Antenna system having beam control members consisting of array of spiral elements

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July 17, 1962 a. ε. marston 3,045,237

ANTENNA SYSTEM HAVING BEAM CONTROL MEMBERS

CONSISTING OF ARRAY OF SPIRAL ELEMENTS

Filed Dec. 17, 1958 4 Sheets-Sheet 1

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L ARTHUR

INVENTOR

E. MARSTON

ANOTHER VIEW OF THE LENS OF FIG.I

ATTORNEY

BY

A. E. MARSTON 3,045,237

ANTENNA SYSTEM HAVING BEAM CONTROL MEMBERS

CONSISTING OF ARRAY OF SPIRAL ELEMENTS

Filed Dec. 17, 1958 4 Sheets-Sheet 2

July 17, 1962

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BY

ATTORNEY

July 17, 1962 a. e. marston 3,045,237

ANTENNA SYSTEM HAVING BEAM CONTROL MEMBERS

CONSISTING OF ARRAY OF SPIRAL ELEMENTS

Filed Dec. 17, 1958 4 Sheets-Sheet 3

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DRIVE MECHANISM

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ROTATIONAL

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INVENTOR

ARTHUR E. MARSTON

RADIO FREQUENCY OPERATIVE DEVICE

ATTORNEY

July 17, 1962 a. e. marston 3,045,237

ANTENNA SYSTEM HAVING BEAM CONTROL MEMBERS

CONSISTING OF ARRAY OF SPIRAL ELEMENTS

Filed Dec. 17, 1958 4 Sheets-Sheet 4

BALANCED TRANSMISSION LINE SPIRAL ANTENNA ELEMENTS

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GROUND PLANE

RADIO FREQUENCY OPERATIVE DEVICE

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SPIRAL ANTENNA LENS SYSTEM EMPLOYING A DIPOLE PRIMARY RADIATOR

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INVENTOR

ARTHUR E. MARSTON

CONFIGURATION SENSE OF SPIRAL ELEMENTS FOR THE ANTENNA

OF FIG. I

BY

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ATTORNEY

3,045,237

Patented July 17, 1962

United States Patent Office

3,045,237

ANTENNA SYSTEM HAVING BEAM CONTROL MEMBERS CONSISTING OF ARRAY OF SPIRAL ELEMENTS

Arthur E. Marston, 718 Putnam Place, Alexandria, Va. Filed Dec. 17, 1958, Ser. No. 781,171

Claims. (Cl. 343—754) (Granted under Title 35, U.S. Code (1952), sec. 266)

The invention described herein may be manufactured and used by or for the Government of the United States 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 antennas having radiation patterns of a highly directive nature.

In many applications of radio frequency energy operative devices it is desirable to have an antenna system capable of producing a radiation pattern having a very high degree of directivity, that is, it has a relatively high response in a selected direction with very low response in other directions. Such may be characterized as a “pencil beam.” Antennas normally produce such a beam by producing the equivalent of a plurality of parallel beams of energy having the same phase front. Thus the energy of all the beams is additive on axis whereas in other directions it is either partially cancelling or otherwise nonadditive. The mechanism whereby a plurality of parallel beams may be produced is subject to considerable variation however the normal means is that of employing a plurality of elements which are spaced apart in directions perpendicular to the desired maximum direction of response. Thus arrays containing many dipoles all of which are operated in specific phase relationship are well known in the art being employed for example in low frequency radar systems. Such a multiple element array has numerous disadvantages however because it is large, heavy and severely restricted as to the frequency range that can be covered without retuning, furthermore, scanning to produce a variation in the direction of the axis of directivity is attended by considerable difficulty.

