WO2012054696A1

WO2012054696A1 - Asymmetric stripline

Info

Publication number: WO2012054696A1
Application number: PCT/US2011/057037
Authority: WO
Inventors: John L. Schadler; Andre Skalina; Gary M. Lytle
Original assignee: Spx Corporation
Priority date: 2010-10-20
Filing date: 2011-10-20
Publication date: 2012-04-26

Abstract

An antenna stripline is provided. The antenna stripline includes a planar body, an input shelf and an output shelf. The planar body includes a straight upper edge, a lower edge and a central region having a plurality of variable width portions. The input shelf is disposed on the straight upper edge and at the central region of the planar body, and is oriented perpendicularly to the straight upper edge of the planar body. The output shelf is disposed on the straight upper edge and at an end of the planar body, and is oriented perpendicularly to the planar body.

Description

ASYMMETRIC STRIPLINE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application Serial No. 61/394,913, filed on October 20, 2010, the disclosure of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates generally to radio frequency (RF)

electromagnetic (EM) signal transmission components. More particularly, the present invention relates to an RF EM signal transfer apparatus, referred to as stripline or a microstrip, having asymmetric impedance loading elements.

BACKGROUND OF THE INVENTION

[0003] Conductors such as single wires, parallel open lines, twisted pairs, shielded pairs, and coaxial lines are variously used for transfer of power and signal energy at frequencies ranging from direct current (DC) to ultra-high frequencies (UHF). At higher frequencies, such as from the very-high frequency (VHF) range, up through UHF and into the microwave range and higher, the abovementioned conductors become unsatisfactory, as losses become more difficult to control and absolute limitations encroach. At these higher frequencies, striplines, microstrips, and waveguides are frequently resorted to despite drawbacks such as cost and tendencies to be fragile in larger sizes, e.g., at a lower extent of their useful range and in greater lengths. Some of these drawbacks are obviated by fiber optics, but fiber can carry very little power, e.g., on the order of a single watt. In contrast, coaxial lines and waveguides can carry power at levels in the megawatts.

[0004] Striplines and microstrips are often used at moderate low power levels and at moderate to high frequencies. Simulation software allows for relatively simple product development of the striplines and microstrips.

[0005] A ubiquitous characteristic of current striplines and microstrips, as they are used in the market and analyzed in academic research, is that each "component," such components being defined by successive width changes, is symmetrical about a longitudinal axes of each of the components. Analytical models developed prior to the contemporary era of rapid computer simulation generally approximated a step width change as a concatenation of strips having different characteristic impedances. In some impedance-matching devices, quarter-wave strips with impedances intermediate between their inputs and outputs, e.g., Z=sqrt(LC), with C being proportional to strip width, were used. Where greater impedance changes were needed, cascaded steps were used, sometimes with strip lengths between steps approximating a wavelength rather than an odd number of quarter wavelengths. Regardless, the layout of the steps has consistently placed an equal share of the change in width on each side of a strip midline, necessitating symmetrical striplines.

[0006] Thus, a particular way to take advantage of certain stripline properties has been neglected in the art. It is these properties that are addressed herein.

SUMMARY OF THE INVENTION

[0001] Embodiments of the present invention advantageously provide an antenna stripline. In one embodiment, the antenna stripline includes a planar body, an input shelf and an output shelf. The planar body includes a straight upper edge, a lower edge and a central region having a plurality of variable width portions. The input shelf is disposed on the straight upper edge and at the central region of the planar body, and is oriented

perpendicularly to the straight upper edge of the planar body. The output shelf is disposed on the straight upper edge and at an end of the planar body, and is oriented perpendicularly to the planar body.

[0002] There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto.

[0003] In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.

[0004] As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is a perspective drawing of an antenna stripline according to an embodiment.

[0006] FIG. 2 is a magnified view of a portion of the stripline shown in FIG. 1.

[0007] FIG. 3 is a perspective drawing of a stripline, adjusted for frequency.

[0008] FIG. 4 is a perspective view of another stripline signal distribution component in accordance with an embodiment.

[0009] FIGS. 5 A, 5B and 6 depict section views of the stripline shown in FIG. 4.

[0010] FIG. 7 is a perspective drawing of an antenna stripline according to another embodiment.

