Abstract:
The present invention relates to a non-rectangular shaped PIFA capable of dual ISM band operation using a single power feed. The dual frequency operation of the PIFA is accomplished by using a slot in the radiating element to quasi partition the radiating element. The dual band performance of the PIFA is realized through the integration of either the microstrip or the Co Planar Waveguide (CPW) feed line to the antenna structure.

Description:
The present application claims the benefit of United States Provisional Patent Application No. 60/390,027 filed Jun. 18, 2002, titled DUAL BAND CIRCULAR PIFA WITH INTEGRATED FEED LINE, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to Planar Inverted F-Antenna (PIFA), and more particularly, PIFA antenna with non-conventional shapes and an integrated feed line on a ground plane: 
     BACKGROUND OF THE INVENTION 
     In wireless radio frequency (“RF”) data communications there is currently a shift in the requirement from the existing single band operation to dual industrial scientific medical (“ISM”) band operation covering, for example, frequency ranges of 2.4-2.5 to 5.15-5.35 GHz. Generally, dual ISM band operation can be accomplished using either external or internal antennas. External antennas are large and susceptible to mechanical damage. Conversely, internal antennas are unseen by the user, smaller, and less susceptible to mechanical damage. However, internal antenna are constrained in effectiveness because of the size and volume restrictions associated with wireless devices 
     In most of the devices, only specified regions with defined volume can accommodate the placement of internal antennas. These regions are usually not of perfect rectangular/square shape or of large size. At times, the available space for internal antennas nearly assumes a circular cylindrical shape of very small area and volume. For optimal performance of the internal antenna, it is desirable that the shape of the radiating structure of the antenna use as much of the allowed area as possible. Dual band ISM internal antenna, however, are generally rectangular in shape, which will be explained in connection with FIG. 9, below. Thus, it would be desirous to develop a non-conventionally shaped PIFA antenna to use more of the available space for internal antenna. 
     There seems to be no work reported on circular shaped either single or dual band PIFAs in open literature Wen-Hsiu Hsu and Kin-Lu Wong, “A Wideband Circular Patch Antenna”, MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Vol. 25, No. 5, Jun. 5, 2000 pp. 328 (hereinafter referred to as Hsu et al) reports a dual band microstrip antenna with a circular radiating element using an air-substrate. The dual frequency operation of the microstrip antenna of Hsu et al is realized through two separate linear slots. The two slots are placed symmetrically with respect to the central axis of the radiating element. The axis of the microstrip feed line is also parallel to the axes of the slots. 
     A dual frequency circular microstrip antenna with a pair of arc-shaped slots has been studied in Kin-Lu Wong and Gui-Bin Hsieh, “Dual-Frequency Circular Microstrip Antenna with a Pair of Arc-Shaped Slots”, MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Vol. 19, No. 6, Dec. 20 1998, pp. 410-412 (hereinafter referred to as Wong et al). The two arc-shaped slots are located on either side of one of the central axes. In the work of Wong et al, the two arc-shaped slots are also symmetrically placed with respect to the referred central axis of the antenna. 
     In both of the above research papers, the size of the radiating element corresponds to half wavelength at the center frequency of the lower resonant band. 
     Circular patch antennas also provide some insight into the present invention. The case studies of circular patches with a single arc or U-shaped slot are described in the work of K. M. Luk, Y. W. Lee, K. F Tong, and K. F. Lee, “Experimental studies of circular patches with slots”, IEEE Proc.—Microw. Antennas Propagation, Vol. 144, No. 6, December 1997, pp. 421-424 (hereinafter referred to as Luk et al). With a single arc shaped slot, the choice of center or offset feed determines the dual or single frequency operation. The choice of a U-shaped slot, as in the paper of Luk et al, results only in a single band operation with a wider impedance bandwidth. 
     Recently there has been a drastic increase in the demand for use of internal antennas in wireless applications. In a variety of options for internal antennas, PIFAs seems to have a greater potential. Apart from extensive utility of PIFA in commercial cellular communications, PIFA continues to find its usefulness in many other systems applications such as WLAN, the Internet, or the like The printed circuit board of the communication device serves as the ground plane of the internal antenna. 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, versatile for optimization, and multiple potential approaches for size reduction. Its sensitivity to both the vertical and horizontal polarization is of immense practical importance in wireless devices because of multi path propagation conditions. All these features render the PIFA to be as good a choice as any internal antenna for wireless device applications. When it comes to diversity schemes, PIFAs have a unique advantage because it can be fashioned into varieties of either Polarization or pattern Diversity schemes. 
