Abstract:
An apparatus for a capacitive feed planar inverted-F (PIFA) multi-band antenna is provided. The antenna structure of the present invention typically comprises of a ground element, a main radiating element, having predefined slits and arranged above the ground element, and a capacitive feed element. The capacitive feed element is electrically connected to an antenna feed and is detached from the main radiating and ground elements. By having additional secondary elements, the bandwidth or the number of resonant frequencies of the antenna can be increased without increasing the overall dimensions of the antenna.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an improved planar inverted-F antenna (PIFA), and in particular to a capacitive feed planar inverted-F multi-band antenna.  
           [0003]    2. Description of Background Information  
           [0004]    Antenna is an essential part of a wireless device. Over the years, wireless devices have been rapidly miniaturizing, thus increasing demand for integrated or built-in antennas. Concurrently, there has been an influx of wireless services and users. To cope with increasing usage and demand, many wireless devices and networks have since migrated from single band operation to dual band (or multi-band) operation to improve network capacity and coverage, and to provide users with seamless quality service.  
           [0005]    A common integrated antenna used in wireless devices is the Planar Inverted-F Antenna (PIFA). The PIFA is a widely favored integrated antenna because it provides for a more compact antenna with an approximate length of λ/4, which is an improvement over a length of λ/2. A typical PIFA is shown in FIG. 1. The PIFA structure shown has a planar radiating element characterized by slits for defining two lips or length portions. Each lip corresponds to a resonant frequency at which the antenna operates. The radiating element has a feed point for directly connecting the radiating element to an antenna feed, and a short circuit point for connecting the radiating element to a ground element arranged below the radiating element. The described antenna structure of FIG. 1 is commonly known as a direct feed PIFA.  
           [0006]    The direct feed PIFA is easy to design and fabricate, but its main disadvantage is insufficient bandwidth to support multi-band operation. Accordingly, there is a need to improve antenna performance by increasing bandwidth of a multi-band antenna while providing for a smaller form factor.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides an integrated capacitive feed planar inverted-F antenna (PIFA) for multi-band operation. A typical embodiment of the present invention comprises a ground element, and a main radiating element arranged at a predetermined height from the ground element, the main radiating element having slits for defining lips. At one end of the main radiating element, it is short-circuited to the ground element. A feed element is arranged in the vertical gap between the ground and the main radiating elements. The feed element is detached (or separated by a gap) from the ground and main radiating elements to create capacitive feeding. For efficient feeding, the feed element may be arranged substantially parallel to the main radiating element. The invention also comprises an antenna feed which is electrically connected to the feed element, but detached from the main radiating and ground elements.  
           [0008]    Secondary (or sub-radiating) elements may also be arranged in the vertical gap and proximate to the feed element for creating an additional resonant frequency or for improving bandwidth performance. The secondary elements are detached (or separated by a gap) from the main radiating, feed and ground elements. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    Embodiments of the present invention will be described with reference to the accompanying drawings, in which:  
         [0010]    [0010]FIG. 1 shows a prior art direct feed PIFA.  
         [0011]    [0011]FIG. 2 shows an antenna structure according to a first embodiment of the present invention.  
         [0012]    [0012]FIG. 3 shows the return loss (lower resonance) of a capacitive feed multi-band antenna in accordance with the first embodiment of the present invention, and a prior art direct feed PIFA.  
         [0013]    [0013]FIG. 4 shows the return loss (higher resonance) of a capacitive feed multi-band antenna in accordance with the first embodiment of the present invention, and a prior art direct feed PIFA.  
         [0014]    [0014]FIG. 5 shows the radiating efficiencies of a capacitive feed multi-band antenna and a prior art direct feed PIFA antenna.  
         [0015]    [0015]FIG. 6 shows an antenna structure according to a second embodiment of the present invention.  
         [0016]    [0016]FIG. 6A is a cross-sectional view of the second embodiment taken from direction A.  
         [0017]    [0017]FIG. 6B is a cross-sectional view of the second embodiment taken from direction B.  
         [0018]    [0018]FIG. 7 shows the return loss of a capacitive feed multi-band antenna employing at least a secondary element for creating an additional resonance.  
         [0019]    [0019]FIG. 8 shows an antenna structure according to a third embodiment of the present invention.  
         [0020]    [0020]FIG. 8A is a cross-sectional view of the third embodiment taken from direction C.  
