the lower beam.By switching an antenna beam alternately between these two positions, the two amplitudes of the received voltages may be compared. If the source is on the boresight axis, the voltages are equal. If the source is above the boresight axis, E,(6) is greater than E and if the source is below the boresight axis, E 9) is greater than E Two additional switching positions are needed to obtain information in the orthogonal coordinate. Thus, a two dimensional sequentially lobing antenna might consist of a cluster of four feed horns illuminating a single reflector, arranged so that the right-left and up-down sectors are covered by successive antenna positions.

Another technique for obtaining tracking information is to continuously rotate an offset antenna beam rather than to discontinuously step the beam among four discrete positions. Apparatus for obtaining a continuously rotating beam includes a parabolic reflector with an offset rear feed rotated about the axis of the reflector. Or, if the antenna is small, the reflector itself may be rotated. In either case, there is mechanical movement between the reflector and the feed to achieve rotation of the beam in space. For large antennas, the problems associated with relative movement between the reflector and feed are increased.

It is, therefore, an object of this invention to provide a rotating antenna field pattern from a single horn and reflector, both of which are stationary with respect to each'other.

SUMMARY OF THE INVENTION In accordance with the present invention, an antenna feed apparatus employs a first means for propagating a first electromagnetic field configuration in response to an input signal at one end. Connected to the other end of the first means for propagating and coupled to a means for generating second and third field configurations is a second means for propagating which propagates not only the first field configuration but also second and third field configurations. A means operable to sequentially add and subtract the first field configuration to the. second and third field configurations in a predetermined manner is connected to the second means for propagating such that the phase center of the resultant field is rotated at the output end of the second means. When the feed apparatus is employed in conjunction with a focusing wave translation means such as a reflector system, a far field pattern is generated which is switched about the focal axis of feed and reflector combination.

DESCRIPTION OF THE DRAWINGS The construction and operation of the invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. IA is a representation of an antenna feed apparatus according to the invention in combination with a reflector;

FIG. IB is a resultant far field pattern of the apparatus of FIG. IA;

FIGS. 2A, 2B and 2C are top, side and end views, respectively, of one embodiment of an antenna feed apparatus according to the present invention;

FIGS. 3A-3D and FIGS. 4A-4G are waveshapes and electric field configurations useful in explaining the theory of operation in the apparatus of FIG. 1;

FIG. 5 is a view of a diode and probe connection employed in the apparatus of FIG. 1;

FIG. 6 is a schematic diagram of a logic circuit employed to scan the phase center of the antenna feed apparatus of FIG. 2; and

FIGS. 7A-7G are waveshapes useful in explaining the operation of the logic circuit of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIGS. 1A and KB, a horn assembly 10 is employed in an electromagnetic wave translation arrangement with a focusing wave translation means such as a parabolic reflector 12, to form a far field antenna pattern in response to a radio frequency signal at the input end 11 of the born 10. When the electrical phase center of the horn I0 is shifted to one side 14 of the horn 10, a first antenna field pattern 20 results. Similarly, when the electrical phase center of the horn 10 is shifted to the other side 18, a second antenna field pattern I6 is generated. By sequentially shifting the electrical center from one side of the horn to the other, sequential lobes (antenna field patterns) are generated about a common focal axis.

While two sequential lobes are shown, it is to be understood that a second set of lobes can be generated by moving the electrical phase center of the horn 10 in a direction orthogonal to the direction dictated by

sides

14 and 18.

An embodiment of a horn 10 according to the present invention is shown in top, side and end views in FIGS. 2A, 2B and 2C, respectively. The horn 10 includes a first section of transmission line, for example, a section of rectangular waveguide 30 having predetermined inner dimensions a, b to support a single electromagnetic field configuration or mode such as the TE mode. Connected to the first section of waveguide 30 is a second section of waveguide 32 of predetermined inner dimensions a', b sufficient to support not only the first mode but also second and third modes such as the TE and the hybrid TE +TM modes. By proper phasing of the TE mode with the TE mode, the horn phase center can be shifted in one plane and similarly by proper phasing of the TE mode with the hybrid TE +TE mode to the phase center can be shifted in an orthogonal plane as will be explained hereinbelow.

