BACKGROUND OF THE INVENTION

This invention relates to antenna systems for radiating wave energy in a desired pattern of radiation amplitude. In particular this invention relates to antennas designed using the pattern synthesis technique to determine the amplitude and phaseof the aperture excitation which will achieve the desired rediation amplitude pattern.

It is well known that any desired radiation amplitude pattern may be approximately achieved using a combination of component antenna beams which result from component aperture excitations. The desired amplitude pattern results from thesuperposition of the component beams in space, and a corresponding composite aperture excitation is determined by the superposition of the component aperture excitations.

In practice it is convenient to choose component aperture excitations which radiate an orthogonal set of component antenna beams. In an orthogonal set of antenna beams, each beam has a direction of maximum radiation associated with it, in whichdirection all other beams in the orthogonal set have a radiation null. Using an orthogonal set of component antenna beams results in there being a corresponding set of directions in space at each of which the amplitude of radiation is determined by theamplitude of a single component aperture excitation.

The pattern synthesis technique may be used to determine an aperture excitation which will cause the antenna to radiate an antenna pattern having any desired amplitude characteristic with direction. As generally applied, the pattern synthesistechnique results in one area of the antenna aperture having all of the excitations in phase. The result is that in this area of the antenna aperture there is a substantial reinforcement of the component electromagnetic fields, which can result indifficulties associated with high-power density. Another disadvantage occurs when the antenna aperture is an array of antenna elements because the elements in the area of phase reinforcement of the orthogonal excitations must have a substantially largeramount of energy coupled to them than is coupled to the other elements in the array. This large amount of coupling creates substantial difficulty when a series feed arrangement is used to couple wave energy to the elements of the array.

OBJECTS OF THE INVENTION

It is, therefore, an object of the present invention to provide a new and improved antenna system for radiating wave energy in a desired radiation pattern using a composite aperture excitation which is the superposition of a plurality ofcomponent aperture excitations.

It is a further object of the present invention to provide such an antenna system wherein the aperture does not have an area where there is substantial phase reinforcement of the component aperture excitations.

It is a still further object of the present invention to porvide such an antenna system using an array of antenna elements wherein the amplitude of the wave energy coupled to the elements has a more uniform distribution among the elements thancould have been achieved using prior art aperture excitations.

In accordance with the present invention, there is provided an antenna system for radiating wave energy in a desired radiation pattern. The antenna system includes an aperture, comprising an array of antenna elements, for radiated wave energypatterns in response to wave energy excitations. The antenna system additionally includes means for supplying wave energy to the elements with predetermined relative phases and amplitudes to develop a composite wave energy excitation on the aperture. The relative phase and amplitude of the wave energy supplied to each of the elements comprises the vector sum of a plurality of component aperture excitations, measured at the location of the element on the aperture, including a reference excitation andother component excitations having both positive and negative phase variation on the aperture with respect to the reference excitation. The component excitations with positive phase variation have, with respect to the reference excitation, an averagephase displacement which is a first monotonic function of the phase variation, and the component excitations with negative phase variation have, with respect to the reference excitation, an average phase displacement which is a second monotonic functionof the phase variation. All excitations have the same sense of average phase displacement. There results a set of element excitations without substantial amplitude reinforcement of the component excitations at any selected one of the antenna elements.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in conjunction with the accompanying drawings and its scope will be pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) illustrate respectively the side view and front view of a linear array antenna system constructed in accordance with the present invention. FIGS. 2(a), and 2(b) and 2(c) illustrate a pattern synthesis technique.

FIGS. 3(a) and 3(b) illustrate respectively the component and composite aperture excitations used in the prior art aperture synthesis technique.

FIGS. 4(a) and 4(b) illustrate respectively the component and composite aperture excitations used in the aperture synthesis technique in accordance with the present invention.

FIGS. 5(a) and 5(b) illustrate respectively the front view and side view of another antenna system constructed in accordance with the present invention.

DESCRIPTION AND OPERATION OF THE FIGURE 1 ANTENNA SYSTEM

The antenna system illustrated in FIG. 1 includes a linear array of dipoles 10(a) through 10(h), mounted on a conductive ground plane 11. Transmission lines 12(a) through 12(h) connect the dipoles 10 to corresponding directional couplers 13(a)through 13h). The directional couplers 13 are in series and are connected to a common input port by transmission line 14. Resistive loads 15 are used to terminate the transmission line 14 and the isolated outputs of the couplers 13.

The dipoles 10, mounted on the ground plane 11, form an antenna aperture which will radiate wave energy patterns in response to wave energy excitations on the aperture. A wave energy excitation is developed on the aperture by supplying to theindividual dipoles 10 wave energy signals having preselected relative amplitudes and phases.

