US3267477A - Dual frequency microwave antenna - Google Patents

Description

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Aug. 16, 1966 o. G. BRICKEY 3,267,477

DUAL FREQUENCY MICROWAVE ANTENNA Filed April 2a, 1964 s sheets-sheet 1 o, X g 4 Q I'f? W i l. "l" :y l #u 1\ u:

Aug 16, 1966 o. G. BRlcKEY 3,267,477

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Aug. 16, 1966 o. G. BRlcKEY DUAL FREQUENCY MICROWAVE ANTENNA 5 Sheets-Sheet 5 Filed April 28, 1964 United States Patent O 3 267 477 DUAL FREQUENCY, MIJROWAVE Al \ITENNA Orville G. Brickey, Indianapolis, Ind., asslgnor to the United States of America as represented by the Secretary ofthe Navy Filed Apr. 28, 1964, Ser. No. 363,309 Claims. (Cl. 343-756) This 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 or therefor.

The present invention relates to a dual frequency microwave antenna and more particularly to a radar antenna which is adaptable for mounting atop the rotor hub of a helicopter. v

Antennas capable of -operating at two different frequencies are not new in the antenna art. The prior art discloses various horn-type antennas that are lused to receive or transmit dual polarized signal waves at separated frequency bands. This has been accomplished in the prior art by employing energizing antennas, or pickup means, depending upon whether the horn is used for transmission or reception, positioned within the small end of a flared waveguide. The two signals of different frequencies are radiated or received with their polarizations crossed in order to eliminate interaction. The hornlike structure is common to both energizing antennas, or pickup means, and the horn-like structure will respond differently to the two frequency bands. Therefore, one frequency band will have a radiation of a given shape and the other frequency band will have a radiation pattern that is different, that is, either sharper or broader than the pattern of the first signal due to a diiference in the frequency response of the horn. This is a decided disadvantage in certain communication systems.

Another disadvantage of horn-type antennas is that it is difficult to suppress side lobes. The princip-al effect of side lobes is to create ghost echoes which can be confusing to the radar operator and which seriously limits the usefulness of radar.

In a preferred embodiment of the .present invention, a dual lfrequency antenna is provided that radiates both Xband (9,60010,000 mcs.) and Lband (1010- 1110 mcs.) power simultaneously. The X-band is horizontally polarized and the L-band is vertically polarized. The antenna is of an X-'band half-cheese configuration with an Lband compound horn being constructed internally therewith and being fed from a coaxial to waveguide transition which protrudes from the back side of a parabolic surface. A series of horizontal slots are provided in the parabolic surface to permit the Lband power to radiate through the parabolic surface. This design .provides low side lobe levels out to 180 degrees on either side of the main |lobe.

It is therefore a general object of the present invention to provide an improved antenna for radiating or receiving separated frequency bands.

Another object of the present invention is to provide a dual frequency antenna that will have low side lobe levels on iboth sides of the main lobe.

Other objects and advantages of the present invention will |be readily `appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with t'he accompanying drawings wherein:

FIGURE 1 is a perspective view showing a preferred embodiment of the present invention with the bottom plate facing upwardly;

FIGURE 2 is a front View of the preferred embodiment;

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FIGURE 3 is a view of the bottom plate partly broken away to illustrate details of construction;

FIGURE 4 is a sectional view taken on line 4-4 of FIGURE 3;

FIGURE 5 is a perspective view showing a plurality of vertical fingers;

(FIGURE 6 is a perspective view showing a hog horn; an

FIGURE 7 is a partial plan View showing the optimum design for slots in a parabolic reector. v

Referring now to the drawings, there is .shown a preferred embodiment of the present invention having a top plate 11 `and a bottom plate 12 that are spaced apart and disposed parallel to one another. Top plate 11 is provided with a parabolic contoured surface 13 and bottom plate 12 -is provided with a similar contoured surface 14. A metallic plate 15 is attached to the edges of parabolic surfaces 13 and 14 thereby forming a parabolic reflector 16.

Referring now to FIGURES 1, 3, and 6, of the drawings, a hog horn 17 is provided to illuminate the parabolic reflector 16 with energy at Xband frequency which is horizontally polarized. As best shown in FIG- URE 6 of the drawings, hog horn 17 is flared in the E plane `in order to provide the proper primary pattern. The fog horn feed illuminating the parabolic reflector provides a constant phase -front that appears parallel to the lattice rectum of the parabolic reflector 16. The rectangular waveguide end 30 of hog horn 17 may be supplied Xband energy in any conventional manner.

The second feed which provides Lband power is mounted behind the parabolic reflector 16 and is, in reality, a compound sectorial horn. The input energy is fed into the feed chamber 18 through coaxial connector 19 and within feed chamber 18 there is a transition to waveguide. As best shown in FIGURE 4 of the drawings, a metallic probe 20 has one end connected to connector 19 and the other end -attached to an insulating support 21. T-he energy introduced by probe 20 is polarized in the vertical direction as indicated by the arrow 22 which represents the electric vector. As the Lband is vertically polarized, a plurality of 'horizontal slots 213 are provided in parabolic reflector 16 to facilitate propagation of the L-band signal through reflector 16. By optimizing the width of slots 23 and the spacing between adjacent slots, the X-band frequency, which is horizontally polarized, sees nothing but the solid parabolic surface. As shown in FIGURE 7 of the drawings, for Xband and LUhand frequencies, the optimum slot width has been found to be a width that is approximately one-fourth the width of the spacing between slots. By Way of example, a slo-t width of 0.100 inch would be used with a spacing between slots of about 0.400 inch.