Coupled to the second section of waveguide 32 is means for generating the second and third modes in combination with the first mode. The means includes a plurality of

probes

34, 35, 36 and 37, respectively, projecting into the second section of waveguide 32 but electrically isolated from its by the insulators 39. The depth and position of the probes and their effect on the antenna lobes relative to the focal axis 22 will be discussed hereinafter.

In general, a section of waveguide has a particular cutoff wavelength. When the frequency of the signal is high enough to permit the transmission of more than one mode, the resultant field is the sum of the fields of the individual mode fields propagating in the guide. If the fields of one mode are stronger than those of the others, this mode predominates.

For example, if a rectangular waveguide, as illustrated in cross section in FIG. 3A, is excited in the TE mode, the electric field variation across the guide is sinusoidal as shown in FIG. 3B. The TE mode can be launched in the waveguide by any of the well-known techniques, several of which are given in the Reference Data for Radio Engineers, Fourth Edition, by International Telephone and Telegraph Corporation. Assume that the b dimension of the guide is less than a half wavelength so that no TE mode can be supported and that the a dimension exceeds one wavelength so that the "IE mode can be transmitted by the guide. The electric field distribution of the TE mode is shown in FIG. 3C.

If only the TE mode is excited, no TE mode will be transmitted. If, however, an asymmetrically located probe projects into and is electrically connected to the guide, as shown in FIG. 3A, the total electric field configuration will become asymmetrical, as shown in FIG. 3A, with a resultant field distribution, as shown in FIG. 3D. When the switch is closed shorting the probe to the waveguide, the probe is a receiving antenna that extracts energy from the incident TE, mode wave and reradiates it so as to excite the TB mode. When the switch 8 is open, the probe merely blocks a small amount of energy.

Note that in FIG. 3D, the phase center of the electric field has been shifted to one side of the waveguide by shorting the probe to the waveguide. When employed in conjunction with a reflector 12, as shown in FIG. 1A, the resultant far field antenna pattern or lobe would be positioned to one side of the focal axis 22.

To shift the antenna pattern about a focal axis in two orthogonal planes, for example, azimuth and elevation planes, two particular modes are required in conjunction with the basic TE mode of FIG. 4A. For azimuth tracking, a mode which radiates two lobes with 180 phase reversal is required. The TE mode illustrated in FIG. 4B satisfies this requirement. Similarly, the hybrid TE -l-TM mode shown in FIG. 4C provides the requisite lobes for elevation information.

A sequentially lobed tracking antenna is obtained if the phase center of the feed is moved to the left and right of the focal axis for azimuth tracking and above and below the focal axis for elevation. The phase center can be shifted in the azimuth plane (left and right) by combining in phase and out of phase the TE and TE modes. By adding the TE and the TE modes, the phase center is shifted to the left as shown in FIG. 40. Similarly, by subtracting (adding out of phase) the TB mode from the TE mode, the phase center is shifted to the right as illustrated by FIG. 4E.

Elevation patterns, above and below the focal axis, are obtained by combining the hybrid TE +TM mode in and out of phase with the TE mode as shown in FIGS. 4F and 4G respectively.

Apparatus for generating and phasing the higher order modes for elevation will be discussed in detail, and the extension to the azimuth plane will be apparent. The smaller section of waveguide 30 of the horn l propagates only the dominant TE mode. At the junction of the small and large sections of waveguide, a number of higher order modes are excited. However, the dimensions a and b are chosen in accordance with well-known principles to suppress all symmetrical higher order modes generated at the symmetrical junction. Thus, if the larger section 32 is symmetrical, only the TE mode will exist in it.

The symmetry of the large section 32 van be eliminated by the addition of the probes 34-37 as shown in FIG. 2. All four probes extend the same distance into the large section 32. By shorting two of the probes, for example, probes 36 and 37, to the structure, an asymmetry is created which converts some of the TE mode energy into TE and TM mode energy. The phase relationship between the TE and the resultant TE -l-TMmode can be controlled in several ways, for example, by physically or electrically changing the length L of the section 32 or by selective shorting and opening of the appropriate probes.

