Arrayed waveguide-to-parallel-plate twist transition with higher-order mode optimization

Abstract

A waveguide-to-parallel-plate twist transition includes at least one waveguide-to-parallel plate twist transition element comprising an input port comprising an input waveguide portion, the input waveguide portion configured to orient an E-field of an electromagnetic wave along a first plane, and an output port comprising a multi-mode parallel plate portion, the multi-mode parallel plate portion configured to orient an E-field of an electromagnetic wave along a second plane, wherein an angle of orientation of the second plane is different from an angle of orientation of the first plane. The twist transition further includes at least one intermediate discrete twist waveguide stage coupling each input waveguide portion to the output multi-mode parallel plate portion, wherein at least one intermediate discrete twist waveguide stage is configured to orient an E-field of an electromagnetic wave along a third plane, wherein an angle of orientation of the third plane is between the angle of orientation of the first plane and the angle of orientation of the second plane.

Claims

1. A waveguide-to-parallel-plate twist transition, comprising: at least two waveguide-to-parallel plate twist transition elements arranged adjacent to each other, the at least two waveguide-to-parallel plate twist transition elements including i) an input port having an input waveguide portion, the input waveguide portion configured to orient an E-field of an electromagnetic wave received by the input port along a first plane; ii) an output port comprising a multi-mode parallel plate transmission line portion, the multi-mode parallel plate transmission line portion configured to orient an E-field of an electromagnetic wave received by the output port along a second plane, wherein an angle of orientation of the second plane is different from an angle of orientation of the first plane; and iii) at least one intermediate discrete twist waveguide stage coupling the input waveguide portion to the output multi-mode parallel plate transmission line portion, wherein the at least one intermediate discrete twist waveguide stage is configured to orient an E-field of an electromagnetic wave received by the at least one intermediate discrete twist waveguide stage along a third plane, wherein an angle of orientation of the third plane is between the angle of orientation of the first plane and the angle of orientation of the second plane.

2. The waveguide-to-parallel-plate twist transition according to claim 1, wherein the first plane is orthogonal to the second plane.

3. The waveguide-to-parallel-plate twist transition according to claim 1, wherein the at least one waveguide-to-parallel plate twist transition element comprises a plurality of intermediate discrete twist waveguide stages, and each intermediate discrete twist waveguide stage of the plurality of intermediate discrete twist waveguide stages is configured to orient an E-field of an electromagnetic wave received by the respective intermediate discrete twist waveguide stage along a respective plane, wherein for each of the intermediate discrete twist waveguide stages an angle of orientation of the respective plane is different from an angle of orientation of other planes of the plurality of intermediate discrete twist waveguide stages.

4. The waveguide-to-parallel-plate twist transition according to claim 3, wherein each of the intermediate discrete twist waveguide stages orients the E-field at (K/(1+N)) degrees relative to an E-field orientation of an immediately adjacent intermediate discrete twist waveguide stage, where K is the angle between the input port and the output port and N is the number of intermediate discrete twist waveguide stages.

5. The waveguide-to-parallel-plate twist transition according to claim 1, wherein the at least one intermediate discrete twist waveguide stage orients the E-field at 45 degrees relative to an orientation of the E-field for the immediately adjacent input port or output port.

6. The waveguide-to-parallel-plate twist transition according to claim 1, wherein at least one intermediate discrete twist waveguide stage comprises at least one of a rectangular or elliptical waveguide.

7. The waveguide-to-parallel-plate twist transition according to claim 1, wherein the waveguide to parallel plate twist transition is formed in a plate structure.

8. The waveguide-to-parallel-plate twist transition according to claim 7, wherein the plate structure comprises a first plate portion and a second plate portion removably attachable to the first plate portion.

9. The waveguide to parallel plate twist transition according to claim 8, wherein a first portion of the input port, a first portion of the output port and a first portion of the at least one intermediate discrete twist waveguide stage are each formed in the first plate structure, and wherein a second portion of the input port, a second portion of the output port and a second portion of the at least one intermediate discrete twist waveguide stage are each formed in the second plate structure.

10. The waveguide-to-parallel-plate twist transition according to claim 1, wherein the input waveguide portion, intermediate discrete twist waveguide stages, and multi-mode parallel plate transmission line portion are formed in a dielectric material.

