Abstract:
A multi-frequency antenna for receiving a first frequency and second frequency signals comprises a grounding element, a first conductive member, a first radiation member, and a second radiation member. The first conductive member connects to the grounding element. The first radiation member and the second radiation member connect to the first conductive member separately. The multi-frequency antenna further comprises a parasitic structure. The parasitic structure structurally encircles the second radiation member and the encirclement is a partial encirclement. Moreover, the parasitic structure connects to the grounding element.

Description:
RELATED APPLICATIONS 
   This application claims priority to Taiwan Application Serial Number 95145782, filed Dec. 7, 2006, which is herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates to an antenna structure and, in particular, to a multi-frequency antenna structure. 
   2. Related Art 
   The connections and communications among various wireless networks, such as wireless personal area networks (WPAN), wireless local area networks (WLAN), and wireless wide area networks (WWAN), or system devices can be implemented with the antennas therein. 
   Generally speaking the antennas of wireless devices can be external or internal. For example, the external antennas of some laptop computers are disposed at the top of the monitors or on the PCMCIA cards. Such external antennas have higher costs because they are exposed to the environment and more susceptible to damages. The other design is to embed antennas inside the laptop computers. 
   The internal antenna designs can avoid drawbacks of external antennas. For example, the computer device can have a better appearance. The antenna is also prevented from accidental damages. However, building the antenna inside a spatially limited computer device may have bad effects on its efficiency. Therefore, the internal antennas have to be appropriately designed in order to fit the space inside the portable computer device and to provide a sufficient efficiency. 
   SUMMARY OF THE INVENTION 
   An objective of the invention is to provide a multi-frequency antenna for wireless devices such as the laptop computer to transmit and receive wireless signals within limited space. 
   In accord with the above-mentioned objective, the invention provides a multi-frequency antenna for receiving signals of a first frequency and a second frequency. The multi-frequency antenna has a grounding element, a first conductive member, a first radiation member, and a second radiation member. The first conductive member has a conductive component and a ground connecting component. One edge of the ground connecting component connects to the conductive component perpendicularly, and its other side connects to the grounding element. The first radiation member receives the first-frequency signal, and connects to the conductive component. The second radiation member receives the second-frequency signal, and connects to the conducive component at a predetermined distance from the first radiation member. The first radiation member is partially disposed between the grounding element and the second radiation member. 
   The multi-frequency antenna is disposed in a three-dimensional space with a first surface, a second surface, a third surface, and a fourth surface. The second surface is roughly perpendicular to the first surface. The third surface is roughly parallel to the second surface, and perpendicular to the first surface. The fourth surface is roughly parallel to the first surface, and roughly perpendicular to the second and third surfaces. The multi-frequency antenna includes a grounding element, a first conductive member, a first radiation member, and a second radiation member. The grounding element is disposed on the first surface. The first conductive member has a conductive component and a ground connecting component. The ground connecting component is disposed on the second surface, with one edge connected to the conductive component and the other edge connected to the grounding element. The first radiation member receives signals of the first frequency and connects to the conductive component. The first radiation member distributes over the second surface and the third surface. The second radiation member receives signals of the second frequency and connects to the conductive component at a predetermined distance from the first radiation member. The second radiation member is disposed on the second, third, and fourth surfaces. The multi-frequency antenna is further installed with a passive element and a parasitic structure to increase the frequency response of the first and second radiation members. 
   Therefore, the disclosed multi-frequency antenna can provide good wireless signal transmission and reception efficiency even in a limited space of a portable computer device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the invention will become apparent by reference to the following description and accompanying drawings which are given by way of illustration only, and thus are not limitative of the invention, and wherein: 
       FIG. 1  is an expanded planar view of the multi-frequency antenna according to an embodiment of the invention; 
       FIGS. 2A to 2E  are planar exploded views of various parts of the multi-frequency antenna; 
       FIG. 3  is a schematic view of the lines for bending the antenna planar structure according to the embodiment; 
       FIGS. 4A to 4D  are three-dimensional views of the multi-frequency antenna from different perspective angles; 
       FIG. 5  shows the sizes of various parts of the multi-frequency antenna; 
       FIG. 6  shows the VSWR of the multi-frequency antenna before the installation of the passive element and the parasitic structure; 
       FIG. 7  shows the antenna efficiency of the multi-frequency antenna before the installation of the passive element and the parasitic structure; 
       FIG. 8  shows the VSWR of the multi-frequency antenna after the installation of the passive element but not the parasitic structure; 
       FIG. 9  shows the antenna efficiency of the multi-frequency antenna after the installation of the passive element but not the parasitic structure; 
       FIG. 10  shows the VSWR of the multi-frequency antenna after the installation of the passive element and the parasitic structure; 
       FIG. 11  shows the antenna efficiency of the multi-frequency antenna after the installation of the passive element and the parasitic structure; 
       FIG. 12  shows the measured result of the S 21  parameter in the band of 0.8 GHZ to 2.5 GHz 
       FIG. 13  shows the measured result of the S 21  parameter in the band of 2 GHZ to 6 GHz; 
       FIG. 14  is an expanded planar view of the multi-frequency antenna according to another embodiment of the invention; and 
       FIG. 15  shows the VSWR of the multi-frequency antenna of  FIG. 14 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same elements. 