         [0021]    [0021]FIG. 8B is a cross-sectional view of the third embodiment taken from direction D  
         [0022]    [0022]FIG. 9 shows an antenna structure according to a fourth embodiment of the present invention.  
         [0023]    [0023]FIG. 9A is a cross-sectional view of the third embodiment taken from direction E.  
         [0024]    [0024]FIG. 9B is a cross-sectional view of the third embodiment taken from direction F. 
     
    
     DETAILED DESCRIPTION  
       [0025]    [0025]FIG. 2 shows an antenna structure according to a first embodiment  200  of the present invention. According to the first embodiment  200 , the antenna structure comprises a ground element  202 , and a main radiating element  201  arranged at a predetermined distance from the ground element  202 . The ground element may be in the form of a planar structure, or may form part of a casing embodying the present invention, or the like. The main radiating element  201  is characterized by slits  207  cut from an edge of the main radiating element  201  to divide the main radiating element  201  into two lips. From the perspective of a feed point  204  (see FIG. 2), the lips have unequal lengths for providing two resonant frequencies for dual band operation. The resonating frequencies of the antenna are dependent on namely the dimensions of the lips, and the dimensions and the number of slits  207 . The resonant frequencies may also be dependent on the vertical gap distance between main radiating element  201  and the ground element  202 . To tune the antenna to operate at a different frequency, the dimensions of any of the lips and slits  207  are varied.  
         [0026]    At one end of the main radiating element  201 , the main radiating element  201  has a short-circuit point  205  for connecting the main radiating element  201  to the ground element  202 . The short-circuit point  205  is typically formed by connecting both elements with an electrically conductive strip or wire.  
         [0027]    The antenna structure  200  also comprises a feed element  203  arranged at a first predetermined height in a vertical gap between the main radiating element  201  and the ground element  202 , and separated from both the main radiating  201  and ground  202  elements (i.e. detached) to create capacitive feeding.  
         [0028]    The feed element  203  is arranged directly below the main radiating element  201  along a lip portion common to both lips (or referred to as a common lip portion). The feed element  203  is illustrated as a rectangular metal strip. If required, the feed element  203  may form an L shape or any shape conforming with a lip portion common to both lips. To achieve a desired bandwidth performance, the feed element  203  may be tuned by varying its dimensions or by varying the gap between the main radiating element  201  and the feed element  203 .  
         [0029]    The feed element  203  has a feed point  204  for electrically connecting to an antenna feed  206  for feeding an input signal. The feed point  204  is positioned at an end closest to the short circuit point  204 . The distance from the short circuit point to the feed point determines the impedance of the antenna system. The feed  206  is also detached from other elements, i.e., ground  202  and main radiating  201  elements, as known to a person skilled in the art.  
         [0030]    As an illustration, the main radiating element  201  used in the present invention is a conductive plate measuring 30 mm by 20 mm to provide for a small form factor. However, it may take other shapes without departing from the invention.  
         [0031]    The vertical gap separating the feed element  203  from the main radiating element  201  is predetermined and will be discussed in greater detail in later paragraphs. The vertical gaps separating the ground element  202  and the feed element  203 , the feed element  203  and the main radiating element  201 , are typically filled with air. If a dielectric is arranged in place of air, parameters on the vertical gap and dimensions of the sub-radiating elements may differ. A smaller antenna form factor may be achieved but may result in a lossy antenna system.  
         [0032]    The present invention is advantageous as it realizes a wider bandwidth at the resonant frequencies while achieving a smaller form factor. A comparison of the bandwidth performance of a direct feed antenna  100  (prior art) and a capacitive feed multi-band antenna in accordance with the present invention is illustrated by FIGS. 3 and 4.  
         [0033]    [0033]FIGS. 3 and 4 show a graphical representation of the return loss of a capacitive feed PIFA according to the present invention and a direct feed PIFA  100  according to the prior art. The return loss of the prior art direct feed PIFA is indicated by curves  301  and  401 . The return loss of a capacitive feed multi-band antenna according to the present invention is indicated by curves  302  and  402 . The return loss of an antenna allows a person skilled in the art to determine resonant frequencies and bandwidth of the antenna. At 7 dB level of FIG. 3 illustrating return loss at a lower resonant frequency, the bandwidth factors of the direct feed antenna  100  and the capacitive feed multi-band antenna are calculated as 7.3% and 8.6% respectively. (Bandwidth factor=Bandwidth/resonant frequency) At 7 dB level of FIG. 4 illustrating return loss at a higher frequency, the bandwidth factors of the direct feed antenna  100  and the capacitive feed antenna are calculated as 4.8% and 5.6% respectively. Clearly, the present invention improves the bandwidth performance at both resonant frequencies.  
