Circular Symmetric Beam Forming Apparatus

Abstract

An antenna is disclosed having an X-section pattern that is circular symmetric, i.e., the antenna produces a beam having equal E and H plane patterns of the type required to achieve circular polarization over a large angular portion of the beam. In general, the antenna is implemented by no less than one slot in a ground plane across the bottom of a conductive cylindrical cup and mutually coupled to and straddled by two dipoles that are normal to the ground plane. In a circularly polarized version of the invention, crossed slots are excited by a rotating electric field.

Description

BACKGROUND OF THE INVENTION

A circular symmetric beam having substantially equal E and H plane patterns is required in many applications such as feeds for circular apertures, or as data links. In U.S. Pat. No. 3,594,806, entitled "Dipole Augmented Slot Raidating Elements", broadband posts were placed intermediate the slots of an array to produce equal E and H plane patterns of the elements and to reduce mutual coupling between array elements. Multiple slots in an array configuration without a cylindrical cup reflector were disclosed in this case. In U.S. Pat. No. 2,846,679, entitled "Beam Forming Antenna", on the other hand, a dipole radiating element was disposed parallel to the bottom surface of a cylindrical cup reflector. In this device there is no slot radiating element mutually coupled to the dipole to equalize the E and H plane field intensity patterns.

SUMMARY OF THE INVENTION

In accordance with the present invention, a slot in a ground plane is excited by a waveguide, for example. Energy radiated from the slots parasitically excites dipoles straddling the slot out of phase with each other. When closely spaced the dipoles radiate very little; as spacing is increased the dipoles radiate more strongly until a drop off occurs when coupling to the slot falls off. The relative phase between dipole radiation and slot radiation is controlled by the length of the dipoles. When short, the dipoles are capacitive and lead the driving voltage. When longer than at resonance, radiation from the dipoles lags the driving excitation. This effect, together with a cylindrical cup reflector, is used to control the antenna pattern. The antenna can be excited by either coaxial or waveguide lines. Also, the dipoles can be driven and the slot parasitically excited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a slot excited embodiment of the present invention;

FIG. 2 illustrates a cross-sectional view through the slot of the antenna of FIG. 1;

FIG. 3 illustrates a plan view of the reflector cup of the antenna of FIG. 1;

FIG. 4 shows a partially sectional schematic view of an antenna in accordance with the invention having driven dipoles and a parasitically excited slot;

FIGS. 5 and 6 show a perspective and end view, respectively, of a circularly polarized version of the antenna of the invention;

FIG. 7 illustrates E and H characteristics of the antenna of FIG. 1; and

FIG. 8 illustrates the manner in which the E field patterns of the antenna of FIG. 1 combine to give the resultant E field.

DESCRIPTION

Referring now to FIGS. 1-3 of the drawings, there is shown a preferred embodiment of the circular symmetric beam forming antenna of the present invention. More particularly, the antenna includes a segment of rectangular waveguide 10 terminated at one extremity with a circular ground plane 11 having a slot 12 symmetrically disposed therein within the cross-section of waveguide segment 10 parallel to the broad side walls thereof. The remaining extremity of waveguide segment 10 is blanked off by means of a conductive end plate 13 disposed transversely thereacross. Coupling to the waveguide segment 10 is provided by a conductive probe 14 inserted through an aperture 15 centrally disposed in a broad sidewall thereof one-quarter wavelength from the inner surface of end plate 13. A coaxial connector 16 attached to the broad sidewall of waveguide segment 10 about the aperture 15 has a center conductor 17 which connects to the probe 14.

Exterior to the waveguide segment 10, dipoles 18, 19 are straddled across the slot 12 midway therealong and normal to ground plane 11. Lastly, a conductive cylinder 20 is disposed about the periphery of ground plane 11 to provide a reflector for the radiating slot 12 and coupled dipoles 18, 19.

