US3413637A - Multifunction antenna having selective radiation patterns - Google Patents

Description

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.6. 1968 F. J. GOEBELS, JR.. ETAL 3,413,637

MULTIFUNCTION ANTENNA HAVING SELECTIVE RADIATION PATTERNS Filed April 12, 1967 3 Sheets-Sheet 1 drrawaw 2 1968 F. J. GOEBELS, JR.. ETAL 3,413,637

MULTIFUNCTION ANTENNA HAVING SELECTIVE RADIATION PATTERNS Filed April 12, 1967 3 heets-Sheet a Nov. 26, 1968 F. J. GOEBELS, JR.. ETAL 3,413,637

MULTIFUNCTION ANTENNA HAVING SELECTIVE RADIATION PATTERNS Filed April 12. 1967 3 Sheets-Sheet :5

United States Patent 3,413,637 MULTIFUNCTION ANTENNA HAVING SELECTIVE RADIATION PATTERNS Frank J. Goebels, Jr., Canoga Park, Charles A. Strider and Alfred T. Villeneuve, Los Angeles, and Charles K. Watson, Manhattan Beach, Calif., assignors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Apr. 12, 1967, Ser. No. 632,507 6 Claims. (Cl. 343-754) ABSTRACT OF THE DISCLOSURE A directional microwave antenna capable of selective operation in two or more distinct radiation modes. An extended radiator such as a slot array provides a linearly polarized planar wave front and resultant narrow-beam radiation pattern. At least one polarization-sensitive lens is disposed in the path of the radiated wave energy and adapted for rotation about an axis normal to the array and parallel to the direction of wave propagation. With the lens oriented in a first direction with respect to the electric field vector of the radiated wave energy, the wave energy propagates therethrough substantially unaffected. When the lens is rotated through an angle of ninety degrees, however, the wave energy propagates therethrough with a phase velocity different than that of free space. Thus, by selecting the proper lens geometry, the phase front of the emerging wave energy can be shaped to yield the desired second radiation pattern.

Field of the invention This invention relates to microwave antenna systems, and more specifically to directional microwave antenna systems having selective diverse radiation patterns.

Description of the prior art Frequently, it is desirable to utilize a single microwave antenna to perform two or more separate functions. It may be advantageous in airborne vehicles, for example, to utilize a single antenna having a first radiation pattern for target acquisition, a second radiation pattern for tracking and a third radiation pattern for terrain mapping.

In the past, antenna systems have been proposed which allow selective operation with either of two diverse radiation patterns. Such systems employ a paraboloid reflector having a small segment thereof which is hinged and which can be pivoted with respect to the remainder of the reflector surface. With the hinged segment in the normal position, the antenna provides a narrow-beam radiation pattern commonly referred to as a pencilbeamf By pivoting the hinged segment, a portion of the beam is spoiled, thereby modifying the radiation pattern to yield a flattened pattern useful for terrain mapping functions.

Although parabolic reflectors have been widely used as elements of directional microwave antennas and are capable of providing narrow-beam radiation patterns, operation of the antenna in the spoiled mode leaves much to be desired. This is due to the fact that since the hinged segment is a portion of a paraboloid, there is little freedom to vary the shape of the spoiled beam. In addition, only single plane spoiling is possible with this technique.

Accordingly, it is a general object of the present invention to provide a microwave antenna capable of enhanced operation in a plurality of selective radiation modes.

Patented Nov. 26, 1968 It is another object of the present invention to provide a directional microwave antenna system capable of selective operation with a wide range of radiation patterns.

A specific application of a multifunction antenna for airborne radar systems involves the use of a single antenna having a first radiation pattern of the pencil beam type and a second radiation pattern of the csc. 0 cos 6 type. In this application the pencil-beam is used for tracking and the csc. 0 cos 0 radiation pattern is used for ground mapping. It was primarly for this application that the spoiled paraboloid antenna, mentioned above, was proposed.

It is therefore a more specific object of the present invention to provide an improved directional microwave antenna system capable of selectively operating in either the pencil-beam or the csc. 0 cos 0 mode.

Summary of the invention In accordance with the principles of the present invention, these objects are accomplished through the use of a linearly polarized planar array radiator. The performance of the planar array antenna is generally considered superior to that of the paraboloid reflector due to its increased efficiency, reduced spill-over and controllable sidelobe level. Interposed in front of the planar array is at least one lens of the type comprising a plurality of parallel-spaced thin conductive plates separated by a dielectric medium.

The lenses are mounted so that they are capable of being rotated about an axis normal to the planar array through an angle of at least ninety physical degrees. When one of the lenses is oriented so that the parallel plates are perpendicular to the electric vector of the radiated wave energy there is no effect on the radiation pattern of the planar array. This is due to the fact that for this orientation, the wave energy is propagated through the lens in the TEM mode at a velocity equal to the velocity of light in the dielectric medium separating the parallel plates. By rotating the lens through an angle of ninety degrees so that the parallel plates are oriented in a direction parallel to the electric vector of the radiated wave energy, a shaped beam is obtained. When the lens is thus oriented the wave energy from the planar array propagates through the lens in the fundamental TE mode.

