Abstract:
An antenna for a folder type communications handset. The handset comprises first and second enclosures pivotably joined to permit rotation of one enclosure relative to the other enclosure. The antenna is disposed over a ground plane formed in a printed circuit board in the first enclosure. The second enclosure also comprises a ground plane. A feed terminal and a ground terminal of the antenna are disposed to limit field coupling between the feed terminal and the ground plane in the second enclosure. The feed and the ground terminals are each connected to corresponding terminals on the printed circuit board by meanderline conductors.

Description:
[0001]     This application claims priority to the provisional patent application filed on Jul. 11, 2003, assigned application No. 60/486,585 and entitled Apparatus for Reducing Ground Effects in a Folder-Type Communications Handset Device. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to antennas for portable communications devices and more specifically to an antenna for limiting ground plane effects on the radiation characteristics of a folder-type communications handset.  
       BACKGROUND OF THE INVENTION  
       [0003]     It is generally known that antenna performance is dependent upon the size, shape, separation distance and material composition of the constituent antenna elements, as well as the relationship between certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These parameters and relationships determine several antenna operational characteristics, including input impedance, gain, directivity, signal polarization, operating frequency, bandwidth and radiation pattern. Generally for an operable antenna, the minimum physical antenna dimension (or the electrically effective minimum dimension) must be on the order of a half wavelength (or a multiple thereof) of the operating frequency, which thereby advantageously limits the energy dissipated in resistive losses and maximizes the transmitted energy. Half-wavelength antennas and quarter-wavelength antennas over a ground plane (which effectively perform as half-wavelength antennas) are the most commonly used.  
         [0004]     The burgeoning growth of wireless communications devices and systems has created a substantial need for physically smaller, less obtrusive, and more efficient antennas that are capable of wide bandwidth operation, multiple frequency-band operation, and/or operation in multiple modes (i.e., selectable radiation patterns or selectable signal polarizations). Smaller packaging for state-of-the-art communications devices, such as cellular telephone handsets and other portable devices, does not provide sufficient space for the conventional quarter and half-wavelength antenna elements. Thus physically smaller antennas operating in the frequency bands of interest and providing other desired antenna-operating properties (input impedance, radiation pattern, signal polarizations, etc.) are especially sought after. Ideally, such antennas are disposed within the handset case so as to avoid possible damage to or breakage of an externally mounted antenna.  
         [0005]     Half-wavelength and quarter-wavelength dipole antennas are popular externally mounted handset antennas. Both antennas exhibit an omnidirectional radiation pattern (i.e., the familiar omnidirectional donut shape) with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. Frequency bands of interest for certain portable communications devices are 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz, and 2.68 inches long at 2200 MHz. The typical antenna gain is about 2.15 dBi. Antennas of this length may not be suitable for most handset applications.  
         [0006]     The quarter-wavelength monopole antenna disposed above a ground plane is derived from a half-wavelength dipole. The physical antenna length is a quarter-wavelength, but when placed above a ground plane the antenna performs as a half-wavelength dipole. Thus, the radiation pattern for a quarter-wavelength monopole antenna above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.  
         [0007]     Several different antenna types can be embedded within a communications handset device. Generally, it is desired that these antennas exhibit a low profile so as to fit within the available space envelope of the handset package. Antennas protruding from the handset case are prone to damage by breaking or bending.  
         [0008]     A loop antenna is one example of an antenna that can be embedded in a handset. The common free space (i.e., not above ground plane) loop antenna (with a diameter approximately one-third of the signal wavelength) displays the familiar donut radiation pattern along the radial axis, with a gain of approximately 3.1 dBi. At 1900 MHz, this antenna has a diameter of about 2 inches. The typical loop antenna input impedance is 50 ohms, providing good matching characteristics.  
         [0009]     Antenna structures comprising planar radiating and/or feed elements can also be employed as embedded antennas. One such antenna is a hula-hoop antenna, also known as a transmission line antenna (i.e., a conductive element over a ground plane). The loop is essentially inductive and therefore the antenna includes a capacitor connected between a ground plane and one end of the hula-hoop conductor to create a resonant structure. The other end serves as the antenna feed terminal.  