DETAILED DESCRIPTION

[001 1] As the terms are used herein, a stripline is a generally flat signal conductor, disposed between two substantially parallel, typically grounded planar surfaces. If the grounded planes extend more than about five times the width of the signal conductor, sidewalls that make the planes into a rectilinear chamber can be ignored for purposes of analysis. For narrower chambers, additional modeling that incorporates the effects of all walls may improve accuracy. The chamber may be filled, such as with, nitrogen gas, air that has been dried or ordinary air, for example, all of which have a relative dielectric constant SR that is close to the dielectric constant of hard vacuum or free space ε₀. Alternatively, the chamber may have one or more dielectric materials, e.g., reinforced polyester circuit board material FR4, polyethylene (PE), polytetrafluoroethylene (PTFE), or ceramics, etc., configured in solid or foam, etc., conditions at least partially filling the volume.

[0012] A stripline carries a signal between locations with good, i.e., low, loss characteristics, but of particular interest is the ease with which passive components can be inserted to modify stripline properties. Unlike coaxial cable, which is mass-produced in large quantities and is commonly relied upon to exhibit uniform properties over various distances, stripline can be useful for purposes such as applying particular filtering functions to UHF signals. For example, narrowing a stripline signal conductor held at uniform spacing between ground planes produces an impedance change equivalent to placing a series inductor into the circuit. Similarly, widening the same stripline conductor produces an impedance change equivalent to connecting a capacitor between the signal line and the ground plane. Thus, by manipulation of these parameters, the resulting adjustments may be treated as transformers, tuned stubs, and other signal-manipulation components. Placing two striplines parallel to each other, either edge-on or broadside, between two grounds allows them to couple signals between each other, with the phase and energy transfer controlled by the dimensions of the apparatus.

[0013] A microstrip is a widely used variant of the stripline, in which a single conductor is positioned parallel to a single ground plane. A microstrip is often used in the design of modern circuit boards, in which signal lines are laid out with reference to a ground on another layer of the board. This controls signal coherence, e.g., relative propagation speeds of harmonics, as well as isolation between signals. Where it is practical to leave a microstrip open to free space on a side away from a reliable ground plane, the microstrip can substitute for a stripline in many applications, and may further lower costs.

[0014] Where strip conductor paths turn, sweeping, constant-width arcs, e.g., with center line radii not less than about three times the strip width, are typically avoided. In lieu of such arcs, square corners are frequently mitered, e.g., are cut at a 45 degree angle. Several empirical models produce comparable miter dimensions to cut away excess metal in compensation for the effective increase in width at a corner, and thus minimize its associated shunt capacitance. Various formulas yield miter dimensions dependent on frequency, SR, strip width, and spacing between the strip and its ground plane(s), for example.

[0015] In the description that follows, like numerals correspond to like elements throughout.

[0016] The present invention provides signal distribution and impedance

manipulation functions using components, the designs of which are based on standard stripline methodology, but are substantially altered in configuration from such methodology. One effect of this change is simplification of production methods while retaining the original electrical properties or better, thereby reducing manufacturing costs while providing improved performance. Another effect introduces unexpected results, as will be disclosed in detail.

[0017] FIG. 1 shows two side-by-side stripline-based antenna signal power distribution inner conductors 10. Each of the two signal power distribution inner conductors 10 is located within a parallel-faced conductive enclosure (FIGS. 5 and 6) that serves as a set of ground planes for a corresponding stripline. The respective stripline inner conductors 10 are each configured to conduct a signal from a respective coaxial input connector 12 to a plurality of output nodes 14 along a path of traverse 16, thereby dividing the signal proximal to the input 12 and again at subsequent tee junctions. For convenience, any part of the signal path may be termed an increment of traverse with reference to the entire path of traverse 16 from an input 12, through any number of splits, to any one of the outputs 14. The stripline inner conductors 10 are readily fabricated from flat stock, such as by cutting; thus, each increment of traverse has two parallel surfaces. The bounds of each increment of traverse are parallel edges of the stripline inner conductor 10, with the exception that the stripline inner conductor 10 is radiused at points of mating with inputs 12 and outputs 14, and mitered at turns, as discussed below.

[0018] At the inputs 12, the direction of signal flow branches as the signal is initially split at connector-adapted tees 18. The divided signals proceed oppositely along paths of traverse 16 thereafter, the paths lying in a first plane 62, and including being repeatedly transformed in impedance by a first set of impedance steps 20, which likewise represent a series of increments of traverse. After these impedance transformations, the paths of traverse 16 include mitered right-angle turns 22 within the first plane 62, are then directed out of the first plane 62, by bending, in the embodiment shown, for example, at bends 24, to a perpendicular rising plane 64, and then into a second plane 66, substantially parallel to the first plane 62, again by bending at bends 26. Thus the bends 24, 26 direct the signal paths 16 into successive additional, non-coplanar increments of traverse.

