Antenna-polarization Means

Abstract

A polarization sensitive phase shifter is applied to selected portions of the aperture of a reflecting antenna so as to produce similar primary and cross-polarized radiation phase patterns.

Description

BACKGROUND OF THE INVENTION

This invention relates to antennas in reflected wave object detection systems and more particularly to antennas including means for manipulating the polarization of radiation incident thereupon.

In radar systems such as monopulse radar, pulses are transmitted, and echo pulses which bounce off an object in the radar field of view are received by a system's antenna. One form of typical antenna comprises a reflector and an element upon which reflected radiation is focused, commonly called a feed. Information is derived from the echo pulse to determine such information as the object's angular position with respect to the boresight of the radar system. A common form of radar tracking comprises the comparison of a sum signal of the system to a defference signal. These signals are derived from the well-known sum and difference radiation patterns of a radar antenna, and may be referred to as the principal patterns. Where a curved reflecting antenna is used, cross-polarization of reflected signals is produced in response to reflection from a curved surface. The feed thus responds to cross-polarized patterns as well as the principal patterns. The linearity of tracking of an object by a monopulse radar system is adversely affected by cross-polarization. If the magnitude of the cross-polarized phase patterns are great enough with respect to the magnitude of the principal patterns, the radar system may even be totally deceived as to the position of the object it is tracking. Various techniques for reducing the adverse effects of cross-polarized returns have been utilized. One such approach is the use of an antenna having as long a focal length as possible, thus reducing the curvature of the antenna over the portion from which returned signals are reflected. Another approach is the use of a polarization screen. Both of these techniques reduce the amplitude of the cross-polarized radiation. Neither technique, however, eliminates the effects of cross-polarization. More specifically, neither technique converts the pattern of the cross-polarized returns to that of the principal return. In addition, cross-polarization may occur due to reflection from the object. No prior technique exists for providing similar primary and cross-polarized phase patterns in response to reflections from an object.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a means in a monopulse radar system to eliminate the effects of cross-polarization and provide for smoother and more accurate angle tracking by a monopulse radar system.

It is a more specific object of the present invention to provide a means for converting the cross-polarized portion of the return to a feed in a monopulse antenna into the same pattern as the principal return.

It is a specific object of the present invention to eliminate the effects of cross-polarization due to curvature of a radar antenna reflector.

It is also an object of the present invention to eliminate the effect of cross-polarization due to reflection of radar pulses from an object or active cross-polarization deception jamming.

Briefly stated, in accordance with the present invention, there is provided an antenna for use in a radar system in which a polarization sensitive phase shifter is applied to selected portions of the aperture of the reflecting antenna so as to produce similar principal and cross-polarized radiation phase patterns.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects and features of novelty are embodied in the invention which is pointed out with particularity in the claims forming the concluding portion of the specification. The invention both as to its organization and manner of operation, may be further understood by reference to the following description taken in connection with the following drawings.

FIG. 1 is a pictorial representation of a use of the present invention;

FIGS. 2a, 2b and 2c are representations of the principal polarization phase, cross-polarization phase, and their resulting composite pattern for an antenna, respectively;

FIG. 3 is a diagram illustrating a typical antenna aperture field distribution, resolved into principal and cross-polarized components.

FIG. 4 is a further representations of components of radiation illustrated in FIG. 3;

FIGS. 5 and 6 are illustrative embodiments of the present invention; and

FIG. 7 is a chart useful in understanding the operation of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Basic Theory

In FIG. 1, an object 1 is tracked by a radar system 2 having an antenna 3 including a feed 4 and a reflector 5. The radar system 2 sights along a boresight 6. For purposes of illustration, the size of the object 1 is greatly exaggerated with respect to the reflector 5. The return from the object 1 in actuality subtends a small portion of the curvature of the reflector 5, and in return is indicated by the reference numeral 7 in FIG. 1.

