US3092834A - Split parabolic radar antenna utilizing means to discriminate against crosspolarized energy - Google Patents

Description

.SEARCH RC June 4, 1963 J. G. MocANN ETAL 3,092,834

SPLIT FARABOLIC RADAR ANTENNA UTILIZING MEANS T0 DISCRIMINATE AGAINST CROSS-POLARIZED ENERGY Filed Deo. 23, 1958 2 Sheets-Sheet 1 0 4 2 JW 5 2 2 www. 2

June 4, 1963 J. G. MGCANN ETAL 3,092,834

SPLIT PARABOLIC RADAR ANTENNA UTILIZING MEANS TO DISCRIMINATE AGAINST cRoss-PoLARzED ENERGY Filed DGO. 23, 1958 2 Sheets-Sheet 2 MG. zz.

INVENToRs Jog 9. Mcm/VN United States Patent Oil ice 3,092,834 Patented June 4, 1963 SPLH PARABOLEC RADAR ANTENNA UTILIZHNG MEANS T DISCRIMINATE AGAI YST CROSS- POLARIZED ENERGY Joe G. McCann, Encino, Robert I. Stegen, Van Nuys, and William llalstrom, Encino, Calif., assignors, by mesne assignments, to Canoga Electronics Corporation, Van Nuys, Calif., a corporation of Nevada Filed Dec. 23, 1958, Ser. No. 782,599 Claims. (Cl. 343-756) This invention relates to airborne radars and, more particularly, to an antenna system for a somewhat unsymmetrical generally dish-shaped or parabolic reflector used in radar tracking systems.

In the past, it has been the practice to use a dish-shaped reflector or radar dish split in half by a plane through its symmetrical axis. The halves are generally kept together for searching operation; however, it has been the practice to rotate these halves through a small angle about a line tangent to the symmetrical center of the parabolic surface in the plane of the split away from each other to a position in which they are maintained stationary with respect to each other for tracking operations. In this case, the halves are illuminated with electromagnetic energy having a plane of polarization or E-plane through the symmetrical axis of the antenna system parallel to the split.

During tracking, both halves are rotated in a conventional manner although their symmetrical axes are maintained at a lxed angle with respect to each other. The split arrangement is employed to broaden the main lobe of the antenna in a plane perpendicular to the E-plane, i.e., the H-plane.

To the present time, a serious disadvantage has accompanied the use of the above-described split reflector during tracking. As is well known, an approximately sinusoidal modulation envelope is normally imposed on received electromagnetic energy by reflection from ay target which is spaced from the axis of rotation during rota-tion in tracking. The angular position of the target relative to the radar transmitter is thus conventionally determined for tracking or other purposes by measuring the magnitude of this modulation and comparing its phase with an alternating analog signal representative of the instantaneous rotating position of the reflector. The magnitude of the modulation envelope, thus, is a function of the angle between the rotational axis and a line through the target and through the point of radiation, and the phase of the modulating envelope is a function of the angular position of a plane through the same line and the axis of rotation and another fixed reference plane through the axis of rotation.

The parabolic split radar dish operates satisfactorily generally when a target exists at a relatively large angle from the axis of rotation, but a phase shift of the modulating envelope occurs at relatively small angles which introduces error. IIn fact, at a certain small angle for a split radar dish of a given geometry, the phase shift error appears to be approximately 90 electrical degrees. The split radar dish thus has become practically useless for accurately tracking targets located close to the axis of rotation of the dish or close to boresight.

The present invention overcomes the above-described and other disadvantages of the prior art by providing in combination therewith, means associated with a split radar dish to discriminate against cross-polarized electromagnetic energy. A deiinition of cross-polarized electromagnetic energy will be given hereinafter. However, even the general manner in which the invention performs its desired function, i.e., the manner in which it eliminates the above-described phase shift error in the modulation envelope produced by off-boresight targets during tracking, cannot be understood without resorting to at least some basic antenna theory.

