US3022506A - Arbitrarily polarized slot antenna - Google Patents

Description

Feb. 20, 1962 F. J. GOEBELS, JR., ETAL 3,022,506

ARBITRARILY POLARIZED SLOT ANTENNA Filed March 27, 1959 e Sheets-Sheet 1 IIIVAMW Mia/V5405 fizz/Ja uar J2, 1214 1 a (44 )6 (law Feb. 20, 1962 F. J. GOEBELS, JR., ETAL 3,

ARBITRARILY POLARIZED SLOT ANTENNA 6 Sheets-Sheet 2 Filed March 27, 1959 Feb. 20, 1962 F. J. GOEBELS, JR, ETAL ARBITRARILY POLARIZED SLOT ANTENNA 6 Sheets-Sheet 3 Filed March 27, 1959 flaw/M7 7 241 Zia-5a: die, lam 74 6. 624

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Feb. 20, 1962 F. J. GOEBELS, JR., ETAL 3,022,505

ARBITRARILY POLARIZED SLOT ANTENNA Filed March 27, 1959 6 Sheets-Sheet 4 Feb. 20, 1962 F. J GOEBELS, JR., ETAL 3,022,506

ARBITRARILY POLARIZED SLOT ANTENNA Filed March 27, 1959 6 Sheets-Sheet 5 Feb. 20, 1962 F. J. GOEBELS, JR., ETAL 3,022,505

ARBITRARILY POLARIZED SLOT ANTENNA 6 Sheets-Sheet 6 Filed March 27, 1959 W I V MM i; M... in w. A Z M I Z W! Z w a m 4 a g! n .llllllll-IIII XVIII/41f. iv :Zfazaazriz, AVI/VVFAJ. A470,

lime/AX United States This invention relates to antennas, and particularly to microwave antennas capable of transmitting and receiving radiation of any polarization and having a planar surface.

No matter what the environment in which an antenna is to be used, it is always, of course, desirable to employ a design which is simply constructed and which has small size and weight. It is recognized that in the antenna art complex functions can be provided by using large and complicated structures, each part of which performs at least a part of the desired functions, In many situations, however, the complex structures cannot be practicably used. Thus in an aircraft a large and heavy antenna may impose weight penalties which limit the use of the aircraft. It is particularly desirable that antennas for use with high speed vehicles be small in size and light in weight, and it is further highly desirable to have antennas which can be mounted flush with a streamlined surface. For this reason, a planar or level antenna which can be mounted flush with an aircraft surface has particular utility. it Will be recognized that if such an antenna has little weight and is of small size it will be of especial benefit.

in many cases, the characteristics of operation of an antenna may be more important than the structural features and the configuration of the device. There is a widespread need for antennas Whose responsiveness and operation is independent of the polarization which they radiate or receive. In radar and in other arts which utilize radiation echoes, for example, it is often found that returned waves do not correspond in polarization to the radiated waves. The polarization of the received wave is affect-ed both by the polarization of the transmitted radiation and by the nature of the object from which it is reflected. Consequently, if an antenna is employed which is sensitive to only one direction of polarization, (e.g. vertical polarization), as many antennas are, the system reception is greatly lessened, and may be subject to excessive error signal components. If, however, an antenna is employed which can operate with any linear polar zation, with circular polarization in either sense of rotation, or with elliptical polarization, optimum polarizaiton detection will always exist. Antennas which display this polarization versatility are usually called arbitrarily polarized antennas. Such an antenna can provide a number of advantages for the system with which it is employed. For example, changes in the polarization of the radiation can be made to achieve optimum response from an obected having given characteristics. Furthermore, its inherent omnipolarization capabilities can improve the system response to received signals. In addition, the ability to change the polarization makes possible a more detailed analysis of the nature of objects from which energy is reflected, due to the fact that various objects have different characteristic effects upon the polarization of the radiation which they reflect.

The combination of these electrical characteristics and operative features with desirable mechanical features has in the past been extremely dificult. The use of an antenna having arbitrary polarization capability but which is small in size is difiicult enough, but the problems are compounded when it is additionally required that the antenna be fed simply and also be of a streamlined configuration. It is further desirable that the antenna be atent C) 31,022,596 Patented Feb. 20., 1952 easily fabricated, and if possible that the antenna use a planar radiating surface so as to avoid the necessity for an extensive front feeding structure. Such an antenna can operate effectively without need for a radome, but still can provide a two-dimensional, as opposed to a linear, array.

It is therefore an object of the present invention to provide a two-dimensional, planar antenna having arbitrary polarization capability.

Yet another object of the present invention is to provide novel antenna forms which may be simply constructed and which can provide selected linear, circular or elliptical polarization.

Yet another object of this invention is to provide lightweight antenna structures capable of generating a pencil beam from a planar surface which can be flush mounted with a supporting body.

Yet another object of this invention is to provide an improved planar antenna for mobile applications Whose pattern response remains invariant with the polarization of operation.

It is a further object of this invention to provide a small two-dimensional microwave antenna which can be simply constructed and which operates to provide ready control of the antennas polarization.

Yet another object of this invention is to provide an improved antenna capable of arbitrarily changing the polarization characteristics of radiated energy.

A further object of this invention is to provide a simply fed and constructed compact antenna capable of being flush mounted in a streamlined structure and suitable for providing selective polarization control.

It is yet another object of this invention to provide an improved antenna for providing a broadside pencil beam pattern from a two-dimensional surface.

A further object of this invention is to provide a planar antenna capable of producing an angularly symmetric pattern independent of the polarization which is radiated or received.

Yet another object of this invention is to provide an improved planar antenna capable of conical scanning.

These and other objects of this invention are achieved by an arrangement in accordance with the invention which utilizes a radial wave guide and which also employs a v composite radiating aperture consisting of at least one annular slot defined by a number of individual elements living in a circlt concentric with the central axis of the radial Waveguide. configuration, such as a crossed slot, which can be excited in any direction in the plane in which they lie. In one form of the invention, the antenna aperture thus defined is excited in a standing wave mode so that each of the crossed slots is excited so as to provide like polarized radiation. This arrangement may use a circular waveguide which is coupled to the radial Waveguide at its central axis and which is excited in its dominant TE mode to provide excitation of the radial waveguide in the E mode. The E radial wave guide mode establishes radial and circumferential currents at the annular slots having like amplitudes but a ninety degree electrical separation. The radial and circumferential attitudm of the crossed slot elements, plus the exciting currents cause the resultant component of the radiation vector at each slot to be equal in magnitude and to lie parallel to the same plane of polarization for any polarization at any given instant of time. The direction of polarization is controlled by the polarization of the TE feed mode. With this arrangement, circularly polarized energy may be provided by establishing orthogonally disposed feed modes which excite the crossed slots equally in amplitude but in time and space quadrature. Elliptical polar- The radiating elements each have av ization is accomplished with a similar feed but with selective control of the relative amplitudes.