11. The waveguide-to-parallel-plate twist transition according to claim 10, wherein interior walls of the input waveguide portion, output parallel plate portion and the at least one intermediate discrete twist waveguide stage comprise a metal plating.

12. The waveguide-to-parallel-plate twist transition according to claim 1, wherein a parallel-plate transmission line is formed integral with the plurality of waveguide-to-parallel plate twist transition elements.

13. The waveguide-to-parallel-plate twist transition according to claim 12 wherein a B-dimension of the parallel plate transmission line is different from a B-dimension of the parallel plate on the output port of the waveguide-to-parallel-plate twist transition elements.

14. The waveguide-to-parallel-plate twist transition according to claim 1, wherein the output port comprises an output waveguide portion arranged between the at least one intermediate discrete twist waveguide stage and the multi-mode parallel-plate transmission line portion, the output waveguide portion having the same E-field orientation as the multi-mode parallel plate transmission line portion and coupling the at least one intermediate discrete twist waveguide stage to the multi-mode parallel-plate transmission line portion.

15. A waveguide-to-parallel-plate twist transition, comprising: at least one waveguide-to-parallel plate twist transition element including i) an input port having an input waveguide portion, the input waveguide portion configured to orient an E-field of an electromagnetic wave received by the input port along a first plane; ii) an output port comprising a multi-mode parallel plate transmission line portion, the multi-mode parallel plate transmission line portion configured to orient an E-field of an electromagnetic wave received by the output port along a second plane, wherein an angle of orientation of the second plane is different from an angle of orientation of the first plane; and iii) at least one intermediate discrete twist waveguide stage coupling the input waveguide portion to the output multi-mode parallel plate transmission line portion, wherein the at least one intermediate discrete twist waveguide stage is configured to orient an E-field of an electromagnetic wave received by the at least one intermediate discrete twist waveguide stage along a third plane, wherein an angle of orientation of the third plane is between the angle of orientation of the first plane and the angle of orientation of the second plane, wherein the output port comprises an output waveguide portion arranged between the at least one intermediate discrete twist waveguide stage and the multi-mode parallel-plate transmission line portion, the output waveguide portion having the same E-field orientation as the multi-mode parallel plate transmission line portion and coupling the at least one intermediate discrete twist waveguide stage to the multi-mode parallel-plate transmission line portion.

16. A waveguide-to-parallel-plate twist transition, comprising: at least one waveguide-to-parallel plate twist transition element including i) an input port having an input waveguide portion, the input waveguide portion configured to orient an E-field of an electromagnetic wave received by the input port along a first plane; ii) an output port comprising a multi-mode parallel plate transmission line portion, the multi-mode parallel plate transmission line portion configured to orient an E-field of an electromagnetic wave received by the output port along a second plane, wherein an angle of orientation of the second plane is different from an angle of orientation of the first plane; and iii) at least one intermediate discrete twist waveguide stage coupling the input waveguide portion to the output multi-mode parallel plate transmission line portion, wherein the at least one intermediate discrete twist waveguide stage is configured to orient an E-field of an electromagnetic wave received by the at least one intermediate discrete twist waveguide stage along a third plane, wherein an angle of orientation of the third plane is between the angle of orientation of the first plane and the angle of orientation of the second plane, wherein the waveguide to parallel plate twist transition is formed in a plate structure comprising a first plate portion and a second plate portion removably attachable to the first plate portion, wherein a first portion of the input port, a first portion of the output port and a first portion of the at least one intermediate discrete twist waveguide stage are each formed in the first plate structure, and a second portion of the input port, a second portion of the output port and a second portion of the at least one intermediate discrete twist waveguide stage are each formed in the second plate structure.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) In the annexed drawings, like references indicate like parts or features.

(2) FIG. 1 illustrates a generic parallel-plate transmission line and a generic waveguide transmission line.

(3) FIG. 2 illustrates a conventional multiple twist waveguides feeding a rectangular parallel-plate transmission line.

(4) FIG. 3 illustrates a conventional tapered horn feeding a rectangular parallel-plate transmission line.

(5) FIG. 4 is a block diagram of an exemplary waveguide twist transition in accordance with the present disclosure.

