Abstract:
A slot antenna assembly for an electronic device comprises an arcuate slot antenna ( 1 ) and a coaxial feeder cable ( 3 ). The slot antenna includes a metal foil ( 2 ) which is bent diagonally to form an arcuate surface ( 20 ). The slot antenna defines an elongated narrow slot ( 21 ) therein. The bent metal foil enlarges radiational scope, to achieve omni-directional radiation as well as increased radiation electric field intensity.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a slot antenna, and in particular to a slot antenna mounted in a portable electronic device and operating in the ISM (Industry, Science, Medicine) frequency band for communicating with various external electronic devices. 
     BACKGROUND OF THE INVENTION 
     There is a growing need for slot antennas for use in wireless communication devices such as notebook computers, Bluetooth apparatus, IEEE802.11 apparatus, wireless LANs, 2.4˜2.5GHz apparatus, and in other applications. Commonly, a slot is formed in a conductive sheet, which is then used as a radiating or receiving element in an antenna. Slot antennas formed with flat metal foil are widely used. 
     FIG. 13 shows a conventional antenna assembly. A flat slot antenna  14  is made from a metal foil  10  having dimensions 68 mm ×11 mm, and has an elongated narrow slot  12  with dimensions 55 mm ×0.7 mm defined therein. A cable  13  has an inner core wire soldered to a flat surface  11  at a first feed-in point  132  near the slot  12 , and an outer shield soldered to the flat surface  11  at a second feed-in point  131  near the slot  12  and opposite the first feed-in point  132 . Each feed-in point  131 ,  132  is 4 mm away from an end of the slot  12 . 
     FIG. 14 shows a measured radiation pattern of the flat slot antenna  14  of FIG. 13 in the X-Z plane. Radiation voids were present at angles 90° and 270° in the X-Z plane. In other words, the X-Z plane has low directivity. Furthermore, a resonant current of the slot in the X-Z plane is small. A measured electric field intensity of the X-Z plane is less than the electric field intensity of the X-Y plane and the Y-Z plane. Thus the radiation in the X-Z plane is weak. The flat slot antenna  14  does not achieve omni-directional radiation. 
     FIG. 15 shows a table of total average gains of the flat slot antenna  14  in the X-Y plane, the Y-Z plane and the X-Z plane at different frequencies when a conventional notebook computer is open. 
     As seen in FIG. 15, the average gain of the flat slot antenna  14  in the X-Z plane is 4˜5 dB less than the average gain in the X-Y plane, and 2˜3 dB less than in the Y-Z plane. 
     A notebook computer with an antenna mounted therein may be used in a variety of different locations. Each location has a different environment which can affect the proper functioning of the antenna. Thus the radiation electric field intensity of the antenna should be sufficiently large and omni-directional to allow the notebook to operate properly in various environments. 
     Hence, an improved antenna is desired to overcome the above-mentioned shortcomings of existing wireless network equipment. 
     BRIEF SUMMARY OF THE INVENTION 
     Therefore, a primary object of the present invention is to provide an antenna assembly having omni-directional radiation. 
     Another object of the present invention is to provide an antenna assembly having an arcuate shape for increasing effective radiation. 
     A further object of the present invention is to provide an antenna assembly occupying minimal space within an electronic device. 
     An antenna assembly in accordance with the present invention comprises an arcuate slot antenna and a coaxial feeder cable. The slot antenna comprises a metal foil which is bent diagonally to form an arcuate surface. The slot antenna defines an elongated narrow slot therein. An inner core of the cable is soldered to the arcuate surface at a first feed-in point near an end of the slot. An outer shield of the cable is soldered to the arcuate surface at a second feed-in point near the end of the slot and opposite the first feed-in point. 
     The slot antenna has a superior scope of maximum directive gain, and a superior total average gain in the X-Y plane, the Y-Z plane and the X-Z plane. 