,In another form of the invention, like polarization efiects can be achieved through the use of traveling wave arrangements with radial waveguides having similarly disposed radiating apertures. rangements, the feed means are disposed on the two plates defining the plane surfaces of the radial waveguide. These feed means mayconsist in one formof a radial feed waveguide and a cylindrical waveguide, and in another form of a pair of radial feed waveguides. The two structures both excite a pair of traveling wave modes within the radial waveguide, and selected relations between the input modes as to amplitude and phase allows any type of polarization.

The novel features of this invention, as well as the invention itself, both as to its organization and method of operation, may best be understood when considered in the light of the following description, when taken in connection with the accompanying drawings, in which like reference numerals refer to like parts, and in which:

FIG. 1 is a perspective exploded view, partially in block diagram form, of a standing wave form of planar radial waveguide antenna capable of operating with arbitrarily selected polarization with a circular waveguide feed in accordance with the invention;

FIG. 2 is a simplified perspective view showing an antenna such as the arrangement of FIG. I mounted flush with a surface of a supporting vehicle and providing a broadside pencil beam pattern (illustrated generally);

FIG. 3 is a plan view of theradial waveguide of FIG. 1, showing vectorial excitation components for the individual elements; 7

FIG. 4, including diagrammatic views FIGS. 4A, 4B

and 4C, shows plan views of the radial waveguide and sectional views of the circular feed waveguide, showing the correspondence between the direction of polarization of energy in the feed waveguide and the'radiation from the individual elements of the radial waveguide;

In the traveling wave ar- 7 J FIG. 5 is a simplified plan view of a radial waveguide and a section view of a circular waveguide of the form of FIG. 1, showing the manner in which circularly o'r elliptically polarized radiation can be provided;

' FIG. 6, comprising fragmentary plan views of radial waveguides similar to FIG; 1, shows diiferent configurations of an annular slot and the manner in which they r are arranged for the radiatingelements which may be utilized to make up the total radiating aperture of the arrangement to reduce side lobes;

FIG. 7 is, an exploded perspective view of another form of antenna in accordance with the invention, utilizing a radial waveguide antenna operating with traveling wave excitation;

FIG. 11 is a fragmentary view'of the center. portion of the arrangement of FIG. 10, showing some of the features in greater detail; a

7 FIG. 12 is a sectional elevation View of the arrangement of FIGS. 10 and ll, showing the exciting modes therein;

FIG.-l3 is a combined perspective and block diagram view of a system configuration utilizing a conical scanning antenna in accordance with the invention; and

FIG. 14 is a fragmentary sectional view of a part of the conical scanning arrangement of FIG. 13.

Arrangements in accordance with the present invention are capable of transmitting and receiving energy about the circle.

which can be arbitrarily polarized. The term arbitrarily polarized is intended to mean that'the antenna is capable of producing a pattern which is independent of the direction of polarization and which, in various forms, may be circularly or elliptically polarized as well as linearly polarized in different directions. The term planar is intended to indicate that the radiating surface of the antenna lies in' a given plane, although the plane may be curved slightly to conform to the configuration of associated structures. Thus these antennas and the patterns they provide are two-dimensional, and provide an additional degree of operational and design freedom over linear arrays. a

One arrangement in accordance with the invention for providing an arbitrarily polarized pattern is illustrated in exploded form in FIG. 1, and as installed in a flush surface in FIG. 2. As may be seen in FIG. 1, the antenna consists principally of a radial waveguide 10 having a central axis and top and bottom plates 11 and 12 which are conductive, circular in form, and which lie substantially normal to the central axis. By referring to the top plate 11, it is not intended to indicate that the antenna must be operated in a given position, but only that this is a normal frame of reference for the radial waveguide 10. The top plate 11, as may be seen in FIG..2, may be mounted flush with a surface of a streamlined vehicle 20, and may provide a pencil beam extending substantially normal to the plane of the surface of the top plate 11.

V The top and bottom plates 11 and 12 are spaced apart a selected distance and joined at their outer periphery by a conductive ring 14 which is affixed to both the plates 11 and 12. The bottom plate 12 includes a central feed aperture 15 for exciting the radial waveguide 10 with a desired mode. With the use of the outer conductive ring 14, the radial waveguide 10 is operated with standing waves. The radial waveguide 10 is dimensioned to support the E mode. I r

The antenna aperture or composite radiating source of the radial waveguide antenna 10 is formed by one or a number of what are here termed annular slots. An annular slot is the convenient basic radiating unit. of a radial waveguide. Each annular slot consists of a number of individual crossed slot elements 16 in the top plate 11. Although, as is described'below, various shapes and dispositions can be employed for the radiating elements 16, the desired pattern can be achieved by the arrangement shown, in which the radiating elements 16 are disposed in an annulus or circle concentric with the central axis of the radial Waveguide 10. A number of the annular slots can be arrayed, as is described in more detail below, to provide the total aperture of the antenna. Each of the crossed slots 16 has a like radial and circumferential attitude with respect to the circle on which the slots lie, and the slots 16 are symmetrically placed about the top plate 11. The attitude of the slots 16 relative to an observer, however, vary progressively Thus, the radiating elements are the crossed slots, which together make up an annular slot.

The term annulus may be used to denote both a radiation aperture and a geometrical configuration.

The feed for this arrangement is intended to excite the radial waveguide in its E standing wave mode. The radial waveguide 10' is fed throughits central feed aperture 15 by a circular waveguide 17 coupled to the central aperture 15. An input source 19 is coupled to the circular waveguide 17 and excites the circular waveguide 17 with input energy in the TB mode which is the dominant circular waveguide mode. As is described below, the polarization of the TE mode may be arbitrarily varied, and the circular waveguide 17 is particularly useful in this respect'because of its insensitivity to directions of polarization of the TE circular waveguide mode. It will be understood, however, that the desired E mode'in the radial waveguide 10 may be established by other feed means. I

In the operation of the arrangement of FIG. 1, the input source 19 excites the circular waveguide TE mode in the circular waveguide 17 (and an E mode is excited in the radial waveguide The standing wave E mode which is established in the radial waveguide 10 provides current regions in the top plate 11 which combine with the attitudes, configurations and disposition of the radiating apertures 16 to provide controlled radiation patterns having the desired polarization characteristics. The operation of this antenna is reciprocal, in that currents which excite the top plate 11 during reception of energy are provided as output in a fashion which is the converse of the transmission operation. The operation is perhaps easier to visualize for transmission, however, and so will only be described in that context although the reciprocal nature will be understood.

The E standing wave mode of a radial waveguide, such as the waveguide 10, provides currents in the top plate 11 in both radial and circumferential bands or regions. In accordance with the standing wave operation established because of the presence of the conductive ring 14, the amplitude of these currents varies sinus'oidally at the frequency of the exciting source. At selected radii from the axis of the radial waveguide 10, the amplitudes of the radial and circumferential currents can be made equal. The currents are, however, displaced ninety electrical degrees. Thus with a standing wave operation the radial and circumferential currents may be considered to be in phase quadrature, but having instantaneous total values which are equal. The magnitudes of the currents at the individual slots 16 vary at any instant dependent upon the position of the selected element 16.

