Abstract:
A compact antenna element and assembly using a directly fed and electromagnetically coupled step probe element for ultra wideband application. It achieves very good impedance match, isolation and pattern stability across a wide frequency band. The compact ultra wideband radiating element covers all known radio frequency bands in the mobile base station industry to date.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a compact dual-polarized antenna element with very good Voltage Standing Wave Ratio (VSWR), isolation and pattern stability across a very wide frequency band. It achieves this via directly feeding and electromagnetically coupling stubs of different lengths similar to a multi-section transformer, in the feed. In the invention, this multiple directly fed and electromagnetically coupled stubs in the feed are shortened to multi-DF&amp;EMC (directly fed &amp; electromagnetically coupled) stepped probe. 
         [0003]    2. Background of the Related Art 
         [0004]    The mobile base station industry is becoming increasingly more competitive. As new frequency bands are being made available, it is a goal of those involved in the design and use of mobile base station antennas and other related systems to maintain or reduce costs, while maintaining or improving upon electrical performance across a broader range of frequency bands. 
         [0005]    United Kingdom Pat. No. GB 2405997B, the entirety of which is incorporated herein by reference, describes a multi-band element designed for multi-band base station antenna arrays operating from 806 MHz to 960 MHz (often referred to as the low band) and 1710 MHz to 2170 MHz (often referred to as the high band). Although it has superior impedance matching performance (VSWR 1.3:1), it exhibits inferior intra-port isolation and cross-polarization, when applied to work in a dual polarized configuration because the elements are fed on or near the edge of the patch. 
         [0006]    Accordingly, there exists a need for a compact dual polarized radiating element with ultra-wideband performance that exhibits good VSWR, good isolation, and a good azimuth pattern across a wide band of operating frequencies whilst still being of inexpensive construction. This invention improves the impedance bandwidth but applied in a balanced configuration to correct for the poor isolation and pattern stability. The multiple directly fed steps to improve the bandwidth is further enhanced significantly by employing additional EM coupling steps to expand the bandwidth and improve the matching performance further. 
       SUMMARY OF THE INVENTION 
       [0007]    It is an object of the invention to provide an improved multiple step probe approach with significantly improved impedance bandwidth and match through additional electromagnetically coupled (EMC) steps by using printed circuit boards (PCB&#39;s) and then balancing the probe through two different techniques to fix the isolation and pattern response across this ultra-wide frequency bandwidth. It is another object of the invention to provide a multi-band element which includes a low band element configured to operate over a frequency band of 695 MHz-960 MHz, and a high band element configured to operate over a frequency band of 1700 MHz-2700 MHz. 
         [0008]    Those and other objects are achieved by an antenna assembly having: a ground plane; a multi-DF&amp;EMC step probes for wide impedance bandwidth enhancements and having a first coupling patch suspended above the ground plane. 
         [0009]    Each multi-DF&amp;EMC step probe may comprise of several vertical and horizontal conductors etched on a microwave quality PTFE substrate. Although a lossy substrate like FR4 (which is a standard PCB material or fiberglass reinforced epoxy laminates that are flame retardant) could be used for the multi-DF&amp;EMC step probe, the design will further implement a distribution feed network on the same substrate and to minimize the insertion loss, a quality PTFE substrate is used. In fact, any conductor, including airline could be used. The multi-DF&amp;EMC step probe may be configured such that the elements form a pair in which each element is fed a signal 180° out of phase. 