In employing the latter technique of shifting the phase center about the focal axis, the probes 34-37 are electrically shorted to or isolated from the horn structure in accordance with the following table:

One means for shorting and opening the electrical connection of a probe to and from the horn structure is shown in FIG. 5 and includes a diode 70 having one end connected to the horn 10 and the other end connected to one of the probes, for example, probe 34. The insulator 39 normally isolates the probe 34 from the horn structure. By applying the appropriate biasing to the diode 70 from a logic circuit to be discussed in detail hereinafter, the probe can be isolated from or shorted to the horn 10.

In a sequential-lobing tracking system, an antenna pattern is sequentially moved in a predetermined pattern, for example, a clockwise pattern among four quadrants, two in azimuth and two in elevation, to determine positional information of a target. One embodiment of a logic circuit for programming the opening and closing of the diodes to cause a clockwise rotation of phase center (and therefore a counterclockwise rotation of the antenna pattern) is shown in FIG. 6 and includes a square wave oscillator 60, the output of which is connected to the input terminal A of a first flip-flop 62. The first output terminal B of flip-flop 62 is connected to a first input terminal J of an exclusive OR with complement circuit 66 such as a Sylvania SG- integrated circuit. The second output terminal C of flip-flop 62 is connected to the input terminal D of a second flip-flop 64 and to a second input terminal K of the exclusive- OR with complement circuit 66. The first output E of the second flip-flop 64 is connected to a third terminal of the exclusive OR with complement circuit 66 and to a first driver circuit 74. The second output terminal F of the second flipflop 64 is connected to the fourth input terminal L OF THE EXCLUSIVE OR with complement circuit 66 and to a second driver circuit 76. The first and second output terminals H and G are connected to

respective driver circuits

72 and 74, and the output terminals of the

drivers

70, 72, 74 and 76 are connected to the diodes associated wit

respective probes

35, 36, 34 and 37.

The waveshape of FIG. 7 will be employed to explain the operation of the logic circuit of FIG. 6. In response to the oscillator 60 output signal, shown in FIG. 7A, the first flip-flop 62 produces a first output signal and its complement at terminals B and C respectively. These signals are shown in FIGS. 78 and 7C and are at a frequency equal to half that of the oscillator 60. Similarly, the second flip-flop 64 produces a first output signal and its complement at its terminals E and F in response to an input signal at its input terminal D from the first flip-flop 62. The output signals, as shown in FIGS. 7D and 7E, are equal in frequency to one-quarter of the frequency of the output signal from the oscillator 60.

The two flip-

flops

62 and 64 are employed to count the oscillator signal down to produce square waves at one-half and one-quarter the oscillator frequency. The output signals of the flip-

flops

62 and 64 are combined in the exclusive OR with complement circuit 66 to produce a second set of signals, as shown in FIGS. 7F and 7G, at one-quarter the frequency of the oscillator signal. The E and F output signals of the second flip-flop 64 and the H and G output signals of the exclusive OR with complement circuit 66 are square waves of one-quarter the oscillator frequency but with four different phase relationships. Assume the diodes are forward biased (shorted) when their respective drivers are in the low voltage, L, output state and the diodes are reverse biased (opened) when their respective drivers are in the high voltage, l-l, output state.

Shown in FIG. 7H is a table indicating the operating condition of the diodes (and therefore the condition of the probes with respect to horn 10) for one cycle of clockwise rotation of the phase center about the focal axis of the horn 10. For example, to position the phase center to the left of the focal axis, the diodes associated with

probes

34 and 36 are forward biased shorting the probes to the horn and conversely the diodes associated with

probes

35 and 37 are reverse biased isolating the probes from the horn. The speed of rotation can be increased or decreased by increasing or decreasing respectively the frequency of the oscillator 60 or the sequence of phase center movement can be changed by connecting the drivers to the appropriate probes.

