Abstract:
A Planar Inverted F Antenna (PIFA) is disclosed comprising a radiating element and a ground plane positioned on a bottom cover. A Radome is positioned over the radiating element and the ground plane with the bottom cover and the Radome enclosing the radiating element and the ground plane. The ground plane is positioned below the radiating element and a conductive shorting strip extends between one end of the radiating element and one end of the ground plane. A feed lead extends from one side of the radiating element and has a base portion which protrudes outwardly of the Radome for connection to the center conductor of a RF power feeding cable. The radiating element includes a first horizontally disposed portion, a second horizontally disposed portion, and a substantially vertically disposed portion extending therebetween. The first substantially vertically disposed portion of the radiating element functions as a first capacitive loading plate with the second horizontally disposed portion of the radiating element functioning as a second capacitive loading plate. A dielectric block is positioned between the second horizontally disposed portion of the radiating element for providing dielectric loading to the radiating element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a Planar Inverted F Antenna (PIFA) and in particular to a method of designing a single band PIFA as an encapsulated module with a localized ground plane and multiple external lead contacts for easy integration to the chassis of a radio communication device. 
     2. Description of the Related Art 
     With the rapid progress in wireless communication technology and the ever-increasing emphasis for its expansion, wireless modems on laptop computers and other handheld radio devices will be a common feature. The technology using a short-range radio link to connect devices such as cellular handsets, laptop computers and other handheld devices has already been demonstrated [Wireless Design On-line Newsletter, Vol. 3, Issue 5, Nov. 22, 1999]. The ISM band (2.4-2.5 GHz) is the allocated frequency band for such applications. The performance of the antenna placed on devices like a cellular handset or a laptop computer is one of the critical parameters for the satisfactory operation of such a radio link. Therefore the performance characteristics of the antenna located on communication devices assumes significant importance in the evolving technology of wireless modems. 
     Recently, in the cellular communication industry, there has been an increasing emphasis on internal antennas instead of conventional external wire antennas. The concept of an 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 to external damage, a reduction in overall size of the handset with optimization, and easy portability. In most internal antenna designs, the printed circuit board of the communication device serves as the ground plane of the internal antenna. Among the various choices for internal antennas, a PIFA appears to have great promise. The PIFA is characterized by many distinguishing properties such as relative light weight, 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. The PIFA also finds useful applications in diversity schemes. Its sensitivity to both vertical and horizontal polarization is of immense practical importance in mobile cellular/RF data communication applications because of 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. 
     A conventional prior art single band PIFA assembly  100  is illustrated in FIGS. 9 and 10. The PIFA  100  shown in FIG. 9 and 10 consists of a radiating element  101 , a ground plane  102 , a power feed hole  103  is located corresponding to the radiating element  101 , a connector feed pin  104 , and a conductive post or pin  105 . The connector feed pin  104  serves as a feed path for radio frequency (RF) power to the radiating element  101 . The connector feed pin  104  is inserted through the feed hole  103  from the bottom surface of the ground plane  102 . The connector feed pin  104  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  is electrically connected to the radiating element  101  at  106  with solder. The body of the feed connector  107  is electrically connected to the ground plane  102  at  108  with solder. The connector feed pin  104  is electrically insulated from the body of the feed connector  107 . A through hole  109  is located corresponding to the radiating element  101 , and a conductive post or pin  110  is inserted through the hole  109 . The conductive post  110  serves as a short circuit between the radiating element  101  and the ground plane  102 , The conductive post  110  is electrically connected to the radiating element  101  at  111  with solder. The conductive post  110  is also electrically connected to the ground plane  102  at  112  with solder. The resonant frequency of the PIFA  100  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  and the conductive post or shorting pin  110 . The impedance match of the PIFA  100  is achieved by adjusting the diameter of the connector feed pin  104 , by adjusting the diameter of the conductive shorting post  110 , and by adjusting the separation distance between the connector feed pin  104  and the conductive shorting post  110 . 
     In the prior art techniques of PIFA design (Murch R. D. et al, U.S. Pat. No. 5,764,190; Korisch I. A., U.S. Pat. No. 5,926,139) the center conductor of the coaxial cable from the RF source is directly connected to the radiating element of the PIFA at the feed point. Further, in all these designs, the feed point of the PIFA is always drawn away from the shorted edge of the radiating element and is located within the central surface of the radiating element. Therefore, the feed cable from the RF source has to pass through the interior region (between the radiating element and the ground plane) of the PIFA. Such a prior art-feeding scheme of the PIFA will prove to be tedious and cumbersome in the final integration process. An alternative scheme of a PIFA design that circumvents such a tedious feed assembly is always desirable. From the structural and fabrication point of view, an avoidance of a feed cable extending through the interior region of the PIFA is preferred. This invention described hereinafter provides an encapsulated PIFA module in which the feed assembly is confined to the exterior of the module and hence overcomes the existing shortcomings in the final integration process of the prior art. 
