US2870440A - Conical scanning antenna systems as used in radar - Google Patents

Description

Jan. 20, 1959 J. 1.. BUTLE'R CONICAL SCANNING ANTENNA SYSTEMS AS USED IN RADAR Filed ua 13, 1954 3 Sheets-Sheet 1 u l0 l2 I I MSTOR Fig. 2 $22 a f Q 3 TRANS MITTER Fig.5

Jesse L. Butler INVENTOR.

Atforney Jan. 20, 1959 J. BUTLER 2,370,440

CON ICAL SCANNING ANTENNA SYSTEMS AS USED IN RADAR Filed May 15, 1954 I 3 Sheets-Sheet 2 Jesse L.Butler uvmvroa.

Jan. 20, 1959 J. L. BUTLER 2,370,440

CONICAL SCANNING ANTENNA SYSTEMS AS USED IN RADAR I Filed May 15, 1954 s Sheets-Sheet 3 RIGHT DOWN Fig. 7

INVENTOR. By Q, 5:1!!!

Attorney Jesse L.Butler' United States Patent CONICAL SCANNING ANTENNA SYSTEMS AS USED IN RADAR Jesse L. liutler, Nashua, H., assignor, by mesne assignments, to Sanders Associates, Incorporated, Nashua, N. H., a corporation of Delaware Application May 13, 1954, Serial No. 429,633 3 Claims. (Cl. 343-755 The present inventionrelates to the art of radiating electromagnetic energy. More particularly, this invention relates to conical scanning antenna systems as used in radar.

In the prior art many systems have been proposed for developing a conical beam of electromagnetic energy by causing beam rotation about the axis of the antenna system. This beam rotation is familiarly termed conical scanning in the art, and is to be distinguished from the azimuth and elevation scanning functions of the system as a whole.

Conical scanning systems as developed in the prior art are characterized by an essentially unbalanced mechanical rotational system (see U. S. Patent No. 2,671,854 of J. Halpern et al., issued March 9, 1954). The speed of conical scanning required by modern radar techniques is unattainable by such systems.

It is therefor an object of the present invention to provide an improved antenna system providing high speed conical scanning.

It is a further object of the present invention to provide a conical scanning antenna system that is electrically and mechanically balanced.

A still further object of the present invention is to pro vide an improved conical scanning antenna system having variable reflectance to electromagnetic energy.

Other and further objects of the invention will be apparent from the following description of typical embodiments thereof, taken in connection with the accompanying drawings.

In accordance with the invention, there is provided an antenna system. The system includes a source of planepolarized electromagnetic energy and a composite lensreflector. The lens-reflector has a first parabolic reflector with a predetermined central axis and a second parabolic reflector disposed in front of the first reflector. The second reflector has a plurality of wave-guide elements, each having one dimension less than a half of a wave-length at the operating frequency. The elements are adjacently so disposed in three substantially 120 sectors, having radiation axes angularly displaced from the first axis, that each sector provides a grating substantially transparent to the energy in certain diametrically opposed positions while being reflective and relatively opaque to the energy in positions at quadrature with the diametrically opposed positions. The passage of the energy is thereby permitted through to the first reflector when in the diametrically opposed positions. Means are provided for directing the polarized energy toward the lens-refiector. Means are further provided for producing relative rotation between the second reflector and the polarized energy, thereby to effect conical scanning of the resultant beam at a frequency three times that of the rotation of the second reflector.

In the accompanying drawings:

Fig. 1 is a schematic diagram illustrating conical scanning as provided by the present invention;

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Fig. 2 is a side view, partly in section, of a preferred embodiment of the present invention;

Fig. 3 is an enlarged, detailed end view of a portion of the embodiment of Fig. 2;

Fig. 4 is a side view, partly in section, of another embodiment of the present invention;

Fig. 5 is an enlarged, detailed end view of a portion of the embodiment in Fig. 4;

Fig. 6 is a series of diagrams illustrating the operation of the invention as shown in Figs. 2 and 3; and

Fig. 7 is a series of diagrams illustrating the operation of this invention as shown in Figs. 4 and 5.

