Abstract:
A multiband antenna includes a 5-6 GHz PIFA surrounded on two or three sides by a 2.4 GHz RFPIFA. The PIFA and RFPIFA are tunable by removing fingers from the PIFA and either removing portions of or creating at least one area in the RFPIFA where inductance may be added. The RFPIFA contains an inductive meanderline. An out-of-plane matching stub is provided between the feed and the ground plane to impedance match the antenna. The PIFA/RFPIFA is supported by a plastic mesa tabletop whose legs are mounted directly to the ground plane of a PCB at the corner of the PCB. Electronic components on the PCB can be mounted underneath the multiband antenna.

Description:
BACKGROUND 
     Mobile communication devices, such as cellular telephones, PDAs, handsets, and laptop computers, require antennas for wireless communication and previously used multiple antennas for operation at various frequency bands. Recent wireless devices, however, use a single antenna to operate in multiple frequency bands. One such frequency range increasing in popularity is the ISM band (2.4 GHz), which covers frequencies between 2.4-2.4835 GHz in the United States with some variations in other countries. Different protocols are used to transmit and receive signals in this band: the Bluetooth Standard published by the Bluetooth Special Interest Group and the IEEE Standard 802.11b published by the Institute of Electrical and Electronic Engineers. The UNII (Unlicensed National Information Infrastructure) band covering the 5-6 GHz range is another frequency band that has been recently allocated (specifically, a 200 MHz block at 5.15 MHz to 5.35 MHz and a 100 MHz block at 5.725 MHz to 5.825 MHz) to alleviate some of the problems that plague the 2.4 GHz band, e.g. saturation from wireless phones, microwave ovens, and other emerging technologies. The UNII band uses IEEE Standard 802.11a, which supports data rates of up to 54 Mbps and is faster than the 802.11b standard, which supports data rates of up to 11 Mbps. In addition, unlike the 802.11b standard, the 802.11a standard departs from spread-spectrum technology, instead using a frequency division multiplexing scheme that&#39;s intended to be friendlier to office environments. Of course, there are many other frequency bands over which wireless devices may operate, including the 800 MHz, GSM and PCS, GSM and DCS, or GPS L1 and L2 bands. 
     As one example of conventional antennas that operate in multiple frequency bands, including the 2.4 GHz range, SkyCross has triband antennas (antennas operating in three frequency ranges) that range in size from 20×18×3 mm to 22.3×14.9×6.2 mm. The smallest antenna has an efficiency of better than 60% but a poor Voltage Standing Wave Ratio (VSWR) of less than 3:1 (the larger antenna has an improved VSWR of 2:1 but an unreported efficiency). Other manufacturers include Ethertronics, having an antenna only matched to −6 dB across the upper band (with a peak efficiency of 75% based on the shown return loss plot), and Tyco Electronics, having a circular antenna of 16 mm diameter and 6 mm height with a better than 2.5:1 VSWR but again, unreported efficiency. 
     Ample room remains for improvement in multiple areas of interest for these antennas for the designer, manufacturer and ultimately consumer with the ever-increasing demand for smaller and lighter (as well as cheaper) consumer electronics. These areas include not only the efficiency and overall performance, but also the cost, size and weight of the antenna. Of course, other conventional antennas used in other mobile communication devices face similar problems; the antenna performance is inherently linked to the size of the antenna as there is a fundamental limit on the efficiency and bandwidth that can be achieved based on the total volume of the antenna. In consequence, manufacturers of consumer electronics, who have little room in their products for antennas given the size and cost pressures, have conflicting interests to improve the device performance. 
     In addition to the size/performance tradeoff noted above, other problems occur when attempting to design antennas using frequency bands that are separated by large amounts, for example an octave or more apart. One such problem is the limiting of the higher frequency bandwidth due to reactive loading by the lower resonance. Adding to this, the antennas must be designed for low cost manufacturing as well as contain low cost materials to be cost effective for use in consumer electronic devices. This has led to the incorporation of the antenna within the package or case for reasons of durability and size. 
     Such wireless devices typically pack a substantial amount of circuitry in a very small package. The circuitry may include a logic circuit board and a radio frequency (RF) circuit board. The printed circuit board (PCB) can be considered an RF ground to the antenna, which is ideally contained in the case with the circuitry. A preferred antenna for use in these wireless devices would be one that can be placed extremely close to such a ground plane and still operate efficiently without adverse effects such as frequency detuning, reduced bandwidth, or compromised efficiency. 
     Various antennas have been developed to provide capability in at least one of the 2.4 and 5-6 GHz ranges. These include Planar Inverted-F Antennas (PIFAs), types of shorted patches, and various derivatives, which may contain meander lines. However, the need to integrate a single, compact, antenna structure that responds (i.e. has resonant frequencies) in both the 2.4 and 5-6 GHz ranges remains. Thus, to date, none of the above antennas satisfy present design goals, in which efficient, compact, low profile, light weight and cost effective antennas are desired. 
