US3414903A - Antenna system with dielectric horn structure interposed between the source and lens - Google Patents

Description

Dec. 3, 1968 H. E. BARTLETT ET AL 3,414,903

ANTENNA SYSTEM WITH DIELECTRIC HORN STRUCTURE INTERPOSED BETWEEN THE SOURCE AND LENS Filed March 10, 1965 IFIGJ.

REFLECTED RAY DUE To PRESENCE of euaoms STRUCTURE LENS PHASE CENTER of LAUNCHER '5 FIG. 3

FIG.4 RESULTANT AMPLITUDE I DISTRIBUTION GUIDE BASE 0|AMETER-l ENERGY THAT WOULD BE LOST IN SPILLOVER LOBES INVENTORS m ABSENCE of DIELECTRIC GU|DE HOMER E.- BARTLETT 8i LEROY PIETSCH B Y Z ATTORNEYS LAUNCHER PATTERN ANTENNA SYSTEM WITH DIELECTRIC HORN STRUCTURE INTERPOSED BETWEEN THE SOURCE AND LENS Homer E. Bartlett, Melbourne, and Le Roy Pietsch, Palm Bay, Fla., assignors to Radiation Incorporated, Melbourne, F la., a corporation of Florida Filed Mar. 10, 1965, Ser. No. 438,582 7 Claims. (Cl. 343753) ABSTRACT OF THE DISCLOSURE An antenna system comprising a source of electromagnetic waves, a lens, and a dielectric horn structure interposed between the'source and lens. The first null of the radiation pattern produced by the source is provided at an angle approximately equal to the sum of the guiding structure taper angle and the complement of the critical angle of the structure dielectric, the critical angle of the structure dielectric being the angle of incidence of internal electromagnetic waves on the boundary of the structure above which total reflection of the incident wave is achieved. Whereby all of the energy in the main lobe of the radiation pattern is confined interiorly of the guiding structure and directed toward the lens.

The present invention relates generally to antenna systems, and more particularly to high aperture efliciency dielectric antennas.

In prior art antenna systems which have been employed to produce a highly directive radiation pattern, such as reflector antennas and lens antennas, numerous proposals have been made for improving aperture efiiciency to the maximum theoretical efliciency of 100 percent. In lens and reflector antennas, the problem'is aggravated by the lossof energy radiated from the feed or exciter because of spillover radiation; that is, that portion of the radiated energy which fails to strike {the lens or reflector. While lens antennas are generallyjsuperior to reflector antennas from a noise temperature standpoint in that the spillover lobes of the former are in a forward direction, nevertheless the directivity characteristics, and hence aperture efliciency, fall significantly below the maximum attainable values for aperture-type antennas. This is a result of the radiation losses caused byfthe nonincidence of the radiated waves on the lens. Hence, efficiency and noise temperature improvements can be obtained by reducing the forward spillover.

In accordance with an embodiment of the present invention, an antenna system is provided which comprises an electromagnetic wave transducer, such as an exciter, a dielectric guiding structure, and a lens. The dielectric guiding structure is interposed between the wave transducer and lens, and is provided with an increasing taper. The dimensions of the source of radiation, or electromagnetic wave transducer, may be predetermined to provide a first null of the radiation pattern at an angle approximately equal to the sum of the guiding structure taper angle and the complement of the critical angle of the structure dielectric. Portions of those waves which are radiated from the transducer at angles between the taper angle and the sum of the taper angle and the critical angle complement, and would not otherwise be incident on the lens in the absence of the guiding structure, strike the boundary between the guiding structure and free space at an angle greater than the critical angle of the boundary, and are totally reflected to the lens, thus significantly reducing spillover. In addition to reduced spillover, the amplitude distribution across the base becomes more ited States Patent nearly uniform with the attendant efficiency increase. The lens itself corrects the phase distribution existing across the base of the guiding structure such that a constant phase distribution is provided thereby.

It is, accordingly, a primary object of the present invention to provide an antenna system having high aperture efiiciency.

It is another object of the present invention to provide a lens antenna having a dielectric guiding structure to reduce spillover.

