Binary phase-scanning antenna with diode controlled slot radiators

BACKGROUND OF INVENTION

In many fields such as that of radar it is common practice to radiate a highly directional beam of electromagnetic energy and to scan the beam so as to controllably vary the direction of beam propagation. It is noted that there have been developed a large number of antennas, antenna arrays and antenna feedand-control systems to achieve the above-identified result. It is stated, for example, in U.S. Pat. No. 3,286,260 to Shirly La Var Howard that it is conventional to employ phase shifting between adjacent elements of an antenna array in order to produce scanning of the beam. By incrementally changing the relative phase of energy radiated from successive elements of an array, the direction of a beam from a broadside array, for example, can be shifted. One manner of changing the relative phase of energy radiated from adjacent elements of an array is to vary the frequency of element energization. Another manner of electronically scanning a beam from an antenna array is to employ phase-shifting devices between elements for changing the phase of energy radiated from separate elements. In this latter category fall ferrite-loaded beam-shifting antennas and the like. The prior art has relied upon some manner of relatively continuously varying the phase between successive elements of an antenna array in order to electronically scan a beam radiated therefrom, and the above-noted patent employs both of the phase-shifting techniques identified above.

While it is recognized that a directional beam can be electronically scanned, it is generally accepted that such scanning requires the utilization of some type of continuous or near continuous phase variation at each of the radiating elements. This requirement is highly disadvantageous in necessitating the utilization of relatively complex structures and circuits.

The present invention provides for the scanning, or controlled variation, in the direction of propagation of the beam by the selective reversal of the phase of energy radiated from separate elements of the antenna array. Thus, in accordance with the present invention, it is not necessary to employ any type of continuous or near continuous phase variation; there is consequently achieved a material simplification of structures and circuits required for electronic-beam scanning.

SUMMARY OF INVENTION

The antenna of the present invention comprises a plurality of radiators which may be physically embodied as dipoles, waveguide slots or the like. These individual radiators are energized in some predetermined or random phase relationship which satisfies the following conditions:

I. phase distributed approximately uniformly over at least two adjacent phase quadrants

2. phase of elements of any group of adjacent elements substantially different

3. aperiodic phase distribution

Selective phase reversal of energy radiated from individual antenna elements is herein employed to first produce a desired beam pattern, and second to produce a desired scanning of, or change in, such beam. In the following description of the present invention the production of a highly directional beam of electromagnetic energy is taken as an example, and the explanation of the invention is referenced to the production of such a beam and to the scanning of same, i.e., the controlled variation in direction of propagation. It is, however, to be appreciated that the present invention is equally applicable to the generation of substantially any desired beam pattern and to controlled changing of the pattern.

The antenna of the present invention produces an aperiodic phase front which suppresses radiation in other than the desired direction, and it is to be noted that this is quite contrary to conventional systems or antennas normally generating a plane or periodic phase front. In accordance with the present invention there is derived a relationship for the relative far field voltage at a predetermined far-field point and containing a phase term. It is herein determined that the far-field pattern is not determined by a unique set of individual element excitation phases, but, instead, is determined by the phase of the algebraic summation of radiation from a number of elements. In accordance herewith the excitation phases of separate elements are considered to be distributed in a nonperiodic manner over the range of 0 to 2ir radians. The phase of each element in the array is then compared in the value of the phase term as described in more detail below, and if the elements' excitation phase differs from this value by more than π/2 radians, the phase thereof is reversed. Consequently, the phase of the resultant of the summation of all elements in a strip statistically approaches the above-noted term as the number of elements is increased.

The present invention may be best described and most easily understood in connection with a series of linear strips of radiating elements, and is thus so described below, [t is, however, to be appreciated that the invention is equally applicable to circular apertures, as is also discussed below.

DESCRIPTION OF FIGURES

FIG. 1 is a schematic illustration bearing notations employed in theoretical considerations upon which the present invention is based;

FIG. 2 is a schematic illustration of an octagonal antenna array in accordance with the present invention;

FIG. 3 is a partial perspective view of a slotted waveguide as may be employed in the present invention;

FIG. 4 is a schematic perspective illustration of a dipole radiator as may be employed in the present invention; and

FIG. 5 is a simple circuit diagram of diode connections for switching in accordance with the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention may be best described and understood by initially considering a planar aperture surface A and radiation in a plane perpendicular thereto. In this respect reference is made to FIG. 1 employing the conventions:

A = aperture surface

N = normal-to-surface at some reference point O 6=angle between the straight lines O-N and O-P L = line on aperture surface defined by the intersection of the plane Q containing the lines ON and OP and the aperture surface

S = narrow strip on aperture surface perpendicular to line L and containing a large plurality of radiating elements

The far-field radiation pattern in plane Q of the array of radiating elements in FIG. 1 is determined by the relationship

AU elements

In the foregoing relationship the terms are defined as follows: E(6 )= relative far-field voltage

β) = a phase term related to the difference in path length from the reference point O to the far-field point P and from the aperture point, i, to the far-field point P. This phase term is a function of the location of the i** element relative to the point O and of the angle Θ.

a, = complex voltage feeding coefficient for the element located at position i.