     BRIEF SUMMARY 
     To achieve the above objectives, in addition to other objectives mentioned herein, combination PIFA/reverse-fed planar inverted F-antennas (RFPIFA) having frequency response in multiple frequency ranges are disclosed in various embodiments below. 
     In one embodiment, the multiband antenna comprises a PIFA having a first resonant frequency and a RFPIFA surrounding the PIFA on two sides and having a second resonant frequency lower than the first resonant frequency. In another embodiment, the multiband antenna the RFPIFA surrounds the PIFA on three sides. 
     In a third embodiment, the PIFA and RFPIFA have first and second resonant frequencies, respectively, (with the RFPIFA resonant frequency lower than the PIFA resonant frequency) as well as being integrally formed from a single piece of conductive material and attached at one end such that dimensions of the multiband antenna are defined substantially by the RFPIFA. 
     Any of the embodiments may contain the elements below. 
     The multiband antenna may comprise an out-of-plane matching stub to impedance match the multiband antenna with external elements. This stub may extend from the feed line. The length and width of the stub as well as distance between the stub and the ground plane (i.e. the height of the stub) is chosen to optimize the impedance match. Similarly, an antenna element that has a third resonant frequency higher than the first resonant frequency may be disposed perpendicular to the ground plane. 
     The conductive material that forms the PIFA and RFPIFA may be separated from a ground plane by two layers having an effective permittivity of about 1 to about 1.7. The PIFA/RFPIFA may be disposed on an undercarriage, which is in turn supported by legs. The thickness of the undercarriage is about 0.3 to 1.0 mm and the overall thickness of the antenna is about 2 mm to 4 mm. The legs contact the ground plane such that the undercarriage is mounted on a printed circuit board (PCB) and the PIFA and RFPIFA are mounted over components mounted on the PCB. The legs may be plastic with metalized contacts positioned on the PCB for solder reflow connection. The multiband antenna may be mounted at an edge of the PCB. 
     The resonant frequencies of the PIFA and RFPIFA may be adjustable by removal of a portion of the PIFA or RFPIFA or addition of inductance at discrete locations including formation of a narrow inductive transmission line in the RFPIFA or between the PIFA and RFPIFA. 
     The multiband antenna may be devoid of dielectric loading and meander lines or may have one or more meanderlines having the same shape. A narrow inductive transmission line may be disposed between the meanderlines. 
     The largest dimension of the RFPIFA is at most {fraction (1/10)} of the second resonant frequency without dielectric loading. The resonant frequency of the PIFA may be 5 to 6 GHz while that of the RFPIFA about 2.4 GHz. 
     The multiband antenna may be relatively insensitive to proximity effects and to changes in ground plane size and component layout on a PCB on which the multiband antenna is mounted. 
     In a fourth embodiment, a method for multiband reception of an antenna comprises communicating in first and second resonant frequencies via a PIFA and RFPIFA, respectively, (with the RFPIFA resonant frequency lower than the PIFA resonant frequency) and limiting an area of the PIFA and RFPIFA such that dimensions of the antenna are defined substantially by the RFPIFA. 
     In a fifth embodiment, a method for multiband reception of an antenna comprises communicating in first and second resonant frequencies via a PIFA and RFPIFA, respectively, (with the RFPIFA resonant frequency lower than the PIFA resonant frequency) and adjusting one of the resonant frequencies by one of removing a portion of the PIFA or RFPIFA or addition of inductance at discrete locations including forming a narrow inductive transmission line in the RFPIFA or between the PIFA and RFPIFA. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross sectional view of a conventional PIFA; 
     FIG. 2 shows a cross sectional view of a RFPIFA; 
     FIG. 3 shows a top view of a PIFA in an embodiment; 
     FIG. 4 illustrates the response of the PIFA; 
     FIG. 5 shows a top view of an antenna of an embodiment; 
     FIG. 6 shows a top view of an antenna of an embodiment; 
     FIG. 7 shows a top view of an antenna of an embodiment; 
     FIG. 8 shows a test setup for a RFPIFA; 
     FIG. 9 shows a test setup for a short; 
     FIGS. 10 a-f  illustrate the electrical characteristics of the RFPIFA and short of FIGS. 8 and 9; 
     FIG. 11 shows the correlation between the RFPIFA and short of FIGS. 8 and 9; 
     FIG. 12 illustrates the return loss of the RFPIFA of FIG. 8; 
     FIG. 13 shows a perspective view of an antenna of an embodiment; 
     FIG. 14 shows a perspective view of an antenna of an embodiment; 
     FIG. 15 shows a bottom view of an antenna of an embodiment; 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As described above, antenna performance must always be weighed against the size of the antenna. With any approach there will be a fundamental limit on the efficiency and bandwidth that can be achieved based on the total volume of the antenna. The multiband PIFA/RFPIFAs of the present embodiments are electrically very small for the efficiency bandwidth product they achieve. 
