US3478362A - Plate antenna with polarization adjustment - Google Patents

Description

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l l "lo ,fs l, INK/Emo 3D LEoN J. Riem RS MILTON L ROSENTHAL TQM United States @arent @ffice ABSTRACT 0F THE DISCLOSURE A low profile antenna is disclosed comprising a plate uniformly spaced from a ground plane. The plate is .ted RF energy at selected opposed points to produce separate current paths in the plate. Polarization may be controled by suitably phasing the currents.

This is a continuation of application Serial No. 519,178 filed Ian. 6, 1966.

This invention relates to antenna systems and methods and means for coupling to an antenna to provide a prescribed radiation pattern about the antenna of linearly or circularly polarized radiation.

Heretofore, the diret-lion of polarization of radiation transmitted by or detf;;,ted by an antenna system has been determined by the orientation of the elements of the antenna relative to the direction of the transmitted or received radiation. For example, it has often been the practice to employ at least two separate antenna elements, each designed for transmitting or receiving radio frequency (RF) energy of a given direction of polarization. These two elements are oriented in space relative to each other, so that their directions of polarization combine to form a resultant direction of polarization as desired. Thus, '5;' energizing two elements with RF energy in the same, or at opposite phase, the direction of the resultant angle of polarization can he determined. Furthermore, this direction can be varied by varying the ratio of amplitudes of RF energy energizing the two elements. In a reciprocal manner, the same two elements can be employed to detect or receive incident radio frequency energy and respond to particular directions of polarization of the incident RF energy by weighting the signals received by each and combining the vieighted signals in the proper phase relationship to make the antenna system responsive to incident RF energy at the particular desired angle of polarization.

The same, or similar structure, has also been employed to transmit or receive radiation 'which is circularly rolarzed or which has a component which is circularly polarized. ln the case of transmission, this is accomplished by feeding the two elements in rhase quadrature. lf the antenna system is used to receive incident circularly polarized radiation, the signals from the elements are detected then combined in phase quadrature. In either event, whether the antenna is used to transmit or receive linearly polarized or circularly polarized radiation, the structz're i as consisted of separately energized elements which are oriented with respect to each other in a prescribed rat-ner.

it is one object of the present invention to provide an antenna system for transmitting or receiving RF energy of a given predetermined polarization employing a single active antenna element.

It is another object of the present invention to provide an antenna system for radiating` or detecting radio frequency energy employing a transmission line system coupled to the active element or' said antenna which may Patented Nov. Il, i959 be operated so that the antenna transmits or detects RF energy which is linearly polarized in'any direction or right or left-hand circularly polarized.

It is another object of the present invention to provide such an antenna and transmission line system, including a single active element.

It is another object of 'l e present invention to provide a low silhouette antenna system, including a single active element with a transmission system coupled thereto and operated so that the anlenna system transmits or receives preferentially radiation ir. any given direction of linear or sense of circular polarization.

It is another object of the present invention to provide an antenna system, the active elements of which follow the contours of the antenna ground plane and are :fparated therefrom by a small fraction of a wavelength of operation of the antenna.

In accordance with the nncipal feature of the presentr invention, a radiating clement is disposed just above the ground plane, through w ith transmission lines proiect for coupling RF energy fo o. from selected points or areas on the element. The number of selected points on the element ineludeat least two pair which are coured to transmit or rexive equipment, so that the system transmits or receives RF energy which is linearly polarized in any predetermined direction or circularly polarized with either lertor right-hand sense. In preferred embodiments, the active antenna element is an eier-Iically conductive plate or film which conforms'to the :om tours of the ground plate and is separated at all points therefrom by a constant distance which is preferably a small fraction o a wavelength of the frequency of operation. For errar-spie, the separation may be as small a ten thousandth of a Wavelength. The points on the plate to which the transmisdon lines couple are preferably in pairs, each pair defining a line such that the lines dened by dierent pairs are orthoeonal in the plane of the plate. In addition, the transmission me system to which the plate couples is such that each point of a pair is in opposte phase with respect to the other point in the sazne pair and means are provided for switching the phase relationship and/or attenuatng RF energy, transmitted to or detected at the points on the plate so that polarization is linear in any direction or circular in either sense.

