Shaped Beam Antenna

Abstract

The apparatus of the present invention provides an antenna adapted to optimize the transmission or reception performance of a satellite subject to the usually encountered weight and volume constraints. The antenna features dual mode operation which provides two independent terminals, each providing the same gain pattern and polarization sense, but having differing senses of phase progression across the beam pattern. In addition, the beamwidth of the antenna is designed to cover a prescribed area on earth wherein minimum gain in this area is maximized rather than the gain at the beam center. The disclosed antenna system may also be used with a single reflector and two orthogonal polarizations, providing a total of four antenna terminals. To provide the second polarization which may be either crossed linear or counter rotating circular, each of the feeds must support the two polarizations, and the two pairs of terminals of a given polarization interconnected via individual hybrids.

Description

BACKGROUND OF THE INVENTION

Antenna systems having aperture shaped excitation functions having beam patterns with flat or saddle tops are well known. These antenna systems, however, invariably require much larger apertures than conventional antennas. A typical linear dimension of such an antenna system is three times that of a conventional system. Because of the large dimensional requirements of a shaped beam satellite antenna system, the volume and weight constraints of the booster may preclude the use of two completely independent antennas to provide the two terminals desired.

SUMMARY OF THE INVENTION

In accordance with the present invention, a dual mode satellite antenna system having a shaped beam is realized by a plurality of linearly disposed off-set feeds at the focal region of a reflector. The feeds are spaced sufficiently close so that the patterns overlap and are fed in a manner to produce differing senses of phase progression across the pattern. Because of this phase progression, the overlapping patterns add vectorially to produce an over-all saddle or flat beam pattern rather than a pattern with a peak in the plane of the linearly disposed feeds. The feed system is adaptable to the outputs from a 90.degree. hybrid, thereby providing two independent terminals to the antenna system. The importance of two independent terminals in transmission is in alleviating the multiplexing problem encountered when multiple transmitters separated in frequency must share the same antenna. When only two transmitters are involved, the provision of two terminals eliminates the requirement for a transmitter multiplexer. When a large number of transmitters is required, the provision of two terminals permits connecting to each a set of transmitters having twice the adjacent channel frequency separation, thereby simplifying the multiplexer. For use in reception, the provision of two antenna terminals reduces the switching required to provide redundancy in the receivers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates dual mode transmitting and receiving antennas in accordance with the invention installed on a satellite;

FIG. 2 shows a cross-section through the feed structure of the antenna of FIG. 1;

FIG. 3 illustrates far field patterns of a single off-center feed, two off-center feed, and the dual mode feed of FIG. 1;

FIG. 4 illustrates a plan view of an off-set feed dual polarization, dual mode, four terminal antenna;

FIG. 5 is an implementation of cross-polarized feed horns for the antenna of FIG. 4; and

FIG. 6 illustrates a two-terminal, four-feed horn system in accordance with the invention.

Referring now to FIG. 1 of the drawings, there is shown a transmitting antenna 10 and a receiving antenna 12 installed on a satellite 14 which includes solar cell panels 15 and 16 at the lower portion thereof, as shown in the drawing. The transmitting and receiving antennas 10, 12 are mounted on a vertical support shaft 18 which is topped by a telemetry and command antenna 20. Cross-section A - A' of antenna 10 is shown in FIG. 2. In particular, antenna 10 includes a conductive parabolic reflector 22 with feeds 23, 24 disposed in the region of the focal point 25 of parabolic reflector 22 and off-set equally from the axis of rotation thereof. Feeds 23, 24 may assume the form of horns or other contemporary radiating element. In the event horns are utilized, the apertures thereof are directed towards the central region of reflector 22. Lastly, transmission lines 26, 27 connect from the body of satellite 14 through a 90.degree. hybrid 29 to the feeds 23, 24, respectively. Similarly, transmitting antenna 12 includes a parabolic reflector 30 with transmission lines 31, 32 connected from the body of satellite 14 through a 90.degree. hybrid 33 to feeds 34, 35, respectively, which are disposed in positions corresponding to the feeds 23, 24 of antenna 10.