Another scheme for controlling the directivity of radio frequency energy is to place a single radiating element on the focus of a parabolic reflector. As is well known, radio frequency energy from the radiating element thus placed is quite effectively directed in parallel paths. The direction of such paths may be readily controlled by movement of the complete unit of element and reflector. This arrangement although effective is subject to some limitation in that the reflector is itself large and heavy. A further possibility with such a reflector however is the movement of merely the single radiating element through a small arc to effectively produce a variation in the direction of emission of radio frequency energy relative to the forward axis of the parabola. With such a scheme as this however, there is a very definite limitation as to the angular variation possible because of the production of “coma” as a variation from the focus occurs producing a distortion of the desired normal pencil beam and introducing substantial side lobe radiation.

A further scheme for controlled radiation is the use of a lens placed in front of a single radiating element to produce a refraction or bending of the radio frequency energy so that the energy traveling outward from the effective point source of the element is collimated to effectively produce a parallel beam of radio frequency energy. The lens scheme has a significant advantage in that scanning by simple motion of the radiating element can produce substantial variation in the angle of the beam without requiring physical motion of the lens. However, the lens itself presents serious problems in that it is normally very heavy and bulky even if simplifications, such as the Fresnel principle, are resorted to. Such a radio frequency lens typically is of a material such as polystyrene which introduces an inherent difficulty in that it will flow of its own weight over a period of time. The con5 elusions to be drawn from the foregoing are that except for very high frequencies where dielectric lenses can be used to some extent, as a practical matter it is not possible to achieve undistorted pencil beam scanning with simple physical motion of a light weight antenna element.

It is therefore an object of the present invention to provide an antenna system of light weight and comparatively small physical size which can produce beamed radio frequency energy in which the angular direction of such beam can be controlled over broad angles and varied at a high rate.

Another object of the present invention is to provide an improved radio frequency lens system.

Another object of the present invention is to provide an improved radio frequency reflector system wherein an essentially planar reflector system may be employed to produce parallel beams of radio frequency energy.

Other objects and many of the attendant advantages of this 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 shows a basic spiral antenna lens system constructed in accordance with the teachings of the present invention;

FIG. 2 shows another view of the lens of FIG. 1;

FIG. 3 shows in general the configuration of an individual spiral antenna element;

FIG. 4 shows a side view of a spiral antenna reflector constructed in accordance with the teachings of the present invention.

FIG. 5 shows a scanning spiral antenna lens constructed in accordance with the teachings of the present invention;

FIG. 6 shows a scanning spiral antenna reflector constructed in accordance with the teachings of the present invention;

FIG. 7 shows a spiral antenna lens system employing a dipole primary radiator; and

FIG. 8 shows the alternate configuration sense of the spiral elements for the antenna of FIG. 7.

In accordance with the basic teachings of the present invention, a radio frequency lens or reflector is provided utilizing the properties of a spiral antenna element typified in FIG. 3 and described subsequently. One of the unique properties of a spiral antenna element is its ability to provide control of the phase of electrical energy in the far field merely by rotation of the element, one degree rotation of the element providing one degree phase change in the far field. Thus if the spiral antenna element is rotated through a typical 20 degrees in one direction, the phase of the radiation field of that spiral antenna element is altered at every point in the far field by precisely that same amount. Thus a plurality of spiral antenna elements are employed and adjusted in angular orientation relative to each other to produce a specific phasing of the field produced thereby regardless of the phasing of excitation of the individual elements. It is possible therefore for a series of spiral antenna elements to produce an outgoing phase front wherein all elements contribute energy which is in-phase or which bears selected relative phasing to produce in effect a beam similar to that produced with light by an optical lens. Unlike the optical lens, however, this particular configuration is characterized by an ability to alter the effective 70 direction of emission of energy by variation of the phase front to produce scanning.

With reference now to FIG. 1 of the drawing, the

3,045,237 apparatus shown therein contains a first spiral antenna element 10 which is coupled through a coaxial cable 11 and a balance to unbalanced converter 12 to a radio frequency operative device 13. The radio frequency energy operative device 13 could be a receiver for reception purposes as well as a transmitter to excite the antenna system for emission of radio frequency energy.