(6) FIGS. 5A and 5B are simplified schematic diagrams illustrating a stepped transition of a waveguide twist transition in accordance with the present disclosure.

(7) FIG. 6 illustrates a waveguide twist transition and a cross section of the waveguide twist transition when formed in a metal plate.

(8) FIGS. 7A, 7B and 7C are top, side and front views of a waveguide twist transition coupled to a waveguide and to a parallel-plate transition that includes a waveguide twist transition in accordance with the present disclosure.

(9) FIG. 7D is a perspective view of a waveguide twist transition formed with a parallel-plate transmission line in accordance with the present disclosure.

(10) FIGS. 8A, 8B, 8C and 8D correspond to FIGS. 7A, 7B, 7C and 7D respectively, with fillets removed.

(11) FIG. 9 is a schematic diagram of two waveguide corporate feeds feeding a parallel-plate transmission line via an array of waveguide-to-parallel plate twist transitions in accordance with the present disclosure.

DETAILED DESCRIPTION OF INVENTION

(12) When a waveguide transmission line is located in the same plane (level) as a parallel plate transmission line and the E-fields of both transmission lines are orthogonal to each other, and the waveguide transmission line is required to carry a relatively wide band or two widely separated bands, a common practice for transitioning RF power between the waveguide transmission line and the parallel-plate transmission line is through a waveguide twist transition or a tapered horn transition. However, and as noted above, such methods result in a device requiring a large package, high complexity and/or high costs.

(13) For example, FIG. 2 illustrates the complexity of a waveguide transition in the form of multiple twist waveguides at the output of the waveguide feed network, while FIG. 3 illustrates how a substantial packaging volume is required to effect a transition from parallel plate transmission line to a waveguide transmission line via the use of a tapered horn.

(14) The device and method in accordance with the present disclosure creates an efficient broad band transmission from a waveguide transmission line to a parallel-plate transmission line. Further, specific unique properties of novel embodiments in accordance with the present disclosure provide potential weight savings, space savings, and reduced design complexity relative to conventional solutions.

(15) A waveguide-to-parallel-plate twist transition in accordance with the present disclosure offers a compact, integrated method of transitioning from a parallel-plate transmission line to a waveguide transmission line, an E-field of the waveguide transmission line being orthogonal to that of the parallel-plate transmission line. In this manner, a more compact, less tolerance-sensitive corporate feed can be used that does not require a labor-intensive joining processes such as conductive bonding or brazing. The waveguide-to-parallel-plate twist transition in accordance with the present disclosure provides the additional benefit of increased ohmic efficiency, reduced part count (and cost), and reduced package size.

(16) Unlike tapered horns, the waveguide-to-parallel-plate twist transition in accordance with the present disclosure is compact and enables use of compact corporate feed networks. Further, unlike discrete waveguide twist components the waveguide-to-parallel-plate twist transition in accordance with the present disclosure can be integrated with the parallel-plate and corporate feed transitions, which allows the waveguide-to-parallel-plate twist transition to be incorporated into the same manufactured part(s) that form these transmission media. The waveguide-to-parallel-plate twist transition in accordance with the present disclosure is advantageous in that it mitigates the packaging and manufacturing limitations of traditional horn transitions and discrete waveguide twist components while providing improved ohmic efficiency. In addition, the device and method in accordance with the invention can uniquely exploit the mutual coupling induced higher-order waveguide modes associated with the close packing/proximity of adjacent waveguide outputs as they couple into the parallel plate. Absent this exploitation, the impedance match of individual embedded waveguide ports would generally be poor (high reflections) thereby limiting the utility of the twist-transition structure.

(17) Referring now to FIG. 4. Illustrated is a block diagram of a waveguide-to-parallel-plate twist transition in accordance with the present disclosure. In the illustrated embodiment, a waveguide 12 is coupled to a waveguide tuning network 64 comprising at least two waveguide ports each of which is in turn coupled to a parallel plate transmission line 10 via a waveguide-to-parallel-plate twist transition element 62 in accordance with the present disclosure. The waveguide tuning network 64, which may include at least one waveguide input, is arranged between the waveguide 12 and each waveguide-to-parallel-plate twist transition element 62. A twist function of the exemplary waveguide-to-parallel-plate twist transition element 62 is formed from a single input port and a single output port and effects the transition between each waveguide of the tuning network 64 and the parallel plate 10. A linear array of such twist elements in conjunction with the multi-waveguide tuning network 64 is employed to transition the single waveguide input port 12 to the (wider) parallel-plate transmission line 10. An output port of the parallel-plate transmission line 10 can operate as a single radiating element (parallel-plate line source) or act as an input to a full CTS or VICTS antenna array of radiating elements.