     A conventional single band PIFA assembly is illustrated in FIGS. 9A and 9B. The PIFA  110  shown in FIG.  9 A 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 in radiation element corresponding to connector feed pin  104   a . Connector feed pin  104   a  serves as a feed path for RF power to the radiating element  101 . 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 point  105   a  with, for example; solder. The body of the feed connector  104   b  is electrically connected to the ground plane at point  105   b  with, for example, 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 in radiation element  101  corresponding to conductive post or pin  107 . Conductive post  107  is inserted through the hole  106 . The conductive post  107  serves as a short circuit between the radiating element  101  and ground plane  102 . The conductive post  107  is electrically connected to the radiating element  101  at point  108   a  with, for example, solder. The conductive post  107  is also electrically connected to the ground plane  102  at point,  108   b  with, for example, 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 . The fundamental limitation of the configuration of the PIFA  110  described in FIG.  9 A and FIG. 9B is the requirement of relatively large dimensions of length (L) and width (W) of the radiating element  101  to achieve desired resonant frequency band. This configuration is limited to only single operating frequency band applications. If PIFA was a dual band PIFA, a slot (not shown) would reside in radiating element  101  to quasi partition the radiating element  101 . 
     As represented by FIGS. 9A and 9B, the majority of PIFA designs focus on PIFA designs having a rectangular or square shape. Thus, it would be desirous to develop a compact dual ISM band internal PIFA having a non-conventional shapes. 
     SUMMARY OF THE INVENTION 
     This invention presents new schemes of designing circular shaped PIFAs with a small ground plane. Deviating distinctly from the routine and conventional feed structure usually employed in PIFA design, this invention also demonstrates that the RF feed line system can be integrated to the PIFAs. 
     To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, planar inverted F antennas are disclosed. The planar inverted F antennas include non-rectangular radiating elements residing on a dielectric carriage, which in turn resides on a ground plane A slot in the radiating element quasi partitions the radiating element. A feed pin, conducting post, and matching stub are used to feed power to the radiating element and tune the PIFA to the appropriate frequency. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the present invention will be apparent upon consideration of the following detailed description, taken in function with the accompanying drawings, in which like reference characters refer like parts throughout, and in which: 
     FIG. 1 is perspective view of a planar inverted F antenna illustrative of an embodiment of the present invention; 
     FIG. 2 is a frequency-response that depicts the characteristics of a particular PIFA constructed in accordance with an embodiment of the present invention; 
     FIGS. 3 a  and  3   b  are measured radiation patterns of the PIFA associated with FIG. 2 for RF frequencies of 2450 and 5250 MHz, respectively. 
     FIG. 4 is a perspective view of a planar inverted F antenna illustrative of another embodiment of the present invention; 
     FIG. 5 is a perspective view of a planar inverted F antenna illustrative of another embodiment of the present invention; 
     FIG. 6 is an exploded view of PIFA  120  associated with Figure 1; 
     FIG. 7 is an exploded view of PIFA  130  associated with FIG. 4; 
     FIG. 8 is an exploded view of PIFA  140  associated with FIG. 5 
     FIG. 9 a  is a top view of a prior art single band PIFA and 
     FIG. 9 b  is a sectional view the FIG. 9 a  prior art PIFA. 
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are now explained with reference to the drawings. While the present invention is explained with reference to certain shapes, such as “Horse Shoe, U- or L-shaped slot,” one of ordinary skill in the art will recognize on reading the disclosure that other shapes are possible, such as “C” shape, elliptical shape, bracket shape, or the like. 
