Patent Publication Number: US-11387567-B1

Title: Multiband antenna with dipole resonant structures

Description:
FIELD OF THE INVENTION 
     This invention relates to dipole antennas. More particularly, this invention relates to dipole antennas with interspersed resonant circuits. 
     DESCRIPTION OF RELATED ART 
     With the ever-increasing need for more compact base station antennas, prior art designs include antennas with multiple arrays of elements, operating on separate frequency bands. These elements from different bands may be close to one another and in a single enclosure and co-located on a single conductive reflector. In some cases, the elements of the different arrays can be located on separate reflectors but they are still very close to one another. 
     In such arrangements, the lower frequency antenna elements which are larger in size, can reside around and above the higher frequency, smaller antenna elements, all in proximity to one another. One issue with such dense collections of arrays at different bands is degraded performance due to parasitic effects of the arrays on the signals emanating from each other, interacting across frequency bands. For example, low frequency elements can have a parasitic effect on the higher frequency elements, and vice-versa. Antenna elements of a first array of elements that operate at one frequency band can appear “electrically large”, for example greater than a half wavelength (Lambda at frequency F is (3*10e6)/(F [Hz]) (meters), at the frequencies of the nearby antenna elements of the other arrays. 
     To reduce this parasitic effect the prior art focuses on essentially two options. The first option is to increase the spacing of the different frequency arrays from one another on the reflector, but this undesirably increases the footprint of the antenna. Another option is to simply operate with the parasitic effects from the interspersed arrays, but this results in less than ideal coverage area for the antenna, including lobes and drop-off zones or “null zones.” 
     There have also been prior art attempts to dampen this parasitic effect by placing “chokes” on the arms of the dipole. A choke is a physical structure on the dipoles that blocks high frequency signals while passing low frequency signals. See for example, U.S. Pat. No. 9,912,076 which includes an array of high frequency elements and an array of low frequency elements on the same reflector. The larger arms of the low frequency dipoles can include RF chokes that provide an open circuit or a high impedance in response to high frequency signals, separating adjacent dipole conductive segments to minimize induced high band currents in the low-band radiator and consequent disturbance to the nearby high band radiating pattern. These RF chokes is resonant at or near the frequencies of the high band. 
     However as illustrated in the prior art FIGS. 1 and 2 from the &#39;076 patent the RF choke resonant structures are in the form of boxes along the arm of the low frequency dipole. This particular implementation of RF filter is relatively large, compared to the elements themselves, due to the lower dielectric constant of air. It also requires high mechanical metalwork accuracy and expertise. 
     OBJECTS AND SUMMARY 
     The present invention overcomes the drawbacks associated with the prior art and provides novel resonant structures that are included in the printed circuit board (PCB) within the arms of dipoles. Moreover, unlike the prior art, the novel PCB resonant structures are of such a design that they may be included within the vertical balun feeds of the dipoles as well. In each case, such resonant structures reduce the parasitic effects of nearby antenna elements of different frequency arrays on the same or nearby reflectors. 
     These resonant structures are placed not only on the arms of the low frequency dipoles but also on the nearby high frequency elements as well. These resonant structures are included not only on the horizontal arms but also on the vertical balun feeds extending perpendicular from the reflector. In some arrangements the resonant structures are in the form of either parallel resonant circuits or series resonant circuits (high pass configuration). 
     To this end the present arrangement provides for an antenna for cellular communications having a reflector and at least a first array of dipole antenna elements on the reflector operating at a first frequency ban. The dipole antenna elements of the first array having a printed circuit construction and composed of a balun feed and dipole arms. At least a second array of dipole antenna elements is provided on the reflector operating at a second frequency band the dipole antenna elements of the first array having a printed circuit construction and composed of a balun feed and dipole arms. 
     The dipole antenna elements of the first array include one or more resonant structures causing a substantially closed circuit at the first frequency band and a substantially open circuit at the second frequency band. The resonant structures on the dipole antenna elements of the first array are located at least in part on the balun feed of the dipole antenna elements. 