Referring again to FIG. 2, the exact position of each of the probes 34-37 relative to the vertical and horizontal centerlines of the horn l0 and the junction of

waveguide sections

30 and 32 is determined by the desired antenna characteristics. The amount of TE energy converted into TE and TM, ,+TE H modes is proportional to the depth, u. of the probes into the large section 32. The ratio of azimuth to elevation shift is determined by the distance x relative to the centerline 33. The shorter the .r distance is, the greater the elevation shift and the less the azimuth shift. The distance .r relative to the centerline is, therefore, a compromise between competing requirements, which requirements are balanced by judicious placement of the probes in accordance with the desired shift in each plane.

The location of the probes relative to the junction of

sections

30 and 32, as indicated by the y dimension in FIG. 2A, is based upon a trade-off of the competing modes, TE and TM -l-TEmodes. Ideally, the probes should be located at the first maximum of the electric field in the waveguide section 32. However, since the maximums for the TE and the TM,,+TEmodes do not occur at the same point, the y distance must be optimized to obtain a desired amount of both modes consistent with the desired phase center shift.

The amount of TE energy converted into the higher order modes is also a function of the diameter D of the probes. In addition t0 the amount of energy conversion, the diameter D of the probes governs the O of the feed apparatus. The larger the diameter D, the lower the Q and the broader the bandwidth of the mode excitation mechanism.

Also included in the second waveguide section 32 of the horn is a conducting septum 31 to reduce residual crosspolarized energy of the hybrid TE,,+TM mode.

For example, an antenna feed apparatus according to the present invention was constructed to operate at 14.4 to 15.2 GHz. The dimensions of the feed as defined in FIGS. 2A-2C are as follows:

a=0.549 inches b=0.256 inches a'=0.940 inches b'=0.676 inches x=0.l25 inches u=0. 147 inches \=0.3 l 2 inches L=2.30 inches D=0.0588 inches Z=0.661 inches The characteristics of this feed apparatus in azimuth (A) including the half power beam width (l-IPBW) of both the primary pattern data (data from the horn l0 alone) and the secondary pattern data (data from the combination of the horn and a reflector assembly) are included in respective tables II and 111.

TABLE II PRIMARY PATTERN DATA HPBW (Degrees) SECONDARY PATTERN DATA (Near Fie1d)f/D=0.5

HPBW (Degrees) j E Plane H Plane Table 111 Continued An antenna feed apparatus has been described in which the phase center can be electrically positioned such that when the feed apparatus is employed with a reflector a resulting antenna pattern can be shifted in space without physical movement of the feed apparatus relative to the reflector.

What is claimed is:

1. An antenna feed apparatus comprising:

first transmission line means having first and second ends for propagating a first electromagnetic field configuration in response to a signal at said first end;

means for generating second and third electromagnetic field configurations;

second transmission line means including a section of waveguide for propagating said first, second and third electromagnetic field configurations having a first end coupled to the second end of said first transmission line means;

said means for generating including a plurality of fixed probes each having one end protruding into said section of waveguide;

means coupled to the other end of said plurality of fixed probes operable to add and subtract in a predetermined order said first electromagnetic field configuration with each of said second and third electromagnetic field configurations propagated in said second means to thereby form a resultant electromagnetic field configuration, the phase center of which moves in a predetermined pattern at the second end ofsaid section of waveguide.

2. An antenna feed apparatus according to claim I wherein:

said first, second and third electromagnetic field configurations are the TE TE and the TE +TM modes, respectively; and

said section of waveguide includes a length of rectangular waveguide having predetermined dimensions, to propagate said TE TE and TE +TM modes.

3. An antenna feed apparatus according to claim 2 wherein said means for generating said second and third electromagnetic field configuration includes:

a plurality of probes having one end protruding into said length of rectangular waveguide and being mounted on the larger sides of said length of rectangular waveguide at predetermined distances from the center and first end of said length of rectangular waveguide; and

means for electrically attaching the other end of said probes to said length of rectangular waveguide in a predetermined pattern to thereby generate said second and third electromagnetic field configurations.

