Abstract:
A diversity antenna comprising two planar inverted F antennas (PIFAs) characterized by: two radiating elements with or without the physical separation between them; the spatially separable radiating elements of the two PIFAs with side-by-side or orthogonal placement with respect to each other are combined to form an equivalent single element consisting of the composite assembly of two radiators; a small ground plane of rectangular or L-shape with or without bending at its opposite ends is common to both the radiating elements; the radiating elements are placed above the unbent common ground plane; the radiating elements are placed above the vertical sections of the bent common ground plane; the shorted ends of the spatially separated radiating elements are placed back to back on the said common ground plane; a common shorting post placed along the common boundary line resulted by the merging of the two radiators with a prior side by side mutual placement; a common shorting post placed within the common boundary surface resulted by the merging of the two radiators with a prior mutual orthogonal orientation.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a diversity antenna system which includes two planar inverted F antennas which have a small common ground plane. Four embodiments of the invention are disclosed herein.  
           [0003]    2. Description of the Related Art  
           [0004]    In its simplest form, the diversity technique, as it applies to antennas for RF data and wireless communication devices, provides a means of achieving reliable and enhanced system performance through the use of an additional antenna. A diversity antenna system utilizes two antennas which sample the RF signal to determine the strongest signal to enable the communication device to utilize the strongest RF signal. To meet the requirement of sustained and fast rate of data transfer, specific emphasis has been recently placed on diversity antennas in RF data communication. Despite the enhanced reliability and the improved performance of an antenna system with the diversity scheme, its adoption to a compact wireless system is not widespread. Theoretically, the spatial diversity technique requires a physical separation of one wavelength between the two antennas. In many practical applications, it may not be feasible to provide the required separation between the two antennas of a spatial diversity scheme. The requirement of a wide separation between the two antennas of a diversity scheme also requires a longer feed cable to the individual antennas from a common RF source point. The resulting longer feed cable leads to the problem of ensuring effective shielding of the cable, the consequent RF power loss in the cable and the undesirable interference effect on system performance particularly at a higher frequency band. The above-mentioned shortcomings apply to diversity schemes consisting of conventional external antennas which have been in existence for a long time as well as with the recently evolving internal antenna. In view of the above constraints associated with the conventional diversity scheme, emphasis is being shifted to arrive at a compactness of the overall spatial diversity scheme which meets acceptable performance standards.  
           [0005]    Of late there has been an increasing emphasis on internal antennas instead of a conventional external wire antenna. The concept of internal antenna stems from the avoidance of a protruding external radiating element by the integration of the antenna into the device itself. Internal antennas have several advantageous features such as being less prone for external damage, a reduction in overall size of the handset with optimization, and easy portability. The printed circuit board of the communication device serves as the ground plane of the internal antenna. Among the various choices for internal antennas, the PIFA appears to have great promise. The PIFA is characterized by many distinguishing properties such as relative lightweight, ease of adaptation and integration into the device chassis, moderate range of bandwidth, Omni directional radiation patterns in orthogonal principal planes for vertical polarization, versatility for optimization, and multiple potential approaches for size reduction. Its sensitivity to both vertical and horizontal polarization is of immense practical importance in mobile cellular/RF data communication applications because of the absence of the fixed antenna orientation as well as the multi-path propagation conditions. All these features render the PIFA to be a good choice as an internal antenna for mobile cellular/RF data communication applications.  
           [0006]    The PIFA also finds useful applications in diversity schemes. Despite all of the desirable properties of a PIFA, the PIFA has the limitation of a rather large physical size for practical application. A conventional PIFA should have the semi-perimeter (sum of the length and the width) of its radiating element equal to one-quarter of a wavelength at the desired frequency. With the rapidly advancing size miniaturization of the radio communication devices, the space requirement of a conventional PIFA is a severe limitation for its practical utility. Further, the internal antenna technology is relatively new and is in an evolving stage of development. The combination of inherent shortcomings associated with the size of the PIFA and the requirement of even larger space or volume for multiple PIFAs seems to be the primary reason for the non-feasibility of the use of PIFA for diversity schemes of modern wireless communication systems.  