     Other objects, advantages and novel features of the invention will become more apparent from the following detailed description of a preferred embodiment when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a slot antenna assembly in accordance with the present invention; 
     FIG. 2 is a perspective view of a notebook computer incorporating the slot antenna assembly of FIG. 1; 
     FIG. 3 shows the slot antenna assembly of FIG. 1 being assembled in the notebook computer of FIG. 2; 
     FIG. 4 is a graph of experimental data obtained for the slot antenna assembly of FIG. 1, showing frequency on the horizontal axis varying with Voltage Standing Wave Ratio (VSWR) on the vertical axis; 
     FIG. 5 shows a measurement coordinate system and orientation of axes for the slot antenna assembly of FIG. 1; 
     FIG. 6 shows a measured radiation pattern in an X-Z plane in vertical polarization of the slot antenna assembly of FIG. 1; 
     FIG. 7 shows a measured radiation pattern in the X-Z plane in horizontal polarization of the slot antenna assembly of FIG. 1; 
     FIG. 8 shows a measured radiation pattern in an X-Y plane in vertical polarization of the slot antenna assembly of FIG. 1; 
     FIG. 9 shows a measured radiation pattern in the X-Y plane in horizontal polarization of the slot antenna assembly of FIG. 1; 
     FIG. 10 shows a measured radiation pattern in a Y-Z plane in vertical polarization of the slot antenna assembly of FIG. 1; 
     FIG. 11 shows a measured radiation pattern in the Y-Z plane in horizontal polarization of the slot antenna assembly of FIG. 1; 
     FIG. 12 shows a table of total average gains of the slot antenna assembly of FIG. 1 in the X-Y plane, the Y-Z plane and the X-Z plane at different frequencies when a lid of the notebook computer of FIG. 2 is open; frequencies when a lid of the notebook computer of FIG. 2 is open; 
     FIG. 13 is a top view of a conventional flat slot antenna assembly; 
     FIG. 14 shows a measured radiation pattern in an X-Z plane of the conventional flat slot antenna assembly of FIG. 13; and 
     FIG. 15 shows a table of total average gains of the conventional flat slot antenna assembly of FIG. 13 in the X-Y plane, the Y-Z plane and the X-Z plane at different frequencies when a lid of a notebook computer is open. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to a preferred embodiment of the present invention. 
     Referring to FIG. 1, a slot antenna assembly in accordance with the present invention comprises an arcuate slot antenna  1  and a coaxial feeder cable  3 . 
     The slot antenna  1  comprises a metal foil  2 . The metal foil  2  is diagonally bent between a first corner  22  and a second corner  23  thereof, and thereby forms an arcuate surface  20 . The metal foil  2  can be bent according to a desired frequency band. In the preferred embodiment, the metal foil  2  has an arc length of 68 mm, and a width of 11 mm. These dimensions are equivalent to the dimensions of the conventional flat slot antenna  14  shown in FIG.  13 . The metal foil  2  defines an elongated narrow slot  21  therein. The slot  21  can be dimensioned for operation at a predetermined frequency range. In the preferred embodiment, the slot  21  has dimensions 55 mm ×0.7 mm. The length of the slot  21  equals a half wavelength of the slot antenna  1 , for operation in the frequency range from 2400 MHz to 2500 MHz. A reference line  25  is defined on the arcuate surface  20  at an end of the slot  21 , and perpendicular to the slot  21 . dimensions 55 mm ×0.7 mm. The length of the slot  21  equals a half wavelength of the slot antenna  1 , for operation in the frequency range from 2400 MHz to 2500 MHz. A reference line  25  is defined on the arcuate surface  20  at an end of the slot  21 , and perpendicular to the slot  21 . 
     An inner core of the cable  3  is soldered to the arcuate surface  20  at a first feed-in point  31  near an end of the slot  21 , and an outer shield of the cable  3  is soldered to the arcuate surface  20  at a second feed-in point  32  near the end of the slot  21  and opposite the first feed-in point  31 . 
     The input impedance of a half-wavelength syntonical slot is 500 Ω, while the characteristic impedance of a coaxial cable is about 50 Ω according to the antenna theory. So feed-in points are generally positioned a distance away from a middle of a slot antenna in order to enable the characteristic impedance of a coaxial cable to match the input impedance of the slot and lower the Voltage Standing Wave Ratio (VSWR) of a slot antenna. In the preferred embodiment, each feed-in point  31 ,  32  is an arc distance of 4 mm away from the reference line  25 . This distance is equivalent to the corresponding distance of the conventional flat slot antenna  14  shown in FIG.  13 . The positions of the feed-in points  31 ,  32  may be modified according to degrees to which the slot antenna  1  is bent. 
     According to antenna theory, the higher a frequency is, the more apparent skin effect is. The thickness of the metal foil  2  affects the characteristics of the slot antenna  1  much more than the area of the metal foil  2 . So under high frequency, the metal foil  2  is required to have a. certain degree of thickness to reduce transmission loss of electromagnetic waves generated by the slot antenna  1 . In the preferred embodiment, the metal foil  2  is at least 35 um thick for the frequency band of 2400 MHz˜2500 MHz. 