     Keeping in pace with the rapid progress in mobile cellular communication technology, the future design of the cellular handset shall have the provision of more than one antenna to fulfill the additional requirement of BlueTooth (BT) applications. The placement of the additional internal antenna should be accomplished without necessitating any change in the overall size of the handset. The consideration of mutual coupling often warrants the placement of the cellular and BT antennas at different locations on the device chassis with a very small volume earmarked for the BT antenna. In cellular communication applications, multiple antennas may be required to utilize the phone chassis as a common ground plane. In such an application., the internal BT antenna will be an integral part of device chassis. Therefore such an additional internal antenna (for BT applications) such as a PIFA should have the desirable feature of simplified adaptability to the device chassis. A design of such an internal PIFA as a separate module with surface mountable features will be of great importance to facilitate a much simplified integration process. 
     SUMMARY OF THE INVENTION 
     A compact, lightweight, single band PIFA has been designed in an encapsulated modular form. The present invention emphasizes the feed assembly of the PIFA confined only to the exterior of the module. In the instant invention, one of the external leads of the encapsulated PIFA module facilitates the connection of the feed point of the PIFA to the RF source point of the radio device. The localized ground plane of the PIFA and the ground potential of the chassis of the radio device are connected by the other external leads. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a perspective view of a cellular telephone handset having the micro-internal antenna of this invention mounted therein; 
     FIG. 2 is a perspective view of the antenna of this invention mounted on a chassis; 
     FIG. 3 is a partial exploded perspective view of the first embodiment of the antenna of this invention; 
     FIG. 4 is a partial perspective view of the antenna of FIG. 3 without the Radome; 
     FIG. 5 is a frequency response chart that depicts the characteristics of the VSWR of the antenna of FIG. 4; 
     FIG. 6 is a perspective view of a second embodiment of the invention; 
     FIG. 7 is an exploded perspective view of the antenna of FIG. 6; 
     FIG. 8 is a frequency response chart that depicts the characteristics of the VSWR of the antenna of FIG. 6; 
     FIG. 9 is a top view of a prior art antenna; and 
     FIG. 10 is a partial sectional view as seen on lines  10 — 10  of FIG.  9 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the numeral  8  refers to a conventional cellular telephone handset including a chassis  9 . In the accompanying text, the numeral  10  refers to the first embodiment of an encapsulated single band PIFA, as seen in FIGS. 2-4. The PIFA  10  includes a radiating element  11  that is located above a ground plane  12 . An external metallic lead  14 , which is a feed tab of the PIFA, serves as an electrical path for radio frequency (RF) power to the radiating element  11 . The feed tab or lead  14  is electrically insulated from the local ground plane  12  by means of the notch  15  formed in the ground plane  12 . The notch  15  formed in the ground plane  12  of the PIFA  10  is such that the feed tab  14  does not touch the ground plane  12 . The feed tab  14  is also electrically insulated from the chassis  9  of the device by means of the notch  16  formed in the device chassis  9 . The location and the size of the notch  16  on the device chassis  9  are such that the base  17  of the feed tab  14  does not touch the device chassis  9  (FIG.  2 ). The notch  16  on the device chassis  9  is realized by the removal of the metallization of the chassis over the area underlying the base  17  of the feed tab  14 . The top end of the feed tab  14  is electrically connected to the radiating element  11  at  18 . A conductive strip  19  serves as a short circuit between the radiating element  11  and ground plane  12 . The conductive strip  19  is electrically connected to the radiating element  11  at  20  arid is electrically connected to the ground plane  12  at  21 . The radiating element  11  is bent 90° at  22  to form a vertical plane  23 . The vertical plane  23  is again bent 90° at  24  to form a lower horizontal plane  25 . The horizontal plane  25  is at a specific distance above the ground plane  12 . The horizontal plane  25  serves a capacitive loading plate for the radiating element  11 . The Radome  26 , which encapsulates the PIFA  10 , includes two separate parts with identical dielectric material property. The top cover  27  of the Radome  26  fully encloses the radiating element  11  and the local ground plane  12  of the PIFA  10 . The top cover  27  of the Radome  26  is designed to have a combination of a flat planar contour  28  and an inclined planar contour  29  resulting in a wedge shaped geometry along  30 . The surface of the top cover  27  of the Radome  26  with flat planar contour  28  is flush with the unbent portion of the radiating element  11 . The surface of the top cover  27  with an inclined planar contour  29  is designed so as to enclose the vertical section  23  and lower horizontal section  25  of the radiating element  11 . The bottom cover  31  of the Radome  26  comprises a flat surface designed to be in flush with the lower surface of the ground plane  12  of the PIFA. The bottom cover  31  of the Radome  26 , the ground plane  12  of the PIFA  10 , the radiating element  11  of the PIFA and the top cover  27  of the Radome  26  are held together at specified height and locations through the two supporting dielectric blocks  32  and  33 . The supporting dielectric block  32  connects the bottom cover  31  and the top cover  27  of the Radome  26  at  34  and  35 , respectively, The supporting dielectric block  32 , while connecting the bottom cover  31  and top cover  27 , passes through a close fitting hole  36  on the ground plane  12  as well as a close fitting hole  37  on the radiating element  11 . The supporting dielectric block  33  holds the lower horizontal section  25  of the radiating element  11  at a predetermined height from the ground plane  12  . The supporting dielectric block  33  with base  38  on the bottom cover  31  passes through a close fit hole  39  on the ground plane  12  and extends vertically up to touch the lower horizontal section  25  of the radiating element  11 . 