     As mentioned above, some prior art designs provide some insight to the present invention. In particular, the following three publications related to prior art antennas are useful: Wen-Hsiu Hsu and Kin-Lu Wong, “A Wideband Circular Patch Antenna”, MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Vol. 25, No. 5, Jun. 5, 2000 pp. 328, Kin-Lu Wong and Gui-Bin Hsieh, “Dual-Frequency Circular Microstrip Antenna with a Pair of Arc-Shaped Slots”, MICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Vol. 19, No. 6, Dec. 20, 1998, pp.410-412, and K. M. Luk, Y. W. Lee, K. F. Tong, and K. F. Lee, “Experimental Studies Of Circular Patches With Slots”, IEEE Proc.—Microw. Antennas Propagation., Vol. 144, No. 6, December 1997, pp. 421-424. Hsu et al. and Wong et al., describe a microstrip antenna where the size of the radiating element corresponds to half wavelength at the center frequency of the lower resonant band. Unlike the Hus et al. and Wong et al. antennas, however, the present invention uses a single slot to yield dual frequency operation of circular PIFA. Further, because of the shorting post associated with the PIFA, the size of the radiating element of the circular PIFA of this invention corresponds only to quarter wavelength or less at the center frequency of the lower resonant band. 
     The present invention uses a U-shaped slot as in Luk et al However, the circular patch antenna of Luk et al. has single band operation with a wider Impedance bandwidth. The present invention employs a single slot to exhibit dual frequency operation. The dual frequency operation of the circular PIFA has been demonstrated with other slot shapes as well, such as, for example, a single arc shaped slot. Further, unlike Luk et al., the dual band operation of the circular PIFA of this invention has been accomplished with a radiating element of quarter wavelength in size corresponding to the mid frequency of the lower band. Finally, the present invention can use a relatively smaller ground plane, such as, for example ground planes ranging from sizes of 30 to 45 mm (L) by 25 to 30 mm (W) thereby accomplishing the compactness of the overall PIFA structure. 
     Referring specifically to FIGS. 1 and 6, a PIFA  120  illustrative of a first embodiment of the present invention is shown. PIFA  120  has a radio frequency (RF) power connector  1 , a ground plane  7 , a radiating element  8 , a dielectric carriage  10 , a slot  11 , a microstrip feed line  13 , and a printed circuit board (PCB)  16 . PCB  16  has a metallic region  17  and a non-metallic region  18 . Dielectric carriage  10  could be many types of dielectric material, such as, for example, an air gap, high density polyethylene, acrolonitrite butadiene styrene, polycarbonates, and the like. Generally, it has been found that dielectric materials with a dielectric contrast in the range of about 2.5 to about 3.5 work well Establishing PCB  16  with metallic and nonmetallic regions is largely a function of design choice. PIFA  120  resides on PCB  16  such that a portion of PIFA  120  is aligned with both metallic ( 17 ) and non-metallic ( 18 ) regions. PIFA.  120  is shown with a majority of the radiating element existing over non-metallic region  18 . It is possible to arrange PIFA  120  so more or less of the radiating element resides over non-metallic region  1 - 8 . Generally, PIFA  120  works better when more of the radiating element is over non-metallic region  18 . In PIFA  120 , while the microstrip feed  13  is on the bottom surface of the PCB  16 , the metallic region  17  is on the top-surface of PCB  16 . 
     While power connector  1  can be any number of equivalent connector, it has been found that a SMA connector is useful. The SMA connector has a center conductor  1   c  and outer conductors  1   a  and  1   b . As shown in FIG. 6, center conductor  1   c  is attached, such as by soldering, to a first end  2   a  of microstrip  13 . A second end  3   a  of microstrip  13  is attached, such as by soldering, to a feed pin  14 . Feed pin  14 , which extends through via holes in ground plane  7  and dielectric carriage  10  (via holes not specifically labeled but shown in FIG.  6 ), is connected to radiating element  8  to provide RF power. 
     Connector  1  generally also has outer conductors  1   a  and  1   b . Outer conductors  1   a  and  1   b  are attached, such as by soldering, to PCB  16 , such as at first solder point  5   c  and second solder point  5   d  are normally arranged such that they are symmetrical with respect to the central axis of the microstrip feed line  13  The locations of first solder point  5   c  and second solder point  5   d  are such that they are symmetrical with respect to the central axis of the microstrip feed line  13 . 
     As best seen in FIG. 6, and describing from PCB  16  to radiating element  8 , ground plane  7  resides on PCB  16  such that the feed via hole in ground plane  7  aligns with second end  3   a  of microstrip  13 . At least third solder point  5   a  and fourth solder point  5   b  connect ground plane  7  to PCB  16 . 
