US2750588A

US2750588A - Wave guide terminating device

Info

Publication number: US2750588A
Application number: US344915A
Authority: US
Inventors: Frank L Hennessey
Original assignee: Individual
Current assignee: Individual
Priority date: 1953-03-26
Filing date: 1953-03-26
Publication date: 1956-06-12
Anticipated expiration: 1973-06-12

Description

J1me 1956 F. L. HENNESSEY WAVE GUIDE TERMINATING DEVICE Filed March 26, 1953 TOR FRANK L. H ENNESSEY ATTORNEY-s United States Patent WAVE GUIDE TERMINATING DEVICE Frankv L. Hennessey, Washingtoml). C., assignor to the United States of America as represented by the Secrelaw of. the Navy Application March 26, 1953', SerialNo. 344,915

6 Claims. (Cl. 343-781) (Granted under Title35, U. S. Code (1952), see. 266) This invention relates to broad band feed systems for microwave antennas utilizing beam concentration reflecting surfaces, and more particularly to terminations for such feedsystems.

More specifically this invention concerns itself with the provision of a vertex feed system for microwave antenna systems that utilize a focal-fed-parabolic reflecting surface. This type of antenna feed system generally comprisesa straight section of waveguide projecting'through the vertex of the reflector and terminated at the focus of the reflector by a radiating'element that directs the energy back to the reflector. Such a system is shown and described in the test Principles of Radar, by Reintijis and C'oate (McGraw-Hili), pp. 956957, wherein a rectangular'waveguide is shown terminated in a so-called' Cutler feed.

When it is desired to rotate the beam produced by such an antenna in a narrow cone in space by rotating the focal endof thewaveguide in a small circle about the focus, such as shown in Patent No. 2,617,029 to Plummet et al., the use of a waveguide of circular cross-section in a vertex feed system is peculiarly advantageous. The circular waveguide may be readily rotated while mainraining a fixed orientation of polarization so that it is only necessary to cock the guide slightly off' the axis. of the reflector near the focus to achieve the desired conical beam. The feed systems that have been developed inthe past are generally restricted to use at a givenv frequency or at best over a narrow band of frequencies. Reflections back into the waveguide that produce a large mismatch and loss of power have invariably resulted when the operating frequency is only slightly removed from the design frequency.

One object of this invention is to provide-an improved vertex feed system for use with afocal-fed reflecting surface wherein a circular waveguide projects through the vertex of the reflecting surface.

Another object of this invention is to provide such a vertex feed system suitable for use over a wide range of frequencies.

A still further object of this invention is to provide a vertexfeed system for use with a focal-fed reflector, producing essentially a point source of radiation to thereflector.

Other objects and features of the present inventionwill become apparent upon consideration of the following;

illustration only. and not as a definition of the limits of the invention, reference for the latter purpose being. had

to the appended claims.

Figure l is a perspective view of an antenna system suitable for practicing the present invention.

" Figures 1a and 1b show side sectional and end elevaducting surface may be described as being a figure of lice.

2 tion views respectively of one embodimentof the present invention.

Figure 1c shows a partial perspective view of a modification of the embodiment of Figures la and 1b resulting in a more simplified construction.

Figure 2a shows an exploded view of another embodiment of the present invention suitable for use when. a sharper point source is desired. Figure 2b is an end view of this embodiment;

Figure 3 is a perspective view of still another embodi ment of the invention that is useful where it is desired both to rotate the feed and to decrease the size of the point source of radiation to the paraboloid.

Briefly the antenna feed structure of the present invention comprises a conducting surface of unique efformation disposed at the focal end of the feed'waveguide so as to intercept and split the wave emanating from the guide, and to redirect it around the terminating edges of, the guide back to the reflector. In more particular the C011? revolution generated by rotating a semi-circle about an axis which is coaxial with the waveguides axis. and which is perpendicular to a line connecting the ends'of the semicircle at, or very near, one end thereof. Such a figure ofrevolution may be further defined as semi-toroidal in shape containing an apex-like protuberance at the axis of revolution. The apex of the generated conducting surface is positioned on the longitudinal axis of the guide so that the annular concave conducting surface of the semi-toroid is disposed aboutv the end of the guide and is'facing the antenna reflector. The apex. of the conducting-surface is thus placed in a region where the magnetic flux of electromagnetic radiation of transverse electric mode of propagation emanating from the guide is very weak. A linearly polarized wave of TE1 modehas a concentration of electric lines of force at the axis of the guide, which lines aredivided by the conducting surface as the waves progress out of. the guide, terrmnatingthereafter on the annular concave face of the semirtoroidal conducting surface and on the edge of the guide untilthe energy is launched into space toward' the. antenna re.- flector. When the conducting surface isappropriat'ely. spaced from the end of the guide, it has beenfound that the wave propagation through the guide is not disturbed, and that electromagnetic energy isdirected toward the antenna reflector with an impedance match that. will give unexpectedly small standingv wave ratios over a Wide variation in operating frequencies. Furthermore it has been discovered that for optimum operation the radius of the semi-circle of revolution above-described. must be. very nearly equal to the radius of the waveguide. While satisfactory operation may be obtained when the ratio of the radius ofthe semi-circleof revolution. to the radius of the guide is other than very nearly unity, there. will be a decrease in the frequency spectrum over which a. given maximum standing wave ratio can be obtained.