     The structure of the present antennas as well as the size and placement of the structure maximize the antenna efficiency and usable space in the consumer device while reducing the sensitivity of the antenna to proximity effects, such as those caused by nearby housing, and to changes in the size of the ground plane and component layout on a printed circuit board (PCB). In addition, the embodiments are relatively cheap to fabricate, having a simple integrated structure that may be stamped, easily modified to adjust the resonant frequencies of the PIFA and RFPIFA, and soldered to the PCB with conventional techniques. Use of injection molding during fabrication also increases repeatability in the thickness direction and reduces the antenna cost by using plastic as the undercarriage. 
     RFPIFA structures have been discussed at length, for example in U.S. provisional patent application serial No. 60/352,113 filed Jan. 23, 2002 and subsequently filed co-pending patent application Ser. No. 10/211,731 filed Aug. 2, 2002, both of which are entitled “Miniaturized Reverse-Fed Planar Inverted F Antenna,” in the names of Greg S. Mendolia, John Dutton, and William E. McKinzie III, commonly assigned to the assignee of the present application, which are incorporated herein by reference in their entirety. Similarly, PIFA structures incorporating frequency selective surfaces (FSS) have be previously discussed in U.S. provisional application serial No. 60/310,655, filed Aug. 6,2001 and subsequently filed co-pending patent application Ser. No. 10/214,420 filed Aug. 6, 2002, entitled “Low Frequency Enhanced Frequency Selective Surface Technology and Applications” in the names of William E. McKinzie, III, Greg Mendolia, and Rodolfo E. Diaz which are incorporated herein by reference in their entirety and commonly assigned to the assignee of the present application. 
     The present embodiments incorporate a normally fed PIFA with a RFPIFA in as single integrated structure without the addition of off-chip components or connections thereof to achieve a compact, efficient, lightweight and cost effective antenna having resonances in multiple bands. In particular, the antennas described herein respond in both the 2.4 and 5-6 GHz frequency ranges. As an example of compactness, using comparable separate non-integrated PIFA and RFPIFAs rather than combining the PIFA/RFPIFA into a single structure, results in an approximately four fold volumetric increase as well as an increase in cost to achieve comparable efficiencies in the frequency range of interest. 
     By way of introduction only, in a conventional PIFA having the cross-sectional view shown in FIG. 1, the PIFA  100  includes a ground plane  102  and a radiating element  104 . The PIFA  100  has a feed  106  positioned between a shorted end  110  and a radiating portion  112  of the radiating element  104 . An RF short  108  electrically shorts the shorted end  110  of the radiating element  104  to the ground plane. The feed engages the radiating element at a feed point which is offset from the RF ground of the radiating element  104 . The feed point is positioned between the RF ground, which engages the radiating element at the shorted end  110  of the radiating element  104 , and the radiating portion  112  of the radiating element  104 . 
     FIG. 2 shows a cross sectional view of a RFPIFA 200. The RFPIFA  200  includes a ground plane  202  and a radiating element  204  which is substantially parallel to the ground plane  202 . The RFPIFA  200  further includes a feed  206  and an RF short  208 . However, in the RFPIFA  200 , the relative positions of the feed  206  and the RF short  208  have been exchanged in comparison to the conventional PIFA. 
     The radiating element  204  includes a feed point  214  at a feed end  210  and a radiating portion  212 , terminating in an open end  216 . The feed  206  engages the feed end  210 , one end of the radiating element. In alternative embodiments, such as those shown in later figures, a stub may extend beyond the feed end  210  of the radiating element  204 . The RF short  208  engages the radiating element  204  beyond the feed point  214 . The effect is that the traditional feed point and ground point, as shown in FIG. 1, are reversed. 
     This arrangement is counter-intuitive, as the energy from the feed  206  now is presented with a short at the RF short  208  before the energy is transmitted to the main radiating portion  212  of the radiating element  204 . Intuition suggests that the energy fed to the RFPIFA  200  would substantially pass to the ground plane  202  through the RF short  208 . This, however, is not the case. The configuration of the RFPIFA  200  is fed from the end of the structure at feed end  210 . There is no alternative path for the energy to flow other than across the RF short  208  in order to reach the radiating portion  212  of the radiating element  204 . By configuring the feed  206  and the RF short  208  as shown in the drawing, the antenna 200 radiates very efficiently when placed close to the ground plane  202 . 