           [0007]    To assist in the understanding of a conventional PIFA, a conventional single band PIFA assembly is illustrated in FIGS. 9A and 9B. The PIFA  110  shown in FIG. 9A and FIG. 9B consists of a radiating element  101 , a ground plane  102 , a connector feed pin  104   a,  and a conductive post or pin  107 . A power feed hole  103  is located corresponding to the radiating element  101 . The connector feed pin  104   a  serves as a feed path for radio frequency (RF) power to the radiating element  101 . The connector feed pin  104   a  is inserted through the feed hole  103  from the bottom surface of the ground plane  102 . The connector feed pin  104   a  is electrically insulated from the ground plane  102  where the pin passes through the hole in the ground plane  102 . The connector feed pin  104   a  is electrically connected to the radiating element  101  at  105   a  with solder and the body of the feed connector  104   b  is electrically connected to the ground plane at  105   b  with solder. The connector feed pin  104   a  is electrically insulated from the body of the feed connector  104   b.  A through hole  106  is located corresponding Goto the radiating element  101 , with the conductive post or pin  107  being inserted through the hole  106 . The conductive post  107  serves as a short circuit between the radiating element  101  and the ground plane  102 , The conductive post  107  is electrically connected to the radiating element  101  at  108   a  with solder. The conductive post  107  is also electrically connected to the ground plane  102  at  108   b  with solder. The resonant frequency of the PIFA  110  is determined by the length (L) and width (W) of the radiating element  101  and is slightly affected by the locations of the feed pin  104   a  and the shorting pin  107 . The impedance match of the PIFA  110  is achieved by adjusting the diameter of the connector feed pin  104   a,  by adjusting the diameter of the conductive shorting post  107 , and by adjusting the separation distance between the connector feed pin  104   a  and the conductive shorting post  107 .  
         SUMMARY OF THE INVENTION  
         [0008]    In this invention, several new embodiments of compact diversity PIFAs having a small and common ground plane are disclosed. This invention demonstrates that it is possible to retain the performance of individual antennas of a spatial diversity antenna scheme even when the separation between the antennas is only a fraction of a wavelength. In the first embodiment of this invention, two PIFAs are placed back to back on a small rectangular ground plane. The two PIFAs are placed such that the shorted ends of the PIFAs face each other. Such an arrangement ensures better isolation between the two PIFAs despite being placed in close proximity to one another. In the second embodiment of this invention, the ground plane is bent at its opposite ends to form vertical sections. The two PIFAs are placed (outward) on the vertical sections at the opposite ends of the ground plane. Such an arrangement of PIFAs allows the placement of some system components between the two vertical sections of the bent ground plane. The distortion of the radiation patterns of the PIFAs is also minimized despite the presence of some components between the two PIFAs. This is mainly due to the blockage effect offered by the vertical sections of the ground plane. With a significantly different design configuration, in the third embodiment of this invention, there is no physical separation between the two PIFAs placed on a common rectangular ground plane. Only a single shorting pin or post partitions the two diversity PIFAs resulting in an extremely simple and compact diversity PIFA. The virtual electrical partitioning between the two radiating elements is realized through the common shorting post. The virtual electrical partitioning between the two radiating elements in lieu of the proposed choice of placement of the shorting post overcomes the need for physical separation between the two radiating elements to serve as separate antennas of a diversity scheme. In the fourth embodiment, which is a modification of the third embodiment, the two PlFAs, which are not physically separated, are placed on a common L-shaped ground plane. The partitioning of the two antennas is again realized through a common shorting post. Unlike the third embodiment, the two PIFAs of the fourth embodiment are oriented orthogonal to each other. The basic concepts proposed in all the embodiments of this invention have been proved through the design of diversity PIFAs for ISM Band applications. In all of the above-described embodiments, good VSWR performance is achieved. The individual PIFAs of the embodiments show satisfactory gain performance. The invention disclosed herein can be extended to other frequency bands of interest.  
           [0009]    One of the principal objects of the invention is to circumvent the requirement of wide separation between the two internal PIFAs of a spatial diversity scheme.  
           [0010]    A further object of the invention is to provide an efficient design of a diversity antenna utilizing only a small ground plane that is common for both the antennas.  
           [0011]    Still another object of the invention is to provide a compact diversity PIFA characterized with the salient feature of the absence of physical partitioning between the two antennas.  
           [0012]    Yet another object of the invention is to utilize the common ground plane of nonrectangular shapes in diversity PIFAs.  
           [0013]    Another object of the invention is to design individual PIFAs of a diversity antenna which are compact in size.  
           [0014]    Still another object of the invention is to provide diversity PIFAs having the desirable features of configuration simplicity, compact size, cost effective to manufacture and ease of fabrication.  
