US3524192A - Scanning apparatus for antenna arrays - Google Patents

Description

Aug. 11, 1970' a. SAKIOTISzETAL 3,524,192v

SCANNING APPARATUS FOR ANTENNA ARRAYS I Filed Dec. 9, 1963 2 Sheets-Sheet 1 Fig. I

PLANE WAVE PLKIT WAVE lnvenfors Nicholas G Sakiofis By .Ben H Aulen g.1 ,197o N.G.SYAKIOTIS ml. 3,524,192

SCANNING APPARATUS FOR ANTENNA ARRAYS Filed Dec. 9, 1963 2 Shets-S'neet 2 INVENTORS Nicholas G. Sakiofis BY Ben H Aufen A/tys.

United States Patent "ice 3,524,192 SCANNING APPARATUS FOR ANTENNA ARRAYS Nicholas G. Sakiotis and Ben H. Auten, Phoenix, Ariz., assignors to Motorola, Inc., Chicago, 11]., a corporation of Illinois Filed Dec. 9, 1963, Ser. No. 328,868 Int. Cl. H01q 3/24 US. Cl. 343854 12 Claims ABSTRACT OF THE DISCLOSURE This invention relates generally to antenna scanning systems and more particularly to a phasing lens and associated circuitry to provide electronic scanning of circular antenna arrays.

Electronic scanning of fixed antenna arrays offers many advantages over mechanical scanning arrangements wherein the radiating structure is rotated. An attractive method for electronically steering the beam of a fixed antenna array is by use of an electromagnetic energy feed system which provides a multiplicity of independent overlapping beams, each of which utilizes the full array aperture. The beam is then steered in discrete steps by sequentially selecting independent feed points for each of the beams.

In some antenna arrays electronic scanning is achieved by utilizing multiport couplers to feed a limited number of array elements. This approach, however, is inherently narrow band and becomes complex for arrays having a large number of elements. Electronically controlled phaseshifters and multipole switches may be incorporated into the system to expand the number of array elements utilized, but such an application becomes expensive and complex for a large number of array elements.

It has been proposed, in conjunction with circular symmetric antenna arrays, to use a two-dimensional microwave lens wherein energy appearing about a given are of its periphery is collimated into a beam diametrically opposite the Wave front of a received plane wave of electromagnetic energy. However, known lenses of this type are provided with an index of refraction (dielectric constant) that varies as a function of lens radius. In addition, scanning systems usually employed with this type of lens require a single feed point, with scanning achieved by causing relative rotation between the antenna array and the lens or by causing rotation of the single feed point relative to the lens. The moment of inertia of the rotating structure and the inherent problems resulting from using mechanically moving parts at high frequencies has placed definite limitations on scanning speed and overall system utility of arrangements of this type. Consequently, it is desirable to provide an energy collimating lens which may be readily incorporated with a multi-element circular antenna array for electronic scanning about a desired azimuth without the use of mechanically moving parts and with a minimum of system complexity.

It is therefore among the objects of this invention to provide an improved antenna scanning system.

Another object is to provide an improved microwave lens construction for simplified electronic scanning of circular symmetric antenna arrays.

Patented Aug. 11, 1970 A further object of the invention is to provide a twodimensional circular-symmetric constant-index of refraction microwave lens particularly suited for enabling simplified electronic scanning of circular antenna arrays.

A feature of the invention is the provision of a homogeneous cylinder of dielectric material clad with metal on both faces thereof to form a parallel plate transmission region. A plurality of arbitrary feedpoints are located about the periphery of the cylinder so that electro-magnetic energy introduced at a selected number of adjacent feedpoints focuses at a single diagonally opposite feedpoint, and a simple electronic switching arrangement is used to cause scanning of a circular symmetric planar antenna array, by changing the location of the focal point about the periphery of the lens.

A further feature is the provision of a two-dimensional circular-symmetric microwave lens having the region between two conducting plates filled with a dielectric material of uniform dielectric constant. A number of feedpoints are equally spaced about its circumference, and angular displacement of the beam produced by an associated circular antenna array is achieved by electronically switching the focal point of the lens about its circumference.

A more specific feature of the invention is the provision, in an electromagnetic energy focusing lens of the above described type, of a low-loss high dielectric constant material disposed between conducting plates to provide a cylindrical transmission region, with the conducting plates spaced for propagation in the TEM mode. This produces a simple and compact construction for broadband operation that has low absorption loss.

