US2928092A - Electromagnetic lens - Google Patents

Description

March 8, 1960 R. M. HATCH, JR

' ELECTROMAGNETIC LENS Filed Oct. 9, 1957 INVENTOR.

' RICHARD M. HATCH, JR.

ATTORNEY Un m Q CIJ LQ 2,928,092 ELECTROMAGNETIC LENS Richard M. Hatch, Jr., West Concord, Mass., assighor, by mesne assignments, to Sylvania Electric Products Inc., Wilmington,'Del., a corporation of Delaware Application October 9, 19-57, Serial No. 89,032

7 Claims. (Cl. 343-753 This invention relates to antenna systemsforgradi: ation and/or reception of ultra high frequency/ electromagnetic waves, and more particularly to antenna systems for maintaining a wide radiation beamiove'r abroad band of frequencies. I c c As is well known in the art, electromagnetic horns of various configurations have long been used as impedance 10 I theouter region of the aperture, thus advancing the cenfin C cording to the main feature of this invention, a radiator of the type just described is modified by a conducting .cylinderdisposed in the'aperture eflectively to divide transformers between the wave guide systems of micrdwave apparatus and free space. The more conventional configuration consists of a section of rectangular wave guide flaredat one end .in one or both planes tdpro'vide a rectangular aperture lying in a flat plane- While ra diators of this type are useful incertain applications, and by suitable modification, as by insertion of septa and the like, can be made to provide specific radiation patterns,.their. radiation patterns are very sensitive to changes in frequency. The radiation beam width'of a simple horn type radiator depends upon the electrical dimensions of the horn, and thus for a given horn, a change in wave length of the transmitted or received-signal results in a corresponding change in the width of the beam.

Thus, a change in wave length of 2:1 would result in the apertureinto two concentric wave guides having by virtue of their different diameters, different propagation velocities. 'With proper'relative spacing between the cylindrical conductor and the inner and outer conductors of the coaxialconductor horn, the relative phase of the energy passing through the central portion of the apert'ure'may be made to advance with respect to that, near acteristics of the various. portions of the lensmay beobtained by the insertion of dielectric materials having dielectric constants greater than thatof free space injone or more of .the concentric wave guides. In a tion will now become apparent, and a better understandroughly a 2:1 change in beam width. Accordingly,'ra-

diators heretofore available are unsatisfactory in certain, communications and radar applications where it is desirable to maintain a wide radiation :bearn over a large frequencyrange. V

Accordingly it is'aprimary' object of the present inventioh to provide a radiator for electromagnetic radiation having a wide {beam width which is maintained over awide range of frequencies. 7 7 Another object of the'inyention is to provide an -electromagne'tic. lens havinga widebeam'width rnaintainable over a broad band of frequencies and which is useful with linearly, elliptic'ally or circularly polarized waves.

Another object of the invention 'is'to provide an anten-v na havingthe foregoing characteristics which'is relatively ern high speed, high altitude aircraft. i The invention is based in part uponthe fact that the phase of field in a space. bounded on at least two sides electric field progresses at a velocity greater, than the ye;

preferred embodiment, the dielectric material also ;fu nc tions as the support for the cylindrical conductors to pro yide an integral rigid structure. .4 The foregoing. andpther objects of thefpresentinven ing of its construction and applications had from the following detailed description when taken in connection with the accompanying drawing in which:

'"fFigs. 11 and g 2 are diagrams illustrating the fieldpajb terns encountered in a coaxial waveguide type of elec-.

tromagneticradiator;

Fig. .3 is a cross-section view of the electromagnetic lens in'its most basic form illustrating the principal or the invention;

Fig. 4 is a cross-section view of a preferred embodi-Q ment of the invention; and

Fig. 5 is a cross-section view illustrating the invention.

as applied to a'circularwaveguide horn radiator.

With referencejnow to the drawing and more particu-Z' larly to. Figs. 1 and 2, in a simple horn radiator of the simple to manufacture and sufiiciently rugged to -with-.. standthe environmental conditions 'encountered in modlocity in free space, this incre'ase jn velocity resulting from the interaction of waves reflected from the boundary,walls. This, field velocity or phase velocity may be controlled by controllingthe spacing of the boundary walls. This phenomena is applied to a horn radiator of the dominant TE mode, the equipha'se suriaces offradiant energy become, increasingly flatas jthe electrical dim mensioir of l the aperture becomes: larg ith: the I q quence that the radiatiombeambecoihes narrower;

waveguide formed by inner conductor 13 and cylinder 14 have difiere'nt propagation velocities. The smaller coaxial waveguide type excited in the dominant TE mode (illustrated in Fig. 2 the equiphase surfaces 10 of radiant energyemanating from the aperture 12 becomeim creasinglyflat as the dimension of the aperture becomes largerwithftheconsequence that the resulting radiation beam. becomes narrower. At constant frequency, of course, the b'eam Width is constant at a width determined by' the dimension at the aperture, but should the. frey I v V I v d y quency of transmitted or received radiation increase',.the, by conductive walls placedv substantially parallel 'to the electrical dimension of the aperture likewisefincreases causing the beam to become narrower. 1