The spacing of the dipoles 10 along the linear array, the length of the linear array, and the number of dipoles 10 required are chosen in accordance with principles which are familiar to those skilled in the art. It will be evident that otherantenna elements besides dipoles may be used to construct the linear array of the FIG. 1 embodiment. Other commonly used antenna elements are feedhorns, waveguide slots and spirals.

In the FIG. 1 antenna, wave energy signals are supplied to the dipoles 10 from the input by means of transmission line 14, directional couplers 13 and transmission lines 12(a) through 12(h). It will be evident to one skilled in the art that theamplitude of the wave energy signals coupled to each of the dipoles 10 is regulated by the coupling coefficients of the various directional couplers 13(a) through 13(h). The phase of the wave energy signals coupled to each of the dipoles 10 isdetermined by the phase length of the input transmission line 14, the directional couplers 13 and the transmission lines 12. It is evident that the structure provides for individual adjustment of the amplitude and phase of wave energy signals that aresimultaneously coupled to each of the dipoles 10. The transmission line 14 used in the FIG. 1 embodiment may be of any type appropriate for use at the operating frequency of the antenna. Typical transmission lines which might be used are waveguides,coaxial lines, and strip transmission lines. The directional couplers 13 may be any type appropriate to the chosen transmission line type. Those skilled in the art will recognize that other means, besides directional couplers, may be used to supplywave energy signals to the dipoles 10 from the input. Examples are reactive power dividers or enclosed multi-mode transmission lines.

FIG. 2(a) indicates a wave energy pattern which may be desired from the FIG. 1 antenna. The amplitude of the wave energy in the desired pattern is constant over a particular range of the angle (A), which is designated in the FIG. 1 drawing. Itis also desired that there be no radiation at angles outside of the desired angular region.

FIG. 2(b) indicates the main lobes of a set of component orthogonal antenna beams which would be radiated by the FIG. 1 antenna when fed with a set of wave energy signals whose amplitude and phases are chosen in accordance with prior arttechniques. The component beams 16(a) through 16(e) would be radiated by component aperture excitations having uniform amplitude at all of the elements and phase distributions which are orthogonal to each other. Orthogonal phase distributions have aphase variation relative to each other which is an integral multiple of 2.pi. across the aperture of the antenna.

In FIG. 2(b) the beam designated 16(c) is a beam which would be radiated by a reference component excitation having equal amplitude and equal phase at all of the elements. Beams designated 16(b) and 16(d) are radiated by other component apertureexcitations with phase variations, with respect to the beam 16(c) excitation, of plus 2.pi. and minus 2.pi., respectively. Beams designated 16(a ) and 16(e) are radiated by component excitations which have phase variations of plus 4.pi. and minus4.pi., respectively, with respect to the reference excitation corresponding to beam 16(c).

FIG. 2(c) represents the composite radiation pattern which results from the superposition of the five beams in FIG. 2(b). This radiation pattern is achieved if the aperture is provided with a composite aperture excitation having an amplitude andphase distribution which is the superposition of the component aperture excitations which result in the beams of FIG. 2(b).

In order to develop the desired composite radiation pattern shown in FIG. 2c from the linear array of FIG. 1 using prior art techniques, such as those described in detail in Section 2.13 of the "Antenna Engineering Handbook" by Henry Jasik(McGraw-Hill, 1961 ), it is necessary to use an aperture excitation which is the superposition of the composite aperture excitations corresponding to the component antenna beams of FIG. 2b. In the illustrated example all of the component antenna beamshave the same amplitude, so it would be appropriate that all of the component aperture excitations have the same amplitude. The phase distributions 17(a) through 17(e) of the component aperture excitations which result in the component antenna beams16(a) through 16((e) of FIG. 2b are illustrated in FIG. 3a. In order to obtain the required phase and amplitude excitations for elements 10(a) through 10(h) it is necessary to make a vector addition of the component aperture excitations at the locationon the aperture of each of the antenna elements 10(a) through 10(h). Since all of the component excitations have uniform amplitude distribution on the aperture, and each has the same amplitude, the amplitude and phase of the required excitation at eachelement is determined by vector addition of five vectors (one for each component aperture excitation) of equal amplitude and each having a phase which is determined by the phase distributions shown in FIG. 3a.

For example, in accordance with prior art techniques, the excitation for element 10(a) is determined by adding five vectors having equal amplitude and phases of approximately 2.pi., .pi., 0, -.pi. and -2.pi.. These phases are determined by thevalue of phase distributions 17(a) through 17(e) at the location of element 10(a) on the aperture. Likewise, for example, the excitation for element 10(d) is determined by adding five vectors having equal amplitude and approximately zero phase asindicated by FIG. 3a.

