US2530098A - Antenna - Google Patents

Description

Nov. 14, 1950 L, c. VAN ATTA 2,530,098

' ANTENNA Filed May 3, 1945 FIG.2 F|G.3

22 I l 27 l42625 .F|G.4 FIG.5.

ATTORNEY l atented Nov. 14, 1950 ANTENNA Lester C. Van Atta, Winchester, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application May 3, 1945, Serial No. 591,743

1 Claim.

This invention relates to antennas for use in high frequency communication systems. It is particularly adapted to antenna systems incorporating radiating elements and reflecting surfaces such as dipoles and parabolic reflectors.

While a smooth, continuous metallic surface is the simplest and usually the best form of microwave reflector from the standpoint of effleient reflection, there are several applications for which some sacrifice of reflector efliciency is justified and for which a partially open reflector in the form of a grating, screen or perforated sheet, according to this invention, will have advantages. Such advantages include:

(1) Reduced wind resistance;

(2) Reduced weight/strength factor;

(3) Increased ease of fabrication and assemy;

(4) Visual transparency; and

(5) Adjustability of reflector shape.

It is one of the objects of the present invention to provide an antenna having a partially open reflector particularly a grating type reflector.

It is another object of the invention to provide an antenna having a reflector whose characteristics are low wind resistance and lightness.

Another object of the invention is to provide an antenna having a parabolic reflector, a radiating element for illuminating the same, and an auxiliary partially open reflector for directing the energy reflected by the parabolic reflector in a divergent path.

Other novel features and advantages of the invention will become apparent as the description proceeds.

In the drawings:

Fig. l is a diagrammatic front elevational view of a partially open reflector according to the invention;

Fig. 2 is a diagrammatic side elevation of an antenna having a reflector according to one embodiment of the invention and the mounting for such antenna;

Fig. 3-is a diagrammatic side elevation of an antenna having a different reflector arrangement according to another'embodiment of the invention; 6

Figs. 4 and 5 are side and front elevations illustrating still another embodiment of the rerange and well above ground or other obstructions for clearest azimuthal view. A mast mounting for the antenna satisfies both conditions on land or on ships. In antennas, particularly those mounted high above the ground or ships deck, an important factor is the wind resistance and lightness of the reflector. As mentioned hereinbefore a partially open reflector is characterized by having the advantages set forth. The following description illustrates by way of example several applications of a partially open reflector according to this invention.

Referring now to Fig. 1, there is shown a reflector 10 for an antenna. Reflector [0 may have a reflecting surface ll formed as a flat plane member or as' a parabolic, hyperbolic or any other suitably shaped surface. The front view configuration of reflector Ii] may be circular, elliptical, rectangular or of any suitable shape.

In conventional antenna systems embodying a reflector it is Well known to form the reflecting surface of a thin conducting sheet of metal or other suitable electrically conductive material. However, according to this invention reflector I0 is formed as a partially open reflector, at least a portion of the reflecting'surface being in the form of a grating, screen or perforated sheet.

For the present description the reflecting sur-.

face ll is shown as preferably being in the form of a grating. Reflecting surface II, or at least that portion formed as a grating, comprises a plurality of uniformly spaced parallel bars, rods 0r strips l2, the forward edges of bars l2 defining at least a portion of the reflecting surface ll.

Bars or strips [2 may be of any suitable shape and size. It has been determined that the best results are obtained when, as in a preferred embodiment, bars l2 are formed of extremely thin substantially flat strips of metal or other suitable conducting material arranged with their planes perpendicular to the plane of the reflecting surface II which is formed by their longitudinal front edges. Such edgewise strip grating is particularly adaptable to fabrication by means of cutting rather than a forming operation. Also such a grating reduces normal'wind resistance by a factorof five or more with negligible sacrifice of reflection efliciency. Another advantage is that mechanical rigidity is quite good with proper bracing. It has been found that round or tubular bars give fairly good results and bars formed of flat strips with their planes lying in the plane of the reflecting surface I I may also be used.