The element 10 which provides inherently a broad pencil beam on both sides is backed by a suitable ground plane 14 which serves to confine the radiation to one direction, typically toward the right in FIG. 1. In this direction relative to element 10 is a group of elements 15-23. Elements 15-23 are connected by means of balanced transmission lines 24-32 to a second group of elements 33-41. The groups of elements are typically separated by approximately one-half wavelength with a ground plane 42 placed between, the transmission lines 24-32 passing through suitable apertures in the ground plane 42.

A specific relationship between the configuration senses of the various elements is important to secure the foregoing result. Thus element 10 has typically a right hand configuration sense when viewed from the direction of the elements 15-23 in which case the elements 15-23 would also have a right hand configuration sense when viewed from the element 10. This insures coupling between the element 10 and the elements 15-23. Furthermore, the elements 33-41 will be of a left hand configuration sense when viewed from a direction looking toward the ground plane 42.

FIG. 2 indicates in a general way a typical rectangular layout of spiral antenna elements and ground plane 42 as viewed typically in the direction of ground plane 42 from the side on which the elements 33-41 are disposed. Thus the array is seen to have depth as well as height providing a high degree of directivity in both planes. In those situations where a greater degree of directivity is desired in one plane than in another it is of course obvious that more antenna elements could be employed in one plane than in the other.

The operation of the apparatus of FIGS. 1 and 2 is as follows, for simplicity of explanation the device being described in terms of a transmitter rather than a receiver, however, it is to be understood that the device is usable for reception as well as transmission. Radio frequency energy produced by the radio frequency operative device 13 is delivered by way of the feed line 11 through the balanced to unbalanced converter 12 where it is fed antiphase to the two conductors of the element 10. Element 10 provides a broad pencil beam in the direction toward the right in FIG. 1 so that radio frequency energy emitted thereby reaches the elements 15-23 but does not proceed beyond the ground plane 42. The radio frequency energy intercepted by elements 15-23 is delivered by corresponding transmission lines 24-32 to elements 33-41. Elements 33-41 are thus energized to produce a broad pencil beam which is confined to the right direction in FIG. 1 by the ground plane 42. With each of the spiral antenna elements 33 through 41 assumed for the present to be radiating in-phase, it is thus seen that the energy traveling in the right direction of FIG. 1 will be additive for all antennas so that a narrow pencil beam will be the overall result of the complete array.

Control of the phasing of the energy emitted by each of the elements 33-41 is effected merely by rotation of each of the elements about its axis perpendicular to the plane of the spiral conductors. Thus each of the elements will be rotated by a selected amount to produce the desired phase shift in the overall path of energy through each of the elements. Thus typically, elements 19 and 37 would be rotated together by a selected amount to place them in a selected initial phase and other elements would be rotated as required to achieve the selected phase shift. This status could be preserved or, if scanning is desired, altered by suitable means to change the phase of the energy transmitted through the element pairs. As has been stated, it is characteristic of the spiral antenna that physical rotation thereof about the axis perpendicular to the plane of the spiral produces a phase change in the far field which is equal in the number of degrees to the angularity through which the element is rotated. Thus, if the element is rotated through a typical 45 degrees the phase in the far field is altered by the same 45 degrees. Where two elements, typically 19 and 37, are connected together as in FIG. 1 and rotated as a unit through a given angle, the net change in the phase in the far field is twice the amount of angular rotation of the unit provided the elements are arranged with configuration senses as described in FIG. 1. Thus rotation of the unit consisting of elements 19, 37 through a typical angle of 45 degrees would cause a phase change of 90 degrees in the far field produced by antenna 37.

In accordance with the basic principles thus set forth it is seen that the exact lengths of the transmission lines 24-32 are not critical nor are the lengths between the element 10 and the elements 15-23, since merely by initial rotation of the various elements 15-23 and 33-41 it is possible to control the phasing of the energy emitted thereby to where the desired in-phase condition may be obtained to produce the pencil beam radiation pattern.