Consider first that the point, P, toward which the peak of the radiation pattern is to be directed has been chosen. This choice determines the plane, Q, and therefore the intersection line L. This, then, also fixes the orientation of the aperture strips such as S. The orientation of these aperture strips may thus be different for different positions of the main beam peak. Making use of the fact that, once the strip orientation has been determined, the elements lying on any strip are

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4 equidistant from the far-field point P, the relative far-field voltage may be written as:

E (9) = 2[ei*«..«z_ai] All All strips elements in a strip where gid,, 6) = a phase term that is the function of the location of the strip and of the angle Θ.

It is to be particularly noted that in accordance with the equation immediately above, the far-field pattern in the plane Q is not determined by a unique set of individual element excitation phases, but, instead, is determined by the phase of the algebraic summation of all elements in the strip. Consequently in order to focus energy toward the far-field point P, it is only necessary that the elements in any strip satisfy the following relation: _____

Phase of [Sad = — g(d_t, e)+C(t)

All elements in a strip where C(t) is any spacial constant and may vary with time. In order to satisfy the requirements of the phase relationship set forth immediately above, the excitation phases of the elements in each strip are considered to be distributed in a nonperiodic manner over the range of 0 to 2rr radians. The phase of each element is then compared to the value of the right-hand term in the phase equation above corresponding to the strip in which the element lies. If the excitation phase of the element differs from this value by more than ir/2 radians, the phase thereof is reversed. The phase of the resultant of the summation of all elements in a strip thereby statistically approaches —g(d, , Θ} =C(t) as the number of elements is increase. The amplitude of the resultant approaches ___________________________

It is to be appreciated that the requirements of the phase equation above does not constrain the distribution of elements within a strip; thus this distribution may be made such that the resultant of the summation of all elements in a strip is considerably less than |s|<n| for all points outside of the plane Q. By making the distribution different for each strip, it is possible to further reduce the radiation outside of the plane Q, and by the proper variation of C(t) it is possible to still further reduce this radiation. In fact, the maximum minor lobe level can be made to be essentially the same as would be generated by a planar phase-front antenna having the same amplitude distribution. This is best understood by considering the far-field electric field to be composed of two components; one in phase with C(t) and one in quadrature with C(t). The in-phase components thus produce a radiation pattern with essentially the same minor lobe level as would be produced by a planar phase-front. The quadrature components produce a pattern which fluctuates about a spacial average power level given by ir²,/8n where n = number of elements.

As the value of C(t) is varied, the quadrature pattern maxima and minima are moved, while the in-phase pattern remains fixed. Therefore, the time average quadrature pattern level approaches the above spacial average at all points in space. This, by increasing the number of elements in the array, the maximum quadrature pattern level may be reduced to an arbitrary amount below the maximum in-phase or planar phase-front level.

A practical embodiment of the present invention may be constructed by the provision of a slotted waveguide with individual radiating elements of the invention being comprised of slots cut in the broad wall of the guide as schematically illustrated in FIG. 3. Energy is propagated in the TE₀₁ mode through the rectangular waveguide 11, and energy is coupled out of the waveguide through slots formed through the broad 5 wall thereof. There is shown in FIG. 3 one pair of slots 12 and 13 in such a waveguide with the slots of each pair being symmetrically disposed on opposite sides of the center line of the guide. It will be appreciated that energy propagated through the waveguide will be coupled out of the guide through these ¹⁰ slots, with pairs of slots being spaced longitudinally along the guide. In accordance with the present invention, provision is made for reversing the phase of energy coupled from any pair of slots. This is herein shown to be accomplished by the location of diodes 14 and 16 in the slots 12 and 13, respectively. ⁵ These diodes are preferably electrically connected in parallel, as illustrated in FIG. 5. Application of a bias voltage in one direction between the terminals 17 and 18 of FIG. 5 will serve to cut off one of the diodes, and oppositely poled bias voltage will cut off the other diode. These diodes serve to open or close the waveguide slots for coupling of energy from the waveguide.

It is possible in accordance with conventional practice to employ diodes in the manner described above to short one or 25 the other of the slots of each pair, so that only the slot which is not shorted will couple energy from the waveguide. It is, however, provided in accordance with the present invention that the diodes 14 and 16 shall be employed only to detune the slots. With the diodes being employed to controllably short 30 the coupling slots, it is necessary for these diodes then to carry a relatively heavy current, but with the diodes placed as shown in FIG. 3 at the ends of the slots, it is provided that conduction of a diode only detunes the slot, rather than fully shorting it. It will be appreciated that a detuned slot does not couple sub35 stantial energy from the waveguide. It is to be further appreciated that employment of the diodes in the manner described above materially reduces the rating required of the diodes, so that it is possible to employ much less expensive diodes for phase reversal herein. It was, in fact, found in one 40 application of the present invention that diodes having a normal capability of operating at 2 billion Hz., when located at the ends of the waveguide slots as shown, provided fully satisfactory switching at 10 billion Hz. It is considered that this is a marked and novel improvement.

