Abstract:
A low profile Planar Inverted-F Antenna (PIFA) comprises a radiating strip, an inductive tuning portion, a vertical feed portion, and a retracted ground plane. The radiating strip is approximately parallel to the ground plane and is suspended above the ground plane by the feed element at a certain distance. Further, the radiating strip, in part or entirely, overhangs the ground plane. In this way, the radiating strip may be suspended very close to the ground plane, but yet exhibits a large bandwidth.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/781,739 filed Mar. 14, 2006, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to antennas and more specifically to a Planar Inverted F-Antenna. 
     BACKGROUND OF THE INVENTION 
     Planar inverted F-antenna (PIFA) has many advantages. It is easily fabricated, simple by design, and cost little to manufacture. Today, the PIFA is widely used in small communication devices such as personal digital assistants and mobile phones. Its popularity is due to its compact size that makes it easy to integrate into a device&#39;s housing, yielding a concealed antenna. PIFA also offers an additional advantage over monopole or whip antenna in terms of radiation exposure. For example, in a mobile phone, a whip antenna has an omnidirectional radiation field, whereas a PIFA has a relatively small radiation field toward the user. Thus making the PIFA more favorable for the health conscious consumers. 
       FIG. 1  illustrates a conventional PIFA  100 . PIFA  100  consists of a ground plane  105 , a radiating element  110 , a feed element  115 , and a shorting or tuning element  120 . PIFA  100  is generally produced on a printed circuit board with ground plane  105  formed thereon. Feed element  115  supplies radio frequency (RF) signals to radiating element  110  which is held substantially parallel to ground plane  105  at a certain distance  125 . The operating frequency or the resonance frequency of the PIFA may be controlled by controlling the size (width or length) of shorting element  120  and the dimensional ratio of radiating element  110 . However, these frequency tuning techniques are less desirable because it may require the relocation of the shorting pin and the redesign of the IC board (not shown). 
     Impedance bandwidth is another important factor one must consider when designing a PIFA. Generally, a PIFA&#39;s bandwidth may be controlled by capacitive or dielectric loading means such as adding a parasitic shorted patch. The added parasitic shorted patch helps increase the impedance bandwidth because it introduces an additional resonant mode to the PIFA&#39;s resonance frequency band, thus creating dual-resonance band PIFA. However, these techniques increase the size and complexity of the antenna which lead to higher cost. In general, the most frequently used technique for increasing a PIFA&#39;s impedance bandwidth is to increase the height between radiating element  100  and ground plane  105 , such as height  125  in PIFA  100 . However, this technique is subjected to the size constraint of the antenna package; thus making it very difficult to increase the PIFA&#39;s bandwidth without increasing the PIFA&#39;s footprint. 
     Accordingly, what is needed is a PIFA where both the resonance frequency and the impedance bandwidth can be controlled and improved without increasing the size of the PIFA and its manufacturing cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The present invention is described with reference to the accompanying drawings. 
         FIG. 1  illustrates a conventional PIFA. 
         FIG. 2  illustrates, in isometric view, an exemplary embodiment of a PIFA according to an embodiment of the present invention. 
         FIG. 3A  illustrates, in isometric view, another exemplary embodiment of a PIFA according to an embodiment of the present invention. 
         FIG. 3B  illustrates a magnified view of a portion of the PIFA shown in  FIG. 3A . 
         FIG. 4  illustrates a top view of the PIFA in  FIG. 3A . 
         FIG. 5  illustrates, in isometric view, an exemplary embodiment of a PIFA according to an embodiment of the present invention. 
         FIG. 6  illustrates a top view of the PIFA in  FIG. 5 . 
         FIG. 7  illustrates, in isometric view, another exemplary embodiment of a PIFA according to an embodiment of the present invention. 
         FIG. 8  illustrates yet another embodiment of a PIFA according to an embodiment of the present invention. 
         FIG. 9  illustrates a detailed view of an antenna portion of the PIFA illustrated in  FIG. 8 . 
     
    
    
     The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the reference number. 
     DETAILED DESCRIPTION OF THE INVENTION 
     This specification discloses one or more embodiments that incorporate the features of this invention. The embodiment(s) described, and references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. An embodiment of the present invention is now described. While specific methods and configurations are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the art will recognize that other configurations and procedures may be used without departing from the spirit and scope of the invention. 