     In another embodiment an antenna for cellular communications has a reflector and at least a first array of antenna elements on the reflector operating at a first frequency band, the dipole antenna elements of the first array having a printed circuit construction and composed of a balun feed and dipole arms. At least a second array of antenna elements is provided on the reflector operating at a second frequency band, the dipole antenna elements of the second array having a printed circuit construction and composed of a balun feed and dipole arms. 
     The printed circuit construction of the antenna elements of the first array include one or more printed circuit resonant structures causing a substantially closed circuit at the first frequency band and a substantially open circuit at the second frequency band. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be best understood through the following description and accompanying drawing, wherein: 
         FIG. 1  is a prior art image of multiple frequency arrays on a reflector with resonant structures; 
         FIG. 2  is a prior art image of a dipole arm with resonant structures; 
         FIG. 3  illustrates an antenna with multiple frequency arrays in accordance with one embodiment; 
         FIG. 4  illustrates an antenna with multiple frequency arrays in accordance with one embodiment; 
         FIG. 5  illustrates a portion of a reflector with dipole elements from different frequency arrays in accordance with one embodiment; 
         FIG. 6  illustrates three dipole antenna elements from different frequency arrays on a single reflector in accordance with one embodiment; 
         FIG. 7  illustrates components of a PCB resonant structure for a dipole element in accordance with one embodiment; 
         FIG. 8  illustrates a vertical ground copper layer in a parallel PCB resonant structure for a dipole element in accordance with one embodiment; 
         FIG. 9  illustrates a microstrip transmission line in a parallel PCB resonant structure for a dipole element in accordance with one embodiment; 
         FIG. 10  illustrates an insulating substrate of a parallel PCB resonant structure for a dipole element in accordance with one embodiment; 
         FIG. 11  illustrates a capacitive copper layer of a parallel PCB resonant structure for a dipole element in accordance with one embodiment; 
         FIG. 12  illustrates a port placement on a parallel PCB resonant structure for a dipole element in accordance with one embodiment; 
         FIG. 13  is a smith diagram for a PCB resonant structure for a dipole element in accordance with one embodiment; 
         FIG. 14  illustrates a PCB resonant structure for a dipole element in accordance with one embodiment; 
         FIG. 15  illustrates a PCB resonant structure for a dipole arm of a dipole element in accordance with one embodiment; 
         FIG. 16  is a smith diagram for a PCB resonant structure for a dipole element in accordance with one embodiment; 
         FIG. 17  illustrates components of a PCB resonant structure for a balun feed of a dipole element in accordance with one embodiment; 
         FIG. 18  illustrates components of a PCB resonant structure for a balun feed of a dipole element in accordance with one embodiment; 
         FIG. 19  illustrates components of a PCB resonant structure for a balun feed of a dipole element in accordance with one embodiment; 
         FIG. 20  illustrates components of a PCB resonant structure for a balun feed of a dipole element in accordance with one embodiment; 
         FIG. 21  is a smith diagram for a PCB resonant structure for a dipole element in accordance with one embodiment; 
         FIG. 22  shows an isolated dipole element with PCB resonant structures in accordance with one embodiment; 
         FIG. 23  shows an isolated dipole element with PCB resonant structures in accordance with one embodiment; 
         FIG. 24  shows an isolated dipole element with PCB resonant structures in accordance with one embodiment; 
         FIG. 25  shows the three dipole element with PCB resonant structures from  FIGS. 21-23  on a common reflector in accordance with one embodiment; 
         FIG. 26  is a smith diagram for one dipole element with PCB resonant structures in accordance with one embodiment; 
         FIG. 27  is an azimuth cut of a radiated pattern plot for one dipole element with PCB resonant structures in accordance with one embodiment; 
         FIG. 28  is a smith diagram for one dipole element with PCB resonant structures in accordance with one embodiment; 
         FIG. 29  is an azimuth cut of a radiated pattern plot for one dipole element with PCB resonant structures in accordance with one embodiment; 
         FIG. 30  is a smith diagram for one dipole element with PCB resonant structures in accordance with one embodiment; 