Switching between waveguide slots 12 and 13 of each pair of slots in the waveguide provides for reversing the phase of energy coupled from each “element of the waveguide. As noted above, the present invention provides for beam scanning by selective phase reversal of energy radiated from individual elements of a large plurality thereof. Thus, the above-described slotted waveguide structure with diode switching for selective slot detuning is capable of carrying out the present invention.

The slotted waveguide structure described above is only one example of structure in accordance with the present invention. Insofar as individual radiating elements are concerned, it is possible to employ various types and structures. Thus, for example, there is illustrated in FIG. 4 a dipole 21 having quarter-wavelength arms 22 and 23 extending outwardly from the outer conductor of a coaxial cable 24 at the upper terminus thereof. This outer conductor is longitudinally slotted from the upper end for a distance of one-quarter wavelength to separate the outer conductor into two portions of such length, with one of the arms 22 or 23 connected to each side of the outer conductor. Switching is accomplished with this structure by the provision of a pair of diodes 26 and 27 connected between a central conductor 28 of the coaxial cable and the separate arms 22 and 23. These diodes are preferably connected in parallel as indicated in FIG. 5, so that it is possible with appropriate biasing to cause either of the diodes to conduct. By the application of energizing voltage between inner and outer conductors of the coaxial cable 24, there is thus applied energization to the arms of the dipole with the phase of such energization being reversible by control of the conduction of the two diodes 26 and 27.

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It is also possible to form the present invention as a pair of spaced plates energized, for example, by a probe extending between the plates at the center thereof and probes from individual radiating elements extending through one of the plates at varying distances radially outward from the energizing probe. Various other conventional types of radiators and radiator-energizing means may be employed in carrying out the present invention.

There is illustrated in FIG. 2 of the drawing an octagonal array of radiators formed, for example, of strips of radiators 31, 32, etc., transversely thereacross, and each of such strips being comprised of a plurality of successive individual radiators. Each strip could, for example, be formed as a waveguide of the type illustrated in FIG. 3, with the individual radiators being then comprised as pairs of waveguide slots, as described above. This configuration of FIG. 2 provides a substantially circular aperture so that the distribution of phases in any strip located a given distance from the center of the aperture, for example, will be independent of the location of the point P at which the beam is to be directed consequently, the far-field pattern shape is independent of the choice of the plane Q of FIG. 1.

It is to be understood that the present invention employs a very substantial number of radiators or radiating elements, and that the elements of any strip or line thereof are energized to radiate in at least tow adjacent phase quadrants so that phase-reversal radiation is achieved in all four phase quadrants. Thus it is possible by reversal of the phase of radiation from selected elements to produce a desired beam direction; consequently, by further selected reversals to scan such a beam. With regard to this accomplishment of phase reversal, it is noted that the preceding description references diode switching; however, it is believed evident that alternative types of switching may be employed. It is furthermore noted that the sequence of diode switching, or phase reversal, and the manner in which such is physically accomplished, is likewise open to wide variation. Electronic or electromechanical means may be employed to control diode switching. With regard to the establishment of a particular desired beam, it is possible to locate a radiation source at a point P with phase C(r) toward which such a beam is to be directed and to measure the radiation at each element of the invention and then to proceed as described above by reversing the phase of those elements differing from the phase of energy received by more than 90°. This may be repeated for different locations of point P throughout a beam scan so as to thus arrive at a phase reversal program, which, when repeated, will direct a beam as desired and scan same in a desired manner. More practically, the programming of reversal may be accomplished by a computer.

With regard to the phase of energy radiated from the elements of an antenna array in accordance with the present invention, it is noted that one highly advantageous arrangement is a random phase distribution between elements. It will be seen that any line 41 drawn across the array of FIG. 2 may be considered as a strip of radiating elements in which elements of the strip radiate in all four quadrants of phase relationship, so that it is possible in accordance with the present invention to reverse the phase of particular elements thereof to generate a beam directed to any far point P. An antenna array of the general configuration of FIG. 2 and having of the order of 400 radiating elements was employed to produce a highly directional beam which was readily and rapidly scanned through a predetermined pattern by the successive reversal of phase of radiation from individual elements of the array.

There has been described above an improved and simplified scanning antenna formed of a large plurality of radiating elements energized to produce radiation in all four phase quadrants and adapted for reversal of phase of radiation from individual elements thereof. In this manner there is produced a desired radiation pattern from the array; and scanning of the beam so produced is accomplished by appropriate switching of the phase of energy radiated by individual elements of the array. There is also provided hereby an advantageous arrangement for reversing the phase of radiation from waveguide slots by the location of diodes at the ends of such slots for detuning a predetermined one of a pair of slots located on opposite sides of the center line of a waveguide. Prior art requirements of frequency variation or substantially continuous phase variations for electronic beam scanning are hereby precluded.

Although the present invention is described herein with respect to particular preferred embodiments thereof, it is not intended to limit the invention to the exact terms of description or details of illustration, but, instead, reference is made to the appended claims for a precise delineation of the true scope of this invention.