Other objects, features and attending advantages of the invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an electronically scanned antenna array utilizing a circular symmetric lens having a focal point on its circumference;

FIG. 2 is a schematic diagram illustrating the manner in which energy is focused by the lens;

FIG. 3 is a plane view of a particular embodiment of a microwave lens according to the invention;

FIG. 4 is an enlarged side View, partially in section, of a portion of the lens of FIG. 3;

FIG. 5 is a partial plane view of another embodiment of the lens;

FIG. 6 is a cross-section of the lens taken along lines 66;

FIG. 7 is a portion of a perspective view of another form of the lens; and

FIG. 8 is a cross-section of FIG. 7 taken along lines 8-8.

The invention includes a microwave lens that focuses energy introduced at a number of adjacent feedpoints on its periphery at one feedpoint. The lens is circular symmetric, having N feedpoints equally spaced around its periphery, with each feedpoint adapted to be connected either to an associated element of a circular symmetric planar antenna array or to utilization means of the overall system by electronic switching means. All feedpoints but one are connected to antenna array elements, with the remaining feedpoint serving as a focal point for electromagnetic energy introduced at the other feedpoints. Scanning of the array elements is achieved by changing the selected focal point on the periphery of the lens.

structurally, the lens consists of two conducting plates parallely disposed, with a dielectric medium therebetween forming a cylindrical transmission region having an index of refraction (or dielectric constant) that is constant throughout. Located symmetrically about the periphery of the transmission region are an arbitrary number of energy feedpoints, each having an associated energy coupling port. It is to be understood that the principle of reciprocity applies so that the feedpoints may function to introduce electromagnetic energy to or to collect electromagnetic energy from the transmission region, depending whether the lens is utilized with a receiving or transmitting antenna array. The energy so introduced (or collected) is then collimated at selected focal points corresponding to the feedpoints positioned around the lens periphery.

Referring now to FIG. 1, the circular symmetric antenna system illustrated includes lens having a multiplicity of N coupling ports 12 representing N possible feedpoints equally spaced around its periphery. Each coupling port is connected by an equal length line to a corresponding array elements 14 of a circular symmetric planar antenna array. A single pole double throw switch 16 is interposed between each coupling port and its associated array element 14, with one leg of each switch connected to a leg or input port of single pole N-throw communtator switch 18. Switch 16 may be biased diode microwave switches or Where feasible from the standpoint of system complexity and cost may be a ferrite circulator. Switch 18 is a multi-throw diode switch which conveniently may be of the type described in the copending application of Philip C. Clar, Ser. No. 308,286, filed Sept. 11, 1963, now Pat. No. 3,223,947 and assigned to the assignee of this application.

The above described switching arrangement enables a selected Nth port of lens 10 to be connected to a selected input port of commutator switch 18, while the remaining ports are connected to N-l elements of the antenna array. Switches 16 and 18 are actuated by a suitable control circuit (not shown), as for example, a selectively triggered DC biasing arrangement when diode switches are used. Scanning is achieved by successively connecting a different single coupling port 12 and hence its associated feedpoint on the periphery of lens 10 to a corresponding input port of commutator switch 18, and then to the common or output port of commutator switch 18 by appropriate control of switches 16 and 18.

Thus, as shown in FIG. 1, N-l coupling ports or feedpoints 12 on the periphery of lens 10 are coupled to array elements 14 by SPDT switches 16, with the Nth feedpoint 12a coupled to a delated Nth port 18a of commutator switch 18. A common port 19 of switch 18 in turn connects the selected Nth port 18a to the output port 20 of the overall system. Accordingly, an incoming plane wave of electromagnetic energy, as shown in dotted lines in FIGS. 1 and 2, is received by an arbitrary number of array elements. The angle between the first and the last array element which receives the incoming plane wave of electromagnetic energy may be considered the array aperture angle. It can be seen that within this array aperture angle a discrete number of individual array elements are provided.

Referring now to FIG. 2, the direction of propagation of the incoming plane wave (or alternately of an outgoing plane wave) is parallel to a diameter of lens 10 taken through its focal point associated with the Nth port 12a. The energy introduced at the remaining N-l feedpoints or coupling ports 12 is propagated through lens 10 and focuses at its Nth feedpoint 12a to be subsequently fed to the system output port 20 via switches 16 and 18. It can be seen from the foregoing that by successively switching other feedpoints 12 to provide other focal points 12a on the periphery of lens 10, the propagation path of a received plane wave is made successively parallel to a diameter of the lens taken through different points about its periphery. This in turn results in steering the antenna array in discrete steps about 360 azimuth.