         [0010]    With those and other objects, advantages, and features of the invention that may be hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims, and to the several drawings attached herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a perspective view of an antenna assembly in accordance with an exemplary embodiment of the invention; 
           [0012]      FIG. 2  is a perspective view of a vertically polarized assembly of the multiple directly fed step probe; 
           [0013]      FIG. 3  is a detailed view of the multiple directly fed step probe element in accordance with an exemplary embodiment of the invention; 
           [0014]      FIG. 4  is a detailed view of the multiple fed step probe element that is fed via electromagnetic coupling in accordance with an exemplary embodiment of the invention; 
           [0015]      FIG. 5  is a detailed view of the multiple directly fed and electromagnetically coupled (multi-DF&amp;EMC) step probes in accordance with an exemplary embodiment of the invention; 
           [0016]      FIG. 6  is a perspective view of a vertically polarized assembly showing both the front view ( FIG. 6(   a )) and the back view ( FIG. 6(   b )) of the multi-DF&amp;EMC step probe with radiating element and ground plane; 
           [0017]      FIG. 7  is a perspective view of a dual polarized assembly showing the multi-DF&amp;EMC step feed with radiating element and ground plane; 
           [0018]      FIG. 8  is a detailed view of the multi-DF&amp;EMC step probes arranged in a balanced configuration for one polarization; 
           [0019]      FIG. 9  is a detailed view of the multi-DF&amp;EMC step probes arrange in a balanced configuration for the other polarization; 
           [0020]      FIG. 10  is a detailed perspective view of the balanced multi-DF&amp;EMC step probe with radiating element and ground plane; and 
           [0021]      FIG. 11  is a perspective view of a pair of elements arranged such that it behaves similar to layout of  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    With reference to  FIG. 1 , an antenna assembly  100  is shown in accordance with an exemplary embodiment of the present invention. The antenna assembly  100  includes a number of high band radiator assemblies  102 , a low band radiator assembly  104 , and, a ground plane  1  (a conductor, generally aluminum). Each of the high band radiator assemblies  102  are formed of high band elements  63 ,  65 ,  67  and a respective high band top plate  64 ,  66 ,  68 . The low band radiator assembly  104  is formed of low band elements  39 ,  49  and a low band top plate  2 . The top plates  2 ,  64 ,  66 ,  68  are aligned with and suspended over the respective radiator elements  39 / 49 ,  63 ,  65 ,  67 . As illustrated, the high band assemblies  102  are about one-half the size of the low band assembly  104 . 
         [0023]    The high band elements  63 ,  65 ,  67  and the low band elements  39 / 49  each include two elongated flat conductive sheets that are coupled together in the form of an X-shape (slant +/−45, dual polarized). The elements  39 / 49 ,  63 ,  65 ,  67  stand upright on their edges, with the top and bottom surfaces facing substantially orthogonal to the ground plate  1 . A plate  2 ,  64 ,  66 ,  68  is placed over each of the elements  39 / 49 ,  63 ,  65 ,  67 , respectively. The high band plates  64 ,  66 ,  68  are generally circular in shape, and the low band plate  2  is rectangular in shape, though any suitable shape can be utilized. An air gap or non-conductive medium (such as plastic or insulator) is positioned between the plates  2 ,  64 ,  66 ,  68  and the elements  39 / 49 ,  63 ,  65 ,  67 . The plates  2 ,  64 ,  66 ,  68  are electromagnetically coupled with the respective elements  39 / 49 ,  63 ,  65 ,  67 , and radiate energy. The plates  2 ,  64 ,  66 ,  68  can be larger (though need not be, and can be smaller) than those elements  39 / 49 ,  63 ,  65 ,  67 . 
         [0024]    Additionally, the high band elements (1700-2700 MHz)  63 ,  64  can be stacked on top of the low band element (695-960 MHz)  39 / 49 , 2 to form a dual-band dual-polarized assembly and those assemblies can be interleaved with one another to form a compact antenna array. The band elements  63 ,  64  directly contact the low band plate  2  and use the low band plate  2  as a ground. The high band elements  65 ,  66  and  67 ,  68  are placed inline with the low band element  39 / 49 , 2 and can share the same ground plane or are suspended above the ground plane on insulators. Thus, the two high band elements  65 ,  67  are placed on the ground plane  1 , with the low band assembly  39 / 49  between them aligned linearly. 
         [0025]    Thus,  FIG. 1  illustrates how the radiating elements using multi-DF&amp;EMC probes can be configured for multi-band operation. The high band radiators  63  using the multi-DF&amp;EMC probes can be stacked above the low band radiator  39 / 49  and also interleaved between the low band radiators  39 / 49 . In the drawing, the high band multi-DF&amp;EMC probes  63 ,  65 ,  67  are arranged such that the probes face each other but fed 180° out of phase. The high band radiators  64 ,  66 ,  68  are excited by the multi-DF&amp;EMC probes  63 ,  65 ,  67 . 
         [0026]    The antenna assembly may further comprise metal radiators disposed above the low band elements, high band elements disposed on the low band radiator, and a high band element disposed between the low band elements. A plurality of such antenna assemblies may be provided in an array. 