FIG. 3(b) shows the resultant amplitudes of excitation for each of the elements 10(a) through 10(h). It will be noted that the amplitudes of excitation for elements 10(d) and 10(e) are greatly in excess of the average amplitude excitation of theelements.

Having determined the amplitude and phase of the excitation required for each of the elements 10(a) through 10(h) of the FIG. 1 array, the desired excitation may be achieved by proper selection of the coupling values for couplers 13(a) through13(h) and transmission lines 12(a) through 12(h). To achieve this it is necessary to compute the percentage of the total power supplied to all of the antenna elements which must be supplied to each of the individual elements. The coupling value foreach of couplers 13(a) through 13(h) is then computed on the basis of the fractional power to be supplied to each element with respect to the power remaining in transmission line 14 at the input to the particular coupler, allowing for power previouslycoupled out. According to the type of transmission line used it may also be necessary to make allowance for power loss in the transmission line 14. The phase of the wave energy supplied to each of the elements 10(a) through 10(h) is adjusted by varyingthe length of the respective transmission lines 12(a) through 12(h). Illustrated in FIG. 3b is the amplitude excitation which would result for each of the elements 10(a) through 10(h) if the prior art synthesis technique were used to achieve thecomposite radiation pattern illustrated in FIG. 2c. As is evident from FIG. 3b and has been discussed above, the phase variations of the prior art synthesis technique tend to cause reinforcement of the amplitude of the excitations of the elements in aparticular region of the aperture. In the set of amplitude excitations illustrated in FIG. 3b elements 10(d) and 10(e) have a much greater amplitude excitation than the remaining elements. In the illustrated case the differences in amplitudeexcitations can be as much as 10:1 in voltage, which results in differences of 100:1 in the power to be supplied to the elements of the array.

DESCRIPTION AND OPERATION OF THE ANTENNA SYSTEM OF FIG. 1 BASED ON THE PRESENT INVENTION

It is an object of the present invention to provide an antenna for radiating a desired radiation pattern without substantial reinforcement of the excitation at any of the elements on the aperture. An antenna constructed in accordance with thepresent invention may be identical in circuit arrangement and detailed design technique to prior art antennas but different component values are used to achieve the required amplitude and phase excitations for the elements. The present invention avoidsthe disadvantage of the prior art aperture designs which result in substantial reinforcement of the energy supplied to particular elements of the array.

FIG. 4(a) indicates the phase distributions 17' (a) through 17' (e) of component aperture excitations which are selected in accordance with the present invention. It should be noted that there is a reference excitation 17' (c) and othercomponent excitations having both positive and negative phase variation with respect to the reference excitation. It should be further noted that the component excitations 17' (a) and 17' (b) which have positive phase variation also have, with respectto the reference excitation 17' (c), average phase displacements x' and x, respectively, and these average phase displacements x' and x are monotonically related to the phase variation of their respective component excitations 17' (a) and 17' (b). Thus,component excitation 17'(a), which has a phase variation of 4.pi. with respect to the reference excitation 17'(c), has a greater average phase displacement x' than component excitation 17' (b), which has a phase variation of 2.pi. and an average phasedisplacement x. Similarly, component excitations 17' (d) and 17' (e), which have negative phase variation, also have, with respect to the reference excitation 17' (c), average phase displacements x and x', respectively, and these average phasedisplacements x and x' are monotonically related to the phase variation of their respective component excitations 17' (d) and 17' (e). The average phase displacements of all excitations with respect to the reference excitation have the same sense. Useof displacements with the same sense prevents phase reinforcement of the component excitations at another point on the aperture.

In the embodiment described by the FIG. 4(a) phase diagram, excitation 17' (a) has the same average phase displacement x' as excitation 17'(e). Similarly, excitation 17' (b) has the same average phase displacement x as excitation 17' (d). Itshould be noted that the average phase displacements of excitations having a positive phase variation may be a different monotonic function of the phase variation than the average phase displacements of excitations having a negative phase variation.

The effect of introducing average phase displacement of the component aperture excitations is to eliminate the point of phase reinforcement in the composite excitation. FIG. 4(b) illustrates the amplitude of the composite excitation at thevarious array elements 10(a) to 10(e) of FIG. 1, which results from the superposition of the component excitations 17' (a) through 17'(e). By comparing FIGS. 3(b) and 4(b), it will be seen that the relative amplitude of excitation for the elements 10(d)and 10(e) has been reduced by approximately 60 percent. The amplitudes of excitation which must be coupled to the remaining elements of the array have been correspondingly increased, resulting in a more uniform amplitude distribution in the compositeaperture excitation. The actual amplitude and phase excitations for an antenna array design in accordance with the present invention is determined in a manner similar to that for the prior art excitation, except that component aperture excitationshaving phase displacements similar to that illustrated in FIG. 4a are used to determine the amplitude and phase excitations for the various elements in the array.