Asbest shown in FIGURES 2 and 3 of the drawings, a plurality of vertical metallic rods are provided between plates 11 and 12 to form the sides of the sectorial horn. Rods 24-28 form one side (indicated by dotted line 29) and rods 31-35 form the other side (indicated by dotted line 36). Two additional rods 37 and 38 -are provided to extend the effective length of the short side of feed chamber 118. The diameter of the metallic rods are kept small in comparison to one X-band wavelength, and the rods are spaced 4such that the magnetic vector at L- band `cannot propagate between the rods while XIband sees very little opposition. Also a plurality of conductive rods 41 are spaced around the outer periphery, between plates 11 and 12, starting at the edge of parabolic reiiector 16 `and continuing around to the rear of hog horn 17. These conductive rods provide a grille that effectively forms a reflecting wall for the vertically polarized energy from the L-band feed. A horizontally placed rod 42 is placed midway between plates 11 and 12 behind hog horn 17 to prevent t-he passage of any stray radiation from the Xband feed.

Referring now to FIGURES 1, 2, 3, and 5, of the drawings, a pair of segmentally-shaped flanges 43 and 44 are attached one each to the front edges of plates 11 and 12, respectively. A plurality of conductive fin-gers 45 are provided on each of flanges 43 and 44. Each finger has one end attached near the arc of the segmentallyshaped flange and the finger rises perpendicular to the flange and then .bends and extends angularly toward the chord of the flange. As best shown in FIGURE of the drawings, the fingers are of varying height with the highest fingers being midway between the ends of the flanges. The conductive fingers 45, in effect, produce a larger aperture for Lband, but due to polarization, are not visible and do not affect Xband. Conductive fingers 45 have a secondary effect of narrowing the elevation beamwidth (Lband) which reduces the backlobe.

In operation, Lband power is applied to the dual frequency antenna through coaxial connector 19 and into feed chamber 1S. This Lband power, which is vertically polarized, passes through slots 23 in the parabolic reflector 16 and is then radiated from the aperture of the antenna. At the same time, Xband power is applied to the waveguide end 30 of hog horn 17 and is directed by hog horn 17 onto parabolic reflector 16 from which it is reflected outwardly through the aperture of the antenna. As the Xband power is horizontally polarized, it is unaffected by slots 23 in parabolic reflector 16.

While the embodiment of the antenna has been described particularly with reference to transmission, it will be apparent that the operation is similar for reception and duplex operation, and operation for transmission or reception or both is to be understood as encompassed by the language of the claims.

It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that numerous modifications or Ialterations may be made therein without departing from the spirit and the scope of the invention. For example, while Xband and Lband power frequencies have been specifically described herein, it is apparent that other frequencies will work equally as well.

What is claimed is:

1. A dual frequency microwave antenna comprising:

first and second conductive plates each having a straight edge and a parabolically shaped edge, said plates being similarly disposed in spaced parallel relation with similar edges co-'adjacent,

a conductive plate attached to said parabolical-ly shaped edges thereby forming a parabolic reflector, said parabolic reflector having a plurality of horizontally disposed slots in the mid-portion thereof,

means positioned forwardly of said parabolic reflector for illuminating said parabolic reflector with a horizontally polarized wave in a first frequency range, 'and means positioned rearwardly of said parabolic reflector for radiating through said slots a vertically polarized wave in a .second frequency range.

2. A dual frequency microwave antenna as set forth in claim 1 whe-rein first and second rows of rods are positioned perpendicularly between said first and second conductive plates, said rows of rods being positioned angularly between said straight edge and said parabolioally shaped edge whereby said first and second rows of rods each form one side of a sectorial horn.

3. A dual frequency microwave antenna as set forth in claim 1 wherein first and second segmentally-shaped flanges are perpendicu-larly mounted one each to the straight edges of said first and second conductive plates and a plurality of conductivey rods of varying height are attached to each said flange whereby said flanges and rods effectively produce a larger aperture for said vertically polarized wave.

4. A dual frequency microwave antenna is set forth in claim 1 wherein said plurality of horizontally disposed slots each have a width about one-fourth the width of the spacing between adjacent slots.

5. A dual frequency microwave antenna as set forth in claim 1 wherein a curved row of rods are positioned perpendicularly between said first and second conductive plates extending on the outer periphery between one end of said parabolic reflector and one end of each said straight edge.

References Cited by the Examiner UNITED STATES PATENTS 2,364,371 12/1944 Katzin 343-756 2,943,324 6/ 1960 Sichak 343-756 3,100,894 8/1963 Giller et al. 343-756 3,148,370 9/1964 Bowman 343-756 GTHER REFERENCES Cumming: A Dual-Polarized Line Source, The Microwave Journal, January 1963, pages 81-86 relied on.

HERMAN KARL SAALBACH, Primary Examiner.

R. F. HUNT, Assistant Examiner.

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