Lens 10, a two-dimensional circular symmetric constant index lens, is comprised of parallel conductive plates defining a cylindrical transmission region filled with a material of uniform dielectric constant. One form, utilizing strip-line techniques and adapted to be utilized in a coaxial line system, is shown in FIGS. 3 and 4. Conducting plate 22, having a generally circular body portion 23 and a scalloped periphery forming horns or tapers 25, is separated from a metallic base plate 24 by dielectric medium 26. A pin or conductor 27 extends through dielectric medium 26 and through an aperture 29 in base plate 24, and is connected to the apex of tapers 25 by a screw 31. Pin 27 terminates such that it is co-extensive with the center conductor of coaxial connector 28, secured to base plate 24. Connector 28 may be a type-N or other suitable coaxial connector. The ratio of the diameter of aperture 29 to pin 27 is preferably chosen to provide the same characteristic impedance of the interconnecting coaxial cable with switches 16 and 18, typically 50 ohms. It is to be understood that a number of pins 27 and coaxial connectors 28 are disposed equally around the periphery of lens 10 to provide a connection with the apex of each of tapers 25. Each connector 28 provides a coupling port for lens 10, with tapers or horns 25 providing a transition for the feedpoints or focal points of energy propagated in the dielectric medium between conductive plate 22 and base plate 24. The taper is utilized to produce a gradual transition to minimize mismatch and hence losses by radiation spillover. Energy supplied to coaxial conductors 28 and thereby introduced at an arbitrary number of points around the periphery of the lens will be directed to a focal point located on a diameter parallel to the direc tion of propagation of a plane wave, as illustrated in FIG. 2.

Conducting plate 22, including body portion 23 and tapers 25, may be of copper, silver, or the like, and may be deposited or plated on dielectric medium 26 by known techniques. Its thickness need only be sufficient to minimize skin losses at the frequencies of interest, in the order of a few thousandths of an inch. Base plate 24 may be copper or brass, and has no critical electrical dimensions. It should be of sufiicient thickness to provide convenient mounting for connectors 28 and to impart mechanical rigidity to dielectric material 26.

Dielectric medium 26 is preferably a material having a high relative dielectric constant and a low loss-tangent. A low loss-tangent will minimize insertion loss of lens 10 in the overall system, while a high dielectric constant reduces physical dimensions to provide a compact unit. Commercially available high alumina ceramics have a relative dielectric constant in the order of or greater, and a loss-tangent of less than 1x10 Ceramic materials, in addition, are machinable to form a desired configuration and a metallic plate 22 may be readily painted, plated or deposited thereon. One such suitable material, having a loss-tangent of .0007 and a relative dielectric constant of is ceramic Nl400Tl10, supplied by the American Lava Corp. Other materials such as strontium titanate (loss-tangent .001, relative dielectric 300) and dielectric materials supplied under the trade names of Eccoceram and Stycast may also be used, and under certain circumstances, hereinafter discussed, air may be used as the dielectric material.

To minimize losses and to provide for losses that are substantially constant over a wide frequency range, it is desirable that the energy be propagated between plates 22 and 24 in the TEM mode. This mode is also advantageous where coaxial cable is used to interconnect array elements 14 and switches 16, 18 in an overall system, as is conventional at UHF and the lower microwave portions of the frequency spectrum. The TEM mode of propagation is readily achieved by making the spacing between conducting plates 22 and 24 less than 7\/2\/e (where E is the relative dielectric constant for dielectric material 26) for the highest frequency of interest. This effectively results in a low pass filter having a cutoff frequency determined by the spacing between plates 22 and 24. And it can be shown that the ratio of lends diameter d to array element diameter D (see FIG. 2) is:

i fi a n D where is a constant chosen to minimize phase error over a given aperture angle at and e is the effective dielectric constant of the lens. Thus it is apparent that an extremely small and compact lens for a given array diameter may be realized by using a dielectric material having a large dielectric constant s.

It is to be noted that the system of FIG. 1 utilizes a 1:1 correspondence lens, that is one having a number of feedpoints equal to the number of antenna array elements. Angular displacement of the focal point results in an equal angular displacement of the array beam and accordingly the antenna array can be steered 360 in azimuth by a complete revolution of the focal point on the lens circumference. Alternatel'y a 2:1 correspondence lens may be utilized, with the number of feedpoints equal to one-half of the array elements. In this instance an additional set of single pole double throw switches are used to connect each port and its associated feedpoint to either one of two diametrically opposite array elements. In this instance, angular displacement of the focal point on the periphery of the lens results in one-half as much angular displacement of the array beam, and accordingly a complete revolution of the focal point on the lens circumference steers the beam 180 in azimuth.