         [0027]    In the following few descriptions, the preferred embodiment of the invention will concentrate on the 695-960 MHz design. This achieves a bandwidth of 32% with a VSWR of 1.35:1. The design can be extended to 1700-2700 MHz. This achieves a bandwidth of 45% with a VSWR of 1.35:1. The feed method described achieves beyond the operating frequency of those 2 bands. However, the bands are limited to operate from 695-960 MHz and 1700-2700 MHz as these are the operating bands for today&#39;s current mobile communications systems. However, the invention can be applied to other suitable designs and applications outside of these ranges. 
         [0028]      FIG. 2  shows wide band impedance performance can be achieved by having a multi-step feed element  3  on a PCB with multiple horizontal/vertical probes or conductors  13 - 16  ( FIG. 3 ). The multi-step feed element  3  of  FIG. 2 , is on a PCB connected to a metal ground plane  1  and coupled with a primary suspended metal radiator  2 . 
         [0029]      FIG. 3  is a more detailed illustration of the multi-step feed element  3  from  FIG. 2 . The horizontal conductors  14 ,  15 ,  16  are parallel to one another and extend substantially parallel with respect to the top edge of the PCB and the ground plane  1 . The vertical conductor  13  extends substantially orthogonal to the top edge of the PCB and the ground plane  1 , and orthogonal to the horizontal conductors  14 ,  15 ,  16 . The feed network  12  is etched on a PCB  11  residing above a ground plane  10 . The solid lines represent the front surface of the PCB  11  and the dashed lines illustrate the back surface of the PCB  11 . The feed network  12  is excited at point  12   a  via a coaxial cable. The inductance of the vertical conductor  13  (or probe) is compensated (i.e., cancelled) by the capacitances of the multiple horizontal conductors  14 ,  15 ,  16  (or probes). The vertical and horizontal conductors (probes) cancel; so if the height of the vertical conductor  13  is increased, the length of the horizontal conductors  14 ,  15 ,  16  needs to be increased, which can be limited by a particular application so that the horizontal conductors don&#39;t run into each other (e.g., see  FIGS. 8 ,  9 ). 
         [0030]    The configuration of the feed element  3  shown in  FIG. 3  achieves a reasonably good performance across a 32% bandwidth from 695-960 MHz and 45% bandwidth from 1700-2700 MHz. However, additional steps (i.e., the horizontal conductors) allow for a larger bandwidth and more freedom of tuning for improved VSWR because of more components to aide in compensation purposes. Unfortunately, it is not possible to increase the number of steps because of two reasons. Firstly, the horizontal conductor  16  must not touch or overlap the ground plane  10  otherwise there will be a severe mismatch as the fields will not be between the conductor  16  and air but predominantly between the conductor  16  and the ground plane  10 . The horizontal conductor  16  is generally kept approximately 5 mm above the PCB ground plane  10 . These are etched on a PCB so therefore can easily be separated. Secondly, the primary radiator  2  is placed approximately 0.12λ above the ground plane  1  in the current configuration for good matching purposes. That is, the VSWR is minimized. To transfer energy from the feed to the radiator, the impedance between these two components must be matched to have a similar impedance. Hence, the vertical conductor  13  and the horizontal conductors  14 ,  15 ,  16  have only a small window to operate. The horizontal conductors  14 ,  15 ,  16  vary from 0.05λ-0.13λ and the vertical conductor  13  varies from 0.03λ to 0.1λ, although values outside this range will also work. 
         [0031]      FIG. 4  shows another configuration for the feed element  3  that can be utilized in conjunction with  FIG. 3 . Shown therein is the multiple steps probe with vertical conductor  17  and horizontal conductors  18 ,  19 . The conductors  17 ,  18 ,  19  together form one single conductor. Since the conductors  17 ,  18 ,  19  are on the back surface of the PCB and the microstrip section  9  is on the front surface of the PCB, the conductors  17 ,  18 ,  19  are electromagnetically coupled with the microstrip section  9 , which is fed from the microstrip feed  12 . These components  17 ,  18 ,  19  are fed from a microstrip feed  12  and energy is delivered via electromagnetic coupling via a large microstrip section  9 . As shown, the vertical conductor  17  on the rear surface of the board is aligned with and overlaps the microstrip section  9  on the front surface of the board, to ensure a strong electromagnetic coupling between those elements. It is noted that any suitable number of conductors can be utilized, though  FIG. 4  shows 2 horizontal conductors and  FIG. 3  illustrates 3 horizontal conductors. The microstrip section  9  couples energy from the feed  12  to the conductors  17 ,  18 ,  19 . The conductor  9  needs to be a certain size and shape to provide a good transition between the feed  12  and the conductors  17 ,  18 ,  19 . That size and shape is optimized on the 3D EM simulator CST Microwave Studio. 
