Patent Publication Number: US-9413051-B2

Title: Radio frequency device with feed structure

Description:
TECHNICAL FIELD 
     The present invention relates generally to radio frequency (RF) devices employing a feed structure, and more particularly area-efficient feeding of transmission-line structures. 
     BACKGROUND ART 
     RF transmission line structures oftentimes include opposing boundary walls between which electromagnetic or RF energy is intended to propagate. Types of RF transmission line structures include open parallel-plate, waveguide and resonant cavity based structures, for example. Frequently the RF transmission line structures are combined with a feed structure configured to introduce RF energy into an area between the opposing boundary walls in order to efficiently and effectively illuminate the RF transmission line structure, tailored to the desired phase and amplitude distribution. Most often, efficient launching or illumination of the RF energy with well-behaved coherency (uniform phase illumination) over a broad operating frequency bandwidth is desired. 
     Current practice for feeding parallel-plate and waveguide-based planar array type RF transmission line structures include: inscribed square/rectangle feed architecture wherein a line-feed or a linear array of couplers (waveguide- or coax-based feed-points oriented along a single line) launch a coherent internal plane-wave that illuminates a generally rectangular region (but leaves exterior regions outside the inscribed rectangular region, but inside the circular boundary, generally un-illuminated/wasted;) discrete perimeter feed architectures which use individual elements or groups of elements oriented along the array perimeter in order to feed a larger proportion of the circular region, but generally support only narrow operating frequency bands and require complex and difficult to package waveguide feeds and launches/transitions in order to provide the requisite phase coherency; and direct-fed waveguide slot antennas wherein a separate complex (rear-mounted) corporate and/or standing-wave-fed waveguide feed is employed to coherently illuminate the desired circular antenna shape in a “scalloped” pseudo-circular form-factor. 
     Notably, in open parallel-plate planar array antenna applications, for example, it is often desired to shape the antenna in a circular or near-circular (elliptical) shape. Examples include planar array surrogates for circular or elliptical parabolic dish antennas (for satellite communication, terrestrial point-to-point communication, radar systems, etc.) However, traditional waveguide-based feed architectures, by their nature, are generally rectilinear in nature and are therefore challenged to efficiently feed a circular shape. An inscribed-square geometrically fills only 64% of a circular area and due to finite limitations, it is generally not possible to feed the antenna all the way to its physical perimeter (i.e. “practical” inscribed-square efficiencies are typically less than 60%.) 
     Generically, the planar array antennas in circular or elliptical form-factors are generally fed via a separate rear-mount (direct-fed waveguide slot antennas) wherein a separate complex (rear-mounted) corporate and/or standing-wave-fed waveguide feed is employed to coherently illuminate the desired circular antenna shape in a “scalloped” pseudo-circular form-factor. Such arrays are inherently limited to narrow frequency-band operation and the bulk and packaging complexity associated with the (typically-multi-level) waveguide corporate feed adds undesired weight and cost. 
     In the special case of parallel-plate transmission-line based planar array antennas such as the Continuous Transverse Stub (CTS) array and Variable Inclination Continuous Transverse Stub (VICTS) array, current state of the (feed) technology has been traditionally to utilize (in ascending order of increased area efficiency and increased cost/complexity) a single linear-feed (“inscribed square/rectangle”;) or multiple parallel linear-feeds (“stepped feed”;) or multiple subarrays (“modularized feed”;) or via discretely-fed perimeter feed slots (“perimeter slot feed”.) While these approaches have varying levels of area-efficiency effectiveness, all suffer from the common inability to completely fill the entire circular extent of the antenna array and (particularly in the case of the latter more complex structures) significantly increase complexity and cost while limiting overall operating frequency bandwidth. 
       FIG. 1  illustrates a typical “inscribed square” feed methodology wherein a single waveguide line-feed  10  represents a linear RF source which coherently launches propagating parallel-plate electromagnetic waves  12  within a bounded parallel-plate region  14  and generally emanating at an angle normal/orthogonal to an axis  16  of the feed  10 . The parallel-plate region  14  has a circular form factor, and the line-feed  10  illuminates a square-shaped or rectangular-shaped region  20  inscribed within the available circular region. Geometrically, this approach excites 64% of the available area, but in practice this figure is generally lower due to practical limitations on the physical extent of the line-feed  10 . 