The position of the radiating slots 16, the attitude of the radiating slots 16, the equal amplitudes of the radial and circumferential current regions and the quadrature relationship of the radial and circumferential currents all contribute to the generation of selectively polarized wave energy with this planar array. The annulus of crossed slots 16 forms the aperture of the antenna. The radial position of the crossed slots 16 with respect to the central axis of the radial Waveguide 10 corresponds to a point at which the amplitudes of the total radial and circumferential currents at that radius are equal. The relationship for the total current for any linear polarization may be expressed as where I is the amplitude of the currents in the radial and circumferential direction relative to the central axis of the radial waveguide 10, as is the angular displacement of each slot from a reference line on the planar surface, 6; and are direction unit vectors, and 'y is the angular displacement of the common plane of polarization from the reference line. Each crossed slot 16, therefore intercepts a portion of the total current flowing and all the slots 16 together give rise to a radiated electromagnetic field having a common plane of polarization.

The resultant vectorial component of radiation from each individual crossed slot 16 is the same. In other words, each crossed slot 16 provides a radiation contribution to the antenna aperture pattern which is of like magnitude and parallel to the same plane of polarization. No cross polarization components are present. These facts may be better understood by reference to FIG. 3, in which the excitation components for the crossed slots 16 are shown in some detail. With the radial and circumferential currents in a quadrature relation and assuming that the polarization to be excited is in a vertical plane (taking a relative position of FIG. 3 as a frame of reference), then the uppermost slot 16 (as seen in FIG. 3) is excited by a radial current but not by a circumferential current, the radial current being of full amplitude and the circumferential current, ninety electrical degrees displaced therefrom being zero. Proceeding clockwise around the crossed slots from the uppermost slot 16, however, the radial and circumferential currents add vectorially to a like resultant component at each individual slot 16. This is due to the sinusoidal distribution of the two currents. At the ninety degree position (still proceeding clockwise from the top), the radial component is zero and the circumferential component a maximum, so that the resultant vectorial component of excitation at this crossed slot 16 is likewise equal in magnitude to the others and parallel to the same plane. The same result holds true for each of the other slot elements 16.

As may be seen in FIG. 4, the plane of polarization of radiated energy from the radial waveguide antenna 10 is dependent upon the polarization of the TE mode in the feed waveguide 17. With reference to FIG. 4 only, we may speak of the angle 7 as relative to the vertical. Thus FIG. 4A shows the relative position when the angle 7 for the feed energy and the excitation is zero degrees, FIG. 4B shows the corresponding feed energy and radiated energy relationship when the angle 7 is 45 degrees and FIG. 4C shows the like reiationships when the angle is ninety degrees. Although the displacement between the radial and circumferential current regions remains the same, the distribution of the currents is shifted angularly through desired values of 'y to achieve these results.

This arrangement may also be utilized to provide circularly or elliptically polarized energy. This result is accomplished in the manner indicated generally in FIG. 5, in which FIG. 5A illustrates the relationship of the modes employed in the feed waveguide 17, and FIG. 53 illustrates in simplified form the radiation resulting from excitation of the individual crossed slots 16.

Circularly polarized microwave energy may be con sidered as consisting of two linearly polarized components in time and space quadrature. For circularly polarized energy, the components are of equal amplitude and when the components are not of equal amplitude, the result is elliptically polarized energy. Accordingly, as shown in FIG. 5A, when the feed waveguide is excited with two orthogonally disposed TE modes which are of equal amplitude, but in time and space quadrature, a rotating E mode is established in the radial waveguide 10.

As may be seen in FIG. 5A, one electric vector E may be used to represent the electric field component of a first TE mode derived from a first input source 21 coupled to the circular feed waveguide 17. Another electric field vector E may be used to represent the corresponding component of a second T5 mode derived from a second input source 22. Energy may be coupled into the circular waveguide 17 through appropriate transition elements, apertures, or excitation elements, none of which are shown but the use of which will be understood.

The sinusoidal variation of the individual vectors E and E with time may be seen to provide a rotation of the resultant vector E of the feed energy, due to the quadrature disposition of the individual vectors. Thus any rotating E mode excites the annular slot aperture of the antenna 10 to cause a rotation with time of the plane of polarization of the radiated electromagnetic waves. The relationship is such that the common plane of the radiation waves completes one full revolution in a distance equal to one free space Wavelength. Now by exciting the T13 modes with unequal amplitudes (due to unequal electric vectors E and E a rotating E mode is established which has an elliptical characteristic as a function or" time and which results in the radiation of elliptical polarization.

Antennas constructed in accordance with these principles are small in size, light and compact and easy to fabricate. They may be made arbitrarily thin and further may be flush mounted with a parallel surface. Thus they can very simply provide functions heretofore supplied only by much more complicated structures.

In addition, however, these mechanical characteristics re accompanied by electrical and operative characteristics which are significant. The antenna is extremely easy to feed, and the polarization which'is radiated or detected may very readily be changed. Antennas conthis fashion inherently display low axial ratio when transmitting or receiving circular polarization. They are two dimensional and consequently, as is described more fully below, permit considerable design freedom in synthesizing and controlling radiation patterns.

Further features may also be employed where specific additional advantages are desired. Reference may be made, for example, to the shapes, radial dispositions and attitudes of the radiating elements shown in the four different top plates illustrated in FlGS. 6A'through 6D. As shown in FIG. 6A, the radiating elements 24 of an annular slot may be made circular. Circular elements may be in some instances be fabricated much more siniply than crossed slots, although each element nonetheless may be excited with any polarization in the plane in which the annulus is located. The radiating elements alternatively, as shown in FIG. 68, may be constructed of crossed slots 25 whose individual arms are diagonally disposed with respect to the concentric annulus on which they lie. Again, these crossed slots 25 may be polarized in any desired direction.

. Control of side lobes may be achieved by the arrangements illustrated in FIGS. 6C and 633, both of which utilize more than one concentric annulus of radiating elements. aninner annulus and slots 27 on an outer annulus which combine to form the'aperture of the antenna. As discussed previously, the requirement for each annulus is that it he on a radius at which the radial and circumferential currents are equal in total amplitude and separated by ninety electrical degrees (in For the selected excitation modes, this condition holds at discrete radii from the central axis. Accordingly, the contribution of each individual element in an annular slot to the total radiation is the same in both polarity and magnitude. A modification of this technique, as shown in FIG. 6D, may employ an amplitude taper in the aperture by using annular slots which contain individual elements of different sizes and different interelement spacing. As seen therein, 'anantenna it of the type discussed here may have an inner annulus of slots 31, a center annulus of slots 39, and an outer annulus of slots 29, the slots 29 of the outer annulus being smaller in size and further apart than the others. Both variations have been shown together but either could be used separately. Such arrangements provide additional degrees of freedom for the selective control of sidelobes.