         [0010]     Printed or microstrip antennas are constructed using patterning and etching techniques employed in the fabrication of printed circuit boards. These antennas are popular because of their low profile, the ease with which they can be formed and their relatively low fabrication cost. Typically, a patterned metallization layer on a dielectric substrate operates as the radiating element.  
         [0011]     A patch antenna, one example of a printed antenna, comprises a dielectric substrate overlying a ground plane, with the radiating element overlying the top substrate surface. The patch antenna provides directional hemispherical coverage with a gain of approximately 3 dBi.  
         [0012]     Another type of printed or microstrip antenna comprises a spiral and sinuous antennas having a conductive element in a desired shape formed on one face of a dielectric substrate. A ground plane is disposed on the opposing face.  
         [0013]     Another example of an antenna suitable for embedding in a handset device is a dual loop or dual spiral antenna described and claimed in the commonly owned application entitled Dual Band Spiral-shaped Antenna, filed on Oct. 31, 2002, and assigned application No. 10/285,291. The antenna offers multiple frequency band and/or wide bandwidth operation, exhibits a relatively high radiation efficiency and gain, along with a low profile and relatively low fabrication cost.  
         [0014]     As shown in  FIG. 1 , a spiral antenna  8  comprises a radiating element  10  over a ground plane  12 . The ground plane  12  comprises an upper and a lower conductive material surface separated by a dielectric substrate, or in another embodiment comprises a single sheet of conductive material disposed on a dielectric substrate. The radiating element  10  is disposed substantially parallel to and a spaced apart from the ground plane  12 , with a dielectric gap  13  (comprising, for example, air other known dielectric materials) therebetween. In one embodiment the distance between the ground plane  12  and the radiating element  10  is about 5 mm An antenna constructed according to  FIG. 1  is suitably sized for insertion in a typical handset communications device.  
         [0015]     A feed pin  14  and a ground pin  15  are also illustrated in  FIG. 1 . One end of the feed pin  14  is electrically connected to the radiating element  10 . An opposing end is electrically connected to a feed trace  18  extending to an edge  20  of the ground plane  12 . A connector (not shown in  FIG. 1 ), is connected to the feed trace  18  for providing a signal to the antenna  8  in the transmitting mode and responsive to a signal from the antenna  8  in the receiving mode. As is known, the feed trace  18  is insulated from the conductive surface of the ground plane  12 , although this feature is not specifically shown in  FIG. 1 . The feed trace  18  is formed from the conductive material of the ground plane  12  by removing a region of the conductive material surrounding the feed trace  18 , thus insulating the feed trace  18  from the ground plane  12 .  
         [0016]     The ground pin  15  is connected between the radiating element  10  and the ground plane  12 . In different embodiments the feed pin  14  and the ground pin  15  are formed from hollow or solid conductive rods, such as hollow or solid copper rods.  
         [0017]     As illustrated in the detailed view of  FIG. 2 , the radiating element  10  comprises two coupled and continuous loop conductors (also referred to as spirals or spiral segments)  24  and  26  disposed on a dielectric substrate  28 . The outer loop  24  is the primary radiating region and exercises primary influence over the antenna resonant frequency. The inner loop  26  primarily affects the antenna gain and bandwidth. However, it is known that there is significant electrical interaction between the outer loop  24  and the inner loop  26 . Thus it may be a technical oversimplification to indicate that one or the other is primarily responsible for determining an antenna parameter as the interrelationship can be complex. Also, although the radiator  10  is described as comprising an outer loop  24  and an inner loop  26 , there is not an absolute line of demarcation between these two elements.  