It is still another object of the present invention to provide an aperture-type antenna having substantially uniform amplitude distribution and constant phase distribution of electromagnetic energy across the aperture.

The above and still further objects, features and attendant advantages of the present invention will become apparent from a consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a perspective view of an exemplary dielectric guiding structure in accordance with the present invention;

FIGURE 2 is an exploded planar diagram of an antenna system employing the dielectric guiding structure of FIGURE 1;

FIGURE 3 illustrates the preferred radiation distribution pattern of an exciter employed in the antenna system of FIGURE 2; and

FIGURE 4 illustrates the resultant amplitude distribution across the base of the dielectric guiding structure.

Referring now to the drawings, FIGURE 1 illustrates an exemplary dielectric guiding structure which may be employed in antenna systems such as will hereinafter be described. In its exemplary form the dielectric medium 10 comprises a solid conical structure for guiding energy radiated from an exciter in the direction of increasing diameter toward a lens (as shown in FIGURE 2). The specific structure of the dielectric guide will, of course, depend upon the shape and dimensions of the associated antenna elements; that is, the electromagnetic wave transducer (exciter or receiving element) and the lens. The guide may readily be constructed of any dielectric material, such as Styrofoam, cross-linked polystyrene (e.g. Rexolite 1422), or artificial dielectric material capable of providing the desired critical angle for a particular application.

The operation of the dielectric antenna system may be best understood by reference to FIGURE 2, wherein heuristic ray tracing theory is employed for purposes of analysis, and wherein is shown, in planar diagrammatic form, an exploded view of the antenna system. An electromagnetic transducer 12, such as a horn, is suitably coupled to a waveguide 13 or other appropriate signal transmission line depending on the particular transducer. A dielectric guiding structure 10 of the type shown in FIG- URE l is inserted into the mouth of horn 12, and a phase correcting lens 15 is disposed adjacent the base of the guiding structure.

Exciter 12 provides a means for transferring electromagnetic energy from the input transmission line, such as waveguide 13, to dielectric guiding structure 10 and may comprise, for example, a metal horn, as shown, a dielectric rod or tube antenna, a helix, a log periodic array, or some other small radiating or receiving element.

The dielectric guiding structure is tapered or flared at an angle on the order of the complement of the critical angle of the particular dielectric employed. As used here, the term critical angle has its usual definition; that is, the angle defining total internal reflection, or the angle marking the dividing point between which a ray incident on the boundary between two dielectric media will either be totally reflected therefrom or totally or partially transmitted therethrough. Total internal reflection will occur for rays incident on the boundary at any angle greater than the critical angle of the particular dielectric material in which the ray is travelling.

The lens 15 is employed to modify the phase distribution of the electromagnetic waves appearing across the base of the guiding structure to provide a constant phase distribution for waves emanating from the system. The lens may be of conventional design, such as a dielectric lens for delaying the electromagnetic wave in accordance with its dielectric constant to convert a spherical wave front into a plane wave front. The focal point 17 of lens 15 is positioned in the dielectric medium of the guiding structure such that, when the antenna components are assembled, it is at the phase center of the exciter. In the event that the lens is fabricated of a dielectric material, its dielectric constant may be selected to be greater than that of guide in order to minimize length and weight, but this is not critical for proper operation.

When the exciter dimensions are so chosen that the first null of the radiation pattern occurs at an angle approximately equal to the sum of the guiding structure taper angle and the complement of the critical angle of the dielectric, an almost uniform amplitude distribution is obtained across the lens (FIGURE 4). In addition, the transfer of energy from exciter 12 to the base of the dielectric structure is accomplished in an extremely efficient manner with an unusually small amount of energy lost in spillover past the lens, resulting in a high directivity for the resultant radiation pattern.