     The frequency of operation of the RFPIFA  200  is defined by at least two dimensions. The first and greatest influence on frequency is the length  220  of the radiating element  204 , from the feed  206  to the open end  216 . The length of the radiating element  204  is approximately one-quarter of a free space wavelength. The second is the position of the RF short  208  with respect to the feed  206 . The position of the RF short  208  or ground return is also used to optimize the impedance match and bandwidth of the antenna  200  as seen from the feed  206 . Based on experiments, the distance between the feed and RF short along the radiating element is approximately {fraction (1/20)} to ⅕ of the total length of the radiating element  204 . The exact position of the RF short is determined to optimize bandwidth, impedance match, and efficiency. 
     The embodiments of the present set of multiband antennas illustrated below are triband antennas. The triband antennas are so called because they integrate a 5-6 GHz element (covering the 802.1 la frequency range of dual reception 5.15 MHz to 5.35 MHz and 5.725 MHz to 5.825 MHz) and a 2.4 GHz element into a single antenna with one RF port. 
     One embodiment of the 5-6 GHz element is shown below in FIG.  3 . The 5-6 GHz element  300  is a planar PIFA  302  with nearly square dimensions. The PIFA  302  is formed from a metal or other conductive material. Any conductive material may be used which is not significantly lossy with respect to transmitting signals along the antenna. Specifically, in these embodiments, the PIFA  302  is fabricated as a single metallic patch. Although FIG. 3 shows a square cutout and diagonal notch in the patch, these sections do not have to be present as they merely alter the resonant frequency of the PIFA by changing the inductance and capacitance, as illustrated in later figures. 
     The feed  304  extends from an edge of the patch rather than the middle of the patch, as in the conventional PIFA of FIG.  1 . As shown, the feed  304  is disposed at approximately the middle of the edge of the PIFA  302 . The feed  304  is connected with a PCB (not shown). The short  306  is connected to a ground plane (not shown). The short  306  is disposed at approximately a corner of the PIFA  302  along the same edge as the feed  304 . While any type of conductor, such as a pin or post, may be used as the feed  304  or short  306 , the feed  304  and short  306  are microstrip lines and are integral with the radiating portion of the PIFA  302 . Thus, the entire antenna  300  may be fabricated using simple, conventional techniques, such as a stamping process, to form the antenna. 
     The PIFA  302  has two radiating modes, one that corresponds to the length of the PIFA  302  and one that corresponds to its width. The resonant modes, i.e. resonant frequencies, are very close to each other in frequency. The PIFA  302  by itself has more than enough bandwidth to cover the 802.11a frequency range at a 10 dB return loss and better than 50% efficiency as shown by FIG.  4 . The microstrip line that feeds this part has approximately 1-1.5 dB of insertion loss at 6 GHz making the return loss approximately 2 dB worse than what is shown and the efficiency approximately 1 dB better. The efficiency is thus better than 60% across the band with the return loss better than 10 dB across the band (and is actually better than 70% over a portion of the band). For the experimental results, the antenna was built on 0.005″ polyimide with a 2.5 mm dielectric spacer made from Rohacell Foam (ε r ). The same measurements performed on an antenna with air under the polyimide rather than a dielectric spacer indicate an efficiency of better than 70% across the band with the return loss better than 10 dB across the band. 
     FIG. 5 shows that a similar 5-6 GHz element (PIFA)  502  is combined with a 2.4 GHz element (RFPIFA)  508  to form the triband antenna  500 . The PIFA  502 , as above contains a feed  504  and short  506 . The triangular cutout at the upper left corner in the figure is not essential. As above, the RFPIFA  508  employs a reverse feed in which the radiating portion  518  of the PIFA  502  forms a stub of the RFPIFA  510 . This is to say that the feed  504  is more proximate to the radiating portion  518  of the PIFA  502  and more distal to the radiating portion  516  of the RFPIFA  508  than the short  506 . The radiating portion  518  of the PIFA  502  and the radiating portion  516  of the RFPIFA  508  are formed on opposite ends of the antenna  500 . 
     In this embodiment, the 2.4 GHz RFPIFA  508  is wrapped around the 5-6 GHz PIFA  502  such that the RFPIFA  508  surrounds the PIFA  502  on essentially two sides of the PIFA  502 . The PIFA  502  and RFPIFA  508  are separated by a slot  512 . There is some coupling across the slot  512  between the PIFA  502  and RFPIFA  508 , but it has a minimal effect on the frequency of the two resonances. The width of the slot  512  is large enough so that the resonant frequencies of the PIFA  502  and RFPIFA  508  are minimally affected by small changes in the slot width due to coupling between the elements. This width is nominally 0.75 mm, but may be decreased to about 0.3 mm. The separation of the higher and lower frequency elements maintains the bandwidth at the upper frequency; that is the loss of bandwidth dramatically increases if the elements are separated. For example, conventional antennas show a 5 db return loss about 650 MHz apart, while in the present embodiments the 5 db return loss is about 1.5 GHz; thus the manner of combination of elements is important to the antenna performance, as discussed below. In this embodiment, the PIFA  502  and RFPIFA  508  are connected through a narrow inductive transmission line  510  formed by increasing the slot  512  to a notch  514  in the area between the two elements thereby decreasing the conductive area connecting the PIFA  502  and RFPIFA  508 . 