     The integration of the encapsulated module of the PIFA  10  to the device chassis  9  is carried out in two steps (FIG.  4 ). In the first step, the PIFA module is placed at the desired location on the device chassis  9  and the external metallic tabs  40  and  41  of the PIFA module are connected to the device chassis  9  at  42  and  43  by solder. In the second step, the center conductor  44  of the RF input cable  45  is connected to the base  17  of the external feed tab  14  at  46 . The outer conductor  47  of the RF input cable  45  is soldered at numerous pre-selected locations on the device chassis  9  to prevent any radiation from the cable. The inner conductor  44  and the outer conductor  47  of the cable  45  are separated from the insulator  48  of the cable  45 . 
     The PIFA  10  configuration illustrated in FIGS. 2-4 functions as an encapsulated single band PIFA. The dimensions of the radiating element  11 , the vertical plane  23 , the lower horizontal plane  25 , the location of the shorting strip  19 , the width of the shorting strip  19 , the material property of the Radome  26  and the relative position of the PIFA  10  on the device chassis  9  are the prime parameters that control the resonant frequency of the PIFA. The bandwidth of the single band PIFA  10  is determined by width of the feed tab  14 , the location of the feed tab  14 , the location of the shorting strip  13 , the width of the shorting strip  19 , the material property of the Radome  26 , and the linear dimensions of the radiating element  11  including the height of the PIFA. The measured resonant frequency is lower than the resonant frequency of the PIFA with only the radiating element  11  alone. The lowering of the resonant frequency of the PIFA  10  is due to the capacitive loading offered by the vertical plane  23  and lower horizontal plane  25 . Further reduction of the resonant frequency is due to the dielectric loading caused by the encapsulation of the entire PIFA  10  within Radome  26 . 
     In its final configuration ready for the integration (FIGS.  2  and  4 ), the encapsulated PIFA  10  module will have three external leads protruding out of the Radome  17 . The RF power input cable  45  is easily assembled to the PIFA module by connecting the center conductor  44  of the cable  45  to the protruding base  17  of the feed tab  14  through a solder connection (FIG.  2 ). The PIFA  10  module can easily be adapted to the device by connecting the external tabs  40  and  41  to the device chassis  9  at  42  and  43 , respectively, by solder (FIG.  4 ). Thus, the proposed modular design of PIFA  10  of this invention greatly simplifies the task of integration of the PIFA to the device. Further, it can easily be inferred that the design of the PIFA  10  module has the distinct advantage of feed assembly which is confined only to the exterior dimensions of the module. The suggested modular design of this invention circumvents the hitherto imposed shortcoming of the feed assembly (cable) passing through the interior region of the PIFA. The result of the tests conducted on the single band PIFA  10 , illustrated in FIGS. 2-4, referred to as the first embodiment of this invention, is shown in FIG.  5 . FIG. 5 illustrates the VSWR plot of the single band PIFA  10  resonating in the ISM band (2400-2500 MHz). The dimensions of the single band PIFA  10  are: Length=16 mm, Width=5.5 mm and Maximum Height=4.5 mm. The projected semi-perimeter of the single band PIFA  10  is 21.5 mm as compared to the semi-perimeter of 30.61 mm of a conventional single band PIFA  110  resonating in the ISM band. 