   An embodiment of the invention is a multi-frequency antenna disposed in a portable electronic device with the wireless communication function, such as a laptop computer or a personal digital assistant (PDA). Such a multi-frequency antenna can receive signals in at least two frequency bands. For the convenience of description, this specification refers exclusively to their central frequencies unless specified. That is, the specification uses a first frequency and a second frequency to represent the two bands. Any person skilled in the art can vary different parameters in the antenna design for different applications according to the need. 
   A planar view of the multi-frequency antenna according to an embodiment of the invention is shown in  FIG. 1 . In this embodiment, the multi-frequency antenna  100  has a grounding element  110 , a first conductive member  120 , a first radiation member  130 , and a second radiation member  140 . The first radiation member  130  receives signals of the first frequency, and the second radiation member  140  receives signals of the second frequency. To increase the frequency response of the first conductive member  120  and the second radiation member  140 , a passive element  136  and a parasitic structure  150  are further disposed on the multi-frequency antenna  100 . The connecting relations and detailed structures of various parts of the invention are given in  FIGS. 2A to 2E . These figures are planar views of various parts of the multi-frequency antenna. Certain parts are not described and labeled with numerals to avoid complications. 
   In  FIG. 2A , the first conductive member  120  has a conductive component  122  and a ground connecting component  124 . The ground connecting component  124  perpendicularly connects to the conductive component  122  with one edge and to the grounding element  110  with the other edge. 
     FIG. 2B  describes the structure of the first radiation member  130 . The first radiation member  130  receives signals of the first frequency, and connects to the conductive component  122 . The first radiation member  130  has a first radiation body  139  and a first connecting part  132 . The first radiation body  139  connects to the conductive component  122 . One end of the first connecting part  132  is connected to the conductive component  124  via a first connecting point  138 . The other end of the first connecting part  132  has a ladder-shaped structure  134 . 
   Besides, the first radiation member  130  further includes a passive element  136  to increase the frequency matching of the first radiation member  130 . The passive element  136  is disposed on the first connecting part  132 . However, whether the passive element  136  should be installed on the multi-frequency antenna  100  is determined by the working bands of the antenna. 
   With reference to  FIG. 2C , the second radiation member  140  has a second radiation body  142  connected with the conductive component  122 . The second radiation member  140  further includes an L-shaped extension  144  connected with the second radiation body  142  and extending from the second radiation body  142  to the first radiation body  139 . The L-shaped extension  144  further includes a first extension  146  extending toward the ladder-shaped structure  134  with a shape corresponding to that of the ladder-shaped structure but not touching the ladder-shaped structure  134 . That is, the first extension  146  and the ladder-shaped structure  134  are separate. 
     FIG. 2D  shows the appearance of the parasitic structure  150 . The parasitic structure  150  is designed to increase the frequency response of the second radiation member  140 . Therefore, whether it should be installed on the multi-frequency antenna  100  depends upon the need. The shape of the parasitic structure  150  corresponds to that of the second radiation member  140 . The parasitic structure  150  and the second radiation member  140  are separate. One end of the parasitic structure  150  has a ground connecting part  152  connected with the grounding element  110 . It will be further described later. In this embodiment, the parasitic structure  150  has a shape encircling the second radiation member  140  to increase the frequency response thereof. In other embodiments, it is also designed according to the shape of the second radiation member  140 . 