     FIG. 6 shows a second embodiment of the antenna. This multiband antenna  600 , has the same basic features as the previous embodiment: PIFA  602 , feed  604 , short  606 , RFPIFA  608  separated from the PIFA  602  by a slot  612  that comes down close to the short  606  but without a narrow inductive transmission line. In this case, however, the short  606  is much wider than that of the previous embodiment and the RFPIFA  608  substantially surrounds the PIFA  602  on three sides of the PIFA  602 , rather than two sides (discounting the 0.6 mm extension of the PIFA  602  shown in the figure, which is about 10% of the total width). In addition, the RFPIFA  608  contains frequency selective surface (FSS) sections  610  and the antenna  600  features an out-of-plane matching stub  614 . Further, unlike conventional antennas, the structure of the antenna  600  permits the ground plane disposed on the PCB underneath the antenna  600 , and to which the short  606  is connected, to be located underneath either the entire antenna  600  or only a portion of the antenna  600  without appreciably affecting the characteristics of the antenna  600 . 
     Use of the FSS  610  in the RFPIFA  608  permits a significant slow wave factor in the modes propagating on the equivalent FSS transmission line, resulting in a low resonant frequency. The size of the RFPIFA can be reduced such that the maximum dimension of the antenna is λ/10 (where λ is the free space wavelength at the lowest resonant frequency). The weight of the structure is also relatively small because bulk dielectric loading is not needed to achieve this decrease in size. The use of an FSS in the RFPIFA additionally decreases the sensitivity of the resonant frequencies to changing environmental factors such as proximity to a human body. 
     The matching stub  614  is out-of-plane with the PIFA  602  and RFPIFA  608 . The matching stub  614  matches the antenna  600  to 50Ω (or to whatever impedance is desired). The matching stub  614  is a stub that extends from the portion of the feed that is not in the same plane as the upper surface of the antenna  600 , on which the PIFA  602  and RFPIFA  608  reside. The matching stub  614  thus extends along the side of the antenna  600  in a length direction of the antenna  600  essentially perpendicular to the upper surface of the antenna  600 . The dimensions of the matching stub  614  as well as the distance between the matching stub  614  and ground plane (not shown) controls the effective impedance thereby permitting realization of a much greater range of impedances than can be compactly realized in the plane of the antenna as well as optimization of the impedance match. The length, width, and thickness of the matching stub  614  are dependant on the design characteristics. The matching stub  614  should be at least 1 mm off ground plane to prevent substantial variations in the impedance due to variations in the fabrication process (that might be present for instance if the matching stub were very close to the ground plane). 
     Because the matching stub  614  is out of plane with the other antenna elements, space is more effectively used by employing the previously unused out of plane area rather than increasing the lateral area in the same plane as the other antenna elements. In this regard, a compact line having substantially lower impedance may be realized using the out of plane matching stub compared to what could be realized by use of a matching stub in the plane of the antenna elements. Further, the use of the matching stub  614  means that additional matching components external to the antenna  600  are not required. In other embodiments that are not shown, another antenna structure having a higher resonance frequency may be disposed on out of plane with the PIFA and RFPIFA elements. Such an out of plane antenna may replace or may be used in addition to the matching stub  614 . 
     In another embodiment shown in FIG. 7, the antenna  600  of the previous embodiment incorporates a mechanical tuning mechanism or means for tuning which permits tuning of the resonant frequencies of the antenna  700  of this embodiment in compensation for fabrication process variations, among other factors. This multiband antenna  700 , has the same features as the embodiment shown in FIG.  6 : PIFA  702 , feed  704 , short  706 , RFPIFA  708  separated from the PIFA  702  by a slot  712  and containing FSS sections  710 , and an out-of-plane matching stub  714 , which have already been discussed. 
     The mechanical tuning mechanism contains multiple different individual mechanisms ( 718  or A 1 ,  720  or A 2 , and  722  or A 3 ) to alter the resonance frequency of the PIFA  702  and RFPIFA  708 . Such mechanisms in the RFPIFA  708  include first and second sets of straps  718 ,  720 . Each of the first and second set of straps  718 ,  720  is formed by a series of holes  724  in the metal of the RFPIFA  708 . These holes  724  extend in a line substantially from one edge of the RFPIFA  708  at least halfway to the opposing edge. Material between holes in the first set of metal straps  718  is cut to form inductive neckdowns  716 , i.e. narrow inductive transmission lines, that increase the inductance and decrease the frequency of the RFPIFA resonance. 