Firstly, portions of the prior art parabolic reilector may be defined as being made up of four identical quadrants divided by perpendicular E- and H-planes through the symmetrical axis thereof. When a plane wave of a given polarization is intercepted by a quadrant, currents flow in planes parallel to both of the E- and H-planes. This may be understood by recognizing that the interception of a plane through a line perpendicular to the axis of symmetry of a parabola of revolution, intercepting the axis at the point of feed or illumination, and the parabola itself delines a predetermined curve. The projection of this curve on a plane perpendicular to the E- and H-planes is then circular. Orthogonal vector currents combining to make for circular current tlow thus explain why currents actually do flow in planes parallel to the H-plane. Electromagnetic energy transmission caused by current flowing in planes parallel to the H- plane on the radar dish conductive surface is called cross-polarized. This phenomenon is not unknown in the prior art and is explained in vol. l2, Radiation Laboratory Series, page 419. Reflection from a radar-dish of cross-polarized energy exists in the above-mentioned four quadrants but radiation from adjacent quadrants are out of phase with respect to each other although, in phase transmission, the energy at 'the common point of feed from which they were derived was in-phase energy. Thus, to the present time, cross-polarized lobes were thought unimportant, firstly, because it was thought they would cancel each other out to a certain extent and, secondly, because they would simply add a second harmonic signal to the fundamental frequency of signal of the modulating envelope produced by an oE-boresight target during the antenna rotation of tracking. In the prior art, the second harmonic signal was theorized because antenna rotation would carry the two nearest-totarget cross-polarized lobes alternately nearer and farther away from a target while the main lobe would be carried toward and away from the target only once during a single cycle of antenna rotation. According to this theory, a selective filter to pass only the fundamental frequency signal of the modulating envelope and to discriminate against the second harmonic signal produced by the cross-polarized lobes should have worked although it did not. Thus, to the present time it has always been assumed that electromagnetic energy received in two adjacent cross-polarized lobes would both add to energy received in the principal plane of polarization in the main lobe.

In accordance with the invention, a theory was worked out, which 4as yet cannot be proved analytically, but which has been proved experimentally, lthat a fundamental frequency phase shift error could be produced by cross-polarized lobes if one added and the other subtracted from energy of the principal polarization that was received in the main lobe. Although there is still no adequate analysis which bears out this theory, the desired result is obtained because when radiation of cross-polarized energy is substantially prevented in accordance with the invention, the split dish operates satisfactorily without introducing noticeable error in the position indication of any oif-boresight targets. It could be argued that reflected or received cross-polarized energy is generally only of one phase and that when this is received in one lobe it is actually added to energy received in the main lobe and when received in an adjacent cross-polarized lobe is subtracted from the energy received in the main lobe; however, there is no physical basis even to support this hypothesis. However, in spite of the misleading presumptions made in the prior art, the invention has proved successful. In accordance with the invention, cross-polarization is completely eliminated and, with it, phase shift error which was produced by the split radar-dish of the prior art.

The invention will be better understood when considered in connection with the following description.

In the accompanying drawings, which are to be regarded as merely illustrative:

FIG. 1 is a rear perspective view of the apparatus of the invention;

FIG. 2 is a schematic diagram illustrating the shape of the reliector shown in FIG. 1;

FIG. 3 is a front perspective view of the apparatus of the invention shown in FIG. 1;

FIG. 4 is a front elevational view of a split radardish made in accordance with the invention;

FIG. 5 is a side elevational view of a ilat edge of one of the halves of the dish shown in FIG. 4;

FIG. 6 is an enlarged sectional view of half the radar reflector taken on the line 6-6 shown in FIG. 5;

FIG. 7 is a front elevational view of a radar-dish made in accordance with an alternative embodiment of the invention;

FIG. 8 is -an enlarged view taken at area 8 shown in FIG. 7;

FIG. 9 is a sectional view taken on the line 9 9 shown in FIG. 8;

FIG. 10 is a front elevational view of still another embodiment of the invention;

FIG. 11 is a side elevational view of one of the halves of the radar-dish shown in FIG. 10; and

FIG. 12 is an enlarged sectional view of half of the antenna system shown in FIG. 11.