The operating characteristics of. the arrangement thus provided should be noted. ,The angular pattern is the same about the central axis, regardless of the type of polarization radiated. With the antenna operating as a linearly polarized radiator there is no cross polarization.

It will be recognized that'the excitation modes for the radial waveguide'which can produce the desired current distributions are not confined to the E mode mentioned. The standing wave modes whichhave the desired characteristic are the E (where n=0, 1, 2 H (where P2 1, 2 modes. The nomenclature adopted in designating these modes corresponds to that used in the Waveguide Handbook by N. Marcuvitz,

' McGriaw-Hill Press, p. 91 (1951).

Arbitrary control of the polarization of adiation pat- V terns can also be achieved by using traveling wave exc ted radial waveguide antennas. With these arrangements,

of the radial waveguide. The desired distributions are Thus, as in FIG. 6C, there may be slots 26 on ...)andthe achieved through the use of a radial waveguide modal pair with one mode having a prescribed. relative phase and amplitude withrespect to the other.

An arrangement for providing traveling wave operation to achieve constant shape pencil beams using any type of polarization is shown in FIGS. 7, 8 and 9, to which figures reference. is now made. A radial waveguide antenna 40 having a top plate 41 and a bottom plate 42 may be arranged in the manner described with reference to the previous figures, and may have a central axis extending through the waveguide antenna 43 in. a direction normal to the plane of the parallel plates 41 and 42. The waveguide antenna 40 may terminate in a circumferential band 43, but'is operated with the traveling wave modal pair through the presence of a ring of resistive material 45 between the plates 41 and-42 at an outer radial portion of the antenna 40. The ring of resistive material 45 may beany suitable microwave attenuating material and tapered inwardly with respect to the waveguide 40 so as to provide a smooth match for the microwave energy therein.

The top plate 41 of the radial waveguide antenna 40 may contain a number of crossed slot elements 46 arranged in separate armuli'concentric with the central axis of the radial waveguide-40. As in the arrangements previously described, the crossed slots 46 in the several .annuli are symmetrically disposed within each of the annuli, and have like radial and circumferential attitudes with respect to the annulus on which they lie.

The structure by which the desired traveling wave modes are established within the radial waveguide antenna 4% consists in this arrangement of a pair of feeds which are structurally. combined. Energy from a first input source 50 is coupled to a first circular waveguide 52 concentric with the central axis of the radial waveguide 49 and, for purposes of description, protruding through it from the bottom surface to beyond the top plate 41. While the first circular waveguide '52 need not be constructed as a unitary element extending entirely through the radial waveguide 40, it will be more conveniently described as such'and the use of alternate methods of construction will be understood to be feasible. For coupling energy through-the bottom plate 42 of the radial waveguide antenna 4! the first circular waveguide 52 includes a number of longitudinal slots '53 positioned on the side of the bottom wall 42 parallel to the central axis and spaced apart from the bottom wall 42. For coupling energy supplied from the side of the top plate 41, the first circular waveguide includes a pair of annular slots 54, 55 inside the radial waveguide antenna 40 and each adjacent inner surface of the bottom and top plate 42, 41 respectively. The first circular waveguide 52 terminates in a short circuiting element 56 on the side ofthe top plate ll. A short circuiting element 57 is also positioned between the elongated slots 53 and the bottom plate 42. Thus the first circular waveguide 52 is separatedinto one section which terminates short of the bottom plate 42 of the radial waveguide antenna 40, and another section which extends through the radial waveguide antenna 49 and'protrudes out the top surface a short distance. The radial waveguide antenna 40, which is the radiating structure of this arrangement, is to be distinguished from a radial feed waveguide 60 coupled to and utilizing the bottomplate 42 of the antenna 40. The radial feed waveguide '60 is concentric with the central axis of the radial waveguide'antenna 40 and with the first circular waveguide 52 and includes as one of its parallel walls an inner radial portion of the bottom plate 42 of the radial waveguide 4t). The radial feed waveguide 60 is electromagnetically coupled to the radial waveguide antenna 40 through radially oriented coupling slots 61 which are all positioned at a constant radius from the central axis of the radial waveguide 40. A rigid coaxial microwave transmission line 62 is formed by the outer surface of the first circular waveguide 52 and the inner surface of a second circular waveguide 63 registering with a central opening in the radial waveguide feed 60 and concentrically encompassing the first circular waveguide 52 for a distance pas-t the longitudinal coupling slots 53. A variable coaxial short consisting of a conductive hollow cylinder 64 is utilized to control the length of the rigid coaxial line 62, being concentric with'the central axis and movable between the two circular waveguides 52, 63.

The portion of the first circular waveguide 52 which is inside the radial waveguide 40 may be supported by a ring 66 of material which is substantially transparent to microwave energy. The ring 66 can include internal members which extend into the annular slots 54, 55 and which provide a precise axial spacing and dimensional control for the slots 54, 55. The ring 66 may also be split for ease of assembly. The axial spacing of the annular slots 54, 55 forces the excitation of a desired mode in the radial waveguide 40. Energy is fed into this portion of the first circular Waveguide 52 from a second input source 70 through a coaxial line 72 having an inner conductor which terminates as a first probe 73 inside the first circular waveguide 52 adjacent its end 56. A second probe 74 extending orthogonal to the first probe 73 may also be utilized to excite the first circular cylinder 52 in an additional mode. The second probe 74, and a coaxial feed line 75,..are shown only in FIG. 8, and illustrate a feed which may be employed to excite any linear, either sense circular, or arbitrary elliptical polarization. With proper feed, therefore, the use of two orthogonal probes 73, 74 provides arbitrary polarization capability, although this immediate example has for simplicity been arranged only for linear polarization in FIGS. -7 and 9.

In operation, considering linear polarization only, the arrangement of FIGS. 7 through 9 operates by exciting the radial waveguide antenna 40 concurrently in the E and H radial waveguide modes. When arranged to have proper phase and amplitude relationships, as is described in greater detail below, these modes excite the three annular slots each composed of crossed slots 46 to provide a common plane of polarization for the radiated field at any given instant of time. The termination 45 may be termed a flat load because it operates substantially without reflection. Means by which adjustments may be made in the direction of polarization have not been included in FIGS. 7 and 9, but it will be understood that the change in position of the polarization can be achieved through mechanical rotation of the feed elements relative to the antenna or through electrical rotation of the energy. In addition, as discussed below, circular and elliptical polarization can be provided because of the feeds for the antenna basically consistof symmetrical modes.

When operating with linearly polarized radiation, input energy is fed to the opposite ends of the first circular waveguide 52 to excite the TE circular waveguide mode therein, this energy being derived from the first input source 50 and a second input source 70. Specifically, the first input source 50 excites the TE mode in that side of the first circular waveguide 52 which is closest to it from the short circuit element 57. The second input source 70 excites the other end of the first circular waveguide 52 through the probe 73 extending from the coaxial line 72. The energy transfer thereafter may best be seen by reference to FIGURE 9.