4. An antenna feed apparatus according to claim 3 wherein means operable to add and subtract said first electromagnetic field configuration with each of said second and third electromagnetic field configurations includes:

a plurality of diodes each having one end connected to the other end of said plurality of probes and the other end connected to said length of rectangular waveguide; and

means connected to said plurality of diodes for selectively forward and reverse biasing certain cones of said plurality of diodes in a predetermined sequence to thereby short the respective probes to said length of rectangular waveguide causing the field configurations to add and subtract in a predetermined pattern.

5. An antenna feed apparatus according to claim 4 wherein:

said plurality of probes includes first, second, third and fourth probes, said first and second probes being mounted on one of the larger sides of said length of rectangular waveguide and said third and fourth probes being mounted diametrically opposite said first and second probes respectively on the other of the larger sides of said length of rectangular waveguide; and

the predetermined pattern produced by said means for electrically attaching the other end of the first, second, third and fourth probes to said length of rectangular waveguide is in accordance with the following table where t through 1 are sequential time intervals to thereby cause rotation of the phase center of the resultant electromagnetic field configuration in a clockwise pattern at the output end of said section of rectangular waveguide.

Claims

1. An antenna feed apparatus comprising: first transmission line means having first and second ends for propagating a first electromagnetic field configuration in response to a signal at said first end; means for generating second and third electromagnetic field configurations; second transmission line means including a section of waveguide for propagating said first, second and third electromagnetic field configurations having a first end coupled to the second end of said first transmission line means; said means for generating including a plurality of fixed probes each having one end protruding into said section of waveguide; means coupled to the other end of said plurality of fixed probes operable to add and subtract in a predetermined order said first electromagnetic field configuration with each of said second and third electromagnetic field configurations propagated in said second means to thereby form a resultant electromagnetic field configuration, the phase center of which moves in a predetermined pattern at the second end of said section of waveguide.

2. An antenna feed apparatus according to claim 1 wherein: said first, second and third electromagnetic field configurations are the TE10, TE20 and the TE11+TM11 modes, respectively; and said section of waveguide includes a length of rectangular waveguide having predetermined dimensions, to propagate said TE10, TE20 and TE11+TM11 modes.

3. An antenna feed apparatus according to claim 2 wherein said means for generating said second and third electromagnetic field conFiguration includes: a plurality of probes having one end protruding into said length of rectangular waveguide and being mounted on the larger sides of said length of rectangular waveguide at predetermined distances from the center and first end of said length of rectangular waveguide; and means for electrically attaching the other end of said probes to said length of rectangular waveguide in a predetermined pattern to thereby generate said second and third electromagnetic field configurations.

4. An antenna feed apparatus according to claim 3 wherein means operable to add and subtract said first electromagnetic field configuration with each of said second and third electromagnetic field configurations includes: a plurality of diodes each having one end connected to the other end of said plurality of probes and the other end connected to said length of rectangular waveguide; and means connected to said plurality of diodes for selectively forward and reverse biasing certain cones of said plurality of diodes in a predetermined sequence to thereby short the respective probes to said length of rectangular waveguide causing the field configurations to add and subtract in a predetermined pattern.

5. An antenna feed apparatus according to claim 4 wherein: said plurality of probes includes first, second, third and fourth probes, said first and second probes being mounted on one of the larger sides of said length of rectangular waveguide and said third and fourth probes being mounted diametrically opposite said first and second probes respectively on the other of the larger sides of said length of rectangular waveguide; and the predetermined pattern produced by said means for electrically attaching the other end of the first, second, third and fourth probes to said length of rectangular waveguide is in accordance with the following table t1 t2 t3 t4 first probe shorted shorted opened opened second probe opened shorted shorted opened third probe shorted opened opened shorted fourth probe opened opened shorted shorted where t1 through t4 are sequential time intervals to thereby cause rotation of the phase center of the resultant electromagnetic field configuration in a clockwise pattern at the output end of said section of rectangular waveguide.