Brief description of the drawings The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a simplified pictorial view of an application of the present invention illustrating two diverse radiation patterns;

FIG. 2 is a simplified pictorial illustration of another application of the present invention illustrating an additional radiation pattern;

FIG. 3 is a simplified pictorial view of one embodiment of the present invention;

FIGS. 4 and 5 are elevation views of the lens of FIG. 3 illustrating the lens orientations for obtaining the dual radiation patterns of FIG. 1; and

FIG. 6 is a simplified pictorial view of another embodiment of the present invention.

Description of the preferred embodiments Referring more specifically to the drawings, the simplified pictorial view of FIG. 1 shows an airborne vehicle such as aircraft It) with which the antenna system of the present invention can be utilized. A first radiation pattern designated Antenna Mode 1 illustrates a forward-looking pencil-beam pattern which is especially useful for target tracking functions. A second radiation pattern designated Antenna Mode 2 represents a so-called csc. 0 cos 0 beam useful for terrain mapping functions. The pattern of Antenna Mode 2 is so named because of its resemblance to the curve derived from a polar plot of the function csc. 9 cos 0, where the angle 0 is measured from the forward bore-sight axis of the antenna system, as shown. This radiation pattern is characterized by substantially uniform gain over a wide angle in a plane parallel to the line of flight of aircraft 10. The antenna system of the present invention is capable of selective operation in either the pencil-beam or the terrain mapping modes.

The simplified pictorial view of FIG. 2 likewise shows two diverse radiation patterns designated Antenna Mode 1' and Antenna Mode 2'. As in FIG 1, Antenna Mode 1' illustrates the pencil-beam function of the antenna system. Antenna Mode 2, on the other hand, is illustrative of forward-looking wide-beam operation. Antenna Mode 2' is useful, for example, in acquiring targets which may be present over a wide sector. Once targets are acquired by use of the wide radiation pattern of Antenna Mode 2', the antenna can be switched to the pencil-beam mode for tracking purposes.

Although only three distinct radiation patterns are illustrated in the pictorial views of FIGS. 1 and 2, it is readily understood that many other alternative radiation patterns can be utilized if desired. For example, sector beam radiation patterns of greater or lesser beam Widths may be advantageous in many applications.

In FIG. 3 there is shown a simplified pictorial view of one embodiment of the present invention. A planar array 30 comprising a plurality of slotted waveguide sections is provided with appropriate feed means such as a waveguide section 31. The open end of waveguide section 31 is, in turn, provided with a coupling flange 32 which facilitates connection to the system with which the antenna is to be utilized. Disposed in the wave path in front of planar array 30 is a rotatable parallel plate lens 33. Although lens 33 is shown displaced some distance in front of planar array 30, this is merely for the purpose of illustration. In practice, lens 33 and planar array 30 can be mounted on a unitary structure or framework with a spacing generally somewhat less than one wavelength. If desired, the entire framework can then be adapted for antenna scanning in the normal manner.

Lens 33 is adapted for rotation about an axis normal to planar array 30. Rotation is facilitated, for example, with the aid of a mounting ring 34 having a section 35, the outer periphery of which is serrated in the manner of a gear. A pinion gear 36, mechanically coupled by means of a shaft 37 to a reevrsible drive motor 38, provides the power necessary to rotate lens 33. As mentioned hereinabove, the rotation of lens 33 need only be through an angle of ninety physical degrees. In the position shown, the plates 39 of lens 33 are oriented in a direction parallel to the electric field vector E of the energy radiated from planar array 30. The shape of lens 33 generally indicated in FIG. 3 provides a csc fi cos 0 radiation pattern when oriented as shown.

In general, the parallel conductive plates 39 which make up lens 33 can be separated and held apart by a suitable low-loss dielectric material, such as a light-weight dielectric foam. For the sake of clarity, however, the dielectric material has been omitted from the pictorial view of FIG. 3. The number of parallel conductive plates 39 and the spacing therebetween is determined primarily by the frequency of intended operation of the antenna system. In

4 general, the spacing between adjacent parallel plates must be large enough to support the first order TE mode for wave energy polarized in the direction shown but small enough so the higher order TE modes are cut off. The spacing between adjacent conductive plates 39 will, therefore, ordinarily range between one-half wavelength and one wavelength for energy propagating in the dielectric medium separating the plates. A more detailed analysis of this type lens may be found, for example, in Microwave Antenna Theory and Design, Radiation Laboratory Series, Vol. 12, McGraw-Hill Book Co., Inc., New York, 1949, at pages 402 et seq., and the references cited therein.