For purposes of explaining the theory of operation of antenna systems in accordance with the present invention, resort is had to the heuristic ray tracing convention. Briefly, this convention consists of drawing vectors perpendicular to surfaces of equal phase and applying to the vectors or rays the optical laws governing light rays. The technique is especially appropriate where antenna dimensions are large in terms of wave lengths of the electromagnetic energy involved. Reference will also be made to the radiation pattern and resultant amplitude distribution illustrated in FIGURES 3 and 4, respectively. As shown in FIGURE 2, those rays striking the guiding structure boundary at an angle greater than the critical angle 3 of the guide dielectric material are totally reflected toward, and are thus incident on the lens. As is well known,

,=arc sin 1/\/s, where e is the dielectric constant of the guide. Such rays would, in the absence of the guiding structure, fail to strike the lens and would represent energy loss in the form of spillover lobes. The guiding structure flare -or taper angle is selected to be on the order of the complement of the critical angle, i.e. -90 and the electromagnetic transducer is designed to provide a first null of the radiation pattern at an angle equal to the sum +(90 to provide an almost uniform amplitude distribution (FIGURE 4) across the lens with very little loss caused by spillover radiation. The result is an aperture efficiency for the antenna system approaching 100 percent.

The darkened portion of the exciter pattern in FIG- URE 3 represents that portion of the energy radiated by the exciter which would normally be lost in spillover lobes in the absence of the' guiding structure. As illustrated in FIGURE 3, the first null of the radiation pattern occurs at approximately an angle of twice the flare angle of the structure.

The amplitude distribution at the base of the dielectric guiding structure as illustrated in FIGURE 4 is the result of the summation of waves incident on the lens in a direct path from the exciter and waves reflected from the boundary of the guiding structure toward the lens.

In practice, any lens may be employed which is capable of converting the spherical wavefront emanating from the exciter to a plane wavefront to produce the desired highly directive antenna system. In one antenna.

system, built in accordance with principles of the present invention as discussed herein, the dielectric guiding structure was a solid cone having a dielectric constant e of 1.02, a base diameter of approximately 18 inches, and a flare or taper angle of approximately 6.5 degrees which was substantially the complement of the structure dielectric critical angle. The antenna system yielded a measured gain of 33 db at 9.4 gc., corresponding to percent aperture efliciency. In a second practical embodiment, the dielectric guiding structure again 'had a dielectric constant of 1.02, a base diameter of approximately 18.5 inches and a flare or taper angle approximately equal to twice the complement of the critical angle or about 15 degrees. In the latter system, the measured gain was 30.8 db at 8 gc. corresponding to about 77 percent aperture efficiency. In other embodiments, the taper angle of the dielectric structure was varied between one-third and three times the critical angle of the dielectric with suitable results.

Wave polarization is not critical in antenna systems according to the present invention, and may thus be of any conventional form, for example, linear, circular, and so forth, as dictated by the exigencies of the specific use or application.

While I have described one particular embodiment of my invention, it will be understood that various changes and modifications in the specific details of construction and operation described may be resorted to without departing from the true spirit and scope of the invention as defined by the appended claims.

I claim:

1. In an antenna system for translating electromagnetic wave energy between a signal line and free space, an electromagnetic wave transducer coupled to said transmission line, lens means for refraction and phase correction of said electromagnetic wave energy, said lens means having a focal point in proximity to the phase center of said transducer, and dielectric guiding means for establishing a boundary between the dielectric medium thereof and free space to substantially confine the propagation of electromagnetic waves between said transducer and said lens means interiorly of said boundary, wherein said dielectric guiding means comprises a solid dielectric mass having a monotonically increasing cross-sectional area from said transducer to said lens, said guiding means having an angle of taper at least approximately equal to the complement of the critical angle of said boundary, where said critical angle is the angle of incidence of said electromagnetic waves on said boundary above which total reflection of the incident wave obtains.

2. A dielectric antenna comprising a source of electromagnetic radiation, a lens for correction of the phase distribution of and for refraction of electromagnetic waves incident thereon from said source, and dielectric wave guide means interposed between said source and said lens for directing electromagnetic waves emanating from said source toward said lens and for inhibiting the escape of electromagnetic wave energy into free space from the region between said source and said lens;

wherein said dielectric waveguide means comprises a solid dielectric structure having a dielectric constant relative to the dielectric constant of free space defining a critical angle at the boundary therebetween such that electromagnetic waves incident on said boundary from within said structure at an angle greater than said critical angle are reflected therefrom to strike said lens;

wherien said dielectric structure has a conical configuration, said lens being disposed at the larger diameter end of said structure and having a focal point within said structure corresponding to the phase center of said source, said conical configuration having an angle of taper of between approximately one-third to three times the complement of the critical angle of said boundary.

