US2969542A - Spiral antenna system with trough reflector - Google Patents

Description

1961 H. P. COLEMAN ETAL 2,969,542

SPIRAL ANTENNA SYSTEM WITH TROUGH REFLECTOR Filed March 30, 1959 RADIO FREQUENCY DEVICE INVENTOR5 'H. PARIS COLEMAN JULIUS A- KAISER.,JR.

P 2,969,542 i Patented J SPIRAL ANTENNA SYSTEM WITH TROUGH REFLECTOR Henri Paris Coleman, Alexandria, Va., and Julius A.

v Kaiser, Jr., Kensington, Md., assignors to the United States of America as represented by the Secretary of the Navy Filed Mar. 30, 1959, Ser. No. 803,037

6 Claims. (Cl. 343-761) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Goverment of the United States 1 of America for governmental purposes without the payment of any royalties thereon or therefor.

' This invention relates to antenna systems in general and in particular to spiral antennas which are utilized for scanning purposes.

In many applications of radio frequency energy operative devices it is desirable to have an antenna system of simple structure which is capable of providing scan- ,ning of a beam of radiated energy over a sector of space. In the prior art such scanning is accomplished mechanically or electrically. When it is accomplished electrically 'it may be provided by switching of the excitation of various antenna elements, however such an arrangement is subject to limitations which in some instances are undesirable. of operation, certain periods necessarily occur in portions of the switching cycle during which the transmitter,

'althoughoperative, may not be connected to the antenna thus providing nonuniform loading of the transmitter and periodic interruption of operation of the complete locator system. In certain other types of operation, scanning is obtained by the continuous manipulation of ,complex variable coupling devices between the radio frequency energy generator and the antenna elements rather For example, in the lobe switching type than the abrupt change from one antenna condition to another. Since the foregoing types of operation are ,frequently undesirable for one reason or another, it is an object of the present invention to provide an electromechanically scanning antenna system of non-complex Another object of the present invention is to provide e 'a spiral antenna system which will scan a selected sector of space.

Another object of the present invention is to provide a spiral antenna system which contains a trough type reflector wherein the spiral antenna is disposed normal to .the bisecting plane of the trough.

7 Another object of the present invention is to provide a spiral trough antenna wherein a simplified mounting .of the spiral elements relative to the trough is possible.

Another object of the present invention is to provide :a spiral trough antenna wherein there is a minimum of interference with the radiated field by the mechanical structure employed to support the spiral elements relative to the trough portion of the antenna.

Other and further objects and features of the present .invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with .the accompanying drawings wherein:

Fig. 1 indicates in a general way a basic spiral antenna connected to a'radio frequency operative device.

Fig. 2 shows such a spiral antenna mounted in proximity to a trough reflector wherein the spiral antenna is disposed normal to the bisecting plane of the trough.

Fig. 3 shows a plurality of spiral antenna elements mounted as in Fig. 2. In accordance'with the basic teachings of the present invention an antenna system is provided wherein a spiral antenna element is disposed in proximity to a reflector in such configuration as to combine the opposite senses of circularly polarized signals produced on opposite sides of the spiral antenna element to obtain linearly polarized radiation. As'a further extension of the teachings of the invention, a plurality of spiral antenna elements are employed in an array wherein the major directivity of such radiation may be shifted merely by rotation of individual spiral antenna" elements. This particular invention is not the first combination of spiral antenna elements and trough reflector. A prior art combination places the spiral antenna elements in the bisecting plane of the right angle trough achieving highly desirable results. Such a'combination however does provide certain inherent characteristics which in certain instances may not be desirable. In particular when spiral antenna elements are disposed in the plane of the bisector of the trough it is normally necessary to provide support and feed for those elements such as rod numbers containing coaxial feed conductors for the spiral, such rod members being disposed perpendicular to the bisecting plane of the trough- In some instances the location of such rod members, which are necessarily conductive, at

least in part, may have an undesired effect upon the radiated field. Also since scanning with spiral antennas is normally obtained by the rotation of the spiral antenna elements in the plane in which they are disposed it is normally somewhat inconvenient to provide rotational apparatus which can engage the rod support members at their terminations through one side of the trough reflector. Where the teachings of the present invention are employed, with the spiral antenna elements disposed in a plane perpendicular to the trough bisecting plane, the support rods can extend toward the apex of the trough, the result being that their effect is symmetrical upon both sides of the trough and also that more simplified forms of rotational apparatus for the individual elements can be provided.

, A spiral antenna element as typified in Fig. 1 is composed of two electrical conductors disposed in a plane and which originate at a central portion spiralling outward to a selected radius. The two conductors terminate at points which are diametrically opposed to each other. Typically the assembly could be formed with thin conductors having a backing member of insulating material by printed circuit techniques. When such an antenna ele ment is energized by out of phase signals applied to the two conductors at the central points, a broad circularly polarized beam is produced on each side of the flat spiral substantially perpendicular to the plane of Fig. 1. The beam radiated from one side of the spiral exhibits circular polarization of the opposite sense from that radiated from the other side of the spiral. Aside from this polarization characteristic, the two radiated beams are subsfantially identical. In many applications it is desirable that such an antenna element radiate to one side of the spiral only. This is accomplished by an appropriate backing of the spiral on one side by a ground plane or a cavity; however in the present instance the beams from both sides of the spiral are both utilized to a particular advantage in a novel combination of spiral antenna elements with a trough reflector.