       BRIEF SUMMARY OF THE INVENTION  
       [0018]     The invention comprises a communications device operative to transmit and receive communications signals, comprising first and second enclosures coupled by a pivotable joint joining the first and the second enclosures along an edge of each of the first and the second enclosures, wherein the first and the second enclosures further comprise respective first and second surfaces, and wherein the communications device is in a closed state when the first and the second surfaces are disposed in a proximate facing relation, and wherein the communications device is in a open state when the first and the second surfaces are disposed in a spaced apart relation by pivoting of the first and the second enclosures with respect to the pivotable joint. The communications device comprises within the first enclosure, a radio frequency signal radiating element comprising a first feed terminal and a first ground terminal, and a first substrate spaced apart from the radiating element and comprising a ground plane having a second ground terminal, the substrate further comprising a second feed terminal. The first enclosure further comprises a first conductive element connected between the first and the second feed terminals and a second conductive element connected between the first and the second ground terminals. A second ground plane is enclosed within the second enclosure. At least one of the first feed terminal and the first ground terminal are positioned on the radiating element to minimize coupling between the radiating element and the second ground plane when the communications device is the open state. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The foregoing and other features of the invention will be apparent from the following more particular description of the invention, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different figures. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
         [0020]      FIGS. 1 and 2  are perspective views of an antenna suitable for use in a handset communications device according to the teachings of the present invention;  
         [0021]      FIG. 3  illustrates an exemplary handset device in a closed position;  
         [0022]      FIG. 4  illustrates an exemplary handset device in an opened position;  
         [0023]      FIGS. 5 and 6  illustrate an antenna constructed according to the teachings of the present invention;  
         [0024]      FIGS. 7-9  illustrate antennas constructed according to other embodiments of the present invention;  
         [0025]      FIGS. 10A and 10B  illustrate the affect of an antenna constructed according to the teachings of the present invention on the specific absorption ratio; and  
         [0026]      FIGS. 11A and 11B  illustrate the affect of an antenna constructed according to the teachings of the present invention on the hand effect phenomenon.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     Before describing in detail the particular antenna and communications apparatus of the present invention, it should be observed that the present invention resides primarily in a novel and non-obvious combination of elements. Accordingly, the inventive elements have been represented by conventional elements in the drawings, showing only those specific details that are pertinent to the present invention so as not to obscure the disclosure with structural details that will be readily apparent to those skilled in the art having the benefit of the description herein.  
         [0028]      FIG. 3  illustrates a so-called folder type communications handset device  50  (a typical cellular telephone handset style) including an embedded antenna  52 . In one example the embedded antenna  52  comprises the spiral antenna  8 , further comprising the radiating element  10  physically and electrically attached to a printed circuit board  56  further comprising a ground plane  58  and a dielectric substrate  60 . Conventionally the ground plane  58  comprises a conductive region disposed on a portion of the printed circuit board  56 , with electronic components  61  and interconnecting conductive traces (not shown) disposed on other regions of the printed circuit board  56 . The feed pin  14  (see  FIGS. 1 and 2 ) is electrically connected between the radiating element  10  and a feed trace (not shown) on the printed circuit board  56 , wherein the feed trace can be connected to one or more of the electronic components and interconnecting conductive traces. The ground pin  15  (see  FIGS. 1 and 2 ) is connected between the radiating element  10  and the ground plane  58 . The feed pin  14  and the ground pin  15  are generally represented in  FIG. 3  by an element  61 , which extends from the circuit board  56  to the radiating element  10 . Note that since the feed pin  14  and the ground pin  15  are laterally adjacent in the embodiment of  FIG. 1 , one is obscured in the side view of  FIG. 3 .  
         [0029]     The radiating element  10  operates in conjunction with the ground plane  58  as in the exemplary antennas described above, causing the embedded antenna  52  to emit radio frequency energy when the handset  50  is operative in a transmitting mode and to receive radio frequency energy when the handset  50  is operative in a receiving mode. The antenna  52  as illustrated herein is intended to include any of the various antenna designs that can be embedded in the handset  50 , including those described above and others known in the art (e.g., an inverted F antenna or a PIFA antenna).  
         [0030]     The handset  50  further comprises a lower case or lower folder  62  enclosing the embedded antenna  52  and the printed circuit board  56 , and an upper case or upper folder  64  comprising a ground plane  65 , an LCD (liquid crystal display)  66  and other elements as known in the art operative in conjunction with the handset  50 . The ground planes  58  and  65  are connected by a flexible cable  67  passing through a suitable opening in each of the upper and lower folders  62  and  64 . The lower folder  62  further comprises a surface  62 A and the upper folder  64  further comprises a surface  64 A as shown.  
