Abstract:
A dual-frequency planar antenna disclosed herein utilizes a main radiating device to produce a resonance mode and excites a parasitic radiating device to produce another resonance mode by the coupling of energy. These two modes can provide sufficiently broad bandwidths, and the present invention is simple in design, which makes it cost effective. Therefore, the planar antenna of the present invention is a competitive alternative for wireless communication applications.

Description:
This application incorporates by reference Taiwan application Serial No. 090132623, filed Dec. 27, 2001. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates in general to a planar antenna, and more particularly to a planar inverted-F antenna of dual frequencies. 
     2. Description of the Related Art 
     Due to developments in communications technology, various wireless products are produced in great quantities. Recently, the Bluetooth system has been developed to enable communications between electronic products, such as computers, printers, digital cameras, refrigerators, TVs, air conditioners, and other wireless products. The frequency range of the ISM (Industrial Scientific Medical) band for Bluetooth is 2.4 to 2.4835 GHz. If more and more wireless products employ the Bluetooth system, the single frequency band of the ISM will not sufficiently support the large volume and transmission rate. The same situation also happens in the other wireless communication systems of ISM 2.4 GHz, such as WLAN (wireless local area network) and HomeRF (Home radio frequency). 
     Therefore, a dual-frequency antenna has been developed to reduce the volume of the wireless communication products by combining two frequencies in an antenna. Furthermore, the product of a dual-frequency antenna will be more competitive if the size of the dual-frequency antenna is minimized. Accordingly, a PIFA (planar inverted-F antenna) is developed to decrease the amount of space occupied, wherein the length of the PIFA is reduced to λ/4, instead of λ/2, which is the length of the traditional planar antenna. This reduction in the size of the planar antenna makes it possible to be concealed within most of the present-day communication devices. 
     Please refer to FIG. 1, which shows the structure of a PIFA (planar inverted-F antenna) according to a traditional design. The PIFA  100  is composed of a radiator  110 , a grounding plane  130 , a medium  150 , a shorting pin  170 , and a feeding means  190 . The medium  150  is used to separate the radiator  110  and the grounding plane  130 , and is positioned between the two. The material of medium  150  can be air, dielectric substrate, or the combination of them. The radiator  110  is coupled to the grounding plane  130  by the shorting pin  170 , which is made of metal. The feeding means  190 , such as SMA connector, can be equipped on the ground and coupled to the radiator  110  to deliver the microwave signal. The radiator  110  and the grounding plane  130  are made of metal, wherein the radiator  110  can be of various patterns, according to the different requirements. 
     Basically, the structures of each PIFA are the same, for instance, the separation of the grounding plane and the radiator by the medium, the coupling of the radiator to the grounding plane by the shorting pin, and the coupling of the feeding means  190  to the radiator. The operational characteristic of the PIFA is determined by the pattern of the radiator. Shown in FIG. 2A is the radiator pattern of a PIFA with dual frequencies, according to a traditional design. The grounding point  271  and the feeding point  291  are, respectively, the parts of the shorting pin contacting with the radiator  210 A and the feeding means contacting with the radiator  210 A, wherein the former is represented by a square and the latter is represented by a circle. The same representations for the grounding point and the feeding point are used in the following figures. 
     In FIG. 2A, an L-shaped slit is embedded in the radiator  210 A, wherein two surface current paths of L 1  and L 2  for the dual frequencies are formed. The radiator  210 A resonates at the higher frequency, such as 5.8 GHz, with the shorter path L 1 , and resonates at the lower frequency, for instance 2.4 GHz, with the longer path L 2 . 
     Please refer to FIG. 2B, which shows a PIFA of dual frequencies according to another traditional design. As the radiator  210 B is excited, the U-shaped slot is responsible for the formation of two current paths in the radiator  210 B, wherein the shorter current path L 1  produces the higher frequency and the longer current path L 2  produces the lower frequency. 
     The detailed configurations of the PIFAs in FIG.  2 A and FIG. 2B are disclosed in “New slot configurations for dual-band planar inverted-F antenna”, Microwave Optical Technology Letters, vol. 28, No. 5, Mar. 5, 2001, pp. 293-298. Such kinds of dual-band PIFA usually cannot afford a sufficiently broad bandwidth. In U.S. Pat. No. 5,764,190, the inverter-F antenna is designed using the capacitive effect or a capacitive feed, which can provide an adequate bandwidth. However, this design is relatively very complicated and the fabrication cost is very high. 