In FIG. 1, a conventional airborne radar is indicated at 10 having special means 20 to open two halves 21 and 22 of dish-shaped reflector 23 to a position shown at 23 in FIG. 2. Halves 21 and 22 may be hinged as indicated at 48. In the position shown in FIG. l, halves 21 and 22 are disposed at an angle indicated at A in FIG. 2, the positions of halves 21 yand 22 being indicated at 21 and 22. The means 20 to open halves 21 and 22 may simply include projections 25 and 26 having ` cables 27 and 28 iixed to them, adapted to be pulled together by any motive power means or mechanical connection therewith. Halves 21 and 22 may even be opened manually since halves 21 and 22 will not be moved with respect to each other during tracking or searching but only at a switching time when it is desired to change from a searching to a tracking operation, or vice Versa. When no force is applied to cables 27 and 28, halves 21 and 22 may be kept normally closed by a spring 29. Dish 23 is illuminated by a feed 30 shown in FIG. 3, which may be conventional, to direct electromagnetic energy toward halves 21 and 22, having a plane of polarization through the axis of feed 30 extending symmetrically through the split between halves 21 and 22. The front elevational view of FIG. 4 shows halves 21 and 22 when they are closed for the searching operation and ar- 4 row 49 indicates the direction of the E-plane while arrow 50 indicates the direction of the H-plane.

As can be seen in FIGS. 5 and 6, each half 21 and 22 of reflector 23 is parabolic in shape having an inner lamination 31 and two outer laminations 32 and 33 all of which may be made of any convenient dielectric fabric material. A plurality of conductors 34 are embedded in a dielectric material 35 to maintain a fixed spaced position. It is to be noted that conductors 34 conform t0 the parabolic shape of reilector halves 21 and 22 but they are aligned in parallel planes substantially parallel to the plane of polarization of energy directed toward halves 21 and 22 by feed 30.

Another embodiment of the invention is shown in FIGS. 7, 8 and 9 where conductive parabolic halves 36 and 37 are made of a sheet metal material having elongated slots 38 therein best illustrated in FIG. 8. It is to be noted that slots 38 are ranged in two sets of rows, the first set being shifted lengthwise from the second set a distance equal to one-half the length of slots 38 plus one-half the length of the distance between slots. This means that cross-polarized energy tending to cause cul'- rents to ilow in the direction of arrow 39 will be interrupted by the slots, whereas current flowing in planes parallel to the direction of polarization and thereby parallel to a symmetrical plane through the center of the reflector halves 36 and 37 indicated at a split 40 will not be interrupted because current induced by such radiation may flow continuously in the direction of arrow 41 between the rows of slots in an uninterrupted fashion.

As shown in FIGS. 10, 1l and 12, a parabolic solid conductive reilector 41 may also be employed when split on the line 42, and semi-.circular dielectric sheets 43 may be mounted by suitable means 44 on halves 45 and 46 of reflector 41. Conductors 47 may be embedded in the material of a dielectric sheet 43 as shown in FIG. 12.

It is to be noted that conductors 47 are perpendicular to line 42 whereas this is not the case in the other embodiments of the invention. This is true for the reason that reflected cross-polarized energy is again freilected back away from reflector 41 by conductors 47 whereas, for example, conductors 34 act as a sieve and crosspolarized energy simply passes through it.

Thus, by use of the conductors of FIG. 5, the slots 38 and reflector halves 36 and 37 shown in FIG. 7, 0r the use of dielectric sheet 43 with conductors 47, crosspolarization can be prevented since current flow in a `direction perpendicular to the principal plane of polarization may be substantially eliminated by the use of parallel spaced conductors. As stated previously, this prevents unwanted phase shift error of a radio frequency signal modulation envelope which accompanies the use of split reliectors such as those indicated in the drawings during nutation.

Although only a few specific embodiments of the invention have been `shown and described, it is to be understood that this invention is by no means limited thereto. Many changes and modifications will, of course, suggest themselves to those skilled in the art. Thus, the invention is not to be limited by the above disclosure, the true scope of the invention being delned only in the appended claims.

What is claimed is:

1. An aircraft radar antenna system comprising: a generally dish-shaped reector split into halves at its center by a plane through its symmetrical axis with each half being rotated in opposite directions away from the other so that the intersecting obtuse angle formed by chords in a common plane subtending the arcs of each of said reflector halves in said common plane is increased so as to produce an antenna pattern having a main lobe broader in its -Hplane than a perfect parabolic reflector of the same size; and means associated with said reflector to discriminate against electromagnetic energy received by said antenna system having a predetermined plane of polarization perpendicular to a plane through said split and through the symmetrical axis of said reflector.