Energy from the first input source 50 is utilizedto cause the forced excitation of an H radial waveguide mode in the radial waveguide antenna 40. The longitudinal coupling slots 53 which extend axially along the first circular Waveguide couple energy into the region between the first circular waveguide 52 and the second circular waveguide 63. The rigid coaxial line 62 defined by the two circular waveguides 52, 63 is excited in a corresponding TE mode. Electric field distributions for the TE mode in a rigid coaxial line correspond roughly to that of a circular waveguide mode, except for the presence of the center conductor, so that no transition elements are needed to in turn excite the E radial waveguide mode. A variable coaxial short 64 is utilized to adjust the amplitude of the TE mode so that it delivers maximum energy to the E radial waveguide mode.

Note that the symmetrical arrangement of the longitudinal slots 53 and of the remaining elements about the central axis permits any type polarization to be excited in the direction normal to the central axis. Energy from the rigid coaxial line 62 couples directly into the radial feed waveguide 66 to excite the E radial waveguide mode therein. As a result, the radially disposed cou pling slots 61 in the bottom plate 42 of the radial waveguide antenna 48' couple energy into the antenna 40, this energy being in the desired H radial waveguide mode.

Excitation of the radial waveguide antenna 46 in the second mode of the modal pair is also initiated by the establishment of a TE circular waveguide mode in the portion of the first circular waveguide 52 which includes the segment passing through the radial waveguide antenna 40. The continuous annular slots 54, 55 concentric with the central axis and spaced 1/2 guide Wavelength apart couple out energy into the radial waveguide antenna 40. Maximum coupling is achieved by positioning the shorting elements 56 and 57 at l/4 guide wavelength and 5/4 guide wavelength respectively from the probe 73 in the circular waveguide 52.

To establish a properly modal pair within the radial waveguide antenna 40 there should in practice be some means of adjusting the amplitude and phase of the individual feed modes. An arrangement for providing control of these parameters has not been shown in conjunction with FIGS. 7 to 9, for simplicity. One arrangement which might be used is however shown in FIG. 13. Other techniques will also suggest themselves to' those skilled in the art.

Thus, by this control the E and H radial waveguide modes are established within the radial waveguide antenna 40, and because absorptive termination of the energy in the resistive termination 45 causes traveling wave operation, the desired current distributions are present over the entire top plate 41. So for the three annular slot aperture the radial and circumferential currents exciting them again produce a radiated field which has a common plane of polarization at any given instant of time.

A desired modal pair may also be established by the use of a difierent feed arrangement, such as that illustrated in FIGS. 10 through 12 and having like numbers for some of the like parts. As shown therein, a radial Waveguide antenna 40 may have top and bottom plates 41, 42 disposed about a central axis, with an internal ring of resistive material forming a flat load 45 within an outer circumferential band 43. In the same manner as the arrangement of FIGS. 7 through 9, crossed slots 46 may be disposed in annuli concentric with the central axis, the crossed slots 75 being symmetrically placed with respect to the circumference on which they lie. is arrangement may utilize a first circular waveguide connected to the bottom plate 42 of the radial waveguide antenna 40 and a second circular waveguide 81 connected to the top plate 41 of the radial waveguide antenna 46. Each of the circular waveguides 8t and 81 may register with a corresponding opening in the associated Wall or plate, 42 or 41 respectively, of the antenna 40. A source (not shown) may be coupled to first circular waveguide 80 for exciting the TE circular waveguide mode, and a separate source (also not shown) may be coupled through a first coaxial line 83 having a center conductor terminating as a probe 84 Within the second circular waveguide 81. To establish circular or elliptical polarization .a separate probe 85 coupled to order to propagate the E another coaxial feed line may be mounted in a direction orthogonal to the probe 84 extending from the first coaxiail line 83. The circular waveguides 80, 81 are thus alike as to the modes which each indirectly provides to the radial waveguide antenna 40. An end plate 86 terminates the second circular waveguide 81.

Each of the circular waveguides 80 and 81 is coupled to a dilferent one of first and secondradial waveguide feeds 90, 94 respectively. These'may also be referred to as radial feed waveguides. Radial waveguide feeds 90, 94 each include a different circular plate 91 and 95 respectively, which is connected by an outer flange 92 and 96 to an inner radial portion of the bottom wall and top Wall 42 and 41 respectively of the radial waveguide antenna 49. The first radial waveguide feed 90 includes a number of crossed slots 23 disposed inan annulus concentric with the central axis of the waveguide antenna 4% and each having a radial and circumferential arm with respect tothe central axis and the annulus upon whichit lies. In likefashion, the second radial waveguide feed94 has crossed slots 97 identically shaped and spaced. A pair or tapered outer rings 98, 99' around the outer periphery of the flanges of the two radial feed waveguides 9t 94 provides a smooth transition for microwave energy between the waveguide feeds 90, 94

y and the associated radial waveguide antenna 40.

In operation, the arrangement thus provided together with external controls such as phase shifters and attenuators in both inputsforces the dependent excitation of the desired E and H radial waveguide modes and with the prescribed relative phase and amplitude in the radial waveguide antenna 49. As may be seen best in FIG.

, 12, the TE mode in the first circular waveguide 80 eshalf the required current distribution necessary to propagate the E and H radialwaveguide modal pair. Concurrentl', the TE mode'excited in the second circular waveguide 81 by the energy in the coaxial line 83 excites an E radial waveguide mode in the second radial feed 7 waveguide 94. This mode in turn provides, through its crossed slots 97 in the second radial waveguide feed 94,

the other half of the required current distribution on the lower plate 4-2 of the radial Waveguide antenna 40, in V and H radial waveguide modal pair.

Again, therefore, the desired modal pair of traveling 'wave modes is established within the radial Waveguide antenna 40. Consequently, the radial and circumferential currents at each of the crossed slots 46, composing the three annular slots, provide at any instant of time a radiated field which is polarized parallel to a common plane of polarization.

Both these traveling wave anrangements'thus operate by properly fixing the amplitude and phase of the two traveling wave modes which are being excited in the-radial waveguide antenna 40. Each establishes the desired curout of phase, and with the prescribed relative amplitude as shown in the next equation a where A and B are the real amplitudes of the E and Id radial waveguide modes respectively, b is theheight of the annuli can'be disposed at smaller'radii 12 radial waveguide, and g is the intrinsic admittance. Having this desired relative phase and amplitude, the radial waveguide modal pair produces a total current flowing over the entire aperture plate that has the form,

It= p[ 0 1 Sill ()l+ 0 1p) cos where and 6,, are unit vectors in the radial and circumferential directions respectively. The radial dependence of the two orthogonal currents is thus determined bythe same expression H (K p), which is a Hankel function of order zero, where Thus under these conditions the radial and circumferential currents of the traveling wave device everywhere have the desired relationships, and'the annuli need not be placed at any specific radii. With the traveling wave device there is no limitation on the number of annuli which The total aperture or source of the antenna consequently can be focused by control of phasing. Additionally, amplitude control can be obtained by selective alteration 'of the size and the interelement spacing of the individual crossed slots in the successive annular arrangements. In this respect, it should be noted that there is usually an inherent amplitude taper in the traveling wave arrays, due to the Hankel function dependence of the outward traveling wave model pair. 7 y

Traveling wave arrays therefore have a number of significant operating features and excellent versatility.

