US 3523298 A
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' `INVENTOR. John Hagen At'rorney J. P. HAGENy BICONICAL HORN AND REFLECTOR ANTENNA Filed May 31, 1968 Aug. l4, 1970 United States Patent O 3,523,298 BICONICAL HGRN AND REFLECTOR ANTENNA John P. Hagen, State College, Pa., assignor, by mesne assignments, to the United States of America as represented bythe Secretary of the Navy Filed May 31, 1968, Ser. No. 733,459 Int. Cl. H01q 19/14 U.S. Cl. 343-775 6 Claims ABSTRACT OF THE DISCLOSURE 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 orv therefor.
This invention relates to antennas and more particularly to lightweight large aperture directional antennas for use in astronomy and satellite tracking systems.
One of the major problems encountered in the design and fabrication of large aperture antennas has been that of maintaining the precise shape of all radiating and reflecting surfaces as the antenna is subjected to dynamic and static structural loading. In the most common of the large aperture antennas, the parabolic antenna, these surfaces must be accurate within a small fraction of the operational wavelength of the array. The feed structure of the paraboloid, which is usually supported from a central portion of the parabolic reflector, must also be precisely positioned with respect to the focus of the parabolic Vreector. Any displacementl of this feed from the focus will not only result in additional deformation of the reflecting surface but also decrease the antenna gain and pointing accuracy.
The antenna of the present invention is a lightweight, relatively low cost, stable structure. Its geometry is that of an extended biconical horn with a cooperating peripheral reflector which redirects the radiation emanating from the mouth of the horn in a single direction. The weightto-size ratio of the present antenna is substantially reduced because its major structural components are relatively thin metal members that are placed'under tension. This tension stabilizes their shape by preventing llexure. These metal members in one embodiment form a geometric structure resembling a rhombus revolved around its minor diagonal. To this structure we give the name double cone. The confronting surfaces of two of these double cones, when suitably spaced, form the biconical horn portion of the present invention.
The annular reflector which cooperates with this horn is also of lightweight design. As in the structural design of the double cone, stability of the shape of the reflector is achieved by tensioning the structure.
The biconical antenna is fed at its center of gravity thus eliminating not only the problems of exure found in construction of parabolic reflectors but also beam pointing discrepancies due to displacement of the feed from the focus.
3,523,298 Patented Aug. 4, 1970 It is accordingly a primary object of the present invention to provide a lightweight antenna of relatively large size whose major structural elements are fabricated from thin metal members.
Another object of the present invention is to provide a lightweight antenna wherein the shape of the critical radiating or reflecting elements are stabilized by having these elements maintained under tension.
A further object of the present invention is to provide a biconical horn antenna with an annular conical reliector to achieve a sharp unidirectional antenna pattern.
A still further object of the present invention is to provide means for directing the flow of energy through a biconical antenna in such a way that energy arriving at the coupling means of the antenna from all parts of the annular aperture will be of the same polarization.
Other objects, advantages and novel features of the invention will become apparent from the following detailed description thereof when considered in conjunction with the drawing.
The single figure shows an antenna assembly which consists of three subassernblies: double cones 1 and 2, whose confronting surfaces define a biconical horn; an annular conical reflector 3, which imparts a unidirectional beam pattern to the apparatus; and a combined internal reflector 12 and feed 15 combination. The term, double cone, hereinafter used, is intended to describe the geometric solid formed by revolving a rhombus about its minor diagonal. When two such double cones are aligned along a common axis and spaced apart, their confronting conical surfaces define a biconical antenna.
The upper double cone 1 of the antenna assembly is constructed from relatively thin metal members 6 and 9, each of which have a similar conical shape. Both cones may be prefabricated or they may be formed by joining two like circular planar sheet members together at their circumferential edges and thereafter displacing their centers an appropriate distance apart. In the latter case, a tapered rigid ring 8 of appropriate diameter is secured between the rim portions of two such sheets. The taper of this ring member determines the size of the double cone assembly.