Referring now in more detail to the drawings and with particular reference to Fig. 1, an antenna system indicated at 1 is depicted as radiating a beam 2 of electromagnetic energy as shown. The main axis 3 of the beam is caused to rotate about the antenna system axis or boresight 4, as shown. The rotating or circular motion of the beam axis 3 is illustrated by the path 5. The extreme lower position of the beam is illustrated by the phantom lines 6. This rotation of the beam thus provides what is known as conical scanning.

Referring now to Figs. 2 and 3, the antenna of the present invention comprises aprimary radiator 7 (for example, a rectangular wave guide) developing a beam of electromagnetic energy for the system. A transmitter 8, coupled to the primary radiator 7, provides a source of electromagnetic energy. The energy radiated by radiator 7 is directed through the lens-reflector 9 and is reflected in the form of a beam such as is illustrated in Fig. 1. A shaft 1%) is mechanically coupled to the lens-reflector 9 and is rotated by a motor 11, as shown. The lens-reflector 9 is attached to and supported by support rods 12. The lensreflector comprises a paraboloidal reflector 13 having a central axis 14. The lens consists of a supporting ring 15 mechanically connected to the reflector 13 as shown, supporting web members 16 and grating members 17.

Referring particularly to Fig. 3, the lens construction is here shown in detail. The members 16 of the supporting web structure are disposed at angles of degrees with respect to each other, as shown. The grating members 17 are disposed less than a half-wave length apart at the operating frequency, for example 10 kilo-megacycles. Wave guide elements 18 are formed by the supporting ring 15, members 16 and members 17. It is to be noted that the wave guide elements 18 are disposed parallel to the central radii of their respective sectors.

In the lens-reflector as shown in Figs. 4 and 5, the construction is similar to the embodiment of Fig. 2 with the exception that the wave guide elements 18 are disposed substantially perpendicular to the central radii of their respective sectors, as shown.

Since the effective transparency of the wave guide element is determined by the surfaces exposed to the primary radiator, it is clear that the modified paraboloidal surface may be eifected with a wire-like construction having substantially no depth.

The operation of the preferred embodiment, as shown in Figs. 2 and 3, can be better understood with particular reference to Fig. 6. Electromagnetic energy, plane polarized such that its electric vector indicated at 19 is vertical as shown, is directed toward the lens-reflector. A wave guide element is substantially opaque to the energy that would pass through it if the element is parallel to the electric vector 19, and is substantially transparent when the element is perpendicular to the electric vector 19.

When the wave guide element is opaque to the energy it reflects the energy. This effect can be more readily understood with reference to Fig. 2. The paraboloidal reflector 13 tends to form a beam having a main axis 14 as shown. If the reflector 13 were displaced so as to aemaao assume a phantom position as indicated at 20, its central axis would be displaced upwardly as shown at 21. Conversely, if the reflector 13 were displaced so as to assume a phantom position as indicated at 22, its central axis 23 would be displaced downwardly. In the present invention electrical asymmetry is effected by causing parabolic sectors of the surface, as defined by the ends of the grating members 17 exposed to the radiator 7, to successively become alternately transparent and then reflective (opaque) to the energy. Since the members 17 are parabolic the effective reflecting surface has the form of a modified paraboloid with a central axis displaced from that of the reflector 13.

The generation of the modified paraboloid may be visualized by considering a parabola having the form:

If the parabola is so displaced that its main axis has a negative slope with respect to the x axis, the modified paraboloid is obtained by rotating the positive parabolic are about the x axis. In the preferred embodiment this is a matter of 1 or 2 degrees with respect to the boresight axis.

The degree to which such a Wave guide element is transparent to the energy tending to pass through it may be expressed:

W=ksin 9 In the above expression, W equals the electromagnetic energy passing through the wave guide element having an electric vector parallel to the vector indicated at 19. The factor k is a constant and the angle 6 equals the angle between the wave guide element and the electric vector 19.