The angular response to the sum and difference patterns in a system in which there are only principal radiation patterns is illustrated in FIG. 2a. In FIG. 2b, the angular response of a system to a cross-polarized radiation is illustrated. FIG. 2c illustrates total response of an antenna. Magnitude of response is plotted against arbitrary units, the waveforms of FIGS. 2a-2c being nominal patterns. The magnitude of the waveforms in FIG. 2b compared to those of FIG. 2a varies with the degree of cross-polarization. Where the magnitude of the cross-polarized pattern is small with respect to the principal pattern, conventional tracking by comparing the sum signal to the difference signal may be accomplished. As the magnitude of the cross-polarized pattern increases, such tracking may become impossible. Cross-polarization produced by reflection from an antenna reflector is illustrated in FIGS. 3 and 4. FIG. 3 illustrates a typical antenna aperture field distribution. The principal components are the vertical vectors and the cross-polarization components are the horizontal vectors. The theory of the aperture-field distribution is explained at Section 12.3 of Silver, Microwave Antenna Theory and Design (McGraw-Hill Book Company, New York, 1949). In order to transform the illustration of FIG. 3 into a more workable form, the illustrations of FIG. 4 and the following convention are used.

The vectors of FIG. 3 are lumped into quadrants. If the phase of the vector in the vertical illumination, i.e., the principal polarization points upward, a plus sign is used. In the case of the cross-polarized, or horizontal component, a plus is used if the vector points to the right, and a minus is used if the vector points to the left. Thus, the representation in illustration a of FIG. 4 represents the principal polarization sum pattern, each quadrant containing a plus sign. In addition, using the above-described convention, the representation illustration b of FIG. 4 is obtained for the cross-polarization sum pattern in which plus signs are in the upper left and lower right quadrants, and minus signs are in the upper right and lower left quadrants.

Construction of the Preferred Embodiments

In accordance with the present invention, a polarization sensitive phase shifter is applied to selected portions of the aperture of the reflecting antenna 3 to produce similar response from the principal and cross-polarized radiation phase patterns. A first embodiment is illustrated in FIG. 5 in which illustration a is a section taken along the line I--I of the elevation of illustration b. The same reference numerals are used to denote elements corresponding to those of FIG. 1.

In the embodiment of FIG. 5 a filter 10 comprising a polarization sensitive phase shifter is interposed between the feed 4 which is a means for receiving radiation and the reflector 5. The filter 10 may take any well-known form, for example, a wire grid, dielectric grid, or any other well-known form. The filter 10 is oriented to affect only the cross-polarized components of the radiation incident upon the reflector 5. The dimensions are chosen such that the phase shift provided will convert the radiation pattern a of FIG. 4 to that of pattern b. This may be accomplished using well-known electro magnetic radiation theory. For example, in the specific embodiment of FIG. 5, a wire grid is used, and the wires are substantially horizontally disposed to match the polarization of incident radiation. The distance between the grid wires is less than one-quarter wave-length and more than one-eighth wavelength of the reflected radiation. The grid 10 is spaced one-quarter wavelength from the reflector 5 and conforms to its contour. The grid 10 reflects radiation. In the present embodiment, the desired phase shift may be achieved over a bandwidth equal to 10 percent of the operating frequency of the radar system 2. A transmissive filter could also be placed between the feed 4 and reflector 5.

In the embodiment of FIG. 6, a filter 11 which transmits radiation comprising first and second pairs of wire grids 12 and 13 is provided on the opposite side of the feed 4 from the reflector 5. In this particular embodiment, the wires within a grid are spaced by three-eighths of a wavelength, and the first and second grid pairs 12 and 13 have their facing sides spaced by one-quarter wavelength. Two pairs of grid wires are used since this arrangement is more convenient for achieving 180.degree. phase shift. Here, again, the frequency bandwidth over which the phase shift filter 11 is effective is approximately 10 percent.

It should be noted that the present invention may be applied to other forms of antennas. The shape of a filter such as the filter 10 or 11, i.e., the portions which must be in registration with corresponding portions of an antenna reflector, is dictated by antenna geometry. Well-known theory may be used to calculate the primary and the cross-polarized patterns. The shape of a filter necessary to convert the cross-polarized radiation to a pattern similar to the primary radiation is a shape including filter portions in registration with portions of an antenna in which vector representations of the primary cross-polarized patterns of the antenna illumination differ. Well-known electromagnetic radiation theory may be used to provide any number of different sorts of planar or other filters to provide the necessary phase shift. Thus while two pairs of planar grids are provided in the specific embodiment of FIG. 6, other numbers of differently shaped grids could be employed.

The filter comprises polarization modifying means in the form of selective phase shifting means. The filter is selective in that it only affects one of the two components of returned radiation. In the embodiments illustrated, only the cross-polarized component is phase shifted.