The shape of the radiating lm or plate, as mentioned above, preferably is such that the plate follows the contours of the ground plane and is separated therefrom at all points by the same distance. Beyond this limitation, however, the plate may have just about any shape. It is preferred, however, that the points or areas on the plat: to which the 'ransmission lines couple be located close to the perimeter of the plate and that the perimeter be at least a wavelength in length.

Other features and objects of the present invention will be apparent from the following specic description taken in conjunction with :he figures in which:

FIGURE l is a pictorial diagram of the plate antenna and block diagram of the transmission line and transmitter or rccei ver system coupled thereto;

FIGURES 2a, b, c and d illustrate a sguare plate autenna, the comers of which couple to the transmission lines and show the type of polarization obtained when these points are energized in particular arzrolitude and phase relationships;

FIGURE 3 illustrates an embodiment of the invention including the flat plate mounted on a cylindrical ground plane, which may represent, for example, the surface of a vehicle such as an airplane, and;

FIGURES 4 and 5 are curves showing the patterns of radiation at dilfere'it directions of polarization for the antenna system illustrated in FIGURES 1 and 3.

t l l Zwem-.s refinements-ln.

Turning first to FIGURE l, there is shown the principal parts of an antenna system embodying features of the invention. The structure includes ai; antenna plate 1 disposed just above the ground plane 2 so that the plate follows the contour of the `ground plane and is spaced from the ground plane at all points by a substantially constant distance S, which is on the order of less than a tenth of a wavelength of the frequency of operation of the antenna. Tue plate 1 as illustrated is substantially square and of a sizesuch that the perimeter of the plate is at least one wavelength of the frequency of operation. The square shape is illustrated for purposes of example and is not a limitation. For example, the plate may be rectangular, circular, oval. or just about any shape awrefl. However, it is preferred that the plate have two transverse Cir-f-nsions which are approximately equal or at least of the same order o? magnitude.

Four points on the plate, denoted A, E, C and D are coupled to separate transmisson lines which extend on the opposite side of the groud plane 2 from the plate. These transmission lines are cach represented in FIG- URE l by double lines extending from the points A, B, C and D through openings 4 to 7 in the ground plane 2. The transmission lines 8 to 11 may be waveguides, 2-elcment transmission lines,` coaxial line, stripline, or any of the well-known .forms of transmission lines suitable for conducting the RF energy.

The transmission lines 8 to 11 couple to a transmission or receive system 12, which is designed and operated to meet the following condition. Points A and C are in opposite phase and points B and D are in opposite phase. Furthermore, either B and A or D and A are in the same phase. When these conditions are met, the antenna system including the antenna and system 12 either transmits or receives linearly polarized radio frequency energy. The direction of polarization is illustratcd by the sketches in FIGS. 2a and 2b. FIGURE 2:. illustrates the direction of the resulting polarization in the case when points A and B are in the same phase. The resultant polarization vector 14 represents the sum of vectors 15 and 16. Vector 15 represents polarization that would be produced by an RF current flow between points A and C and vector 16 represents polarization that would be produced by currents flowing between B `nd D in the phases mentioned above.l The transverse dirvtion of polarization represented by vector 1S in FIG. 2b is produced when points A and D are at the same phase. ln this case, vector 16 is reversed relative to the direction in FIG. 2a, and so the resultant polarization direction is switched to the transverse position.

The transmission system 12 may be so designed that during operation the resultant polarization direction, such as represented by vectors 14 or 18 may be switched by the simple expedient of reversing the phase between a pair of points on the surface of the plate such as pair AC or pair BD. This may be accomplished employing an electrical or a mechanical phase shifter.

The antenna plate illustrated in FIGS. l and 2, aS already mentioned, is shown as a square and the points designated A to D are at the corners of the square, close to the periphery of the plate. As a result, the pairs of points AC and BD define orthogonally related polarization directions. Furthermore, since the plate is symmetricai on diagonals and the transmission lines which couple to the points A to D are equal in all respects, the patterns of radiation representing each of the polarization directions AC and BD are equal. Accordingly, the magnitude of the vectors such as 15 and 16 representing these radiation patterns are equal and orthocnal. This explanation of the operation is o'ered as an attempt to explain or justify the empirical results. It is not, however, suggested that the antenna functions entirely in such a simple manner and that all operations of such an antenna, or a similar antenna, can be explained by such a simple analog of the antenna.