       FIG. 2  illustrates a variant of the inscribed square of  FIG. 1 , wherein multiple rectangular regions of propagating parallel-plate waves  12  are created, each fed by its own dedicated single waveguide line-feed  10 . This method can provide marginally higher area efficiencies as compared to the inscribed-square, but at the expense of significantly higher component count and overall packaging complexity. In addition the foreshortened length of the wave/mode paths within each rectangular region can result in unintended consequences, for example constraints on antenna radiator coupling as well as undesired antenna sidelobe artifacts associated with the imperfect “blending” (discontinuities) between adjacent regions in the case of a planar array antenna. 
     A further extension of the rectangular approach (not shown) is known, wherein the feed is “modularized” into individual subarray regions with their own corresponding feeds. Such extension has the benefit of added area efficiency (filling of the available circular form factor) but again at the expense, for example, of antenna radiator coupling and sidelobe degradation in the case of a planar array antenna. 
       FIG. 3  illustrates a “Perimeter Discrete” feed method wherein individual feed elements  22  are introduced along the perimeter (in this case the left half) of the circular form factor of the parallel-plate region  14 . The individual feed elements  22  launch the propagating parallel-plate waves  12  across the left half, and (as an option) a waveguide line-feed  10  located in the middle of the circular form factor launches the parallel-plate waves  12  across the right half. Again, this method realizes good improvement in area efficiency (fill-factor), but with substantial added feed network complexity for the individual feed elements  22 . In the case of a planar array antenna type RF transmission line structure, again there is associated antenna sidelobe degradation. 
     In view of the above-noted shortcomings, there is a strong need in the art for an RF device which includes a more efficient feed arrangement for illuminating an RF transmission line structure in the case of a non-rectilinear form factor. 
     SUMMARY 
     According to an aspect, a radio frequency (RF) device is provided which includes an RF transmission line structure including opposing boundary walls with a non-rectilinear form factor; and a feed structure configured to introduce RF energy into an area between the opposing boundary walls to illuminate the RF transmission line structure with the RF energy across the non-rectilinear form factor. The feed structure includes a plurality of traveling-waveguide-fed leaky line-segment structures, each configured to launch the RF energy into the area with a propagation direction having an oblique angle relative to an axis of the line-segment structure. 
     According to another aspect, the plurality of leaky line-segment structures are positioned proximate a perimeter of the non-rectilinear form factor. 
     In accordance with another aspect, the non-rectilinear form factor is circular or elliptical. 
     According to yet another aspect, the plurality of leaky line-segment structures are positioned along corresponding chords of the circular or elliptical form factor. 
     According to still another aspect, two or more of the plurality of leaky line-segment structures are oriented at oblique angles to one another. 
     In yet another aspect, two of the plurality of leaky line-segment structures are oriented at an oblique angle to one another and extend from a common vertex. 
     According to another aspect, two of the plurality of leaky line-segment structures are oriented at an oblique angle to one another and the feed structure further includes one or more feed segments which separate the two plurality of leaky line-segment structures and are configured to launch the RF energy into the area with a propagation direction having a non-oblique angle relative to an axis of the feed segment. 
     In accordance with another aspect, one or more of the plurality of leaky line-segment structures is an end-fire leaky waveguide. 
     In still another aspect, the end-fire leaky waveguide includes at least one of a continuous broadwall coupling slot, an array of discrete broadwall slots or apertures, or an array of discrete sidewall slots or apertures. 
     Regarding another aspect, the end-fire leaky waveguide includes a meandering slot. 
     In yet another aspect, the end-fire leaky waveguide has a variation in the “a” (broadwall) dimension along a length of the end-fire leaky waveguide. 