         [0031]     In a closed state or closed position illustrated in  FIG. 3  the surface  62 A is proximately spaced-apart from and generally parallel to the surface  64 A. The lower and upper folders  62  and  64  are mechanically coupled by a rotatable or pivotable joint  68 , permitting the upper folder  64  to be pivoted with respect to the lower folder  62  into an operational (or open) position as illustrated in  FIG. 4  where the surface  62 A is spaced away from the surface  64 A.  
         [0032]     Continuing with the description of  FIG. 4 , for the embedded antenna  52 , a maximum current region  70  is present at a location where the current feeds the radiating element  10 , e.g., where the feed pin  14  is in conductive communication with the radiating element  10 . Due to substantial current flow in the region  70 , when the handset device  50  is in the open or operational position of  FIG. 4  there is considerable electric field coupling between the ground plane  65  of the upper folder  64  and the radiating element  10 . The coupling, indicated by field lines  72 , detunes the operational frequency of the embedded antenna  52  and can affect other operational antenna parameters. Generally, the embedded antenna  52  is designed to operate in conjunction with the ground plane  58 . However, when configured to the opened position of  FIG. 4 , the ground plane  65  is also coupled to the antenna  52 , causing the aforementioned detuning effects.  
         [0033]     For example it has been demonstrated that with the handset  50  in the closed position (as in  FIG. 3 ) the antenna  52  exhibits a resonant frequency peak at about 875 MHz. When the handset  50  is configured in the open position (as in  FIG. 4 ), the resonant frequency peak shifts (i.e., the antenna is detuned) to about 825 MHz. Accordingly, the coupling between the radiating element  10  and the ground plane  65  shifts the antenna operative frequency by about 50 MHz. Such a considerable frequency shift can significantly degrade performance of the handset  50 .  
         [0034]     Note the coupling effect is absent when the lower and upper folders  62  and  64  are in the closed orientation, since the ground plane  58  is interposed between and thus blocks the effects of the ground plane  65  on the radiating element  10 . Of course, the handset  50  is not designed for operation in the closed position.  
         [0035]     According to the teachings of the present invention, the region of substantial current flow is relocated away from the ground plane  65  when the handset  50  is in the open position to reduce coupling between the antenna  52  and the ground plane  65 . Thus, when the handset  50  is opened for operation the antenna performance characteristics will not be substantially altered. To reduce the coupling, one or both of the feed and ground terminals on the prior art radiating element  10  is relocated to minimimze coupling between the radiating element and the ground plane  65  when the handset  50  is in the open state. The extent to which the coupling is minimized according to the teachings of the present invention is dependent on the physical construction and separation distances of the various elements of the handset  50 .  
         [0036]     It is generally considered advantageous to retain the location of the feed and ground terminals on the printed circuit board  56  (to which the feed and ground terminals of the radiating element are connected) such that an antenna constructed according to the teachings of the present invention constitutes a pin-for-pin replacement for a prior art antenna that exhibits the frequency detuning effects described above. Further, the coupling effect that causes antenna detuning is not substantially affected by the location of the feed and ground terminals on the printed circuit board  56 .  
         [0037]     As illustrated in the top view of  FIG. 5 , the printed circuit board  56  comprises a feed terminal  80  and a ground terminal  82 , which are shown in exemplary locations on the printed circuit board  56 . An antenna  78  constructed according to the teachings of the present invention, as illustrated in both the top view of  FIG. 5  and the side view of  FIG. 6 , comprises conductors  84  and  86  connected between the feed and ground terminals  80  and  82  on the printed circuit board  56 , and feed and ground terminals  88  and  90  on a radiating element  79  of the antenna  78 . Preferably, the conductors comprise meanderline conductors,  84  and  86 . Meanderline conductors are generally defined as conductive structures disposed over a ground plane with a separating dielectric material therebetween, where the conductor&#39;s electrical length may not be equal to its physical length. Thus in the embodiment of  FIGS. 5 and 6 , the meanderline conductors  84  and  86  are suspended between the radiating element  79  and the printed circuit board  56 , as illustrated in the side view of  FIG. 6 , such that there is an underlying ground plane (i.e., the ground plane  58 ) and a dielectric material between the conductor structures and the ground plane (i.e., an air gap dielectric). Use of a dielectric material other than air increases the effective electrical length of the meanderline conductors compared to the effective electrical length with an air dielectric. Thus the physical length of each one of the meanderline conductors  84  and  86  can be made shorter when a dielectric material other than air is employed, yet the meanderline conductors  84  and  86  will exhibit the appropriate electrical length relative to the wavelength of the signal transmitted or received by the antenna  78 .  