           [0015]    These and other objects will be apparent to those skilled in the art. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is an illustration of the design configuration of compact diversity PIFAs according to the first embodiment of the present invention;  
         [0017]    [0017]FIG. 1A is an isometric view of the compact diversity using PIFAs according to the first embodiment of the present invention;  
         [0018]    [0018]FIG. 1B is a top view of the design configuration of the compact diversity PIFAs according to the first embodiment of the present invention;  
         [0019]    [0019]FIG. 1C is a sectional view of the design configuration of the compact PIFAs taken along the line C-C′ of FIG. 1B;  
         [0020]    [0020]FIG. 2 is a frequency response chart that depicts the characteristics of the VSWR of the embodiment of FIG. 1;  
         [0021]    [0021]FIG. 2A is a frequency response chart that depicts the characteristics of the VSWR of the first PIFA (Port # 1 ) of the embodiment of FIG. 1;  
         [0022]    [0022]FIG. 2B is a frequency response chart that depicts the characteristics of the VSWR of the second PIFA (Port # 2 ) of the embodiment of FIG. 1;  
         [0023]    [0023]FIG. 3 is an illustration of the design configuration of compact diversity PIFAs according to the second embodiment of the present invention;  
         [0024]    [0024]FIG. 3A is an isometric view of the compact diversity PIFAs according to the second embodiment of the present invention;  
         [0025]    [0025]FIG. 3B is a top view as well as the end view of the second embodiment of the present invention;  
         [0026]    [0026]FIG. 3C is a side view of the second embodiment of FIG. 3B;  
         [0027]    [0027]FIG. 4 is a frequency response chart that depicts the characteristics of the VSWR of the embodiment of FIG. 3;  
         [0028]    [0028]FIG. 4A is a frequency response chart that depicts the characteristics of the VSWR of the first PIFA (Port # 1 ) of the embodiment of FIG. 3;  
         [0029]    [0029]FIG. 4B is a frequency response chart that depicts the characteristics of the VSWR of the second PIFA (Port # 2 ) of the embodiment FIG. 3;  
         [0030]    [0030]FIG. 5 is an illustration of the design configuration of a compact diversity PIFA according to the third embodiment of the present invention;  
         [0031]    [0031]FIG. 5A is an isometric view of the design configuration of compact diversity PIFAs according to the third embodiment of the present invention;  
         [0032]    [0032]FIG. 5B is a top view of the third embodiment of the present invention;  
         [0033]    [0033]FIG. 5C is a sectional view taken along the line C-C′ of FIG. 5B;  
         [0034]    [0034]FIG. 6 is a frequency response chart that depicts the characteristics of the VSWR of the embodiment FIG. 5;  
         [0035]    [0035]FIG. 6A is a frequency response chart that depicts the characteristics of the VSWR of the first PIFA (Port # 1 ) of the embodiment of FIG. 5;  
         [0036]    [0036]FIG. 6B is a frequency response chart that depicts the characteristics of the VSWR of the second PIFA (Port # 2 ) of the embodiment of FIG. 5;  
         [0037]    [0037]FIG. 7 is an illustration of the design configuration of compact diversity PIFAs according to the fourth embodiment of the present invention;  
         [0038]    [0038]FIG. 7A is an isometric view of the fourth embodiment of the present invention;  
         [0039]    [0039]FIG. 7B is a top view of the fourth embodiment of the present invention;  
         [0040]    [0040]FIG. 7C is an end view of the embodiment of FIG. 7B;  
         [0041]    [0041]FIG. 7D is another end view of the embodiment of FIG. 7B;  
         [0042]    [0042]FIG. 8 is a frequency response chart that depicts the characteristics of the VSWR of the embodiment of FIG. 7;  
         [0043]    [0043]FIG. 8A is a frequency response chart that depicts the characteristics of the VSWR of the first PIFA (Port # 1 ) of the embodiment of FIG. 7;  
         [0044]    [0044]FIG. 8B is a frequency response chart that depicts the characteristics of the VSWR of the second PIFA (Port # 2 ) of the embodiment of FIG. 7;  
         [0045]    [0045]FIG. 9A is a top view of a prior art single band PIFA; and  
         [0046]    [0046]FIG. 9B is a sectional view taken along the line B-B of FIG. 9A.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]    In the accompanying text describing the compact diversity PIFAs using a small and common ground plane covered under the first embodiment of this invention, refer to the FIGS.  1 A- 1 C for illustrations. The compact diversity PIFA antenna  10  includes two radiating elements  11  and  12  that are placed above the common and small ground plane  13 . The PIFA including radiating element  11  is designated as antenna  1 . A conducting post  14  connects the ground plane  13  and the radiating element  11  and serves as a short circuiting element. The conducting post  14  is connected to the radiating element  11  at  15   a  by solder and the conducting post  14  is also connected to the ground plane  13  at  15   b  by solder. A coaxial cable  16  serves as an electrical path for radio frequency (RF) power to the radiating element  11  is extended through a hole in the ground plane  13 , as seen in FIG. 1C. The inner conductor  16   a  of coaxial cable  16  forms a feed conductor and the top end of the feed conductor  16   a  is electrically connected to the radiating element  11  at  17   a.  The outer conductor  16   b  of the feed cable is connected to the ground plane  13  at  17   b.  The feed conductor  16   a  is insulated from the outer conductor  16   b  by means of an insulator of the RF cable. The bottom end of the feed conductor  16   a  of cable  16  is terminated with a SMA connector  16   c.  The connector  16   c  forms the Port # 1  of the diversity PIFA  10 . Radiating element  11  is bent 90° at  18  to form a vertical plane  11   a.  