     To solve the problems mentioned above, the present invention discloses a PIFA with broad bandwidth, simple structure, and low cost. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a dual-frequency PIFA with the advantages of broad bandwidth and simple structure. 
     In accordance with the object of the invention, a dual-frequency PIFA is disclosed, wherein the said PIFA has a first operational band, such as 2.4 GHz ISM band, and a second operational band, such as 5.8 GHz ISM band. The dual frequency PIFA comprises a grounding plane, a main radiating device, a parasitic radiating device, a medium, two shorting pins and a feeding means, wherein the main radiating device and the parasitic radiating device are coupled to the grounding plane through shorting pins, respectively. The feeding means positioned on the grounding plane is coupled to the main radiating device for transferring the microwave signal. The excitation of the main radiating device triggers the excitation of the parasitic radiating device by the coupling of the electromagnetic energy. The first resonance mode of the main radiating device enables the PIFA to operate in the first operational band and the first resonance mode of the parasitic radiating device enables the PIFA to operate in the second operational band. Thus, the PIFA can operate in dual frequencies. 
     Please note that the structure of the present invention is not limited to the PIFA. It is also applicable in a planar antenna. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The description is made with reference to the accompanying drawings. 
     FIG. 1 it shows a structure of the PIFA according to a traditional design. 
     FIG. 2A shows the radiator pattern of the PIFA with dual frequencies according to a traditional design. 
     FIG. 2B shows a PIFA of dual frequencies according to another traditional design. 
     FIG. 3 shows a dual-frequency PIFA according to a preferred embodiment of the present invention. 
     FIG. 4 shows the return loss of the PIFA according to the preferred embodiment of the present invention. 
     FIG. 5A shows the measurements of the H-plane radiating pattern and E-plane radiating pattern as the PIFA operates at 2.4 GHz according to the preferred embodiment of the present invention. 
     FIG. 5B shows the measurements of the H-plane and E-plane radiating patterns as the PIFA operates at 5.8 GHz according to the preferred embodiment of the present invention. 
     FIG. 6A shows the relationship between gain and frequency as the PIFA operates in the 2.4 GHz band according to the preferred embodiment of the present invention. 
     FIG. 6B shows the relationship between gain and frequency as the PIFA operates in the 5.8 GHz band according to the preferred embodiment of the present invention. 
     FIG. 7 shows the condition that a slot is embedded in the radiator according to the preferred embodiment of the present invention. 
     FIG. 8A shows the structure that the main radiating device is circular and the parasitic radiating device is annular according to the preferred embodiment of the present invention. 
     FIG. 8B shows the structure that the main radiating device is a smaller annular structure and the parasitic radiating device is a larger annular structure according to the preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the present invention, the radiator of the PIFA (planar inverted-F antenna) consists of a main radiating device and a parasitic radiating device, wherein the main radiating device is equipped with a feeding means. As the main radiating device is excited, some part of the energy of the electromagnetic wave is coupled to the parasitic radiating device. Then, the parasitic radiating device is also excited, and the PIFA can operate in dual frequencies, wherein the band of the first frequency is operated in the first resonance mode of the main radiating device and the band of the second frequency is operated in the first resonance mode of the parasitic radiating device. Please note that the characteristics of the present invention are not limited to the PIFA, and it is also applicable in any planar antenna operated in dual frequencies. 
     For example, consider the ISM band. To produce the operational band of 2.4 GHz (2400˜2500 MHZ), the parasitic radiating device is excited by the main radiating device through the coupling of the electromagnetic wave. The operational band of 5.8 GHz (5725˜5850 MHz) is produced by exciting the main radiating device. The bandwidth of the 2.4 GHz and the 5.8 GHz are both wide enough for use. 
     Please refer to FIG. 3, which shows a dual-frequency PIFA according to a preferred embodiment of the present invention. The basic structure is similar to that of the traditional design, wherein a medium  150  is positioned between a grounding plane  130  and a radiator, and is composed of air and a microwave substrate. In addition, the radiator of the present invention consists of a main radiating device  31  and a parasitic radiating device  32 , wherein the parasitic radiating device  32  has a concave side which is opposite and partially surrounds the main radiating  31 , as shown in FIG.  3 . 