     Generally, a PIFA such as PIFA  100  has the ability to send and receive electromagnetic signals in both vertical and horizontal polarized fields. For this reason, PIFA usage in mobile phones has been very popular. On a high level, PIFA  100  sends and receives electromagnetic radiation by taking advantage of its natural resonance frequency. PIFA&#39;s  100  resonance frequency can be modified by adjusting the dimension and shape of radiating element  110  or by moving the location of feed element  115  with respect to tuning element  120 . Further, the resonance frequency of PIFA  100  can also be slightly adjusted by modifying the width and height of shorting or tuning element  120 . 
     As shown in  FIG. 1 , PIFA  100  resonance or operating frequency is fixed by the shape, location, and size of radiating element  110 , feed element  115 , and tuning element  120 , respectively. To this end, the FR4 substrate or the circuit board (not shown) in which PIFA  100  is formed thereon must be specifically designed for PIFA  100 . For example, a hole must be formed in the circuit board underneath ground plane  105  at a certain location where feed element  115  is to be connected to a coaxial feed line (not shown). Similarly, the location of landing areas  135  and  140  must be taken into account when designing and fabricating the circuit board. Thus, from a manufacturing and designing perspective, it is impractical and expensive to re-tune PIFA  100  to a resonance frequency that is outside of its original design. Further, to improve the impedance bandwidth of PIFA  100 , height  125  must be made larger. However, an increase in height  125  leads to an undesirable size increase of the overall antenna package size. 
     The present invention incorporates a PIFA design where the impedance bandwidth can be improved without increasing the size of the antenna package. Additionally, the frequency tuning process can be easily done without the need to relocate the feed location and/or redesign the circuit board. 
       FIG. 2  illustrates a PIFA  200  according to an embodiment of the present invention. PIFA  200  includes a ground plane  205  formed on a substrate  230 , a radiating element  210 , a feed element  215 , and a tuning or shorting element  220 . Tuning element  220  is coupled to a landing surface  235  that is electrically coupled to ground plane  205 . In an embodiment, tuning element  220  is L-shaped with one of the legs coupled to surface  235  and the other leg coupled to feed element  215 . In this way, PIFA  200  may be tuned simply by changing the height of the tuning element  220  without increasing the height of the overall PIFA profile. Specifically, the height or length of a leg portion  260  of tuning element  220  may be increased or decreased. By varying the height of tuning element  220 , the current path length from surface  235  to surface  240  and to feed element  215  is varied. In this manner, the inductive characteristic of PIFA  200  is affected thus allowing PIFA  200  to be tuned. 
     In an alternative embodiment, tuning element  220  is U-shaped (or V-shaped), with one of the legs coupled to surface  235  and the other coupled to surface  240 . Although L and U shapes are described, other shapes could also be used to increase the current path length as would be understood by one skilled in the art. 
     In PIFA  200 , feed element  215  is coupled to a surface  240 . Surface  240  is electrically isolated from ground plane  205 . Although not shown, feed element  215  is coupled to a coaxial feed line underneath ground plane  205  and substrate  230 . The coaxial feed line provides radio frequency (RF) signals to the feed element which in turns feeds RF signals to radiating element  210 . In an alternative embodiment, feed element  215  is coupled to a microstrip line, embedded microstrip line, slotline, or coplanar line located on the same layer or a layer below of feed element  215 . 
     Radiating element  210  is suspended above substrate  230  by feed element  215  at a certain distance  225 . For example, in one embodiment, radiating element  210  is suspended in parallel with substrate  230 . In general, the impedance bandwidth of PIFA  200  may be affected by varying distance  225 . Up to a certain height threshold, an increase in distance  225  corresponds to an increase in the impedance bandwidth of PIFA  200 . However, this technique is disadvantageous because it increases the overall antenna package size. Alternatively, PIFA  200  may be capacitively or dielectrically loaded. These techniques are also disadvantageous because they add complexity and cost to the PIFA. In PIFA  200 , the impedance bandwidth is increased by suspending radiating element  210  such that an edge  245  of radiating element  210  extends pass an edge  250  of ground plane  205 . In other words, ground plane  205  is retracted with respect to substrate  230  and/or radiating element  210 . Further, from a different perspective, edge  245  falls outside of a perimeter image of ground plane  205 , if such an image is projected onto the same horizontal plane of radiating element  210 . 