In order to separate the centers of these sheets by controllable amounts, an extensible hub structure 13 is secured between these points. This axial movement, it will be appreciated, has the effect of placing under tension those portions of sheets 6 and 9 which are between the rigid ring member and the stationary central hub. It is this tension, as noted hereiubefore, which stabilizes the subassernbly, while at the same time permitting it to be fabricated of otherwise normally flexible members.
The hub structure of cone 1 includes an extensible strut 13 adapted to coact with pads 11 and 12 which distribute the mechanical load of sheets 6 and 9 attached thereto. Adjustment of the tension in sheets 6 and 9 is accomplished by increasing the separation between pads 11 and 12. Pad 12, in addition to providing anchoring means for sheet 6, includes a lower surface which functions as an internal reflector. Pad 12 may be threaded to receive strut 13 which when turned increases the separation between pads 11 and 12. Turning of strut 13 is accomplished by turning that portion of strut 13 which passes through and projects above pad 11. It will be appreciated that as the size of the antenna is increased other tensioning devices may be employed to form the uniform double conical surface.
The degree of regularity of the reflecting surfaces of the double cone will depend on the guage of the material, its elastic constant, its density, its extent and the force applied thereto.
Double cone 2 is similar in construction to ,cone 1. Sheets 7 and 10 are secured to ring 18 at their peripheral sides and edges and centrally to a hub consisting of hollow strut 14 slidably mounted in hollow pad 16. Here the hub is hollow to allow access of coupling means to the throat of the biconical horn. A horn 15 is mounted through the hollow hub and is secured to the inner face of hollow strut 14 to provide for coupling of electromagnetic energy into and out of the antenna.
While the ligure shows the use of mechanically actuated, variable length struts as the apparatus for developing tension in the sheets, conventional pneumatic counterparts may be employed in place of the struts. In its collapsed form, the double cone under tension will resemble two drumheads. It is possible to feed the drumheads at their centers and derive an omnidirectional radiation pattern.
Double cones 1 and 2 are separated and held in alignment along a common axis by a multiplicity of insulated posts distributed around the periphery of the horn. Alternatively, this separation may be achieved by the utilization of a cylinder constructed of a low-loss dielectric material in place of the posts. A further means for maintaining the proper separation includes the use of a toroidal lens made of a low-loss dielectric material in place of the posts.
The second subassembly, the reflector, redirects the omnidirectional output of the biconical horn into a single direction. When cones 6 and 7 are excited at their confronting apices by cone feed 15, maximum radiation is obtained at right angles to axis 5, the common axis of the double cones. The pattern of energy emanating from the mouth of this biconical radiator is omnidirectional in a plane 4 perpendicular to axis 5 and passing through the center of the array.
In the transmitting mode, an annular conical reflector 3 is positioned adjacent the mouth of the biconical radiator formed by cones 6 and 7 to reect the omnidirectional radiation emanating therefrom into a unidirectional beam 30 parallel to axis 5. This beam issues forth from an aperture which is the annulus, r, formed by the periphery of cone 6 and that part of reliector 3 adjacent this peripheral edge. Since the polarization of the wave front appearing in the annular aperture r must be maintained the same al1 over the aperture, the Wave propagating in the biconical radiator must be cylindrical. This cylindrical wave is reflected by reflector 3 and emerges from the antenna as a plane wave of one polarization. Reector 3 is itself the truncated portion of a right isosceles cone, the surface of which forms an angle of 45 with respect to plane 4. The surface of this reilector may include a metallic sheet placed under tension by supports 22 and 23 secured to base 21. The tensioning of the metallic sheet also adds stability to the array. If, however, the wave were guided from the transmitter (not shown) through the device to the aperture r, the polarization on one side of the antenna would oppose that on the other side and a null would appear in the direction of propagation rather than the desired maximum. This diiculty is obviated by the third subassembly, the coupling retiector and horn, 12 and 15.