Of particular significance in the present invention is the characteristic of the lens-reflector as described, whereby a single rotation through 360 degrees eflects three rotations of the resultant beam of energy as will be presently shown. In the Diagrams a through 2, the lensrefleetor of Fig. 3 is schematically illustrated by the central wave guide elements A, B and C disposed such that each of the angles AOB, ECG and COA equal 120 degrees, as shown. The effect of increasing the reflectance of each element is to cause the resultant beam to be refracted in the direction of the elements exhibiting minimum reflectance (minimum opacity).

Thus, in the Diagram [1 the element A is positioned at zero degrees with respect to the electric vector and reflects 1; units of energy. In accordance with the expression for W above, the elements B and C each reflect .25k units of energy; the total energy reflected is 1.5k units. Since the elements B and C are symmetrically disposed about the vertical axis, there exists no tendency for the axis of the resultant beam to be directed to the right or left. Since the element A exhibits maximum reflectance, the principal axis of the resultant beam will be directed down as at 6 in Fig. 1.

In the Diagram b the element A has been rotated 30 degrees. The element C is then precisely perpendicular to the electric vector and reflects Zero units of energy. The elements A and B each reflect .75k units of energy and are symmetrically disposed about the horizontal axis. In this case, the main axis of the resultant beam will be directed to the left.

In the Diagram the element A has been rotated 60 degrees with respect to the electric vector 19. The element B is precisely parallel to the electric vector 19 and accordingly reflects k units of energy. The elements B and C each reflect .25k units of energy; consequently, the main axis of the resultant beam is directed up.

In the Diagram d the element A is shown rotated 90 degrees with respect to the electric vector 19 and accordingly reflects zero units of energy. The elements B and C each reflect .75k units of energy; thus, the main axis of the resultant beam is directed to the right. In the Diagram e the element A is shown rotated 120 degrees with respect to the electric vector 19. The element C is now positioned such that the operation of the system as described with respect to the element A above is repeated.

In the Diagram 1 the locus of the main axis of the resultant beam due to the rotation of the lens through an angle of 120 degrees is illustrated. The points W, X, Y and Z relate to the positions as illustrated by the Diagrams a, b, c and d, respectively. By this analysis, it is clear that the main axis of the beam rotates through 360 degrees three times while the lens mechanically r0 tates through 360 degrees once.

The operation of the system as shown in Figs. 4 and 5 can be better understood with particular reference to Fig. 7. In the Diagrams a through 2 the lens-reflector of Fig. 6 is schematically illustrated by the wave guide elements A, B and C abstracted from each sector and disposed substantially in the form of an equilateral triangle, as shown. I

In the Diagram a the element A is positioned at zero degrees with respect to the electric vector 19 and reflects k units of energy. The elements B and C each reflect .25k units of energy and are symmetrically disposed about the horizontal axis. Since the centers of the elements B and C are disposed to the left of the vertical axis, the main axis of the resultant beam is directed to the left.

In the Diagram [2 the element has been rotated 30 de grees and accordingly reflects .75k units of energy. The element C is precisely perpendicular to the electric vector 19 and therefore reflects zero units of energy. The element B also reflects .75k units of energy. Since the elements A and B are symmetrically disposed about the vertical axis, the main axis of the resultant beam is directed up.

In the Diagram 0 the element A has been rotated 60 degrees with respect to the electric vector and accordingly reflects .25k units of energy. By symmetry the element C also reflects .25k units of energy. The element B is precisely parallel to the electric vector 19 and therefore reflects k units of energy. The main axis of the resultant beam is directed to the right.

In the Diagram of the element A has been rotated degrees with respect to the electric vector and reflects zero units of energy. The elements B and C each reflect .75k units of energy; accordingly, the main axis of the resultant beam is directly down.

In the Diagram e the element A has been rotated degrees with respect to the electric vector 19. The element C is positioned such that the operation of the system as described with respect to the element A above is repeated.