         [0034]    Another advantage of the present invention employing a capacitive feed is a higher radiating efficiency. FIG. 5 is a graphical representation of radiating efficiency with respect to frequency and is obtained from a simulation performed using IE3® from Zeland Software, Inc.  
         [0035]    [0035]FIG. 5 shows a comparison of radiating efficiency curves between a direct feed antenna  100  and a capacitive feed multi-band antenna having separately 2-mm (millimeter), 3-mm and 5-mm gaps. The gap refers to the vertical gap distance between the main radiating element  201  and the feed element  203 . Their radiating efficiencies are indicated by curves  501 ,  502 ,  503  and  504  respectively. FIG. 5 shows that a direct feed antenna  100  has a lower radiating efficiency while a capacitive feed multi-band antenna, according to the present invention, has a higher radiating efficiency. Among the efficiency curves of a capacitive feed antenna, FIG. 5 shows that a 5-mm vertical gap provides an optimized radiating efficiency curve.  
         [0036]    The return loss and radiating efficiency curves shown in FIGS. 3, 4 and  5  are based on a capacitive feed multi-band antenna  200  according to a first embodiment of the present invention and a direct feed antenna  100 . Both antenna structures have identical dimensions and conditions for the main radiating element  201 , ground element  202  and the antenna feed  206 . FIGS. 3, 4 and  5  show that the bandwidth performance and radiating efficiency of a capacitive feed multi-band antenna is higher than a prior art direct feed antenna  100 . Thus, it follows that to achieve similar performance as a prior art direct feed PIFA  100 , the dimensions of a capacitive feed multi-band antenna are smaller than those of a direct feed PIFA  100 . Accordingly, the dimensions of a capacitive feed multi-band antenna may be optimized for achieving both improved bandwidth performance and smaller form factor.  
         [0037]    The foregoing description and advantages of a capacitive feed antenna for a dual band antenna are also applicable to embodiments employing secondary (or sub-radiating) elements, which will be described in the following paragraphs. The presence of secondary elements increases the bandwidth of the antenna and/or creates additional resonance for triple or quad-band operation. Examples of triple-band operation include Global Standard for Mobile Communication (GSM), Digital Communication System (DCS) and Personal Communication Service (PCS)).  
         [0038]    [0038]FIG. 6 shows an antenna structure according to a second embodiment  600  of the present invention. The structure and arrangement of the second embodiment  600  is similar to the first embodiment  200 . Additionally, the second embodiment  600  has a first secondary element  601 . The first secondary element  601  is arranged at a second predetermined height in the vertical gap separating the main radiating element  201  and the ground element  202 . The second predetermined height may be the same as the first predetermined height of the feed element  203  to form a substantially same planar surface. However, the secondary element can be arranged at a different height.  
         [0039]    As an illustration, the first secondary element  601  is shown as an L-shaped element. One arm of the L-shaped element is arranged proximate to the feed element  203  and separated by a gap. The L-shaped element may be formed by cutting away from a corner of a rectangular plate during the tuning process. In FIG. 6, the first secondary element  601  is shown as a flat structure, but it can be folded or contoured to conform to a shape required of a device embodying the invention. The shape and arrangement of the secondary element  601  should allow coupling with the main radiating element  201  and/or the feed element  203 .  
         [0040]    The first secondary element  601  is detached from other elements, such as, the feed element  203 , main radiating element  201 , ground element  202  and feed  206 . Preferably, the gap separating the feed element  203  and the first secondary element  601  allows sufficient coupling between the two elements.  
         [0041]    [0041]FIGS. 6A and 6B illustrate a cross-sectional view taken from directions A and B respectively. It is understood by a person skilled in the art that the feed  206  is detached from the ground element  202 .  
         [0042]    [0042]FIG. 7 shows the return loss of an antenna having at least a secondary element to create an additional resonance.  
         [0043]    [0043]FIG. 8 shows an antenna structure according to a third embodiment  800  of the present invention. For purposes of illustration, the main radiating element  201  have slits  207  to provide two lips. In addition to the structure described for the second embodiment, the third embodiment has a second secondary element  801 .  