Additionally, it has been found that a circularlypolarized wave may be redirected toward, the antenna: reflector, with a standing wave ratio equally low asv that obtained. with linearly polarized waves. The reason for, this is that orthogonally polarized modes may be readily re-. directed by the feed system because of the axial sym: metry of the conducting surface. Inasmuch as a circu-. larly polarized wave-may be considered to be the sum of two orthogonal linearly polarized waves, circular.

of paraboloidal reflector 1 is mission of electromagnetic energy from a conventional external source. The waveguide is terminated at the focus of the paraboloid by the conducting surface provided by the present invention and indicated in general at 117.

With reference now to Figure in, there is shown in detail one embodiment of the present invention wherein components corresponding to those of Figure I bear corresponding identifying numerals. Circular waveguide 101 is of three distinct sections, two

cylindrical sections

103 and 107 being joined by a truncated conical section 105, cylindrical section 103 having a larger inner diameter than cylindrical section 107. Fitted within waveguide section 107 is a dielectric plug member 109, typically made of polystyrene. The outer diameter of this member is the same as the inner diameter of waveguide section 107 along the length of waveguide section 107, but tapers to a point 111 within conical section 105. At the end of dielectric member 109 opposite the tapered point 111 is an end section 113 of rather critical dimensions. End section 113 is of semi-toroidal shape, the radius of curvature of the surface of the toroid being approximately half the outer diameter D of the waveguide section 107. As has been previously indicated, the semi-toroidal surface may be considered to have been generated by revolving around the axis of the guide a semi-circle of diameter A tangent to said axis so as to form an apex thereat. Fitted snugly over end section 113 is a metal cap 117, of Monel metal or any other metal that is a good electrical conductor. It should be noted that waveguide 101, dielectric member 109 and metal cap 117 are distinct members, and may be readily disassembled each from the other.

The outer dimensions of cap 117 are not particularly critical but the inner curved dimensions are quite critical; for optimum results the cap must fit very snugly over dielectric end section 113 and its central point, or apex, 115 must be very exactly positioned on the axis of waveguide section 107. The diameter A of the curved sur face of metal cap 117 and dielectric end section 113 (twice the radius of curvature of the toroidal surface of end section 113) need not be exactly the same as the outer diameter of waveguide section 107, but it has been found that this relationship gives optimum results. For every dimension of diameter A there is a distance L between the tip of the apex 115 of metal cap 117 and the transverse plane of terminating edge 121 of the waveguide that will give optimum results as described below. This dimension L" must be determined experimentally for every metal cap by axially moving dielectric member 109 within guide 107 until the best impedance match is obtained. Generally, as the ratio of A to D is increased, it has been found that the apex 115 should project farther into the waveguide 107. Typically at an operating frequency of 9,375 mc., a distance L of .043 inch has been found satisfactory for a cap having a radius of curvature of .33 inch, and a major or outer diameter B of 1.345 inches, using a waveguide of .685 inch outer diameter and .05 inch wall thickness. It has been found that a difference of 7% between A and D, or of .02 inch in the axial positioning of a cap of the dimensions given above is enough to produce a decided adverse eifect on the impedance match.

Using a waveguide and a, metal cap of the dimensions and relative positioning given above, a standing wave ratio of less than 1.4 was obtained over a frequency range of about 7800 to about 10,800 mc. Prior art devices could be operated with a standing wave ratio of less than 1.4 only over a 3% frequency range, typically of 9150 mc. to 9450 me.

The function of the dielectric member 109 is primarily to provide both mechanical support for metal cap 117 and a means to axially position said metal cap 117. It

a waveguide 101 for transshould be noted however that another useful purpose is served by member 109 in that its presence in the feed guide 101 permits a reduction in the permissible diameter of the waveguide section 107 where the dielectric member is positioned over the diameter of guide section 103 where an air dielectric is employed. The significance of this reduction in diameters is that a point source of radiation is more nearly achieved at the focus of the reflector. The tapered section 111 of the dielectric plug 109 and conical waveguide section 105 are utilized to provide a good impedance match between

waveguide sections

103 and 107. Dielectric member 109 including end section 113 may be eliminated if desired so long as other means are provided for positioning the metal cap 117 which do not adversely affect the propagation of the radiation.

The operation of the embodiment of Figure 1 is believed to be as follows. Assume that a vertically polarized TEu wave is being propagated in the waveguide. Upon reaching the point of the metal cap 117, electric lines of force of the wave will be progressively divided by the metal cap starting at the tip 115. Effectively, the wave is then transmitted around terminating edge 121, the electric lines of force being terminated on edge 121 and on the interior of the toroidal conducting surface of the metal cap 117. It has been found that the waves are divided and swung around terminating edge 121 in such a manner as to be almost entirely redirected toward the paraboloidal reflector.