In operation, a signal to be transmitted is applied through coaxial connector 16 to the probe 14 to excite the cavity formed by the waveguide segment 10, which, in turn, causes energy to be radiated from slot 12. The energy radiated from slot 12 has an E-plane pattern 22, FIG. 8, which has a circular configuration. The E-plane energy radiated by slot 12 parasitically excites dipoles 18, 19 out of phase. The H-plane pattern of the dipoles 18, 19 vary approximately as cos .theta. as shown by the patterns 23, 24, FIG. 8, for the dipoles 18, 19, respectively. The amplitude of the dipole patterns 23, 24 is controlled by the spacing of the dipoles 18, 19 from the slot 12. When closely spaced, the dipoles 18, 19 radiate very little. As spacing is increased, radiation increases until a drop-off occurs due to falling off of coupling with slot 12. The relative phase between radiation from dipoles 18, 19 and radiation from slot 12 is controlled by the length of the dipoles 18, 19. When short relative to resonant length, the dipoles 18, 19 are capacitive and lead the driving voltage. When longer than at resonance, radiation from the dipoles 18, 19 lags the driving excitation. A length is selected for the dipoles 18, 19 to make the phase of the radiation therefrom differ by 180.degree. from that of the E-plane pattern from slot 12. When this is the case, the H-plane patterns 23, 24 subtract, reducing the E-plane slot pattern 22 to a resultant pattern 25. The dipoles 18, 19 are symmetric in the H-plane and, being out of phase, do not radiate in this plane. Thus, by appropriate adjustment of the spacing of the dipoles 18, 19 from the slot 12, the E-plane pattern for slot 12 is made substantially equal to the H-plane pattern therefor.

Referring to FIG. 7, there is illustrated an E-plane pattern 30 and an H-plane pattern 32 for a circular symmetric beam forming antenna of the type described in connection with FIGS. 1-3 wherein the dipoles 18, 19 have a height of three-eighths wavelength and a spacing of 0.22 wavelength, and the conductive cylinder 20 has a length of one-quarter wavelength and a diameter of 0.9 wavelength.

Referring to FIG. 4 there is shown a partially cross-sectional schematic drawing of a circular symmetric beam forming antenna in accordance with the present invention wherein like reference numerals refer to like elements. In this case, the dipoles 18, 19 are driven 180.degree. out of phase by appropriate connections through coaxial lines 34, 36, respectively, from the outputs of a 180.degree. hybrid 38 having an input 40. The slot 12 in ground plane 11 is enclosed by a cavity 42 on the side thereof opposite from the dipoles 18, 19. Operation is the same as that of the antenna described in connection with FIGS. 1-3 with the exception that the slot 12 is now parasitically excited by the dipoles 18, 19 through mutual coupling thereto rather than the dipoles 18, 19 being parasitically driven by radiated energy from the slot 12. Relative phase between the radiated energy from the dipoles 18, 19 and that from the slot 12 may be controlled by selecting the perimeter of slot 12, i.e., by controlling the resonant length of slot 12. The size of the cavity 42 should, of course, be sufficient to support oscillations at the resonant frequency of the slot 12.

Referring to FIGS. 5 and 6 there are shown two views of a circular symmetric beam forming antenna 50 for generating a circularly polarized beam. More particularly, antenna 50 includes cylindrical waveguide section 51 intermediate a rectangular-to-cylindrical conversion section 52 and a ground plane 53. Ground plane 53 includes a centrally disposed slot 54 parallel to the broad sides of the rectangular waveguide portion of section 52 and a centrally disposed slot 55 that is normal to the slot 54. A conductive cylinder 56 is disposed about the periphery of ground plane 53 in a direction extending away from the cylindrical wave guide section 51 thereby to form a cylindrical cup reflector for energy radiated from the slots 54, 55. Dipoles 57, 58, 59 and 60 are erected within the cylindrical cup reflector equidistant from the center and normal to the ground plane 53 along the bisectors of the quadrants between the slots 54, 55. Lastly, a dielectric slab 62 constituting a one-quarter wave plate is disposed across a diameter and along a central portion of the cylindrical waveguide section 51 at a 45.degree. angle with the slots 54, 55.

In operation, energy propagating in the rectangular waveguide in the TE.sub.10 mode is converted to the TE.sub.11 mode in the cylindrical waveguide section 51 by the rectangular-to-cylindrical conversion section 52 with the electric field normal to the slot 54. In traversing the one-quarter wave plate 62, the energy in the incident TE.sub.11 mode is divided into two orthogonal TE.sub.11 modes which have a 90.degree. phase difference. The two orthogonal TE.sub.11 modes excite the slots 54, 55 in a manner to generate a circularly polarized wave. The dipoles function in the same manner as the dipoles 18, 19 in the antenna of FIGS. 1-3.