In accordance with the present invention, a ing cylinder 14, preferably of metal, is disposed in..the aperture of the radiator concentrically with the inner conductor 13 and the outer conductor 15 of the coaxial waveguide. As shown in Fig. 3, the cylinder 14 may be tapered slightlyin the same direction as the taperof, the flared opened end 15a of the outer conductor 15 of the waveguide, and is of such a dimension that the coaxialis ofsmaller dimension than the waveguide, formed by cylinder 14 andthe flared portion 15a. Thus; it is'seen that cylinder 14 divides the aperture intotwo'iconcentric waveguides,'fwhich by virtue of their diflerent diameters Patented Mar. 8,

conduct;

guide has a greater propagation velocity than the concentric outer guide, and therefore advances the relative phase of energy passing therethrough, that is, along the path A, with respect to that in the outer guide, designated by the arrows B. The center of the equiphase surface emanating from the lens is thus advanced and the total surface becomes more nearly spherical with the result that the radiation beam is widened. By proper selection of the length, diameter, and position of a' single cylinder 14 in the aperture, it is possible to maintaina. beam width of approximately 70 substantially constant over a range of frequency of 2:1.

While the structure illustrated at Fig. 3 is useful in explaining the principle of operation of the antenna, and has utility. in certain applications, to maintain substantially uniform band width over a wide frequency range the diameter of cylinder 14 necessarily is so small that the cutofif frequency of the inner waveguide is approached within the operating frequency range of the antenna. At frequencies near orbelow the cutoff frequency, the cylinder introduces objectionable reflections, obviously an undesirable condition. This disadvantage is overcome by an extension of the basic concept hereinabove described, and the introduction of means to delay the field at the periphcry of the aperture and not the field at the center of the aperture. In a preferred embodiment, illustrated in Fig. 4, this is accomplished by the introduction of dielectric materials having relatively high dielectric constantnear the periphery of the aperture, and the use of concentric conducting cylinders to divide the aperture into concentric waveguides in the manner discussed in connection with Fig. 3.

More specifically, the electromagnetic lens of Fig. 4 comprises a coaxial waveguide having an outer conductor 16 and inner conductor 18, the outer end of the outer conductor being flared as shown at 20. Instead of the single cylinder of Fig. 3, the embodiment of Fig. 4 includes two conducting cylinders mounted within the aperture concentrically with inner conductor 18. A first of these cylinders, designated 22, is of frusto-conical shape and has a flare angle somewhat less than that of the flared open end 20 of the outer conductor 16. Cylinder 22 is supported relative to the flare 20 by a mass of rigiddielectric material 24 of generally torroidal shape, which is in turn, supported inside the flange 20. Dielectric material 24 may be Teflon or polystyrene, which have dielectric constants of approximately 2.0-2.5, or preferably a metal powder loaded plastic foam material, which has recently become available, and whose dielectric constant may 'be adjusted to any value between 1.0and about 4.0 by controlling the proportions of the plastic-metal mixture. It will be noted that the diameter of the smaller end of cylinder 22 is slightly larger than the inner diameter of conductor 16, and that the dielectric material 24 extends radially beyond the larger diameterof cylinder 22.

Fitted within cylinder 22 and supported on the cylinder and on a portion of dielectric 24 along the surface 26 is a second mass of similar dielectric material 28, also of generally torroidal form, on which is supported a second cylinder 30 also of frusto-conicalshape. The smaller diameter of'cylinder 30 is substantially equal to the smaller diameter of cylinder 22 and has a similar angle of flare whereby cylinders 22 and 31 together define a passage for wave energy having substantially parallel sides. It will be evident that in fabrication, dielectric material 24 may be deposited in the flared open end of the coaxial waveguide and the cylinder 22 fitted in place, and the dielectric member 28 and its associated cylinder 39 separately fabricated and machined and then inserted inplace within cylinder 22. The outer surface of dielectric members inner conductor 18 and cylinder 30, a second guide between cylinders 22 and 30, and a third guide between the flared portion 20 and the cylinder 22. The first-mentioned waveguide is unloaded, and is of such dimensions that of the three it has the highest propagation velocity.