         FIG. 31  is an azimuth cut of a radiated pattern plot for one dipole element with PCB resonant structures in accordance with one embodiment; and 
         FIG. 32  illustrates multiple frequency arrays on a single reflector panel with dipole elements each having PCB resonant structures in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In one embodiment of the present invention as illustrated in  FIGS. 3 and 4 , a multi frequency band antenna  10  is shown. In this example, antenna  10  is a three panel omni directional antenna, but the invention is not limited in this respect. The salient features described below can be used in conjunction with any antenna that has two or more different frequency arrays on a single reflector or nearby reflector. In the example of  FIGS. 3 and 4 , antenna  10  has three reflector panels  12 , each of which has three different frequency arrays, a low frequency array of elements  14 , a mid frequency array of elements  16 , and a high frequency array of elements  18 . As shown in the top elevation  FIG. 4  as well as  FIG. 3 , each of arrays,  14 ,  16 , and  18  include multiple dipole elements  14   a ,  16   a , and  18   a  respectively, each part of their own respective frequency array. 
       FIG. 5  shows another partial representation of an exemplary panel  12  to show an arrangement of dipole elements  14   a ,  16   a , and  18   a  forming the three frequency arrays. As illustrated in this figure, the various elements are closely interspersed in physical proximity to each other. As explained above, the larger low frequency elements have a parasitic effect on the higher frequency elements and vise versa. 
       FIG. 6  is an exemplary schematic view of one segment of panel  12 , with one low frequency element  14   a , one mid frequency element  16   a , and one high frequency element  18   a .  FIG. 6  shows one elements  14   a ,  16   a , and  18   a , without the inventive resonant structures for the purposes of giving an exemplary spatial context to the elements on reflector panel  12 , and also to provide base structures on which to build on to explain the salient features of the resonant structures. 
     In this example low frequency element  14   a  is a 0.86 Ghz band element, mid frequency element  16   a  is a 2.2 GHz band element, and high frequency element  18   a  is a 3.6 GHz band element. As seen in this schematic, three exemplary elements  14   a ,  16   a , and  18   a  are shown with overlapping or nearly overlapping footprints which, as explained in the background, would result in parasitic effects by one element on the adjacent two. For the purposes of illustration these three frequency bands are used, however the structures described herein can be modified to be used for other frequency bands. 
     As shown in the  FIG. 7  an exemplary resonant structure  50  in the form of a Parallel LC structure (high impedance), is sized for acting as an open circuit at 2.2 GHZ (mid band) or sized as an open circuit at 3.6 GHZ (high band). For the purposes of illustration, parallel resonant structure  50  shown in  FIG. 7  and described in more detail below is for the 2.2 Ghz open circuit resonance in the balun feed of low band dipole  14   a  (resonant structure  51  described below is for the 3.6 Ghz open circuit resonance in the balun feed of low band dipole  14   a ). The balun feed involves a vertical signal feed in two parallel columns running vertically to the dipole arms. Resonant structure  50  has two halves  50   a  and  50   b  one structure for each of balun feed columns. As will be described more fully in connection with another embodiment of the invention, a resonant structure for the balun feed of  14   a  is sized for acting as an open circuit at the high band 3.6 Ghz. 
       FIGS. 8 through 11  show the iterations and layers to form resonant structure  50 .  FIG. 8  represents a vertical balun ground plane  52  forming the ground plane gap and an inductive connection  54 . Inductive connection is a narrow copper line  54  that transverses the vertical balun ground gap. There are two such copper lines  54  on both sides of resonant structure  50   a / 50   b , symmetrically located around a ground center point, and connecting the upper ground and the lower ground.  FIG. 9  shows the next layer which is essentially just the microstrip transmission line of the vertical balun feed of the dipole.  FIG. 10  shows a vertical balun feed PCB substrate  56  substrate (e.g. 0.5 mm thick).  FIG. 11  illustrates a vertical balun feed resonant capacitance top plate  58  (for bridging ground plane gap). 
       FIG. 12  shows the resonant structure above from  FIGS. 7 through 11 , with added explanations for the resonant circuit ports. Transmission line ports  60   a  (bottom) and  60   b  (top) are just the entry and exit ports for the balun feed circuit itself. Port  62  illustrates the open circuit resonance port that lies across the ground plane gap. The resonant frequency is determined as follows:
 