Although the operation of the embodiment of FIG. 3 will now be described in terms of its transmitting mode, it is readily understood that operation in the receiving mode is also contemplated. In operation, the wave energy to be transmitted is coupled into planar array 30 by means of waveguide section 31. The slots in planar array 30 are oriented to provide a substantially planar wave front with the electric field vector oriented in the vertical plane as shown by arrow E. In the absence of the lens, the wave front provided by planar array 30 results in a radiation pattern of the pencil-beam type. Upon passing through lens 33 the phase velocity of the radiated wave energy is increased. By selecting the proper spacing and geometry of parallel plates 39 the shape of the phase front of the wave energy emerging from lens 33 can be varied over a wide range. The phase distribution of the wave energy necessary to produce a given radiation pattern can be computed using the synthesis technique given by A. S. Dunbar in an article entitled On the Theory of Antenna Beam Shaping, appearing in the Journal of Applied Physics, vol. 23, August 1952, at pp. 847-853.

Operation of the embodiment of FIG. 3 can be more readily understood with reference to the elevation views of FIGS. 4 and 5. Specifically, FIG. 4 is an elevation view of lens 33 as seen from the axis of the antenna structure. In FIG. 4 lens 33 is shown oriented so that plates 39 are parallel to the electric field vector E of the radiated wave energy. When the lens is so oriented the wave energy propagates therethrough in the fundamental TE mode. Since the phase velocity of energy in this mode is greater than that of free space, the lens has an effective index of refraction less than unity. If the dielectric constant of the material separating the plates is the same as that of air, the index of refraction is given by where x, is the free space wavelength and a is the distance separating adjacent plates 39.

By rotating lens 33 through an angle of ninety degrees by means of drive motor 38, the parallel conductive plates 39 are oriented as shown in FIG. 5. In this position the conductive plates 39 are substantially perpendicular to the electric field vector E of the radiated wave energy. With such an orientation the wave energy passing through the lens is propagated in the TEM mode. Since the velocity of propagation of wave energy in this mode is substantially equal to that of free space, there is substantially no relative phase shift of the wave energy emerging from different portions of the lens. Thus, with the lens 33 oriented as shown in FIG. 5 the pencil-beam radiation pattern of the planar array is unchanged.

The operation of the present invention is based on the use of a lens which is sensitive only to the components of wave energy having a given polarization and is insensitive to components of the wave energy which are orthogonally polarized. Although the radiation pattern of the antenna system of FIG. 3 is selected by rotating the lens with respect to the fixed planar array, it is understood that in some applications it may be desirable to rotate the planar array and keep the lens fixed.

If the selective use of more than two diverse radiation patterns is desired, the principles of the present invention may be extended to a multi-lens structure, such as that shown in the simplified pictorial view of FIG. 6. In FIG. 6 a basic planar array 60 is provided with a coupling waveguide section 61. First, second and third lenses designated 62, 63 and 64 are coaxially disposed in front of planar array 60 in the path of the radiated wave energy. In the embodiment of FIG. 6, lenses 62, 63 and 64 are depicted as cylindrical disks. The orientation of the parallel plates utilized in each of the lenses are indicated by arrows 65, 66 and 67 on the face of lenses 62, 63 and 64, respectively.

As indicated in FIG. 6, lenses 62 and 63 are oriented so that they have substantially no effect on the wave energy radiated from planar array 60. Lens 64, on the other hand, is indicated with an orientation which places it into operative coupling with the antenna system. As in the case of the embodiment of FIG. 3, reversible drive motors 68, 69 and 70 are utilized in conjunction with lenses 62, 63 and 64 respectively, to provide the selective orientation of the lenses. With the selective operation of these drive motors, any one, or combination of one or more lenses, can be operatively coupled into the antenna system.

In all cases it is understood that the above-described embodiments are merely illustrative of but a small number of the many possible specific embodiments which can represent applications of the principles of the present invention. Numerous and varied other arrangements, including those using radiators other than planar arrays, can be readily devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A multifunction microwave antenna system cornprising, in combination, means for radiating linearly polarized electromagnetic wave energy in a given direction of space, said radiating means being characterized by a first predetermined radiation pattern, means for selectively altering the shape of said radiation pattern, said means comprising at least one polarization-sensitive lens disposed in the path of said radiated wave energy and adapted for selective rotation about an axis substantially parallel to said given direction.

2. The microwave antenna according to claim 1 wherein said radiating means comprises a planar array.

3. The microwave antenna according to claim 1 wherein said polarization-sensitive lens comprises a plurality of spaced-apart parallel conductive plates.

4. In a directional microwave antenna system of the type having a radiator capable of radiating linearly polarized electromagnetic wave energy in a given direction of space, said radiator being characterized by a predetermined radiation pattern, and means for selectively changing the shape of said radiation pattern, said means comprising at least one polarization-sensitive lens disposed in the path of said radiated energy, and adapted for selective rotation about an axis substantially parallel to said given direction.

5. The antenna system according to claim 4 wherein said radiator comprises a planar array.

6. The antenna system according to claim 4 wherein said polarization-sensitive lens comprises a plurality of spaced-apart parallel conductive plates.

References Cited UNITED STATES PATENTS 2,712,067 6/1955 Kock 343753 2,818,563 12/1957 Butler 343756 2,680,810 6/1954 KOrrnan 343-756 RICHARD A. FARLEY, Primary Examiner.

RICHARD E. BERGER, Assistant Examiner.

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