[0019] The portions of the original signal directed into the second, e.g., upper parallel, plane 66 are each redivided at another tee 28. The respective tees 28 also introduce impedance compensation. Since there are two loads acting electrically in parallel after the tees 28, the output stripline increments of traverse are made narrower, i.e., have higher impedance, than the input increments of traverse. The tee 28 output segments 30 are then stepped up 32 to approximately the previous width, and therefore approximately the previous impedance, in the embodiment shown. A final mitered turn 34 leads to a final tee 36 power division.

[0020] There are no step decreases in width in the embodiment shown between the input connectors 12 and output load points 14 except in conjunction with tees 18, 28, 36. Width increases introduced to the stripline 10 at several points in steps 20, 32 occur at intervals typically separated by an amount close to n quarter-waves with reference to an assumed path of traverse 16 of an electromagnetic signal from the respective locations of inputs 12 to outputs 14. These steps 20, 32 adjust impedance, and can be modeled as transformers and simulated using any of several well-known EM propagation simulation software products.

[0021] FIG. 2 shows an enlarged image 40 of a portion of FIG. 1. Some of the shown impedance steps 32 is seen here to be symmetric, i.e., a center line 42 of an increment of traverse in the form of a stripline segment 44, for example, as it leads up to a step 32, is aligned with a center line 46 of a next increment of traverse in the form of a stripline segment 48 that continues from the step 32. All the flat-way right-angle turns 50 shown are mitered or, alternatively, are termed chamfered, beveled, etc., 52 in accordance with at least one of the various known calculation methods that approximate constant impedance in the vicinity of the turn 50.

[0022] Of interest with reference to the present invention is an exception to the symmetry discussed above. An arrangement for impedance adjustment that precedes half of the output legs 54 may be contrasted with the arrangement at the other half of the output legs 56. The last pair of legs 54, 56 terminates a feed line 10, after a mitered 52 right-angle turn 50, and followed by a tee junction 36. Note that the tee 36 is stepped 58 on its output side, so that the respective legs 54, 56 have unequal widths, and thus unequal impedances. From the tee 36, the legs 54, 56 lead in opposite directions. One leg 54 is positioned alongside and thus coupled to the feed line 10, while the other leg 56, is directed away from the feed line 10, and thus interacts therewith less than the leg 54. By engineering option, the two signal legs 54, 56 are rendered electrically comparable by a gap of width G between the doubled-back leg 54 and the feed line 10, and a single-sided impedance step 60 in the remaining leg 56. In this instance, the single-sided step 60 was realized by experiment and simulation as a solution to a layout challenge. The step 60 provided a simpler layout than alternatives that were considered, while its electrical behavior as modeled was free of deleterious effects. Spacers 68 minimally affect circuit behavior.

[0023] FIG. 3 shows a perspective view of a pair of stripline-based antenna signal power distribution components 70, of which the impedance-compensation elements are laid out to apply the instant invention more extensively. Here, it may be observed that one complete edge 72, 74 of each continuing segment of the stripline component 70 is straight, while a series of steps 76, 78, is present in the other edge. Flat turns 80 are mitered or, alternatively, are chamfered, beveled, etc., as above. The function of the stripline components 70 directly corresponds to that of the stripline components 10 of FIGS. 1 and 2. Dimensions differ in view of design for a higher frequency regime. It is noteworthy that the increase in the scaled size of steps 76, 78, necessary for keeping comparable impedance changes with a straight edge 72, 74, introduces negligible losses, so dimensions are generally proportional to those of the design of FIGS. 1 and 2. [0024] FIG. 4 is a perspective view of an antenna stripline 100 for which output power levels are unequal. It will be noted that a balanced, i.e., equal signal level, power tee could be realized with a single input and two outputs, all edge-connected, and with no impedance adjustments, for example. However, in order to realize unbalanced output power levels, as shown in the divider of FIG. 4, the stripline 100 has unequal widths W_{1 }} W₂, as is described in greater detail below with reference to FIG. 5.

[0025] As shown in FIG. 4, the stripline 100 includes a substantially planar body 114. The planar body extends away from a central region 113, which includes a tee point 106, variable width portions 121, and an input shelf 1 15 extending perpendicularly from the an upper edge of the planar body 1 14. A feed point 1 12 is connected to the input shelf 115, and receives a connector, such as a coaxial connector, for example, to supply a signal to the stripline 100. Straight upper edges (108, 1 10) extend away from the central region 1 13 toward output shelves (1 17, 119) disposed at distal ends of the planar body 114. As shown in FIG. 4, the output shelves (117, 119) are oriented substantially perpendicular to the body portion 1 14. Lower edges (109, 1 1 1) are disposed opposite the straight upper edges (108, 1 10), respectively, on the planar body. In contrast to the straight upper edges (108, 1 10), the lower edges (109, 1 1 1) are not straight, in that they include steps, e.g., variable width portions 121 , that change a width of the planar body 1 14, as will be described in greater detail below with reference to FIGS. 5A and 5B. Put another way, the antenna stripline 100 is asymmetric about a longitudinal axis of the planar body 1 14. As described in greater detail above, the width changes 121 of the planar body 1 14 alter the impedance of the stripline 100, which thereby affects a signal, applied to the input shelf 1 15, as it travels along the planar body 1 14 towards the respective output shelves (1 17, 119).