         [0038]     The meanderline conductors  84  and  86  are so-called slow wave structures where the physical dimensions of the conductor are not equal to its effective electrical dimensions. Generally, a slow-wave conductor or structure is defined as one in which the phase velocity of the traveling wave is less than the free space velocity of light. The phase velocity is the product of the wavelength and the frequency and takes into account the material permittivity and permeability, i.e., c/((sqrt(ε t )sqrt(μ t ))=λf. Since the frequency remains unchanged during propagation through a slow wave structure, if the wave travels slower (i.e., the phase velocity is lower) than the speed of light in a vacuum (c), the wavelength of the wave in the structure is lower than the free space wavelength. Thus, for example, a half-wavelength slow wave structure is shorter than a half-wavelength conventional structure in which the wave propagates at the speed of light. The slow-wave structure de-couples the conventional relationships among physical length, resonant frequency and wavelength, permitting use of a physically shorter conductor since the wavelength of the wave traveling in the conductor is reduced from its free space wavelength.  
         [0039]     Slow wave structures are discussed extensively by A. F. Harvey in his paper entitled  Periodic and Guiding Structures at Microwave Frequencies,  in the IRE Transactions on Microwave Theory and Techniques, January 1960, pp. 30-61 and in the book entitled  Electromagnetic Slow Wave Systems  by R. M. Bevensee published by John Wiley and Sons, copyright 1964. Both of these references are incorporated by reference herein.  
         [0040]     A transmission line or conductive surface overlying a dielectric substrate exhibits slow-wave characteristics, such that the effective electrical length of the slow-wave structure is greater than its actual physical length according to the equation, 
 
 l   c =(ε eff   ½ )× l   p .
 
 where l c  is the effective electrical length, l p  is the actual physical length, and ε eff  is the dielectric constant (ε t ) of the dielectric material proximate the transmission line. 
 
         [0042]     The meanderline conductors  84  and  86  should also exhibit appropriate impedance matching characteristics and present the required electrical length for producing the desired characteristics for the antenna  78 . Additionally, in one embodiment the length of the meanderline conductor  84  (which connects the feed terminal  80  on the printed circuit board  56  to the feed terminal  88  on the radiating element  79 ) may have to be shorter than about λ/8, where λ represents the wavelength of the signal carried by the meanderline conductor  84 . If longer than λ/8, the meanderline conductor  84  can disadvantageously act as radiating structure, causing significant energy coupling with the radiating element  79  and thereby reducing the efficiency (gain) of the antenna  78 .  
         [0043]     In another embodiment, the meanderline conductors  84  and  86  are supported by an underlying dielectric substrate  91  as illustrated in the partial side view of  FIG. 7 . Use of the dielectric substrate  91  allows for physically shorter meanderline conductors  84  and  86  (because the dielectric constant of the substrate  91  is greater than the dielectric constant of air) and also promotes repeatability during the manufacturing process to ensure proper physical placement of the meanderline conductors  84  and  86 .  
         [0044]     In yet another embodiment, the meanderline conductors  84  and  86  are formed within and on one or more surfaces of a dielectric substrate or carrier  92  that substantially fills the region between the radiating element  79  and the printed circuit board  56 . See  FIG. 8  where only the meanderline conductor  84  is illustrated as the meanderline conductor  86  is hidden from view. Segments  84 A and  84 C of the meanderline conductor  84  are disposed on surfaces  92 A and  92 C of the dielectric substrate  92 . The segment  84 C is connected to the feed terminal  80  on the printed circuit board  56 . A segment  84 B is disposed internal the dielectric substrate  92 . The radiating element  79  is disposed on a surface  92 B. The dielectric substrate  92  and the conductive elements can be formed according to known masking and subtractive etching techniques such as those used to form conductive patterns on single-layer and multi-layer printed circuit boards. The embodiment of  FIG. 8  further promotes repeatable manufacturing and accurate placement of the meanderline conductors  84  and  86  and the radiating element  79 .  