     From yet another perspective, a portion of the perimeter of radiating element  210  overhangs edge  250  of ground plane  205  if such perimeter portion is projected onto ground plane  205  horizontal plane. Stated another way, a portion of radiating element  210  is above ground plane  205  and a portion is above substrate  230 . In this way, PIFA  200  impedance bandwidth is increased because a portion of radiating element  205  is further away from ground plane  205  as compared to when radiating element  205  is fully inside of ground plane&#39;s  205  perimeter. In an alternative embodiment, the radiating element  210  is suspended such that substantially all of radiating element  210  falls outside of ground plane  205  perimeter&#39;s projection. In other words, radiating element  210  is not directly below or above ground plane  205 . Additionally, ground plane  205  may be sandwiched between substrate  230  and a dielectric layer (not shown) formed on top of ground plane  205 . 
     As illustrated in  FIG. 2 , PIFA  200  may be tuned simply by replacing tuning element  220  with a smaller or larger tuning element. For example, the length of leg portions  255  and  260  of tuning element  220  may be increased to affect the current path. In this way, the positional change of feed element  215  is simulated without having to actually reposition feed element  215  and surface  240  with respect to tuning element  220 . Even though tuning element  220  is shown to have a “L” shape, other shapes could also be used to increase the current path as would be understood by one skilled in the art. 
       FIG. 3A , illustrates a PIFA  300  according to an embodiment of the present invention. PIFA  300  includes a retracted ground plane  305  and a retracted substrate  330  that corresponds to ground plane  305 . Ground plane  305  and substrate  330  are horizontally retracted with respect to radiating element  310 . In this way, an edge or portion  345  of radiating element  310  is not directly above a surface of ground plane  305 , and also is not above substrate  330 . In PIFA  300 , radiating element  310  is C-shaped. In this configuration, PIFA  300  may be made smaller while radiating element  310  still has a sizeable surface area. Further, retracted ground plane  305  and substrate  330  have a boundary line  350  that tracks along the general shape of radiating element  310  along boundary line  350 . Further, PIFA  300  impedance bandwidth is increased because radiating element  310  tracks boundary line or edge  350 . 
     As shown in  FIG. 3B , feed element  315  in PIFA  300  is shaped like the letter “U”. More specifically, feed element  315  shapes like an unbalanced “U”. The bottom feed element  315  is coupled to surface  340  and to a coaxial feed line (not shown). The longer leg of feed element  315  is coupled to radiating element  315 . The shorter leg of feed element  315  is coupled to tuning element  320 . This leg portion is adjusted in height according to the height of tuning element  320 . In this configuration, PIFA  300  may be tuned simply by changing the shape and size of feed element  315  and tuning element  320  without having to move surfaces  335  and  340 , and also without effecting radiating element&#39;s  310  height with respect to ground plane  305 . 
       FIG. 4  illustrates a top view of PIFA  300  that includes radiating element  310  having a perimeter border line  410 , and ground plane  305  having a corresponding perimeter border line  445 . As shown in  FIG. 4 , border line  410  does not overlap border line  445  and is completely outside of ground plane&#39;s  305  perimeter. In an alternative embodiment, from the top view perspective, radiating element  310  is partially located directly above ground plane  305  such that border line  410  can be seen inside of ground plane  305 . Even though radiating element  310  is being described and shown as having a C-shaped configuration, other shapes could also be used to affect the PIFA resonance frequency as would be understood by one skilled in the art. 
       FIG. 5  illustrates a PIFA  500  according to another embodiment of the present invention. PIFA  500  may include all of the features of PIFA  200 . As shown, PIFA  500  includes a rectangular ground plane  505 , a radiating element  510 , and a rectangular substrate  530 . In PIFA  500 , ground plane  505  and substrate  530  are flushed with one another at the perimeter. As illustrated in  FIG. 6 , a top view of PIFA  500 , radiating element  510  partially overhangs ground plane  505 . In this configuration, a edge  610  of radiating  510  is located, from a horizontal perspective, beyond a edge  620  of ground plane  605 . In this way, PIFA  500  can have an increased impedance bandwidth without having to increase the vertical height of the overall antenna package. 