2. An aircraft radar antenna system comprising: a dish-shaped reflector split at the center to provide two halves, each of said halves being rotated in opposite directions away from the other so that the intersecting obtuse angle formed by chords in a common plane subtending the arcs of each of said reflector halves in said common plane is increased, said reflector having conductor means extending substantially only in one direction parallel to the direction of the split of said reflector.

3. An aircraft radar antenna system :comprising: a dish-shaped reflector split in the center for providing two halves, each half having its respective split edge disposed away from the split edge of the other halt` at an angle located in a predetermined plane perpendicular to the direction of said split so that the intersecting obtuse angle formed by chords in a common plane subtending the arcs of each of said reflector halves in said common plane is increased so as to produce an antenna pattern having a main lobe broader in said perpendicular plane than a perfect parabolic reflector of a similar size, said reflector having conductor means extending substantially only in one direction parallel Ito the direction of the split of said reflector.

4. An aircraft radar antenna system comprising: first and second reflector halves, each of said reflector halves having a flat edge and being in the shape of one-half of a parabola of revolution about its axis of symmetry, each said reflector half having its respective flat edge disposed at an angle away from the flat edge of the other said reflector half in a predetermined plane passing through said axes of symmetry so that the intersecting obtuse angle formed by chords in a common plane subtending the arcs of each of said reflector halves in said common plane is greater than if said halves were in a perfect parabolic position, said reflector halves comprising a dielectric material and a plurality of spaced linear conductors having shapes similar to the intersection with a parabola of revolution of planes parallel to said plane of symmetry.

5. An aircraft radar antenna system comprising: first and second reflector halves, each of said reflector halves having a flat edge and being in the shape of one-half of a parabola of revolution about its axis of symmetry, each said reflector half having its respective llat edge disposed at an angle away from the flat edge of the other said reflector half in a predetermined plane passing through said axes of symmetry so that the intersecting obtuse langle formed by chords in a common plane subtending the arcs of each of said reflector halves in said common plane is greater than if said halves were in a perfect parabolic position, said reflectors being made entirely of a oonductive material with a plurality of first :and second alternate rows or' elongated slots therethrough, the axis of each of said rows of slots extending in parallel planes parallel to said plane of symmetry, said first rows being spaced lengthwise from said second rows a distance approximately equal to one-half of the length of the slot plus oneh-alf the distance between slots.

6. An aircraft radar antenna system comprising: first and second reflector halves, Ieach of said reflector halves having a flat edge and being in the shape of one-half of a parabola of revolution about its yaxis of symmetry, each said reflector half having its respective llat edge disposed at yan angle away from the llat edge 4of the other said reflector half in a predetermined plane passing through said axes of symmetry so that the intersecting obtuse angle formed by chords in a common plane subtending the arcs of each of said reflector halves in said common plane is greater than if said halves were in a perfect parabolic position, said rellectors being made entirely of a conductive material with a dielectric supporting means affixed to the peripheral edge of said reflector so as to cover it, and a plurality of linear spaced conductors lixed 6 to said dielectric support means in parallel planes perpendicular to said plane of reference.

7. A radar antenna system comprising: a reflector including two individual substantially identical halves, said rellector having a generally parabolic shape when said halves are maintained in a first position contiguous to each other, each of said reflector halves having coincident symmetrical axes 'and a predetermined plane of separation when maintained in said first position, said reflector halves being rot-atable from said first pois-tion to a second position such that each of Isaid reflector halves is rotated at an angle in a direction opposite to and away from the other so that the intersecting obtuse angle `formed by chords in -a common plane subtending the -arcs of each of said reflector halves in said common plane is increased, said rellector halves being rotatable about an axis perpendicular to said coincident symmetrical axes, said rotatable axis being in said plane of separation; and means associated with said reflector to discriminate against electromagnetic wave energy having a plane of polarization perpendicular to said plane of separation.