General considerations are the same for screens and perforated sheet as for gratings since a screen can be thought of as two gratings at right angles. Screens of round wire are better than those of flat strips provided that intersections are electrically good in both cases. Egg crate construction would be best of all, especially if it were desired to reduce wind resistance to a very low value.

For a flat reflecting surface H, bars l2 are straight while for a parabolic or other surface having a curvature, bars l2 will be curved in the desired manner so that their forward portions Will form the desired shape of the reflecting surface. Bars [2 may be supported andsecured to a suitably shaped frame I3 in any desired manner such as by screws, welding, brazing-soldering, etc.

It is desirable that the percent open area of reflector Ill be large relative to the areas of the edges of bars or strips I2. Preferably the space between adjacent'bars 12 is less than half the wavelength and slightlygreater than aquarter ofthe wavelength (in air) ofthe energy directed on reflecting surface -I I It is-also desirablethat the longitudinal edges -'of bars or strips [2 lie in planes substantially parallel to the E-vector of the energy impinging thereon. The E-vector of the, primary field impinging upon'the reflector is thusoriented to be parallel with the strips, and considering the space betweentwostrips, the energy tending to propagate therethrough' willbe attenuated ac- -=cofdingto the attenuation of a wave guide designed beyond cutoff. The amount of attenua- --tion will-be proportional to the depth of the strip andtheresultant effectis asif thev average reflectingsurface ofthegrating reflector is located -sli'ghtlyback from the surface formed by the front edges of .the strips. This arises because of =the fact that:the.energy cannot be immediatelyattenuated upon reaching this inter-strip -region but -must be attenuated exponentially,

thusresulting.inan effective reflecting surface displaced inward .a given amount. .Bars or strips L2, .when associated with a backing re- -flector,- rnay also be arranged to be in, planes perpen'dicular orat an angleto the ,E-vector which 'angulaiurelation may be,determined according tolthedesiredpolarization characteristics of the reflected -energy.

.-Fig. 2.illustrates one applicationof the antenna embodyinga reflector according to this .:invention. Reflector 2G is shown as having a paraboloidal reflecting surface formed as a gratingainthe manner described hereinbefore. Reflector Ellis positioned in a conventional manner .to be illuminated by a radiating element -such as a dipole feed 2|, or a horn, located at the approximate focal point of reflector 2U. Dipole feed 2! is fed and excited in a well known man- -ner such as by a wave guide or coaxial line (not shown). Reflector 28 is mounted in any suitable -manneron a support such as mast 22 and may be rotated about a substantially vertical axis for azimuthal coverage in a well known manner.

Fig. 3 illustrates another application of the reflector according to this invention; Instead of the radiating element and reflector being mounted near the top of a mast, as illustrated in Fig. 2, a smooth surface paraboloidal reflector 25 illuminated by a radiating element 23 is mounted near the bottom of the mast '21 with its axis substantially perpendicular'to the ground plane in such manner that radiant energy is directed vertically upward. An auxiliary reflector '28 is mounted near the top of mast?! to rotate about reflector 28.

1 advantages.

4 a vertical axis substantially coincident with the axis of reflector 25. Auxiliary reflector 28 preferably has a flat reflecting surface formed as a grating in the manner described hereinbefore with reference to Fig. 1. The surface of reflector 28 is arranged at an angle, preferably of 45, to a plane arranged perpendicularly to the axis of reflector 25, whereby the upwardly directed radiant energy is reflected in a direction substantially perpendicular to the energy impinging on It will be understood that the reflecting surface of auxiliary reflector 28, instead of being flat, may also be of parabolic, hyperbolic or any other desired shape so that it is adapted to reflect energy in a beam pattern of any-conventional manner, preferably. atthesame speedof rotation as auxiliary reflector 28.

An arrangement according toFig. 3 has several It affords a-lighter construction, reduced-wind resistance, eliminateslong transmission lines and consequent connections, and makes the radiating element and its reflector more accessible for repairs and-replacement.