     The material between the holes  724  is cut such that the holes  724  in the first set of straps  718  are joined one by one as necessary to increase the inductance to the desired value. The first set of straps  718  and associated inductance of the narrow inductive transmission lines  716  is formed at various locations in the RFPIFA  708 ; between the FSS sections  710 , between the RFPIFA  708  and the PIFA  702 , and between the main body  726  and the end section  728  of the RFPIFA  708 . In the embodiment above, the first two of these straps have holes that extend substantially from one edge of the RFPIFA  708  almost to the opposing edge, while the holes of the last of these straps extends about halfway to the opposing edge. The last of these straps may be used to control both the resonance frequency of the RFPIFA and the impedance matching between the RFPIFA and the PIFA. The first set of straps  718  may each be altered one at a time for greater control. By tuning the inductance at the three points shown in FIG. 7, the lower resonance can be shifted slowly down by a maximum of about 250 MHz. 
     The second set of straps  720 , which increase the frequency coarsely, is slightly different from the first set of straps  718 . The second set of straps  720  have holes that extend all the way across the end  728  of the RFPIFA  708 , from the slot  712  to the opposing outer edge of the RFPIFA  708 . To adjust the frequency of the RFPIFA  708  using the second set of straps  720 , the strap closest to the end of the RFPIFA  708  (i.e. the end of the RFPIFA  708  most proximate to the matching stub  714 ) is completely cut through and the material removed such that the RFPIFA  708  is shortened. Tuning is effected by consecutively cutting through the second set of straps  720  one by one thereby consecutively removing the material closest to the end of the RFPIFA  708  and shorting the length of the RFPIFA  708 . This coarse tuning increases the RFPIFA  708  frequency by up to a maximum of about 300 MHz. Using the first and second set of straps  718 ,  720 , the frequency of the antenna  700  in the 2.4 GHz band may be adjusted down finely and up coarsely, respectively, over a range of about 550 MHz. The number and placement of both the first and second set of straps  718 ,  720  are variable depending on design considerations or convenience as well as the ultimate mechanical tolerance of the fabrication technique. For example, the conventional stamping process requires a minimum of 0.2 mm trace and a 0.2 mm gap between straps. 
     The resonance frequency upper 5-6 GHz band may be tuned by cutting or otherwise removing fingers  722  off of the edge of the PIFA  502 . The twelve fingers  722  extend in parallel from the edge of the PIFA  702  most distal to the connection between the PIFA  702  and the RFPIFA  708  towards this connection. Each finger  722  that is removed shifts the upper resonance by about 30-40 MHz. If all the fingers  722  are removed, the total tuning range is about 500 MHz assuming the initial resonance is approximately 5 GHz. The number of fingers is alterable as desired within the minimum tolerance of the fabrication technique, as above, and with a larger number of fingers each providing a smaller change in frequency and a smaller of fingers each providing a larger change in frequency. Note that in any of the tuning mechanisms, the material can be easily cut or removed to alter the frequency because the material is exposed at the top of the overall antenna structure and has an undercarriage underneath the material that supports the material, as discussed below. Variations of the tuning mechanism may be found in a currently pending related U.S. application serial number entitled “Method of Mechanically Tuning Antennas for Low-Cost Volume Production,” filed Oct. 16, 2002 in the names of Greg S. Mendolia and James Scott and commonly assigned to the assignee of the present application, incorporated herein by reference in its entirety. 
     Turning to the electrical characteristics of the RFPIFA, the reactance of the short will dominate the reactance of the open circuited line unless the open circuited line is at or near its resonant length. Assuming that the short can be represented by a small inductance to ground and that the 2.4 GHz element can be represented by an open ended transmission line 90 degrees long at 2.4 GHz, the reactance of the 2.4 GHz element with the short may be written as follows (where Z tline  is the impedance of the transmission line, L short  is the inductance associated with the short, ω is 2π*frequency, β is 2π*frequency/propagation velocity of the transmission line in meters per second, and 1 is the length of the transmission line):          Z     2   ,   4       =     1     (       (       Z   tline     /     (       -     jZ   tline                     cot                   (     β                 1     )       )       )     +     (       Z   tline     /     (       jL   short                   ω     )       )       )                 Γ     2   ,   4       =       (       Z     2   ,   4       -   1     )       (       Z     2   ,   4       +   1     )                              
     The electrical characteristics of the RFPIFA and short are shown in FIG. 10 a-f . The measured RFPIFA and short are illustrated in FIGS. 8 and 9, respectively. The RFPIFA and short of FIGS. 8 and 9 were placed on a 2.5 mm dielectric spacer made from Rohacell Foam, as the PIFA above, and then measured. FIG. 10 a  shows the reactance of a 0.025 nH shorted inductor in a 100Ω system plotted from 2.4-2.5 GHz. FIG. 10 b  shows the reactance of a 100Ω transmission line that is 100 degrees long (lossless) at 2.4-2.5 GHz. FIG. 10 c  shows the reactance of the parallel combination of the open ended transmission line and the shorted inductor. Note the two elements together are resonant but there is no loss in the system. Similarly, FIG. 10 d  shows the reactance of a shorted inductor from 5-6 GHz. FIG. 10 e  shows the reactance of a 100Ω open ended transmission line that is 90 degrees long (lossless) at 2.45 GHz. FIG. 10 f  shows the reactance of the parallel combination of the open ended transmission line and the shorted inductor from 5-6 GHz. 