The effect of the average phase displacement of the component aperture excitations is a corresponding phase change in the component radiated beams. Therefore, if the component excitations of FIG. 4(a) are used, antenna beam 16(b) of FIG. 2(b)will result from excitation 17' (b) of FIG. 4(a). The component antenna beam 16(b) will have a phase difference from the reference component antenna beam 16(c) equal to the average phase displacement x of the component excitation 17' (b). The effect ofthe phase differences among antenna beams on the composite antenna pattern is small, since the phase difference between adjacent antenna beams is small. In the FIG. 4(a) embodiment adjacent beams are displaced in phase by approximately .pi./2. Thisphase difference between adjacent beams may cause an increase in the "ripple" effect on the composite antenna pattern as shown in FIG. 2(c). The magnitude of the ripple effect increases with increased phase difference between adjacent beams.

The desired shape of the radiation pattern may be other than uniform amplitude as in the FIG. 2 example. Specific applications may require radiation patterns which are tapered or even multi-lobed. In such case, the desired pattern may besynthesized using component excitations with different relative amplitudes. In some cases component excitations may even have opposite polarity. However, the present invention may be applied to these cases without difficulty.

It will be evident to those skilled in the art that finer pattern detail and more precise correspondence between the composite antenna pattern and the desired antenna pattern will result from the use of a larger antenna aperture withcorrespondingly smaller component antenna beams. The selection of aperture size would naturally involve a tradeoff between the lower cost of a small aperture and the better correspondence to the desired pattern available from a large aperture.

It will be evident to one skilled in the art that using the basic principles of the invention, as they are disclosed above, one can use the present invention to construct antenna systems of many different forms. For example, the linear array ofthe FIG. 1 embodiment may be combined with other linear arrays to form a planar or cylindrical array antenna using the present invention. The technique for formulating the composite aperture excitation may also be applied in the perpendicular plane ofan antenna system which uses a plurality of the FIG. 1 linear arrays. It will also be evident that it is not necessary that the antenna aperture be an array of elements. Once a desired composite aperture excitation has been formulated using thesynthesis technique, it is possible to achieve that illumination on an aperture consisting of a transmissive or reflective focusing means excited by conventional means, such as a plurality of feed elements. In such an application, each feed elementforms a component excitation on the focusing means and a composite excitation on the aperture would result from simultaneous excitation of all of the feed elements.

DESCRIPTION OF THE FIG. 5 ANTENNA SYSTEM

FIG. 5 illustrates another linear array antenna system constructed in accordance with the present invention. In the FIG. 5 antenna system the antenna elements consist of slot elements 20 located in a side wall of a rectangular waveguide 19. Inthis embodiment the amplitude of the wave energy supplied to each of the slot elements 20 is determined by the fraction of the wave energy in the waveguide coupled to the slot element 20, which is a function of the angle of the slot element 20 in theside wall of the waveguide 19. The phase of the wave energy coupled to each of the slot elements 20 is determined by the phase of the wave energy in the wave guide 19 at the location of the slot element 20. The phase of any slot element 20 may bechanged by 180.degree. by reversing the slot inclination angle. Once a desired composite aperture excitation, amplitude and phase has been determined with the invention as described above, it is possible to locate the slot elements 20 along thewaveguide 19 so that wave energy signals introduced at the input end of the waveguide 19 are coupled to the slot elements 20 with the required phase; the inclination angle of each slot element 20 is adjusted so that the slot elements 20 will have therequired amplitude of wave energy signals coupled to them.

The present invention is particularly advantageous in the FIG. 5 embodiment since there is a practical limit to the fraction of the energy in the waveguide 19 which may be coupled to each slot element 20. The present invention facilitates theimplementation of the FIG. 5 embodiment by allowing the use of an aperture excitation which has a more uniform amplitude distribution of the wave energy signals coupled to each of the elements.

In describing the various embodiments above, reference has been made to transmitting antenna systems, but it will be recognized by those skilled in the art that the principles of the present invention can also be applied to receiving antennasystems. Accordingly, the appended claims are intended to be construed as covering both transmitting and receiving antenna systems regardless of the descriptive terms actually used therein.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from theinvention and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

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