         [0045]     In still another embodiment illustrated in  FIG. 9 , a dielectric substrate  94  comprises two conductive vias  95 A and  95 B with the meanderline conductor  84  connected therebetween. The conductive via  95 A is further connected to the radiating element  79  and the conductive via  95 B is further connected to the feed terminal  80  on the printed circuit board  56 .  
         [0046]     Use of meanderline structures for the meanderline conductors  84  and  86  can advantageously reduce the size of the antenna  78 , as a meanderline structure exhibits electrical dimensions that are greater that its physical dimensions, as discussed above.  
         [0047]     Since the location of the feed terminal  88  on the radiating element  79  (a region of relatively high current) in  FIG. 5  is farther from the ground plane  65  (when the handset  50  is disposed in the opened position) than the embodiment of  FIG. 4 , the coupling between the radiating element  79  and the ground plane  65  is reduced, especially in the high current region  70  of  FIG. 4 . With reduced coupling, the ground plane detuning effects created by the ground plane  65  are reduced. In one embodiment the frequency shift is reduced from the  50  MHz referred to above to about 10-20 MHz. Yet this advantage is attainable without increasing the overall antenna size due to the use of meanderline conductors for connecting the feed and ground terminals  88  and  90  on the radiating element  79  to the feed and ground terminals  80  and  82  on the printed circuit board  56 .  
         [0048]     It has also been determined that there is a beneficial reduction in the specific absorption ratio (or SAR, a measure of the amount of radiation to which the user of a cellular telephone is subjected when the telephone is in the operational position near the user&#39;s head) when the connections of the feed and ground terminals to the radiating element  10  are as illustrated in the various embodiments described above. This effect is illustrated in  FIGS. 10A and 10B , (the upper folder  64  is not shown for clarity) where the magnitude of the antenna near-field electromagnetic radiation is indicated by the length of an arrowhead  100  and a region of maximum surface current is indicated by reference characters  102  and  103 . The surface current maximum occurs in the region  102  ( FIG. 10A ) when the feed and ground terminals are as illustrated in  FIGS. 3 and 4 . Note the near-field radiation reduction illustrated in  FIG. 10B , where the surface current maximum  103  occurs at the feed and ground terminals  88  and  90  on the radiating element  10 , as illustrated in  FIG. 5 .  
         [0049]     The “hand” or “body” effect is a known phenomenon that should be considered in the design of antennas for handheld communications devices. Although an antenna incorporated into such devices is designed and constructed to provide certain ideal performance characteristics, in fact all of the performance characteristics are influenced, some significantly, by the proximity of the user&#39;s hand or body to the antenna when the communications device is in use. When the hand of a person or another grounded object is placed close to the antenna, stray capacitances are formed between the effectively grounded object and the antenna. These capacitances can significantly detune the antenna, shifting the antenna resonant frequency (typically to a lower frequency) and can thereby reduce the received or transmitted signal strength. It is impossible to accurately predict and design the antenna to completely ameliorate these effects, as each user handles and holds the handset communications device differently.  
         [0050]     According to the teachings of the present invention, the hand effect is reduced due to the location of the feed and ground terminals  88  and  90  on the radiating element  79  as illustrated in  FIG. 5 . As illustrated in  FIG. 11A , a finger  119  of a user&#39;s hand  120 , when holding the handset  50  in the operational mode, is proximate the surface current maximum region  102 . For an antenna constructed according to the teachings of the present invention, i.e., as illustrated in  FIG. 5 , the surface current maximum occurs in the region  103  and the hand effect and the frequency detuning caused thereby is reduced. See  FIG. 11B .  
         [0051]     An antenna has been described as useful in a communications handset device. Specific applications and exemplary embodiments of the invention have been illustrated and discussed that provide a basis for practicing the invention in a variety of ways and in a variety of circuit structures. Numerous variations are possible within the scope of the invention. Features and elements associated with one or more of the described embodiments are not to be construed as required elements for all embodiments. The invention is limited only by the claims that follow.