     According to another aspect, the plurality of leaky line-segment structures are positioned at least one of between the opposing boundary walls, adjacent an outer surface of one or both of the opposing boundary walls, or adjacent an opening between the opposing boundary walls along a perimeter of the non-rectilinear form factor. 
     According to still another aspect, the RF transmission line structure comprises at least one of a parallel-plate transmission structure, a partially open transmission structure having a lower-plate covered in a dielectric layer, a waveguide, or a resonant cavity. 
     In still another aspect, the plurality of leaky line-segment structures are configured to launch the RF energy in coherent waves. 
     According to another aspect, at least one of the plurality of leaky line-segment structures comprises a curved waveguide including at least one of a linear continuous broadwall coupling slot, a linear array of discrete broadwall slots or apertures, or a linear array of discrete sidewall slots or apertures. 
     In yet another aspect, the curved waveguide has a constant “a” (broadwall) dimension. 
     In accordance with another aspect, a leaky line-segment structure is provided which includes a curved waveguide, and formed in the curved waveguide at least one of a linear continuous broadwall coupling slot, a linear array of discrete broadwall slots or apertures, or a linear array of discrete sidewall slots or apertures. 
     According to another aspect, the at least one of the linear continuous broadwall coupling slot, the linear array of discrete broadwall slots or apertures, or the linear array of discrete sidewall slots or apertures is formed in a flat wall of the curved waveguide. 
     To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the annexed drawings, like references indicate like parts or features: 
         FIG. 1  is a schematic illustration in partial cutaway of a first example of a conventional RF device having a feed structure; 
         FIG. 2  is a schematic illustration in partial cutaway of a second example of a conventional RF device having a feed structure; 
         FIG. 3  is a schematic illustration in partial cutaway of a third example of a conventional RF device having a feed structure; 
         FIG. 4A  is a top-view schematic illustration in partial cutaway of a first exemplary embodiment of an RF device having a feed structure arrangement in accordance with the present invention; 
         FIG. 4B  is a schematic cross-sectional illustration of the RF device shown in  FIG. 4A ; 
         FIG. 4C  is bottom-view schematic illustration of the RF device shown in  FIG. 4A ; 
         FIG. 5  is a top-view schematic illustration in partial cutaway of a second exemplary embodiment of an RF device having a feed structure in accordance with the present invention; 
         FIG. 6  is a graph showing the theoretical area efficiency of an RF device in accordance with the embodiment of  FIGS. 4A-4C ; 
         FIG. 7  is a top-view schematic illustration in partial cutaway of a third exemplary embodiment of an RF device having a feed structure in accordance with the present invention; 
         FIG. 8  is schematic cross-sectional illustration of a fourth exemplary embodiment of an RF device in accordance with the present invention; 
         FIG. 9  is a top view schematic illustration of a leaky line-segment structure according to an exemplary embodiment; 
         FIG. 10  is a top view schematic illustration of a leaky line-segment structure according to an alternative exemplary embodiment; 
         FIG. 11  is a graph illustrating the computed beam angle ( 8 ) for An exemplary leaky line segment structure as a function of frequency; and 
         FIG. 12  is a top view schematic illustration of a leaky line-segment structure according to another alternative exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Provided is an RF device having a more efficient feed arrangement for illuminating an RF transmission line structure with a non-rectilinear form factor. The device departs from the traditional use of one or more rectilinear line-segment structures emanating RF energy at an angle normal/orthogonal to an axis of the line-segment structure. Instead, the device employs multiple line-segment structures which emanate RF energy at an angle which is oblique relative to the axis of the line-segment structure. The multiple line-segment structures may be obliquely angled relative to one another in order to more efficiently inscribe and feed/illuminate the desired non-rectilinear form factor in a piece-wise linear manner. The line-segment structures are traveling-waveguide-fed leaky line-segment structures, each configured to launch the RF energy with a propagation direction having an oblique angle relative to an axis of the line-segment structure. These replace generally more complex conventional multi-level feed architectures with a resultant benefit in size, weight, complexity, and cost. Moreover, the traveling-waveguide-fed leaky line-segment structures can exhibit unusual beneficial properties in terms of improved operating frequency bandwidth as compared to conventional feeding techniques. 