For a given size aperture, they have greater aperture emciency. They also have wide bandwidth. For many applications where low sidelobes are of extreme importance, their arraying potential gives them particular attractiveness.

The, examples shown are merely to illustrate the arrangement of structures in accordance with the invention and it will be understood that other variations are possible. As with the standing Wave array, the traveling Wave radial waveguide antenna may provide linear polarization of any direction, circular polarization of either sense of rotation, or elliptical polarization. Two'mod'al pairs are employed for such polarization characteristics. It may be seen in' the arrangements of 'FIGS.'8, l0 and 11, that the crossed probes 73, '74 (FIG. 8) and 84, 85 (FIGS. 10 and 11) may be utilized to provide such dual excitation. It will also be understood that the E and H modal pair is illustrative of only one specific relationship in a general class of relationships which may be employed. In the general case, the E and H modal pairs can be used, wheren=l,2,3.. V

A number of other arrangements will also suggest themselves to those skilled in the art. Where it is desired to have a .much more compact traveling wave arrangement, for example, the tapered flat load shown in the two travel ing wave arrangements can be replaced by a more compact' termination. Additionally, the central feed structure can be made more compact, if desired, so that the It will also be recognized that the modal pair can be excited without utilizing a protruding feed; The entire ,feed may be accomplished from one's'ide of the radial waveguide, so that the structure would be completely flush'with an associated It is nonetheless apparent that the small promajority of circumstances.

There is illustrated, in FIGS. 13 and 14, another arrangement showing the manner in which a radial waveguide antenna 100 may be employed in a system context. These figures provide an exemplification of a conical scanning arrangement and are also illustrative of the manner in which desired modes can be established and controlled as to amplitude and phase. Other arrangements will suggest themselves to those skilled in the art, but the provision of this specific example will assist in making the arrangement clear.

The antenna 100 itself is best seen in FIG. 14. It is intended to operate as a standing wave device, and has one annulus consisting of a number of crossed slots 101, only a few of which have been illustrated for simplicity. The antenna 100 is fed by a circular waveguide 103 which is coupled to an opening in the central axis of the waveguide 100 in the manner prew'ously described. The waveguide 100 also includes an internal dielectric member 102 which is eccentrically placed with respect to the central axis of the antenna 100. The dielectric member 102 acts as a phase shifter within the radial waveguide antenna 100, and serves to slow waves therein to an extent determined by the amount of dielectric in the radial path. The shape of the dielectric member 102 is not a circle, but is varied in accordance with functions apparent to those skilled in antenna synthesis to provide a desired phase variation at the individual elements of the aperture. The net result is that the antenna pattern is tilted slightly off the central axis of the antenna 100. The tilting is in the direction opposite to the greatest radial displacement of the dielectric member 102 from the central axis. If, therefore, the antenna 100 can be rotated about its central axis, a conical scanning arrangement will be provided.

As seen in FIG. 13, this antenna may be mounted flush with an associated surface 105, and may contain a peripheral gear 106 which is driven by an associated gear 107 which is in turn rotated by a motor 108. The motor 108 may rotate the antenna 100 continuously, or may provide incremental angular positions.

The arrangement shown for exciting the antenna 100 provides a general illustration of the manner in which amplitude and phase control may be provided in the other arrangements previously discussed. A source of energy 109 provides microwave energy into two different channels. One channel contains a first variable phase shifter 110 and a first variable attenuator 114, and the other channel contains a second variable phase shifter 111 and a second variable attenuator 115. The outputs from the two variable attenuators 114 and 115 may constitute a pair of orthogonally disposed probes 117 and 118 in a circular waveguide section 119. This circular waveguide section is coupled to the circular waveguide feed 103 of the antenna 100 through a microwave rotary joint 120. e The arrangement thus provided illustrates both conical scanning and the control of excitation of the antenna 100 through amplitude and phase adjustments. The antenna 100 provides its olf-axis beam during rotation by the motor 108, for conical scanning. The symmetrical arrangement of the feed waveguide 103 and the coupled rotary joint 120 is independent of the direction of polarization of the radiation coupled to the antenna 100. The orthogonally disposed probes 117, 118, however, can establish direction of linear polarization, circular polarization of either sense of rotation, or elliptical polarization in the associated waveguide 119. Such variations are effected by the two variable phase shifters 110, 111 and the two variable attenuators 114, 115. The attenuators 114, 115 determine amplitude, and the phase shifters 110, 111 determine the phase of the excitation modes. For a rotating polarization, the phase shifters would be operated cyclically.

The scanning antenna thus provided is substantially mechanically symmetrical and thus is largely free of inertial effects. This fact in combination with the substantially planar structure make it possible to use the antenna 100 in a forward part of a lightweight and streamlined system without complicating balance and weight distnibution problems. Conical scanning might also be achieved through the use of a thin standing wave arrangement which is slightly tilted with respect to the central axis, or which has a slight bend in the feed waveguide. Such arrangement can be made arbitrarily thin and extremely small.

Thus there has been provided a new family of planar surfaced antennas having arbitrary polarization capabilities and providing broadside pencil beams. Antennas constructed in accordance with this invention are compact, lightweight and have versatile operating characteristics.

We claim 1. A planar surfaced antenna capable of operating with like patterns independently of the polarization characteristic of the radiation being transduced, the antenna comprising: a radial waveguide having parallel conductive plates, one of the plates including at least one annular aperture defined by a number of crossed slots arranged on an annulus disposed about the center of the radial waveguide, the crossed slots having like radial and circumferential attitudes with respect to the annulus on which they lie; and means coupled to the radial waveguide at the central axis thereof for exciting the radial waveguide surface containing the crossed slots with radial and circumferential current components which are equal in amplitude at the annulus containing the crossed slots and which are displaced from one another by ninety electrical degrees.

2. An arbitrarily polarized planar antenna array comprising a radial waveguide having an antenna aperture defined by a plurality of individual radiating elements disposed concentrically about the center of the radial wavequide in one surface thereof, the radiating elements comprisin individual apertures shaped to provide radiation components in any direction in the plane of the surface in which they are positioned; and wave energy feed means coupled to the radial waveguide at the central axis thereof and energizing the radial waveguide in at least one radial waveguide mode for establishing radial and circumferential currents in the surface thereof containing the radiating elements, the position of the radiating elements being selected such that the total vectorial radiation component at each aperture is like that at the others in magnitude and direction.