- The third contains more dielectric material or material of higher dielectric constant than the second whereby energy at the periphery of the aperture is delayed more than in either of the second or first guides. As a consequence, the total radiation pattern from the lens has a wide beam and substantially spherical wave front. The lens structure being circularly symmetric, the polarization of the energy may rotate about the axis of the structure without effect upon the radiating properties of the lens, and is thus equally well adapted to elliptically or circularly polarized waves as to linearly polarized waves.

In applications of the antenna where dust may be encountered, it may be preferable to cover the aperture, a

' cap 31 formed of thin, radiation permeable plastic material being shown for this purpose. Internally of its crown,the cap is preferably-provided with a boss 32 having an opening therein for receiving the tip 18a of the center conductor of the coaxial waveguide. Thus, in addition to sealing the lens, the cap provides lateral support for conductor 18.

The invention has been described in connection with two possible physicalembodiments; namely the single conducting cylinder of Fig. 3 and the double-cone, dielectric-loaded embodiment of Fig. 4. It will be understood that in arriving at a desired radiation pattern it maybe possible to devise many alternative combinations. For example, a two-cone Iens without dielectric loading may be useful in some applications, and it will be appreciated by those skilled in the art that considerable adjustment of the angle of taper of the conducting'cylinders, their locations relative to each other and relative to the flared open end of the coaxial waveguide, and the angle of flare of the coaxial waveguide (as well as thenumber of conducting cylinders) are subject to individual adjustment without departing from the spirit of the invention. Likewise, the structure of Fig. 4 is subject to modification, such as the relative angles of flare of the outer conductor and the two cylinders, as well as the external shape of the dielectric material.

Although the circular cylindrical conductors positioned in the aperture have been described as rigid elements, it'is within the contemplation of the invention to deposit conductive material on the surfaces of dielectric members 24 and 28 on which the cylinders 22and 30, are

positioned, such as by plating or painting. 'It can be,

expected that in certain applications it may be desirable to havea very thin conducting boundary for one or more of the concentric transmission lines; it is possible with this technique to reduce the overall weight of the antenna system. Accordingly, it is intended that the term cylinder should cover this alternate construction.

It will now be recognized by ones skilled in the art that the principles of the invention may also be applied to a horn radiator of the type shown in Fig. 5 comprising a flaredhollow circular Waveguide 34. In this configuration also, when excited in the'dominantv TE mode, the equiphase surfaces of wave energy emanating from the aperture 36 become increasingly flat as the diameter of coaxial waveguide divide the aperture of the radiator into 1 the aperture increases, resulting in a narrowing of the beam. This elfect can be minimized and the beam width maintained substantially constant over a wide range of frequencies by the insertion of one or more conducting cylinders, such as 38, in the aperture of the horn to divide the aperture into concentric waveguides having relative propagation velocities so asto produce the desired phase distribution across the aperture. As in the coaxial waveguide embodiment, a two-cone lens may be used, with or without'diel'ectric loading, with the angle of flare and position of the cylinders in the aperture subject to considerable.- modification. a

as/tense reference to the transmission, of radio energy it will be;

'at least onhollow cylindrical conductor of substantially rifrus'to-conical shape" flared in the same direct-ion as the apparent to those skilled in the art that the constructions descnbed herein may also" be used for the reception if desired since a radio horn is in essence a reversibl Further modifications of the inventive concepts disclosed herein may now become apparent to those skilled in this art. It' will be understood that the scope oft he present invention is to be regarded as subject only to hqse m ta en tqf e ppende m What is claimed is: p 7

1. An electromagnetic lens comprising, a coaxial waveguide having an outer conductor of circular section, an open end of which is flared to provide an aperture, and an inner conductor of circularisection coaxial with said outer conductor and extending beyond the flared open end of said outer conductor, and a pair of hollow con-- ductors of frusto-conical shape disposed in said aperture coaxially with said inner and outer conductors with the smaller ends thereof directed inwardly and spaced apart along the extending portion of said inner conductor, said hollow conductors being insulated from each other and from said inner and outer conductors and spaced to divide said aperture into three concentric waveguides having successively increasing propagation velocities from the u outer conductor and extending beyond the flared open end of said outer conductor, a pair of hollow conductive cylinders of substantially frusto-conical shape both flared in the same direction as the flared open end of said outer conductor disposed in said aperture coaxially'with said device.

i'laz jed endo'tsaid outer conductor disposed in aperture coaxially with said inner" and outerconductors insulated; theretrom; the smaller diameter of saidl frustoconical conductor' being substantially equalto the" diameter: of'saicl 'outerTcoriductor and thelarger diameter rthereoflbeisng slightly smaller than the diameter-of: Said taperturedrh rebvsai .c n lrco dustq d e in at lea t w ma enta; waveg i e m ll r :and, larger diameters of said 'frusto-conical condukitor being so related to the diameters of said outer conductor and said aperture. that the innermost of said concentric waveguides has the higher propagation velocity.