Vertical plane  11   a  forms the capacitive loading plate of the radiating element  11 . The capacitive loading element  11   a  is designed for lowering the resonant frequency of the radiating element  11  without increasing the size of the PIFA. The PIFA with the radiating element  11  explained above and illustrated in FIGS.  1 A- 1 C functions as a single band PIFA. The dimensions of the radiating element  11 , the length of the vertical plane  11   a,  the location of the shorting post  14 , the diameter of the shorting post  14 , and the relative position of the radiating element  11  on the common ground plane  13  are the prime parameters that control the resonant frequency of the radiating element  11  of the PIFA. The bandwidth of the single band PIFA with radiating element  11  is determined by: the location of the feed conductor  16   a,  the location of the shorting post  14 , the diameter of the shorting post  14  and the linear dimensions of the radiating element  11  including the height (distance between the radiating element and the ground plane) of the PIFA. The distance of separation between the radiating elements  11  and  12  is also an additional parameter of importance (for both the resonant frequency and bandwidth of the radiating element  11 ) since the close proximity of the two radiating elements  11  and  12  influence each other. The resonant frequency of the PIFA with the vertical capacitive loading section is lower than the resonant frequency of the PIFA with the radiating element  11  alone.  
         [0048]    The PIFA with the radiating element  12  is designated as antenna  2  of the diversity antenna  10 . A conducting post  19  connects the common ground plane  13  and the radiating element  12  and serves as a short circuiting element. Conducting post  19  is electrically connected to the radiating element  12  at  21   a  by solder and the conducting post  19  is electrically connected to the ground plane  13  at  21   b.  A coaxial cable  22  that serves as an electrical path for radio frequency (RF) power to the radiating element  12  is drawn through a hole in the ground plane  13 , as seen in FIG. 1C. The inner conductor  22   a  of coaxial cable  22  forms a feed conductor for the radiating element  12  and the top end of the feed conductor  22   a  is electrically connected to the radiating element  12  at  23   a.  The outer conductor  22   b  of the feed cable is electrically connected to the ground plane  13  at  23   b.  The feed conductor  22   a  is insulated from the outer conductor  22   b  through an insulator of the cable  22 . The bottom end of the feed conductor  22   a  of the RF cable  22  is terminated with a SMA connector  22   c.  The connector  22   c  forms the Port # 2  of the PIFA antenna  10 .  
         [0049]    The radiating element  12  is bent 90° at  24  to form a vertical plane  12   a.  The vertical plane  12   a  forms the capacitive loading plate of the radiating element  12 . The capacitive loading element  12   a  is designed for lowering the resonant frequency of the radiating element  12  without increasing the size of the PIFA. The PIFA configuration with radiating element  12  described above and shown in FIGS.  1 A- 1 C functions as a single band PIFA. The prime parameters that control the resonant frequency of the radiating element  12  of the PIFA are: the dimensions of the radiating element  12 , the length of the vertical plane  12   a,  the location of the shorting post  19 , the diameter of the shorting post  19 , and the relative position of the radiating element  12  on the common ground plane  13 . The bandwidth of the single band PIFA with the radiating element  12  is determined by: the location of the feed conductor  22   a  on the radiating element  12 , the location of the shorting post  19 , the diameter of the shorting post  19  and the linear dimensions of the radiating element  12  including the height of the PIFA. The distance of separation between the radiating elements  12  and  11  is also an additional parameter of importance (for both the resonant frequency and bandwidth of the radiating element  12 ) since the close proximity of the two radiating elements  11  and  12  influence each other. To achieve the overall size reduction of the diversity antenna, the distance between the radiating elements  11  and  12  has been decreased considerably. To overcome the shortcomings such as enhanced mutual coupling associated with the close placements of the radiating elements  11  and  12 , the shorted ends (edges) of the two radiating elements  11  and  12  are designed to face other. Based on the first embodiment of this invention, a compact schematic design for diversity PIFAs with a common and small ground plane has been developed for ISM band (2400-2500 MHz). The two separate PIFAs constituting the two antennas with Port # 1  and Port # 2  of the diversity antenna  10  according to the first embodiment of this invention have been designed and fabricated. The results of the tests conducted on the compact diversity antenna  10  comprising the PIFAs  1  and  2  illustrated in FIGS.  1 A- 1 C are shown in FIG. 2. The VSWR Characteristics of the first PIFA (with the radiating element  11  and RF input designated as Port # 1 ) are shown in FIG. 2A. Analogous to the first PIFA with input as Port # 1 , the VSWR characteristics of the second PIFA (with the radiating element  12  and RF input designated as Port # 2 ) are shown in FIG. 2B. As can be seen from the FIGS. 2A and 2B, good impedance match has been achieved for both the PIFAs of the diversity antenna  10  outlined in the first embodiment of this invention. The size of the common ground plane  13  is 18 mm (wide) and 42 mm (length). The projected semi-perimeter of the radiating elements  11  and  12  is 28 mm as compared to the semi-perimeter of 30.61 mm of a conventional PIFA radiating element without the capacitive loading feature. From the above description, it can be seen that a compact layout for a diversity scheme comprised of two PIFAs with separate input ports has been realized.  