     Referring to FIGS. 2-3, a notebook computer  4  comprises a mainframe  41  and a lid  42  containing a Liquid Crystal Display (LCD) panel  43 . The LCD panel  43  is mounted on a back plane of the lid  42 , and is spaced apart from a main inner surface  44  of the lid  42 . The slot antenna  1  is attached to a side of the inner surface  44 , as shown by dotted lines beside the LCD panel  43 . The slot antenna  1  is soldered to the inner surface  44  of the notebook  4  at opposite ends of the slot antenna  1 . All the following experimental data and radiation patterns were obtained when the LCD panel  43  was open. 
     FIG. 4 is a graph of experimental data, showing frequency on the horizontal axis varying with the VSWR on the vertical axis. The VSWR determines the rate of power feeding into the antenna and the characteristics of the antenna under a certain frequency. Generally, the VSWR is reasonable when it is greater than 1.0. When the VSWR is less than 2.0, up to 10% of energy is reflected back and becomes transmission loss, and remaining energy is radiated out through the antenna. Antenna designers accordingly seek to attain a VSWR greater than 1 and less than 2 under the desired frequency band. According to the experimental data in the table in FIG. 4, if the frequency is 2400 MHz, the VSWR is 1.789 (see triangle  1 ). If the frequency is 2450 MHz, the VSWR is 1.211 (see triangle  2 ). If the frequency is 2500 MHz, the VSWR is 2.096 (see triangle  3 ). These results are reasonable under the desired frequency band 2400 MHz˜2500 MHz. 
     FIGS. 1 and 5 show a measurement coordinate system and orientation of axes for the preferred embodiment. Measured radiation patterns in different directions are shown in FIGS. 6-11, and are based on such system and orientation. The measured radiation patterns were all obtained at a frequency of 2450 MHz. 
     FIGS. 6 and 7 show radiation patterns in the X-Z plane. The maximum directive gain of the slot antenna  1  was approximately −6 dB in horizontal polarization, and approximately 0 dB in vertical polarization. At radiation angles 90° and 270°, the directive gain was respectively −34 dB and −48 dB in horizontal polarization, and −4 dB and −11 dB in vertical polarization. As is clearly shown in FIGS. 6 and 7, no significant radiation void existed in the directive gain. 
     FIGS. 8 and 9 show radiation patterns in the X-Y plane. The maximum directive gain of the slot antenna  1  was approximately 0 dB in horizontal polarization, and approximately −6 dB in vertical polarization. When the radiation angle was in the range of 0° to 360°, the directive gain was in the range of 0 dB to −42 dB. As is clearly shown in FIGS. 8 and 9, no significant radiation void existed in the directive gain. 
     FIGS. 10 and 11 show radiation patterns in the Y-Z plane. The maximum directive gain of the slot antenna  1  was approximately −9 dB in horizontal polarization, and approximately −3 dB in vertical polarization. When the radiation angle was in the range of 0° to 360°, the directive gain was in the range of −9 dB to −35 dB. As is clearly shown in FIGS. 10 and 11, no significant radiation void existed in the directive gain. Thus the radiation patterns in the X-Y plane, Y-Z plane and X-Z plane were all standard at the frequency of 2450 MHz. 
     The table in FIG. 12 shows total average gains of the slot antenna  1  in he X-Y plane, the Y-Z plane and the X-Z plane at different frequencies when the lid  42  of the notebook computer  4  was open. The table in FIG. 15 shows corresponding data for the conventional flat slot antenna  14  shown in FIG.  13 . 
     Comparing FIG. 12 to FIG. 15, the average gain of the slot antenna  1  in the X-Z plane is 1˜3 dB more than the average gain of the conventional flat slot antenna  14  in the X-Z plane. Furthermore, the average gain of the slot antenna  1  in the X-Y plane is approximately 1˜3 dB more than the average gain of the conventional flat slot antenna  14  in the X-Y plane. Moreover, the average gain of the slot antenna  1  in the Y-Z plane is approximately 0.11 dB more than the average gain of the conventional flat slot antenna  14  in the Y-Z plane. Thus, the total average gain of the slot antenna  1  is 1˜2 dB more than that of the conventional flat slot antenna  14 . 
     In addition, under microscopic view, the slot antenna  1  can be seen to be the equivalent of a synthesis of hundreds of thousands of small flat slot antennas uniformly arranged to face progressively different directions. Moreover, as mentioned before, the foil is diagonally bent in this embodiment. Alternately, a twisted foil may be another performed type used in the invention. 
     In the summary, the experimental data proves that the slot antenna  1  of the present invention achieves omni-directional radiation and increased radiation electric field intensity. 
     It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.