         [0044]    In the antenna structure of FIG. 8, the slits  207  and short circuit point  205  are defined differently from the previous embodiments to allow different arrangements of the secondary elements. Similar to the first  200  and second  600  embodiments, a feed element  203  is arranged at a first predetermined height in the vertical gap between the main radiating element  201  and the ground element  202 , and below a lip portion common to both lips. The feed element  203  has a feed point  204  for connecting to the antenna feed  206 . Similarly, the feed element  203  is detached from but proximate to the main radiating element  201  to create capacitive feeding. The feed element  203  is also detached from the ground  202  and other secondary elements ( 203 ,  601  and  801 ). The antenna feed  206  is electrically connected to the feed element  203  and detached from the ground  202  and other secondary elements ( 601  and  801 ).  
         [0045]    Similar to the second embodiment, a first secondary element  601  is arranged in the vertical gap between the main radiating element  201  and ground element  202  at a second predetermined height. The first secondary element  601  is detached from and proximate to the feed element  203  as described for the second embodiment. The first secondary element  601  is also detached from the main radiating  201 , ground  202  and other secondary elements ( 203  and  801 ).  
         [0046]    As described earlier, the feed element  203  and the first secondary element  601  can be arranged at a same predetermined height to form a substantially same plane with the feed element  203 . Alternatively, both secondary elements can be arranged at different predetermined heights, but should create coupling with the feed element  203  and/or the main radiating element  201 .  
         [0047]    A second secondary element  801  is arranged at a third predetermined height in the vertical gap between the main radiating element  201  and the ground element  202 . The second secondary element  801  may be arranged to form a substantially same plane with the feed element  203  and/or the first secondary element  601  at the same height in the vertical gap. Alternatively, the second secondary element  801  may be arranged at a different height, but should create coupling with other secondary elements and/or with the main radiating element  201 .  
         [0048]    In FIG. 8, the second secondary element  801  is illustrated as an L-shaped member. One arm of the L-shaped element is arranged proximate to the feed element  203  and separated by a gap. The L-shaped element may be formed by cutting away from a corner of a rectangular plate during the tuning process. Similar to the first secondary element  601 , the second secondary element  801  is detached from other elements ( 201 ,  203 ,  206 ,  601 ).  
         [0049]    [0049]FIGS. 8A and 8B illustrate a cross-sectional view taken from directions C and D respectively. It is understood by a person skilled in the art that the feed  206  is detached from the ground element  202 .  
         [0050]    [0050]FIG. 9 shows an antenna structure according to a fourth embodiment  900  of the present invention. The structure and arrangement of the fourth embodiment is similar to that of the third embodiment  800 . Additionally, the fourth embodiment  900  has a third secondary element  901 . The third secondary element  901  is arranged at a predetermined height in a vertical gap between the feed element  203  and the ground element  202 . The third element  901  is arranged with at least a portion common with or overlapping with the feed element  203  to create coupling.  
         [0051]    The fourth element  901  is illustrated in FIG. 9 as an E-shaped element, where the middle arm of the E-shaped element is common with the feed element  203  (i.e., the feed element  203  overlays the middle arm of the E-shape element). Alternatively, the fourth secondary element  901  may take other shapes. Similar to the first  601  and second  801  secondary elements, the third secondary element  901  is detached from and proximate to the other secondary elements, and is also detached from the main radiating  201 , ground  202  element and feed  206 .  
         [0052]    For efficient coupling, the secondary elements ( 203 ,  601 ,  801  and  901 ) may be arranged substantially parallel to the main radiating element  201 .  
         [0053]    Preferably, each described secondary element ( 203 ,  601 ,  801 ,  901 ) has a surface area smaller than the main radiating element  201 , and made of electrically conductive materials.  
         [0054]    The described main radiating  201 , ground  202 , and secondary elements ( 203 ,  601 ,  801 ,  901 ) are illustrated herein as having flat structures. However, they may be folded or contoured to conform to an external casing of an internal structure of a device embodying the invention.  
         [0055]    Typically, the antenna in accordance with the present invention may be incorporated in electronic devices with wireless communication capabilities, such as, phones, headphones, Wireless Digital Assistants (WDAs), organizers, portable computers, keyboards, joysticks, printers, and the like.