Referring to FIG. 3(a), there is shown a far field pattern 40 for a single off-center feed 23 or 34. In FIG. 3(b) there is shown far field pattern 40 plus a far field pattern 42 for the single remaining feed 24 or 35. Under normal circumstances with the electric fields both in phase, the intersecting point 43 of far field patterns 40, 42 would represent 0.5 the peak amplitude shown by dashed line 44. In the present case, however, the far field patterns 40, 42 are added "vectorially" and, in particular for the antenna of FIG. 1, are added at 90.degree.. That is, the 90.degree. hybrids 29, 33 provide two terminals for each of the antennas 10, 12 and excite the feed horns 23, 24 and 34, 35, respectively, with signals that are equal in intensity and 90.degree. out of phase. Under these circumstances, the far field patterns 40, 42 add to give the "shaped" pattern 46, FIG. 3(c) for each terminal of the 90.degree. hybrids 29 and 33. The beam dimension thus developed permits "shaping" the beam in its long angular dimension while using a conventional beam shape in its short dimension, by adjusting the off-set of the beam centers generated by the feeds 23, 24 and 34, 35. This off-set may, for example, be approximately .+-. 1.6 degrees.

In a typical installation, receiving antenna 12 is designed to receive signals in the 5.925 - 6.425 GHz band and the transmitting antenna 10 is designed to transmit signals in the 3.7 - 4.2 GHz band. By making the diameter of the reflectors 22, 30 24 wavelengths, the angular dimensions of the beam generated by both antennas 10, 12 is roughly an ellipse with a major axis of 6.degree. and a minor axis of 2.5.degree.. Under these circumstances, the reflector 22 of transmit antenna 10 is approximately 6 feet in diameter while the reflector 30 of receive antenna 12 is 4 feet in diameter. This beam configuration can be oriented to provide coverage over the contiguous 48 states of the United States from the stationary satellite's altitude of 19,300 nautical miles.

The size required for the transmitting antenna reflector may preclude the use of two separate antennas. In this instance, the use of the antenna system of FIG. 4 is required when two independent terminals are required. Referring to FIG. 4, there is shown a dual-polarization, dual-mode, four-terminal antenna 50 in accordance with the present invention. Antenna 50 comprises a reflector 52 which constitutes a circular section of a paraboloid having a diameter extending outwards from the center thereof. Dual polarization feeds 54, 55 are off-set horizontally from the axis of rotation of the paraboloid in the region of the focal point thereof, as viewed in the drawing. Terminals 56, 57, are connected by transmission lines 58, 59, respectively, through a 90.degree. hybrid 60 to common polarization inputs to feeds 54, 55. Similarly, terminals 61, 62 are connected by transmission lines 63, 64, respectively, through a 90.degree. hybrid 65 to the remaining common polarization inputs of feeds 54, 55. Referring to FIG. 5, there is shown an implementation of the dual-polarization feeds 54, 55 of FIG. 4. In this case, waveguides 68, 69 constitute the transmission lines 58, 59 and connect from terminals 56, 57 to feed horns 54, 55 in a manner to excite vertically polarized signals therein, as viewed in the drawing. Waveguides 73, 74, on the other hand, constitute the transmission lines 63, 64 and connect from terminals 61, 62, respectively, to feed horns 54, 55, respectively, in a manner to excite horizontally polarized signals therein, as viewed in the drawing. The use of contra-rotating circularly polarized signals instead of orthogonally polarized signals is considered to be within the spirit and scope of the present invention.