(18) With reference to FIGS. 5A and 5B, illustrated are simple schematic diagrams showing the concept of an intermediate discrete twist waveguide stage (also referred to as a waveguide twist transition or waveguide twist) for a waveguide-to-parallel-plate twist transition element 62 in accordance with the disclosure. For ease of understanding, the portions of the waveguide-to-parallel-plate twist transition element 62 are illustrated in FIGS. 5A and 5B as separate parts that are spaced apart from one another and/or in different positions. However, in practice the portions illustrated in FIGS. 5A and 5B are joined/adjacent to one another to form an integrated waveguide-to-parallel-plate twist transition element.

(19) Functionally, a purpose of the waveguide-to-parallel-plate twist transition is to (1) behave in the desired frequency band as an array of matched low-loss 2-port devices (elements) transitioning the waveguide transmission line media to the open-structure parallel-plate transmission line media; and (2) in the desired frequency band, twist the E-field to allow for the use of lower-loss, lower cost waveguide feed network elements on the waveguide side of the transition.

(20) The waveguide-to-parallel-plate twist transition element 62 includes an input port 80, an output port 82 and at least one intermediate discrete twist waveguide stage 84 arranged between the input port 80 and the output port 82. The input port 80 includes an input waveguide portion 80a that is configured to receive and/or orient an E-field of an electromagnetic wave along a first plane. For example, the first plane may be oriented to correspond to an E-field orientation of a received electromagnetic wave from a waveguide feed network. The output port 82 includes a multi-mode parallel plate transmission line portion 82a that is configured to output and/or orient an E-field of an electromagnetic wave along a second plane, where an angle of orientation of the second plane is different from an angle of orientation of the first plane. For example, the second plane may be oriented to correspond to an E-field orientation of an electromagnetic wave that is output by an array of output ports 82 (elements). In one embodiment, the first plane is orthogonal to the second plane. The output port 82 may further include an output waveguide portion coupled to the multi-mode parallel-plate portion, the output waveguide portion forming a connection means between the multi-mode parallel-plate portion and the at least one intermediate discrete twist waveguide stage 84.

(21) As used herein, a multi-mode parallel-plate transmission-line is defined as a broad open structure, comprised of two separated similarly-sized conductive generally parallel plates/surfaces, each with transverse extent (physical width) that is generally greater than two free space wavelengths and generally eight times or greater than the physical separation between upper and lower conductive plates/surfaces. Unlike conventional dominant-/single-mode (TE10) Rectangular Waveguide structures, multi-mode parallel-plate structures support multiple simultaneous low-loss propagating modes (TE1, TE2, . . . and TM1, TM2, . . . ) which provide additional degrees-of-freedom (and design flexibility) for realization of more complex field distributions and applications (including antenna feed, diplexer, and filter devices) In addition, these structures are generally lower-cost to manufacture and lower in Radio Frequency (RF) loss as compared to rectangular waveguide-based structures.

(22) The waveguide-to-parallel-plate twist transition element 62 also includes at least one intermediate discrete twist waveguide stage 84, which includes a waveguide 84a that couples the input waveguide portion 80a to the output parallel plate portion 82a (or to the output waveguide portion). As used herein, an intermediate discrete twist waveguide stage is defined as a waveguide portion that creates a step change in an orientation of the E-field created by the intermediate discrete twist waveguide stage relative to an orientation of the E-field created by an immediately adjacent waveguide, waveguide portion, or parallel plate. Such step change in orientation of the E-field is in contrast to a continuous change in orientation of the E-field as provided by the waveguide twists 40 described in FIG. 2. The intermediate discrete twist waveguide stage 84 may include a waveguide portion having a rectangular shape, an elliptical shape, or any other shape known in the art.