It is further a property of the apparatus of FIG. 1 that once the elements are set up to produce in-phase radiation in the far field so that a narrow pencil beam is produced, the direction of the narrow pencil beam may be varied over a wide angle without distortion of the beam merely by rotation of the individual elements. This rotation, although of a very simple nature which can be easily accomplished by electrical or mechanical drive means or merely by the manual positioning of the elements, must be done according to a prescribed pattern which is described in the following. Typically to produce a shift of the pattern in the upward direction each of the elements displaced from the central element must be rotated through precise relative angles, typically elements 19 and 37 would not be rotated whereas the paired elements 18 and 36 would be rotated in one direction through an angle Θ, elements 17 and 35 would be rotated in the same direction through an angle 20, elements 16 and 34 would be rotated in the same direction through an angle 3Θ, and so forth. Elements on the opposite side of the elements 19 and 37 would be rotated in the opposite direction, for example 20 and 38 would be rotated through an angle — Θ, elements 21 and 39 through an angle —2 fl, elements 22 and 30 through an angle —3Θ, etc. To produce a shift of the direction of the beam in the opposite direction, the above direction of angularity of rotation of the various elements need merely be reversed. With such an antenna array as the foregoing it is readily possible to move the antenna directivity pattern through an angle of 45 degrees in each direction or a total of 90 degrees in each plane without any serious distortion of the basic narrow pencil beam pattern. As typical examples of the value of Θ to produce a given angular displacement of the beam, the beam will be displaced by an angle which is approximately 6Θ where Θ is the basic rotation of the first displaced pairs of elements 18 and 36, 20 and 38.

In summary therefore with regard to FIGS. 1 and 2, it is seen that an array is provided having for all practical purposes the properties of a lens since merely by rotation of the various spiral antenna elements constituting the array in accordance with basic principles set forth above it is possible to produce desired phasing of the energy leaving the elements 33-41. It is thus possible to simulate a lens of a desired refractive power as well as a desired area to produce a beam of desired sharpness. Such a lens is of considerable value in such diverse fields as radar and radio astronomy.

The spiral antenna element indicated more or less schematically in FIG. 3 appears to behave as if it were a two wire transmission line which gradually by virtue of its

3,045.237 spiral geometry transforms itself into a radiating structure or antenna. A spiral antenna element as typified in FIG. 3 is a planar assembly consisting of two interspaced conductors disposed layer upon layer in such a manner as to present a spiral configuration having a first or a second sense depending upon whether the outward spiral from the center is in a clockwise or counter-clockwise direction. For example, the two conductors could be printed circuit conductors disposed on a base member or disc of non-conductive supporting material. Each conductor has a starting point near the center of the disc and a termination near the periphery of the disc, the terminations of the two conductors occurring at diametrically opposed portions of the periphery. Such a spiral antenna element may be energized at the center by means of a balanced feed system or a coaxial cable with one conductor of the coaxial cable connected to one conductor of the element and the other conductor of the coaxial cable connected to the second conductor of the element. In such a coaxial cable arrangement however it is ordinarily desired that a balanced to unbalanced converter be inserted between the element conductors and the coaxial cable. When such an element is energized by radio frequency energy it radiates a broad circularly polarized pencil beam to each side of the plane of the element. Each radiated beam is normal to the plane of the element and the sense of circularity of polarization of the beam on any one side corresponds to the winding sense of the element as viewed from the opposite side. Accordingly, the two radiated beams are substantially the same except that the sense of polarization of the radiated field on one side is the opposite of that on the other. In many applications such as with the elements of the present invention it is desirable that the elements radiate to one side only, such being readily accomplished by appropriately backing the elements on one side with a ground plane to produce reflection of energy, or with a cavity to produce absorption of energy. Where a ground plane is used it is normally preferable to space each element and the ground plane apart by a distance equal to a quarter wavelength or an odd multiple thereof, thus, in-phase reflection occurs.