     As can be seen, the parallel combination of the shorted inductor and the open ended transmission line shown in FIG. 10 f  is nearly identical to the response of the short alone. The result suggests that the short in the 5-6 GHz element can be replaced by a RFPIFA without degrading the performance from 5-6 GHz thereby inviting the combination of a PIFA and a RFPIFA for use as a multi-band antenna. In general, when attempting to realize multi-band performance from PIFA elements with the resonances being an octave or more apart, the lower resonance will reactively load the higher frequency element and tend to limit the bandwidth of the upper resonance. The lower frequency element is electrically long at the upper resonance and the reactance of the lower frequency element will change quickly with frequency relative to the response of the upper resonance. However as can be seen by the electrical characteristics above, using a RFPIFA for the lower resonance eliminates this problem because the response of the RFPIFA is dominated by the response of the short when the reverse fed element is not resonant. The higher frequency element does not generally present a problem to the lower frequency element because the higher frequency element is electrically short in the lower band. 
     This is further shown by the measurements of FIGS. 11 and 12, which illustrate the correlation between a short on the surface of the antenna versus a reverse fed PIFA over frequency. One can see from these figures that there is a very good correlation between the RFPIFA and the short from 3.5 GHz to 6.25 GHz, which again suggests that the PIFA can be easily integrated with the RFPIFA that is resonant in the lower frequency range without significantly compromising the bandwidth of the higher frequency element. 
     FIGS. 13-15 illustrate three-dimensional views of FIG. 7 without a supporting structure or with an undercarriage. In general, the antenna  700  can be placed on any low dielectric material and mounted on a PCB. Low dielectric material is one or more layers having a total permittivity of the material is between 1 and about 1.7, preferably between about 1-1.4. An example of such a solid material is foam, for instance, as used in the test structures shown in FIGS. 8 and 9. Although the antenna  700  as illustrated in FIG. 13 (shown with conductive mounting feet  732 ) could be mounted directly on the PCB, the overall antenna structure would be relatively weak and easily damaged most frequently during mounting. The antenna  700  is thus formed with an undercarriage  730  to reinforce the structural integrity. 
     Details of the fabrication technique may be found in co-pending U.S. non-provisional patent application filed Oct. 2, 2002, entitled “Method of Manufacturing Antennas using Micro-Insert-Molding Techniques” in the names of Greg S. Mendolia and Yizhon Lin which is incorporated herein by reference in its entirety and commonly assigned to the assignee of the present application. Briefly however, the antenna  700  may be fabricated by stamping the antenna  700  design in metal. The metal is then placed in an injection mold, which is belly up with the metal disposed at the bottom of the mold. Liquid crystal polymer is then injected into the mold to form the plastic undercarriage  730  including legs  734 . The injection of the polymer forces the metal to the surface of the mold and thereby makes the antenna structure highly repeatable. Standard surface mount techniques are used to assemble these antennas on the PCB (not shown); that is, introducing solder paste on mounting pads within the PCB, placing the antenna  700  on these pads with the conductive mounting feet  732  in contact with the solder, and melting the solder to form a permanent electrical connection between the antenna  700  and the PCB. The antenna  700  thus does not require any cables, connectors, tuning, or matching components and can be fabricated in a high volume production process without hand assembly. 
     After fabrication, the PIFA/RFPIFA is disposed about 3 mm from the ground plane. In general, the height of the structure, i.e. the distance of the PIFA/RFPIFA from the ground plane, can vary between about 2 mm to about 4 mm. This height is chosen according to design considerations that balance decreased separation between the PIFA/RFPIFA and the ground plane, which decreases the performance of the antenna, and increased separation, which increases the overall size of the antenna and may result in the antenna not meeting the height specifications of the electronics. The above separation of about 3 mm includes about 0.5 mm plastic undercarriage supporting the antenna and about 2.5 mm of air between the undercarriage and the ground plane. As above, the composite permittivity between the PIFA/RFPIFA and the ground plane is between 1.1 and 1.4. 