A similar arrangement is illustrated inFigs. 4 and 5. -A conventionalparaboloidalreflector '38 issuitably supported with itsaxis preferably perpendicular to the ground plane. Reflector-5D may be mounted'to be-stationary or it may be mounted .to rotate about'its axis. Thus for exampia-reflectorBfi may berigidly-or freelysupported'in a shallow cylindrical frame-51%| mounted to'ibe free to-rotate on ball or roller bearings 32 carried by a base 33. Any suitable means may :be provided for rotating frame -3I such as a motor34 driving agear-35 meshing witha ring gear 36 secured around the periphery of frame 3|.

Reflector 3G isillluminated in a conventional manner by a radiating element such as-dipole and parasitic elements! fed by a wave guide or coaxial transmissionline 38 and mounted at the apparent focal center of reflectoriill to rotate about the reflector axis. "Suitably and rigidly mounted oneedge portions of frame 3lare upstanding supporting members 39, here shown as two beams of any desired length, mounted on diametrically opposite sides of frame 3 I.

Anauxiliary partially open reflector? fill], having a reflecting surface preferably formed bygrating bars 4| substantiallyashereinbefore described with referencatotFig. ;l,=is secured and preferably mounted between supporting members 39 such as by shafts, bolts, or lugs 42. The surface of reflector 341] may beflator suitably shaped to shape the reflected beam .as hereinbefore described with reference to Fig. 3. Reflector '40 is arranged so that a plane perpendicular to-its axis is at an angle,preferably of 45, to the latus rectum plane-of reflectorte with the planes of grating bars'or strips 41 substantially parallel to the axis of reflector 3fl or preferably parallel t the E-vector of theradiant energy,

'Aswill'be understood reflector 353 is. adapted to. direct-radiant energy upwardly andwith. re-

flector 40 set at a 45 angle thereto, the latter will reflect energy in a substantially horizontal direction. In order that reflector 40 may efiiciently intercept the incident beam, it is preferable that reflector 40 be substantially elliptical in shape with the major axis substantially coincident with the axis of the supporting shafts 42.

It will be understood that with the arrangement as described with reference to Figs. 4 and 5 reflector 40 Will rotate about the axis of reflector 30 at substantially the same speed of rotation therewith. It will also be understood that reflector 30 need not be rotated but may remain stationary in which case frame 3| is not secured to reflector 30 but is arranged to rotate relative thereto. However, in order to maintain the proper relation between the bars of reflector 40 and the E-vector of the radiant energy it is necessary that radiating element 31 rotate at the same speed as reflector 40.

While preferred embodiments of this invention have been particularly described and illustrated it will be understood that various other modifications and improvements may be made without departing from the spirit of the invention. Therefore, it is not desired that the invention be limited to the precise details'set forth.

What is claimed is:

An antenna for a high frequency communication system comprising a paraboloidal reflector arranged with its axis perpendicular to a ground plane, radiating means located at the focal point of said reflector, said radiating means and said paraboloidal reflector being adapted to direct electromagnetic waves of energy vertically upward, a base, a frame rotatably supported on said base and surrounding said reflector, means for rotating said frame and said radiating means in the same direction and at the same speed, a plurality of supporting members secured to and extending upwardly from said frame, and an auxiliary reflector supported by said'members near their upper extremities, said reflecting surface comprising a plurality of parallel substantially flat strips of conducting material arranged with their planes parallel to the E-vector of the radiant energy, and wherein the distance between the adjacent strips is not more thanhalf the wave length of the radiant energy, said reflecting surface being disposed at an angle of relative to the latus rectum plane of said paraboloidal reflector whereby said auxiliary reflector is adapted to reflect said upwardly directed radiant energy in a horizontal direction.

LESTER C. VAN A'I'IA.

REFERENCES CITED Thefollowing references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 744,897 Braun Nov. 24, 1903 1,548,958 Sperry Aug. 11, 1925 1,931,980 Clavier Oct. 24, 1933 2,042,302 Frantz et al. May 26, 1936 2,043,347 Clavier et al. June 9, 1936 2,115,788 Scharlau May 3, 1938 2,423Z648 Hansell July 8, 1947 FOREIGN PATENTS Number Country Date 642.385 Germany July 7, 1937

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