Although the exact theory of operation of the spiral antenna is not rigorously established at the present time, a possible explanation is that each spiral antenna element behaves as if it were a two wire transmission line which gradually by virtue of its spiral geometry transforms itself into a radiating structure or antenna. Ordinarily a two wire transmission line wherein the wire spacing is a small fraction of a wavelength yields a wholly negligible amount of radiation when excited at its terminals. This is due to the fact that currents in the two wires of the line at any normal cross-section are 180° out of phase so that the radiation from one line is essentially cancelled by the radiation from the other. In such an antenna element as that shown in FIG. 3, if the spacing between adjacent conductors is substantially smaller than the radius of the outer turn of the element, the difference in length between the two conductors from the origin to a point in the outermost circle is approximately equal to half the circumference of the element. With anti-phase excitation of the conductors at the center, the phasing gradually changes along the length of the two conductors proceeding outwardly so that where the radius of the outer conductor is λ 2π the currents in the two conductors are precisely in-phase and radiation is at a maximum. Such a spiral antenna element when excited at higher frequencies wherein the outer conductor radius is greater than λ ~2?

would achieve such an in-phase condition at a smaller radius than the periphery so that portions of the conductors located at the smaller radius produce maximum radiation. Such an antenna thus is characterized by wide 5 band operation with respect to frequency because selected portions thereof become effective at different portions of the frequency band.

With reference now to FIG. 4 of the drawing, the lens principles of FIG. 1 are shown applied to a reflector. 10 The apparatus shown therein comprises a radio frequency operative device 50 which as previously indicated can be a transmitter or a receiver, the explanation which follows being more easily carried out by speaking of device 50 as a transmitter. The radio frequency operative device 50 15 is connected to spiral antenna element 51 by means of a coaxial cable 52 preferably through a balanced to unbalanced converter 53. As before the element 51 is backed by a suitable ground plane device 54 which typically is spaced therefrom by a quarter wavelength. Dis20 posed in front of element 51 are a plurality of spiral antenna elements 55-63. Spiral antenna elements 55-63 are spaced a quarter wavelength from and supported by a ground plane member 64. The supports of the elements are numbered 65-73 and in addition to providing sup25 port of the elements are also balanced transmission lines terminated in a short circuit which are thus purposefully made reflective. Thus energy intercepted by the elements 55-63 travels down the transmission lines 65-73, is reflected by the short circuit from which it travels back to 30 the elements 55-63 and is reradiated. In accordance with the basic principles outlined in connection with FIG. 1, the elements 55-63 may be oriented with careful attention to the angular phasing thereof as determined by the position of the conductors of the elements so that 35 positive control of the phase of the energy reradiated by each element is possible. To apply the principles of FIG. 1 however requires attention to certain details and principles. Thus to permit the elements 55-63 to couple to the element 51, attention to configuration sense is required, 40 in that element 51 and elements 55-63 must have the same configuration sense.

The operation of FIG. 4 apparatus may thus be summarized as the reradiation of energy emitted by a point source, such reradiation occurring with a selected control ⁴⁵ of phasing of the energy so that the outgoing or reradiated energy from each element has a selected phase. Typically where all of the energy is emitted in the same phase a narrow pencil beam will result in a direction perpendi5θ cular to the elements 55-63. As before, the direction of this pencil beam may be controlled or varied by proper relative rotation of the individual spiral elements so that any degree of angularity within the limits of the beam of the individual antenna elements may be obtained.

FIG. 5 shows an arrangement of apparatus utilizing the basic lens structure of FIG. 1 with lengthened transmission lines 75-83, and two ground planes 84, 85 having a finite spacing to allow the insertion of a drive mechanism 86 to provide programmed rotation of the in60 dividual spiral antenna elements in forward and reverse directions and in desired relative angular rates as set forth in connection with FIG. 1. Drive mechanism 86 could simply be a suitable set of gearing utilizing well known ratio principles to provide the angular relation65 ships plus and minus θ, 2Θ, 39, etc., discussed in connection with FIG. 1. It is understood of course that angular variations in both directions of FIG. 2 would be necessary to produce scanning of the output beam in the γθ typical azimuth and elevation planes. Thus radiation from the complete apparatus of FIG. 5 would be in a direction generally corresponding to that indicated to the right in FIG. 5 by arrow 87 whereas it could also be caused to be displaced therefrom in any direction typically 75 as indicated by arrow 88 by the rotation of the individual

3,045,237 spiral antenna element pairs according to principles previously set forth.