Although at the present time there is no universally accepted theory for the operation of the spiral antenna element itself, Fig. 1 indicates certain dimensions which in accordance with the following discussion will be of assistance in understanding the design possibilities or this antenna. The two conductor spiral antenna behaves as a 'two wire transmission line which gradually by virtue of its spiral geometry transforms itself into a radiatingstructure or antenna. It is well known that a two wire transmission line with closely spaced wires yields negligible radiation when excited anti-phase at its terminals. This is due to the fact that the currents of the two wires of the line in any normal cross-section are always 180 electrical degrees out of phase so that radiation from one line is essentially cancelled by radiation from the other. Such is true if the lines'are closely spaced in terms of the excitation wavelength and are equal in length. The situation is different when a two conductor transmission .line is disposed in a spiral configuration as Fig. 1 and energized anti-phase by radio frequency signals; the adjacent conductors of the spiral are not always 180 'degrees out of phase. With point 15 of Fig. 1 selected as a point on one of the conductors of the line which is at a selected distance along the conductor from the initial point 16 of that conductor, point 17 of the other conductor will be located at a similar distance from its starting point 18. Point 17 is diametrically opposite to point 15 with respect to the center 19 and both points 15 and 17 lie on the circumference of a circle centered at 19. This implies that the point 15 and point 20 on the other conductor directly alongside the point 15 lie at such distances from points 16 and 18 respectively that the difference of this distances is precisely the arc length 17 to 20 along the spiral. If the spacing between points 15 and 20 along a radius from 19 to point 20 is much less than the radial spacing of point 19 and point 15, the arc length 17 to 20 is approximately equal to 'n'R. Assuming that each conductor supports a progressive wave of current and that these current waves are antiphase at points 16 and 18, where a suitable balanced line feeds the spiral, it is clear that the difference in phase of the two current elements at any region 15 through 20 on the two conductor line, measured in radians, is

(1r Z'Ir where )l is the current wavelength on the spiral and R is the radial distance from the center 19 to the point 15. Thus neighboring current elements are anti-phase at the points 16 and 18 and gradually come into phase as one proceeds outwardly along the sprial two wire line. When R is M2, these currents are precisely in-phase and radiation is a maximum. When R is M21, the phase change is M 2 and the circumference is )1. Where a spiral is taken having a circumference greater than R there will be inner portions where the circumference is a full wavelength for a path length difference of M2 for a band of frequencies rather than for a single frequency. This quality makes the spiral antenna an inherently broadband device with regard to frequency, the basic requirement being only that the maximum radius be large enough to allow a half wavelength of phase shift between current elements on adjacent conductors at the minimum frequency of the band.

With careful design, construction, and feed of such a single spiral antenna element, circular symmetry about the spiral axis can be obtained which allows rotation of the spiral about its axis to produce a change in phase of the radiated field everywhere in space without variation in the far field amplitude. With such rotation, one degree of mechanical rotation of spiral produces a corresponding change of one electrical degree in the phase at any point in the far field. Electrical connection to the conductors may be by balanced leads 21 which go to a receiver or transmitter radio frequency device 21A as desired. These leads typically may be the inner and outer conductors of a coaxial cable or the two conductors of a balanced line, the latter being preferable, however it is readily understood that the feed could he basically a coaxial cable for simplicity of rotation, with an unbalanced to balanced converter placed between the coaxial line and thespiral conductors. The desirability of such variations and in mechanism for their accomplishment are well known to those skilled in the an.

When a spiral antenna element such as that shown in Fig. 1 is placed in proximity to a special form pf reflector, such as a right angle trough 25 shown in Fig. 2, in such position that the plane of the spiral lies in a plane perpendicular to the plane bisecting the angle between the sides of the trough, the energy radiated at one side of the spiral is reflected by the right angle trough and passes outward to space combining with the field from the opposite side of the spiral antenna element. The result is the combination of two circularly polarized fields of opposite sense which produce a combined field which is linearly polarized. Since the phasing of the reflected field is dependent at least in part upon the spacing of the trough and the spiral element it is possible to control the direction of the plane of the linear polarization by adjusting the spacing between the trough and the spiral antenna element. 7

Such a device as Fig. 2 then retains the basic spiral antenna property of control of the phase of radiation at any point in the far field by rotation of the spiral so that it is possible to vary the phase of the energy at any point in the far field merely by rotating the spiral antenna element about an axis perpendicular to the plane thereof.