Fres=1/(2*π*(sqrt( L*C )))[Hz]
 
     For example, in an exemplary calculation for parallel resonance at 2.2 GHz, the following values can be used:
 
 L= 3 nH
 
 C= 1/(((Fres*2*π){circumflex over ( )}2)* L )=1.74 [pF]
 
     where, the inductance is a narrow copper trace of approximately 3 mm in length, and the parallel plate capacitance is calculated from the formula:
 
 C=ϵr*ϵo*A/D [ F ]
 
     Where: 
     ϵr=relative permittivity of dielectric 
     ϵo=8.854*10{circumflex over ( )}(−12) [F/m] (permittivity of free space) 
     A=capacitor area [m{circumflex over ( )}2] 
     D=dielectric thickness [m] 
     Note that in the implementation shown in  FIGS. 7 to 12 , there are 2 parallel inductances and 2 series capacitances, the values referenced above are the total resultant inductance and capacitance (L and C). 
     In the case of the parallel resonance at 3.6 GHz, the following values can be used:
 
 L= 3 nH
 
 C= 1/(((Fres*2*π){circumflex over ( )}2)* L )=0.65 [pF]
 
     Note also that the transmission line has the effect of adding capacitance in parallel with the parallel L-C network, so it adds to the total resultant capacitance. The final configuration may be simulated with a CAD tool to take into account all effects of the circuit, and optimized for proper performance. 
       FIG. 13  shows a smith diagram of the frequency response for resonant structure  50  as it would be applied on the balun feed of dipole  14   a  showing a closed circuit at approximately 0.86 Ghz but “open” at the approximate mid band frequency of 2.2 Ghz. It is noted that that by “open” in this context and as used throughout regarding the resonant structures through the application is “open” or “near to an open” below resonance, at least enough to reduce induced parasitic currents and coupling to the lower frequency adjacent elements. For example, marker  5  (M 5 ) shows a port impedances close to 50 Ohms of 1.00 and marker  1  (M 1 ) shows low transmission loss along the transmission line (e.g. from port  60   a  to  60   b  from  FIG. 12 ). However, the open circuit resonance (e.g. at port  62  from  FIG. 12 ) is open at markers  3 ,  4 , and  5  (M 3 , M 4 , and M 5 ). As noted above, the present example is for a resonant structure  50  for the balun feed of dipole  14   a , where is it sized and dimensioned to provide an open circuit at 2.2 Ghz, but the same concepts hold true for a differently dimensioned resonant structure  50  for 3.6 Ghz that could also be placed on the balun feed of element  14   a  and would result in a similar smith diagram showing a closed circuit at approximately 0.86 Ghz and open circuit at 3.6 Ghz. For example, altering the resonant frequency can be achieved by reducing the, an L or C or both L and C portions of copper line  54  to open circuit resonate at the higher frequency of 3.6 Ghz using the equation above. 
     For example, in  FIG. 13 , the plotted line at (s(3,3)) on the smith chart passes near the upper part of the graph at marker m 2  (1.7 GHz, which is the low end of the mid frequency band), then goes near to an open circuit (the middle right hand side of the graph) near marker m 4  (2.2 GHz) which is the middle of the mid frequency band), and then, the frequency increases by marker m 3  (2.7 GHz, which is the high end of the mid frequency band). This line actually travels clockwise around the outer edge of this smith chart, even starting out near the middle left hand side of the chart, which is near a short circuit (that is at the low frequency band). However, if the line for resonant structure  50  were to be swept to even high frequency, the line would continue in a clockwise direction, past marker m 3 , and actually head towards a short circuit again (e.g. at the middle left hand side of the chart). This may occur near the high frequency band. As such, the balun feed of low frequency elements  14   a  include not only resonant structures  50  to be open at the mid band frequency of 2.2 Ghz, but also separate resonant structures  51  to be open at the high band frequency of 3.6 Ghz. 
     For example,  FIG. 14  shows resonant structure  51  and how the LC structure generates the closed circuit at the low band (in this example) and the open circuit at high band (using the same sub-structures as labeled in  FIG. 12  and with structure  50 ). As shown in  FIG. 14  The (C+C)∥(C+C) (see location  64 ) creates a capacitance Cr. The L∥L (see location  66 ) creates a resultant inductance Lr. Lr is in parallel with Cr and resonates to an open circuit and Lr is small enough to not perturb the microstrip feed impedance across port  60   a  and  60   b.    