The metal cap 117 may be eliminated if desired by coating the exterior surface of dielectric end section 113 with a metal film. Such a device is shown in Figure 10, wherein numeral 123 denotes the metal film or coating over the end section 113. Care must be taken to insure that the conductive coating goes to the tip of cavity 125 so that the waves will be properly divided and redirected. The operation of the device is the same as described above for Figure 1a, conductive coating 123 performing the function of the concave, interior surface of metal cap 117.

It should be noted that apex 115 need not come to a sharp point, but may be blunted somewhat. In the specific example given above it will be noted that there is .025 inch difference between the outer diameter B of the toroidal surface and the diameter A of the figure of revolution. This difference is the diameter of the blunted apex.

It has been found that if linearly polarized waves are used a point source of radiation may be more nearly achieved and the antenna gain considerably increased by cutting away part of the end section 113 and metal cap 117 in the manner shown in Figure 2. For satisfactory results, the cut-away sections have a radius of curvature the same as that of the outside diameter B of dielectric end section 113, but this dimension is not critical. The. radii of curvature are centered so as to cut only to the outer surface of waveguide section 107. For correct operation using this feed, it is necessary to polarize the radiation along the major axis of the resulting end-section, as shown by polarization indicating arrows 133. Under this condition the section of the full feed of Figure 1a that is cut-away in the cut feed of Figure 2 would have little effect on the incident wave.

The spoked cap embodiment shown in Figure 3 combines the better features of the cut feed of Figure 2 with the ability of the full feed of Figure l to transmit radiatron of any polarity. In this embodiment the metal cap 117 comprises a solid base member 129 snug-fitted ever end section 113 and having a diameter essentially equal to the outer diameter of waveguide section 107. Uniformly radially extending from the base member 129 are a number of spokes or fins 131 snug-fitting on the outer portion of end section 113. spokes nearly perpendicular to the E-vector have little eifect on this incident wave, only the spokes more nearly It has been found that the parallel to the electric field direction through the axis of the guide being effective to redirect the radiation.

Although the embodiments disclosed in the preceding specification are preferred, other modifications Will be apparent to those skilled in the art which do not depart from the scope of the broadest aspect of the present invention.

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 thereof or therefor.

What is claimed is:

1. In a vertex feed system for an antenna having a beam concentrating reflector and a section of tubular waveguide projecting through the vertex thereof to the focus thereof; a conductive member disposed at the end of said waveguide, said member having a concave surface facing the reflector, said surface being generated as a figure of revolution formed by revolving a semi-circle about an axis perpendicular to the diameter of said semicircle and coaxial with the waveguide axis thereby to form an apex on said axis; and adjustable positioning means operable to position the apex of said surface along the axis of said waveguide at the focal end thereof with said apex pointing toward said beam concentrating reflector.

2. In a vertex feed system for an antenna having a beam concentrating reflector and a section of tubular waveguide projecting through the vertex thereof to the focus thereof; a conductive member disposed at the end of said Waveguide, said member having a concave surface facing the reflector, said surface being generated as a figure of revolution formed by revolving a semi-circle about an axis perpendicular to the diameter of the semicircle and coaxial with the axis of the waveguide thereby to form an apex at said axis, said conducting member being symmetrically cut away on two opposite sides thereof up to the projection of said waveguide on said conducting surface, and adjustable positioning means operable to position the apex of said surface along the axis of said waveguide at the focal end thereof with said apex pointing toward said reflector.

3. A termination for a tubular waveguide for redirecting electromagnetic radiation around an end thereof comprising a conductive reflecting surface having a concave reflective surface configuration substantially that of a surface of revolution derived by rotating a semi-circle about an axis tangential to said semi-circle at a diametric end thereof forming an apex at the said axis, and means operative to position said reflecting surface at the end of said waveguide symmetrically with respect to the axis thereof with said apex pointing toward said Waveguide.

4. A termination for a tubular waveguide for redirecting electromagnetic radiation around an end thereof comprising a conductive reflecting surface having a concave reflective surface configuration substantially that of a surface of revolution derived by rotating a semi-circle about an axis tangential to said semi-circle at a diametric end thereof forming an apex at the said axis.

5. In a vertex feed system for an antenna having a beam concentrating reflector and a section of tubular Waveguide projecting through the vertex of the reflector to the focus thereof; a conductive member disposed at the end of said waveguide, said member having a concave surface facing the reflector, said surface being generated as a figure of revolution formed by revolving a semicircle having a diameter substantially equal to the outer diameter of said waveguide about an axis perpendicular to the diameter of said semi-circle and coaxial with the waveguide axis thereby to form an apex on said axis; and adjustable positioning means operable to position the apex of said surface along the axis of said waveguide at the focal end thereof with said apex pointing toward said beam concentrating reflector.

6. The feed system described in claim 1 wherein the said conductive member is a finular member with the fins thereof radially extending from the waveguide axis and said fins are individually shaped to collectively form the described surface configuration.

References Cited in the file of this patent UNITED STATES PATENTS 2,206,923 Southworth July 9, 1940 2,483,575 Cutler Oct. 4, 1949 2,505,424 Moseley Apr. 25, 1950 2,535,331 Swarts Dec. 26, 1950 2,617,029 Plummer et al. Nov. 4, 1952