     The main radiating device  31  and the parasitic radiating device  32  are coupled to the grounding plane  130  through shorting pin  317  and shorting pin  327 , respectively. The shorting pin  317  and the shorting pin  327  are made of a metal pin. The grounding point  312  is the part of the shorting pin  317  contacting with the main radiating device  31 , and the grounding point  322  is the part of the shorting pin  327  contacting with the parasitic radiating device  32 . 
     Please note that a feeding means  190 , equipped on the grounding plane  130 , is a SMA connector and is only coupled to the main radiating device  31 , wherein a feeding point  311  is the point of feeding means  190  connecting to the main radiating device  31 . After a microwave signal is fed into the main radiating device  31  through the feeding means  190 , the main radiating device  31  is excited. The electromagnetic energy is coupled to the parasitic radiating device  32  by irradiating, and the parasitic radiating device  32  is then excited. Therefore, the PIFA of the present invention has the characteristics of dual frequencies. 
     As shown in FIG. 3, the main radiating device  31  is smaller than the parasitic radiating device  32 . Both of the main radiating device  31  and the parasitic radiating device  32  resonate at λ/4, and thus the former provides the operational bandwidth of higher frequency, such as 5.8 GHz, and the latter provides the operational bandwidth of lower frequency, such as 2.4 GHz. While the main radiating device  31  is larger than the main radiating device  32 , the former and the latter provide the operational bandwidth of lower frequency and higher frequency, respectively. 
     Referring to FIG. 4, it shows the return loss of the PIFA according to a preferred embodiment of the present invention. With the parasitic radiating device, the PIFA operates in the 2.4 GHz band, which is the first resonance mode of the parasitic radiating device and has a bandwidth of 132 MHz (2383˜2515 MHz) according to the definition of an impedance bandwidth in 1:2.5 VSWR. With the main radiating device, the PIFA operates in the 5.8 GHz band, which is the first resonance mode of the main radiating device and has a bandwidth of 695 MHz (5370˜6065 MHz) according to the definition of an impedance bandwidth in 2:1 VSWR. These two modes of the present invention resonate in λ/4, and the characteristics of the corresponding antennas are improved. 
     Referring to FIG. 5A, it shows the measurements of the H-plane and E-plane radiating patterns as the PIFA operates at 2.4 GHz, wherein the principal polarization pattern is represented by the thicker line and the cross polarization pattern is represented by the thinner line. Additionally, the H-plane is the x-z plane and the E-plane is the y-z plane. 
     Referring to FIG. 5B, it shows the measurements of the H-plane and E-plane radiating patterns as the PIFA operates at 5.8 GHz. As in FIG. 5A, the principal polarization pattern and the cross polarization pattern are represented by the thicker line and the thinner line, and the H-plane and the E-plane are the x-z plane and the y-z plane, respectively. Please refer to FIG.  6 A and FIG. 6B, they show the relationship of the gain and the frequency as the PIFA operates in the 2.4 GHz and 5.8 GHz bands, respectively. 
     Referring to FIG. 7, which shows the condition that slits  715  are embedded in the main radiator, wherein the path of the exciting surface current path is lengthened and the resonance frequency is decreased. To maintain a constant resonance frequency, the size of the radiator embedded with slits will be smaller than that of the radiator without slits. Therefore, the volume of the PIFA can be decreased by applying a slot. For the same reason, the size of parasitic radiating device  72  will be decreased and the path of the exciting surface current will be lengthened by embedding a rectangular slot  725  therein. Please note that, in FIG. 7, the resonance frequency of the main radiating device  71  is lower than that of the parasitic radiating device  72  due to the difference of their sizes. In addition, the main radiating device  71  has a concave side which is opposite and partially surrounds the parasitic radiating device  71 , as shown in FIG.  7 . 
     Besides a rectangular shape, the radiating device can be implemented by another shape. For instance, as shown in FIG. 8A, the main radiating device  81 A is circular and the parasitic radiating device  82  is annular to surround the main radiating device  81 A. In FIG. 8B, the main radiating device  81 B is a smaller annular structure and the parasitic radiating device  82  is a larger annular structure surrounding the main radiating device  81 B. 
     While the invention has been described by way of example and in terms of the preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiment. To the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.