FIG. 6 shows an extension and combination of the principles enumerated in connection with the reflector of FIG. 4 and the lens of FIG. 5. A drive mechanism 89 is placed in back of the ground plane 90 and the members 91 through 99 are extended at least as far as the mechanical portions thereof are concerned to provide programmed orientation of the spiral antenna elements 100-108 to produce control of the direction of the pencil beam of the array. Thus scanning can be obtained with the element 109 held in a fixed position and the various antenna elements 100-108 rotated through plus or minus angles in the relationship of θ, 29, 39, etc., to produce changes in the antenna direction. It is seen therefore that a large structure employing many hundreds or more of spiral antenna elements may be mounted with a suitable ground plane 90 and a rotational mechanism 89 to provide a device for radio astronomy without requiring any complicated supporting structure or rotating mechanism to position a huge, weighty, parabolic reflector.

Thus far the invention has been described with spiral antenna elements as the basic feed, 10, 51,109 to facilitate an understanding of the coupling of the energy in the antenna systems, but with these systems, the single spiral feed results in the production of emitted energy having circular polarization. It is not always desirable to employ circular polarization, for example, linear polarization in one plane or another may be desirable in certain instances. The apparatus of FIGS. 7 and 8 shows the linear polarization arrangement as applied to the basic apparatus of FIGS. 1 and 2. Two differences are present, namely the spiral antenna element 10 of FIG. 1 has been replaced by a dipole 10-A in FIG. 7, and all the spiral antenna elements on each side of the ground plane 42 no longer have the same configuration sense, but alternate on a checker board pattern basis.

The operation of the antenna of FIGS. 7 and 8 thus incorporate the principles of the spiral doublet antenna wherein the dipole 10 emits linearly polarized energy which couples to the elements 15-23 regardless of their configuration sense. Energy then couples out from each of the elements 33-41 with circular polarization however of opposite sense from those elements arbitrarily marked L (left hand configuration sense) than it does from those elements having the opposite configuration sense, R. The two polarizations combine in the far field, the result being linear polarization.

The phasing dependency of energy in the far field upon rotation of the elements is maintained however so that scanning is obtained by rotation of the elements as with the previously described devices. In general however, elements marked R and L must be rotated in opposite directions to provide scanning, thus in keeping with the previous scanning scheme, in FIGS. 7 and 8, the horizontal row including element 37 (and its mate 19 on the opposite side of ground plane 42) would remain stationary, the horizontal row including element 36 would be rotated through 9 degrees in first directions, typically clockwise, for elements marked R and counter-clockwise for elements market L whereas the horizontal row including element 38 would be rotated in opposite directions, counter-clockwise for elements marked R and clockwise for elements marked L, the horizontal row including element 35 through 29 in appropriate directions clockwise, etc. This would produce scanning in the plane of the elements 33-41 with energy having polarization as determined by the dipole 10-A.

Likewise scanning in the plane perpendicular to the plane of elements 33-41 is obtained by differential rotation of the vertical rows of elements in FIG. 8.

For the foregoing discussion the groups of spiral elements have been described as being in planes and the term ground plane has been used. It is to be understood that the principles of the present invention could be ap plied to arrays having a ground plane which is not a true plane, for example, it may be an ellipsoid of revolution, in which case advantages may accrue from disposition of the planes of the various spiral elements in each group in positions other than in the same plane. In such case control of the phase front is still possible by rotation of the individual spiral elements.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.