       FIG. 7  illustrates a PIFA  700  according to another embodiment of the present invention. PIFA  700  is similar to PIFA  200 . PIFA  700  may include some or all of the features of PIFA  200 . As illustrated in  FIG. 7 , PIFA  700  includes a top dielectric layer  710 , a support pad  720 , and a support structure  730 . Dielectric layer  710  is formed on top of ground plane  205 . In this way, ground plane  205  is sandwiched between dielectric layer  710  and substrate  230 . Dielectric layer  710  provides a couple of functions. One of the functions is to isolate feed pad or surface  240  and support pad  720  from ground plane  205 , the other function is to provide a support surface. 
     As eluded to above, support pad  720  is anchored to dielectric layer  710 . Although not shown, no portion of ground plane  205  is located beneath support pad  720 . In this way, current traveling through radiating element  210  and support structure  730  remains isolated from ground plane  205 . In an embodiment, support pad  720  has a rectangular shape. In an alternative embodiment, support pad  720  has a regular polygonal or an irregular polygonal shape as shown in  FIG. 7 . The shape and size of support pad  720  is primarily determined by the tuning requirements of PIFA  700 , which will be discussed below. 
     Support structure  730  provides additional support for radiating element  210 . In PIFA  200 , radiating element  210  is cantilevered from support structure  215 . Considering the size and scale of PIFA  200 , the length of radiating element  210  is very short. Thus structural integrity is not an issue. However, through handling and packaging of the PIFA  200 , radiating element  210  may be accidentally bent for example. Support structure  730  allows PIFA  700  to be more versatile. Thus accidental bending or other physical deformation will less likely occur during manufacturing and/or packaging process. Another added benefit of support structure  730  is the increased current path length. The additional current path length may help to reduce the overall height of radiating element  210  by allowing feed element  215  to be shorter, while keeping the total current path length the same. 
     As previously discussed, PIFA  200  may be tuned by changing the length or height of leg portion  260  of tuning element  220 . By varying the height of tuning element  220 , the overall current path length from surface  235  to surface  240  and to feed element  215  is varied. In this manner, the inductive characteristic of PIFA  200  is affected thus allowing PIFA  200  to be tuned. Similarly, the inductive characteristic of PIFA  700  may also be varied by changing the height of support structure  730 . 
     In an embodiment, the inductive characteristic of PIFA  700  may be varied by changing the shape and/or size of support pad  720 . In this way, PIFA  700  may be tuned simply by extending a side of support pad  720 . For example, as shown in  FIG. 7 , a portion of a side of support pad  720  is extended. This extension serves as an extension to radiation element  210  and/or support structure  730 . In this way, the overall current path length of PIFA  700  is changed, thus allowing PIFA  700  to be properly tuned to any desired frequency band. In an alternative embodiment, instead of extending a portion of a side of support  720 , the full length of the side is extended. Support structure  730  can be made with any conducting material. Preferably, support structure  730  and radiating element  210  comprises the same material such as a wire element or metal traces. Support pad  720  may also be made from the same material as radiating element  210  and/or support structure  730 . 
       FIG. 8  illustrates a PIFA  800  according to another embodiment of the present invention. PIFA  800  is similar to PIFA  700  but also includes an extension (toe)  810  to support structure  730 . In general, extension or toe  810  extends in the direction radiating element  210 . In other words, if radiating element  210  has a semi-circular shape, then extension  810  will also take the form of an arc to add on to the semi-circular shape of radiating element  210 . As shown in  FIG. 8 , radiating element  210  has a rectangular shape. Thus, extension  810  is also a rectangular structure that adds onto the length of radiating element  210  and support structure  730 . Extension  810  may also have other shapes (i.e., shape substantially different than radiating element  210 ), as long as the overall current path length is changed. In this way, PIFA  800  may be tuned to any desired frequency band. 
       FIG. 9  illustrates a detailed view of support structure  730  and extension  810 . As shown, support structure  730  includes an extended portion  910  that is used to anchor support structure onto substrate layer  230  below. This is accomplished by threading portion  910  through a via in dielectric layer  710  and support pad  720 . 
     CONCLUSION 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.