Referring now to FIG. 6 of the drawings, there is shown an embodiment of the invention wherein a linear array of four feed horns 81, 82, 83, 84 are disposed in the region of the focal point of a parabolic reflector, not shown, and including magic-T's 85, 86, 87 and a 90.degree. hybrid 88. Magic-T's are characterized by four terminals designated 1, 2, 3, and 4, as labeled on magic-T's 85, 86, and 87, wherein there is no direct coupling between terminals 1 and 4 and terminals 2 and 3. Terminal 1 is also designated the summation or .SIGMA. input and terminal 4 the difference or .DELTA. input. Signals applied to the summation terminal divide equally between terminals 2 and 3 and have the same phase, whereas signals applied to the difference terminal divide equally between terminals 2 and 3, but are of opposite phase. On the other hand, the sum of signals applied to terminals 2 and 3 appear at the summation terminal and the difference appear at the difference terminals. Returning now to FIG. 6, antenna terminals 90, 91 are connected through 90.degree. hybrid 88 to the summation terminals of magic-T's 86, 85, respectively; difference terminals of magic-T's 85, 87 are connected to terminals 2 and 3, respectively, of magic-T 86; terminals 2 and 3 of magic-T 85 are connected to feed horns 82, 83, respectively; terminals 2 and 3 of magic-T 87 are connected to feed horns 81, 84, respectively; the summation terminal of magic-T 87 is terminated; and the difference terminal of magic-T 86 is terminated.

Thus, in operation, a signal E, applied to antenna terminal 90 of 90.degree. hybrid 88 results in applying signals

to summation terminals of magic-T's 86, 85, respectively. The signal

applied to the summation terminal of magic-T 86 divides equally between terminals 2 and 3 thereof to provide the signal

at each of these terminals. The signal

applied to the difference input of magic-T 87 results in a signal

applied to feed horn 81 and a signal

applied to feed horn 84. The signal

applied to the difference terminal of magic-T 85 along with the signal

applied to the summation terminal results in the signal

Summarizing, signal E applied to terminal 90 results in the following signals:

Feed horn 81 0.36 E.vertline.90.degree. Feed horn 82 0.61 E.vertline.35.6.degree. Feed horn 83 0.61 E.vertline.-35.6.degree. Feed horn 84 0.36 E.vertline.-90.degree.

the resulting beam constitutes the vector addition of the overlapping portions of the individual beam patterns. In the event a signal is applied to the antenna terminal 91, the signal amplitudes are the same but the phase progression occurs in the opposite direction.

Claims

What is claimed is:

1. A satellite antenna system comprising a parabolic reflector having a focal point; first, second, third and fourth feed horns disposed in the order named along a linear array in the region of said focal point facing said reflector; dual terminal means responsive to a signal at either terminal for generating first and second output signals at first and second output terminals, respectively, said first and second output signals being of equal power and having a 90.degree. phase difference; first, second and third magic-T's each having terminals 1, 2, 3, and 4, terminals 1 of said first and second magic-T's being connected to said first and second output terminals, respectively, of said dual terminal means, terminals 2 and 3 of said second magic-T being connected to said second and third feed horns, respectively, terminals 2 and 3 of said first magic-T being connected to terminal 4 of said second and third magic-T's, respectively, terminals 2 and 3 of said third magic-T being connected to said first and fourth feed horns, respectively; and means for terminating terminal 4 of said first magic-T and terminal 1 of said third magic-T.

2. The satellite antenna system as defined in claim 1 wherein said dual terminal means constitutes a 90.degree. hybrid.

3. A dual mode, dual polarized satellite antenna system comprising a parabolic reflector having a focal point, means including first and second dual polarized feeds disposed in the region of said focal point facing said reflector and offset symmetrically from the axis of rotation thereof for generating overlapping field intensity patterns, means for feeding said first and second feeds with a first set of equal amplitude signals of 90.degree. phase difference having a predetermined polarization, and means for feeding said first and second feeds with a second set of equal amplitude signals of 90.degree. phase difference and of a polarization orthogonal to said predetermined polarization.