     Referring to  FIGS. 4A-4C , shown is an RF device  30  in accordance with a first embodiment. The RF device  30  includes an RF transmission line structure  32  including opposing boundary walls  32 A,  32 B. The RF transmission line structure  32  has a non-rectilinear form factor, in this particular embodiment circular although other non-rectilinear form factors are equally possible (e.g., elliptical, non-rectilinear polygonal, etc.). In this embodiment the RF transmission line structure is an open parallel-plate transmission structure including boundary walls  32 A,  32 B made up of parallel conductive plates within which can propagate parallel-plate RF waves and modes. According to an alternative embodiment, the RF transmission line structure  32  can instead be any other transmission structure having opposing boundary walls through which can propagate RF waves and modes. For example, the RF transmission line structure  32  may be a partially open transmission structure having a lower-plate covered in a dielectric layer, a waveguide, resonant cavity, etc., (each having opposing boundary walls) without limiting the scope of the RF device  30  described herein. 
     The RF transmission line structure  32  may include, but is not limited to, homogeneously or inhomogeneously filled parallel-plates representing boundary walls  32 A,  32 B. The parallel-plates may or may not be strictly parallel but are suitably parallel to enable suitable transmission of parallel plate waves. One or both of the parallel plates representing the boundary walls  32 A,  32 B may include corrugated conductors on the surface thereof. 
     The RF device  30  further includes a feed structure  36  configured to introduce RF energy into an area  37  between the opposing boundary walls  32 A,  32 B to illuminate the RF transmission line structure  32  with the RF energy across the non-rectilinear form factor. Most preferably, the feed structure  36  is configured to illuminate the RF transmission line structure  32  with coherent propagating parallel-plate electromagnetic plane waves  12  with a desired amplitude distribution which may or may not be uniform. 
     As a particular example, the RF device  30  may represent a parallel-plate array antenna or feed element. One or both of the boundary walls  32 A,  32 B may include an array of slots (not shown) or the like designed to extract and radiate RF energy provided from the electromagnetic waves  12 . Use of such slots or other type apertures is well known in the art and therefore further description will be omitted for sake of brevity. 
     The feed structure  36  includes an arrangement of traveling-waveguide-fed leaky line-segment structures  38 , in this embodiment leaky line-segment structures  38 A,  38 B. As is described in more detail below, each of the leaky line-segment structures  38  is configured to launch RF energy into the area  37  with a propagation direction having an oblique angle θ relative to an axis  16  of the line-segment structure  38 . The leaky line-segment structures  38  can be any type of transmission line which is leaky in the sense that RF energy is continuously coupled (or “leaked”) from the line-segment structure such that a desired amplitude distribution is realized ideally with a minimum amount of power remaining at the perimeter of the RF transmission line structure  32 . In the exemplary embodiment, the leaky line-segment structures  38  are conventional end-fire oriented rectangular waveguides. However, other type line-segment structures are also suitable, such as homogeneously or inhomogeneously filled rectangular waveguides, single- or doubly-ridged waveguide, post-wall waveguide, suspended air stripline, etc. 
     Most preferably, the leaky line-segment structures  38  are configured to launch the RF energy into the area  37  as coherent propagating parallel-plate plane waves  12 . In the embodiment of  FIGS. 4A-4C , two leaky line-segment structures  38 A, 38 B launch coherent parallel-plate waves at a designed oblique angle θ relative to their corresponding feed axis  16 . The oblique orientation of these line-segment structures  38 A, 38 B serve to more efficiently conform to the circular form-factor of the parallel-plate region and therefore illuminate a larger percentage of the available area  37 . Further, the RF phenomenology of the employed “end-fire” oriented line-segment structures  38 A,  38 B exhibits unusually stable beam position (the angle at which the RF waves launch relative to the axis of the feed) and therefore exemplary operating frequency bandwidth. 