5. An electromagnetic lens comprising, a coaxial waveguide having a hollow 'ou-ter conductor of circular section, an open end ofwhich'is flared outwardly to provide a circular aperture, and an inner conductor of circular section coaxial with said outer conductor and extending beyond the flared open end of said outer conductor, a pair of similar hollow conductive circular cylinders of substantially frusto-conical shape both flared in the same direction as the flared end of said outer conductor disposed in said aperture coaxially with said inner and outer conductors, the smaller diameter of said cylinders being substantially equal to the diameter of said outer conductor, said cylinders being spaced apart along the extending portion of said'inner conductor and from the flared open end of said outer conductor by a rigid dielectric medium having a dielectric constant greater than1.0.

6. Anelectromagnetic lens comprising, a coaxial wave- "guide having a hollow outer conductor" of circular secinner and outer conductors, the smaller ends of both said cylinders being substantially the same diameter-as said outer conductor and the larger ends of both having smaller diameters than that of the flared opened end of said outer conductor, said cylinders being spaced apart along the extending portion of saidinner conductor and separated from the flared open end of said outer conductor by a rigid dielectric material having a dielectric constant greater than '1.0, said' dielectric material extending beyond the outer diameter of said concentric cylinders and having an outer surface tapered inwardly from the periphery of the flared open end of said outer contion, an open end of which is flared outwardly to provide a circularaperture, and an inner conductor, of; circular section coaxial with said outer conductor ;and extending beyond the flared open end of said outer conductor, a pair of similar hollow conductive cylinders of substantially frusto-conical shapefboth flared in the same direction as the flared end of said outer conductor disposed in said aperture coaxially with said inner and outer condnctors, the smaller diameter of said cylinders being substantially equal to the diameter of said outer con-- ductor and the largerdiameter of said cylinders being ductor to the periphery of the larger end of the furthest extending of said cylindrical conductors;

cylinders of substantially frusto-conical shape both flared in the same direction asvthe flare on the outer end of said outer conductor disposed in said aperture coaxially with said inner and outer conductors, said cylinders being spaced apart along the extending portion of said inner conductor to divide said aperture into three concentric waveguides, the outer two of said three concentric waveguides being filled with rigid dielectric material having a dielectric constant greater than 1.0, the outer surface of said dielectric material being flared inwardly from the periphery of the flared opened end of said outerconductor to the outer periphery of the furthest extending one of said cylindrical conductors.

4. An electromagnetic lens comprising, a coaxial waveguide having a hollow outer conductor of circular secapproximately equal-to-thefdiameter of said aperture,

said cylinders being spaced apart along the extending 7' portion of said inner conductor and from the flared open end of said outer conductor by a dielectric medium having a dielectric constant greater than 1.0,' thespacing between the first of said cylinders and the flared outer end ofsaid outer conductor being greater. than the space ing between said cylinders and between the second cylini'der and said inner conductor thereby todivide' said aperture into three concentric waveguides havingsuccessively 6i) open end of which is flared outwardly to provide a cir-- 'increasing propagation velocity from the outer portion of said aperture to the innerportion of said aperture. Y

7. An antenna for the production of a wide beam cular aperture of diameter greater than the diameter of said outer conductor, and an inner conductor of circular section coaxial with said outer conductor and extending beyond the flared open end of said outer conductor, a pair of hollow conductive cylinders of substantially frustoconical shape both flared inqthe same direction as the flared open end of said outer conductor disposed in said aperture coaxially with said inner and outer conductors and spaced apart along the extending portion of said inner conductor, the smaller diameter of said cylinders being substantially equal to'the diameter of said outer conductor and the larger diameter of said cylinders being substantially equal and somewhatjsmaller than the diameter of said aperture, said cylinders being separated from each other and from the flared portion of said outer con ides he pe' aaaaoaa ductor by a rigid dielectric material extending inwardly to the cylindrical plane of said outer conductor and ex furthest extending of said cylindrieal conductors, the dielectric constant of said material being greater than 1.0, and a radiation permeable cap closing the'open end of said furthest extending cylindrical conductor.

References Cited in the file of this patent UNITED STATES PATENTS Skellett Apr. 3, 1951 Linderblad June 5, 1951 1955 Rust Sept. 25, 1956 Maybury July 30, 1957 FOREIGN PATENTS.

Great Britain Dec. 31, 1952

Images