         [0050]    In the accompanying text describing the diversity antenna  20  of PiFAs using a common and compact ground plane covered under the second embodiment of this invention, refer to the FIGS.  3 A- 3 C for illustrations. In the second embodiment of this invention, the compact diversity antenna  20  consists of a ground plane bent at the opposite ends which are situated along the direction of the length of the ground plane. As shown in FIGS.  3 A- 3 C, the common ground plane  13  is bent 100° down at  25  forming a vertical section  13   a  of the ground plane. Similarly the common ground plane  13  is also bent 100° down at  26  forming another vertical section  13   b  of the ground plane. In the diversity PIFA  20 , the first PIFA with the radiating element  11  is placed outwardly with respect to the vertical section  13   a  of the ground plane  13 . The radiating element  11  and the vertical section  13   a  of the ground plane  13  are separated by a predesired distance. Further in the diversity PIFA  20 , the second PIFA with the radiating element  12  is also placed outwardly with respect to the vertical section  13   b  of the ground plane  13 . Similar to the first PIFA, there exists a pre-desired distance of separation between the radiating element  12  and the vertical section  13   b  of the ground plane. All the other elements of the compact diversity antenna  20  consisting of the two PIFAs are similar to the diversity antenna  10  which has already been explained under the first embodiment of this invention and the further description of the diversity antenna  20  will therefore be omitted.  
         [0051]    The PIFA configuration with a radiating element  11  explained above and referred to in FIGS.  3 A- 3 C functions as a single band PIFA. The dimensions of the radiating element  11 , the length of the vertical plane  11   a,  the location of the shorting post  14 , the diameter of the shorting post  14 , and the relative position of the radiating element  11  on the vertical section  13   a  of the common ground plane  13  are the design parameters that control the resonant frequency of the radiating element  11  of the PIFA. The bandwidth of the first PIFA with the radiating element  11  is determined by: the location of the feed conductor  16   a,  the location of the shorting post  14 , the diameter of the shorting post  14  and the linear dimensions of the radiating element  11  including the height of the PIFA.  
         [0052]    Similar to the first PIFA (designated as antenna  1  with RF input Port # 1 ) with the radiating element  11  of FIGS.  3 A- 3 C, the second PIFA (designated as antenna  2  with RF input Port # 2 ) with the radiating element  12  also functions as a single band PIFA. The dimensions of the radiating element  12 , the length of the vertical plane  12   a,  the location of the shorting post  19 , the diameter of the shorting post  19 , and the relative position of the radiating element  12  on the vertical section  13   b  of the common ground plane  13  are the important factors that determine the resonant frequency of the radiating element  12  of the PIFA. The bandwidth of the second PIFA with radiating element  12  is determined by: the location of the feed conductor  22   a  on the radiating element  12 , the location of the shorting post  19 , the diameter of the shorting post  19  and the linear dimensions of the radiating element  12  including the height of the PIFA. The two separate compact PIFAs constituting the two antennas with Port # 1  and Port # 2  of the diversity antenna  20  according to the second embodiment of this invention have been designed and fabricated.  
         [0053]    Invoking the design concept enunciated under the second embodiment of this invention, compact diversity PIFAs with a small and common bent ground plane has been developed for ISM band (2400-2500 MHz). The results of the tests conducted on the compact diversity antenna  20  consisting of the two PIFAs shown in FIGS.  3 A- 3 C are illustrated in FIG. 4. The VSWR Characteristics of the first PIFA (with the radiating element  11  and designated RF Input Port # 1 ) are shown in FIG. 4A. Analogous to the first PIFA with input as Port # 1 , the VSWR characteristics of the second PIFA (with the radiating element  12  and designated RF Input Port # 2 ) are shown in FIG. 4B. As can be seen from the FIGS. 4A and 4B, a good impedance match has been obtained for both the PIFAs of the diversity antenna  20  described in the second embodiment of this invention. The size of the common ground plane is 17 mm (wide) and 30 mm (length). The projected semi perimeter of the radiating elements  11  and  12  is 28 mm as compared to the semi perimeter of 30.61 mm of a conventional PIFA radiating element without the capacitive loading feature. The significant advantage of the compact diversity antenna  20  of the second embodiment of this invention is the possibility for the placement of some of the system components between the vertical sections  13   a  and  13   b  of the ground plane  13 . Through the above illustrations and discussions, yet another novel compact layout for a diversity scheme comprising the two compact PIFAs with separate input ports has been realized with a small and common ground plane.  