8. A radar lantenna system comprising: a rellector including two individual substantially identical halves, said reflector h-aving a generally parabolic shape when said halves `are maintained in a lirst position contiguous to each other, each of said reflector halves having coincident symmetrical axes and a predetermined plane off separation when maintained in said first position, said reflector halves being rotatable from said first position to a second position such that each of said reflector halves is rotated at an an-gle in ra `direction opposite to and away from the other so that the chords in a common plane subtending the arcs of each of said rellector halves in said common plane is increased, said reflector halves being rotatable about an axis perpendicular to said coincident symmetrical axes, said rotatable axis being in said plane of separation, said rellector halves comprises a dielectric material and a plurality of spaced linear conductors having shapes similar to the intersection of planes parallel to said plane of separation and a parabola of revolution.

9. A rad-ar antenna system comprising: a reflector including two individual substantially identical halves, said reflector having Ia generally parabolic shape when said halves are maintained in a lirst position contiguous to each other, each of said reflector halves having coincident symmetrical axes and a predetermined plane of separation when maintained in said first position, said reflector halves being rotatable from said first position toa second position such that each of said reflector halves is rotated at an angle in a direction opposite to and -avvay vfrom the other so that the intersecting obtuse angle formed by chords in a common plane subtending the arcs of each of said reflector halves in said common plane is` increased, said reflector halves being rotatable about an axis perpendicular to said coincident symmetrical axes, said rotatable 4axis being in said pla-ne of separation, said reflector halves being made entirely of a conductive material with a plurality of first and second alternate rows of elongated slots therethrough, the axes of said rows of slots extending in planes parallel Ito said plane fof separation when said reflector halves are maintained in said first position, said first rows being spaced lengthwise from said second rows a distance approximately equal to one-half the length of a slot plus one-half the longitudinal distance between slots.

l0. A radar antenna system comprising: a reflector including two indivi-dual substantially identical halves, said rellect-or having a generally parabolic shape when said halves are maintained in a first position contiguous to each other, each of said reflector halves having coincident symmetrical axes and a predetermined plane of sepaf 7 8 the other so that the intersecting obtuse angle formed in parallel planes perpendicular to said plane of separaby chords in a common plane subtending the arcs of each tion. of said reector halves in said common plane is increased, References Cited in the fue of this patent said reflector halves -being rotatable about `an taxis perpendicular to said coincident symmetrical axes, said rotatable 5 UNITED STATES PATENTS axis being in said plane of separation, s'aid reflector halves 2,408,373 Chu Oct. 1, 1946 being made entirely of =a conductive material, -a dielectric 2,522,562 Blitz Sept. 19, 1950 supporting means fixed fto said reflector halves in 'a po'si- 2,597,339 Krutter May 20, 1952 tion to cover said ree'ctor halves, anda plurality of linear 2,790,169 Sichak Apr. 23, 1957 spaced conductors fixed to said ydielectric support means 10 2,870,440 Butler Jan. 20, 1959

Claims (1)

1. AN AIRCRAFT RADAR ANTENNA SYSTEM COMPRISING: A GENERALLY DISH-SHAPED REFLECTOR SPLIT INTO HALVES AT ITS CENTER BY A PLANE THROUGH ITS SYMMETRICAL AXIS WITH EACH HALF BEING ROTATED IN OPPOSITE DIRECTIONS AWAY FROM THE OTHER SO THAT THE INTERSECTING OBTUSE ANGLE FORMED BY CHORDS IN A COMMON PLANE SUBTENDING THE ARCS OF EACH OF SAID REFLECTOR HALVES IN SAID COMMON PLANE IS INCREASED SO AS TO PRODUCE AN ANTENNA PATTERN HAVING A MAIN LOBE BROADER IN ITS H-PLANE THAN A PERFECT PARABOLIC REFLECTOR OF THE SAME SIZE; AND MEANS ASSOCIATED WITH SAID REFLECTOR TO DISCRIMINATE AGAINST ELECTROMAGNETIC ENERGY RECEIVED BY SAID ANTENNA SYSTEM HAVING A PREDETERMINED PLANE OF POLARIZATION PERPENDICULAR TO A PLANE THROUGH SAID SPLIT AND THROUGH THE SYMMETRICAL AXIS OF SAID REFLECTOR.

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