Assuming that the above analog depicting operation of the antenna is correct and can be extended to predict and/or explain operation when the points A to C are 'not disposed at the corners of a square or the transmission lines coupled to these points are not equal, then the expedient of s 4'itchin hase betwce a r ts such as AC o Bllgwiil not cause the resultant polarizaion direction to switch to the transverse direction. For example, if the transmission lines 8 and 10, which couple to points A and C, respectively, are more lossy than the transmission lines 9 and 11 which couple to points B and D, respectively, then the vector 15 will be of smaller magnitude than vector 16. The resultant of two such unequal vcctors will not be switched to the transverse direction by the mere expedient of switching phase of one of the vectors. ln fact, the resultant will switch to a dihw tion which is less than 1 from the original direction of the resultant. Similarly, if two of the points, such as A and C, are closer together than the points B and D, the same thing will happen; namely, the resultant polarization vector will switch in ('irection less than 90 when the phase between a pair of the points is switched.

In accordance with another feature of the invention. the transmission system 12 may be so designed that the resultant polarization vector rotates to the right, illustrated in FIG. 2c, or to the left, as illustrated in FIG. 2d. Thus, the antenna system will transmit or receive radio frequency energy which is circularly polarized, 0r at least has a strong component which is circularly polarized. It has been found that the left-hand circularly polarized response of the antenna system illustrated in FlG. 2c will be obtained when the pairs of points AC and BD are energized so that the vectors 15 and 16 in the analog constructed as already described, are in phase quadrature, `with vector 1S leading vector 16 in phase by 1r/2 radians. When this occurs, the resultant polarization vector Z'i, as viewed looking toward plate 1 and the ground plane 2, will rotate clockwise as shown in FSG. 2c. On the other hand, when vector 16 leads vector 15 by ir/2 radians, the circular polarization is in the opposite sense, as shown by vector 22 in FIG. 2d. Again, it will be remembered that the analog constructed here t0 explain the observed operation of the antenna system is olered for convenience and does not preclude another explanation of the same observed phenomena. based upon D another or a different analog of operation.

Returning again to FIG. I, and more particularly to details of the transmission system 12, there is shown in a general form a system for operating the antenna either to transmit or to receive in any of the manners a1- ready described with reference to FIGS. 2a to 2d. For four transmission lines 8 to 11 which couple to the points A to D respectively. Each of these transmission lines extends to a balun structure, the length of each line between the balun struct-ure and the point on the plate 1 being electrically the same. Thus, points A and C are coupled by transmission lines 8 and 10, respectively, to balun structure 23 and points B and D are coupled via transmission lincs 9 and 11, respectively, tc balun structure 24. The balun structures 23 and 24 are so constructed that they divide power in opposite phase between the two transmission lines connecting the balun structurel to the points on the antenna plate 1 and, in reciprocal?, fashion, these balun structures combine radio frequencyi energy of opposite phase conducted from the points so that the energy adds. Art example of a balun structure which would be suitable is the magic Tec hybrid which would be used if the transmission lines 8 to 11 are waveguides. Another type of balun structure used fre-l quendy with multi-element transmission lines such as b cficial lino or tr l' the hybrid referred to as a hybrid ring or more commonly the rat lace. Any of these may be employed with the appropriate transmission line to perform the function of the balun structures 23 and 24.