In the Diagram f the locus of the main axis of the resultant beam due to the rotation of the lens through an angle of 120 degrees is illustrated. The points W, X, Y and Z relate to the positions as illustrated by the Diagrams a, b, c and a, respectively. By this analysis it is clear that the main axis of the beam rotates through 360 degrees three times, while the lens mechanically rotates through 360 degrees once.

From the above descriptions it is to be noted that the systems as described are inherently electrically and mechanically symmetrical. Since the system is mechanically balanced, the physical speed of rotation may be 30 increased that conical scanning rates may be increased from a typical value of 50 cycles per second to as high as 1,000 cycles or more per second.

Although the invention as described is related to the propagation of radio frequency electromagnetic energy, it is clear that the principles of the invention are readily applicable to other electromagnetic radiations. For example, the construction of a useful analogous system employing a polarized light source and polarized lenses is obvious.

The present invention greatly enhances the effectiveness of modern radar techniques as used in the detection and control of supersonic aircraft.

While there has been hereinbefore described what is at present considered preferred embodiments of theinvention, it will be apparent that many and various changes and modifications may be made with respect to the embodiments illustrated, without departing from the spirit of the invention. It will be understood, therefore, that all such changes and modifications as fall fairly within the scope of the present invention, as defined in the appended claims, are to be considered as a part of the present invention.

What is claimed is:

1. An antenna system comprising, in combination, a source of plane-polarized electromagnetic energy; a composite lens-reflector comprising a first parabolic reflector having a predetermined central axis, and a second parabolic reflector, disposed in front of said first reflector and having a plurality of wave guide elements each having one dimension less than half of a wave length at the operating frequency and adjacently so disposed in three substantially 120 degree sectors having radiation axes angularly displaced from the first said axis, that each sector provides a grating substantially transparent to said energy in certain diametrically opposed positions while being reflective and relatively opaque to said energy in positions at quadrature with said diametrically opposed positions, thereby permitting the passage of said energy therethrough to said first reflector when in said diametrically opposed positions; means directing said polarized energy towards said lens-reflector; and means producing relative rotation between said second reflector and said polarized energy eflecting, thereby, conical scanning of the resultant beam at a frequency three times that of the rotation of said second reflector.

2. An antenna system comprising, in combination, a source of plane-polarized electromagnetic energy; a composite lens-reflector comprising a first parabolic reflector having a predetermined central axis, and a second parabolic reflector, disposed in front of said first reflector and having a plurality of wave guide elements each having one dimension less than one half of one wave length at the operating frequency and adjacently, radially disposed in radiation axes angularly displaced from the first said axis, each sector providing a grating substantially transparent to said energy when said dimension is parallel to the direction of polarization of said energy, while being reflective and relatively opaque to said energy when said dimension is orthogonal to said direction of polarization; means directing said energy towards said lensreflector; and means producing relative rotation between said second reflector and said polarized energy effecting, thereby, conical scanning of the resultant beam at a frequency three times that of the rotation of said second reflector.

3. An antenna system comprising, in combination, a source of plane-polarized electromagnetic energy; a composite lens-reflector comprising a first parabolic reflector having a predetermined central axis, and a second parabolic reflector, disposed in front of said first reflector and having a plurality of wave guide elements each having one dimension less than half of a wave length at the operating frequency and adjacently disposed in parallel in three substantially degree sectors with said elements perpendicular to the central radius of their respective sector, each sector providing a grating substantially transparent to said energy when said dimension is parallel to the direction of polarization of said energy, while being reflective and relatively opaque to said energy when said dimension is orthogonal to said direction of polarization; means directing said energy towards said lensreflector; and means producing relative rotation between said second reflector and said polarized energy effecting, thereby, conical scanning of the resultant beam at a frequency three times that of the rotation of said second reflector.

References Cited in the file of this patent UNITED STATES PATENTS 2,472,782 Albersheim June 14, 1949 2,510,020 Iams May 30, 1950 2,543,130 Robertson Feb. 27, 1951 2,554,936 Burtner May 29, 1951 2,677,056 Cochrane Apr. 27, 1954 2,790,169 Sichak Apr. 23, 1957

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