         [0032]    Thus,  FIGS. 3 and 4  essentially do the same thing, except that  FIG. 4  is EM coupled and  FIG. 3  is directly fed.  FIGS. 3 and 4  are combined to provide the configuration shown in  FIG. 5 , with  FIG. 3  (shown in solid lines) provided on the front of the PCB and  FIG. 4  (shown in dashed lines) provided on the back of the PCB.  FIG. 5  provides a larger bandwidth and more freedom to tune because instead of one set of steps (i.e.  FIG. 3  or  FIG. 4 ) to match the probe to the radiator, you have additional steps (probes) which could be tuned to work across a slightly higher or lower frequency and more options to tune for improved VSWR because there are more steps/stubs to adjust. 
         [0033]    Shown in  FIG. 5  is the multiple directly fed (DF) steps probe (i.e.,  FIG. 3 ) but with additional electromagnetically coupled (EMC) vertical conductor  17  and multiple horizontal conductors  18 ,  19  (from  FIG. 4 ) placed on the back of the PCB (as represented by the dashed lines). The additional EMC vertical and horizontal conductors  17 ,  18 ,  19  provide improved impedance matching (i.e., better VSWR across the band) and increase the bandwidth as additional lengths are employed. The vertical and horizontal conductors  17 ,  18 ,  19  on the back of the PCB need not be (though can be) aligned with the conductors  13 ,  14 ,  15 ,  16  on the front of the PCB. Any arbitrary shaped can be used. Preferably, however, the vertical conductor  17  on the rear surface of the board is aligned with and overlaps with the vertical conductor  13  on the front surface of the board to ensure a strong electromagnetic coupling between those elements. The PCB used in this design is  0 . 8 mm thick, though any suitable thickness can be used. 
         [0034]      FIG. 6(   a ) shows the front view and  FIG. 6(   b ) shows the back view of the antenna element assembly with the ground plane  1 , the primary radiator  2 , and the multiple direct fed and electromagnetically coupled (multi-DF&amp;EMC) step probe  20 , for single polarization. The step probe  20  corresponds to the probe element of  FIG. 5 , but the probe elements  3  of  FIGS. 3 and 4  can also be utilized. 
         [0035]      FIG. 7  provides a view on the setup for a dual-polarized application. In this design, the multi-DF&amp;EMC step probes  20   a ,  20   b  are arranged such that the probes  20   a ,  20   b  are arranged in a ±45° configuration. The probes  20   a ,  20   b  are in contact with a ground plane  1  and are coupled with a low band top plate  2 . The probes  20   a ,  20   b  are each the same as the probe element  20  shown in  FIGS. 5 and 6   a ,  6   b . In  FIG. 6 , the design is vertically polarized and the feeds are arranged in a slant +/−45 degree configuration for dual polarization. As with the single vertically polarized configurations of  FIG. 4 , the VSWR on the slant  45  dual polarized configuration is very good owing to the broad band design of the multi-DF&amp;EMC step probe. However, because the primary radiator (or patch)  2  is excited on the edge, the isolation between  20   a ,  20   b  is very poor. This is typically on the order of −12 dB. Apart from the poor isolation, the pattern is less stable and often squint over a large frequency band. 