   Furthermore, the multi-frequency antenna in this embodiment can be installed with a third radiation member  210  to increase the applicable wireless standard of the multi-frequency antenna. Therefore, the ground connecting part  152  of the parasitic structure  150  further extends out a second conductive member  154 . The third radiation member  210  connects to the second conductive member  154  via a second connecting point  156 . In other words, the third radiation member  210  of the multi-frequency antenna connects to the parasitic structure  150 . The structure of the third radiation member  210  is shown in  FIG. 2E . In this embodiment, the third radiation member  210  includes a first portion  212  and a second portion  216 . The first portion  212  and the second portion  216  receive signals of a third frequency and a fourth frequency, respectively. The first portion  212  and the second portion  216  are connected via a third conductive member  214 . The third radiation member  210  can have different shapes in accord with different wireless standards in other embodiments. 
   In practice, the multi-frequency antenna in this embodiment is disposed in a three-dimensional space inside a wireless device. Therefore, the above-mentioned structure bends along some specific line. Please refer to  FIG. 3  showing the antenna structure bending along a line. The multi-frequency antenna structure in this embodiment bends along three lines A, B, and C to form a three-dimensional structure. 
   Please refer to  FIGS. 4A to 4D , showing the multi-frequency antenna of this embodiment in different perspective angles.  FIGS. 4A and 4B  are three-dimensional views from different angles.  FIGS. 4C and 4D  are side views of both ends of the antenna.  FIG. 4A  shows the multi-frequency antenna after it bends along the three lines. 
   The three-dimensional space of the multi-frequency antenna has four surfaces, a first surface  410 , a second surface  420 , a third surface  430 , and a fourth surface  440 . The second surface  420  is perpendicular to the first surface  410 . The third surface  430  is parallel to the second surface  420  and perpendicular to the first surface  410 . The fourth surface  440  is parallel to the first surface  410  and perpendicular to the second surface  420  and the third surface  430 . As  FIGS. 4A to 4D  are for different viewing angles, the specification uses the X, Y, and Z axes to define the four surfaces. The negative Y axis points to the first surface  410 . The positive Y axis points to the third surface  430 . The negative X axis points to the first surface  420 . The positive X axis points to the fourth surface  440 . Since the connecting relations of various components in the antenna have been described before, they are not repeated hereinafter. 
     FIGS. 4A and 4B  show how various components are distributed in the three-dimensional structure of the antenna. The grounding element  110  is disposed on the first surface  410 . The first conductive member  120  distributes over the second surface  420 , the third surface  430 , and the fourth surface  440 . The first connecting part  132  exists on the second surface  420 . The first radiation body  139  distributes over the second surface  420  and the third surface  430 . 
   The second radiation body  142  and the parasitic structure  150  are located on the fourth surface  440 . The second conductive member  154  exists on the fourth surface  440 . The parasitic structure  150  extends via the third surface  430  to the second surface to increase the frequency response of the second radiation member  140 .  FIG. 5  shows that the ground connecting part  152  and the grounding element  110  are connected in the three-dimensional space, so that the entire antenna structure has all the fourth surfaces connected. 
   The first portion  212  of the third radiation member  210  is also located on the fourth surface  440 . The third conductive member  214  is located on the third surface  430 . The second portion  216  is located on the second surface  420 . The first portion  212  and the second portion  216  are connected via the third conductive member  214 . 
   The L-shaped extension  144  is located on the third surface  430 , extending from the second radiation body  142  toward the first radiation body  139 . The first extension  146  extended from the L-shaped extension  144  is located on the second surface  420 . 
   As shown in  FIGS. 4C and 4D , the components on the second surface  420  are not disposed on the same plane. It consists of surfaces  422 ,  424 ,  426 , and  428 . Please refer simultaneously to  FIG. 4A . Surface  428  is a ground connecting component  124 . Surface  426  has the conductive component  122  and the first connecting part  132 . Surface  422  includes the second portion  216 , the first radiation body  139 , the first extension  146 , the first connecting part  132 , and the parasitic structure  150 . The parasitic structure  150  extends to part of the first surface, but does not bend to reach surface  426 . It bends at a different position to produce surface  424 . 
   To fully understand the functions of the disclosed multi-frequency antenna, this embodiment is applied to the working bands of a wireless wide area network (WWAN). The working bands of the WWAN are about 824˜960 MHz and 1710˜2170 MHz. The sizes of various components of the antenna are shown in  FIG. 5  in units of millimeters (mm). The drawing also shows the voltage standing wave ratio (VSWR) and efficiency of the antenna. In the VSWR plot, the horizontal axis is the frequency and the vertical axis is the return loss. In particular, point A has a frequency of 824 MHz, point B has a frequency of 960 MHZ, point C has a frequency of 1710 MHz, and point D has a frequency of 2170 MHz. The antenna efficiency plot has the frequency as its horizontal axis and the efficiency as its vertical axis. According to the VSWR plot, the return loss of the antenna in the WWAN working bands is expected to be lower than 2, ensuring a good impedance matching. 