     Radiating element  8  contains slot  11 , a conducting post  15 , and a matching stub  9 . Slot  11 , which is a horse-shoe shaped slot, can be located in a number of locations to quasi partition radiating element  8 . Slot  11  is formed on the radiating element  8  by making a trace from a point located on the left hand side of feed pin  14  to a point positioned on the right hand side of conducting post  15 . In this case, slot  11  has an arc of about 270 degrees, but the arc could be from about 180 degrees to about 300 degrees depending on the placement of the feed pin and conducting post. Conducting post  15  is attached to radiating element  8  and extends through a via hole in dielectric carriage  10 . Conducting post  15  is connected to ground plane  7 , but not microstrip  13  (i.e., conducting post  15  is grounded). Matching stub  9  attached to radiating element  8  at  8   a  also extends along the outer sidewall of the dielectric carriage  10  without attaching to ground plane  7 . As one of skill in the art would recognize on reading the disclosure, the size, shape and placement of slot  11 , feed pin  14 , conducting post  15 , and matching stub  9  control the operation frequencies of the dual band ISM PIFA. In particular, controlling the arc radius of slot  11  (more or less arc radius) has a pronounced effect on the upper frequency of PIFA  120 . The lower frequency is generally tunable by varying the dimensions and placement of the matching stub  9 . The locations as well as the sizes of the conducting post  15  and feed pin  14  have small effects on resonant frequencies of PIFA  120 . FIGS. 2,  3   a  and  3   b  show plots of VSWR and gain of PIFA  120  with a radius of 7.5 mm and height of 7.5 mm. The radius and height can vary between 4 to 10 mm for radius and 4 to 8 mm for height. Also, the radius and height do not have to be equal. 
     Referring to FIGS. 4 and 7, a PIFA  130  illustrative of a second embodiment of the present invention is shown. PIFA  130  is similar to PIFA  120 , however, PIFA  130  has,an alternative slot design. As one of skill in the art would recognize on reading this disclosure, the circular PIFA can have many slot configurations and the slots shown in the figures are exemplary and non-limiting. 
     In particular, PIFA  130  has a connector  38 , a microstrip  35 , a PCB  34 , a ground plane  26 , a dielectric carriage  29 , a radiating element  27 , a slot  30 , a feed pin  36 , a conducting post  37 , a matching stub  28 . PCB  34  has a metallic region  32  and a non-metallic region  33 , PIFA  130  resides on PCB  34  such that a portion of PIFA  130  is aligned with both metallic ( 32 ) and non metallic ( 33 ) regions. PIFA  130  is shown with a majority of the radiating element existing over non-metallic region  33 . It is possible to arrange PIFA  130  so more or less of the radiating element resides over non-metallic region  33 . Generally, PIFA  130  works better when more of the radiating element is over non-metallic region  33 . 
     Referring to FIG.  7  and using an exemplary SMA connector for power connector  38 , a center conductor  20   c  is attached to a first end  21   a  of microstrip  35 . Outer conductors  20   a  and  20   b  are attached to PCB  34  at points  24   c  and  24   d . A second end  22   a  of microstrip  35  is attached, such as by soldering, to a feed pin  36 . Feed pin  36 , which extends through via holes in ground plane  26  and dielectric carriage  29  (via holes not specifically labeled but shown in. FIG.  7 ), is connected to radiating element  27  to provide RF power. 
     Outer conductors  20   a  and  20   b  are attached, such as by soldering, to PCB  34 , such as at first solder point  24   c  and second solder point  24   d . The locations of solder points  24   c  and  24   d  are such that they are symmetrical with respect to the central axis of the microstrip feed line  35 . 
     As best seen in FIG. 7, ground plane  26  resides on PCB  34  such that the feed via hole in ground plane  26  aligns with second end  22   a  of microstrip  35 . At least third solder point  24   a  and fourth solder point  24   b  connect ground plane  26  to PCB  34 . 