         [0036]      FIG. 8  show a balanced configuration  39  whereby the multi-DF&amp;EMC step probes  35   a ,  35   b  are fed 180° out of phase. Here, the probe  20  of  FIG. 5  is mirrored with itself and joined together to form a single one-piece elongated probe  39  having two nearly identical halves  20 . The vertical conductors  17 ,  13  are positioned toward the outside portions of the board  38  so that they are further away from each other, and the horizontal conductors  14 ,  15 ,  16 ,  18 ,  19  extend inward toward the center of the board  38 . However, the vertical conductors  13 ,  17  can be positioned toward the center of the PCB at the inside of the respective probes  20 , with the horizontal conductors extending outward. The only difference between the probe halves  20  is that the length of the conductive track  32  (on the left probe half in the embodiment shown) is 180° of phase longer than the length of the conductive track  31  (on the right probe half in the embodiment shown). This is done because the radiator is approximately half a wavelength, the opposite ends  2   a ,  2   c  ( FIG. 7 ) needs to be fed 180 degrees out of phase otherwise the electric fields cancel. 
         [0037]    The configuration of the probe  39  offers high performance. The probe  39  is balanced electrically because the radiator  2  is fed at both ends  2   a ,  2   c  as oppose to transferring energy from the probes to the radiator at one end  2   a  only. With this configuration, the VSWR is still very good across a wide frequency band but the isolation has improved markedly to better than −30 dB from −12 dB and the radiation pattern is very stable a very wide frequency band. The feed network  30  is excited at point  30   a  and resides above a ground plane  33  on the PCB  36 . Power from an input port  30   a  is then split equally (preferable but not always the case) at junction  30   b.  The power is then carried to multi-DF&amp;EMC step probes  35   a  and  35   b  via conductive tracks  31  and  32  respectively. 
         [0038]      FIG. 9  shows a balanced configuration for a probe  49  whereby the multi-DF&amp;EMC step probes  45   a ,  45   b  are fed 180° out of phase. The feed network  40  is excited at point  40   a  and resides above a ground plane  43 . Power from input port  40  is then split equally (preferable but not always the case) at junction  40   b . The power is then carried to multi-DF&amp;EMC step feeds  45   a  and  45   b  via conductive tracks  41  and  42  respectively. The length of the conductive track  42  is 180° longer than the length of conductive track  41 . 
         [0039]    The probe  49  is nearly identical to the probe  39  of  FIG. 8 , except as to the slots  34 ,  44 . As shown in  FIG. 8 , the PCB  38  has a slot  34  extending vertically downward from the top of the PCB  38  at the middle of the probe  39  to divide the probe  39  in half. The slot  34  extends nearly to the bottom of the PCB  38 . And as shown in  FIG. 9 , the PCB  46  has a slot  44  that extends vertically upward from the bottom of the PCB  46  at the middle of the probe  49  to divide the probe  49  in half. The slot  44  extends only slightly upward by a distance that is about the same (or slightly greater than) as the distance from the slot  34  to the bottom of the PCB  36  in  FIG. 8 . Accordingly, the slots  34 ,  44  from  FIGS. 8 and 9  respectively mate together so that the probes  39 ,  49  to form an X-shaped cross. The slot  44  slides down through slot  34  so that the probes  39 ,  49  engage one another in a friction fit. 
         [0040]      FIG. 10  shows the balanced configuration whereby the balanced multi-DF&amp;EMC step probes are mirrored and fed 180° out of phase with each other, for dual polarization. By feeding the radiator  2  on the edge, the currents across the radiator  2  will be different at the opposite ends  2   b ,  2   d  of the radiator  2  along the diagonal. The result is poor cross polar discrimination and poor pattern stability. This improves by mirroring the probes  20   a , b and feeding the probes 180 degrees out of phase. 
         [0041]      FIG. 10  show the dual polarized balanced multi-DF&amp;EMC step feeds arranged in a ±45° configuration. A coupling radiator patch (i.e., low band top plate)  2  is a flat sheet of metal that resides above the mated configurations  39 ,  49 . Here, the coupling patch  2  is approximately 0.12× above the ground plane  1 . The coupling patch  2  may have any other arbitrary shape that is appropriate for the application for which it is desired. This shape could have bent up walls or shaped like a box. Alternatively, additional radiating patches can be stacked for further bandwidth enhancements although it is not required in this design as it already meets the operating frequency bandwidth without the additional coupling patches.  FIG. 10  resolves the drawback of  FIG. 7  where the isolation and pattern stability becomes an issue over a wide frequency band. That is, the multi-DF&amp;EMC probes are now exciting both ends  2   a ,  2   c  and  2   b ,  2   d  of the radiator  2 . It is also fed 180° out of phase because the radiator is approximately ½ a wavelength long. The currents are therefore more balanced than in  FIG. 7 , where it is only excited at one end (end  2   a  for one polarization and end  2   d  for the other) of the radiator. Because of this configuration, patterns show better stability and isolation whilst still maintaining the wide impedance matching characteristics of the multi-DF&amp;EMC probes. 