Of course, while the filters 10 or 11 are oriented to convert the cross-polarized radiation pattern to the primary pattern, they may be oriented to do the opposite. In either case, a smooth response such is that of FIG. 2a is provided, rather than that of FIG. 2c.

OPERATION

In FIG. 7, desired distributions of a two lobe antenna system are illustrated in column a, columns b and c illustrate the radiation phase distribution for vertical and horizontal components of radiation; and columns d and e represent the radiation phase distribution of the vertical and horizontal components for a corrected antenna using the embodiment of FIGS. 5 or 6. The symbol for the well-known pattern to which each representation in FIG. 7 corresponds is marked next to the representation. The azimuth difference pattern is denoted .DELTA.Az and the elevation difference pattern is denoted .DELTA.E1. It will be noted that the corrected antenna patterns agree with the desired distribution patterns. A convention similar to that of FIG. 4 is used. Upward pointing vectors of the vertical component correspond to a plus, and downward pointing vectors correspond to a minus for the vertical component. For the horizontal component, arrows pointing to the left correspond to a plus, to the right correspond to a plus and the arrows pointing to the left correspond to a minus.

In order to illustrate the operation of the present embodiment, the response to the sum signal as illustrated in FIG. 6 is examined. Note that the horizontal component representation, the upper right and lower lefthand vectors, point to the left, corresponding to phase shifting means. For example, filter 10 of FIG. 5 or filter 11 of FIG. 6 operates only on these portions of the antenna to change the cross-polarized component orientation by 180.degree. to provide the uncorrected horizontal representation to the corrected horizontal representation. The invention operates similarly on the other representations derived from the returned signal.

The present invention is especially useful in situations in which the cross-polarization is produced by the object 1 (FIG. 1). Where a reflected signal is cross-polarized due to reflection from an object on the boresight 6, for example, the return will be the same as the primary return of an object which is off boresight. Thus a radar system tracking a target producing cross-polarized reflection would respond as though the target were off boresight. Consequently, a tracking radar would be deceived into scanning a portion of the sky other than where the object 1 is. The present invention eliminates this problem.

In addition, the invention provides for smoother monopulse tracking even where the cross-polarization is due only to the geometry of the antenna reflector. As seen in FIG. 2a, a smoother sum to difference ratio is obtained than for the representation of FIG. 2c.

The present invention is not limited to the standard lobe radar, but may also be incorporated in conical scan-lobe on receive only (LORO) and conical scan radar systems. The invention may also be practiced in accordance with the above teachings on non-reflective antennas such as aperture and lens antennas and various types of arrays.

Claims

What is claimed as new and desired to be secured by Letters Patent of the

1. In a radar antenna, a polarization selective antenna illumination phase shifter comprising:

polarization modifying means having portions placed in registration with portions of an antenna in which a principal polarization vector representation of antenna illuminating differs from a cross-polarization

2. In a parabolic reflector antenna, means for eliminating cross-polarized return comprising:

selective phase-shifting means for shifting the phase of one component of returned radiation 180.degree., said phase-shifting means including portions in registration with first and second opposite quadrants of said

3. The arrangement of claim 2 in which said phase shifting means is

4. The arrangement of claim 3 in which said phase-shifting means is of parabolic contour and spaced one-quarter wavelength from said reflector.

5. The arrangement of claim 2 in which said phase-shifting means is planar, disposed substantially perpendicularly to the boresight of said antenna, and on the opposite side of the feed of said antenna from said reflector.

6. An antenna comprising, in combination:

a. a feed for receiving radiation;

b. a reflector for focusing radiation on said feed; and

c. selective phase shifting means for converting the phase distribution of one component of antenna illumination, said selective phase shifting means including a portion in registration with a portion of said reflector in which a principal polarization vector representation of antenna illumination differs from a cross-polarized vector representation of

7. An antenna according to claim 6 in which said selective phase shifting

8. An antenna according to claim 6 in which said selective phase shifting means is disposed on the opposite side of said feed from said reflector.

9. An antenna comprising in combination:

a. a surface for illumination by radiation; and

b. selective phase shifting means for converting the phase distribution of one component of antenna illumination, said selective phase shifting means including a portion in registration with a portion of said surface in which a principal polarization vector representation of antenna illumination differs from a cross-polarization vector representation of antenna illumination.