3. A dielectric antenna comprising a source of electromagnetic radiation, a lens for correction of the phase distribution of and for refraction of electromagnetic Waves incident thereon from said source, and dielectric Waveguide means interposed between said source and said lens for directing electromagnetic waves emanating from said source toward said lens and for inhibiting the escape of electromagnetic wave energy into free space from the region between said source and said lens,

wherein said dielectric guide means comprises a solid dielectric structure having a dielectric constant relative to the dielectric constant of free space defining a critical angle at the boundary therebetween such that electromagnetic waves incident on said boundary from within said structure at an angle greater than said critical angle are reflected therefrom to strike said lens,

wherein said dielectric structure has a conical configuration, said lens being disposed at the larger diameter end of said structure and having a focal point Within said structure corresponding to the phase center of said source, said conical configuration having an angle of taper of between approximately onethird to three times the complement of the critical angle of said boundary,

wherein said lens comprises a solid dielectric medium having a convex surface upon which said electromagnetic waves emanating from said source are incident, and having a dielectric constant greater than said dielectric constant of said dielectric structure for convert-ing a spherical wave front to a plane wave front.'

4. In an antenna system for translating electromagnetic wave energy between the signal transmission line and free space:

source means coupled to said transmission line for directing electromagnetic wave energy in a given direction and in a radiation pattern having at least a first radiation null at a predetermined angle relative to said given direction;

lens means for correcting the phase distribution of and for retracting the electromagnetic wave energy generated by said source means; and

' solely dielectric waveguide means interposed between said source means and said lens means for interiorly confining all of the electromagnetic wave energy generated by said source within said predetermined angle, said dielectric waveguide means comprising a dielectric mass having a monotonically increasing cross-sectional area in proceeding from said source means to said lens means, and having an angle of taper approximately equal to the complement of the critical angle at the boundary of said dielectric waveguide means,

said critical angle being the angle of incidence of internal electromagnetic waves on said boundary above which total reflection of the incident wave occurs.

5. In an antenna system for translating electromagnetic wave energy between the signal transmission line and free space:

source means coupled to said transmission line for directing electromagnetic wave energy in a given direc- 5 tion and in a radiation pattern having at least a first radiation null at a predetermined angle relative to said given direction;

lens means for correcting the phase distribution of and for refracting the electromagnetic wave energy generated by said source means; and v solely dielectric Waveguide means interposed between said source means and said lens means for interiorly confining all of the electromagnetic wave energy generated by said source within said predetermined angle, said dielectric waveguide means comprising a dielectric mass having a conical configuration with a monotonically increasing cross-sectional area' in proceeding from said source means to said lens means, and having an angle of taper approximately equal to the complement of the critical angle atthe boundary'of said dielectric waveguide means,

said critical angle being the angle of incidence of internal electromagnetic waves on said boundary above which total reflection of the incident wave occurs,

said lens means having a focal point within said dielectric mass corresponding to the phase center of said source means.

6. In combination:

an electromagnetic wave launcher,

lens means positioned relative to said launcher to modify the phase distribution of electromagnetic waves supplied thereto from said launcher,

a tapered solid dielectric waveguide extending in in= creasing girth from said electromagnetic wave launcher to said lens and coupled to said wave launcher to receive electromagnetic wave energy therefrom said waveguide having a dielectric constant substantially greater than one, and having an angle of taper approximately equal to the complement of the critical angle for totalinternal reflection of electromagnetic waves emanating from said launcher from the boundary formed by the tapered surface of said waveguide.

7. The combination according to claim 6 wherein said launcher has a phase center at the focal point of said lens means.

.' References Cited UNITED STATES PATENTS 2,596,190 5/1952 Wiley 343 7s3 2,822,541 2/1958 Sichak e161 343-786X 3,005,983 10/1961 Chandler 943-5753 3,321,763 5/1967 1mm et al. 343454 FOREIGN PATENTS 9/1953 Germany.

4/ 1960 Germany. 11/1963 Canada.

ELI LIEBERMAN, Primary Examiner,

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