3. An antenna comprising: a conductive surface plate providing a radiating source defined by a number of individual radiating elements disposed on an annulus about a center point, the radiating elements being of a form having equal radial and circuferential excitation capabilities with reference to the center point; and means electromagnetically coupled to the surface plate for exciting the radiating elements with radial and circumferential currents thereat which are equal in total amplitude at the annulus but relatively displaced at the individual elements so that the resultant vector of radiation provided at each element is caused, by the attitude of each individual radiating element and the current excitation thereat, to provide like resultant components from each of the radiating elements.

4. An antenna having a planar radiating surface and being capable of being mounted flush with a supporting structure, the antenna providing a pencil beam pattern of a substantially like characteristic independently of the polarization characteristics of the radiation which is transduced by the antenna and comprising: a radial waveguide having a central axis and defined by a pair of parallel conductive plates, one of the plates containing a plurality of crossed slots which together define an annular antenna aperture, the crossed slots being disposed in circles about the central axis of the radial waveguide and the circles lying at selected radial distances from the central axis, the crossed slots being symmetrically placed about and having like attitudes with respect to the circles on which they lie; termination means positioned between 7 guide having parallel conductive plates and a central axis extending normal to the plates, one of the plates having a number of crossed slots therein, the slots lying in at least one annulus about the central axis and having sea lectediattitudes and positions; wave'encrgy termination means coupled between the plates at a radius further from the central axis than the crossed slots; and means coupled to the radial waveguide for exciting modes therein which establish at least two current regions in the platecontaining the crossed slots, the current regions and the attitudes and positions of the crossed slots combining to provide uniform operation independent of the direction of excitation of the crossed slots.

6. A planar antenna for providing a broadside pencil beam pattern and for operating substantially independently of polarization characteristics, the antenna comprising: a radial waveguide including a pair of parallel conductive plates and a circumferential conductive band for providing standing wave operation, one of the plates having at least one annular radiating aperturedefined by a number of crossed slots of like configuration disposed symmetrically on at least one annulus about the center of the radial waveguide, each of the crossed slots having a like radial and circumferential attitude with respect to the annulus on which it lies; and wave energy feed means coupled to one of the plates of the radial Waveguide at the center thereof for providing coupling to a selected radial waveguide mode in which radial and circumferential currents of equal amplitude but ninetydegree relative displacement are provided at the annuli containing the crossed slots, such that the excitations of each of the'crossed slots are like in amplitude and direction.

waveguide having a number of radiating elements symmetrically placed about a selected point in the Waveguide, the elements having nonpolarization sensitive configurations; and means at the selected point in the waveguide for coupling energy thereto which provides excitation of the individual elements with orthogonally disposed components. i

8. An antenna comprising: a radial waveguide includaxis and encompassing the plate and a radiating surface plate coupled to the terminating flange and being substantially parallel'to the first plate, the radiating surface plate including a plurality of crossed slots disposed in at least one annulus concentric with the axis' of the radial waveguide, the crossed slots having like angular positions with respect to the annulus on which they lie; and means coupled to the first plate of the radial waveguide for feeding Wave energy thereto to establish radial and circumferential currents of "90 phase disposition at each of the crossed slots in the radiating plate.

7. An antenna comprising: a standing wave radial sing a first plate concentrically positioned about a central c axis, a conductive terminating flange concentric with the 9. An antenna for providing a circularly or elliptically' polarized pencil beam pattern broadside to a planar surface and comprising: astanding wave radial waveguide having parallel'top and bottom plates, the top plate having a total antenna aperture defined by at least one annular slot consisting of a circular arrangement of crossed slots disposed concentrically about the center of the radial waveguide, the crossed slots each having like radial and circumferential attitudes with respect to the center of the radialwaveguide, the radial waveguide being dimensioned to support the E mode; a circular-feed waveguide coupled to the radial Waveguide; means coupled to the feed waveguide for exciting a first TE mode therein, so that a first E mode is excited in the radial waveguide to excite the crossed slots with radial and circumferential currents which are equal in amplitude but displaced by ninety electrical degrees to provide a first like instantaneous ,vectorial component at each crossed slot; and means coupled to the feed waveguide for exciting a second TE mode therein which'is orthogonal with respect to the first TE mode and has a selected amplitude relationship, the second TE mode exciting. a second E mode in the radial waveguide which is in quadrature with the first E mode, so that secondlike instantaneous vectorial'components are provided at each annular slot, the two instantaneous vectorial components varying with time to providea total rotating electromagnetic wave energy component having a degree of el-lipticity determined by the relative magnitudes of the exciting modes.

10. A planar antenna for transducing between a confined energy mode and arbitrarily polarized space radiation, said antenna comprising: a radial waveguide including a first circular conductive plate'concentric with a given axis, a short circuiting conductive band about the first conductive plate and fixed thereto, and a secondconductive plate parallel to and coextensive with the first con ductive plate, and attached to the conductive band to define with the first conductive plate a radial waveguide dimensioned to support the E standing wave mode of a radial waveguide, the second conductive plate having at least one annulus of crossed slots disposed concentrically about the axis at symmetrically spaced radial positions, the crossed slots defining an annular aperture and having like relative positions with respect to the annulus on which they lie, thefirst conductive plate having a central feed aperture; a circular waveguide coupled to the central feed aperture of the first conductive plate of the radial waveguide and concentric with the axis thereof; and means for exciting the circular waveguide in the dominant TE mode of a circular waveguide for establishing the E standing wave mode in the radial waveguide.

11. An antenna comprising: a radial waveguide having top and bottom conductive plates, the top plate having at least, one'group of radiating apertures each having a crossed slot configuration and lying in an inner region of the radialwaveguide; termination means positioncd'between the top and bottom plates in an outer region of the radial waveguide and thereby providing a traveling wave array; first means coupled to the bottom plate of the radial Waveguide for exciting therein a first traveling wave mode; and second means coupled to the top plate of the radial Waveguide for exciting therein a second traveling wave mode having selected amplitude and phase relationships With respect to the first traveling wave model 12. An antenna comprising: a radial waveguide antenna'having a number of radiating elements in one wall thereof spaced from the center of the radial waveguide, each of the apertures being excitable in both radial and circumferential directions relative to the radial waveguide antenna; feed means coupled to the radial waveguide antenna at the center thereof for exciting therein a pair of modes having selected phase and amplitude relationships; and dissipative termination means within the, radial waveguide adjacent the periphery thereof for operating the radialwaveguide antenna in a traveling wave mode.

13. A radial Waveguide antenna for providing a pencil beam patternand operating substantially independently of the polarization characteristics of the radiated wave, said antenna comprising: a radial waveguide having a plurality of crossed slots in one wall, the slots having like attitudes relative to'the central axis of the radial waveguide;

exciting the radial waveguide in a pair of traveling wavemodes providing equal total current amplitudes and hav- 1 7 ing a selected phase separation, such that the excitation of each of the crossed slots is equal in magnitude and parallel to a selected plane.