     The thickness of the undercarriage is chosen to balance the mechanical stability of the structure, which decreases with decreasing thickness, and the ability of the structure to straddle electronic components disposed underneath on the PCB, which decreases with increasing thickness (assuming that the overall thickness remains constant). In addition, the use of minimal plastic also helps to reduce the effect of the plastic on the resonance frequencies as well as variations caused by fluctuations in the dielectric of the plastic when the ratio of volume of plastic to volume of air is low (up to about 20-25%). Further, thinner plastic permits thicker metal for the antenna, feed, and short, which decreases overall resistive losses without overall increase in thickness. With these considerations, the thickness of the undercarriage is between about 0.4 mm to 1.0 mm, preferably about 0.3-0.5 mm. 
     The use of multiple legs promotes stability and robustness of the structure. In the antenna of the present embodiments, four legs are formed, which helps to stabilize the antennas when mounted and decrease the susceptibility of the antenna structure to inadvertently applied external force that may distort or destroy the antenna structure. The legs  734  have isolated islands of metal (the mounting feet  732 ) at the ends of all but one of the legs. As above, these small flat pieces of metal  732  are used as solderable surfaces to create mounting pads at the bottom of each leg  734 . The last leg  736  has metal contacts that are directly connected between the main antenna  700  and the PCB (the ground plane and signal feed), and thus does not use the isolated mounting pads  734 . The wider short  706  permits easier soldering to the ground plane, but does not significantly benefit the performance of the antenna  700 . The antenna is mounted on an edge or corner location of the PCB for optimal performance: movement of the antenna to the sides of the board, away from the corner, results in a 2 to 3 dB loss in efficiency and movement to the center of large boards decreases the efficiency even further. 
     The antenna size after fabrication is relatively small, typically 10×14×2.4 mm and weighs a maximum of 0.18 g. The mounting area on the PCB required for a typical antenna is 140 mm 2 , the total contact area on the PCB is 2.0 mm 2 , and the maximum height of components under the antenna is 1.7 mm. 
     To determine the appropriate embodiment for a particular application, antenna samples are mounted to location on a PCB as required by the particular design along with all surrounding or underlying components. A standard surface mount technique with 5 mils thick solder paste on all mounting pads is used. The antenna performance is measured including resonant frequency and bandwidth. Components used during this measurement should be no greater than 1.0 mm in height from the PCB ground layer. The embodiment is determined based on measured return loss. 
     The reduction in size enabled by the antennas in the above embodiments makes these antennas particularly well suited for applications with densely populated PCBs. The electrical characteristics of the antenna, as shown above, are ideal for Bluetooth and 802.11b/g products particularly since they are often used in different environments ranging from ground planes the size of a thumbnail (for products such as wireless hands-free kits) to large ground planes (for applications such as printers or laptops). Also, due to the very low profile of the antenna, the antenna is well suited for demanding portable Bluetooth devices with severe restriction on total height. 
     Furthermore antennas can ultimately be fabricated as an integral part of the RF module; that is the antennas can be fabricated with a complete Bluetooth RF multi-chip module (MCM) system embedded inside the antenna. The antennas can be designed to accommodate both passive and active RF components within their form factor without any significant degradation of performance. In addition to being surface mountable directly on the board, components such as front-end modules or filters can be directly placed inside the antenna volume. Subsequently, the antenna can be seamlessly integrated into the radio frequency (RF) front end without adversely affecting performance. 
     In summary, the antenna is electrically small given that its largest dimension is λ/10. Size reduction is achieved without any dielectric loading, but instead by designing the antenna with built-in inductive and capacitive features to act as a slow wave structure. The antenna design does not use dielectric loading or traditional meander lines to reduce size, thus efficiency is maximized for minimum Q-factor. Such internal loading also allows the resonant frequency to be insensitive to proximity effects (of users, components such as integrated circuits or passive chips, or the loading effects of plastic housings), to temperature and humidity changes, and to changes in ground plane size and component layout. Further, these low profile antennas can be surface mounted directly onto a ground plane. This saves board space, permits components to be mounted beneath the antenna, and enables board area on the opposite side of the PCB to be used for additional components. 
     In addition, the antenna may be produced by repeatable high-volume manufacturing techniques using lightweight molded plastics and assembled using standard surface mount technology processes in which cables or connectors are not required. 
     Although antennas for multiple frequencies within the 2.4 and 5-6 GHz ranges are described above, there is no physical reason why the above structure cannot be scaled (and perhaps the FSS modified) for different frequencies and different applications. One example would be to use a RFPIFA structure of about 7 mm for reception and transmission in the 800 MHz range and incorporate a PIFA structure as the 1.9 or 2.4 GHz element. 
     While particular embodiments of the present invention have been shown and described, modifications may be made by one skilled in the art without altering the invention. It is therefore intended in the appended claims to cover such changes and modifications which follow in the true spirit and scope of the invention.