The trough reflector is of right angle configuration having an area typically approximately ten times the total projected area of the spiral antenna element. The trough itself is indicated in general by numeral 24 and the two sides or planes which produce the trough are indicated by the numerals 25 and 26. The included angle between the planes of the sides 25 and 26 is degrees. The spiral antenna element 27 is supported and fed by a rod type member 28 which typically is placed in the plane bisecting the angle between the planes 25 and 26, the inward end of rod 28 projecting through a suitable aperture at the apex of the trough to provide connection to the radio frequency device 21A of Fig. l as well as provide an extension which is readily available to facilitate rotation of the spiral antenna about the axis provided by rod 28 as well as to permit e'asy adjustment of the spacing between the spiral antenna element and the apex of the'trough.

Positioning of the element 27. is provided by drive unit 29 which may be operative mechanically, electrically, or manually or by a combination thereof. 7

When a plurality of similar spiral antenna elements 30 and 31 are placed in a single trough reflector 34 as shown in Fig. 3 and provided with suitable means 33 for rotation of the spiral antenna elements and anti-phase elec trical excitation of the two conductors of each spiral, each spiral antenna element contributes to the energy in the far field however when the energy combines in the far field the net result is a composite field which will be at a maximum in a direction perpendicular to the apex of the trough only when the fields produced by the two spiral antenna elements are in phase in the far field. Where such fields are not in phase in the far field the maximum for the composite field will be achieved in some direction other than that perpendicular to the apex of the trough, the angular displacement being dependent upon the exact phase relationship between the 'fields emitted by the two spiral antenna elements. Such a phase change can be brought about deliberately by physical rotation of one of the spiral antenna elements in the plane of the spiral by suitable means 33. Typically only one antenna need be revolved or it may be desirable to revolve both in opposite directions. Where more than two antenna elements are employed typically the center element would not rotate for scanning purposes where each additional element to one side of the center element would rotate through an angle in one direction and the elements on the opposite side thereof would rotate through the same angles but in the opposite direction. Typically the first removed elements would rotate through plus and minus angles of 0, the second elements through angles 20 and the third elements through angles 30, etc. Additional elements provide sharper beam, however the scan angle is not effected substantially by the addition of other elements.

From the foregoing it is obvious that considerable variation of the specific structure is possible without exceeding the scope of the invention as defined in the appended claims, for example more than two elements 30 and 31 could be incorporated in an array with the elements being rotated at selected angular relationships and directions as described above to produce particularly desired forms of scanning and beam configuration all of which may be readily predicted by considering the effect of the rotation upon the relative phasing of the energy from the various elements in the far field.

What is claimed is:

1. An antenna system comprising, a spiral antenna element, a right angle trough reflector, and means mounting said spiral antenna element in proximity to the trough in a plane normal to the plane bisecting the right angle of the trough reflector.

2. An antenna system comprising, a two conductor planar spiral antenna element, a right angle trough reflector, and means symmetrically mounting said element in proximity to the reflector in a plane normal to the plane bisecting the right angle of the trough reflector.

3. An antenna system comprising, a base member, of insulating material, first and second conductors supported;

thereby in planar configuration relatively insulated from each other, each said conductor being in the form of a spiral from a portion having a small radius to a portion having a larger radius, a right angle trough reflector, and means mounting said base member and said first and second conductors in proximity to the reflector with the plane thereof substantially normal to the plane bisecting the right angle of the trough reflector.

4. An antenna system comprising, a base member of insulating material, first and second conductors supported thereby in a planar configuration relatively insulated from each other, said conductors being in the form of a spiral from a portion having a small radius to a portion having a large radius, a right angle trough reflector, means mounting said base member and said first and second conductors in proximity to the reflector with the plane thereof substantially normal to the plane bisecting the right angle of the trough reflector, and means for rotating the base member about an axis substantially parallel to the plane bisecting the right angle of the reflector.

5. An antenna system comprising, a plurality of spiral antenna elements, a right angle trough reflector, and means for rotatably supporting said elements in front of the reflector, the elements being disposed substantially normal to the plane bisecting the trough and rotatable about an axis substantially parallel to the bisecting plane.

6. An antenna system comprising, a plurality of base members composed of insulating material, each of said base members having first and second conductors supported thereby in a planar configuration relatively insulated from each other, said conductors supported by each member being in the form of a spiral from a portion having a small radius to a portion having a larger radius, a right angle trough reflector, and means for rotatably supporting said base members in front of the reflector, the conductors thereof being disposed substantially symmetrically and perpendicular to the plane bisecting the trough and rotatable about axes in the bisecting plane.

References Cited in the file of this patent UNITED STATES PATENTS 2,460,326 Woodrutf Feb. 1, 1949 2,663,869 Adcock et a1 Dec. 22, 1953 2,773,254 Engelrnann Dec. 4, 1956 2,856,605 Jacobsen Oct. 14, 1958 2,863,145 Turner "1. Dec. 2, 1958 2,863,148 Gammon et al. Dec. 2, 1958 2,897,496 Woodward July 28, 1959 2,935,746 Marston et al May 3, 1960 FOREIGN PATENTS 642,385 Germany July 7, 1937 OTHER REFERENCES Aviation Week, July 14, 1958, pages 75, 77, 79, 81, and 82, Airborne Spiral Antenna Minimize Drage, Klass.

Antenna (McGraw-Hill, 1950), Kraus, page 338.

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