In the simplest embodiment of the invention, the balun structures 23 and 24 are coupled by appropriate transmission lines 2.5 and 26 to a power divider 27 which couples to a transmitter or receiver 28, depending upon whether the system is used to transmit or receive. The power divider 27 and the transmission lines coupling the power divider to the balun structures are such that the balun structures are fed equal power during transmission cr such that equal power from the balun structures are combined and fed to the receiver when the system is employcd to receive. If additional polarization modes of operation are desired and it is desired to switch from one polarization direction to another, as described above with reference to FIGS. 2a to 2d, means such as a phase shifter 29 is provided between one of the balun structures 23 or 24 and the power divider 27. For example, when it is desired to switch linear polarization direction from one direction to the transverse direction, as shown in FIGS. 2a and 2b, then the phase shifter 29 must be operated to reverse the phase at balun structure 14 relative to balun structure 23 and this is the case whether the system is used to transmit or receive. For this purpose, phase shifter 29 may be .lectrisally or mechanically operated to shift the relative vphases at the balun structures by 180. On the other hand, when circularly polarized transmission or reception -by the antenna system is desired, the phase shifter 27 must shift phase so that radio frequency energy at the baluzi structures 23 and 24 is in quadrature. For this purpose, he phase shifter 29 may be designed so that when operated one way or another it will shift phase 90 or 270 and, thus, produce right-hand or left-hand polarization of the antenna system. If it is dea sired to switch from linear to circular polarization, the phase shifter may be designed to produce phase differences at balun structures 23 and 24 of 0 or 90.

Typical radiation patterns obtained with an antenna structure such as described above, with reference to FIGS. l and '2., are shown in FIGS. 4 and 5. Each of the FIGS. 4 and S include plots of relative radiation intensity in decibel; vs. direction angle coordinates. The direction 4 angles are defined as 0 and p which are orthogonal angular coordinates taken as illustrated in FIG. 3. FIGURE 4 shows tlc radiation patterns as a function of decibels in each of the angular directions 0 and (broken and solid lines, respectively) for radiation linearly polarized in the o direction and FIG. 4 shows the same type patterns for radiation linearly polarized in the e direction.

The radiation patterns shown in FIGS. 4 and 5 are typically those obtained employing, for example, a inch square plate or film antenna 31 mounted centrally on a metal ground plane cylinder 32, which is 4 feet in diameter and 3 feet high. The radiation pattern is obtained from measurements made when the antenna is energized as described above with reference to FIGURE 1 at 250 mc. The ground plane cylinder 32 might represent the fuselage or external confirmation of some part of a missile or an airplane. The plate antenna 31 is preferably fixed to the outside of this surface by a suitable dielectric adhesive so that it is spaced a few thousandrhs of a wavelength (at 250 mc.) from the surfat all points. Since the periphery of the plate 31 is about 60 inches, quite clearly, the periphery is more than a. wavelength nt the operating frequency of 250 mc. A transmission line structure such as already described above with reference tc FIG. l, preferably is located inside the ground plane cylinder 32 and energizes the antenna as already described, so that radiation issuing from the antenna plate is polarized in e 0 direction er in the o direction. Perpendicular to plate 3l, 0 polarization is substantially parallel to axis 33 of cylinder 32, whereas o polarization is substantially parailel 'ro the circumference line 34 of the cylinder. The coordinates 9 and qb defining a spatial point 35 at unit distances 36 are measuredas demonstrated in FIGURE 3 and it is these measurements which are plotted as the abscissa in the radiation paticrus shown in FIGS. 4 and 5. The ordinate (Db) is a measure of the intensity of the polarized radiation at the unit distance.

As'can be seen from FIGS. 4 and 5, the intensity of polarized radiation is greatest in the direction directly perpendicular to the plate 31 extending from the origin 37 along a line out of the page. Both of the coordinate angles 0 and p are measured from this line and so along this line, both 0 and are zero. Each angular value of 0 and p is defined by a line from the origin 37 to the point in space such as '3S at the unit distance. As can be seen from the radiation patterns, in both the and the d coordinates, substantial intensities of radiation polarized in either direction are produced at 0 or qb equal to or -90. In fact, at -lor 90 in either case, the radiation level is only down between 20 to 30 db. Thus, the radiation pattern if depicted in three dimensions would appear to wrap around the radiating plate 31, somewhat similar to a cardioid pattern.

Various embodiments of the present invention may be combined to perform a multitude of functions. For exarn ple, a plaze such as 31 could be mounted almost ush with the surface of an airplane and separated therefrom by only a small fraction of an inch or less and energized from within the airplane to create radiation patterns ex tending perpendicular from the plate of the various linear or circular polarizations described and the polarization direction could be switched readily between dierent types of linear or circular polarization. This structure would be an advantage in, for example, a weather radar system on board an airplane, where a beam of circularly polariycd radiation is launched with some directivity to detect rain. It is the unique characteristics of rain drops to reiiect much oi the incident circularly polarized radiation and reverse the direction of circular polarization. Thus, the antenna system could be used to transmit circularly polarized radiation rzone sense and an instant later, or simultaneously, operated to receive echo signals polarized in the opposite sense, just as already described above with reference to FIG. l and FIGS. 2c and d. Of course, the same antenna structure could be employed to transmit or receive linearly polarized radiation, as already described.