14. A planar antenna array for providing an arbitrary polarization pattern and comprising: a radial waveguide antenna defined by a pair of parallel "plates concentric with a central axis, the radial waveguide antenna including a plurality of radiating elements having a configuration such that they may be excited in any direction in the plane of the surface in which they lie; means coupled to at least one of the plates for exciting the radial waveguide antenna in an E mode, where n=0, 1, 2'. and means coupled to the other of the plates of the radial waveguide antenna for exciting the radial waveguide antenna in an H mode, where n=0, 1, 2 the two modes being separated by and arranged to provide relatively equal total current amplitude.

15. An antenna comprising: a radial waveguide antenna arranged to operate in a traveling wave mode and having a plurality of crossed slots in a first wall thereof and disposed symmetrically about the central axis of the radial waveguide; a centrally disposed circular waveguide coupled to the radial waveguide along the central axis thereof and protruding through both walls of the radial waveguide, the first circular waveguide including annular slots within the radial waveguide for exciting a selected mode within the radial waveguide antenna, and longitudinal slots in the side which protudes from the second wall; means coupled to the first circular waveguide adjacent the first wall of the radial waveguide for feeding waves of desired amplitude and phase to the circular waveguide, thereby to excite the radial waveguide in a mode of selected amplitude and phase through the annular slots; 2 second circular waveguide encompassing the first circular waveguide at the protruding portion which extends 'from the second wall of the radial waveguide and being coupled thereto by the longitudinal slots in the first circular waveguide and forming therewith a coaxial waveguide; a radial feed waveguide coupled to the coaxial waveguide and to the second wall of the radial waveguide antenna and electromagnetically coupled by radially oriented slots therein to the interior of the radial waveguide antenna; and means coupled to the portion of the first circular waveguide which protrudes from the second wall of the radial waveguide antenna for exciting the first circular waveguide in a selected mode, so that energy in the first circular waveguide excites the coaxial waveguide and in turn the radial feed waveguide so that a second mode of selected characteristics is established in the radial waveguide antenna.

16. An antenna for providing a pencil beam broadside to a substantially planar surface, the antenna operating substantially independently of the polarization characteristics of the space propagated wave upon which it operates and comprising: a radial waveguide antenna having top and bottom conductive plates disposed about a central axis normal to the plates, the top plate including a plurality of crossed slots disposed in concentric annuli about the central axis, the crossed slots in each of the annuli being symmetrically disposed and having like radial and circumferential attitudes with respect to the annulus on which they lie, the bottom plate including radially oriented slots disposed symmetrically about the central axis of the radial waveguide at an inner radius thereof; a ring of resistive material disposed between the top and bottom plates of the radial waveguide and at a further radius from the central axis than the outermost annulus of crossed slots, the ring of resistive material having a taper of increasing height in the outward direction so as to provide traveling wave operation of the radial waveguide antenna; a radial feed waveguide coupled to the bottom plate of the radial waveguide and electromagnetically coupled to the radially oriented slots therein; a first circular waveguide concentric with the central axis and coupled to the radial feed waveguide to form the outer conductor of a rigid coaxial line; a second circular waveguide extending through the first circular waveguide and through the radial waveguide from the bottom plate side to the top plate side and terminating at the top plate side; the second circular waveguide including a ring of longitudinal slots which are elongated in the direction of the central axis and further disposed from the radial waveguide than the radial feed waveguide and within the length of the first circular waveguide, the second circular waveguide also including annular circumferential slots adjacent the inner surfaces of the top plate and the bottom plate within the radial waveguide antenna; a conductive short circuiting element extending across the interior of the second circular waveguide between the radial waveguide and the longitudinal slots in the second circular waveguide; feed means coupled to the portion of the second circular waveguide protruding beyond the top plate of the radial waveguide; feed means coupled to the second circular waveguide on the bottom plate side thereof; and a cylindrical coaxial shorting member movably positioned between the outer surface of the second circular waveguide and the inner surface of the first circular waveguide for controlling the length of the rigid coaxial transmission line extending along the two circular waveguides in a direction away from the radial waveguide and the radial feed waveguide.

17. An antenna comprising: a radial waveguide antenna arranged to operate in a traveling wave mode having a pair of walls, a first of which includes a plurality of crossed slot apertures; means including a first radial feed waveguide coupled to the first wall for exciting a first radial waveguide mode in the radial waveguide antenna; and means including a second radial feed waveguide coupled to the second wall of the radial Waveguide antenna for exciting a second radial waveguide mode therein, the two radial waveguide modes having a selected phase separation and having equal total current amplitudes.

18. A planar-surfaced antenna capable of providing a pencil beam pattern broadside to the planar surface and substantially independent in characteristics of the nature and direction of the polarization upon which the antenna operates, the antenna comprising: a radial waveguide antenna structure comprising a pair of conductive plates parallel to each other and symmetrically disposed about a central axis, a first of the plates providing the planar surface of the antenna and including crossed slot radiating apertures therein, the apertures being disposed in symmetric fashion in annuli concentric with the central axis, the crossed slots having like radial and circumferential attitudes with respect to the annulus upon which they lie; a terminating load ring positioned between the plates of the radial waveguide at an outer radius thereof and providing traveling wave operation of the radial waveguide; a first circular waveguide coupled to the outer surface of the first plate at the central axis; a first radial feed waveguide coupled to and completed by the inner surface of the first plate of the radial waveguide antenna and concentric with the central axis, the first radial feed waveguide being fed by the first circular waveguide and including a plurality of crossed slots disposed in its innermost surface in a feed annulus concentric with the central axis; a second circular waveguide coupled to the second plate of the radial antenna waveguide at the outer surface thereof; a second radial feed waveguide coupled to and completed by the inner surface of the second plate of the radial antenna waveguide and including a plurality of crossed slots disposed in a feed annulus about the central axis; means coupled to the first circular waveguide for exciting the first circular waveguide in a TE mode,

thereby to excite the E radial waveguide mode inthe V first radial feed waveguide for establishing an E radial waveguide mode in the radial antenna Waveguide; and

means coupled to the second circular waveguide for exciting "the second circular waveguide in a TE mode to establish an E radial waveguide mode in the second radial feed Waveguide, thereby to excite the radial waveguide antenna in an H radial waveguide mode, the

:20 means for exciting the circular waveguides being an ranged to provide a selected phase-difierence between the E and H modes in the radial waveguide-antenna, and

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3322 506 February 20, 1962 Frank J. Goebels, Jr, v at a1;

It is hereby certified that error appears in the above numbered patenl'. requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 57 for "objected" read object column 2 line 47 for "circlt" read circle column 7, line l9 strike out "'be" first, occurrence; column 9 line 53 strike out "of" first occurrence; column ll lines 2 and 3 for "coaxiaiP' read m coaxial column 12, line 13 to 14 for that portion 01' the equation reading read W b b Column line 51, for "circuferential" read circumferential --o Signed and sealed this 28th day of August 1962 (SEAL) Attest;

ESTON G; JOHNSON DAVID L LAUU Atltesting Officer Commiesioner of Patents

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