   Please refer to  FIGS. 6 and 7 .  FIG. 6  shows the VSWR when the multi-frequency antenna does not have the passive element and the parasitic structure.  FIG. 7  shows the antenna efficiency of the same. Most of the return loss between point A and point B is above 2. The situation is the same between point C and point D. In  FIG. 7 , the working efficiencies of the antenna in the frequency bands 824˜960 MHz and 1710˜2170 MHz are not very high. This means that the disclosed multi-frequency antenna can still work even without the passive element and the parasitic structure. However, it can be improved in the working bands of the WWAN. 
   To increase the frequency response of the antenna at high frequencies, the first connecting part is connected with a passive element, such as a capacitive passive element, inductive passive element, or resistive passive element.  FIGS. 8 and 9  show the VSWR and the antenna efficiency after the passive element is installed. As shown in  FIG. 8 , the return loss in most of the band between point C and point D is lower than 2. However, the low-frequency response between point A and point B is still inappropriate for applications in WWAN.  FIG. 9  shows that the antenna efficiency in the two bands has a significant improvement. 
   To further enhance the frequency response of the antenna at low frequencies, a parasitic structure is provided in the antenna, extending from the grounding element and encircling the second radiation member.  FIG. 10  gives the result of the VSWR of the antenna.  FIG. 11  shows the antenna efficiency in this case. The frequency response in either high or low frequencies is almost all below 2. Therefore, the antenna is suitable for WWAN applications after the installation of passive element and parasitic structure. As shown in  FIG. 11 , the antenna has good efficiencies in the two bands used for the WWAN. 
   In addition to the first radiation member and the second radiation member, the multi-frequency antenna in this embodiment is further provided with a third conducive member connected to one end of the parasitic structure. When the antenna is used in a WWAN, the first radiation member and the second radiation member receive signals in high and low frequencies. In this embodiment, the third conductive member uses the design of the first portion and the second portion to receive signals of the wireless area network (WAN). Nevertheless, there should be sufficient separation between the antennas for the WWAN and the WAN in order to ensure the normal operations of the two antennas.  FIGS. 12 and 13  provide the measured result of the parameter S 21  of the antenna. The vertical axis indicates the S 21  parameter in units of dB. The horizontal axis is the frequency. The drawing shows that, with the installation of the WAN antenna in the disclosed multi-frequency antenna, S 21  in the band of 0.8˜1 GMHZ is mostly below −20 dB, meaning that the separation in this band is mostly smaller than −20 dB. S 21  in the band of 1 G˜6 is mostly below −10 dB, meaning that the separation in this band is mostly smaller than −10 dB. Therefore, the two antennas for the WWAN and the WAN have a good separation. 
   Of course, in addition to being used as the WAN antenna, the third radiation member in other embodiments can be used for other wireless communication protocol by tuning its parameters and shape. Such wireless communication protocols include Ultra-wideband (UWB), worldwide Interoperability for Microwave Access (Wi-MAX), and Digital Video Broadcasting. 
   Besides, the invention can have another embodiment.  FIG. 14  is a planar view of the antenna structure. In this embodiment, the second radiation member  910  and the parasitic structure  920  are changed into a long-stripe structure. Other components are the same as the previously mentioned embodiment. A passive element is also installed to increase the frequency response of the first radiation member. After the antenna is bent according to the bending lines mentioned before, its VSWR is shown in  FIG. 15 . The VSWR of the antenna in certain bands can go below 2.5. Although its efficiency is not as good as the embodiment of  FIG. 1 , it can nevertheless be used as an antenna for other bands or be improved for better impedance matching in specific bands by varying its parameters. 
   In all embodiments of the invention, the first connecting point is the signal feeding point of the first radiation member and the second radiation member. The second connecting point is the signal feeding point of the third radiation member. Besides, the disclosed multi-frequency antenna can be made of a thin metal or a soft printed circuit. A plastic solid can be disposed in the central region of the three-dimensional structure for better structural support. 
   The multi-frequency antenna structure of the invention can provide wireless signal transmission and reception within limited space inside a wireless device. The parasitic structure and the passive element are employed to increase the frequency matching of the radiation members. A subsidiary antenna structure can be further attached to the parasitic structure, so that the multi-frequency antenna has wider applications. With the installation of parasitic structure and passive element of appropriate sizes, experiments indicate that the disclosed multi-frequency antenna have good performance in the working bands of the WWAN. 
   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 embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.