     The second embodiment of the invention is illustrated in FIGS. 6 and 7. The single band PIFA  50  illustrated in FIGS. 6 and 7 is similar to the PIFA  10 , but has an additional slot  51  formed in the radiating element  11  (FIG.  7 ). Further, there is a dielectric block  52  of pre-desired dielectric constant placed between the lower horizontal section  25  and the ground plane  12 . The supporting block  33  passes through a tight fit hole  53  on the dielectric block  52  in addition to passing through the tight fit hole  39  on the ground plane  12 . Also, the external leads  40  and  41  of PIFA  50 , for connecting the ground plane  12  of the PIFA  10  to the device chassis  9 , are absent. Therefore, the ground plane  12  of the PIFA  50  module is not connected to the ground potential of the device chassis  9  resulting in the physical isolation of the PIFA  50  from the device chassis  9 . As a consequence, the effective size of the ground plane for the optimum performance of the PIFA  50  is merely the size of the localized ground plane  12  itself. This is in contrast to the relatively large effective ground plane for the PIFA  10  of the first embodiment of this invention where the localized ground plane  12  of the PIFA  110  is directly connected to the device chassis  9 . Therefore, the significant feature of the design of PIFA  50  is the extremely small size of the ground plane  12 . In actuality, the size of the ground plane  12  is comparable to the linear dimensions of the radiating element  11  of the PIFA  50 . The size of the ground plane  12  has significant effect on the resonance characteristics and the gain performance of the PIFA. To achieve the resonance in the ISM band despite the miniaturization both in size of the PIFA  50  and the size of the ground plane  12 , the dielectric loading of the PIFA  20  has also been incorporated through the dielectric block  52 . Provision has been made for connecting the outer conductor  47  of the RF input cable  45  to the external tab  54  to offer a ground potential to the PIFA  50 . The external tab  54  is a protrusion of the ground plane  12  of the PIFA  50 . All the other elements of the single band PIFA  50  illustrated in FIGS. 6 and 7 are identical to the single band PIFA  10  illustrated in FIGS. 2-4 which has already been explained while covering the first embodiment of this invention. Further redundant explanation of the single band PIFA  50  illustrated in FIGS. 6 and 7 will therefore be omitted. 
     The slot  51  is positioned between the vertical plane  23  and the shorting strip  19  and is located corresponding to a position on the radiating element  11  of the PIFA  50  as illustrated in FIG.  7 . The choice of the location of the slot  51  illustrated in FIG. 7 has been with a specific purpose to offer reactive loading effect to the radiating element  11 . Hence the size and position of the slot  51  will control the resonant frequency of the PIFA  50 . In its final configuration ready for the integration (FIGS.  6  and  7 ), the encapsulated PIFA  50  module will have two external leads protruding out of the Radome  26 . The RF power input cable  45  is easily assembled to the PIFA module by connecting the center conductor  44  of the cable  45  to the protruding base  17  of the feed tab  14  through a solder connection (FIG.  7 ). The shield (outer conductor)  47  of the cable  45  is soldered to the protruding external tab  54 . From this, it can easily be inferred that the design of the PIFA  50  module has the distinct advantage of feed assembly, which is confined only to the exterior dimensions of the module. The suggested modular design of this invention circumvents the hitherto imposed shortcoming of the feed assembly (feed cable) passing through the interior region of the PIFA. The result of the tests conducted on the single band PIFA  50  illustrated in FIGS. 6 and 7 referred to as the second embodiment of this invention is shown in FIG.  8 . FIG. 8 illustrates the VSWR plot of the single feed multi-band PIFA  50  resonating in the ISM band (2400-2500 MHz). The dimensions of the single band PIFA  50  are: Length=16 mm, Width=5.5 mm and Maximum Height=4.5 mm. The projected semi-perimeter of the multi-band PIFA  50  is 21.5 mm as compared to the semi-perimeter of 30.61 mm of a conventional single band PIFA  110  resonating in the ISM band only. 
     As can be seen from the foregoing discussions, a novel scheme to design a single band PIFA in a modular form has been proposed and demonstrated. The suggested design of the PIFA in a modular form has the distinct advantage and the desirable feature of easy and much simplified integration to the device chassis. In the PIFA designs of this invention, the feed assembly is confined only to the exterior of the module resulting in enhanced fabrication ease. The proposed design also overcomes the tedious feed assembly of the prior art techniques of the PIFA design. The radiating element, the shorting strip, the feed tab, and the ground plane of the PIFA are so configured to facilitate the formation of the PIFA in one process of continues and sequential bending of a single sheet of metal resulting in improved manufacturability. The resonance of the PIFA in ISM band has been achieved without increasing the effective area of antenna, thereby accomplishing the miniaturization of the size of the PIFA. The concept of the slot loading technique and the partial dielectric loading has also been invoked in this invention to achieve the reduction of resonant frequency of the PIFA without increasing the size of the PIFA. The concept of partial dielectric loading involving the dielectric block over a small and selective area of the PIFA reduces the weight and cost of the PIFA. The partial dielectric loading also results in a relative reduction of the dielectric loss and hence contributes to the enhanced radiation efficiency of the PIFA. The encapsulated single band PIFA  10  and PIFA  50  as of this invention are lightweight, compact, cost-effective and easy to manufacture. 
     Thus the novel design technique of encapsulated single band PIFA in a modular form of this invention has accomplished at least all of its stated objectives.