Horn 15 establishes a plane wave front at its opening or mouth 25. This plane wave propagates out of horn 15 and is reflected by the lower portion-of pad 12 which extends to, but not into, mouth 25. This lower portion is metal coated and is in lthe form of a right isosceles cone. If the apex of cone 12 extends into mouth 25, a plane Wave is prevented from forming. If this apex is spaced from the mouth, maximum rellection of the plane wave will not be achieved. After reflection by the right isosceles cone, the Iplane wave propagates down the biconical radiator as the required cylindrical wave.
In the receiving mode, reflector 3 directs an incoming plane wave 30 towards the center of the array where it is reected by the right isosceles cone. The polarization of the incoming Wave is preserved as it is reconstructed at opening 25 of horn 15 by conical pad 12.
The beamwidth of the antenna will be determined by the outside diameter of the array. This beamwidth is given by -where 0=beamwidth in degrees D=the aperture ofthe antenna Azoperational wavelength of the antenna.
The amount of energy in the minor lobes of the antenna pattern will be alfected by the width of the annular aperture r. The gain of the antenna will be proportional to the area of this aperture. For large aperture antennas this gain G is given by where G=the gain over an isotropic radiator A=area of the aperture =operational wavelength of the antenna.
The antenna thus constructed may be mounted and gimballed in the conventional manner. Trunnions 24, mounted on base member 21, are provided to receive a yoke 26 for this purpose. Because of the lightweight stable construction, tortional loading produced by steering or the encountering of wind will be minimized and Iiexure of the radiating surfaces reduced.
It will -be appreciated that an alternate method of constructing the metallic double cones lies in the substitution of metallic spokes (not shown) secured to the central hub and the peripheral ring for the thin metal members. An array thus constructed, in addition to having a further reduced weight-to-size ratio, also reduces the elfect of wind on the array.
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.
What is claimed is:
1. An antenna comprising:
a pair of hollow, double cones in spaced relationship such that their confronting surfaces form a biconical horn assembly,
the conical surfaces of each of said pair of double cones being constructed from relatively thin, lightweight material;
means for placing each of said double cones under tension such that the conical surfaces of said cones are made rigid and resist deformation, the throat portion of said horn assembly corresponding to the confronting apices of said spaced double cones; and
means for coupling electromagnetic energy into and from said throat portion.
2. An antenna comprising:
a pair of double cones in spaced relationship such that their confronting surfaces form a biconical horn assembly, the throat portion of said horn assembly corresponding to the locus of the confronting apices of said spaced double cones; and
means for coupling electromagnetic energy into and from said throat portion, said coupling means including a right isosceles cone secured to and centered on one of said confronting surfaces and means for forming a plane wave at the center of the other of said confronting surfaces, such that plane waves formed at the center of said other surface are reflected by said right isosceles cone outwardly through said biconical horn.
3. The antenna as recited in claim 2 wherein said plane wave forming means is a conical waveguide positioned so that its mouth projects through said other confronting surface into said throat such that the center of said mouth lies at the apex of said right isosceles cone.
4. The antenna as recited in claim 2 further comprising means located at the open end of said biconical horn for reflecting radiation propagating outwardly therefrom parallel to the axis of alignment of said double cones and for reecting radiation incident on said reflecting surface to said coupling means.
5. The antenna as recited in claim 4 wherein said reflecting means is a truncated right isosceles conical surface under tension positioned such that its edge of smaller circumference coincides with the peripheral edge of one of said double cones and such that the portion of the conical surface adjacent the peripheral edge of the other of said double cones is spaced therefrom.
6. In a biconical antenna the combination of:
a pair of axially aligned metallic double cones,
each of said double cones comprising two circular plates of equal diameter joined at their peripheral edges, a rigid ring attached to said joined edges and means adjustable in length and coacting with the centers of said plates to separate them so as to form a double conical 5 surface under tension;
means for spacing said double cones such that their adjacent surfaces form a biconical horn; and means at the center of said horn to couple electromag- 10 netic energy into and out of said antenna.
References Cited UNITED STATES PATENTS 2,471,021 5/ 1949 Bradley 343-774 15 2,532,551 12/1950 Jarvis 343-774 ELI LIEBERMAN, P rimaryExaminer U.S. Cl. X.R.