Many other uses and modifications of various features of the present invention will be apparent to those skilled in the art. Accordingly, the embodiments described herein are made only by way of example and do not limit the scope of the invention set forth in the accompanying claims.

What is claimed is:

1. An antenna system comprising a ground plane and a layer of conductive material disposed opposing the ground plane and substantially uniformly spaced therefrom,

said layer serving to conduct separate RF current paths which cross, and

a separate transmission line coupled to the layer at each end of each current path,

whereby the antenna sysem has a directional pattern of RF radiation hav'ng a. rotating direction of polarization when the cu rreuts n the separate paths are not ir. phase.

2. An antenna system as in claim 1 andin which,

said uniform spacing is substantially less than a wave length of said RF radiation.

3. An antenna system as in claim 1 and in which there are four of said separate transmission lines coupled tn said layer, each at a different point.

4. An antenna system as in claim 3 and in which, said points detine a square.

5. An antenna system as in claim 4 and in which,

said points are at diago Jal corners of said square and said points at diagonal corners are at opposite RF phase.

6. An a1 tenna system as in claim 1 and in which,

said RF currents in separate paths are in phase n uadrature.

7. An antenna system as in claim 1 and in which,

said transmission lines are part of a transmiring s5stem for energizing said antenna system to produce said directional pattern of RF radiation.

8. An antenna system as in claim 1 and in which,

said transmission lines are part of a receiver system responsive to said directional pattern of RF radiation.

9. An antenna system comprising a ground plane, a layer of electrically conductive material which is continuous in two dimensions defining an Iarea of the layer, the layer being disposed opposing said ground plane and substantially uniformly spaced therefrom, and at least four transmission lines, each coupled to a different point along the perimeter of said area of conductive material, whereby said antenna system has a directional pattern of polarized radiation.

10. An antenna system as in claim 9 and in which said pattern is of greatest intensity in direction substantially perpendicular to said layer of conductive material.

11. An antenna system as it: claim 9 and in which said uniform spacing is substantially less than a wavelength of said. radiation.

17.. An antenna system as in claim 9 and in which the perimeter of said area of conductive material is at least as great as a wavelength of said radiation.

13. .An antenna system as in claim 9 and in which said perimeter of said area of conductive material is substan tially coincident with the edge of said layer of conductive material.

14. An antenna system as in claim 9 and in which said four different points are spaced at comers of a square.

15. An antenna system as in claim 414 and in which said points are at diagonal corners of `said square and couple to transmission lines such that the diagonal points are energized at opposite phase of said radiation.

16. An antenna system as in claim 15 and in which the points at opposite diagonal corners of said square A 8 couple to transmission lines which are part of a transmission line system such that the antenna system responds radiation incident upon the conductive plate which energizes opposed diagonal points of said square in opposite phase.

17. An antenna system as in claim 16 and in which said transmission line system includes means for switching phase of at least one pair of said diagonally opposed points of said square.

18. An antenna system as in claim 16 and in which said transmission line system includes means for switching phase response of at least one pair of said diagonally opposed points of said square.

19. An antenna system as in claim 15 and in which two of said diagonally opposed points of said square are energized in phase quadrature with respect to the other two points.

20. An antenna system as in claim 16 and in which said transmission line system is such that the antenna system responds to radiation incident upon the conductive plate which energizes two o said diagonally opposed points of said square in phase quadrature with respect to the other two points.

References Cited UNITED STATES PATENTS 2,791,769 5/1957 Lindenblad 343-769 2,826,756 3/1958 Cary 343-829 2,990,547 6/1961 McDougal 343-908 3,086,204 4/ 1963 Alford 343-769 3,165,743 l /1965 Hatkin 343-767 X ELI LIEBERMAN, Primary Examiner U.S. C1. X.R. 343-708, 857, 908

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