     Radiating element  27  contains slot  30 , a conducting post  37 , and a matching stub  28 . Slot  30 , which in this case is a is a “U” or bracket shaped slot, can be located in a number of locations to quasi partition radiating element  27 . Slot  30  is formed on the radiating element  27  such that the contour of the slot is positioned away from the center of the circular PIFA. The placement of the U-shaped slot is determined by the positions of feed and shorting posts. The length and the width of the U-shaped slot as well as its relative positions with respect to the locations of the feed/shorting posts are determined by the desired frequency tuning. In the embodiment shown, the line connecting the feed post and the shorting post is internal to the profile of the U-shaped slot. Conducting post  37  is attached to radiating element  27  and extends through a via hole in dielectric carriage  29 . Conducting post  37  is connected to ground plane  26 , but not microstrip  35  (i.e., conducting post  37  is grounded). Matching stub  28  attached to radiating element  27  at  27   a  also extends along the sidewall of the dielectric carriage  29  without attaching to ground plane  26 . As one of skill in the art would recognize on reading the disclosure, the size, shape and placement of slot  30 , feed pin  36  conducting post  37 , and matching stub  28  control the operation frequencies of the dual band ISM PIFA. In particular, controlling the placement and size of slot  30  has a pronounced effect on the upper resonant frequency of PIFA  130 . The lower resonant frequency is generally tunable by varying the dimensions and placement of the matching stub  28 . The locations as well as sizes of the conducting post  37  and feed pin  36  have small effects on resonant frequencies of PIFA  130 . The radius and height for PIFA  130  can vary between 4 to 10 mm for radius and 4 to 8 mm for height. Also, the radius and height do not have to be equal. 
     Referring now to FIGS. 5 and 8, PIFA  140  of a third embodiment of the present invention will be described. PIFA  140  is similar to PIFAs  120  and  130 . But unlike PIFAs  120  and  130 , PIFA  140  eliminates the via holes in the ground plane by strategic locations of the feed pin, shorting post and the choice of the Co Planar Waveguide (CPW) feed line instead of microstrip feed line, as explained below. 
     PIFA  140  comprises a connector  56 , a PCB  54 , CPW  55 , a radiating element  47 , a dielectric carriage  49 , and a ground plane  46 . PCB  54  contains a metallic region  52  an d a non-metallic region  53 . In this example, PIFA  140  resides on non-metallic region  53  of PCB  54 . The CPW  55 , thus, extends from the connector  56  over the metallic region  52  to the interface between the metallic region  52  and non-metallic region  53 . It would be possible to arrange PIFA  140  with portions over metallic region  52 . But in this configuration, it has been shown that PIFA  140  works better when it resides over the non-metallic portion of PCB  54 . 
     As shown best in FIG. 8, and again using the standard SMA connector for connector  56 , a center conductor  40   c  is attached to a first end  41   a  of CPW  55 . Outer conductors  40   a  and  40   b  of the RF connector  56  are attached to PCB  54  at first solder point  44   a  and second solder point  44   b . A second end  42   b  of CPW  55  is connected to feed strip  42 . Feed strip  42  extends along the sidewall of the dielectric carriage  49  and is connected to radiating element  47 . Because feed strip  42  extends along the sidewall of carriage dielectric  49 , the via holes in ground plane  46  and dielectric carriage  49  can be eliminated. Similarly, a conducting post  43  is attached to the radiating element  47 , extends along the sidewall of the dielectric carriage  49 , to be attached to ground plane  46 . A matching stub  48  also attached to radiating element  47  extends along the outer wall of the dielectric carriage  49 . The feed strip  42 , the conducting post  43  and the matching stub  48  are in flush with the sidewall of the dielectric carriage  49 . 
     Slot  40  is L-shaped. The segment of the L-shaped slot  40  with an opening or gap (open end) in the circumference of the radiating element forms the horizontal section of the L-slot. The axis of the horizontal section of the L-slot is perpendicular to the axis of the CPW  55 . The vertical section of the L-slot  40  has a closed end. The axis of the vertical section of the L-slot is parallel to the axis of the CPW  55 . As one of skill in the art would recognize on reading the disclosure, the size, shape, and placement of slot  40 , feed strip  42 , conducting post  43 , and matching stub  48  control the operation frequencies of the dual ISM band PIFA  140 . The radius and height for PIFA  140  can vary between 4 to 8 mm for radius and 4 to 8 mm for height. Also, the radius and height do not have to be equal. 
     While the invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those of ordinary skill in the art that various other changes in the form and details may be made without departing from the spirit and scope of the invention. Further, while particular configurations of the present invention have been illustrated and described, other configurations are possible.