[0026] A conventional, i.e., symmetric, power divider not incorporating the embodiments shown in FIG. 4, for example, would need a more complex layout, such as a requirement to place the feed point 1 12 into the center of the planar body 114 to permit steps that are symmetrical about the longitudinal axis thereof, for example. [0027] As shown in FIG. 4, insulating spacers 116 attached to the flat surface provide positioning for components. Mitered corners 1 18 provide impedance control on this design as on the above examples.

[0028] FIG. 5 A is a plan view of a device 80, with the stripline 100 of FIG. 4 equipped with connectors 120, and showing an enclosure 122 in cutaway. FIG. 5B is a plan view showing dimensions of an embodiment. The uneven power division of the tee 106 due to width differences Wi and W5 from the tee point 106 is readily seen in these views. The embodiment shown affords power division approximating 70:30, with a reflected signal on the order of -30 dB, i.e., about 97% passes to the load. Other features of the tee power divider 102 of FIG. 4 are the same in this view and in the one that follows, and are so labeled.

[0029] In one embodiment, the dimensions of the stripline 100 are as shown in FIG. 5B. Specifically, as the planar body 114 extends outward from the tee point 106, i.e., from the input shelf 1 15 towards the respective output shelves 1 17 and 1 19, widths Wl and W5 are 0.231 and 0.950 inches, respectively, and correspond to lengths LI and L5, respectively, which are 1.550 and 1.633 inches, respectively. Widths W2 and W6 are 0.625 and 1.055 inches, respectively, and correspond to lengths L2 and L6, respectively, which are 1.450 and 1.361 inches, respectively. Widths W3 and W7 are 1.108 and 1.237 inches, respectively, and correspond to lengths L3 and L7, respectively, which are 1.500 and 1.509 inches, respectively. The remaining lengths of the planar body 114 extending to the output shelves 117 and 1 19, respectively, have widths W4 and W8 of 1.287 inches each. As shown in FIG. 5B, the overall length of the planar body is therefore approximately 37.605 inches, and is divided approximately into two halves at the tee point 106.

[0030] FIG. 6 is an end view of the device 80 of FIG. 5. The relation between the straight edges 108, 1 10 and the maximum stripline width edges 124, on one hand, and proximal parts 126 of its enclosure 122, on the other hand, is defined according to standard stripline rules, namely that a chamber of which the width Wc is less than about 500% of the stripline width Ws may require that the relevant dimensions be included in any simulation models in order to retain close correlation between calculated and tested device 102 electrical behavior. Spacers 1 16 and connector 120 attachment to the divider 102 may be sufficient to position all components mechanically, or other spacing components in addition to or in lieu of those shown may be required, depending at least in part on consideration of vibration and other mechanical stress to which an individual design may be subjected.

[0031] FIG. 7 shows a two-terminal device 130 to be housed between a pair of ground planes (not shown, comparable to that shown in FIGS. 5 and 6). The device 130 has a single input 132 and a single output 134, and a series of steps 136 that differ in size, spaced at roughly quarter- wavelength intervals Q. Each width reduction 138 can be modeled as a serial inductance and each width increase 140 as a shunt capacitance, or a transformer model may be applied, as determined by user preference and available simulation software options. Input and output impedances are functions of stripline segment heights Hi, ¾, and thus differ somewhat from each other in the embodiment shown. Mitered input and output corners 142 function as described above to limit change of impedance at turns.

[0032] Referring again to FIG. 5, manufacturing of devices of the types shown can employ a particular technology-based process to produce high quality with relatively low labor and wastage costs. A laser cutter is preferentially used in to shear the sheet metal of the stripline tee 102, which is thereupon bent to final form, has spacers 116 and other

components attached to it, and is joined with its enclosure 122.

[0033] By making one of every two stripline 102 edges straight, the invention allows the number of production steps to be literally reduced, if a complete edge can be used with no cuts, or to be simplified, if long segments can be cut with no interruptions for changes in direction.