     Continuing to refer to the embodiment of  FIGS. 4A-4C , the leaky line-segment structures  38  are rear-mounted on the boundary wall  32 B. Each of the line-segment structures  38  includes a continuous tapered or meandering (varying offset) slot  40  in its upper waveguide broadwall. The slot  40  extends through the boundary wall  32 B thus enabling RF energy which leaks from the line-segment structures  38  to launch into the area  37  within the RF transmission line structure  32 . The slot  40  is centered near the location of the upper feed  42  of the waveguide (for minimum coupling) and increases in offset monotonically relative to the centered axis  16  (increasing coupling) towards its lower extreme. An absorptive load (not shown) may be placed at the end of the waveguide  38 A,  38 B in order to absorb a small amount of uncoupled RF energy. When using the RF device  30  as a transmitting antenna, for example, RF energy is introduced into each of the waveguides  38 A,  38 B through its respective feed terminal  42  using conventional waveguide feed techniques. The RF energy then propagates through the respective waveguide  38 A,  38 B toward the end of the waveguide. During such time, the RF energy from each waveguide  38 A,  38 B is continuously coupled (or “leaked”) from the line-segment structure  38  through the slot  40  such that a desired amplitude distribution at the desired oblique angle θ is realized within the transmission line structure  32 . 
     As in the other embodiments described herein, the leaky line-segment structures  38  may be positioned proximate a perimeter of the non-rectilinear form factor of the RF transmission line structure  32 . By selecting an appropriate oblique angle θ for each of the line-segment structures  38 , the feed  36  is better able to illuminate efficiently the RF transmission line structure  32  with coherently propagating RF energy across the entire non-rectilinear form factor. The non-rectilinear form factor may be circular, elliptical, etc. The leaky line-segment structures  38  may be positioned along corresponding chords of the circular or elliptical form factor as exemplified in  FIGS. 4A-4C . Moreover, the leaky line-segment structures  38  may be oriented at oblique angles to one another as exemplified in  FIGS. 4A-4C . For example, two leaky line-segment structures  38  may be oriented at an oblique angle to one another and extend from a common vertex  46 . 
     Those having ordinary skill in the art will appreciate that in an alternative embodiment the slot  40  may instead (or also) include an array of discrete broadwall slots or apertures, an array of discrete sidewall slots or apertures, etc. The leaky-line segment structures  38  need only be oriented properly relative to the RF transmission line structure  32  so that the RF energy may be launched appropriately into the area  37 . 
     Referring now to  FIG. 5 , shown is another exemplary embodiment of an RF device denoted as  50 . This embodiment varies from the embodiment in  FIGS. 4A-4C  in that the feed  36   a  further includes a feed segment  52  which separates the leaky line-segment structures  38 A,  38 B and is configured to launch the RF energy into the area  37  with a propagation direction having a non-oblique angle relative to an axis of the feed segment  52 . As shown in  FIG. 5 , the feed segment  52  again is a rectangular waveguide which includes one or more slots  54  in its broadwall which extend through the boundary wall  32 B thus enabling RF energy to leak from the feed segment  52  to launch into the area  37 . Similar to the waveguide line-feeds  10  in conventional devices, the feed segment  52  is configured to launch the parallel-plate waves in a direction normal to its axis. Combined with the leaky line-segment structures  38 A,  38 B located adjacent to the line feed segment  52  yet configured to launch the RF energy into the area  37  with a propagation direction having an oblique angle θ, the area efficiency of the non-rectilinear form factor is improved as compared with the embodiment of  FIGS. 4A-4C . Furthermore, the operating frequency bandwidth of the device  50  is improved (based of the resultant smaller physical length of the leaky line-segment structures  38 A,  38 B and greater flexibility in selection of the oblique angle θ.) 
     According to a variation of the embodiment in  FIG. 5 , the feed segment  52  is composed of n (e.g., 20) waveguide coupling elements fed via a (n+2)-way (e.g., 22-way) waveguide corporate feed structure. The outermost ports of the waveguide feed segment  52  (the 1st and (n+2)th ports) serve to feed the inclined leaky line-segment structures  38 A,  38 B via the individual waveguide feeds  42 . 