Claims

What is claimed is:

1. A circular symmetric beam forming antenna comprising a ground plane; no less than one slot in said ground plane; first and second conductive posts disposed on one side of said ground plane on opposite sides of and mutually coupled to each of said no less than one slot; means disposed on the side opposite from said one side of said ground plane for providing a cavity including said no less than one slot in a side wall thereof; a conductive cylinder of a height less than that of said first and second conductive posts disposed symmetrically about said no less than one slot in contact with and on said side of said ground plane thereby to provide an antenna element; and means coupled to said antenna element for exciting said element with electromagnetic energy to be transmitted.

2. The circular symmetric beam forming antenna as defined in claim 1 wherein said means coupled to said antenna element for exciting said element with energy to be transmitted constitutes a 180.degree. hybrid element having an input and first and second outputs, said first and second outputs being connected to said first and second conductive posts, respectively, whereby a signal applied to said input excites said antenna element.

3. The circular symmetric beam forming antenna as defined in claim 1 wherein said means coupled to said antenna element for exciting said element with energy to be transmitted constitutes means for exciting said cavity thereby to excite said no less than one slot in a side wall thereof.

4. The circular symmetric beam forming antenna as defined in claim 1 wherein said no less than one slot in said ground plane constitutes first and second crossed orthogonal slots and said means coupled to said antenna element for exciting said element with energy to be transmitted includes cylindrical waveguide means for exciting said first and second crossed orthogonal slots with two TE.sub.11 orthogonal modes of a phase difference equal to 90.degree. thereby to provide a circularly polarized output from said antenna element.

5. The circular symmetric beam forming antenna as defined in claim 1 wherein the diameter of said conductive cylinder is less than one free space wavelength of said electromagnetic energy to be transmitted.

6. A circular symmetric beam forming antenna comprising a section of rectangular waveguide; a ground plane having first and second sides, said ground plane being disposed transversely across one extremity of said section of rectangular waveguide, in contact with said first side, the portion of said ground plane enclosed by said waveguide having a centrally disposed slot parallel to the broad side walls of said waveguide; first and second conductive posts of substantial dipole length disposed on said second side of said ground plane on opposite sides of and mutually coupled to said slot; a conductive cylinder of a length less than the height of said first and second conductive posts disposed symmetrically about said slot in contact with said second side of said ground plane thereby to provide an antenna structure; and means for launching a fundamental mode electromagnetic wave in said section of rectangular waveguide whereby a circular symmetric beam is radiated by said antenna structure.

7. The circular symemtric beam forming antenna as defined in claim 6 wherein said first and second conductive posts are of a length that differs from said dipole length to produce a 180.degree. phase difference in a signal radiated therefrom as compared to a driving signal radiated from said slot.

8. A circular symmetric beam forming antenna comprising a ground plane having a slot therein; a cavity in contact with one side of said ground plane whereby a portion of said ground plane provides a side wall for said cavity, said portion including said slot; first and second conductive posts of dipole length disposed through said ground plane from said one side thereof on opposite sides of said slot thereby to effect mutual coupling therewith; a conductive cylinder of a length less than said dipole length disposed symmetrically about said first and second conductive posts in contact with the side opposite said one side of said ground plane; and a 180.degree. hybrid having an input and first and second outputs, said first and second outputs being connected to said first and second conductive posts, respectively, on said one side of said ground plane thereby to drive said first and second conductive posts in response to a signal applied to the input of said hybrid with signals differing in phase by 180.degree..

9. A circularly polarized circular symmetric beam forming antenna comprising a section of cylindrical waveguide; a ground plane having first and second sides, said ground plane being disposed transversely across one extremity of said cylindrical waveguide with said first side thereof in contact therewith, said ground plane additionally having first and second crossed orthogonal slots centrally disposed within the area thereof enclosed by said cylindrical waveguide; first, second, third and fourth conductive posts of substantially dipole length disposed on said second side of said ground plane in respective quadrants thereof formed by said crossed slots; a conductive cylinder of a length less than said dipole length disposed symmetrically about said first, second, third and fourth conductive posts in contact with said second side of said ground plane; and means for launching a signal in the form of first and second orthogonal TE.sub.11 modes 90.degree. out of phase along said cylindrical waveguide towards said first and second crossed orthogonal slots in said ground plane thereby to radiate a circularly polarized circular symmetric beam.