(23) The waveguide 84a of the at least one intermediate discrete twist waveguide stage 84 is configured to orient an E-field of an electromagnetic wave along a third plane, where an angle of orientation of the third plane is between the angle of orientation of the first plane and the angle of orientation of the second plane. For example, if the first plane orients the E-field at 0 degrees and the second plane orients the E-field at 90 degrees, then the third plane orients the E-field between 0 degrees and 90 degrees, e.g., 45 degrees.

(24) Preferably, the waveguide-to-parallel-plate twist transition element 62 includes a plurality of intermediate discrete twist waveguide stages 84, where each intermediate discrete twist waveguide stage 84 is configured to orient an E-field of an electromagnetic wave along a respective plane. More particularly, for each intermediate discrete twist waveguide stage 84 an angle of orientation of the respective plane is different from an angle of orientation of other planes of the plurality of intermediate discrete twist waveguide stages. For example, if the first and second planes are orthogonal to each other, three intermediate discrete twist waveguide stages 84 can be placed between the input port 80 and output port 82, where each intermediate discrete twist waveguide stage orients the E-field at 22.5 degrees relative to an orientation of the E-field for an immediately adjacent intermediate discrete twist waveguide stage and/or waveguide or parallel plate portion. This relationship between the E-field orientation of an intermediate discrete waveguide twist stage 84 relative to an immediately adjacent intermediate discrete waveguide twist stage 84 may be expressed as (K/(1+N)) degrees, where K is the E-field orientation in degrees between the input port 80 and the output port 82 and N is the number of intermediate discrete twist waveguide stages 84.

(25) By including a plurality of intermediate discrete twist waveguide stages, the twist operation becomes smoother, resulting less reflections and thus improving efficiency. As will be appreciated, the specific number of intermediate discrete twist waveguide stages implemented in the waveguide to parallel plate twist transition 62 can vary depending on the application requirements.

(26) A plurality of waveguide-to-parallel-plate twist transition elements 62 arranged adjacent to one another (arrayed) form a waveguide-to-parallel-plate twist transition. Such configuration: 1) enables the wider area of a parallel plate transmission line to be efficiently filled with the electromagnetic wave, 2) allows for controlled excitation of the parallel plate structure through appropriate selection of phase and amplitude excitation of individual waveguide-to-parallel-plate twist transition elements, and 3) allows for scanning of the wave front within the parallel plate structure via appropriate phasing of individual waveguide-to-parallel-plate twist transition elements.

(27) The input waveguide portion 80a, output parallel plate portion 82a and waveguide 84 may be defined by a length, width and height as required by the specific application. Preferably, the length of the input waveguide portion 80a, output parallel plate portion 82a and waveguide stage 84a are minimized to provide a small overall footprint for the waveguide-to-parallel-plate twist transition element, but should also be sized taking into account higher-order modes with the goal of minimizing reflections of the waveguide-to-parallel plate twist transition as integrated and/or arrayed with applicable input port waveguide tuning network(s) and/or applicable output port parallel plate tuning networks, CTS arrays, or other antenna arrays.

(28) According to one embodiment, a parallel-plate transmission line 10 is formed integral with the output port 82 of the at least one waveguide-to-parallel-plate twist transition element 62. By forming the parallel-plate transmission line 10 integral with the waveguide-to-parallel-plate twist transition elements 62, further space savings can be realized as mounting means for attaching the parallel-plate transmission line 10 to the waveguide-to-parallel-plate twist transition elements 62 is not needed.

(29) With reference to FIG. 6, the waveguide-to-parallel-plate twist transition 62 may be formed from a conductive metal plate structure, where the plate may include two pieces, e.g., a first or top plate portion 86 and a second or bottom plate portion 88 that is removeably attachable to the first plate portion. Parts of each of the input port 80, the output port 82 and the at least one intermediate discrete twist waveguide stage 84 can be formed in a part 86a of the first plate portion 86, while a remaining part of the input port, the output port and the at least one intermediate discrete twist waveguide stage can be formed in part 88a of the second plate portion 88. In this regard, parts of the input port 80, output port 82 and intermediate discrete twist waveguide stage 84 may be machined within the first plate portion 86 and the second plate portion 88, such that when the two plates are mated to one another the complete input port 80, output port 82 and intermediate discrete twist waveguide stage 84 are defined between the two plates (e.g., the air space defined by 86a and 88a in FIG. 6). Other logical combinations of manufacturing methods (e.g. injection molding the plates with subsequent metalization) are also possible. Further, as shown in FIG. 6, each intermediate discrete twist waveguide stage may optionally be configured as an approximated shape of a rectangular waveguide as opposed to a perfect rectangle as a means to improve manufacturability and reduce RF reflections of the waveguide-to-parallel plate twist transition as a whole.