     In the case of the low band element  14   a , each vertical balun feed open circuit parallel resonant circuit at port  62  is tuned to provide, in the vertical direction, an open circuit balun ground at either high band 3.6 Ghz (structure  51 / FIG. 14 ) or mid band 2.2 Ghz (structure  50 / FIG. 12 ) and closed at low band of 0.86 Ghz with a well matched (i.e. 50 Ohms) on the microstrip balun feed ports  60   a  and  60   b.    
     The above description of resonant structure  50  (mid band open) for use on the balun feed of low band dipole  14   a  is essentially the same for resonant structure  51  (high band open) shown in  FIG. 14 . Regarding the resonant structures to be used on the arms of the diploes, such as low band dipole  14   a , work on essentially the same principle as resonant structures  50 / 51 . 
       FIG. 15  shows an exemplary resonant structure  70  for mid band open circuit to be used on the arms of low band dipole  14   a . (Resonant structure  71  will be used to refer to the resonant structure or high band open circuit to be used on the arms of low band dipole  14   a ). 
     As with resonant structure  50 , resonant structure  70  provides low impedance at low band 0.86 Ghz and an open circuit at either mid band (2.2 Ghz—structure  70 ) or high band (3.6 Ghz—structure  71 —not shown) depending on the dimensions and arrangement of the L and C elements. Such resonant structure  70  likewise has a copper wire  74  (forming L component—inductance) and capacitance plate  78  (forming C component). The impedance is defined along the diploe arm of element  14   a . The L∥L inductance and the C+C capacitance create a parallel L-C network, same as in the resonant structure  50 . As shown in the related smith diagram of  FIG. 16  marker M 1  shows an open circuit at the mid band frequency of about 2.2 Ghz with a closed circuit across the dipole arm of element  14   a  at near the low band frequency of 0.86 Ghz. 
     The above examples of resonant structure  50 / 51  for the balun feed of low band elements  14   a  and resonant structures  70 / 71  for the diploe arm of low band elements  14   a  can be likewise used on mid band element  16   a  as resonant structure  80  for the balun feed (3.6 Ghz open—closed at 2.2 Ghz) and resonant structure  81  for the dipole arm of element  16   a  (also 3.6 Ghz open—closed at 2.2 Ghz). Element  16   a  does not need to have a resonant structure at 0.86 Ghz low band because of limited space on the mid-band element and also the parasitic effect of the mid-band element  16   a  on the low band element  14   a  is somewhat less. However, it is noted that the series resonant circuit analogous to the one used on the high band element (described below) could also be used on the mid-band element if significant degradation of the low band element is seen in the presence of the mid-band element. This would create a mid band element with parallel resonant circuits resonating at high band as well as series resonant circuits resonating at mid-band. 
     Regarding the resonant structure on the high band elements  18   a , these are series resonant structures instead of parallel resonant structures as used on elements  14   a  and  16   a . Such series resonant structures on the high band elements  18   a  are essentially high pass filters to prevent the high frequency elements from interfering with the signals from the low and mid band structures. For example,  FIGS. 17-19  show the layered structure of a series resonant structure  90  for use on high band element  18   a  that is closed at 3.6 Ghz but having high impedance at the lower 0.86 Ghz and 2.2 Ghz.  FIG. 17  illustrates a vertical balun ground plane including 92.  FIG. 18  shows the layer of the vertical balun feed resonant series (C-L-C shape) with a capacitor top plate  96  and thin copper line inductor  94 , which are not connected to ground plane.  FIG. 19  shows the balun feed transmission line.  FIG. 20  illustrates resonant structure  90  with ports  98   a  and  98   b  for the balun feed transmission line and port  100  for resonant structure  90 . As with the resonant structures  50 ,  70 , and  80  for balun feeds of elements  14   a  and  16   a , resonant structure  90  may likewise be modified (resonant structure  91 ) to be on the arms of elements  18   a.    
       FIG. 21  shows smith diagram for resonant structure  90  that provides low impedance at high frequency 3.6 Ghz and high impedance at 0.86 Ghz and 2.2 Ghz. For example, the Smith diagram shows transmission line characteristic impedance close to 50 ohms at ports  98   a  and  98   b  (marker  2 —M 2 ) and low transmission loss along the transmission line (from port  98   a  to  98   b —marker  3 —M 3 ). The diagram also shows that structure  90  resonates to a short circuit (low impedance) across port  100 . However, structure  90  has high impedance across port  100  at lower frequency 0.86 Ghz and 2.2 Ghz. (markers  4  and  5 —M 4  and M 5 ). 