         [0054]    In the diversity antennas  10  and  20  described under the first and second embodiments of this invention, the two PIFAs of a diversity antenna have their radiating elements physically separated from each other. The resulting improvement in isolation between the two RF input ports of the diversity antenna is primarily due to the physical separation between the radiating elements. From the configuration simplicity point of view as well from the fabrication ease consideration, it is always desirable to arrive at a structure of diversity PIFAs devoid of physical partitioning between the radiating elements of the respective PIFAs. The design concept of a single feed dual band PIFA without the physical partitioning of the original single band structure has been addressed by applicants in the paper [G. R. Kadambi et al., “A New Design Method for Single Feed Dual Band PIFA”, URSI symposium, Salt Lake City, 2000, pp. 221]. In the above-cited paper, through the selective choice of the shorting post on the PIFA structure, dual band PIFA operation has been realized without the physical partitioning of the structure. The proposed selective placement of the shorting post imparts the virtual electrical partitioning of the PIFA structure there by resulting in the dual resonance characteristics. The above concept of realizing the virtual electrical partitioning of the PIFA structure by a shorting post has been extended to the design of diversity PIFAs as explained in the subsequent embodiments of this invention.  
         [0055]    In the following text describing the compact diversity layout  30  of PIFAs using a small and common ground plane covered under the third embodiment of this invention, refer to the FIGS.  5 A- 5 C for illustrations. As shown in the FIGS.  5 A- 5 C, the two PIFAs with the radiating elements  11  and  12  exhibit no physical separation between them. Both the radiating elements are placed over a common ground plane  13 . The radiating elements  11  and  12  of the PIFAs merge (combine) together along a simple line contour A-A′. The line contour A-A′ also forms a common boundary to both the radiating elements  11  and  12 . A shorting post  14  placed along A-A′ serves as a common short-circuiting element to both the radiators  11  and  12 . The virtual electrical partitioning between the two radiating elements  11  and  12  in lieu of the proposed choice of placement of the shorting post  14  overcomes the need for physical separation between the two radiating elements to serve as separate antennas of a diversity scheme. The proposed choice of placement of the shorting post  14  circumvents the need for physical separation between the two radiating elements to serve as separate antennas of a diversity scheme. All the other elements of the diversity antenna  30  illustrated in the FIGS.  5 A- 5 C are similar to the diversity antennas  10 ,  20  of the first and second embodiments which have already been explained. Therefore further redundant detailed explanation of the diversity antenna  30  will not be provided to avoid the repetition.  
         [0056]    The PIFA configuration with a radiating element  11  illustrated in FIGS.  5 A- 5 C functions as a single band PIFA. The resonant frequency of the radiating element  11  of the PIFA depends on: The dimensions of the radiating element  11 , the length of the vertical plane  11   a,  the location of the shorting post  14 , the diameter of the shorting post  14 , and the relative position of the radiating element  11  on the common ground plane  13 . The parameters that determine the bandwidth of the single band PIFA with radiating element  11  are: the location of the feed conductor  16   a,  the location of the shorting post  14 , the diameter of the shorting post  14  and the linear dimensions of the radiating element  11  including the height of the PIFA. The resonance and the bandwidth characteristics of the first PIFA with the radiating element  11  are also significantly influenced by the second PIFA with the radiating element  12  because of the absence of physical separation between them. This also suggests an increased mutual coupling and reduced isolation between the two ports of a diversity scheme. However, the major advantage of the third embodiment of this invention is that the two PIFAs of the diversity antenna  30  can be fabricated as a single element resulting in the enhanced ease of fabrication. Similar to the PIFA with the radiating element  11  (designated as antenna  1  and RF input Port # 1 ) of FIGS.  5 A- 5 C, the PIFA with the radiating element  12  (designated as antenna  2  and RF input Port # 2 ) also functions as a single band PIFA. The dimensions of the radiating element  12 , the length of the vertical plane  12   a,  the location of the shorting post  14 , the diameter of the shorting post  14 , and the relative position of the radiating element  12  on the common ground plane  13  determine the resonant frequency of the radiating element  12  of the PIFA. The bandwidth of the single band PIFA with radiating element  12  is dependent on: the location of the feed conductor  22   a  on the radiating element  12 , the location of the shorting post  14 , the diameter of the shorting post  14  and the linear dimensions of the radiating element  12  including the height of the PIFA. To prove the novel design concept explained under the third embodiment of this invention, a compact schematic layout for diversity PIFAs with a common and compact ground plane has been developed for ISM band (2400-2500 MHz). The two separate compact PIFAs constituting the two antennas with Port # 1  and Port # 2  of the diversity antenna  30  according to the third embodiment of this invention have been designed and fabricated.  