         [0042]    Referring to  FIGS. 1 and 10 , high band assemblies  102  are added to the configuration of  FIG. 10  to provide  FIG. 1 . A high band assembly  102  is added to opposite sides of the low band assembly  104  on the ground plane  1 , with the low band assembly  104  therebetween. In addition, a high band assembly  102  is stacked on top of the low band assembly  104 , as mentioned above. The high band elements  102  are typically close to twice the frequency of operation as the low band elements. This turns the dual polarized element of  FIG. 10  to a multi-band dual polarized configuration of  FIG. 1  operating from 695-960 MHz and 1700-2700 MHz. 
         [0043]    Referring to  FIG. 11 , alternatively from a further cost reduction point of view, the structure of  FIG. 7  can be made to emulate  FIG. 10  by employing two radiating assemblies and then feeding them 180° out of phase. Here, the radiating elements  50   a ,  50   b  and their respective top plate  51  make up one radiating assembly and radiating elements  50   c ,  50   d  and top plate  52  make up another radiating assembly. The top plates  51 ,  52  have respective ends or corners  51   a - d,    52   a - d.  The radiating elements  50   a ,  50   b ,  50   c ,  50   d  can either be high band or low band, as with  FIG. 10 . The surfaces of the PCBs of the radiating elements  50   a ,  50   b ,  50   c ,  50   d  are generally facing inward toward one another. The radiating elements  50   a ,  50   b  form a first pair and are separated from each other; and the radiating elements  50   c ,  50   d  form a second pair are also separated from each other. Thus, the radiating elements  50   a ,  50   b ,  50   c ,  50   c  generally form the sides of a square or rectangular shape but are open on the corners so that they are not directly connected with each other. In addition, the vertical conductors of each pair are at the side of the radiating elements  50   a, b, c, d  that are furthest away from each other. Thus, for instance, the vertical conductor of element  50   c  is at the far side of the element  50   c  with respect to element  50   d  in that pair; likewise, the vertical conductor of element  50   d  is at the far side of that element  50   d  with respect to element  50   c.    
         [0044]    Although it is still being fed at one end or corner  51   a ,  52   c , and  51   d ,  52   b  of the radiating patch, the combination behaves like a single balanced element of  FIG. 10 . In the mobile station industry, sidelobe suppression can still be maintained by employing pairs of elements fixed in an array. With reference to  FIG. 11 , if multi-DF&amp;EMC step feeds  50   a  is fed 180° out of phase with multi-DF&amp;EMC step feed  50   c , the end result is an element with higher gain as there are now two elements in the array but it&#39;s behavior is very similar to that of the balanced feed structure of  FIG. 10  but using only half the PCB substrate. 
         [0045]    Similarly, if multi-DF&amp;EMC step feeds  50   b  is fed 180° out of phase with multi-DF&amp;EMC step feed  50   d , the end result is an element with higher gain as there are now two elements in the array but it&#39;s behavior is very similar to that of the balanced feed structure of  FIG. 10  but using only half the PCB substrate. Note that multi-DF&amp;EMC step feeds  50   a  and  50   c  have the same polarization, designated as P 1 . Multi-DF&amp;EMC step feeds  50   b  and  50   d  have the same polarization, designated as P 2 . The multi-DF&amp;EMC step feds can be fed via coaxial cable, PCB&#39;s or airline. The low band radiators  51 ,  52  are conductors usually made of aluminum but a PCB with a metal ground plane etched on it will provide the same function. The ground plane  53  is also a conductor, typically aluminum although any conductor will do. An airline is a metal conductor that is suspended in air a short distance above the ground plane. Because air is the medium, the insertion of the network using airline is very low. 
         [0046]    The foregoing description and drawings should be considered as illustrative only of the principles of the invention. The invention may be configured in a variety of shapes and sizes and is not intended to be limited by the preferred embodiment. Numerous applications of the invention will readily occur to those skilled in the art. Therefore, it is not desired to limit the invention to the specific examples disclosed or the exact construction and operation shown and described. Rather, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.