     Owing to the structures  50 ,  51 ,  70 ,  71 ,  80 ,  81 ,  90 , and  91  described above, resonant structures can be implemented directly into the PCB structure of balun feeds as well as the arms of diploes  14   a ,  16   a , and  18   a . Not only does this simplify the resonant structures over the prior art designs, because of the smaller PCB application, they are easily integrated into the balun feeds as well, whereas prior art designs were unable to be used in such a manner. The balun feeds themselves as vertical impediments can cause just as significant parasitic effects, and this is not addressed in the prior art. The present arrangement provides a solution to that issue as well as being of smaller, more compact, and robust construction. 
     Turning now to the placement of the resonant structures on diploes  14   a ,  16   a , and  18   a ,  FIG. 22  illustrates a single dipole element  14   a  having a series of resonant structures  50 ,  51 ,  70 , and  71  thereon. Resonant structures  50  (mid band) and  51  (high band) are found on the balun feed and structures  70  (mid band) and  71  (high band) are found on the diploe arms of diploe  14   a.    
     Regarding the locations of parallel LC resonant circuits  50  and  51  as well as  70  and  71 , they are ideally arranged to break up the conductor of element  14   a  into pieces smaller than a half wavelength at the parasitic frequency of which they are attuned. For example, the location of mid band resonant structures  50  and  70  on the balun feed and dipole arms respectively, are arranged to break up those metallic structures of diploe  14   a  into segments that are smaller than ½ wavelength at 2.2 Ghs, as much as possible given space limitations. Even at segments that are at or larger than ½ wavelength there are positive effects, but in some implementations that may not be attainable due to space constraints. In any case, resonant structures  51  and  71  are arranged to break up those metallic structures of diploe  14   a  into segments that are smaller than ½ wavelength at 3.6 Ghs. This location arrangement is done because metallic objects such as diploe  14   a  can resonate at frequencies from mid and high band dipoles  16   a  and  18   a  when they approach a dimension of a half wavelength. This causes severe perturbation of adjacent elements operating at that frequency. 
       FIG. 23  shows the exemplary placement of resonant structures  80  (balun) and  81  (arm) on diploe  16   a  set to reduce parasitic effects at 3.6 Ghz that would otherwise impair the function of high band dipole element  18   a .  FIG. 24  shows the placement of series resonant structures  90  (balun) and  91  (arm) on high frequency dipole  18   a .  FIG. 25  shows all three diploe elements  14   a ,  16   a , and  18   a  (as contrasted with schematic  FIG. 6  showing the same three dipole elements without resonant structures). 
       FIGS. 26 and 27  illustrates two charts showing the resonant structures  50 ,  51 ,  70 , and  71 , implemented on dipole  14   a  do not negatively affect in-band dipole input impedance match ( FIG. 26 ) and radiated pattern shape ( FIG. 27 ) within the low band.  FIGS. 28 and 29  illustrates two charts showing the resonant structures  80  and  81 , implemented on dipole  16   a  do not negatively affect in-band dipole input impedance match ( FIG. 28 ) and radiated pattern shape ( FIG. 29 ) within the mid band. 
       FIGS. 30 and 31  illustrates two charts showing the resonant structures  90  and  91 , implemented on dipole  18   a  do not negatively affect in-band dipole input impedance match ( FIG. 30 ) and radiated pattern shape ( FIG. 31 ) within the low band. The final  FIG. 32  shows a full panel  12  with three frequency range arrays  14  (low band 0.86 Ghz),  16  (mid band 2.2 Ghz), and  18  (high band 3.6 Ghz), composed of elements  14   a ,  16   a , and  18   a  respectively, to be used in an antenna, such as antenna  10  of  FIGS. 3 and 4  or any other multi-frequency cellular antenna. In  FIG. 32 , arrays  14 ,  16  and  18  include elements  14   a ,  16   a , and  18   a , arranged as single column arrays (2× 14   a,  4× 16   a,  4× 18   a ). Owing to the resonant structures on elements  14   a ,  16   a  and  18   a , each of arrays  14 ,  16 , and  18  are implemented on the single common reflector  12 , reducing the overall array volume. 
     While only certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes or equivalents will now occur to those skilled in the art. It is therefore, to be understood that this application is intended to cover all such modifications and changes that fall within the true spirit of the invention.