         [0057]    The results of the tests conducted on the compact diversity antenna  30  consisting of the two PIFAs depicted in FIGS.  5 A- 5 C are shown in FIG. 6. The VSWR characteristics of the first PIFA (antenna  1  with the radiating element  11  and designated RF input as Port # 1 ) are shown in FIG. 6A. Analogous to the first PIFA (antenna  1  with the radiating element  11  and designated RF input as Port # 1 ), the VSWR characteristics of the second PIFA (antenna  2  with the radiating element  12  and designated RF input as Port # 2 ) are shown in FIG. 6B. As seen from the FIGS. 6A and 6B, good impedance match is evident for both the PIFAs of the diversity antenna  30  explained in the third embodiment of this invention. The size of the common ground plane is 16 mm (wide) and 42 mm (length). The projected semi perimeter of the radiating elements  11  and  12  is 28 mm as compared to the semi perimeter of 30.61 mm of a conventional PIFA radiating element without the capacitive loading feature. The single utmost advantage of the compact diversity antenna  30  covered under the third embodiment of this invention is equivalent emergence of the two PIFAs as a single element and the consequent ease of fabrication. Through the above illustrations, the proposed novel design concept of compact layout for a diversity scheme comprising the two PIFAs devoid of physical partitioning between them has been demonstrated.  
         [0058]    In the first three embodiments of the diversity PIFAs, a common feature is the rectangular shape of the common ground plane. However, in some system applications, the optimal utilization of the available volume for the diversity scheme with internal antennas (PIFAS) may warrant a choice of common ground plane of non-rectangular shapes. With such a design study in view, this invention extends the concept proposed in the third embodiment of this invention to include the case of a common ground of L-shape. The design of compact diversity PIFAs with radiating elements oriented orthogonal to each other and placed on a common ground plane of L-shape forms the thrust of the fourth embodiment of this invention. In the accompanying text describing the compact diversity antenna  40  including PIFAs using a small and common ground plane covered under the fourth embodiment of this invention, refer to the FIGS.  7 A- 7 D for illustrations. As illustrated in the FIGS.  7 A- 7 D, the two PIFAs with the radiating elements  11  and  12  exhibit no physical separation between them. The radiating elements of both the PIFAs are placed over a common ground plane  13  of L-shape. Similar to the diversity antenna  30  of the third embodiment, the two radiating elements  11  and  12  of the PIFAs in the compact diversity antenna  40  of the fourth embodiment of this invention also merge. In the case of diversity antenna  30 , the two radiating elements merge along a simple line contour A-A′ with the contour A-A′ also forming a common boundary to both the radiating elements  11  and  12  (FIG. 5B). In the diversity antenna  40  of fourth embodiment of this invention, the two radiating elements merge along a surface with contour A-A′-B-B′ with the surface contour A-A′-B-B′ forming a common boundary to both the radiating elements  11  and  12  (FIG. 7B). A shorting post  14  placed at the center of the common boundary serves as a common short circuiting element to both the radiators  11  and  12 . As stated previously while explaining the diversity antenna  30 , the virtual electrical partitioning between the two radiating elements  11  and  12  is realized through the common shorting post  14 . The virtual electrical partitioning between the two radiating elements  11  and  12  in lieu of the proposed choice of placement of the shorting post  14  overcomes the need for physical separation between the two radiating elements to serve as separate antennas of a diversity scheme. All the other elements of the diversity antenna  40  illustrated in the FIGS.  7 A- 7 D are similar to the diversity antennas  10 ,  20  and  30  of the earlier embodiments which have already been explained. Therefore further redundant detailed explanation of the diversity antenna  40  will not be attempted.  