(30) Instead of forming the input port 80, output port 82 and intermediate discrete twist waveguide stage 84 in metal plate structures 86, 88, the respective components can be formed in a dielectric material, such as plastic or the like, via an injection molding process or other dielectric forming process. Walls within the resulting structure that define the input port 80, output port 82 and at least one intermediate discrete twist waveguide stage 84 then could be plated with a metal film to provide waveguides through the dielectric material. An advantage of forming the waveguide-to-parallel-plate twist transition via injection molding is that cost and weight can be significantly reduced while still providing comparable performance to that of a structure formed in metal plates.

(31) Moving to FIGS. 7A-7D, illustrated are top, side, front and perspective views, respectively, of a waveguide-to-parallel-plate twist transition element 62 integrally formed with a parallel-plate transmission line 10 using an additional waveguide output port 82 whose E-field is parallel to that of the parallel-plate transmission line 10. Note that the figures are presented in a transparent format to reveal hidden lines. Multiple tuning shapes (grooves, protrusions) are used to effect the necessary twisting function of the transition.

(32) As seen in FIGS. 7A-7D, a waveguide 90 feeds the waveguide portion 80a of the input port 80 with an electromagnetic wave. The input port 80 and waveguide portion 80a may be separate from the waveguide 90, or may form an end portion of the waveguide 90. The input port 80, via the waveguide portion 80a, provides the electromagnetic wave to the at least one intermediate discrete twist waveguide stage 84 with an E-field having a first orientation. The at least one intermediate discrete twist waveguide stage 84 then alters an orientation of the E-field as described herein, and provides the electromagnetic wave to the waveguide portion 82a of the output port 82. The waveguide port 82a further alters the E-field of the electromagnetic wave as described herein, and provides the electromagnetic wave to the parallel-plate transmission line 10. The result is the E-field of the electromagnetic wave has its orientation, as provided by the input port 80, changed relative to the E-field as output by the output port 82. As noted earlier, in some embodiments, output port 82 may be in the form of a parallel plate in lieu of a waveguide, further compacting the transition.

(33) The parallel-plate transmission line 10 may optionally include transformer sections 92 to transition from the parallel plate B-dimension to a convenient waveguide narrow wall B-dimension that is consistent with a desired frequency band of interest. FIG. 7D shows an isometric view where the transformer sections 92 can be seen. The B dimension is the gap separating the upper and lower parallel-plate conductors (see FIG. 7D). In one embodiment, the B-dimension of the parallel plate transmission line 10 is different from the B-dimension of the parallel plate on the output port 82 of the waveguide-to-parallel-plate twist transition elements.

(34) FIG. 9 shows an embodiment that includes two waveguide corporate feeds 100, 102 (one dedicated to low-band operation and a second dedicated to high-band operation) each supporting two separate frequency bands feeding a parallel plate diplexer 104 from opposite ports. Each feed network is comprised of a series of E-plane bends and Tee's that constructively combine the RF fields into a single waveguide port, and each uses a linear array of twist transitions 62 to couple from waveguide 100, 102 to parallel plate 104. The waveguide twist transition 62 as described herein can be used to reduce the size of the structure of FIG. 9.

(35) Other embodiments comprising arbitrary feed networks or combinations of arbitrary feeding networks to combine an arbitrary number of Waveguide to Parallel-Plate Transitions with arbitrary tuning networks are possible.

(36) The described invention can be employed in a Variable Inclination Continuous Transverse Stub (VICTS) and/or Continuous Transverse Stub (CTS) antenna arrays and more generally in any antenna feed that employs parallel-plate fed with two or more waveguides. Immediate and future applications include X-, K-, Ka-, Q-, V- and W-band phased array antennas, VICTS arrays and CTS arrays. Immediate and future applications include Fixed, Ground-Mobile, and Aeronautical embodiments of CTS and VICTS arrays for sensors and communications.

(37) 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.