     Those having ordinary skill in the art will appreciate that any number of leaky line-segment structures  38  along with any number of traditional line-feeds  50  may be combined in a device. The line-segment structures  38  and line-feeds  50  may be distributed, preferably about a perimeter of the non-rectilinear form factor in order to most efficiently illuminate the area within the boundary walls  32 . Moreover, each leaky line-segment structure  38  may be designed for its own particular oblique angle θ. Namely, the value of the oblique angle θ is selected based on the particular orientation of the line-segment structure  38  relative to the other line-segment structures and the desired direction of the coherent parallel-plate waves. 
     Regarding the area efficiency metrics for the embodiment of  FIGS. 4A-4C  as a function of oblique angle θ,  FIG. 6  illustrates that theoretically the area efficiency is maximized (at a value of 88%) for angles θ between 55 and 60 degrees. For the embodiment of  FIG. 5 , it can be shown that this theoretical area efficiency increases to approximately 92% and at a smaller angle θ of approximately 45 degrees. 
     Referring briefly to  FIG. 7 , another embodiment of an RF device is denoted as  60 . The embodiment is essentially identical to that of the embodiment in  FIG. 5 ; however, the RF device  60  in this case includes an RF transmission line structure  32  which has a non-rectilinear form factor different from a circle. In this embodiment, the RF transmission line structure  32  is an octagon although it will be appreciated that virtually any other non-rectilinear form factor is equally possible. 
       FIG. 8  illustrates another embodiment of an RF device denoted as  70 . The embodiment is the same as the embodiment in  FIGS. 4A-4C  with the following exceptions. In this embodiment, the leaky line-segment structures  38  are positioned between the opposing boundary walls  32 A,  32 B rather than being rear-mounted (i.e., adjacent an outer surface of one or both of the opposing boundary walls). The leaky line-segment structures  38  again are configured with at least one of a continuous broadwall coupling slot, an array of discrete broadwall slots or apertures, or an array of discrete sidewall slots or apertures, so that the RF energy introduced via the feeds  42  may leak from the line-segment structures  38  to launch into the area  37  within the RF transmission line structure  32  at a desired oblique angle θ. According to yet another embodiment, the leaky line-segment structures  38  may be located adjacent an opening between the opposing boundary walls  32 A,  32 B along the perimeter of the non-rectilinear form factor. In other words, the leaky line-segment structures  38  need not be located directly in between the opposing boundary walls  32 A,  32 B. 
       FIG. 9  is a top view schematic illustration of a leaky line-segment structure  38  according to an exemplary embodiment and is shown in larger detail. The line-segment structure  38  is realized as a rectangular waveguide section with a continuous tapered (varying offset) slot  40  in its upper waveguide broadwall. The central linear axis of the waveguide section is represented by axis  16 . The slot  40  is centered along the axis  16  near the feed  42  location (minimum coupling), and increases in offset monotonically from the axis  16  (increasing coupling) towards its opposite end (where an absorptive load, not shown, is typically placed in order to absorb a small amount of uncoupled RF energy). 
     The desired amplitude distribution along the length of the leaky line-segment structure is generally driven by a number of factors including compensation for the varying lengths of the propagation paths  12 , desired tapering of the amplitude towards the edges of the array in order to reduce antenna pattern sidelobes, and conservation of RF energy along the leaking RF paths such that sufficient energy is available at the end/terminus of the leaky-wave path. The amount of coupling (amount of RF energy leaked per unit length along the feed path) is regulated primarily by the relative mechanical offset of the coupling slot  40  relative to the center-line of the feed  16  (increasing offset producing increasing coupling). Other factors including the selected width and thickness of the slot, the physical internal height and width (characteristic impedance) of the leaky line-segment and the height and physical details of the parallel-plate (characteristic impedance and effective dielectric constant) also play a in determining the leaky-wave coupling (leakage per unit length) factor. Similarly, the oblique angle of the energy emanating from the leaky line-segment is determined primarily by the internal width (cut-off frequency, fc, as shown in  FIG. 11 ) for the leaky line-segment structure and the effective dielectric constant (Er as shown in  FIG. 11 ) of the parallel-plate structure (generally dictated by the specific geometry of any physical corrugations or dielectric material properties employed in the parallel-plate region) though the other aforementioned design details also can have second-order effects on the specific oblique angle. Based on the disclosure herein, one having ordinary skill in the art will readily appreciate the application of these principles in order to arrive at the specifically desired oblique angle θ. 