[0034] In view of many decades of established stripline practice, wherein

symmetrical impedance step positioning has been used exclusively, the practical

interchangeability between the invention and established practice is unexpected. It could be anticipated that employing the invention, with its evident asymmetry, would result in propagation path spreading, reflections, higher current densities, or the like. Such

phenomena would lead to greater return losses, reduced bandwidth, etc. In practice, however, electrical performance degradation has been found to be essentially nonexistent. [0035] At the same time, asymmetric arrangements are potentially simpler, quicker, and/or less costly to design and manufacture, as is readily understood by comparing FIGS. 1 and 3. The invention further permits at least some embodiments to be more compact than corresponding symmetrical implementations, potentially providing such additional benefits as lower weight and wind loading. Depending on the application, this can lower mechanical stress on mounts, leading to higher reliability, and/or can shrink support structure, leading to further economy.

[0036] This can be contrasted with historical efforts to optimize the shape of miters used on flat turns. To cite one example, simple, e.g., unmitered, right-angle turns are known to introduce shunt-capacitive lumps as the effective stripline width varies around the turn. Sources note a variety of empirical and analytical algorithms for choosing a 45 degree miter size, the effect of which is to approach a minimum lump, but not to eliminate such a lump. To cite another example, other sources note that circuit-board mask generators, including those used for microstrips, commonly use a round spot of light for exposing photographic film, with the insides of corners sharply square, while the outsides of corners have an arc with a radius equal to half of the trace width. While still not a perfect no-lump shape, this is often electrically superior to a square corner. Still other sources suggest still other alternative corner arrangements. Despite such evidence of academic inquiry into minutiae of striplines, nowhere is there evidence of consideration of asymmetric stripline shapes.

[0037] The many features and advantages of the invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to that fall within the scope of the invention.

#103953816 1

Claims

What is claimed is:

1. An antenna stripline, comprising:

a planar body including a straight upper edge, a lower edge and a central region having a plurality of variable width portions;

an input shelf, disposed on the straight upper edge and at the central region of the planar body, oriented perpendicularly to the straight upper edge of the planar body; and an output shelf, disposed on the straight upper edge and at an end of the planar body, oriented perpendicularly to the planar body.

2. The antenna stripline of claim 1, wherein the planar body is asymmetrical along a longitudinal axis.

3. The antenna stripline of claim 1 , further comprising a feed point, connected to the input shelf, to receive a signal connector.

4. The antenna stripline of claim 1, wherein the plurality of variable width portions are disposed on the lower edge of the planar body.

5. The antenna stripline of claim 4, wherein the lower edge of the planar body is disposed opposite to the straight upper edge of the planar body.

6. The antenna stripline of claim 4, wherein the plurality of variable width portions includes a plurality of steps that change the width of the planar body.

7. The antenna stripline of claim 6, wherein each step has a different width.

8. The antenna stripline of claim 7, wherein the width of the step that is located closer to the end of the planar body is greater than the width of the step that is located further from the end of the planar body.

9. The antenna stripline of claim 6, wherein each step is a different length.

10. The antenna stripline of claim 6, wherein each step is spaced at approximately quarter-wavelength intervals.

1 1. The antenna stripline of claim 1, further comprising a plurality of insulating spacers attached to the planar body.

12. The antenna stripline of claim 11, wherein the spacers are attached between the central region and the end of the planar body.

13. The antenna stripline of claim 1, wherein each end of the planar body includes a mitered corner.

14. The antenna stripline of claim 1 , further comprising an additional output shelf disposed on the straight upper edge at an opposite end of the planar body.

15. An antenna system, comprising:

an antenna; and

an antenna stripline, coupled to the antenna, including:

an input shelf, disposed on the straight upper edge and at the central region of the planar body, oriented perpendicularly to the straight upper edge of the planar body; and

an output shelf, disposed on the straight upper edge and at an end of the planar body, oriented perpendicularly to the planar body.

16. The system of claim 15, wherein the planar body is asymmetrical along a longitudinal axis.

17. The system of claim 15, further comprising a feed point connected to the input shelf to receive a signal connector.

18. The system of claim 1, wherein the plurality of variable width portions are disposed on the lower edge of the planar body and opposite to the straight upper edge of the planar body.

19. The system of claim 15, wherein the plurality of variable width portions include a plurality of steps that change the width of the planar body, each of the plurality of steps including a different width and a different length.

20. An antenna stripline, comprising:

means for carrying a signal between locations that includes a plurality of variable width planar portions;

an input shelf disposed on the means for carrying the signal; and

an output shelf disposed on the means for carrying the signal.