         [0059]    The PIFA configuration with a radiating element  11  explained above and illustrated in FIGS.  7 A- 7 D functions as a single band PIFA. The dimensions of the radiating element  11 , the length of the vertical plane  11   a,  the location of the shorting post  14 , the diameter of the shorting post  14 , and the relative position of the radiating element  11  on the common ground plane  13  are the prime parameters that control the resonant frequency of the radiating element  11  of the PIFA. The bandwidth of the single band PIFA with radiating element  11  is determined by: the location of the feed conductor  16   a,  the location of the shorting post  14 , the diameter of the shorting post  14  and the linear dimensions of the radiating element  11  including the height of the PIFA. The resonance and the bandwidth characteristics of the first PIFA with the radiating element  11  are also significantly influenced by the second PIFA with the radiating element  12  because of the absence of physical separation between them there by suggesting an increased mutual coupling and reduced isolation between the two ports of a diversity scheme. The orthogonal orientation of the two PIFAs with respect to each other in the diversity antenna  40  helps to achieve relatively better isolation between the two ports as compared to the case of diversity antenna  30 . Similar to the case of the third embodiment, the two PIFAs of the diversity antenna  40  has the advantage of being amenable for fabrication as a single element resulting in the cost-effective manufacturing.  
         [0060]    Similar to the PIFA with the radiating element  11  (designated as antenna  1  and RF input Port # 1 ) of FIGS.  7 A- 7 D, the PIFA with the radiating element  12  (designated as antenna  2  and RF input Port # 2 ) also functions as a single band PIFA. The dimensions of the radiating element  12 , the length of the vertical plane  12   a,  the location of the shorting post  14 , the diameter of the shorting post  14 , and the relative position of the radiating element  12  on the common ground plane  13  are the prime parameters that control the resonant frequency of the radiating element  12  of the PIFA. The bandwidth of the single band PIFA with radiating element  12  is determined by: the location of the feed conductor  22   a  on the radiating element  12 , the location of the shorting post  14 , the diameter of the shorting post  14  and the linear dimensions of the radiating element  12  including the height of the PIFA. Based on the design concept explained under the fourth embodiment of this invention, a compact schematic design for diversity PIFAs with a compact and common ground plane of L-shape has been developed for ISM band (2400-2500 MHz). The two separate PIFAs constituting the two antennas with Port # 1  and Port # 2  of the diversity antenna  40  according to the fourth embodiment of this invention have been designed and fabricated. The results of the tests conducted on the compact diversity antenna  40  consisting of the two PIFAs depicted in FIGS.  7 A- 7 D are shown in FIG. 8. The VSWR Characteristics of the first PIFA (antenna  1  with the radiating element  11 ) with RF input designated as Port # 1  are shown in FIG. 8A. Analogous to the first PIFA (antenna  1  with the radiating element  11 ) with RF input as Port # 1 , the VSWR characteristics of the second PIFA (antenna  2  with the radiating element  12 ) with RF input designated as Port # 2  are shown in FIG. 8B. As depicted in the FIGS. 8A and 8B, good impedance match has been achieved for both the PIFAs of the diversity antenna  40  explained in the fourth embodiment of this invention. The size of the two sections forming the L-shaped common ground plane is 13 mm (wide) and 29 mm (length). The semi-perimeter of the common boundary A-A′-B-B′ is 18.5 mm and the projected semi-perimeter of the radiating elements  11  and  12  is 26.75 mm. The novelty of the diversity antenna  40  of the PIFAs is the distinct deviation adopted in the choice of the shape of the ground plane and the resulting orthogonal orientation of the radiating elements. The fore most advantage of the compact diversity antenna  40  covered under the fourth embodiment of this invention is equivalent emergence of the two PIFAs as a single element and the consequent ease of fabrication. Through the above illustrative typical case study, the proposed novel design concept of compact layout for a diversity scheme consisting of the two PIFAs oriented orthogonal to each other and devoid of physical partitioning between them has been demonstrated.  
         [0061]    As can be seen from the foregoing discussions, several novel schemes for the design of compact diversity antennas including PIFAs with a small and common ground plane have been developed and demonstrated. To achieve the overall compactness of the lay out of proposed diversity scheme, special emphasis is placed on the utilization of a small ground which is common to both the PIFAs. The concept of capacitive loading has been invoked in this invention to achieve the reduction in the resonant frequency of the PIFAs. The reduction in the resonant frequency is achieved without increasing the physical size of the PIFA. The absence of physical partitioning between the two PIFAs of the proposed schemes realize further compactness of the overall size of the diversity antenna. The diversity antenna  10 , the diversity antenna  20 , the diversity antenna  30  and the diversity antenna  40  are lightweight, compact and easy to manufacture. In the diversity antenna  30  as well as in the diversity antenna  40 , further configuration simplicity is evident because of the absence of physical separation between the PIFAs. In these schemes, the two PIFAs can be fabricated as a single element resulting in the further ease of fabrication. The novel design techniques of the compact diversity antenna consisting of the compact PIFAs of this invention have accomplished all of its stated objectives.