       FIG. 10  illustrates another embodiment of a leaky line-segment structure  38 , in this case denoted  38   a . Again the line-segment structure  38  is made up of a rectangular waveguide section, but in this case with the slot  40  being linear (straight) and the waveguide itself “curving” in order to realize the desired variable slot offset. The slot  40  preferably is formed in the broadwall of the waveguide, with the waveguide curving in a plane perpendicular to the broadwall. In this case, the linear slot  40  itself represents the axis of the line-segment structure  38   a  and the axis  16  instead represents the axis of curvature of the waveguide. 
     In other words, when the embodiment of  FIG. 10  is employed, the oblique angle θ may be defined by the axis, or main line of direction of the line-segment structure, represented by the (straight) slot  40 . In the case of the curved slot  40  in the embodiment of  FIG. 9 , the oblique angle θ may be defined relative to the straight axis  16 , again representing the main line of direction of the waveguide. 
     With respect to the embodiment of  FIG. 9 , it may be desirable to employ a slight variation in the “a” (broadwall) dimension along the length of the waveguide. This varies the propagation constant within the waveguide and thus is useful in order to compensate for non-linear phase “error” which may be introduced by the curved slot geometry. More specifically, the variation in the “a” dimension may be selected so as to vary the propagation constant such that the cumulative phase (integrated propagation constant along the length of the waveguide) conjugates (cancels the phase error) introduced by the curved slot. Conversely, when employing the curved waveguide as in the embodiment of  FIG. 10  the “a” dimension may be constant (for a constant propagation constant). 
     As will be appreciated, in either of the embodiments of  FIGS. 9 and 10 , the linear continuous slot  40  can equally be adapted or replaced with a linear array of discrete broadwall slots or apertures, a linear array of discrete sidewall slots or apertures, or some combination thereof. 
       FIG. 11  shows the computed oblique angle θ (degrees from end-fire) for a leaky line-segment structure  38  as a function of frequency (the quotient f/fc, the frequency divided by the cutoff frequency for the waveguide) and for various effective dielectric constants within the parallel-plate region area  37 . Also shown on this graph is the computed beamwalk (beam stability) expressed as the expected angle change (in degrees) per percent of operating frequency change. Optimal bandwidth performance (minimum beamwalk, e.g. minimum variation in launch angle as frequency is varied) is achieved at the highest (f/fc) values and for the highest effective dielectric constant (0.18 degrees/percent bandwidth for Er=1.8 and (f/fc)=1.9) This beam stability value is approximately 4× better (75% smaller) than the beamwalk expected in a typical line-feed as employed in the above-described conventional inscribed-square design. 
       FIG. 12  illustrates another example of a leaky line-segment, in this instance one formed by a post-wall waveguide  38   b . The spacings between the posts  65  along one of the walls are varied in order that the RF energy introduced via the feed  42  may leak from the line-segment structure  38   a  at the desired oblique angle θ. 
     As described herein, the RF device  30  utilizes a combination of features in order to efficiently feed an RF transmission line structure including opposing boundary walls with a non-rectilinear form factor. The opposing boundary walls preferably are parallel or semi-parallel plates to form parallel/semi-parallel plate regions. The RF device can be any parallel/semi-parallel plate RF structure, but is particularly well suited for circularly-shaped Continuous Transverse Stub (CTS) arrays and Variable Inclination Continuous Transverse Stub arrays. 
     Although the invention has been shown and described with respect to a certain embodiment or embodiments, equivalent alterations and modifications may occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.