A modification of lens -10 of FIGS. 3 and 4 is shown in FIGS. 5 and 6, and provides an in-line rather than a perpendicular coaxial to strip line transition. Conducting plate 22, including body portion 23 and tapers 25, is plated or depositioned on dielectric material 26 in the manner previously discussed. Dielectric material 26 is made circular in shape and has a diameter approximately equal to the spacing between the apexes of two diametrically opposite tapers 25. Thus the apex of each taper 25 terminates at the periphery of dielectric material 26. Base plate 24 is also circular and has the same diameter as dielectric material 26. A plurality of mounting brackets 35 are secured to the annular edge of circular base plate 25, and extend perpendicularly across the annular edge of circular dielectric material 26, mounted coaxial with base plate 24. Connectors 28 are in turn mounted on brackets 35 and positioned so that their center conductors 28a rest on the top surface of dielectric material 26 and at the apexes of tapers 25. Each connector 28 may be a standard coaxial type-N connector with its center pin shortened so that it just contacts the apex of a taper 25, and an electrical connection such as soldering made at that point.

A still further modification wherein waveguide feed is provided for lens is shown in FIGS. 7 and 8. This modification is particularly useful at X-band and higher microwave frequencies where coaxial interconnections of the overall system becomes undesirable. In this modification plates 22 and 24 both identical in structure, are made circular in configuration. A groove 37 is provided around the outer periphery each of plates 22 and 24, and a plurality of holes 39, parallel to the major axes of the lens 10, are provided in grooves 37.

A number of hollow rectangular members 40, taking the form of short sections of waveguide, are disposed around the periphery of plates 22 and 24. One end of the top and bottom walls of hollow members 40 are provided with lands 42 which mate with grooves 37. Lands 42 are in turn bolted or screwed to the holes 39 in grooves 37 to maintain plates 22 and 24 in a fixed spaced relationship. As before, the spacing between the plates 22 and 24 is maintained less than A/ZV? to provide propagation in the TEM mode. The other end of hollow members 40 are provided with a flange 44, which may be a standard waveguide flange for mating with interconnecting waveguide of the overall system.

The lens periphery is at the outer periphery of circular plates 22 and 24, and a feedpoint is provided at points about this periphery equidistant between the side walls of hollow members 40. The space between the plates may be air filled, or may be filled with a suitable dielectric material as discussed. The dielectric reduces the physical size of the lens structure while air dielectric reduces losses which may be troublesome at higher microwave frequencies.

Because of the simplified construction of the above described focusing lens and of the simplified associated switching circuitry used therewith, circular antenna arrays having or more elements may be conveniently scanned in discrete steps about a 360 azimuth by providing a lens with a multiplicity of focal points on its periphery. By utilizing microwave diode switches controlled by bistable multivibrator circuits connected as a ring counter and synchronized by a clock pulse, scanning rates between 200 r.p.s. and 20,000 r.p.s. are possible. A large number of array elements provides a high resolution so that angular separation of the two beam widths for individual array elements is readily distinguishable.

The invention provides, therefore, an improved antenna scanning system utilizing a simple and compact two-dimensional focusing lens. Electronic scanning of circular symmetric antenna arrays is readily achieved with a minimum of circuit complexity. The lens is readily adaptable to systems having a large number of independent array elements by use of a simple switching arrangement. Use of a high dielectric low loss material having a constant index of refraction for the lens simplifies construction and ensures reliable loW loss operation over a wide band of frequencies in the UHF and microwave regions.

We claim:

1. A system for scanning a circular array of antenna elements including in combination, electromagnetic energy focusing means comprising a homogeneous cylinder of dielectric material having conductive boundary surfaces for electromagnetic energy disposed on the opposite ends thereof, a plurality of electromagnetic energy feedpoints spaced at discrete points around the periphery of said cylinder, with a selected one of said feedpoints providing a focal point for a plane wave of electromagnetic energy having a direction of propagation parallel to a diameter of said cylinder taken through said selected feedpoint, and circuit means for selectively coupling said one feedpoint to a common terminal while coupling the other feedpoints to elements of said antenna array, said circuit means operable to sequentially couple a different selected feedpoint to said common terminal to rotate said focal point around the periphery of said cylinder, thereby causing scanning of said antenna array elements.

2. A system for scanning a circular array of antenna elements including in combination, focusing means for electromagnetic energy comprising a pair of spaced apart conductive plates with a dielectric material of constant index of refraction disposed therebetween to provide a cylindrical transmission region for said electromagnetic energy, a plurality of electromagnetic energy feedpoints equally spaced at discrete points around the periphery of said cylindrical transmission region, with a selected one of said feedpoints providing a focal point for a plane wave of electromagnetic energy having a direction of propagation parallel to a diameter of said cylindrical region taken through said selected feedpoint, and switching means operable to sequentially select different focal points while coupling remaining feedpoints to said array elements, thereby providing scanning of said array elements.

3. A system for scanning a circular array of antenna elements including in combination, a microwave lens comprising a pair of spaced apart conductive plates with a dielectric material of constant index of refraction disposed therebetween to provide a cylindrical transmission region for high frequency electromagnetic energy, a plurality of electromagnetic energy coupling ports, a plurality of transitions connected to said coupling ports to provide energy feedpoints about the periphery of said cylindrical transmission region, single-pole multi-throw switching means having a common port and a plurality of further ports, and a plurality of single-pole double-throw switching means for connecting one of said electromagnetic energy coupling ports to said further ports while at the same time connecting the remaining ones of said electromagnetic coupling ports to elements of said antenna array, said single-pole multi-throw switching means and said singlepole double-throw switching means operable to rotate the energy feedpoints of said cylindrical transmission region about its periphery to thereby cause scanning of said antenna array elements.

4. A focusing lens for electromagnetic energy comprising a homogeneous cylinder of dielectric material, conductive surfaces on each end of said cylinder defining a transmission region within said cylinder, and a plurality of energy coupling means spaced at discrete points around the periphery of said cylinder, with a selected one of said energy coupling means providing a focal point for a plane wave of electromagnetic energy having a direction of propagation parallel to a diameter of said cylinder taken through said selected energy coupling means.

5. A microwave lens including in combination, first and second conductive plates with a dielectric medium of uniform dielectric constant disposed therebetween, said plates and said dielectric medium providing a cylindrical transmission region having a constant index of refraction, and a plurality of feedpoints for electromagnetic energy disposed at discrete points around the periphery of said cylindrical transmission region, with a selected one of said feedpoints providing a focal point for a plane wave of electromagnetic energy appearing at the other feedpoints and having a direction of propagation parallel to a diameter of said cylinder taken through said selected feedpoint.

6. The lens of claim wherein the spacing of said conductive plates is less than one-half of the effective wavelength of the electromagnetic energy propagated in said cylindrical transmission region.

7. A microwave lens including in combination, a body of dielectric material having first and second parallel major surfaces and having a constant index of refraction throughout, first and second conductive members disposed on said major surfaces, at least one said conductive memher having a circular body portion to thereby define a cylindrical transmission region within said dielectric material, a plurality of electromagnetic energy coupling ports, and a plurality of electromagnetic energy transitions equally spaced about the periphery of the circular body portion of said one conductive member, each said transition connected between the periphery of the circular body portion of said one conductive member and one of said coupling ports to thereby provide a plurality of equally spaced energy feedpoints about the periphery of said cylindrical transmission region, with each said feedpoint providing a focal point for a plane wave of electromagnetic energy having a direction of propagation parallel to a diameter of said cylindrical transmission region taken through such feedpoint.

8. The microwave lens of claim 7 wherein said dielectric material has a relative dielectric constant substantially greater than unity.

9. The microwave lens of claim 7 in which said coupling ports are coaxial transmission line connectors and said transitions are comprised of generally triangular conductive members having an apex connected to the center conductor of said coaxial connector and a base contiguous with the periphery of the circular body portion of said one conductive member.

10. A microwave lens including in combination, first and second disc shaped metallic plates, said metallic plates coaxial with one another and spaced apart to provide a cylindrical transmission region therebetween, and a plurality of wave guide transition elements disposed around the periphery of said cylindrical transmission region, said transition elements providing a plurality of feedpoints for electromagnetic energy around the periphery of said cylindrical transmission region, with individual ones of said feedpoints providing a focal point for a plane wave of electromagnetic energy having a directional propagation parallel to a diameter of said cylindrical transmission region taken through such feedpoint.

11. The microwave lens of claim 10 wherein said cylindrical transmission region has a relative dielectric constant of unity and has a constant index of refraction throughout.

12. A microwave lens of claim 10 wherein said cylindrical transmission region has a relative dielectric constant substantially greater than unity and has a constant index of refraction throughout.

References Cited UNITED STATES PATENTS 2/ 196-5 Rotman. 4/ 1965 Abbott.

US. Cl. X.R.

Images